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Smart Wheelchair Scale
Design #1: Single Load Cell (SLC)
By:
Roee Ramot
Ahmad Paintdakhi
Beth Showers
March 29, 2004
Introduction:
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 and 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 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 weightings 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 weight control.
Design:
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 weightings

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
Spring
Load
Cell
36"
Note: Vertical scale different from
horizontal scale for higher visibility
Figure 1: Side view of scale unit
2"
34"
30"
Side
Panel
Platform
2"
Load
Cell
12"
36"
1.4"
12"
Note: Vertical scale different from
horizontal scale for higher visibility
Figure 2: Another side view of scale unit - entrance ramps visible
Equipment
Load cells are the primary measurement instruments in weighing devices. Load
cells consist of multiple strain gages (see figure 3), 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 3). 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.
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.
(*Information taken from Omega Engineering, 2004)
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. 5).
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 5: 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
.
Load
Power supply
Force Transducer
Signal Filter
Signal Amplifier
User Input
MIcroprocessor
LCD Unit
Figure 6: Block diagram of scale 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 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.
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 7: User interface flowchart
Figure 7 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.
Static Analysis:
A static’s 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 static’s 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 7: 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. In both equations, A) and B), FW = 800 lbs
* 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.
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 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.