GB2165647A - Method of conducting measurement using an electro-mechanical transducer - Google Patents

Abstract

A method of conducting measurement of a physical parameter using a transducer whose electrical properties are modified by the parameter, comprising the steps of: subjecting the transducer to a DC excitation voltage at two different levels; detecting output signals from the transducer representative of the parameter; and combining the output signals to derive a composite signal as a measurement of the parameter so as to substantially eliminate the effect of system off-set voltages. As shown, power MOSFETS 3-6 apply equal amplitude opposite polarity voltages in turn to a strain gauge bridge 1 used for sensing load or pressure, and the output signals are subtracted to eliminate offset. Filters 12 in negative feedback loops maintain high frequency stability for the MOSFET switching. Resistors 14A, 14F in the current supply to the bridge monitor the resistance and thus temperature thereof; the latter is used with a look-up table to correct the output signal. <IMAGE>

Description

SPECIFICATION Methods of conducting measurement using an electromechanical transducer This invention relates to methods of measurement using an electromechanical transducer, and in particular, methods which result in the avoidance of system off-set voltage error and/or compensate for the effects of temperature variations.

Electromechanical transducers are used increasingly in industry as the electronic display or control of physical properties becomes more common. The strain gauge, in the form of an electrical bridge circuit, is used in a wide range of transducers, including pressure and load measurement.

Transducers with which the present invention is concerned, and of which the stain gauge type is an example, require an external supply voltage (excitation voltage) applied to the device. A signal voltage is then generated by the transducer which is proportional to the excitation voltage, provided that the transducer is under constant physical conditions. For a constant excitation voltage, the signal voltage will generally be an approximately linear function of the property to be measured, e.g. pressure, with relatively small effects due to the variation of other physical conditions, e.g. temperature, acceleration, elapsed time, previous pressure history (hysteresis).

The transducer forms part of the measurement system, which also comprises the instrumentation used to generate the excitation voltage and to process and display the measurement signal voltage, and the interconnecting cables. The overall accuracy of the system is therefore effected by the linearity and drift characteristics of the instrumentation, any thermal voltages generated in the interconnecting cables, and extraneous noise coupled into the system.

For measurement accuracy of approx. 1% of the full scale reading, the error due to nonlinearity, and other physical conditions can generally be ignored. Conventional analog instrumentation can be employed with little difficulty at this level of accuracy. Such analog instrumentation consists of a constant D.C. excitation voltage applied to the transducer via the interconnecting cable. If the transducer is remote, and hence the cable of significant resistance, two extra cables to sense the voltage directly at the transducer may be employed to ensure accurate excitation voltage.

The signal from the transducer passes to an instrumentation amplifier, and then through appropriate filter circuits to the display or output circuits.

It is quite common to eliminate any offset voltages within the instrumentation itself by the use of an auto-zero technique. This technique requires the inputs to the instrumentation to be shorted and the instrumentation offset voltage may be ascertained from the resultant reading.

This can then be used to correct the transducer reading. Auto-zero techniques are used internally in some integrated circuits, or can be applied when the analog instrumentation is controlled by digital circuitry. However this method only corrects the instrumentation offset voltage.

One known method that eliminates offset voltages in the entire measurement system uses A.C. excitation of the transducer. Typically the transducer is energised with a sinusoidal voltage, and the resulting sinusoidal signal is amplified and passed through a high-pass filter, so eliminating any D.C. offset voltages in the system. In this case any thermal emfs (voltages) generated in the interconnections to the cable and the transducer are eliminated. However, there are significant problems in processing the A.C. signal to give the degree of accuracy required, particularly if fast overall response is required.

Another method that reduces the offset voltages within the measurement system uses low duty-cycle, high voltage pulses to excite the transducer. Provided that the transducer can withstand the voltage, the low duty cycle will prevent the overheating and destruction of the device that would occur if the peak voltage were to be continuously applied. The apparatus used in the method is designed to sample the signal during the applied voltage peak. This method reduces the effect of offset voltages, which are not a function of excitation voltage, because the signal voltage is proportional to the higher excitation voltage. It does however require expensive high voltage power supplies and switching components. The high voltages used are not consistent with the need for safety if the transducer is operating in a hazardous area, (risk of explosive gas) as found in many industrial environments.

When increased precision is required, it becomes necessary to take into consideration the nonlinearity of the transducer, and the effects of physical parameters other than the one being measured. The physical parameter which usually causes the major measurement error is temperature.

The linearity of the transducer can be corrected in analog circuitry by suitable non-linear signal conditioning, or a computation can be performed on the results either by computer or manually.

Temperature effects are typically treated by using a second transducer to sense the temperature of the original transducer, and using this second signal to correct the results of the original transducer. This correction can again be applied using analog, digital or manual techniques.

The present invention in a first aspect seeks to overcome the problems of system off-set voltages and in a second aspect temperature drift in high accuracy transducer systems.

In a first aspect the present invention provides a method of conducting measurement of a physical parameter using a transducer, the electrical properties of which are modified by said parameter, comprising subjecting the transducer to a d.c. excitation voltage at two different values, combined the resultant signals to derive a composite signal as a measurement of said parameter in which the effect of the system off-set voltages is eliminated.

Preferably the two values of the excitation voltage are of the same magnitude and of opposite polarity, the composite signal being the subtraction of the resultant signals.

The method works because the transducers under consideration exhibit a linear relationship between the signal output and the excitation voltage, and since the system off-set voltages are essentially independent of the excitation voltage, measurements taken at two or more excitation voltages can be corrected to remove the off-set.For example, if measurement M1 is recorded for excitation voltage +V, and M2 is recorded at excitation voltage -V, and the true signal is s*Excitation voltage (s times excitation voltage), and there is a constant offset o, then M1=s*V+o -(1 M2=--s"V+o -(2, and subtracting, Ml -M2=2s*V Although A.C. excitation of transducers eliminates offsets in a similar way, the invention relates to the use of switched D.C. which allows the direct use of Analog to digital conversion, without the difficulty of rectifying and smoothing an A.C. signal, or generating a precision A.C.

excitation voltage. The method enables an extremely high degree of accuracy to be achieved together with a fast sampling rate. This becomes particularly important if many transducers are to be multiplexed into one instrumentation system, so that the system can rapidly scan from one transducer to the next.

Although any two D.C. voltages could be chosen as the excitation voltages, and one of which could be zero, (in which case system off-sets may be measured directly) the optimum signal level for a given maximum excitation voltage within a system is obtained by inverting the excitation voltage to provide the second voltage. One case where the excitation voltage is limited to maximum values is when all cable runs to the transducer have to pass through safety barriers (hazardous area operations). In this case by switching the excitation voltage polarity, an effective doubling of the signal can be obtained over a conventional constant-DC.-excitation system.

It should be noted that all off-sets in the system are corrected for using this method, no just the ones within the instrumentation itself. These other off-sets commonly occur in the interconnections and cable runs out to the transducer.

A further aspect of this invention applies to the need to sense the temperature of the transducer in order to apply temperature related corrections.

In a second aspect the present invention provides a method of conducting measurement of a physical parameter using a transducer, the electrical properties of which are modified by said parameter, and the resistance of which is, to at least a limited extent, temperature dependent wherein the measurement comprises detecting the signal produced as a result of an excitation voltage applied to the transducer, characterised in that the method further includes the step of measuring the current produced in the transducer as a result of the excitation voltage, and on the basis of this value determining the temperature of the transducer to derive a correction factor to the measured value of the physical parameter.

Preferably the excitation voltage is supplied to the transducer via voltage controlled resistance element, wherein negligible current flows in the control line.

Such resistance element may comprise power MOSFETS.

In this aspect of the invention, the temperature of the transducer is obtained by measuring the current drawn by the transducer. Since the instrumentation uses a known excitation voltage, this current can be used to compute the resistance of the transducer. Most transducers exhibit a temperature dependent excitation resistance, and if this is known, the temperature of the transducer can be obtained with sufficient accuracy to correct the primary measurement.

This method has several advantages over the use of a second transducer to sense the temperature of the primary transducer. Since the temperature transducer and the primary transducer are one and the same, thermal differentials are eliminated. No extra transducer housing, or extra cable runs are required, which simplify installation and reduce cost. This method is of particular advantage if the transducer is operating in a hazardous area, and all cable connections have to be made through individual safety barriers.

The technique of temperature correction according to the invention could be applied to any method of transducer excitation, and the switched d.c. method of transducer excitation could be applied independently of the current sensing temperature correction.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing which is a circuit diagram of a transducer and signal processing circuitry.

Refering to the figure, a strain gauge bridge transducer 1 is connected by a cable to the electronic signal generating and processing circuitry. An excitation voltage is applied to points 1A and 1B of the transducer bridge via power mosfet drivers 3,4,5,6. The excitation voltage is sensed at the bridge by a second pair of cables, and returned to operational amplifiers 9 and 10 via switches 7A and 7B or 8A and 8B. The bridge signal generated at points 1C and 1D is routed to an instrumentation amplifier 2 and subsequently to an Analog to Digital converter (not shown) for further processing. Control signals A and B for the switches 7 and 8 respectively are generated by a microprocessor system (not shown). Only one of these signals is produced at any one time.When control signal A is produced switches 7A,B,C,D close, and the transducer will be driven with excitation voltage V+ at point 1B, and V- at point 1A. When control signal B is activated, and switches 8A,B,C,D close, the transducer will be driven with excitation voltage V- at point 1B, and V+ at point 1A.

The excitation current for the transducer is drawn from the +ve supply via a resistor 14A, and from the -ve supply via a resistor 14F. The voltage drop induced in these resistors due to the excitation current is coupled to an instrumentation amplifier 11 via a resistor network 14B,C,D,E. The output of amplifier 11 is then proportional to the transducer supply current, and this current output signal is routed to the Analog to Digital converter for subsequent processing by the microprocessor.

When control signal A is produced, and switches 7A,B,C,D are closed, then control signal B must be inactive, and hence switches 8A,B,C,D are open. The output of op-amp 9 is routed to the power mosfet 5 via an R.C. filter network 12A,B,C and switch 7C. Similarly the output of op-amp 10 is routed to mosfet 4 via an R.C. filter network 13A,B,C and switch 7D. The bridge sense voltage signal from 1B is routed back to op-amp 9 via the switch 7A, and the bridge sense voltage signal from 1A is routed back to the op-amp 10 via the switch 7B. This completes the two control loops with overall negative feedback that maintains point 1B of the bridge at V+, and point 1A at V-.

The filter networks 12 and 13A,B,C,D,E are necesary to maintain high frequency stability, since the op-amps are connected in a positive feedback configuration, with the overall negative feedback within the control loop being produced by the inverting effect of the power mosfets.

The supply current to mosfet 5 is drawn from the +ve supply via the resistor 14A. The supply current to mosfet 4 is drawn from the -ve supply via the resistor 14F. Mosfets 3 and 6 are off while the switches 8C,D are open. Due to the construction of power mosfets, negligible current flows from the gate terminal, (which is connected to the op-amps), and in addition, the low input current of the op-amps, and low leakage current of the switches ensure that to a high degree of accuracy the current in resistors 14A and 14F exactly match the bridge excitation current. In principle, the voltage drop across either of the resistors 14A or 14F could be measured directly by the instrumentation amplifier. However practical limitations on the common mode input voltage of most amplifiers preclude this approach.The potential dividers 14D and 14C, and 14B and 14 E, reduce the common mode voltage at the input of amplifier 11 to midway between the supply voltage, while summing the voltage drops across 14A and 14F.

Reference voltages V+ and V- are provided by conventional voltage reference circuits (not shown). The bridge output signal and the current output signal are digitised, and used by the controlling microprocessor. This controller first selects control signal A, and stores the values of the bridge output signal and current output signal. Next control signal A is de-selected, and control signal B is produced. This closes switches 8A,B,C,D,- and energises the transducer with the reverse polarity. The bridge output signal will then be of reverse polarity, and the current output signal will be the same polarity as before. These values are stored by the controller.

Processing of the results can now take place. The values of the current reflect the resistance of the transducer, and hence its temperature. The temperature is derived directly through the use of look-up tables relating this temperature to the current signal. Any off-set voltages in the cable, interconnections or instrumentation are eliminated by subtracting the second bridge output signal value from the first bridge output signal value. The true transducer signal, having changed polarity from the first to second readings, will effectively ADD together (subtracting a negative value), the constant offset voltages will be subtracted, and hence eliminated (subtracting a positive value). The signal value thus obtained is corrected for system offsets. This value is used in a second look-up table, in conjunction with the temperature information, to obtain a temperature corrected value of considerable accuracy.

Claims (6)

1. A method of conducting measurement of a physical parameter using a transducer, the electrical properties of which are modified by the parameter, comprising the steps of: subjecting the transducer to a DC excitation voltage at two different levels; detecting resultant output signals from the transducer representative of the parameter; and combining the resultant signals to derive a composite signal as a measurement of the parameter in such a way that the effect of the system off-set voltages is substantially eliminated.

2. A method as claimed in Claim 1 wherein the two values of the excitation voltage are of the same magnitude and of opposite polarity, the composite signal being the arithmetic sum of the resultant signals.

3. A method as claimed in Claim 1 or Claim 2 wherein the electrical properties of the transducer are at least partially temperature dependent characterised in that during the step of subjecting the transducer to an excitation voltage a signal is derived by from the excitation voltage or a function thereof representative of the temperature of the transducer to enable a correction factor to be derived whereby the effect of the transducer temperature on the measurement of said parameter can be compensated for.

4. A method as claimed in Claim 3 wherein the or each DC excitation voltage is supplied to the transducer by way of a power MOSFET device.

5. A method as claimed in any preceding claim wherein the transducer is a strain gauge bridge network.

6. A method as hereinbefore described with reference to the accompanying drawing.