Download Mechanical methods of measuring pressure

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Force & Pressure Measurements (1)
2011
Force & Pressure
Measurements (1)
Lecture Notes
Systems & Biomedical Engineering Department Faculty of Engineering, Cairo
University
Prof. Bassel Tawfik
Biomedical Measurements
1/1/2011
Force & Pressure Measurements (1)
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Lecture Outline
1.
2.
3.
4.
Force, Pressure, Stress, and strain
The Strain Gauge: Introduction & Construction
Other Methods
Applications in Medicine
1. Force, Pressure, Stress & Strain1
1.1 Basic Physics
Figure 1: Cylindrical model of a wire under tensile
strength.
When an external force is applied to a
stationary object, stress and strain result.
Stress is defined as the applied force (F)
divided by the cross sectional area (A), i.e.
A
F
L
L + L
Stress () = F/A
Stress is the same as pressure except that
the latter is defined in only one direction
(inwardly or acting to compress the object). It is, therefore, measured in units of
pressure such as “Pascal” or “bar”.
Revision of units:
Strain, on the other hand, may be thought
of as the deformation (elongation or Variable
Units
N (Kg m/s2)
compression) resulting from stress. It is Force
Pa = N/m2
either compressive or tensile and is Stress (Pressure)
Strain
Dimensionless
typically measured by a strain gage2. 1 bar = 105 Pa (Atmospheric pressure at sea level)
Strain is defined as the amount of
deformation (L) relative to the total original length (L) of an object, i.e.
Strain () = L / L
1
Most of the material presented here is taken from
http://www.omega.com/literature/transactions/volume3/strain3.html
2
Also called “gauge” in British English.
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2. The Strain Gage: A Brief Introduction
Figure 2: One possible design of a capacitive strain gage. Courtesy Boeing Corporation. (Source: Strain
Gage Users’ Handbook - 1992).
In principle, all strain gages are designed to
convert mechanical deformation due to applied
force (displacement) into an electrical signal. To
do this, an electrical property such as
capacitance, inductance, or resistance must be
used to indicate this deformation.
For instance, in a capacitive strain gage (figures
2, 3), the distance between opposite plates of a
capacitor indicates the amount of displacement
caused by the applied force. It is to be noted,
however, that capacitive and inductive strain
gages are not very popular because of their
sensitivity to vibration, special mounting
requirements, and circuit complexity.
Figure 3: Electrical equivalent circuit and
mechanical schematic of the strain gage
in figure 2.
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Resistive strain gages are simpler to construct and are not affected by electromagnetic
interference (EMI). The working concept is simple: when a wire is held under tension, it
gets slightly longer and its cross-sectional area is reduced (see figure 1). This changes its
resistance (R) in proportion to the strain sensitivity (S) of the wire's resistance. The
strain sensitivity, called the gage factor (GF), is given by:
GF = [R / R] / [L / L]
3. Types of Strain Gages & Testing of Materials
The deformation of an object can
be measured not only be measuring
Sample
changes in electrical properties of
materials, but also by mechanical,
Ruler
optical, acoustical, and pneumatic
means. The earliest strain gauges
were mechanical devices that
Load
measured strain by measuring the
change in length and comparing it
to the original length of the object.
For example, the extension meter Figure 4: One possible design of a mechanical
extensiometer (here used with textiles).
(extensiometer) uses a series of
levers to amplify strain to a
readable value. A simple version of an extensiometer is shown in figure 4. In general,
however, mechanical devices provide low resolutions and their readings cannot be
readily stored and processed digitally.
When selecting a strain gage, one must consider not only the strain characteristics of
the sensor, but also its stability and temperature sensitivity. Unfortunately, the most
desirable strain gage materials are also sensitive to temperature variations and tend to
change resistance as they age. For tests of short duration, this may not be a serious
concern, but for continuous measurements, the designer must account for temperature
and drift characteristics.
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Each strain gage
wire material has its
own characteristic
gage
factor,
resistance,
temperature
coefficient, thermal
coefficient
of
resistivity,
and
stability. The most
popular alloys used
for strain gages are
copper-nickel alloys
and nickel-chromium
alloys.
Load cell
Actuator
Control
Panel
Figure 5: This testing machine can be used to obtain Stress/Strain curves
in both tension and compression modes. It can also collect fatigue data
when used with a cyclic input. The force applied to the sample is
measured with a Load Cell (strain gauge balance) on the stationary top
cross-head. The bottom cross-head holds the hydraulic actuator which
loads the sample. The material deformation is determined from either
the moving grip position or by means of a strain gauge extensiometer.
The machine is operated via the Control Panel or the computer. Data
may be collected and displayed during acquisition using dedicated
software.
(Courtesy:
www.princeton.edu/~humcomp/sophlab/m&mla_30.htm)
In the mid-1950s,
scientists at Bell Labs
discovered
the
piezoresistive
characteristics
of
semiconductors.
Although
the
materials exhibited
substantial
nonlinearity
and
temperature
sensitivity, they had
gage factors more
than fifty times, and
Mechanical methods of measuring pressure have been known for
sensitivity more than
centuries. U-tube manometers were among the first pressure indicators.
a 100 times, that of Originally, these tubes were made of glass, and scales were added to
metallic wire or foil them as needed. But manometers are large, cumbersome, and not well
suited for integration into automatic control loops. The above figure
strain gages. Silicon shows one type of mechanical pressure measurement, namely the
wafers are also more Bourdon Tube which comes in different designs.
elastic than metallic ones. After being strained, they return more readily to their original
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shapes. Finally, the size is much smaller and the cost much lower than for a metallic foil
sensor.
In summary, the ideal strain gage is small in size and mass, low in cost, easily attached,
and highly sensitive to strain but insensitive to ambient or process temperature
variations.
4. Resistance-type Strain Gages
Bonded Resistance Gages
Bonded semiconductor strain gages are the
most popular method of measuring strain.
The gage consists of a grid of very fine
metallic wire, foil, or semiconductor
material bonded to the strained surface by
a thin insulated layer of epoxy (Figure 6).
When the surface is strained, the strain is
transmitted to the grid material through
the adhesive. The variations in the
electrical resistance of the grid are Figure 6: Bonded Resistance Strain Gage
Construction
measured as an indication of strain. The
grid shape is designed to provide maximum gage resistance while keeping both the
length and width of the gage to a minimum.
Bonded resistance strain gages have a good reputation. They are relatively inexpensive,
can achieve overall accuracy of better than +/-0.10%, are available in a short gage
length, are only moderately affected by temperature changes, have small physical size
and low mass, and are highly sensitive.
Bonded resistance strain gages can be used
to measure both static and dynamic strain.
Typical metal-foil strain gages
In bonding strain gage elements to a strained
surface, it is important that the gage
experiences the same strain as the object.
With an adhesive material inserted between Figure 7: Metal foil strain gage
the sensors and the strained surface, the
installation is sensitive to creep due to
degradation of the bond, temperature influences, and hysteresis caused by thermoBiomedical Measurements | Bassel Tawfik
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elastic strain. Because many glues and epoxy resins are prone to creep, it is important to
use resins designed specifically for strain gages.
The bonded resistance strain gage is suitable for a wide variety of environmental
conditions. For instance, it can measure strain at very high temperatures and in
cryogenic fluid applications at temperatures as low as -452*F (-269*C). It has low mass
and size, high sensitivity, and is suitable for static and dynamic applications. Foil
elements are available with unit resistances from 120 to 5,000 ohms. Gage lengths from
0.008 in. to 4 in. are available commercially.
The three primary considerations in gage selection are: operating temperature, the
nature of the strain to be detected, and stability requirements. In addition, selecting the
right carrier material, grid alloy, adhesive, and protective coating will guarantee the
success of the application
Measuring Circuits
Strain gages are used to measure
displacement, force, load, pressure,
torque and weight. Modern strain-gage
transducers usually employ a grid of four
strain elements electrically connected to
form a Wheatstone bridge measuring
circuit. (Figure 8)
A Wheatstone bridge is a divided bridge
circuit used for the measurement of static
or dynamic electrical resistance. The
output voltage of the Wheatstone bridge
is expressed in mV output per volt (V)
input. The Wheatstone circuit is also well
suited for temperature compensation.
Figure 8: Wheatstone bridge circuit schematic
In Figure 8, if R1, R2, R3, and R4 are equal, and a voltage, VIN, is applied between points
A and C, then the output between points B and D will show no potential difference.
However, if R4 is changed to some value which does not equal R1, R2, and R3, the
bridge will become unbalanced and a voltage will exist at the output terminals (B, D). In
a so-called G-bridge configuration, the variable strain sensor has resistance Rg, while the
other arms are fixed value resistors.
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The sensor, however, can occupy one, two, or four arms of the bridge, depending on the
application. The total strain, or output voltage of the circuit (VOUT), is equal to the
difference between the voltage drop across R1 and R4, or Rg. This can also be written
as:
The bridge is considered balanced when R1/R2 = Rg/R3 and, therefore, VOUT equals zero.
Any small change in the resistance of the sensing grid will throw the bridge out of
balance, making it suitable for the detection of strain. When the bridge is set up so that
Rg is the only active strain gage, a small change in Rg will result in an output voltage
from the bridge. If the gage factor is GF, the strain measurement is related to the
change in Rg as follows:
As mentioned above, the
number of active strain
gages connected to the
bridge depends on the
application. For example, it
may be useful to connect
gages that are on opposite
sides of a beam, one in
compression and the other
in
tension.
In
this
arrangement, the bridge
output is doubled for the
same strain. In installations
where all of the arms are
connected to strain gages,
temperature compensation
is automatic, as resistance
Strain gauge
elements
Fixed resistors
Strain gauge
elements
Diaphragm
Pressure
Diaphragm
Movable
block
Pressure
Strain gauges
Fixed points
WIRE RESISTANCE STRAIN GAUGE
DOUBLE BONDED STRAIN GAUGE
Figure 9
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change due to temperature variations will be
the same for all arms of the bridge.
In a four-element Wheatstone bridge,
usually two gages are wired in compression
and two in tension. For example, if R1 and
R3 are in tension (positive) and R2 and R4
are in compression (negative), then the
output will be proportional to the sum of all
the strains measured separately. For gages
located on adjacent legs, the bridge
becomes unbalanced in proportion to the
difference in strain. For gages on opposite legs, the bridge balances in proportion to the
sum of the strains.
Whether bending strain, axial strain, shear strain, or torsional strain is being measured,
the strain gage arrangement will determine the relationship between the output and
the type of strain being measured. As shown in Figure 8, if a positive tensile strain
occurs on gages R2 and R3, and a negative strain is experienced by gages R1 and R4, the
total output, VOUT, would be four times that due to a single gage.
The Chevron Bridge
The Chevron Bridge is
illustrated in Figure 10. It is a
multiple
channel
arrangement that serves to
compensate for the changes
in bridge-arm resistances by
periodically switching them.
Here, the four channel
positions are used to switch
Figure 10: Chevron Bridge Circuit Schematic
the digital voltmeter (DVM)
between G-bridge (one active gage) and H-bridge (two active gages) configurations. The
DVM measurement device always shares the power supply and an internal H-bridge.
This arrangement is most popular for strain measurements on rotating machines, where
it can reduce the number of slip rings required.
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Four-Wire Ohm Circuit
Although the Wheatstone bridge is
one of the most popular methods of
measuring electrical resistance, other
methods can also be used. The main
advantage of a four-wire ohm circuit is
that the lead wires do not affect the
measurement because the voltage is
detected directly across the strain
gage element.
Figure 11: Four-Wire Ohm Circuit Schematic
A four-wire ohm circuit installation might consist of a voltmeter, a current source, and
four lead resistors, R1, in series with the gage resistor, Rg (Figure 11). The voltmeter is
connected to the ohms sense terminals of the DVM, and the current source is
connected to the ohms source terminals of the DVM. To measure the value of strain, a
low current flow (typically one mA) is supplied to the circuit. While the voltmeter
measures the voltage drop across Rg, the absolute resistance value is computed by the
multimeter from the values of current and voltage.
The measurement is usually done by first measuring the value of gage resistance in an
unstrained condition and then making a second measurement with strain applied. The
difference in the measured gage resistances divided by the unstrained resistance gives a
fractional value of the strain. This value is used with the gage factor (GF) to calculate
strain.
The four-wire circuit is also suitable for automatic voltage offset compensation. The
voltage is first measured when there is no current flow. This measured value is then
subtracted from the voltage reading when current is flowing. The resulting voltage
difference is then used to compute the gage resistance. Because of their sensitivity,
four-wire strain gages are typically used to measure low frequency dynamic strains.
When measuring higher frequency strains, the bridge output needs to be amplified. The
same circuit also can be used with a semiconductor strain-gage sensor and high speed
digital voltmeter. If the DVM sensitivity is 100 V, the current source is 0.44 mA, the
strain gage element resistance is 350  and its gage factor is 100, the resolution of the
measurement will be 6 microstrains.
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Constant Current Circuit
Resistance can be measured by
exciting the bridge with either a
constant voltage or a constant current
source. Because R = V/I, if either V or I
is held constant, the other will vary
with the resistance. Both methods can
be used.
While there is no theoretical
advantage to using a constant current
source (Figure 12) as compared to a Figure 12: Constant Current Circuit Schematic
constant voltage, in some cases the
bridge output will be more linear in a constant current system. Also, if a constant
current source is used, it eliminates the need to sense the voltage at the bridge;
therefore, only two wires need to be connected to the strain gage element.
The constant current circuit is most effective when dynamic strain is being measured.
This is because, if a dynamic force is causing a change in the resistance of the strain gage
(Rg), one would measure the time varying component of the output (V OUT), whereas
slowly changing effects such as changes in lead resistance due to temperature variations
would be rejected. Using this configuration, temperature drifts become nearly
negligible.
Sources of Interference
The output of a strain gage circuit is a very low-level voltage signal requiring a sensitivity
of 100 V or better. The low level of the signal makes it particularly susceptible to
unwanted noise from other electrical devices. Capacitive coupling caused by the lead
wires' running too close to AC power cables or ground currents are potential error
sources in strain measurement. Other error sources may include magnetically induced
voltages when the lead wires pass through variable magnetic fields, parasitic
(unwanted) contact resistances of lead wires, insulation failure, and thermocouple
effects at the junction of dissimilar metals. The sum of such interferences can result in
significant signal degradation.
Shielding
Most electric interference and noise problems can be solved by shielding and guarding.
A shield around the measurement lead wires will intercept interferences and may also
reduce any errors caused by insulation degradation. Shielding also will guard the
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measurement from capacitive coupling. If the measurement leads are routed near
electromagnetic interference sources such as transformers, twisting the leads will
minimize signal degradation due to magnetic induction. By twisting the wire, the fluxinduced current is inverted and the areas that the flux crosses cancel out. For industrial
process applications, twisted and shielded lead wires are used almost without
exception.
Guarding
Guarding the instrumentation itself is just as important as shielding the wires. A guard is
a sheet-metal box surrounding the analog circuitry and is connected to the shield. If
ground currents flow through the strain-gage element or its lead wires, a Wheatstone
bridge circuit cannot distinguish them from the current generated by the current source.
Guarding guarantees that terminals of electrical components are at the same potential,
which thereby prevents extraneous current flows.
Connecting a guard lead between the test specimen and the negative terminal of the
power supply provides an additional current path around the measuring circuit. By
placing a guard lead path in the path of an error-producing current, all of the elements
involved (i.e., floating power supply, strain gage, all other measuring equipment) will be
at the same potential as the test specimen. By using twisted and shielded lead wires and
integrating DVMs with guarding, common mode noise error can virtually be eliminated.
Lead-Wire Effects
Strain gages are sometimes
mounted at a distance from
the measuring equipment.
This increases the possibility
of
errors
due
to
temperature
variations,
lead desensitization, and
lead-wire
resistance
changes.
In a two-wire installation
(Figure 13-A), the two leads
are in series with the straingage element, and any
change in the lead-wire
resistance (R1) will be
Figure 13: Alternative Lead-Wire Configurations
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indistinguishable from changes in the resistance of the strain gage (Rg) (which is a
problem).
To correct for these lead-wire effects, an additional, third lead is introduced to the top
arm of the bridge, as shown in Figure 13-B. In this configuration, wire C acts as a sense
lead with no current flowing in it, and wires A and B are in opposite legs of the bridge.
This is the minimum acceptable method of wiring strain gages to a bridge to cancel at
least part of the effect of extension wire errors.
Theoretically, if the lead wires to the sensor have the same nominal resistance, the
same temperature coefficient, and are maintained at the same temperature, full
compensation is obtained. In reality, wires are manufactured to a tolerance of about
10%, and three-wire installation does not completely eliminate two-wire errors, but it
does reduce them by an order of magnitude. If further improvement is desired, fourwire and offset-compensated installations (Figures 13-C and 13-D) should be
considered.
In two-wire installations, the error introduced by lead-wire resistance is a function of
the resistance ratio R1/Rg. The lead error is usually not significant if the lead-wire
resistance (R1) is small in comparison to the gage resistance (Rg), but if the lead-wire
resistance exceeds 0.1% of the nominal gage resistance, this source of error becomes
significant. Therefore, in industrial applications, lead-wire lengths should be minimized
or eliminated by locating the transmitter directly at the sensor.
Temperature and the Gage Factor
Strain-sensing materials, such as
copper, change their internal
structure at high temperatures.
Temperature not only can alter
the properties of a strain gage
element, but can also alter the
properties of the base material to
which the strain gage is attached.
Differences
in
expansion
coefficients between the gage
and base materials may cause
dimensional changes in the
Figure 14: Gage-Factor Temperature Dependence
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sensor element.
The gage factor reflects the strain sensitivity of the sensor. The manufacturer should
always supply data on the temperature sensitivity of the gage factor. Figure 14 shows
the variation in gage factors of the various strain gage materials as a function of
operating temperature. It is apparent that Copper-Nickel alloys (such as Advance) have
gage factors that are relatively insensitive to operating temperature variations, making
them the most popular choice for strain gage materials.
Apparent Strain
Apparent strain is any change in gage
resistance that is not caused by the
strain on the force element.
Apparent strain is the result of the
interaction of the thermal coefficient
of the strain gage and the difference
in expansion between the gage and
the test specimen.
The variation in the apparent strain
of various strain-gage materials as a
function of operating temperature is Figure 15: Apparent Strain Variation with temperature
shown in Figure 14. In addition to the temperature effects, apparent strain also results
from aging, instability of either the metal or the bonding agent. Such effects must be
compensated for in the design.
Stability Considerations
It is desirable that the strain-gage measurement system be stable and does not drift
with time. In calibrated instruments, the passage of time always causes some drift and
loss of calibration. The stability of bonded strain-gage transducers is inferior to that of
diffused strain-gage elements. Hysteresis and creeping caused by imperfect bonding is
one of the fundamental causes of instability, particularly in high operating temperature
environments.
If stable sensors are used, such as deposited thin-film element types, and if the forcedetector structure is well designed, balancing and compensation resistors will be
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sufficient for periodic recalibration of the unit. The most stable sensors are made from
platinum or other low-temperature coefficient materials. It is also important that the
transducer be operated within its design limits. Otherwise, permanent calibration shifts
can result. Exposing the transducer to temperatures outside its operating limits can also
degrade performance. Similarly, the transducer should be protected from vibration,
acceleration, and shock.
Strain Gage Installation Methods
Figure 16: Strain Gage Installation Alternatives
In Figure 16-A, a vertical beam is subjected to a force acting on the vertical axis. As the
force is applied, the support column experiences elastic deformation and changes the
electrical resistance of each strain gage. In another configuration, the strain gage may
be bonded to a cantilever to measure the bending moment (Figure 16-B). The strain
gages mounted on the top of the beam experience tension, while those on the bottom
experience compression. The transducers are wired in a Wheatstone circuit and are
used to determine the amount of force applied to the beam.
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Strain-gage elements are also used widely in the design of industrial pressure
transmitters. Figure 16-C shows a bellows type pressure sensor in which the reference
pressure is sealed inside the bellows on the right, while the other bellows is exposed to
the process pressure. When there is a difference between the two pressures, the strain
detector elements bonded to the cantilever beam measure the resulting compressive or
tensile forces.
In the fourth type, a diaphragm-type pressure transducer is created when four strain
gages are attached to the diaphragm (Figure 16-D). When the process pressure is
applied to the diaphragm, the two central gage elements are subjected to tension, while
the two gages at the edges are subjected to compression. The corresponding changes in
resistance are a measure of the process pressure. When all of the strain gages are
subjected to the same temperature, such as in this design, errors due to operating
temperature variations are reduced.
5. Applications in Medicine
4.1 Blood pressure Monitoring
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4.2 Biomechanics
4.2.1 Force Plates/Dynamometers
A force plate (shown to the right with its
computer interface) is a device that
measures the ground reaction forces
(GRF) exerted by a subject standing (or
walking) on it. Force plates are used for
gait analysis, diagnosis of foot
impairment, studies of balance, sports
medicine, and design of medical shoes.
Force plates consist of a top plate which is
separated from the bottom frame by
force transducers at each corner.
Courtesy: Neurocom International
The forces exerted on the top surface (of
the plate) are transmitted through the
force tri-axial transducers (operating in
transverse (Z), antero-posterior (X) and
vertical (Y) directions).
Foot pressure during heel down, toe off gait
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Concept of Isokinetic measurements using dynamometers. Courtesy:
http://www.pt.ntu.edu.tw/hmchai/Kines04/KINmotion/Musculature.htm
4.2.2 Weighs & Scales
Weighing applications vary, in sometimes very interesting ways. For instance, most
food processing systems use known proportions of material inputs and feed rates in
order to obtain the final mix. Since weight is the fundamental variable, material must
be weighed in order to reach accurate and repeatable proportions. Medical applications
involving weighing include baby and adult scales, lab and pharmacy sensitive scales, in
addition to alarm systems whose alarm
sets off when the applied weight exceeds
a certain threshold value as in couches of
CT scanners and MRI’s.
In all these devices, there is a sensor
which converts force or weight into an
electrical signal. This sensor is the load
cell which is classified as a force
transducer. The strain gage is the heart of
a load cell.
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The figure to the right shows
a simplified (but useful) block
diagram of a complete weighscale system. As shown, a
voltage
signal
is
first
generated at the bridge,
which is fed into an OP AMP,
then a low pass filter, and
finally and analog-to-digital converter to obtain digital display of the weight. The
system’s power supply produces a 5V DC to support different functions.
5. General Applications
Intrusion Detection (Security)
Pressure Switch
Medical Gas Systems
Water treatment Systems
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Appendix (A)
Article in MDDI
Advances in Load Cell Technology for Medical
Applications
Miniaturization and automation are paving the way for new uses for load cells in
medical devices.
Javad Mokhbery
Load cells are essentially transducers that convert force or weight into an
electrical signal. They have been widely used for measuring and sensing
applications in virtually every industry for decades. At the heart of most
load cells is a strain gauge. This element changes resistance when pulled
or pushed (placed under tension or compression). Foil strain gauges are
the most common and are created from an ultrathin heat-treated metallic
foil, which is either chemically etched on a thin dielectric layer or attached
using vacuum deposition or sputtering techniques to bond the materials
molecularly. The latter technique is commonly known as thin film.
Desirable strain gauges are small in size, low in cost, very sensitive to
strain in the load direction, and insensitive to surrounding environment
temperature changes.
An S-beam load cell
sensor is used for
various medical
applications, such as
on a blood transfusion
bag.
To measure strain with a strain gauge, an electric circuit is used that is
capable of measuring extremely low resistance changes from induced
microstrain. Strain gauge transducers typically employ four strain gauge
elements that are electrically connected to form a Wheatstone bridge
circuit. The optimal choice for strain measurement, a Wheatstone bridge
circuit is a four-leg parallel divided bridge circuit that measures electrical changes resulting from
resistance changes. Its output voltage is expressed in millivolts per volt of input (mV/V). A
Wheatstone bridge is also well suited for temperature compensation.
Types of Load Cells and Corresponding Technologies
The normal configuration for a Wheatstone bridge circuit comprises four strain gauges. But some
load cells use 8, 16, 32, or more gauges, while other devices only use one or two. The precise
positioning of the gauges, the mounting, and the materials used define the performance of any
load cell. The analog output of the transducer is normally signal conditioned, amplified, and
digitized to display the force, load, pressure, displacement, or applied torque.
Foil strain gauges have distinct advantages, including reduced size, a variety of gauge patterns,
and temperature compensation. Low production cost and flexibility for installation on surfaces that
are flat, curved, or slotted, or that are inside holes, also support creative design requirements. For
this reason, foil strain gauges are the most common type in use today.
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The latest emerging technology is microelectromechanical systems (MEMS). MEMS are
microsized silicon structures etched in the forms of beams, diaphragms, or plates that can
function as sensors within a load cell. MEMS are fabricated using bulk and surface
micromachining, just like any integrated circuit manufacturing process. They can be mass
produced, because thousands of sensor elements can be fabricated on a single wafer with
integrated supporting circuits. Although millions of sensors can be mass produced at a very low
price (as low as a few dollars), their applications are still limited compared with foil strain gauges.
The most popular MEMS applications in the medical industry are in the area of low-cost,
disposable products that are manufactured in lots of millions. These include disposable blood
pressure sensors and angioplasty devices used to measure pressure in balloon catheters.
Moving from Industrial to Medical
In most cases, size and cost are the foremost issues when making the transition from standard
industrial to medical applications. The basic technology does not change in terms of such
capabilities as range and reliability. Medical applications typically require measurement of loads
in ounces, grams, and milligrams, whereas in industrial settings, the load is typically in pounds,
kilonewtons, or tons. The only exception to this rule is in physical rehabilitation devices, where
standard-sized load cells are used.
All medical load cells must be highly precise and packaged to be portable and lightweight,
particularly when they need to be attached directly to patients. If the cell is used inside a machine
integrated with another medical device for monitoring, standard packaging materials such as
stainless steel and anodized aluminum are used. If it is in contact with the human body or with
fluids, special autoclavable stainless-steel or disposable sensors can be used.
Early medical load cell applications included mechanical measurements such as bed-weight
monitoring. Until the early 1980s, nurses had to physically monitor patients to track critical weight
fluctuations. By affixing load cells to hospital beds, the beds could effectively transmit accurate
patient weight to a handheld instrument. Typically four load cells, one under each leg of the bed,
fed data to a junction box that was connected to a related instrument or controller.
Small load cells were soon integrated into another area that was susceptible to human error: the
infusion pump for administering drugs. Originally, a hanging bag held fluid, medication, or
nutrients that were infused to the patient via gravity through a flexible line. Various clamping
methods were used to regulate the flow as precisely as possible. Again, this required continual
attendant monitoring to ensure that the solution was being delivered properly, that the bag was
properly filled at all times, and that no back bleeding was occurring. Integrating a load cell and
monitoring system to the basic infusion-delivery method removed guesswork from the process.
The load cell measured the exact weight of the bag and immediately sent a warning to a
connected device if the weight of the infusion varied from its prescribed path. Normally a small
bending-beam load cell sensor with a 100-g to 1-lb capacity was placed in a cartridge under the
flexible tube used to deliver the infusion. The sensor detected the changes in tube weight during
the flow and communicated with the electronic controller.
The integration of load cells into previously mechanical methods made feedback to other devices
possible. Introducing automation to many medical applications enabled a reduction in human
error. Data provided by the load cells were permanently recorded, which also greatly improved
tracking of medical processes for liability assurance.
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Force & Pressure Measurements (1)
Application Examples
Today,
load
cells in medical
devices range in
size from 3–4 in.
in diameter for
physical therapy
applications
down to smaller
than a dime.
Measurement
ranges run from
milligrams
to
hundreds
of
pounds and are
not affected by
the
physical
dimensions
of
load cells. The
smallest
load
cell offers the
same
range,
accuracy,
and
repeatability of
its
larger
cousins.
At
some
point,
however,
size
does begin to
limit the capacity
of the sensors,
but
most
medical
applications do
not require the
weight range of
a large load cell.
Modern
computerized
automation,
wireless
interfaces, and
Figure 1
the
shrinking
size
and
enhanced capabilities of semiconductors and electronic circuit devices have greatly expanded the
reach of load cell technology in medical applications. The previously mentioned fluid-delivery
methods and bed-weight measurement are still widely used, but both are now much more
sophisticated with integral automated monitoring equipment (see Figure 1).
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Force & Pressure Measurements (1)
Medical load cells are used in a wide range of delicate fluid-monitoring applications including
blood transfusions, kidney dialysis, and blood donation. In such applications, the load cells
ensure that the amount of fluids entering, leaving, or being replaced in the body are being started,
stopped, or recirculated at the right time and in the proper dosage or ratios. Therefore, doctors
and nurses can now monitor far more patients in various applications than in the past when
everything was done manually.
Kidney Dialysis. A typical kidney dialysis system may depend on one or several load cells to
ensure that the filtration system has perfect balance and timing. The dialysis system must remove
contaminated blood, clean it, and recirculate the clean, reoxygenated blood. Any malfunction can
be disastrous. Load cells used for this type of system are typically in-line, small, and work by
monitoring flow changes by sensing the weight of a hanging bag to ensure the dialysis procedure
is performed safely every time.
This process is a noninvasive measurement, since body fluid is not in contact with any part of the
sensor. A load cell known as an S-beam Jr. is typically used in a dialyzer. It has a 5–10-lb
capacity range with 1000% overload protection and is about the size of a quarter. This load cell is
attached to the end of a hanging bag. The bag is connected to the dialysis machine via two
flexible tubes. One tube is used for the flow inlet and the other is for discharge. Some systems
use several bags and require multiple load cells. Each load cell is connected to a programmable
logic controller or computer to monitor the flow by weight measurement. Using the load cell
information, the system automatically processes and controls the dialysis procedure while
collecting data for further analysis when needed.
Endoscopic Surgery. Endoscopy is a unique area aided by load cell technology. In endoscopic
surgery, the pressure of instruments can be highly critical and the incision depth needs to be very
precise. Load cells can monitor the force of these instruments against the tissue, thereby greatly
improving surgical accuracy.
On the front end, during product development of endoscopic tools, small, button-type load cells
about 3/8 in. OD or smaller are used to help in improvement of tool design by minimizing the
force required for the tool during piercing and penetration. They cover the range of 50–100 lb.
These load cells have helped to reduce required forces from 75 lb in earlier tools to about 25 lb or
lower. Reducing the amount of required force means that surgeons exert less force and patients
experience less pain.
In the final production of such tools, MEMS sensors in very small package sizes (typically 0.4
mm) are integrated within the tool to help surgeons monitor and control piercing and penetration
forces during actual surgery. This protects against excessive force and also collects data that can
be used for further analysis when needed.
Rehabilitation. Large load cells (2–4 in.) are used in physical therapy to monitor muscle
recovery. They are normally integrated with a hand-gripping device of some type to monitor the
rehabilitation progress in those who have an injury, arthritis, or have had strokes.
The same theory is used with tension devices to measure leg pushing and contraction values
against a surface. A load cell attached to a gripping or tension device can indicate exact changes
in an affected muscle and how much progress is being made after each therapy session. This
allows the therapist to customize the types of therapy to the needs of the patient. These load cells
vary in size from 1 to 4 in. diameter, with measurement ranges of 50–1000 lb. There are many
system configurations designed for this purpose, but all have two things in common: the patient
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Force & Pressure Measurements (1)
exerts force against some object that is connected to the load cell, and the load cell sends the
resulting measurement to a readout device or computer. The computer then converts the signal
from analog to digital to produce an accurate, real-time display.
Orthopedics. A unique application in the area of corrective orthopedics is placing a very-lowprofile flat-plate or load-button load cell into shoe heels with a connection to a headset radio
device. When the wearer is walking correctly with proper balance and posture, the load cell
activates the radio device and music plays. If the subject falls into an irregular foot pattern,
throwing off the correct balance of the body, the radio device will stop playing, thus training the
person to correct improper stance and walking patterns.
Monitoring MRI Movement. To control or monitor patient movement during magnetic resonance
imaging (MRI), special hand-grip load cells are used to detect movement and monitor any loss of
strength. The results will also indicate if the patient is losing consciousness. Proprietary
manufacturing processes and material selection are used to develop special, nonmagnetic load
cells that can be used in the MRI environment.
Premature Birth Detection. Load cell sensors are also used to monitor irregular pressure
changes during pregnancy to help prevent premature birth defects in high-risk pregnancies. One
company created a device that includes a bending-beam load cell to monitor these pressure
changes. The device uses a special belt that is attached externally to the abdomen of the
pregnant woman. The belt is equipped with a microprocessor control, a load cell, and a modem.
When irregular pressures are sensed, the device calls the nursing center via the wireless modem
to alert the gynecologist if changes in uterine pressure indicate emergency care is required.
Applications on the Leading Edge
Micro Load Cells. Load cells are
used in various ways in both knee
and hip joint replacement, both in
R&D and during surgery. Small,
customized S-shaped shear-beam
load cells are used to measure
torsional forces of tendons and
ligaments
during
surgical
procedures. Also, customized clip-on
soft-tissue load cells accurately
measure the forces of the knee
extensor mechanism interoperatively.
Customized load cells used for R&D
during the development of implants,
such as for the patella ligament, are
also used within the implants to
measure the tension of a ligament
(see Figure 2).
Figure 2
Load cells used directly in knee
implants include an implantable knee
simulator that measures patellofemoral force on the patella implant and an implantable simulator
for the knee and tibia. This device is designed to interface with trial knee implants and to measure
the loads between the tibia and the femur. Assessment of the load balance of the tibiofemoral
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Force & Pressure Measurements (1)
joint can aid in balancing soft-tissue structures. This results in the precise positioning of implants
and greater knee stability.
The miniaturization of load cells has also been instrumental in aiding dental researchers and
equipment manufacturers identifying the bite strength of each tooth under various conditions. This
has enabled improvement of the materials used in dental work as well as dentures and implants.
A small, 50-lb-capacity strain gauge load cell, or a customized denture, which includes a sensor
under each tooth, provides information to a readout device or remote data acquisition device.
That information allows dental technicians to accurately read the positioning and pressure points
of the subject’s jaw and teeth to create what is needed for a well-aligned and healthy bite.
Pancake and Multicomponent Load Cells. The manufacturers of artificial joints and robotic
limbs have borrowed a technique from standard industrial quality control applications. Multiple
pancake-type load cells are used in both hip and knee simulator machines, which allow friction
and wear tests of multiple joints simultaneously for endurance testing and determining mean time
between failure. Testing durability provides a way to develop better, stronger, and more-flexible
devices with a goal of lifetime use with no problems.
Rod-End Load Cells. Prosthetic arms and legs have played a big role in mobilizing the
handicapped. However, with the absence of the muscles and nerve systems, many abilities are
very limited. Manufacturers are now using special rod-end load cells to monitor and display
forces, and work is under way to enable such data to be relayed to a patient’s brain, creating a
closed-loop process.
The Future
Miniaturization and automation are radically changing the face of medical load cell applications.
No area has changed more than surgery, thanks to automation. With the growing sophistication
of surgical robots, load cells are the key to opening up this cost-effective alternative to ever more
delicate procedures. Giving such robots a sense of touch will make them truly effective. Load
cells in a variety of sizes, shapes, and ranges are making these advances possible. It is
anticipated that in the future, miniaturized and smarter load cells with interchangeability and
communication capabilities will be used to give robots sensing ability equal to, if not superior to,
human touch. This technology will expand the reach of surgical procedures while lowering the
overall cost, bringing life-saving options to more patients than ever before.
Load cells will also be seen in a range of preventive medical devices. Miniaturized load cell
devices are already being used to show people the way to correct bad habits and to prevent
injury. This can be as simple as integrating a load cell into a golf bag or strength-training
equipment to prevent lifting strain. It can be as complicated as the use of multiple miniaturized
load cells in robotic artificial limbs to create toes and fingers that can feel pressure and send that
data to the brain so the wearer can respond.
At the rate technology is progressing today, by the next decade the common load cell may be the
key to opening new avenues in a range of medical procedures that are only being imagined
today.
Javad Mokhbery is the founder and CEO of Futek Advanced Sensor Technology. He can be
reached at [email protected].
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Force & Pressure Measurements (1)
Appendix (B)
Photoemulsification (Phaco)
Cataract surgery is usually fast, comfortable, and
quite successful. Surgery can and usually is
performed as outpatient surgery. Anesthesia
consists of minimal sedation and a local block,
usually by drops alone for comfort. The surgery
usually takes about 20 minutes. You return home in
2-3 hours total and most normal activity can be
resumed. All surgical work is done through a selfsealing (sutures are not used) opening into the eye
that is about the size of a pen tip. Lasers are NOT ordinarily used to remove Cataracts; rather
the preferred method uses ultrasound energy.
The cloudy lens is removed with an instrument that loosens the cloudy lens protein (emulsifies)
and gently vacuums it out of the eye. The instrument is called a phacoemulsifier and is not a
laser. The phacoemulsifier uses ultrasound energy to loosen the cataract. Once the cloudy lens
is removed, a lens implant is necessary to restore the focus of the eye. The lens implant is
folded
and
inserted
through
the
same
tiny
opening
into
the
eye.
The lens is placed through the pupil, behind the colored iris to replace the natural human lens.
The lens is permanent and restores the focus of the eye.
Cataract Lens Implants
When the cataract, the cloudy lens, is
removed, the haze is eliminated.
However, the focus of the eye must be
restored. Intraocular lens implants are
made of a type of plastic, acrylic or
silicone.
Haptic
The lens implants can be folded to permit
placement inside the eye through the tiny
incision already made for cataract removal. The power of the lens implant is usually calculated
to give the best distance vision possible without glasses. The design of the intraocular lens is
shown to the right. The tail-like endings of the IOL are called haptics. Part of the design process
is to make sure that these haptics can withstand certain forces during surgery. Such procedure
is done by using an extensiometer (see lecture).
Biomedical Measurements | Bassel Tawfik