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Final Report
Table of contents
Abstract…………………………………………………
Introduction……………………………………………………
3
Table 1: Existing Wheelchair Scale Products [3-5]………………
Purpose……………………………………………………………...
Method………………………………………………………………
Outline of Report……………………………………………………
Design 1………………………………………………………………
Figure 1: Side view of scale unit………………………………………
Figure 2: Another side view of scale unit - entrance ramps visible…….
Equipment………………………………………………………………
Figure 3: Strain gage array and Wheatstone bridge circuit………
Figure 4: Beam load cell, model LCEC – 1k…………………………
Specification……………………………………………………………
Static Analysis………………………………………………………….
Figure 5: FW = 800 lbs at maximum loading………………………….
Block Diagram of System……………………………………………
Design 2………………………………………………………………...
Figure 6: Front view of scale with four load cells……………………
Figure 7: Side view of scale with four load cells……………………….
Equipment………………………………………………………………
Specifications…………………………………………………………..
Static Force Analysis…………………………………………………...
Assumptions…………………………………………………………….
Figure 8: Free body diagram of four-load-cell design…………………..
Circuit Analysis…………………………………………………………
Block Diagram of System……………………………………………….
Flow chart of the Microprocessor……………………………………….
Figure 9: Flowchart of processing and display unit………………….….
User interface flowchart…………………………………………………
Figure 10: User interface flowchart……………………………………..
Conclusion………………………………………………………………
Design 3…………………………………………………………………
Figure 11: Side view of weighing platform with FTUs near corners…..
Figure 12: Top view of weigh platform………………………………...
Figure 13: FTU diagram showing bending material, strain gage,
and leg pieces causing 3-point bending………………………………...
Static Analysis…………………………………………………………..
Tentative part List……………………………………………………….
Wheatstone Bridge Circuit………………………………………………
Circuit Analysis………………………………………………………….
Block Diagram of System…………………………………………….…
Flow chart of the Microprocessor……………………………………….
Figure 14: Flowchart of processing and display unit……………………
User interface flow chart………………………………………………..
Figure 15: User interface flowchart……………………………………..
Conclusion………………………………………………………………
Choosing the Optimal Design………………………………………….
Optimal Design………………………………………………………….
Objective……………………………………………………………
Figure 16: Front view of scale with four load cells………………….
Figure 17: Side view of scale with four load cells…………………
Figure 18: Top view of optimal design with four load cells………
Equipment……………………………………………………………
Static Force Analysis…………………………………………………
Figure 18: Beam view simulating platform bending behavior……..
Figure 19: Beam view simulating distributed 500 lb load…………
Analysis……………………………………………………………
Circuit Analysis……………………………………………………
Block Diagram of System……………………………………………
Flow chart of the Microprocessor…………………………………
Figure 20: Flowchart of processing and display unit………………
User interface chart…………………………………………………
Figure 21: User interface flowchart………………………………
Final Design Product
Circuit Analysis ................................................................................
Mechanical Design ...........................................................................
Scale Design-Finished Product...........................................................
figures 22-35
Interface Design.................................................................................
Engineering Standards and Environmental Considerations…………..
IEEE Code of Ethics [11]……………………………………………..
Parts List………………………………………………………………
Timeline……………………………………………………………….
Conclusion…………………………………………………………….
References…………………………………………………………….
Acknowledgments…………………………………………………….
Abstract:
A major problem with the weighing of the wheelchair-bound patient is
convenience and ease of use of a wheelchair scale. It is necessary for patients having lung
disorders such as COPD (Chronic Obstructive Pulmonary Disorder) or emphysema to
watch their weight closely, and as a result, a wheelchair scale is needed that can weigh
users once or twice a day and store a history of weights for tracking progress. Since it is
inconvenient to exit the wheelchair and be weighed conventionally, most wheelchair
scales weigh both user and wheelchair, then subtract the weight of the latter. Wheelchair
scales are typically large, bulky, and not designed for home use. The wheelchair-bound
population is one of the largest minorities in the nation and the prohibitive price-tag on
current wheelchair scales create a significant market for a wheelchair scale that can be set
up in one’s home and used as easily as a television.
The typical wheelchair scale consists of a weighing platform, access ramps to the
platform and a display with an interface. Most wheelchair scales have a price between
$1400 and $3200 and exhibit a complex interface that does not log previous weights. The
wheelchair scale outlined in this report will exhibit this unique feature, allowing the user
to assess improvement. Furthermore, the total cost will be below the average wheelchair
scale price, making it affordable and increasing the market potential. Three potential
designs for the wheelchair scale are discussed, and an optimized version of one is
presented in-depth.
Introduction:
Background:
COPD is a disease in which the lungs have difficulty keeping their shape and the
patient to inhale and exhale. COPD is irreversible, and as a result many patients are
permanently wheelchair-bound and must also carry oxygen tanks. Weight gain is
common with COPD sufferers and being underweight or overweight can adversely affect
the patient’s health [1-2]. The device to be designed is intended to help wheelchair-bound
COPD patients assess their relative health through their weight, as physical exercise is
absolutely essential to keep COPD patients healthy. Another goal is to further increase
the convenience encountered by wheelchair-bound users of weighing one’s self at home.
The device to be designed is intended to help wheelchair-bound COPD patients
assess their relative health through their weight, as physical exercise is absolutely
essential to keep COPD patients healthy. Wheelchair scales have been designed and
brought to market: A search on the U.S. Patent and Trademarks Office website resulted in
numerous examples of devices intended to adapt a standard weighing platform or scale to
wheelchair use [6]. The purpose of most scale adapting methods is to check the forward
motion of the wheelchair by its wheels; many contain supports for contact with the
wheelchair chassis itself to keep the platform surface from contacting either the back set
or front set of wheels. Rishel (U.S. Pat. No. 5,448,022) describes a tip-on wheelchair
scale adaptor consisting of a support on which the front of the wheelchair chassis is
tipped and a support holding the back of the wheelchair so that both wheels are off the
platform.
Disadvantages of platforms adapting scales to wheelchair-bound individuals
include significant wobble [6], which may cause discomfort to older persons. Also, this
arrangement, as do many of the patented wheelchair scale platforms, requires “only one
assistant” for each weighing operation. An improvement on the design is found in U.S.
Pat. No. 6,268,572, where Wilson outlines a design for a platform with an access ramp to
be placed on a standard weighing device, so that the individual can access the scale
independently. However, the weight may need to be read by an assistant, as it is visible
through a slot cut out in the platform (placed over the scale) and can be under the
wheelchair chassis [6].
Based on the prior art of patented wheelchair scale adaptors patented, as well as
the prior art status of any weighing device using a strain gage or similar measuring
method, it can be concluded that the device cannot be patented based on its ability to
weigh a wheelchair-bound individual. Many products exist on the market for the
weighing of individuals in wheelchairs; most of these designs are lacking compatibility
with standard computer systems and an interface that is easily accessible to individuals
with hearing or vision impairments. The ability of the product to be patented is limited;
however, it can offer options that cannot be found in the market yet.
There is a large variety of products on the market of wheelchair scales
incorporating various features. Wheelchair scales can be mechanical (typically balancebeam scales) or electronic (transducer-based), and have features added such as wheels
and folding parts for easy storage and setup and smart-card functionality and/or memory
for storing previous weights and weight of the wheelchair for taring. The wheelchair
scale model by Seca model 664 [4] is a state of the art wheelchair scale that is
lightweight, portable, and has the option of being able to store the weights of wheelchairs
belonging to multiple users on smart cards. The weighing platform is large enough to
accommodate any wheelchair user and can be entered using low-grade ramps from either
the front or back. Shallow protective railings on either side of the weighing platform
opposite the access ramps on either side are alternatively used as handles and the entire
scale folds into a compact unit for side storage. The unit costs $1875 dollars plus $65 for
shipping/handling [4].
Detecto, a division of Cardinal Scale Manufacturing, offers several models,
mechanical and digital [3], with large rubberized-mat weighing platforms (minimum 26”
by 30”) and a capacity and resolution standard in the industry (500lb and 0.2 lb, or
1000lb and 0.5 lb res. respectively). The FHD133II/FD144II Geriatric/Bariatric digital
wheelchair scale incorporates a keypad with the display unit for typing in the wheelchair
weight for taring. An ABLEDATA database search for wheelchair scales turned up 12
devices, even including a “do it yourself” entry on how to build a plywood platform to
adapt a bathroom scale to a wheelchair [5]. A typical wheelchair scale solution costs
within the range of $1400 to $3200 and most scales have load limits around 500 lb and
800 lb with resolutions of 0.2 lb. Most wheelchair scales are accessed by ramps leading
to the platform, usually with safety barriers on either side of both ramp and platform.
Table 1: Existing Wheelchair Scale Products [3-5]
Found
Using
Abledata
Model
Fold and
Roll Weigh
6550
Manufacturer
Detecto
Capacity
800 lb
Resolution
0.2 lb
Horizontal
Dimensions
Features
Limitations
Cost
28" * 32"
Option of
preset or
keyed-in tare
weight,
wheels for
easy moving,
0.7" LCD
screen
battery
powered
$2,315
Abledata
Google
search
Model 6495
Stationary
Wheelchair
Scale
Seca Model
664
Detecto
Precision
Weighing
Balances
400 lb
600 lb
0.2 lb
0.2 lb
30" * 26"
Optional AC
adapter,
padded
handrail
Immobile,
only one
entrance
ramp
$2,272
36" * 40.5"
Smart Card
compatibility
option, HOLD
function for
keeping
weight on
screen after
descending,
extremely flat,
batteries or
AC adapter,
folding design
hardly
any; one
of the best
models
found
$1,875
plus
$65
S&H
Purpose:
The device to be designed is a smart weighing scale that wheelchair-bound persons can
use to weigh themselves with daily frequency while inside a wheelchair and that will
store previous weights along with other information. The need for this device is exhibited
in patients with COPD (Chronic Obstructive Pulmonary Disorder) and other lung
disorders such as emphysema. The individuals suffering from such diseases may have to
monitor their weight frequently to avoid medical complications. Since long term
monitoring of a patient’s weight may have to occur at home, it should not be assumed
that assistance is always available and the device must be designed for a home
environment.
The device will meet market standards such as an 800-900 lb weighing capacity
and a resolution of 0.2, and will go above and beyond the current market standards for the
user’s ability to store previous weights, use previous wheelchair weights for taring, and
accessibility to hearing impaired or vision impaired users. To be competitive in the
wheelchair scale market, the device must have a cost below the average cost of $2000 for
a wheelchair scale (see Table 1).
The scale must be durable enough to provide long-term accurate weighing of the
user and should keep the user as stable during the weighing operation as he or she is on
solid ground. No assistance should be required for any level of operation for the device
except installation. Taring (the process of weighing the individual by subtracting the
wheelchair weight from the weight of wheelchair plus occupant) must be a process the
user can execute unassisted. The system should have a simple, easy to use interface that
can remember previous weighings of a user and also the weight of the wheelchair for
taring. Interface components such as buttons and displays should be large and accessible.
The cost of the device should be affordable compared to other marketed models filling
the same need, and should create a pleasant weighing experience to help motivate
individuals with respect to weight control.
Method:
Since the device consists of a supporting surface for a wheelchair and an access
path to bring the wheelchair to it, wheelchair engineering standards must be applied to
determine the size, height, and static and dynamic requirements of both. The tipping
angle, the sideways angle at which a static wheelchair will tip over (measured for side,
front and back) is of primary importance. The device must be designed so that the
maximum wheelchair angle possible, with different sides of a wheelchair on the ground
and highest point on the device respectively, must be lower than the smallest tipping
angle. The wheelchair may be manual or powered; access ramps must be designed so that
the effort for a manual wheelchair user to mount the platform is less than that required to
ascend typical wheelchair-accessibility ramps to most buildings. If the wheelchair mounts
the platform at certain high speeds, lateral stability tests for common wheelchairs should
dictate under what limits the device should respond [7].
A budget of $2000 is provided for the design and implementation of the
wheelchair scale. The total amount of money including the cost of parts, fabrication, and
testing cannot exceed $2000. To be manufactured industrially, the device must be made
out of parts, materials or assemblies that are commercially available from reliable
suppliers. Examples of these are standard compression load cells, plywood boards, sheet
metal, rubberized mat sections, wooden beams, etc… It must be possible to assemble all
the parts into the device using conventional manufacturing methods.
A set of tests must be employed (e.g. voltage/load, max. load, beam deflection) to
determine beyond a doubt that the device performs according to specifications without
destroying the functionality of the device. The operation of the device by wheelchairbound individuals and perhaps by walking individuals cannot endanger the health of
either party. Safety measures such as smooth edges, barriers to block the slipping of
wheelchairs and surfaces with traction should be implemented to prevent injury.
Repeated use should not adversely affect the health of the user.
The most common method of load sensing involves a compression load cell. The
selection of this component is by far the most critical in all aspects of the design of the
scale. A single load cell with a 1000 lb capacity will cost between $300-$500, and four
load cells with a 250 lb capacity would each cost no less than $150, effecting a total cost
of over $600. A four load cell design would enable more distributed placement of the
platform on the cells and would perhaps enhance accuracy. Electronics equipment is
normally inexpensive – a printed PCB board and microprocessor for the display and
computation unit would cost around $150 including cords, display and housing. Building
materials such as plywood, sheet metal and fittings may cost around $200 total. Together,
the estimate of the total cost of the device to manufacture is $1800. This device can be
built with the given budget of $2000.
Outline of Report:
Three different designs for the wheelchair scale are presented in sequence,
denoted as Design 1, Design 2, and Design 3. Each design is introduced with a short
description of the system and specific design issues. Labeled diagrams of the top, side
and front views of the system are then provided with specific measurements. An
equipment section describes the necessary materials and parts and their specifications.
Analysis of the system follows in a section divided into static analysis and electronic
analysis. The benefits and disadvantages of the system are then summarized in a
conclusion section.
Design 1
: Single Load Cell:
Description:
A single load cell will be used to determine the weight resting on the platform,
since this component is particularly expensive, although a design featuring four load cells
at the corners would be more stable and accurate. The corners of the platform will be
supported by springs or another elastic element such that the platform must be depressed
by some threshold weight before contacting the top of the cell under it. Access ramps on
opposite sides of the scale with ramp angle no greater than 10 degrees will be level with
the platform, which will have height no greater than 2 inches. The springs, load cell, and
ramp will rest on or be supported by a crosspiece-assembly under the platform. The
signal from the load cell will travel through a cable to a processing and display unit.
The processing and display unit will have a memory capable of storing 11
numerical values with a precision of four decimal places (10 historical weights, 1
wheelchair weight for taring, accuracy of 0.2 lb) and will be connected to an LCD screen
sufficiently large enough for low-vision accessibility. Large buttons and audible cues
through a speaker will also make the device more accessible to handicapped individuals.
The processor will hold a written software program containing:

an interface through which users can weigh themselves, store weight, key in
wheelchair weight and cycle through previous weighings

an algorithm to calculate actual weight based on stored or inputted
wheelchair weight and signal from load cell
This processor and other components mentioned will be in a circuit powered by
an AC adapter or standard batteries and will turn on and off by a switch or by a threshold
load placed on the platform.
34"
30"
Platform
2"
Load
Cell
Spring
36"
Note: Vertical scale different from
horizontal scale for higher visibility
Figure SEQ Figure \* ARABIC 4: Side view of scale unit
34"
30"
Side
Panel
Platform
2"
Load
Cell
12"
36"
12"
Note: Vertical scale different from
horizontal scale for higher visibility
Figure SEQ Figure \* ARABIC 5: Another side view of scale unit - entrance ramps
visible
1.4"
Equipment
Load cells are the primary measurement instruments in weighing devices. Load cells
consist of multiple strain gages (see figure 1), which measure resistance when it is put
under a compression or tension force. Strain gages are made up of thin metallic foils
called a “Gage Patch.” The multiple gages patches are connected tighter and attached
inside of the load cell. When the surface of the cell experiences a force the metallic wires
experience the same force. The multiple strain gages that make up a load cell are wired
together to form the legs of a Wheatstone bridge (see figure 1). The Wheatstone bridge
creates an electric current that results from the response of a load that is applied to the
cell in the form of an analog signal. A microprocessor is then used to convert the analog
signal to a digital signal, so the output from the load cell can be determined and read on
an electronic LCD display [8].
Figure 3: Strain gage array and
Wheatstone bridge circuit
Figure 4: Beam load cell, model LCEC – 1k
“Model LCEC load cells are designed to operate in all weather or
washdown environments. Their low profile and high side load capability simplify
mechanical installation considerations. Their weather sealing, high precision and
repeatability make them ideally suited for rugged industrial applications such as
testing, batching, weigh pits and other applications exposed to the elements…”
SPECIFICATIONS
Excitation: 10 Vdc (15 V max)
Output: 3 mV/V nominal
Calibration: NIST Traceable
Linearity: ±0.03% FS
Hysteresis: ±0.02% FS
Repeatability: ±0.01% FS
Creep (after 20 minutes): ±0.03%
Zero Balance: ±1% FS
Operating Temp Range:
–55 to 90°C (–65 to 200°F)
Compensated Temp Range:
–15 to 65°C (0 to 150°F)
Thermal Effects:
Zero: ±0.0015% Rdg/°F max
Span: ±0.0008% FS/°F max
Safe Overload: ±150% of Capacity
Ultimate Overload: ±300% of Capacity
Input Resistance: 350 +50/–3.5 
Allowable Side Load at Rated Load:
50% Rated Capacity
Output Resistance: 350 ±3.5 
Construction: High Carbon Steel
Electrical: 5 ft. (1.5 m) insulated
4-conductor shielded color coded cable
Metric Ranges Available - Consult Engineering *See Section D For Compatible
Meters
Ordering Examples: 1) LCEC-250 is a 250 lb capacity load cell, $265.
2) LCEC-1K is a 1000 lb capacity load cell, $285.
( *All information taken from Omega Engineering, 2004 [8])
Static Analysis:
A statics analysis is a valid method of calculating the forces acting on the weighing
platform because we can assume there is no significant dynamic behavior. The scale unit
is a passive weighing device, and it is assumed the wheelchair user moves onto the
platform relatively slowly. Two statics properties are used here – the sum of moments
around a point and the sum of forces acting on the platform, both equaling zero.
Assumptions:
1) platform of equal length and width greater than 30 in.
2) platform is supported at five points – at four corners by springs, and at center by
load cell
3) axial centers of springs make 30 in. * 30 in. square representing platform
modeled here
4) load cell is contacted by platform only if load is greater than some threshold, i.e.
springs must undergo some displacement before load cell begins supporting
FW
F4
F3
FL
F1
F2
30"
60
B
A
Platform
Load
Cell
Spring
30"
Figure 5: FW = 800 lbs at maximum loading
A) Sum of moments about one corner, F1 :
M
F1
 30 * ( F 4  F 2)  30 2 * ( F 3)  15 2 * ( FL)  A2  B2 * ( FW )  0
B) Sum of vertical forces
F
Y
F1  F 2  F 3  F 4  FL  Fw  0
* Load cell must be able to measure maximum weight of 800 lbs, as per design
specifications.
* Assume when midpoint between wheels is over center of load cell, most accurate
reading is obtained.
Since the platform is suspended in part by elastic elements and only supported
occasionally by a stable fulcrum (the load cell), the tilting of the platform must be
accounted for.
C) Spring Constant:

Assume maximum acceptable platform tilt angle = 5 deg.

Side view of platform: worst case scenario is 800 lb load applied to middle of
one edge of platform. At this point, platform must have angle of no more than 5
deg.

Assume that two-spring system with springs supporting one-dimensional lever
with load cell in middle as fulcrum approximates actual scenario – from
symmetry, load reduces to 400 lbs

Assume that lever with one end as fulcrum, spring on other end, load applied at
spring is an approximation of the one-dimensional lever.

Assume Hooke’s law applies to any displacement of spring:
Max angle = 5 deg.
Tan(5 deg) = 0.08749 = (Spring displacement) / (Lever length of 30 in.)
= ( 400 lbs / k) / (Lever length of 30 in)
Solving for k, the spring constant:
k = 150 lbs/in. , approximately
This constant provides a good indication of what springs will be suitable for
supporting the platform. Any elastic element that is short and wide enough to allow
movement only in a vertical direction (with tall springs, torsional and shear forces may
cause significant movement to the sides) and will furnish approximately 150 lbs of force
per inch of compression or tension from equilibrium is suitable to install at the four
corners of the platform and scale base support so that an 800 lb force at the middle of one
edge of the platform will not tilt the platform more than five degrees. If the platform is
allowed to tilt, the wheelchair may roll to the side or off the scale, or the resulting gap
between the platform and ramp could damage the scale or tip the wheelchair.
Block Diagram of System:
Load
Power supply
Force Transducer
Signal Filter
Signal Amplifier
User Input
MIcroprocessor
LCD Unit
Design 2
: Four Load Cells:
A set of four load cells will be used to determine the weight resting on the
platform. The load cells will be positioned at the four corners of the platform, supporting
the platform statically and inhibiting movement. Since all force exerted on the platform
must be sensed by the cells, they must be the only support for the platform; however,
there cannot be a significant horizontal component to the forces exerted on the cells.
Access ramps on opposite sides of the scale with ramp angle no greater than 10 degrees
will be level with the platform, which will have height no greater than 2 inches. The load
cells and ramp will rest on or be supported by a lower platform or crosspiece assembly
under the platform. The signal from the load cell will travel through a cable to a
processing and display unit.
The processing and display unit will have a memory capable of storing 11
numerical values with a precision of four decimal places (10 historical weights, 1
wheelchair weight for taring, accuracy of 0.2 lb) and will be connected to an LCD screen
sufficiently large enough for low-vision accessibility. Large buttons and audible cues
through a speaker will also make the device more accessible to handicapped individuals.
The processor will hold a written software program containing:

an interface through which users can weigh themselves, store weight, key in
wheelchair weight and cycle through previous weightings

an algorithm to calculate actual weight based on stored or inputted
wheelchair weight and signals from load cells
This processor and other components mentioned will be in a circuit powered by
an AC adapter or standard batteries and will turn on and off by a switch or by a threshold
load placed on the platform.
34"
30"
Platform
Load
Cell
Load
Cell
2"
36"
Note: Vertical scale different from
horizontal scale for higher visibility
Figure 6: Front view of scale with four load cells
34"
30"
Side
Panel
Platform
Load
Cell
12"
Load
Cell
36"
Note: Vertical scale different from
horizontal scale for higher visibility
Figure 7: Side view of scale with four load cells
2"
12"
1.4"
Equipment
Load cells are the primary measurement instruments in weighing devices. Load cells
consist of multiple strain gages (see figure 1), which measure resistance when it is put
under a compression or tension force. Strain gages are made up of thin metallic foils
called a “Gage Patch.” The multiple gages patches are connected tighter and attached
inside of the load cell. When the surface of the cell experiences a force the metallic wires
experience the same force. The multiple strain gages that make up a load cell are wired
together to form the legs of a Wheatstone bridge (see figure 1). The Wheatstone bridge
creates an electric current that results from the response of a load that is applied to the
cell in the form of an analog signal. A microprocessor is then used to convert the analog
signal to a digital signal, so the output from the load cell can be determined and read on
an electronic LCD display [8].
Series 350
Shear beam load cells for
Industrial weighing
Capacities 300 kg to 5,000 kg
5 year warranty
Simple low cost installation
Alloy version fully potted and sealed to IP 66
Alloy (350a) or stainless steel (350i) load sensor
Optional load feet, mounting plates, load buttons
And supports
Industry standard configuration
Corrosion resistant
Stainless steel version fully welded and
Hermetically sealed to IP68
Optional anti-vibration loading assembly available 3000 divisions OIML R60
Class C
Specifications [8]
Load Cell Capacity Units
Standard Load Ranges 300, 500, 750, 1000, kg
1500, 2000, 3000, 5000 kg
Rated Output 2 mV/V±0.1%
Accuracy Class 3000 n.OIML
Combined Error <±0.017 %*
Non-repeatability <±0.015 %*
Creep (30 minutes) <±0.016 %*
Temperature Effect on Zero Balance <±0.01 %*/°C
Temperature Effect on Span <±0.006 %*/°C
Compensated Temperature Range -10 to +40 °C
Operating Temperature Range -20 to +70 °C
Safe Overload 150 %*
Ultimate Overload 200 %*
Zero Balance <±2 %*
Input Resistance 400 ±30
Output Resistance 350 ±1.5
Insulation Resistance >5000 M@ 100V
Recommended Supply Voltage 10 V
Maximum Supply Voltage 15 V
Environmental Protection 350i=IP68 350a=IP66 –
Load Foot Accessories for Series 350 Shear Beam [9]
The cells could be excited (via a junction box) by a cost effective instrument such as
our XT1200 unit.
JB4T JUNCTION BOX
Pricing from Thames Side-Maywood
Hole size 4mm
European
X350A-300
US
SHEAR BEAM LOAD CELL
108
COST EACH
£68.00




TOTAL
QUANTITY 4
X35913-FOOT
$125.12
£272.00
$500.48
£9.00
$16.56
80
50
CAPACITY 300KG (660lbs)
LEVELLING FOOT
COST EACH
QUANTITY 4
120
TOTAL
£36.00
$66.42
ALL DIMENSIONS IN MILLIMETRES (Not to Scale)
JB4T
JUNCTION BOX (IP65)
4 WAY SUMMING
55
COST EACH
£40.00
$73.60
The resolution required would easily be achieved (and much better). The load
cell is approved to OIML R60 C3 which is the European Weights and Measures accuracy
requirement. It would be important that the wheel chair platform structure that the cells
would support is very rigid in nature because any bending or flexing would produce
accuracy errors.
Data and quotes provided by Thames Side-Maywood (logo below) [9]:
Static Force Analysis:
Since weighing a moving vehicle in the form of a two-wheeled or four-wheeled
wheelchair is assumed to occur at low speeds, a dynamic analysis of the four-load-cell
system is not considered necessary. The static analysis is based on the zero-sum totals of
the moments about any point and the forces in any direction. The weight of the
wheelchair and user is considered here as one force vector in position (A, B) from one
corner, and the dimensional measurement of 30 inches square is specified from the center
of each load cell to form a square.
The part of the platform not inside the square formed by the supporting points of
each load cell is considered unloaded, or free. The platform, load cells, and supporting
structure for the cells are assumed to be relatively level. Since a maximum operating
weight of 800 lbs (center loading) is specified, this value is used to calculate the worstcase scenario for static loading. Each load cell is fastened to the platform and supporting
structure, making the entire structure immobile. Because of this, each loading point
should have six degrees of freedom (3 translational forces, 3 rotational moments);
however, only 1 translational force and 3 moments are considered in this analysis.
Assumptions:
1) Dynamic effects are negligible
2) Wheelchair weight can be approximated as single downward vector
3) All relevant forces are vertical
4) The platform is fastened securely to each load cell
FW
F4
F3
F1
F2
30"
60
B
A
Platform
30"
Load Cell
Figure 8: Free body diagram of four-load-cell design
Sum of vertical forces:
Equation SEQ Equation \* ARABIC 1
F
Y
F1  F 2  F 3  F 4  FW  0
Sum of moments about point at F1:
Equation SEQ Equation \* ARABIC 2
M
F1
 30 * ( F 4  F 2)  30 2 * ( F 3)  A2  B 2 * ( FW )  0
FW = 800 lbs maximum:
* Assuming platform weight is negligible, worst case scenario can be assumed to be
entrance loading. The wheelchair must at some point during the process rest entirely on
the outer edge, presumably between the two front load cells. In this case, the weight is
distributed almost entirely between the two load cells, making the necessary maximum
loading weight 800 / 2, or 400 lbs. However, this is different from the maximum load that
can be measured. If it is assumed that the scale system can detect when the average
loading of the wheelchair is in the approximate center of the scale, an accurate average
reading can be taken and the total reaction force on each load cell is approximately 200
lbs. It is this maximum force that must be measurable. The load cells described in the
Equipment section have a maximum measurable load of 660 lbs.
* The sum of all the load cell signals from the platform equals the total weight of the
platform, assuming only vertical forces. This quadruples the possible accuracy of the
scale unit and also makes the unit more stable than a four-spring, single load cell design.
The readings from the load cells enable the processor to ascertain the position of the user
on the scale as well, which should ideally be in the middle of the platform for the most
accurate reading.
Circuit Analysis:
-5V
Load Cell
+5V
R1
Input
Output
+5V
R1
R2
R3
R4
In1
In2
In3
In4
Output
+5V
-5V
Microprocessor
out1
out2
out3
out4
out5
out1
out2
out3
out4
out5
LCD Display
vcc
Input
vcc
R2
ground
Load Cell
+5V
ground
-5V
Load Cell
+5V
R3
Input
+5V
+5V
Output
+5V
-5V
Load Cell
+5V
R4
Output
vcc
1
2
3
4
+5V
+5V
Input
User Interface
ground
The above circuit is a generic view of a real circuit that is going to be mounted on our
design. The design might have changes in terms of resistors values and the type of
MosFets we are going to use in our optimal design. Also the real design might have a
less inputs in order to make the design more accurate and affordable. The basic approach
to the circuit is very simple, the four main outputs from the load cells are going through
filters so the voltage values entering the microprocessor are without any noise and
interference, also these voltage values go through some resistance in order to make them
suitable for the requirements of the microprocessor, different microprocessors have
varying input voltage acceptance values. The microprocessor also gets input values from
the User Interface, all these inputs of the microprocessor go through calculations and
memory storage sequences and then the result is displayed on the LCD Display.
Block Diagram of System:
The above block diagram is the electrical part of the design; the power supply is
shown supplying a voltage to the load cell (force transducer) and at the same time, the
load cell’s resistance changes in proportion to the load applied axially. A Wheatstone
bridge circuit is used to detect the extremely small changes in resistance, and this
constitutes the output signal from the cell. Once the load cell signal reaches the linear or
steady state, then the analog signal is sent to the Signal filter where it is transformed from
a noisy signal to a continuous one and then this output is sent to the Signal Amplifier for
possibly voltage or current adjustments before the signal goes to the Microprocessor.
The Microprocessor gets two inputs one from the user and the other from the Signal
Amplifier. Once the Microprocessor is done with its arithmetic and all the memory
storing and sorting, then the final weight is displayed onto the LCD display. The scale
turns on as the user moves onto the platform with some minimum weight or when the
power switch is toggled.
Flow chart of the Microprocessor:
The computation and user interface for the weighing system is
accomplished by a microprocessor with user input switches and an LCD monitor, as well
as possibly a piezoelectric speaker. The following is a microprocessor flow chart, which
shows the different functions performed by the user and the microprocessor while
computing the weight of the person (see Fig. 4).
Analog
output from
the Load
Cell
The analog signal coming
from the Load Cell is being
converted to digital signal
using the A/D component
of the Microprocessor
The A/D input of the
Microprocessor
The user enters
the value of the
wheel chair and
then presses
enter.
The digital signal coming
from the microprocessor is
being sent to the input of
the ALU component of the
Microprocessor where the
subtraction of the wheel
chair weight from the total
weight takes place.
Wheel Chair
input from
the User
The digital value of the weight being
converted from the Load Cell is stored
into some register ‘x’ and then the user
value entered is being stored in some
register ‘y’. The ALU component of the
microprocessor does the calculations
and stores the final value in the register
‘z’, which is also stored in RAM, so it
could be accessed again.
ALU functions of the
Microprocessor
Up to 10 different weights
of the person are
registered in this memory,
for the records of the user.
Random Access
Memory (RAM)
of the
Microprocessor
Digital to Ib/
kg converter
The output of the
Microprocessor is
displayed on the LCD
display.
The LCD display
Figure 9: Flowchart of processing and display unit
User interface flowchart:
The user pushes the
button on the panel which
brings up the previous
measurements of the
patient.
Accessing the
previous
measuremtns
.
Scale Process
Menu:
1-Power
2-Weigh
3-History
4-Change
wheelchair weight
Switch on
Switch off
Filtered weight
If time > 5 min
Access
wheelchair
weight
Yes
display
No
display
Hold till scale is
stable
Display: “No wheelchair
weight stored.” “Store
wheelchair weight?”
No wheelchair
weight stored
Yes
No
Input
wheel chair
weight
Go to
Menu
If time > 2 min
Filter weight
“Error too
much
movement”
Store Wheelchair
weight as 1-10
Display
“Your
weight
is__.”
Go to
Menu
“Store to
memory?”
Yes
Store in next
available space,
bump out oldest
entry if full
No
Go to
Menu
Figure 10: User interface flowchart
Figure 5 above shows the user interface flowchart enabling the user to navigate
through four options – turn the scale on/off, weigh, check weight history, and change
wheelchair weight. There are few menu levels, making the interface easy to use for most
people. The memory is accessed by the processor whenever the user selects the option to
actually store their weight, and the oldest entry for weight is deleted and all entries are
moved back to make room for the new one. The user must know their wheelchair weight
and type it on only once as it is stored in the memory. The machine memory must be able
to handle repeated and constant use, and must not fail in a power outage. A battery can be
provided to ensure that stored data is not lost.
Conclusion:
This design offers many beneficial features that make this product highly accurate. This
design includes a higher cost aspect but involves a more accurate approach to weight
measurements with the addition of four load cells. The combination of four load cells
provides an in-depth average of the individual’s weight, because it will not matter where
exactly the individual is placed on the scale providing easier access. This design also
does not include springs to act as supports; instead the four load cells will act as supports
for the platform with their placement in load feet.
The display unit will be large enough for an individual with moderately low
vision to see and operate. The operation of the scale and display unit will also be user
friendly and allow enough memory to store collected weightings for up to 10 times. The
circuit analysis in this design is more accurate and affordable to build. The usage of
fewer resistors and capacitors is taken into consideration.
Design 3
: Force Transducer Units:
A set of four force transducer units (FTUs) will be used to determine the weight
resting on the platform. The units will be positioned near the four corners of the platform,
supporting the platform statically and inhibiting movement. Since all downward force
exerted on the platform must be sensed by the FTUs, they must be the only support for
the platform; however, there cannot be a significant horizontal component to the forces
exerted on the cells. Access ramps on opposite sides of the scale with ramp angle no
greater than 10 degrees will be level with the platform, which will have height no greater
than 2 inches. The FTUs and ramp will rest on or be supported by a lower platform or
crosspiece assembly under the platform. The signal from the FTUs will travel through
wiring to processing circuitry and a computing / display unit.
The processing and display unit will have a memory capable of storing 11
numerical values with a precision of four decimal places (10 historical weights, 1
wheelchair weight for taring, accuracy of 0.2 lb) and will be connected to an LCD screen
sufficiently large enough for low-vision accessibility. Large buttons and audible cues
through a speaker will also make the device more accessible to handicapped individuals.
The processor will hold a written software program containing:

an interface through which users can weigh themselves, store weight, key in
wheelchair weight and cycle through previous weightings

an algorithm to calculate actual weight based on stored or inputted
wheelchair weight and signals from force transducers
This processor and other components mentioned will be in a circuit powered by
an AC adapter or standard batteries and will turn on and off by a switch or by a threshold
load placed on the platform.
Diagrams:
Figure 11: Side view of weighing platform with FTUs near corners
Figure 12: Top view of weigh platform
Figure 13: FTU diagram showing bending material, strain gage, and leg pieces causing
3-point bending
Static Analysis:
At each corner of the weighing platform, a beam of steel with assumed uniform
properties undergoes 3-point bending. The bending moment of the bottom of the beam
(tension) directly under the loading sleeve is proportional to the transverse load on the
beam, and the bending strain produced on the outer surface of the beam is in turn
proportional to this moment. A strain gage is attached to this area of the bending beam
and records the strain produced. Four strain readings are each read using a quarter-bridge
Wheatstone bridge circuit, filtered, converted into load readings, and summed to calculate
the total downward force on the platform.
Bending bar material in Force Transducer Unit (FTU): A36 hot-rolled (structural) steel
Given (to 3 significant digits):
Length of each side of weighing platform:
0.889 m (35.0 in.)
Total length of bending bar in FTU:
0.180 m (7.09 in.)
Length of bar experiencing free bending moment:
0.150 m (5.91 in.)
Width of bar:
0.02 m (0.787 in.)
Thickness of bar:
0.003 m (0.118 in)
Maximum measuring load on center of bar:
200 lbs (889 N)
Maximum load on center of bar:
400 lbs (1780 N)
Yield strength of A36 hot-rolled structural steel:
150 Map compressions
250 Map tension
Elastic modulus of A36 hot-rolled structural steel:
200 Gpa
Calculations:
 M  0  F1* R1  F 2 * R2  F 3 * R3  F 4 * R4  Fw1* Rw1  Fw2 * Rw2
F
y
 0  F1  F 2  F 3  F 4  Fw1  Fw2
1) Calculate forces F1, F2, F3, and F4 for wheelchair entrance to ramp (loading on edge
of platform, see Diagram 3 in notes):
Edge-loading, 800 lbs - each wheel assumed to transfer 400 lbs:
Fw1 = Fw2 = 400 lbs
Center-edge loading assumed: A = (0.889 m – C) / 2, B = (0.889m + C) / 2
Assume wheelchair wheel width of 0.7 m, A = .095 m, B = 0 m
Symmetry: F1 = F2, F3 = F4
Sum of moments about axis through F1, F2 = (400 + 400lbs)*(0.0635m) + (F3 +
F4)*(0.762m) = 0
F3 = F4 = -67 lbs, downward force (tension)
Sum of vertical forces = -400 - 400 – 67 – 67 + F1 + F2 = 0
F1 = F2 = 467 lbs, upward force
2) Calculate forces F1, F2, F3, and F4 for wheelchair in center of ramp:
Fw1 and Fw2 are centered: A = 0.095, B = 0.381 m
Four-way symmetry: F1, F2, F3, F4 all equal
Sum of vertical forces = -400 – 400 + F1 + F2 + F3 + F4 = 0
F1 = F2 = F3 = F4 = 200 lbs
3) Calculate maximum shearing stress in bending beam:
Factor of safety (Nfs) = 1.3: 467 * 1.3 = 600 lbs
Reactions at either end of beam: R1 = R2 = 600 / 2 = 300 lbs
Max. shear in beam, Vmax = 300 lbs / cross sectional area
Convert to Newton’s: 300 lbs * 0.4536 kg/lb * 9.8 m/s2 = 1334 N
Shear: 1334 N / (0.02*0.003m) = 22.2 Map
4) Calculate maximum bending strain on underside of beam:
Factor of safety (Nfs) of 1.3: 467 * 1.3 = 600 lbs
Convert to Newton’s:
m 
Mc
I
600 lbs * 0.4536 kg/lb * 9.8 m/s2 = 2670 N
I  bh 3
M 

LF
8
m
E
Here, b = width of beam, h = thickness, c = h / 2, L is length of beam, F is
the vertical load in the center of the beam, E is the elastic modulus, and M is the
bending moment in the center of the beam. The bending moment equation is a
generalized equation assuming ideal beam behavior, and yields the maximum
bending moment in the beam, happening in the center. The maximum strain as
produced under a 600 lb load is given as:
 max 
Mc
LF

IE 16Eaw 2
where L is the bending length of the beam, a is the width of the beam, and w is
the thickness.
For a load of 600 lbs (2670 N), a free-bending length of 0.150 m, a width of 0.020
m, and a thickness of 0.003 m (see given specifications above),
Maximum bending strain = (2670 N) (0.150) / (16*200e9*0.02*0.003^2)
= 6.953e-4 = 695 micro strain. = (delta L / L)
Analysis:
I.
None of the stresses calculated exceeded the minimum yield strength for A36
structural steel, 150 MPa in compression. It is assumed that any stress
exceeding this value will permanently deform the FTU, leading to failure or
inaccurate readings.
II.
Strain gages typically measure 1-1000 or 1-1500 micro strain. The amount of
micro strain calculated for maximum-loading conditions with a safety factor
of 1.3 was 695 micro strains. Since the maximum loading for a centered
wheelchair would be one-third this value, the maximum strain to be measured
is 232 micro strains. This is very much within the operating conditions of a
strain gage.
Tentative Parts List:
Part
Qty
Fasteners, 1/4" bolts with nuts
36
beams
4
support squares
8
sleeves
4
weigh platform
1
support platform
1
ramps
2
LCD display
1
Strain gages
5
A/D converter
4
Filters
4
Wheatstone bridge Circuit:
The strain gages are measured using the Wheatstone bridge circuit analysis, where four
resistors are placed of equal value, if the fourth resistor which is the strain gage itself
doesn’t experience any changes then the circuit will generate no voltage, on the other
hand if the circuit experience a slight resistance change in the strain gage resistor then the
circuit will generate a voltage in millivolts. Since we are using five strain gages all their
outputs are being added together and then send to the A/D converter of the
microprocessor [10]. The fifth strain gage is placed in the circuit so the issue of
temperature affecting the circuit is dealt with, the fifth strain gage is placed on the piece
of metal similar to the one used for the other four strain gages, and one the temperature
changes the resistance of the strain gage then we know how much calibration value we
should add to the main circuit. The would be circuit of the strain gage is displayed below
using only one strain gage [10];
W heatstone bridge circuit using one strain gage
Vout in mvolts
R3
Strain gage resistance
R1
+5V
R2
The above circuit shows only one part of the Wheatstone bridge analysis, the way it is
designed, resistors R1, R2, R3, and strain gage resistor are all equal in value, which
means no voltage generated to the output but if there is a slight resistance change in strain
gage resistor due to strain then the output terminal gets a voltage value.
Circuit Analysis:
-5V
Load Cell
+5V
R1
Input
Output
+5V
R1
R2
R3
R4
In1
In2
In3
In4
Output
+5V
-5V
Microprocessor
out1
out2
out3
out4
out5
out1
out2
out3
out4
out5
LCD Display
vcc
Input
ground
R2
vcc
+5V
ground
-5V
Load Cell
Load Cell
+5V
R3
Input
+5V
+5V
Output
+5V
-5V
Load Cell
+5V
R4
Input
Output
vcc
1
2
3
4
+5V
+5V
ground
User Interface
The above circuit is a generic view of a real circuit that is going to be mounted on our
design. The design might have changes in terms of resistors values and the type of
MosFets we are going to use in our optimal design. Also the real design might have a
less inputs in order to make the design more accurate and affordable. The basic approach
to the circuit is very simple, the four main outputs from the load cells are going through
filters so the voltage values entering the microprocessor are without any noise and
interference, also these voltage values go through some resistance in order to make them
suitable for the requirements of the microprocessor, different microprocessors have
varying input voltage acceptance values. The microprocessor also gets input values from
the User Interface, all these inputs of the microprocessor go through calculations and
memory storage sequences and then the result is displayed on the LCD Display.
Block Diagram of System:
The above block diagram is the electrical part of the design; the power supply is
shown supplying a voltage to the load cell (force transducer) and at the same time, the
load cell’s resistance changes in proportion to the load applied axially. A Wheatstone
bridge circuit is used to detect the extremely small changes in resistance, and this
constitutes the output signal from the cell. Once the load cell signal reaches the linear or
steady state, then the analog signal is sent to the Signal filter where it is transformed from
a noisy signal to a continuous one and then this output is sent to the Signal Amplifier for
possibly voltage or current adjustments before the signal goes to the Microprocessor.
The Microprocessor gets two inputs one from the user and the other from the Signal
Amplifier. Once the Microprocessor is done with its arithmetic and all the memory
storing and sorting, then the final weight is displayed onto the LCD display. The scale
turns on as the user moves onto the platform with some minimum weight or when the
power switch is toggled.
Flow chart of the Microprocessor:
The computation and user interface for the weighing system is
accomplished by a microprocessor with user input switches and an LCD monitor, as well
as possibly a piezoelectric speaker. The following is a microprocessor flow chart, which
shows the different functions performed by the user and the microprocessor while
computing the weight of the person (see Fig. 4).
Analog
output from
the Load
Cell
The analog signal coming
from the Load Cell is being
converted to digital signal
using the A/D component
of the Microprocessor
The A/D input of the
Microprocessor
The user enters
the value of the
wheel chair and
then presses
enter.
Wheel Chair
input from
the User
The digital signal coming
from the microprocessor is
being sent to the input of
the ALU component of the
Microprocessor where the
subtraction of the wheel
chair weight from the total
weight takes place.
The digital value of the weight being
converted from the Load Cell is stored
into some register ‘x’ and then the user
value entered is being stored in some
register ‘y’. The ALU component of the
microprocessor does the calculations
and stores the final value in the register
‘z’, which is also stored in RAM, so it
could be accessed again.
ALU functions of the
Microprocessor
Up to 10 different weights
of the person are
registered in this memory,
for the records of the user.
Digital to Ib/
kg converter
The output of the
Microprocessor is
displayed on the LCD
display.
Random Access
Memory (RAM)
of the
Microprocessor
The LCD display
Figure 14: Flowchart of processing and display unit
The user pushes the
button on the panel which
brings up the previous
measurements of the
patient.
Accessing the
previous
measuremtns
.
User interface flowchart:
Scale Process
Menu:
1-Power
2-Weigh
3-History
4-Change
wheelchair weight
Switch on
Switch off
Filtered weight
If time > 5 min
Access
wheelchair
weight
Yes
display
No
display
Hold till scale is
stable
Display: “No wheelchair
weight stored.” “Store
wheelchair weight?”
No wheelchair
weight stored
Yes
No
Input
wheel chair
weight
Go to
Menu
If time > 2 min
Filter weight
“Error too
much
movement”
Store Wheelchair
weight as 1-10
Display
“Your
weight
is__.”
Go to
Menu
“Store to
memory?”
Yes
Store in next
available space,
bump out oldest
entry if full
No
Go to
Menu
Figure 15: User interface flowchart
Figure 5 above shows the user interface flowchart enabling the user to navigate
through four options – turn the scale on/off, weigh, check weight history, and change
wheelchair weight. There are few menu levels, making the interface easy to use for most
people. The memory is accessed by the processor whenever the user selects the option to
actually store their weight, and the oldest entry for weight is deleted and all entries are
moved back to make room for the new one. The user must know their wheelchair weight
and type it on only once as it is stored in the memory. The machine memory must be able
to handle repeated and constant use, and must not fail in a power outage. A battery can be
provided to ensure that stored data is not lost.
Conclusion:
This design offers many beneficial features that make this product highly accurate. This
design includes a higher cost aspect but involves a more accurate approach to weight
measurements with the addition of the strain gage analysis. The combination of five
strain gages provides an in-depth average of the individual’s weight, because it will not
matter where exactly the individual is placed on the scale providing easier access the
weight of the individual will be calculated very accurately. This design also does not
include springs to act as supports; instead the five strain gages will act as supports for the
platform with their placement in different part of the platform.
The display unit will be large enough for an individual with moderately low
vision to see and operate. The operation of the scale and display unit will also be user
friendly and allow enough memory to store collected weightings for up to 10 times. The
circuit analysis in this design is more accurate and affordable to build. The usage of
fewer resistors and capacitors is taken into consideration.
Choosing the optimal design:
Optimal design is chosen because it provides the best solution and at the same time it is
more cost efficient compare to the other projects designed. The first design lacks static
stability, while the third design would cause measurement error in terms of calculating
weights because of the strain gages used in the design. The strain gages are very delicate
and the amount of bending they go through could affect the correct measurement of the
weight of the person and at the same time the strain gages go through abrupt temperature
changes that would cause instability in the process of weight measurement. For these
reasons, the second project was our best choice, in terms of selecting the optimal design
even though we still made some changes to the static analysis part of it, especially the
amount of stress the platform is going to go through.
Optimal Design:
Objective:
A set of four load cells will be used to determine the weight resting on the
platform. The load cells will be positioned at the four corners of the platform, supporting
the platform statically and inhibiting movement. Since all force exerted on the platform
must be sensed by the cells, they must be the only support for the platform; however,
there cannot be a significant horizontal component to the forces exerted on the cells.
Access ramps on opposite sides of the scale with ramp angle no greater than 10 degrees
will be level with the platform, which will have height no greater than 2 inches. The load
cells and ramp will rest on or be supported by a lower platform or crosspiece assembly
under the platform. The signal from the load cell will travel through a cable to a
processing and display unit.
The processing and display unit will have a memory capable of storing 11
numerical values with a precision of four decimal places (10 historical weights, 1
wheelchair weight for taring, accuracy of 0.2 lb) and will be connected to an LCD screen
sufficiently large enough for low-vision accessibility. Large buttons and audible cues
through a speaker will also make the device more accessible to handicapped individuals.
The processor will hold a written software program containing:

an interface through which users can weigh themselves, store weight, key in
wheelchair weight and cycle through previous weightings

an algorithm to calculate actual weight based on stored or inputted
wheelchair weight and signals from load cells
This processor and other components mentioned will be in a circuit powered by
an AC adapter or standard batteries and will turn on and off by a switch or by a threshold
load placed on the platform.
Rest Bar
¼ in
5.1 in
1.24 in
18.3 in
2.5 in
3.2 in
27.5 in
18 in
36 in
Ramp
Load Foot
Load Cell
Figure 16: Side view of optimal design with four load cells
36 in
0.25 in
1.5 in
Rest Bar
2.5 in
1.24 in
3.2 in
2 in
Weigh
Platform
Load Cell
1.24 in
Load Foot
Figure 17: Front view of optimal design with four load cells
Figure 18: Top view of optimal design with four load cells
Equipment
Load cells are the primary measurement instruments in weighing devices. Load cells
consist of multiple strain gages (see figure 1), which measure resistance when it is put
under a compression or tension force. Strain gages are made up of thin metallic foils
called a “Gage Patch.” The multiple gages patches are connected tighter and attached
inside of the load cell. When the surface of the cell experiences a force the metallic wires
experience the same force. The multiple strain gages that make up a load cell are wired
together to form the legs of a Wheatstone bridge (see figure 1). The Wheatstone bridge
creates an electric current that results from the response of a load that is applied to the
cell in the form of an analog signal. A microprocessor is then used to convert the analog
signal to a digital signal, so the output from the load cell can be determined and read on
an electronic LCD display.
Series 350
Shear beam load cells for
Industrial weighing
Capacities 300 kg to 5,000 kg
5 year warranty
Simple low cost installation
Alloy version fully potted and sealed to IP 66
Alloy (350a) or stainless steel (350i) load sensor
Optional load feet, mounting plates, load buttons
And supports
Industry standard configuration
Corrosion resistant
Stainless steel version fully welded and
Hermetically sealed to IP68
Optional anti-vibration loading assembly available 3000 divisions OIML R60
Class C
Specifications
Load Cell Capacity Units
Standard Load Ranges 300, 500, 750, 1000, kg
1500, 2000, 3000, 5000 kg
Rated Output 2 mV/V±0.1%
Accuracy Class 3000 n.OIML
Combined Error <±0.017 %*
Non-repeatability <±0.015 %*
Creep (30 minutes) <±0.016 %*
Temperature Effect on Zero Balance <±0.01 %*/°C
Temperature Effect on Span <±0.006 %*/°C
Compensated Temperature Range -10 to +40 °C
Operating Temperature Range -20 to +70 °C
Safe Overload 150 %*
Ultimate Overload 200 %*
Zero Balance <±2 %*
Input Resistance 400 ±30
Output Resistance 350 ±1.5
Insulation Resistance >5000 M@ 100V
Recommended Supply Voltage 10 V
Maximum Supply Voltage 15 V
Environmental Protection 350i=IP68 350a=IP66 –
Load Foot Accessories for 350
The cells could be excited (via a junction box) by a cost effective instrument such as
our XT1200 unit.
JB4T JUNCTION BOX
Pricing from Thames Side-Maywood
Hole size 4mm
European
X350A-300
US
SHEAR BEAM LOAD CELL
108
COST EACH
£68.00




TOTAL
QUANTITY 4
X35913-FOOT
$125.12
£272.00
$500.48
£9.00
$16.56
80
50
CAPACITY 300KG (660lbs)
LEVELLING FOOT
COST EACH
QUANTITY 4
120
TOTAL
£36.00
$66.42
ALL DIMENSIONS IN MILLIMETRES (Not to Scale)
JB4T
JUNCTION BOX (IP65)
4 WAY SUMMING
COST EACH
£40.00
$73.60
The resolution required would easily be achieved (and much better). The load
cell is approved to OIML R60 C3 which is the European Weights and Measures accuracy
55
requirement. It would be important that the wheel chair platform structure that the cells
would support is very rigid in nature because any bending or flexing would produce
accuracy errors.
Data and quotes provided by Thames Side-Maywood (logo below):
Static Force Analysis:
Static Analysis: Optimized Design (4 Load Cells)
Calculations (assuming central and edge two-point symmetrical loading from wheelchair
wheels, at loading of 1000 lbs):
1) Maximum (surface) flexural stress in weigh platform
2) Shearing stress on bolts fastening platform to rest bar
3) Flexural stress in rest bar
4) Horizontal force on load cell
Yield strength of A36 hot-rolled structural steel:
150 Mpa compression
21.8 ksi
250 Mpa tension
36.3 ksi
Elastic modulus of A36 hot-rolled structural steel:
200 Gpa
Platform Dimensions
29000 ksi
36” x 36” x 0.25”
Rest Bar Dimensions (see diagram):
Top:
Side:
1.5” x 36”
2.5” x 36”
Bottom: 2” x 36”
Thickness: 0.25”
Useful Equations:
m 
Eq. 1)
Mc
I
Eq. 2)
I  bh 3
M 
Eq. 3)

Eq. 4)
Eq. 5)
Eq. 6)
LF
8
m
E
I  Ic  ax 2
I rec tan gle  bh 3
1)
Assume platform is fastened to rest bars such that no moment can be developed
about the joints (low bolt tension) and is supported along the joints evenly in the ramp
direction. Wheelchair wheels are modeled with two 500 lb forces (factor of safety, F.S. =
0.8) spaced 18 in. apart, positioned symmetrically in center of weigh platform. This
creates pure-bending (constant bending moment) between the downward forces.
Used BeamView Java Applet (Virginia Tech, freely available online) to simulate
platform bending behavior:
Figure 19: Beam view simulating platform bending behavior
Maximum Bending Moment: 4500 lb*in
By Eq. 1, maximum bending stress = (4500lb*in)(0.125 in) / (0.5625 in^4)
= 1000 psi = 1 ksi
2)
Assume shearing force on bolts attaching rest bars to platform is flexural
stress of 1000 psi (above) multiplied by bolt projection area
Flexural stress distributed through 4 bolts:
250 psi
Shearing force on 0.25” thick bolt holding 0.25” platform:
= (250 psi)(0.25 in)(0.25 in) = 15.6 psi
3)
Assume the cumulative 500 lb load on the bottom portion of the rest bar is
loaded evenly across the 36” length:
Moment of inertia of rest bar: Split into 3 rectangles of 1.25”x0.25”,
0.25”x2.5”, and 1.75”x0.25”:
I[1] = 1.25*0.25^3+1.25*.25*(1+0.125)^2
I[2] = 0.25*2.5^3
I[3] = 1.75*0.25^3+1.75*0.25*(1+0.125)^2
Total I = I[1] + I[2] + I[3] = 8.07 in^4
BeamView Java Applet used to simulate distributed 500 lb load:
Figure 20: Beam view simulating distributed 500 lb load
Maximum flexural stress = (1500 lb*in)(0.125 in) / (8.07 in^4) = 23.2 psi
4)
Assume flexural stress multiplied by thickness is horizontal force on
bolts attaching load cells to rest bar
Fx = (23.2 psi)(36 in)(0.25in) / 2 = 100.4 lbs
Analysis:
I.
The flexural stress in the platform developed from two 500 lb concentrated
loads spaced 18 inches apart and symmetrically in the center of the platform is
1 ksi, and the yield stress is 21.8 ksi. The stress developed is less than 5% of
the yield strength, indicating the platform is able to support over 20 times the
maximum loading for the scale.
II.
The shearing force of 15.6 psi developed on the bolts securing the platform to
the rest bars is negligible and indicates that 4 bolts is enough to secure the
platform for maximum loading
III.
The flexural stress in the rest bar is extremely small compared to yield
strength, since the moment of inertia I is very large and the load is assume to
be equally distributed. This indicates the rest bar will support the platform
without yielding during maximum loading
IV.
The total horizontal force acting on the part of the load cell attached to the
rest bar is 100.4 lbs. Typical shear-beam load cells will not tolerate sideloading (defined as loads perpendicular to the axis of the load measured) that
is greater than 100% of the rated maximum axial load. The maximum load for
the load cell is 660 lbs (300 kg), meaning that the sideload percentage is
100/660 or 15%, which is safely below the limit.
This indicates that at a maximum loading of 1000 lbs, the horizontal
forces acting on the load cells in a direction toward the center of the platform
are within safe limits of operation for the load cell. This force also introduces
a moment about the axis parallel to the load cell’s largest dimension.
Although not permanently damaging to the load cell if inside safe limits (such
as in this case), moments about this axis and sideloads will affect the
measurement of the load cell, making the creation of a calibration curve
necessary along with extensive testing of the scale accuracy.
Circuit Analysis:
-5V
Load Cell
+5V
R1
Input
Output
+5V
Output
+5V
-5V
Microprocessor
out1
out2
out3
out4
out5
out1
out2
out3
out4
out5
LCD Display
vcc
Input
R1
R2
R3
R4
In1
In2
In3
In4
vcc
R2
ground
Load Cell
+5V
ground
-5V
Load Cell
+5V
R3
Input
+5V
+5V
Output
+5V
-5V
Load Cell
+5V
R4
Input
Output
vcc
1
2
3
4
+5V
+5V
ground
User Interface
The above circuit is a generic view of a real circuit that is going to be mounted on our
design. The basic approach to the circuit is very simple, the four main outputs from the
load cells are going through filters so the voltage values entering the microprocessor are
without any noise and interference, also these voltage values go through some resistance
in order to make them suitable for the requirements of the microprocessor, different
microprocessors have varying input voltage acceptance values. The microprocessor also
gets input values from the User Interface, all these inputs of the microprocessor go
through calculations and memory storage sequences and then the result is displayed on
the LCD Display.
Block Diagram of System:
The above block diagram is the electrical part of the design; the power supply is
shown supplying a voltage to the load cell (force transducer) and at the same time, the
load cell’s resistance changes in proportion to the load applied axially. A Wheatstone
bridge circuit is used to detect the extremely small changes in resistance, and this
constitutes the output signal from the cell. Once the load cell signal reaches the linear or
steady state, then the analog signal is sent to the Signal filter where it is transformed from
a noisy signal to a continuous one and then this output is sent to the Signal Amplifier for
possibly voltage or current adjustments before the signal goes to the Microprocessor.
The Microprocessor gets two inputs one from the user and the other from the Signal
Amplifier. Once the Microprocessor is done with its arithmetic and all the memory
storing and sorting, then the final weight is displayed onto the LCD display. The scale
turns on as the user moves onto the platform with some minimum weight or when the
power switch is toggled.
Flow chart of the Microprocessor:
The computation and user interface for the weighing system is
accomplished by a microprocessor with user input switches and an LCD monitor. The
following is a microprocessor flow chart, which shows the different functions performed
by the user and the microprocessor while computing the weight of the person (see Fig.
21).
Analog
output from
the Load
Cell
The analog signal coming
from the Load Cell is being
converted to digital signal
using the A/D component
of the Microprocessor
The A/D input of the
Microprocessor
The user enters
the value of the
wheel chair and
then presses
enter.
Wheel Chair
input from
the User
The digital signal coming
from the microprocessor is
being sent to the input of
the ALU component of the
Microprocessor where the
subtraction of the wheel
chair weight from the total
weight takes place.
The digital value of the weight being
converted from the Load Cell is stored
into some register ‘x’ and then the user
value entered is being stored in some
register ‘y’. The ALU component of the
microprocessor does the calculations
and stores the final value in the register
‘z’, which is also stored in RAM, so it
could be accessed again.
ALU functions of the
Microprocessor
Up to 10 different weights
of the person are
registered in this memory,
for the records of the user.
Digital to Ib/
kg converter
The output of the
Microprocessor is
displayed on the LCD
display.
Random Access
Memory (RAM)
of the
Microprocessor
The LCD display
The user pushes the
button on the panel which
brings up the previous
measurements of the
patient.
Accessing the
previous
measuremtns
.
Figure 21: Flowchart of processing and display unit
User interface flowchart:
Scale Process
Menu:
1-Power
2-Weigh
3-History
4-Change
wheelchair weight
Switch on
Switch off
Filtered weight
If time > 5 min
Access
wheelchair
weight
Yes
display
No
display
Hold till scale is
stable
Display: “No wheelchair
weight stored.” “Store
wheelchair weight?”
No wheelchair
weight stored
Yes
No
Input
wheel chair
weight
Go to
Menu
If time > 2 min
Filter weight
“Error too
much
movement”
Store Wheelchair
weight as 1-10
Display
“Your
weight
is__.”
Go to
Menu
“Store to
memory?”
Yes
Store in next
available space,
bump out oldest
entry if full
No
Go to
Menu
Figure 22: User interface flowchart
Figure 22 above shows the user interface flowchart enabling the user to navigate
through four options – turn the scale on/off, weigh, check weight history, and change
wheelchair weight. There are few menu levels, making the interface easy to use for most
people. The memory is accessed by the processor whenever the user selects the option to
actually store their weight, and the oldest entry for weight is deleted and all entries are
moved back to make room for the new one. The user must know their wheelchair weight
and type it on only once, as it is stored in the memory. The machine memory must be
able to handle repeated and constant use, and must not fail in a power outage. A battery
can be provided to ensure that stored data is not lost.
Scale Design-Finished Product
This chronicles the designing and building process we went through to deliver the
finished product, overcoming small and large obstacles and making the changes that were
necessary to arrive at the end result.
Our load cells arrived the first day we met as a group to build the project, giving
us the chance to immediately test them and inspect them. We used a clamp, table, and
labeled weights as well as a multimeter and voltage source to make a plot of voltage vs.
load. Our project parts were obtained as needed instead of all at once, throughout the
semester. The second step was ordering the voltage source and writing code to use the
LCD. I decided to use freely available code written by professional developers after
failed attempts to write it myself. The basic building blocks of the program were written
one by one until the main code could be simplified to calling the functions as needed and
hence easily changed.
During this development, we decided to simplify the design by splitting the circuit
into two: an amplifying circuit to acquire, amplify and filter the analog signal and the
interface circuit to read the signal and communicate with the user. There were a total of
six conductors – power, ground, and four load cell signals – needed between the two,
enabling us to use any standard wiring cable with more than 6 conductors. We made
heavy use of oscilloscopes to determine the signal-to-noise ratio and noise difficulties in
the amplifier circuit caused us a huge delay. If the power supply had generated less noise
or the filters able to keep the peak-to-peak range of the signal below +/- 2.5 mV, the 10bit A/D converter on the microcontroller would not be affected by it because it reads in
4.9mV increments. This would enable us to weigh in microseconds. However, the signalto-noise ratio was large enough to necessitate the use of an algorithm which waits for 5
equal weighings in a row (sample frequency 2.865 kHz) to use the value as a legitimate
weight. This process ended up taking close to 30 seconds in the worst cases in the final
unit; however, we a large object such as a wheelchair actually statically stabilized the
scale to the degree that the weighing time was reduced to less than a second.
Creating the PCB designs were straightforward for the amplifier circuit, and this
circuit was assembled without any major problems. However, the protoboard of the
interface circuit would not receive a steady keypad input, and it took an inordinately long
amount of time to find out that this was due to floating voltages. This was easily solved
by changing the PCB schematic to include 10K pull-down resistors on four pins. We
were all surprised when the interface circuit, connected to the tested amplifier circuit with
scale, worked correctly the first time.
Testing the scale with known weights revealed that the scale took very long in
certain situations to register a value, and that sometimes this value was not consistent. We
found steps to improve consistency and accuracy, such as making sure the floor is a level
surface, adjusting the load cells to be collinear with their bolts, and dampening the
vibrations from the podium bar. When the cord to the interface unit is dangling, it creates
a moment on the bar that is increased through leverage down to the adjacent load cell,
offsetting the result by as much as 4 lbs. We also found that the load cells actually take
upwards of 10 seconds to equilibrate to the correct weight value, and included this in the
Owner’s manual. In conclusion, the scale actually works best when loaded with a
wheelchair, and meets every single one of our Project Specifications.
The design of the scale changed significantly from the original design from
everything to the setup and the placement of the load cells. The most significant change
occurred with the actual placement of the load cells. The original design called for the
load cells to be attached under a Z shaped piece of metal that the main platform would be
attached to. After learning more about load cells, it was determined that the load cell
would need to actually be placed directly to the main platform. In order for the load cells
to measure the tension produced by the load on the platform, the Z shaped piece of metal
would have greatly reduced the overall load that would be distributed to the load cells
and then measured, thus resulting in inaccurate readings of the person and their
wheelchair. So the new design had all four load cells attach to the corners of the
platforms.
Side view of scale-figure 22
Top view of scale-figure 23
Block Diagram of System:
FigureA1: Block Diagram of the system
The above block diagram is the electrical part of the design; the power supply
shown is supplying a voltage to the load cell (force transducer) and at the same time, the
load cell’s resistance changes in proportion to the load applied axially. A Wheatstone
bridge circuit is used to detect the extremely small changes in resistance, and this
constitutes the output signal from the cell. Once the load cell signal reaches the linear or
steady state, then the analog signal is sent to the Signal Amplifier where the signal is
being amplified. After the signal is being amplified it is sent to the Signal filter where it
is transformed from a noisy signal to a continuous one and then this output is sent to the
Microprocessor. The Microprocessor gets two inputs, one from the user and the other
from the Signal Amplifier. Once the Microprocessor is done with its arithmetic and all
the memory storing and sorting, then the final weight is displayed onto the LCD display.
The scale turns on as the user moves onto the platform with some minimum weight or
when the power switch is toggled.
Circuit Analysis:
The circuit analysis is based on the designs of the Power Supply and general circuit
which contain four Load Cells, Amplifiers, filters, resistors, capacitors, and voltage
regulator. The Power Supply generates +12 volts and -12 volts of voltage, which feeds
the whole circuit. The +12v and -12v voltages needs to be filtered before they are drawn
to the rest of the circuit because the amount of noise generated by them could alter the
results of the design as a whole. In order to do that Power Supply filter analysis is used
below which describes the design process of linear filters for the output of the Power
supply.
a) Power Supply filter analysis:
By hooking capacitors to ground shunts all the high frequencies to ground, while passing
all the low frequencies so basically this works as a passive low-pass filter. The
impedance is dependent on the frequency as the frequency increases the impedance
decreases. The capacitors acts like a bypass capacitor, where the AC component of the
signal is directed towards ground while the DC component of the signal is passed through
the circuit.
Vin is +12v and four capacitors in parallel are added together .1+.1+.1+.1 = 0.4µF for C.
I = Cdv/dt
V (t) = 1/C ∫tt0 i dt + v(0) since to is usually zero
Xc = 1/2ΠfC
With certain Xc values the frequency (f) goes to zero which is the DC value.
Following is the signal shown before the power supply is being filtered with a noise value
of approximately 215mV peak to peak.
Figure A2: +12 volts signal before it is being filtered:
Below is the signal when the power supply is being filtered, with a noise value of
approximately 36mV from peak to peak.
Figure A3: +12 volts signal after it is being filtered:
b) Load Cell signal analysis:
The Load Cell uses a Wheatstone circuit analysis in order to compute the voltage desired
by the user. The circuit for the Wheatstone analysis is shown below;
Figure A4: Wheatstone Bridge
By looking at the circuit it is certain that there are two output voltages which are in
microvolt range, therefore computing the difference between those two voltages requires
a Differential OP-Amp which computes the difference and then amplifies the difference
with a gain of 702. The gain was found by finding the voltage to pound ratio which is
shown below;
Weight (Ib)
0
15
25
Voltage (volts)
0.11
0.22
0.30
By taking the average we find out that 31µV of voltage is generated by a 1Ib force.
c) Filter Design:
The voltage coming out of the Load Cell is in microvolt range, which impounds the
possibility of noise mixed with the signal. If the signal is mixed with noise that will
dramatically decrease the effectiveness of the design. So in order to control the noise a
Low-pass Active filter must be designed. Active filter because in order to obtain a sharp
response with a passive filter, we would need to cascade several passive stages. Each
cascaded stage, however, loads the previous stages. This loading attenuates the desired
part of the signal, i.e., frequencies within the filter pass-band, as well as unwanted
frequency content within signal. This problem is commonly known as insertion loss.
Active filtering practically eliminates insertion loss due to the high input impedance and
the low output impedance of an Op-Amp.
Filter Analysis:
The following First-Order Unity-Gain Low-Pass Active filter was
used to filter the noise coming out of the differential Op-Amp. The Vout of this filter is
the signal going into to the Microprocessor. The following filter is being used for our
design where all the filter analysis was acquired from the following useful website:
http://users.telenet.be/educypedia/electronics/analogfil.htm
We want frequencies of less than 5 Hz passed through the circuit therefore the signal is
being filtered in order to cutoff frequencies higher than 5 Hz. For our design the
following parameters were used;
F0 = 0.22 Hz
C = 22µF
R = 1/ (2Π*C*f0 )
R = 1/ (2Π*22µF*0.22) = 33KΩ
The following is the signal before the output of the differential OpAmp is being filtered
with a noise of approximately 30mV from peak to peak.
Figure A5: Noise level before the filter:
Below is the signal after the output of the differential Op-Amp is being filtered with a
noise of approximately 20mV from peak to peak. The 20mV noise value is not exactly
20 mV its much lesser than that, by looking at the waveform of the signal attained from
the oscilloscope, and taking the standard deviation of all the spikes, the voltage value of
the noise seems to be more like around 2mV.
Figure A6: Noise level after the filter:
By taking the standard deviation of the different voltages of the waveform supplied by
the oscilloscope we see that the noise is brought down tremendously;
Record Length
Sample Interval
Trigger Point
2.50E+03
1.00E-04
1.25E+03
Source
CH1
Vertical Units
V
Vertical Scale
5.00E-02
Vertical Offset
2.40E-02
Horizontal Units s
Horizontal Scale 2.50E-02
Pt Fmt
Y
Yzero
0.00E+00
Probe Atten
1.00E+00
Firmware Version FV:v6.08
-0.125
-0.1249
-0.1248
-0.1247
-0.1246
-0.1245
-0.1244
-0.1243
-0.1242
-0.1241
-0.124
-0.1239
-0.1238
-0.1237
-0.1236
-0.1235
-0.1234
-0.1233
-0.1232
-0.1231
-0.123
-0.1229
-0.1228
-0.1227
-0.1226
-0.1225
-0.1224
-0.1223
-0.1222
-0.1221
-0.122
-0.1219
-0.1218
-0.1217
-0.1216
-0.1215
-0.1214
-0.1213
-0.1212
-0.002
-0.01
0.004
0.002
0.004
0.002
0.004
0.002
-0.01
0.002
0.002
0.002
0.004
0.004
0
-0.002
0
0.006
0.002
0
-0.002
0
0.002
0.002
-0.002
0.002
0
0
0
0
0
0.004
0
0.002
0
0
0
0.002
0.002
Standard
Mean
Deviation
0.000714
0.002413691
The standard deviation of the output signal shows how much noise is being generated by
the differential Op-Amp which is about 2mV.
The 2mV noise would not affect the measurement of our design and this was proved by
testing the design.
After the circuit analysis part of the design was complete then the circuit as a whole was
designed
on the Express schematic as shown below.
Figure A7: Schematic Circuit
After completing the schematic of the circuit, the circuit was linked to the PCB part of
the Express software where the PCB board of the circuit was created as shown below;
Figure A8: PCB board
The PCB part of the design is being done so that our PCB board could be ordered and
finally placed on the final design.
Mechanical Design
Ideally we want the scale that we designed to be very user friendly so that
individuals in a wheelchair will be able to appropriately monitor their weight on a daily
basis to keep track on the weight losses and gains. When designing this scale a
requirement was that we would like to see the scale be able to be transported by another
person for an easy monitoring use. The transportation of this device is vital whether it is
used in a clinical setting or used in an individual’s home. Another requirement was that
this scale would be very durable and withhold a large amount of weight. The scale will
be very sturdy and resilient through out its lifespan. The scale is made out of low carbon
steel. Steel is a very strong, robust, long lasting material that can handle many
situations. However with the decision to use such a material that can endure physical
abuse, the weight of the scale had to be increased. Steel is also a very heavy material
hence it’s durability. Design changes were definitely constructed so that such a solid
material could be used. The new design eliminated almost one half of the original
designs material and thus it’s weight. Therefore with the addition of a resilient scale the
requirement of having the scale be transportable was still maintained.
The new design actually allows the scale to break down into 5 major components
plus the nuts and bolts that allow the scale to be transported. The 5 major component
produce the restriction on the individual transporting the scale, so that they won’t carry
more than one component at the time and possibly cause harm to themselves by carry
more things than they realistically can.
Additionally eliminating half of the overall raw materials also reduces the cost
significantly.
The design process involved many stages of machining. Many holes were drilled
in the angle iron and platforms through the use of the Miller and drill press. Sheets of
metal were also cut and bended using the vertical band saw and the Betenbender. A
small amount of welding was also needed for bolts and podium attachments. This process
produced obstacle along the way but ones that were always overcome.
Here is a look of different stages in the design development:
This is how it all began with the cutting of the angle iron-figure 24
Next the Miller was used to drill into the angle iron-figure 25
The angle iron with all the completed holes-figure 26
The raw metal sheets that have yet to be machined-figure 27
The metal sheet cut down, and then with bended sides to increase the strength of the
platform. Also all holes in the platforms have been drilled, and the back guardrail has
been attached. Figure 28
A close-up of the welded bolts to the platform that go into the load cells, which allow for
less pieces during disassembly. Figure 29
The full scale view, showing all welded bolts-figure 30
And then after a lot of hard work and machining the main base of the scale was ready for
assembly. Figure 31
Close up of the underside of the platform, showing load cell and attachment nuts. Figure
32
The swivel, adjustable podium for the user interface and the LCD-figure 33
The LCD holder and the user interface-figure 34
The objective of this design is to create a scale that is easily accessible to
wheelchair users. We want a handicapped wheelchair bound individual to have easy
access to an affordable scale that will not require much assistance from others. The scale
will have a clear easy readout display, to allow for the individual to keep track of their
weight. This objective has been fulfilled.
The scale appearance is not outlandish or eccentric. It is a soft dark gray that will
fit nicely in a commercial or home environment, having an exterior with the same appeal
of an appliance. The overall scale is made out of a very durable and strong metal steel,
that will enhances the scale’s longevity and resilience.
Interface and Program Design
Measuring Techniques:
Two different software approaches were tried in order to obtain a
consistent weight value. The first approach was to employ a signal average,
and the second was to close the weighing algorithm in a loop until an identical
weight value was obtained a number of times in a row. Both algorithms
summed all four 10-bit values corresponding to analog load cell outputs to
obtain a total platform weight.
The first algorithm measured a platform weight, then added this to a
variable space 16 times. The number of weights that could be added was
limited because of the maximum value that two variables can accommodate.
Later, this number was increased to 256*16 or 4096 in an attempt to increase
the consistency of the result. In both cases, the weights obtained by the A/D
converter was not consistent enough for our needs, although the computing
time was very fast and was the same each time. Each time a weighing cycle
was performed, the weight would vary by as much as +/- 2 divisions (here, a
division is defined as a single bit of accuracy from the A/D converter; the 10bit onboard A/D converter of the PIC16F874 thus yields 2^10 or 1024
different divisions). This translated to as much as +/- 3 lbs, giving a maximum
difference between adjacent measurements of 6 lbs. This was wholly
unacceptable.
One possible reason that the signal averaging technique was not working
was the nature of the signal noise. Instead of white noise, the signal was
composed of brief, equally spaced pulses of noise in between which there was
an area of much less noise. Although the RMS value of the amplified and
filtered load cell signal was equal to the DC component of the signal, the
peak-to-peak maximum values for the signal were the most important aspect.
Because this value was almost +/- 100mV in the final PCB circuit installed in
the scale, averaging would have had to sample many values to cover one
period of the signal, and we were not able to do this. The error range doubled
when the circuit was tested on PCB, presumably because of the large number
of parallel traces carrying noisy +12V and -12V of power very close to traces
carrying the weak load cell signal (see Filter Analysis).
The second approach was to use a continuous loop that returned a value
only when it had been exactly repeated 5 times in a row. This algorithm
differed in that it could theoretically go on forever, if this condition is not met.
A future software version should incorporate a break option for the user, if
he/she feels too much time has been spent in a weighing cycle. The algorithm
works as follows: first, a counter is set to zero and the platform weight is
obtained twice and stored in two separate variables. If the values are exactly
equal, the counter is incremented and the least recent value is deleted with the
most recent value occupying that variable. Then, another weighing cycle is
performed and compared to the current value. If they are equal, the counter is
incremented again and this repeats until the counter is equal to a previously
defined “tolerance” value, declared in the microprocessor code as “Tol” (see
Microprocessor Code, under label “Weighloop”).
We found that we could trade off speed for consistency, and settled on a
tolerance value of 4 in the final code. This translated to the requirement of 5
equal weights in a row before the loop could return the value. In a vibrationfree, stable, EMF-noise-free environment, the loop could return a stable
weight from the scale as quickly as 5 seconds, or in the worst cases, as long as
5 minutes. However, it is more important that the right weight is obtained
each weighing cycle than the speed with which this occurs. If the user feels
otherwise, it is a simple matter to reprogram the chip with a smaller tolerance
value. A tolerance value of 3 was found to finish weighing almost 10 times
faster.
The same weighing algorithm is used for all parts of the program that need
a weight, and this value is stored as a 16-bit value, between two 8-bit
registers. All the weights are stored as a binary value corresponding to the
value obtained by the A/D converter minus the taring weight and the stored
wheelchair weight. These values are only converted to pounds when displayed
on screen. This ensures that the original bit values are a standard in the
calculations performed by the program.
Using Microcontroller Memory
Two values are subtracted from the weight when it is to be displayed on
the screen after a weighing cycle (in menu option 1) and when it is to be
stored. These are the tare weight (“Tareh”, “Tarel” variable names) and the
wheelchair weight (“Wheelh”, “Wheell” variable names). Both are set to zero
by default upon startup. The wheelchair weight can be specified by the user in
menu option 2, done by manually typing in the weight value in pounds. This
decimal value is stored as five decimal digit values in five variables
designated as incoming decimal digits from the keypad. These are converted
to binary in a subroutine and divided by a conversion factor to convert to the
same scale as the A/D converter. The tare weight is set with menu option 4,
which calls the weighing loop and stores the resulting weight as a tare weight.
It is important to note that the weighing loop always returns the absolute
weight, consisting of the platform itself, any objects on it, and any electrical
offset in the amplifier circuit. This simplifies taring operations. The binary
value stored in memory is actually the absolute weight minus both tare and
wheelchair weights.
Since the binary values obtained from the converter are not in units of
pounds, a conversion factor was needed. A subroutine was written to multiply
the digital value between 0 and 1023 divisions by a number less than 1 to
obtain a number between 0 and 800 lbs. This conversion factor has four
significant decimal digits (9999 divisions) and is sufficient in accuracy for this
application. The inverse of the conversion factor is stored as well, to convert
from a binary representation of an input in pounds to the appropriate decimal
value. The conversion factor was successively approximated by testing the
scale with known weights, and the final version contains a conversion factor
accurate to the nearest 0.2 lbs
The microprocessor’s data memory can hold 82 individual bytes without
interfering with declared variables, translating to 41 different weight values. If
EEPROM were to be implemented in a future software version for the same
chip, the capacity would be 64 different weight values. Also, these values
would stay in storage even during a power outage; currently, the memory is
dependent upon a power supply. The block of memory is initialized to zero
upon startup, and a new value is stored as follows: the values are all shifted
downward in the block, as in a stack, with the first value moved to the second
space, and so on until the last value is eliminated. Then, the weight to be
stored is stored as the first value. In this manner, the memory acts like a
shifting stack, keeping the most recent values as the first and maintaining
itself by eliminating the oldest values. With a capacity of 41 weight values, a
user can monitor his/her weight for well over a month if used every day.
Testing and calibration techniques were made as rigorous as possible with
the precision of available weights. The scale was considered to be in working
condition only when three weighing cycles in a row returned the same value.
Testing for hysteresis was performed by loading and unloading the scale with
a known weight several times in a row and verifying the equality of the
alternating loaded and unloaded values.
The tools used to develop the firmware for the microcontroller as well as
downloading into the chip were MPLAB IDE and the Qik Start Education
Board, from Microchip and Diversified Engineering Inc., respectively.
Engineering Standards and Environmental Considerations:
It is important that as engineers we take into account our own ethical
standards. In creating this design we made sure that we have and will continue to uphold
the IEEE code of ethics and that as engineers our current and future designs will not be
harmful to any individuals or the environment itself. In continuation of our design and
our future work as engineers we will maintain that when something ethically wrong is
occurring we will, however possible stop any harm from occurring.
IEEE Code of Ethics [11]
1. to accept responsibility in making engineering decisions consistent with the safety,
health and welfare of the public, and to disclose promptly factors that might endanger the
public or the environment;
2. to avoid real or perceived conflicts of interest whenever possible, and to disclose them
to affected parties when they do exist;
3. to be honest and realistic in stating claims or estimates based on available data;
4. to reject bribery in all its forms;
5. to improve the understanding of technology, its appropriate application, and potential
consequences;
6. to maintain and improve our technical competence and to undertake technological
tasks for others only if qualified by training or experience, or after full disclosure of
pertinent limitations;
7. to seek, accept, and offer honest criticism of technical work, to acknowledge and
correct errors, and to credit properly the contributions of others;
8. to treat fairly all persons regardless of such factors as race, religion, gender, disability,
age, or national origin;
9. to avoid injuring others, their property, reputation, or employment by false or
malicious action;
10. to assist colleagues and co-workers in their professional development and to support
them in following this code of ethics
Ideally this design should not produce anything that is harmful to the environment
during the production phase, while it is in use, and after the life of the device has ended,
therefore minimizing the devices short and long term effects on the environment. This
design needs to take into account all past disasters and problems with other engineering
designs and make sure that these previous problems are not recreated through this design
or that this device does not create any new exploitations to the environment and when
possible minimize the resources that the environment provides. The optimal design for
the smart wheelchair scale does not have a short lifespan and will ideally be able to be
used to the extent of an individuals entire life to monitor there weight. With the use of
load cells, a LCD and a sturdy, durable material for the scale minimal operating
malfunctions and deterioration of this product will exist. Disposal of the smart
wheelchair scale and the end of its life will still contain materials that maybe stripped and
recycled for use in other electronic products. For the remaining product, it will be able to
be disposed of at you local landfill and it will not produced harmful substances into the
environment because of the use of non-toxic materials. The smart wheelchair scale will
not be a burden on the environments energy consumption. A small amount of electrical
energy will be needed to operate the device and it will not require any fuel to operate.
The ADA Standards for Accessible Design pamphlet specifies regulations for
streets, walkways, buildings, etc. for the safe day-to-day accessibility of wheelchairbound persons. In addition to minimum widths and lengths for wheelchair-accessible
areas, the document describes maximum angles for access ramps. The design presented
here is within the bounds of these regulations. The American Disabilities Act specifies a
32” minimum length or width for a wheelchair accessible area and a maximum 1:12 ramp
slope, both of which encompass the optimized design described in this report [12].
Parts List:
Part
Load Cell
Load Foot
Junction Box
Platform
Bolts
Nuts
Hinges
Microprocessor
LCD display
Assorted electronic
parts
Timeline:
Name
X350A-300
X35913-FOOT
JB4t
A36 Structural Steel
Carriage Bolt, Plain 18-8 1/4-20 Stainless
Finished Hex Nut, Plain 18-8 1/4-20
Stainless
Manufacturer
Thames Side-Maywood
Thames Side-Maywood
Thames Side-Maywood
Machine Shop
Wink Fasteners, Inc.
Qty.
4
4
1
1
16
Total
Cost
$500.48
$66.24
$73.60
$100.00
$1.53
16
$0.80
A217 16-gauge 1-1/16
TBD
TBD
Wink Fasteners, Inc.
Austin Supply and Hardware,
Inc.
TBD
TBD
4
TBD
TBD
$13.16
TBD
TBD
TBD
TBD
TBD
TBD
Conclusion:
This design offers many beneficial features that make this product highly accurate. This
design includes a lower cost aspect but involves a more accurate approach to weight
measurements with the addition of four load cells. The combination of four load cells
provides an in-depth average of the individual’s weight, because it will not matter where
exactly the individual is placed on the scale providing easier access. This design also
does not include springs to act as supports; instead the four load cells will act as supports
for the platform with their placement in load feet.
The display unit will be large enough for an individual with moderately low
vision to see and operate. The operation of the scale and display unit will also be user
friendly and allow enough memory to store collected weightings for up to 10 times. The
circuit analysis in this design is more accurate and affordable to build. The usage of
fewer resistors and capacitors is taken into consideration.
References:
1) Diseases and Conditions Index. “What is Chronic Obstructive Pulmonary
Disease?” National Heart, Lung, and Blood Institute. Medline Plus Copyright ©
2004, http://www.nhlbi.nih.gov/health/dci/Diseases/Copd/Copd_WhatIs.html
2) National Lung Health Education Program. “Lung Treatment of COPD and
Asthma”. Copyright © 2002-2004, http://www.nlhep.org/lung_trtmnt.html
3) Detecto, division of Cardinal Scales Mfg. Co. “Wheelchair Scales” (product
inventory). Webb City, MO: http://www.detectoscale.com/wheelchairs.htm
4) “Seca 664C Smart Card…” (product inventory). Copyright © 2003 Precision
Weighing Balances, http://www.balances.com/seca/medical-wheelchairscale.html
5) ABLEDATA keyword search, “wheelchair scale”. ABLEDATA, Silver Spring
MD.
6) USPTO Quick Search, keyword “wheelchair scale”. United States Patent and
Trademark Office, Last Modified: 2004
7) Douglas A. Hobson, PhD. “Development and Application of Wheelchair
Standards” Powerpoint File, Sept. 1999, NIDRR,
http://www.wheelchairnet.org/WCN_WCU/SlideLectures/DAH/WCStds1099.pdf
8) Product Search: LCEC Minibeam Load Cells. © 2001 Omega Technologies,
http://www.omega.com/ppt/pptsc.asp?ref=LCEC&Nav=pref08
9) Series 350 Shear Beam Load Cell. © 2004 Thames Side-Maywood Ltd.,
http://www.thames-side-maywood.com
10) “The Strain Gage”. Transactions in Measurement and Control. Vol. 3, Omega
Technologies, http://www.omega.com/literature/transactions/volume3/strain.html
11) IEEE Code of Ethics. IEEE Board of Directors, © 2004 Institute of Electrical and
Electronic Engineers,
http://www.ieee.org/portal/index.jsp?pageID=corp_level1&path=about/whatis&fi
le=code.xml&xsl=generic.xsl
12) Department of Justice. “ADA Standards For Accessible Design”. 28 CFR Part 36,
Revised July 1994, http://www.usdoj.gov/crt/ada/adastd94.pdf
Acknowledgements:
We would like to thank the people in the UConn machine shop, which include
Tom, Serge and Rich for their advice, suggestions, and extreme motivation and support
towards this project. Also the Rehabilitation Engineering Research Center (RERC) has
made an outstanding contribution in order to fund this project. Many other professors on
campus whose extremely important advice and guidance were appreciated include John
Ayers who provided us with important circuit analysis guidance, Jeff Meunier who
provided important programming advice, our advisor Dr. John Enderle, Bi Zhang, and
Dr. Christian Davis.
We would also not have been able to complete this project without the help of
Christopher Liebler and Franscisco Rodriquez for all of the support and advice given
throughout the development of the final product.
Thanks so much, we greatly appreciate all of the help!