Force, Torque, Tactile Sensors/Measurements

Mechatronic systems in automated manufacturing environments require extensive environmental information to make intelligent decisions. Such information relates to the tasks of material handling, machining, inspection, assembly, painting, etc. Assembly tasks and automated handling tasks require controlled operations like grasping, turning, inserting, aligning, orienting, and screwing.

Every situation has somewhat different sensing requirements.

This section discusses some of the techniques used for force and torque sensing. A precise measurement of strain is an important consideration in measurement. Strain measurement is used as a secondary step in the measurement of many process variables, including flow, pressure, weight, and acceleration. Electrical-resistance strain gauges are widely used to measure strains due to force or torque. When a force is applied to a structure, it undergoes deformation. The gauge, which is bonded to the structure, is deformed by strain, and its electrical-resistance changes in a nearly linear fashion.

If a piece of metal wire is stretched, not only does it get longer and thinner, but its resistance increases. The greater the strain experienced by the wire, the greater is the change in resistance.

There are a number of ways in which resistance can be changed by a physical phenomenon.

The resistance, R, of a metal depends on its area, length, and electrical resistivity. it's possible to express the resistance of a conductor at a constant temperature, T, as […]

Where […]

rho = resistivity, _-m Ro = sample resistance, ohm; l = length, m; Ao = cross-sectional area, m^2

1 Sensitivity of Resistive Transducers

If a specimen is subjected to tension, causing an increase in length, its longitudinal dimension will increase, and its lateral dimension will decrease. If a resistance gauge made of this conducting material is subjected to a positive strain, its length increases while its cross-sectional area decreases.

Since the resistance of the conductor is dependent on its length, cross-sectional area, and specific resistivity, the change in strain is due to the change in dimension or specific resistivity.

For a circular wire of length, L; cross-sectional area, A; and diameter, D, the resistance of the wire before straining is

Let us subject the wire to tension which causes the strain. Tension increases length and reduces the diameter, which in turn reduces the area of cross section. Let the stress applied to the strain gauge be s in N/m^2. Additional definitions are […]

_L = change in length of wire

_A = change in area of cross-section

_D = change in diameter

= resistivity

_ = Poisson's ratio

Gauge factor also can be expressed as […]

The change in resistivity occurs because of the piezoresistive effect, which is explained as an electrical resistance change which occurs when the material is mechanically deformed. In some cases, the effect is a source of error. If the change in resistivity or piezoresistive effect of the material is neglected, the gauge factor becomes

The gauge factor gives an idea of the strain sensitivity of the gauge in terms of the change in resistance per unit strain. Although strain is a unit-less quantity, it's a common practice to express strain as a ratio of two units as m/m. Poisson's ratio for all metals is between 0 and 0. The gauge factor for metal can vary from 2 to […]. For semiconductors, it can vary between 40 to 200. Some common materials and their gauge factors are listed in Tbl. 2.

TBL. 2

Material:

Nickel Manganese Nicrome Constantan Soft Iron Carbon Platinum

Gauge Factor:

_12.6

0.07 2.0 2.1 2 20 8

The gauge factor is normally supplied by the manufacturer from a calibration made of a number of gauges from a sample batch. The gauge factor for various metals ranges from _12 for nickel to 4 for soft iron. This indicates that changes in resistivity of a material could be quite significant while measurements are made.

2 Strain Gauges

A resistance strain gauge consists of a grid of fine resistance wire of about 20 m in diameter. The elements are formed on a backing film of electrically insulating material. Current strain gauges are manufactured from constantan foil, a copper-nickel alloy, or single-crystal semiconductor materials. The gauges are formed either mechanically or by photochemical etching. Strain-gauge transducers are of two types: unbonded and bonded.

Unbonded Strain Gauges In an unbonded strain gauge (Ill. 32(a)), the resistance wire is stressed between the two frames. The first frame is called the fixed frame, and the second is called a moving frame. The wires in the unbonded gauges are connected such that the input motion of one frame stretches one set of wires and compresses another set of wires.

As an example, a 20 _m diameter wire is wound between insulated pins with one attached to a stationary frame and the other to a movable frame. For a particular stress input, the winding experiences either an increase or decrease in stress, resulting in a change in resistance. The output is connected to a Wheatstone bridge for measurement. With this type of strain gauge, measurement of small motions as small as a few microns can be made.

ILL. 32 STRAIN GAUGES

(a) Stretched unbonded; Fine strain-gauge wire; (a) Bonded wire strain gauge

Bonded Strain Gauge Bonded strain-gauge transducers are widely used for measuring strain, force, torque, pressure, and vibration. The gauges have a backing material. Bonded strain gauges (Ill. 32(b)) are made of metallic or semiconductor materials in the form of a wire gauge or thin metal foil. When the gauges are bonded to the surface, they undergo the same strain as that of the member surface. The coefficient of thermal expansion of the backing material should be matched to that of the wire.

Strain gauges are sensitive devices and are used with an electronic measuring unit. The strain gauge is normally made part of a Wheatstone bridge, so the change in its resistance due to strain either can be measured or used to produce an output, which can be displayed. Strains as low as a fraction of a micron can be measured using strain gauges. Tbl. 3 presents characteristics of bonded strain gauges.

For precise measurement, the strain gauges should have the following properties.

• A high gauge factor increases the sensitivity and causes a larger change in resistance for a particular strain.

• The gauge characteristics are chosen so that the variation in resistance is a linear function of strain. If the gauges are used for dynamic measurements, the linearity should be maintained over the desired frequency range. High resistance of the strain gauge minimizes the effect of resistance variation in the signal-processing circuitry.

• Strain gauges have a low temperature coefficient and absence of the hysteresis effect.

= = = TBL. 3 BONDED STRAIN GAUGES

Material:

Nichrome, Ni:80%,Cr:20% Constantan, Ni:45%,Cu:55% Platinum Silicon Nickel

Gauge Factor:

2.5

2.1 8

_100 to 150

_12

Resistance, ohm

- 100 50 200

Resistance- Temperature Coefficient _/_/°C

Comments:

For use under […]; For high temperature use 400 ° C 1200 °C

= = =

EXAMPLE 7

A compressive force is applied to a structure causing the strain, __5(10)_6 Two separate strain gauges are attached to the structures, where one is a nickel wire stain gauge of gauge factor of _12.1 and another is a nicrome wire strain gauge of gauge factor of 2. Calculate the value of resistance of the gauges after they are strained. The resistance of strain gauge is 120 _.

Solution

Let us consider tensile strain as positive and compressive strain as negative.

Change in resistance for Nickel strain gauge;

Change in resistance for Nichrome strain gauge;

The value of resistance of nickel strain gauge increases, whereas, the value of resistance of nichrome strain gauge decreases.

EXAMPLE 8

A resistance wire strain gauge with a gauge factor of 2 is bonded to a steel structure member subjected to a stress of 100 MN/m^2. The modulus of elasticity of steel is 200 GN/ m^2. Calculate the percentage change in value of the gauge resistance due to the applied stress.

Bridge Circuit Arrangement The Wheatstone bridge circuit is used to measure the small changes in resistance that result in most strain-gauge applications. The change in resistance either can be measured or provided as an output that is processed by the computer. Ill. 33 shows an arrangement of a bridge circuit. In the balanced bridge arrangement, strain-gauge resistance, R1, forms one arm of the Wheatstone bridge, while the remaining arms have resistances R2, R3, and R Between the points A and C of the bridge, there is a power supply; between points B and D, there is a precision galvanometer. The galvanometer gives an indication of the presence of current through that leg. For zero current to flow through the galvanometer, the points B and D must be at the same potential. The bridge is excited by the direct current source with voltage, V and Rg is the resistance in the galvanometer. The bridge is said to be balanced when there is no current flowing through the galvanometer.

The condition of balance is […]

If R1 changes due to strain, the bridge (which is initially in the balanced condition) becomes unbalanced. This may be balanced by changing R4 or R2. The change can be measured and used to indicate the change in R1. This procedure is useful for measuring static strains.

ILL. 33 BRIDGE CIRCUIT WITH STRAIN GAUGE

In the unbalanced bridge arrangement, the current through the galvanometer or the voltage drop across it's used to indicate the strain. This is useful for measuring dynamic as well as static strains.

3 Offset Voltage

As shown in Ill. 33, G is a null deflector that is used to compare potentials of point-B and D.

The potential difference between points B and D is […]. If all the resistance values (R1, R2, R3, R4) chosen in the bridge circuit are same, then the voltage at points B and D are the same, _V will be zero, and the bridge is balanced.

Let us consider R1 as the strain gauge. If R1 is strained, its resistance value changes, and the bridge becomes unbalanced, causing a nonzero _V. If any other resistance value is adjusted, the bridge can be brought back to a balanced condition. The adjusted value of any resistor needed to force _V to zero is equal to the strained value of the strain gauge. The current flowing through the bridge arms is computed as

Signal Enhancement Strain-gauge devices with signal-conditioning equipment are designed to balance the bridge automatically and provide the strain value in terms of microstrains. Data acquisition systems for force and strain measurement are programmed to provide the unbalanced offset voltage, which is proportional to the gauge resistance.

Ill. 34 shows an arrangement of an instrumentation amplifier to be connected to the input channels of the data acquisition system.

ILL. 34 BRIDGE CIRCUIT WITH INSTRUMENTATION AMPLIFIER

Possible Strain-Gauge Arrangement When more than one arm of the bridge circuit contains strain transducers and their resistances change, the bridge output is due to the combined effect of these changes. More than one strain gauge, if suitably arranged, can lead to a higher signal-enhancement factor and a larger change in output voltage for a given strain.

e.g., in Ill. 33, R3 is the original strain gauge, and if we use R1 as another strain gauge placed in a location such that it has same strain as R3 the bridge output will be double the value obtained for a single gauge. In many experimental situations, there are areas of tension and compression in the same object with similar strain but of opposite sign. In such situations, care must be taken in arranging strain gauges in such a way that the adjacent arms of the bridge have strains of opposite nature.

In Ill. 35, R1 measures changes due to axial tensile strain. In Ill. 36, strain gauge R1 is bonded to the elastic member to measure axial tensile strain. R1 changes due to axial tensile strain. R2 measures changes due to transverse compressive strain. In the arrangement shown in Ill. 37, both R1 and R3 are subjected to axial tensile strain of the same amount, and R1 and R3 form opposite arms of the bridge. This causes a signal enhancement factor of 2.

ILL. 35 POSSIBLE ARRANGEMENT STRAIN GAUGES TO MEASURE P

ILL. 36 POSSIBLE ARRANGEMENT OF GAUGES TO MEASURE P

ILL. 37 POSSIBLE ARRANGEMENT OF GAUGES TO MEASURE TENSION

ILL. 38 CANTILEVER DEFLECTION MEASUREMENT

In the example shown in Ill. 38, R1 has tensile strain, and R2 has compressive strain. R3 also has tensile strain, and R4 has compressive strain. Strain gauges R1, R2, R3 and R4 are bonded at the root of the cantilevers, where the bending stresses are maximum. In the arrangement shown in Ill. 39, four active gauges are used with R2 and R4 arranged at right angles to R1 and R3 to pro duce a signal enhancement factor of 2(1 _), where = denotes Poisson's ratio.

ILL. 39 ALTERNATE ARRANGEMENTS

In the arrangement shown in Ill. 40, the strain gauges are arranged in such a way that R1 and R3 measure axial strains, while R2 and R4 measure the circumferential strains, which have strain of the opposite nature.

ILL. 40 HOLLOW CYLINDER WITH AXIAL LOADING

Temperature Effects in Strain Gauges The strain-gauge measuring environment is often influenced by temperature changes. The electrical resistivity of most alloys changes with temperature, increasing as temperature rises and decreasing as it falls. As shown in Tbl. 3, metals used in strain gauges have a temperature coefficient ( 0 ) of the order of 0.004/°C. The resistance at temperature T is given as […]

Resistance change due to change in temperature _T is […]

e.g., if the temperature changes by one degree, the change in resistance is calculated as

_T = 1°, _0 = 0.004/°C, , and .

When a strain gauge is bonded to the member being tested, its resistance will be affected by a change in temperature. This effect is independent of any strain applied to the gauge. The recording instrument can't differentiate between the changes in the resistance due to temperature and strain.

In addition, unless the coefficient of the linear expansion of the gauge is the same as that of the material to which it's bonded, the temperature change during measurement also will be a source of false strain due to differential expansion.

Temperature Compensation: Temperature compensation is achieved in two manners:

1. Using a dummy gauge.

2. Using more than one active gauge with proper arrangement of gauges.

If active and dummy gauges are mounted on the adjacent arms of a bridge, variation in temperature will not affect the bridge. The active gauge is subjected to strain as well as temperature change, while the dummy gauge is subjected to temperature change only. Since active and dummy gauges form adjacent arms of the bridge, the output due to temperature change is zero, as both active and dummy gauges change identically due to temperature. Furthermore, it's desirable to choose a gauge material with a coefficient of thermal expansion very close to that of the material under test.

Since it's inconvenient to calculate and apply temperature correction after the measurement is made, the temperature compensation can be made in the experimental setup itself. The gauges are suitably arranged so that adjacent arms have strains of opposite nature. This procedure ensures signal enhancement as well as temperature compensation.

Acceleration Sensing Using Strain: Gauges Strain gauges are used in a variety of electrical transducer devices. Their advantages include ease of instrumentation, high accuracy, and excellent reliability. One of the most common configurations used in pressure, force, displacement, and acceleration transducers is the cantilever configuration with strain gauges mounted at the base, shown in Ill. 41. A point mass of weight W is used as the acceleration-sensing element, and the cantilever (mounted with gauges) converts the inertial force into a strain.

ILL. 41 ACCELERATION SENSING

Ill. 42 presents a photograph of a load cell being used in a force measurement application.

ILL. 42 LOAD CELL

Semiconductor Strain Gauges: Semiconductor strain gauges are very useful in low strain applications. Use of semiconductor silicon has notably increased during the last few years. In a semiconductor gauge, the resistivity changes with strain as well as with physical dimensions. Changes in electron and hole mobility with changes in crystal structure as strain is applied results in larger gauge factors than possible with the metal gauges. Gauge factors of semiconductor gauges are between 50 and 200.

Semiconductor strain gauges physically appear as a band or strip of material with an electrical connection. The gauge is either bonded directly to the test element, or if encapsulated, it's attached by the encapsulation material. Signal conditioning is essentially a bridge circuit with temperature compensation.

There is also a need to linearize the output, because the basic characteristic of resistance verses strain is nonlinear. For good linearity of the output voltage with respect to strain, it's desirable to maintain a constant gauge current. This is accomplished by maintaining constant voltage excitation or by suitable modification, which produces constant current in the bridge arm in addition to constant voltage.

The benefits of semiconductor strain gauges are low power consumption and low heat generation. In addition, the mechanical hysteresis is negligible.

The resistance of semiconductor strain gauges varies from 1000 to 5000 _. They are usually made from p- or n-type silicon material.

4 Tactile Sensors

Tactile sensors are used in many applications ranging from fruit picking to monitoring human prosthetic implants; however, the major area of application is in the biomedical field. Tactile sensors are used for the following.

• Study the forces developed by the human foot during motion.

• Study the forces developed during various types of hand functions.

• Monitor the artificial knee and sense the forces developed.

Other areas of application include the field of robotics, where tactile sensors can be placed on the gripper of the manipulator to provide feedback information from the workpiece. Besides being used as a touch sensor, gripping force sensors detect the force with which the object is gripped, pressure sensors detect the pressure applied to the object, and slip sensors can detect if the object is slipping.

In addition, other industrial applications of the tactile sensors include the study of forces developed by fastening devices.

A tactile sensing system has the ability to detect the following.

1. Presence of a part

2. Part shape, location, and orientation

Contact pressure distribution

Force magnitude and direction The major components of tactile sensors include:

• Touch surface

• Transducer

• Structure and control interface

ILL. 43 PHOTOGRAPH OF THE TACTILE SENSOR

Some tactile sensors are designed using piezoelectric films. Piezoelectric (Piezo) film consists of poly-vinylidene fluoride (PVDF) that has undergone special processing to enhance its piezoelectric properties. Piezo film develops an electrical charge proportional to induced mechanical stress or strain. As a result, it produces a response proportional to the rate of stress rather than to the stress magnitude. This sensor is passive-that is, its output signal is generated by the piezoelectric film without the need for an excitation signal. The piezoelectric tactile sensor can be fabricated with the PVDF film strips imbedded into a rubber skin. To measure surface vibration, the film is bonded to the surface. As the surface vibrates, it stretches the surface in a cyclical manner, generating a volt age. Piezo-film voltage output is relatively high.

A resistive tactile sensor known as a force sensing resistor (FSR) can be fabricated using material whose electrical conductivity changes with strain. FSR consists of a material whose resistance changes with applied pressure. Such materials are known as conductive elastomers fabricated of silicone rubber, polyurethane, and other compounds. The basic operating principle of elastomeric tactile sensors is based either on varying the contact area when the elastomer is squeezed between two conductive plates or on changing the thickness. When the external force varies the contact area at the interface of the elastomer, changes result in a reduction of electrical resistance. Compared with a strain gauge, the FSR has a much wider dynamic range. Miniature tactile sensors are used extensively in robotic applications where good spatial resolution, high sensitivity, and wide dynamic range are required.

SUMMARY Strain Gauges

The resistance, R, of a resistance wire depends on its area, length, and electrical resistivity.

Where […]

_ resistivity, _-m R0 = sample resistance, = l = length, m A0 = cross-sectional area, m^2 Sensitivity or gauge factor, Gf, is defined as the ratio of unit change in resistance to unit change in length.

Bonded Strain Gauges

Bonded strain gauges (Ill. 44) are made of metallic or semiconductor materials in the form of a wire gauge or thin metal foil. When the gauges are bonded to the surface they undergo the same strain as that of the member surface.

ILL. 44 BONDED WIRE STRAIN GAUGE

Strain gauges are very sensitive devices and are used with an electronic measuring unit. The resistance strain gauge is normally made part of a Wheatstone bridge (Ill. 45) so that the change in its resistance due to strain can either be measured or used to produce an output which can be displayed or recorded.

Features

Strain gauges should have the following features:

• A high gauge factor increases its sensitivity and causes a larger change in resistance for a particular strain.

• High resistance of the strain gauge minimizes the effect of resistance variation in the signal processing circuitry. Choose gauge characteristics such that resistance is a linear function of strain.

ILL. 45 BRIDGE CIRCUIT ARRANGEMENT

• For dynamic measurements, the linearity should be maintained over the desired frequency range.

• Low temperature coefficient and absence of the hysteresis effect add to the precision.

Applications

• Strain-gauge transducers are used for measuring strain, force, torque, pressure, and vibration.

• In some applications, strain gauges are used as a primary or secondary sensor in combination with other sensors.

Tactile Sensors

• Tactile sensors are used in applications ranging from fruit picking to monitoring human prosthetic implants.

• Biomedical applications include the study of forces during human foot motion, during various types of hand functions, and monitoring and sensing the forces developed in knee implants

• In robotics, the tactile sensors are placed on the gripper of the manipulator to provide feedback; pres sure sensors detect the pressure applied to the object, and slip sensors can detect slip

[omitted math content]

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