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1 Force & Pressure Measurements (1)

Figure 1: Cylindrical model of a wire under tensile strength.

Revision of units:

Variable UnitsForce N (Kg m/s2)Stress (Pressure) Pa = N/m2

Strain Dimensionless1 bar = 105 Pa (Atmospheric pressure at sea level)

L

L + L


Lecture Outline

1. Force, Pressure, Stress, and strain2. The Strain Gauge: Introduction & Construction3. Other Methods4. Applications in Medicine

1. Force, Pressure, Stress & Strain1

1.1 Basic Physics

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.

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”.

Strain, on the other hand, may be thought of as the deformation (elongation or compression) resulting from stress. It is either compressive or tensile and is typically measured by a strain gage 2 . 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.Biomedical Measurements | Bassel Tawfik

A F

http://www.omega.com/literature/transactions/volume3/strain3.html

Figure 2: One possible design of a capacitive strain gage. Courtesy Boeing Corporation. (Source: Strain Gage Users’ Handbook - 1992).

Figure 3: Electrical equivalent circuit and mechanical schematic of the strain gage in figure 2.


2. The Strain Gage: A Brief Introduction

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.

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:

Biomedical Measurements | Bassel Tawfik

Figure 4: One possible design of a mechanical extensiometer (here used with textiles).


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 changes in electrical properties of materials, but also by mechanical, optical, acoustical, and pneumatic means. The earliest strain gauges were mechanical devices that measured strain by measuring the change in length and comparing it to the original length of the object. For example, the extension meter (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.


Ruler

Load

Sample

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)

Mechanical methods of measuring pressure have been known for centuries. U-tube manometers were among the first pressure indicators. Originally, these tubes were made of glass, and scales were added to them as needed. But manometers are large, cumbersome, and not well suited for integration into automatic control loops. The above figure shows one type of mechanical pressure measurement, namely the Bourdon Tube which comes in different designs.


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.

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 sensitivity more than a 100 times, that of metallic wire or foil strain gages. Silicon wafers are also more elastic than metallic ones. After being strained, they return more readily to their original shapes. Finally, the size is much smaller and the cost much lower than for a metallic foil sensor.


Load cell

Control Panel

Actuator

http://www.princeton.edu/~humcomp/sophlab/m&mla_30.htm

Figure 6: Bonded Resistance Strain Gage Construction

Figure 7: Metal foil strain gage


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 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 the sensors and the strained surface, the installation is sensitive to creep due to degradation of the bond, temperature influences, and hysteresis caused by thermo-elastic strain. Because many glues and epoxy resins are prone to creep, it is important to use resins designed specifically for strain gages.


Figure 8: Wheatstone bridge circuit schematic


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.

In Figure 8, if R1, R2, R3, and R4 are equal, and a voltage, V IN, 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.


Pressure

Diaphragm

Fixed points

Movableblock

Pressure

Diaphragm

Strain gauges

Strain gaugeelements

Fixed resistors

Strain gaugeelements

WIRE RESISTANCE STRAIN GAUGE DOUBLE BONDED STRAIN GAUGE

Figure 9


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 change due to temperature variations will be the same for all arms of the bridge.


Figure 10: Chevron Bridge Circuit Schematic


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 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.


Figure 11: Four-Wire Ohm Circuit Schematic


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.

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.


Figure 12: Constant Current Circuit Schematic


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 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 (VOUT), 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


Figure 13: Alternative Lead-Wire Configurations


reduce any errors caused by insulation degradation. Shielding also will guard the 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 flux-induced 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.

GuardingGuarding 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 strain-gage element, and any change in the lead-wire resistance (R1) will be


Figure 14: Gage-Factor Temperature Dependence


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, four-wire 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 sensor element.


Figure 15: Apparent Strain Variation with temperature


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 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 force-detector structure is well designed, balancing and compensation resistors will be 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


Figure 16: Strain Gage Installation Alternatives


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

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.

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


Courtesy: Neurocom International

Foot pressure during heel down, toe off gait


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.

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).



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.

The figure to the right shows a simplified (but useful) block


Concept of Isokinetic measurements using dynamometers. Courtesy: http://www.pt.ntu.edu.tw/hmchai/Kines04/KINmotion/Musculature.htm

http://www.pt.ntu.edu.tw/hmchai/Kines04/KINmotion/Musculature.htm


diagram of a complete weigh-scale 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 SwitchMedical Gas SystemsWater treatment Systems



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.

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.

The latest emerging technology is microelectromechanical systems (MEMS). MEMS are microsized silicon structures etched in the forms of beams, diaphragms, or plates that can


An S-beam load cell sensor is used for

various medical applications, such as

on a blood transfusion bag.

http://www.devicelink.com/mddi/archive/06/08/mddi0608p110b.jpg


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.


Figure 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 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).


http://www.devicelink.com/mddi/archive/06/08/mddi0608p82b.jpg


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 exerts force against some object that is connected to the load cell, and the load cell sends the


Figure 2


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-low-profile 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).

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 joint can aid in balancing soft-tissue structures. This results in the precise positioning of implants and greater knee stability.


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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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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 self-sealing (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.

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).


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