1、CHAPTER 4STRAIN.FORCE.PRESSURE.AND FLOWMEASUREMENTS1.STRAIN GAGES The most popular electrical elements used in force measurements include the resistance strain gage,the semiconductor strain gage,and piezoelectric transducers.The strain gage measures force indirectly by measuring the deflection it pr
2、oduces in a calibrated carrier.Pressure can be converted into a force using an appropriate transducer,and strain gage techniques can then be used to measure pressure.Flow rates can be measured using differential pressure measurements which also make use of strain gage technology.STRAIN GAGE BASED ME
3、ASUREMENTS Strain:Strain Gage,PiezoElectric Transducers Force:Load Cell Pressure:Diaphragm to Force to Strain Gage Flow:Differential Pressure Techniques The resistance strain gage is a resistive element which changes in length,hence resistance,as the force applied to the base on which it is mounted
4、causes stretching or compression.It is perhaps the most well known transducer for converting force into an electrical variable.Unbonded strain gages consist of a wire stretched between two points.STRAINSENSINGWIREAREA=ALENGTH=LRESISTIVITY=RESISTANCE=R LLGFRRALR Force acting on the wire(area=A,length
5、=L,resistivity=)will cause the wire to elongate or shorten,which will cause the resistance to increase or decrease proportionally according to:R=L/A And LLGFRR/where GF=Gage factor(2.0 to 4.5 for metals,and more than 150 for semiconductors).The dimensionless quantity L/L is a measure of the force ap
6、plied to the wire and is expressed in microstrains(1=10-6 cm/cm)which is the same as parts-per-million(ppm).FORCE Bonded strain gages consist of a thin wire or conducting film arranged in a coplanar pattern and cemented to a base or carrier.The gage is normally mounted so that as much as possible of
7、 the length of the conductor is aligned in the direction of the stress that is being measured.Lead wires are attached to the base and brought out for interconnection.Bonded devices are considerably more practical and are in much wider use than unbonded devices.Strain gages can be used to measure for
8、ce,as in Figure where a cantilever beam is slightly deflected by the applied force.Four strain gages are used to measure the flex of the beam,two on the top side,and two on the bottom side.The gages are connected in an all-element bridge configuration.Recall from Section 2 that this configuration gi
9、ves maximum sensitivity and is inherently linear.This configuration also offers first-order correction for temperature drift in the individual strain gages.Strain gages are low-impedance devices;they require significant excitation power to obtain reasonable levels of output voltage.A typical strain-
10、gage based load cell bridge will have(typically)a 350 impedance and is specified as having a sensitivity in terms of millivolts full scale per volt of excitation.The load cell is composed of four individual strain gages.For a 10V bridge excitation voltage with a rating of 3mV/V,30 millivolts of sign
11、al will be available at full scale loading.The output can be increased by increasing the drive to the bridge,but self-heating effects are a significant limitation to this approach they can cause erroneous readings or even device destruction.+VB+SENSE+VOUT-VOUT-SENSE-VB Many load cells have sense con
12、nections to allow the signal conditioning electronics to compensate for DC drops in the wires.Some load cells have additional internal resistors which are selected for temperature compensation.Pressures in liquids and gases are measured electrically by a variety of pressure transducers.A variety of
13、mechanical converters(including diaphragms,bellows,and manometer tubes)are used to measure pressure by measuring an associated length,distance,or displacement,and to measure pressure changes by the motion produced.The output of this mechanical interface is then applied to an electrical converter suc
14、h as a strain gage or piezoelectric transducer.Unlike strain gages,piezoelectric pressure transducers are typically used for high-frequency pressure measurements(such as sonar applications or crystal microphones).There are many ways of defining flow(mass flow,volume flow,laminar flow,turbulent flow)
15、.Usually the amount of a substance flowing(mass flow)is the most important,and if the fluids density is constant,a volume flow measurement is a useful substitute that is generally easier to perform.One commonly used class of transducers,which measure flow rate indirectly,involves the measurement of
16、pressure.Flow can be derived by taking the differential pressure across two points in a flowing medium-one at a static point and one in the flow stream.Pitot tubes are one form of device used to perform this function.The flow rate is obtained by measuring the differential pressure with standard pres
17、sure transducers as shown in Figure.Differential pressure can also be used to measure flow rate using the venturi effect by placing a restriction in the flow as shown in Figure.FLOWRESTRICTION2.BRIDGE SIGNAL CONDITIONING CIRCUITS An example of an all-element varying bridge circuit is a fatigue monit
18、oring strain sensing circuit as shown in Figure below.The full bridge is an integrated unit that can be attached to the surface on which the strain or flex is to be measured.In order to facilitate remote sensing,current excitation is used.The OP177 servos the bridge current to 10mA around a referenc
19、e voltage of 1.235V.The strain gage produces an output of 10.25mV/1000.The signal is amplified by the AD620 instrumentation amplifier which is configured for a gain of 100.Full-scale strain voltage may be set by adjusting the 100gain potentiometer such that,for a strain of-3500,the output reads-3.50
20、0V;and for a strain of+5000,the output registers a+5.000V.The measurement may then be digitized with an ADC which has a 10V fullscale input range.The 0.luF capacitor across the AD620 input pins serves as an EMI/RFI filter in conjunction with the bridge resistance of lk.The corner frequency of the fi
21、lter is approximately 1.6kHz.Another example is a load cell amplifier circuit shown in Figure below.+15V0 TO+10.000V FS-15V350 LOAD CELL100mV FS A typical load cell has a bridge resistance of 350.A 10.000V bridge excitation is derived from an AD588 precision voltage reference with an OP177 and 2N221
22、9A used as a buffer.The 2N2219A is within the OP177 feedback loop and supplies the necessary bridge drive current(28.57mA).To ensure this linearity is preserved,an instrumentation amplifier is used.This design has a minimum number of critical resistors and amplifiers,making the entire implementation
23、 accurate,stable,and cost effective.The only requirement is that the 475 resistor and the 100 potentiometer have Low temperature coefficients so that the amplifier gain does not drift over temperature.The Figure below shows a precision load-cell amplifier that is powered from a single supply.19628.7
24、 The excitation voltage to the bridge must be precise and stable,otherwise it introduces an error in the measurement.In this circuit,a precision REF195 5V reference is used as the bridge drive.The REF195 reference can supply more than 30mA to a load,so it can drive the 350 bridge without the need of
25、 a buffer.The dual OP213 is configured as a two op amp in-amp with a gain of 100.For optimum common-mode rejection,the resistor ratios must be precise.High tolerance resistors(0.5%or better)should be used.The AD7730 24-bit sigma-delta ADC is ideal for direct conditioning of bridge outputs and requir
26、es no interface circuitry.The simplified connection diagram is shown in Figure below.The entire circuit operates on a single+5V supply which also serves as the bridge excitation voltage.Note that the measurement is ratiometric because the sensed bridge excitation voltage is also used as the ADC refe
27、rence.Variations in the+5V supply do not affect the accuracy of the measurement.The AD7730 has an internal programmable gain amplifier which allows a fullscale bridge output of 10mV to be digitized to 16-bit accuracy.The AD7730 has self and system calibration features which allow offset and gain errors to be minimized with periodic recalibrations.The effective input voltage noise RTI is approximately 264nV peak-to-peak.This corresponds to a resolution of 13 ppm,or approximately 16.5-bits.Gain linearity is also approximately 16-bits.
