WO1988006719A1

WO1988006719A1 - Transducer signal conditioner

Info

Publication number: WO1988006719A1
Application number: PCT/GB1988/000122
Authority: WO
Inventors: Barry Lascelles Welsh; Clive Robert Pyne
Original assignee: The Secretary Of State For Defence In Her Britanni
Priority date: 1987-03-05
Filing date: 1988-02-24
Publication date: 1988-09-07
Also published as: GB8803669D0; GB2201791B; GB8705192D0; GB2201791A

Abstract

A temperature compensation circuit for use with a resistance-bridge type pressure transducer (1), particularly useful for wind tunnel experiments. In one embodiment, a differential amplifier (7) senses the transducer output and produces an output signal referenced to an intermediate voltage generated by an operational amplifier (8). The voltage across a sense resistor (R1?) connected between the transducer and a power supply (2) varies in accordance with transducer temperature. These voltage variations are applied to the output signal by means of a summing amplifier (10) to compensate for thermal offset in the transducer output. Thermally-induced sensitivity variations in the transducer output may be further compensated by incorporating a thermally sensitive resistor (R3?) placed in proximity to the transducer and a gain adjustment amplifier (12). In a digital embodiment, the transducer output and sense resistor voltage are digitised and fed into a signal processor (16) which corrects the transducer output to compensate for thermal offset sensitivity variation using calibration data from a digital memory (15).

Description

TRANSDUCER SIGNAL CONDITIONER

This invention relates to signal conditioning circuitry and particularly to such circuits suitable for use in conjunction with pressure and force transducers.

In the case of resistance bridge transducers, such as foil-type or semi-conductor type strain gauges operating on the Wheatstone bridge principle, problems often arise during measurement due to variations in the transducer's temperature. Temperature variations affect this type of transducer in two ways, namely its zero balance and its sensitivity. In an ideal Wheatstone bridge each resistive element has the same temperature coefficient, thus cancelling the effects of any temperature changes. In practice, however, this is not generally the case and these differences in temperature coefficients coupled with the non-linear expansion of the transducer housing lead to an offset in the output and temperature dependant sensitivity characteristics. These effects are particularly serious for steady pressure measurements.

A known method for compensating for these temperature effects relies on a passive resistor network. Each network has a low temperature coefficient of resistance and is adjusted to correct for both zero balance offset and for sensitivity variations of the transducer. However, these compensated transducer networks still do not have ideal thermal characteristics and are unsuitable for use in, for example, an aerodynamic wind tunnel where the mean air temperature can vary ±2ºC typically within an operating range between 0°C and 50°C.

A further known technique for performing temperature compensation and having better thermal stability than that employing resistor networks is described in UK Patent No 1,591,620. This patent describes a signal conditioning circuit for use in combination with a transducer of the Wheatstone bridge type comprising an amplifier for receiving an output signal from the transducer via a gain control, temperature compensation means adapted to receive an input signal, separate from the transducer signal, which is dependent upon the transducer temperature and to generate from the input signal a gain control signal and a DC offset voltage which is applied to the transducer output signal such that compensation is made for the variations of the transducer output signal with temperature.

Two disadvantages of this particular circuit are that a second power supply separate from the transducer bridge excitation supply is required, and that the setting-up procedure is laborious.

The present invention provides a signal conditioning circuit which can be powered from the bridge excitation supply. It seeks to provide a conditioning circuit which is easy and quick to set up and which has good thermal stability over a wide temperature range.

According to a first aspect of the present invention, a signal conditioning circuit for use in combination with a transducer of the resistance bridge type comprises means for sensing the transducer output and for producing a transducer voltage level with respect to a reference voltage, means for producing a signal dependant on transducer temperature and means for applying this temperature-dependant signal to the transducer voltage level in order to compensate for variations in the transducer output with temperature. Preferably, the means for sensing the transducer output and producing the referenced transducer voltage level is a differential amplifier.

Two operational amplifiers may be connected between the transducer outputs and the differential amplifier in order to act as impedance buffers .

The reference voltage may be generated by means of an operational amplifier.

Preferably, the means for applying the temperature-dependant signal to the transducer voltage level comprise a potentiometer and summing amplifier.

Preferably, variations in the transducer temperature are detected by sensing variations in the resistance of the transducer. In the case of a transducer excited by a constant voltage supply, this may be achieved by connecting a sense resistor, preferably a temperature-stable resistance, in series with the constant voltage supplied to the transducer and detecting changes in the voltage drop across the sense resistor resulting from variations in the current drawn by the transducer bridge due to temperature variations thereof. The voltage is then used in accordance with the first aspect of this invention to control a DC offset voltage applied to the transducer voltage level. Hence, the transducer's zero balance offset may be corrected. According to a further feature of the invention, temperature-induced sensitivity variations in the transducer output may be compensated by the addition of transducer-voltage-level gain control means comprising an operational amplifier whose gain is varied by a temperature-sensitive resistor mounted in close proximity to the transducer.

According to a second aspect of the invention, a signal conditioning circuit suitable for use in combination with a transducer of the resistance bridge type comprises a resistor connected in series with a transducer voltage supply the voltage across the resistor thus varying as a function of the temperature of the transducer, means for converting the output of the transducer and the voltage across the resistor into digital signals to produce a transducer output value and a resistor output value respectively, a digital memory for storing calibration data and a signal processor for receiving the transducer output value, the resistor output value and the calibration data and for computing a temperature-compensated transducer output value by applying correction terms involving the resistor output value and the calibration data to the transducer output value.

If the signal conditioning circuit is to be used close to the transducer, ie in the same thermal environment, then it Is preferable for the electronic components which comprise the circuit to have high thermal stability. Otherwise the full capabilities of the conditioning circuitry will not be realised.

If the circuit is to be used In a thermally stable environment, remote from the transducer, then the constraints on thermal stability of the components can be relaxed.

The circuit has the advantage that it may be added to any standard uncompensated transducer bridge without making any alterations to the transducer bridge circuit itself.

The absence of serious thermal hysteresis effects enables the compensated transducer to respond rapidly to thermal shocks without significant loss of accuracy, limited only by the thermal mass of the transducer itself. Furthermore, the frequency response of the circuit is such that it does not limit the dynamic performance of the transducer. A transducer bridge operating in conjunction with a circuit according to the Invention has the advantage of being able to give an output signal which Is substantially independent of temperature over a range -160°C to 100°C. The sense resistor voltage provides an accurate measurement of the transducer temperature and is useful as a thermometer when alternative methods of temperature measurement are unreliable or difficult to install.

The invention will now be described with reference to the drawings, of which Figure 1 shows a first embodiment of the invention,

Figure 2 shows a second embodiment of the invention, and

Figure 3 shows calibration curves pertinent to the embodiment of Figure 2.

Figure 1 shows a circuit diagram of a signal conditioning circuit for a transducer bridge 1. The transducer bridge comprises a four active arm Wheatstone bridge of the miniature semi-conductor type and is excited from a regulated constant voltage source 2. The transducer bridge is configured as a pressure transducer suitable for use in the simultaneous measurement of both steady and unsteady pressures within an aerodynamic wind tunnel. Hence the output signal from the transducer bridge 1 contains DC and AC components representing the steady and unsteady components of pressure respectively. This signal is fed via lines 3 and 4 into operational amplifiers 5 and 6 respectively, outputs from which are fed into a differential amplifier 7 to which a reference signal from a voltage reference amplifier 8 is also applied. A temperature-stable sense resistor R₁ is connected in series between the transducer bridge 1 and the voltage source 2. A change in the temperature of the transducer, bridge 1 is reflected by a change in the bridge resistance which produces a corresponding change in the bridge excitation current. However, the transducer bridge 1 is configured such that changes in pressure at a constant temperature do not change the excitation current. Hence the sense resistor R₁ only senses changes in temperature. A typical value for R₁ is 10% of the transducer bridge resistance. The sense voltage signal on line 9 is applied to the inputs of a summing amplifier 10 via a potentiometer R₂. This voltage is summed with the reference signal from the reference amplifier 5 and with the output of the differential amplifier 7 to give a temperature-compensated output on line 11. This output and the reference signal form a differential pair.

As all the amplifiers in the signal conditioning circuit shown in Figure 1 are powered from the single-sided source 2, It is necessary to employ the voltage reference amplifier 8 to derive an intermediate voltage which can be regarded as a signal zero or reference. All signals within the conditioning circuit are measured with respect to this reference and hence are allowed to assume both positive or negative values. The actual value of this signal zero is arbitrary provided that the output swing of each amplifier is not limited. However, it is important for the voltage reference amplifier 8 to have a stable output of sufficiently low impedance, typically 200Ω, to render it insensitive to circuit load changes.

The foregoing description with reference to Figure 1 relates to circuitry which is used to compensate for thermally induced zero balance offset. The circuit shown in Figure 1 also provides means for compensating for thermally induced sensitivity variations in the transducer's output.

This is achieved by incorporating an additional operational amplifier 12 whose inputs are fed from the output of the amplifier 8 directly and from the output of the amplifier 10 via a resistor R₃. The resistor R₃ is a thermally sensitive semi-conductor resistor and is placed in close proximity to the transducer bridge 1 such that it experiences the same thermal environment.

The gain of the amplifier 12 is set by a feedback resistor R₄ and the thermally sensitive resistor R₃, a change (ΔR₃) in the value of R₃ as a function of temperature being reflected as a change in the gain G of amplifier 12 as follows:

where R₄, has a value which is selected so that the ampli fier gain compensates for any thermally-induced increase in sensitivity of the transducer bridge 1.

Compensation for sensitivity variations may not be required for all applications, in which case the resistors R₃ and R and the amplifier 12 would be omitted from the circuit shown in Figure 1.

Temperature compensation for zero balance offset is achieved by performing a two-point calibration procedure under conditions of constant pressure. Firstly, all amplifiers are nulled at a first (datum) temperature. (Subsequent temperature changes are then detected by sensing changes in the transducer's supply current). Secondly, at a second (set) temperature, the sense voltage is scaled and summed with the transducer's output signal to actively correct for thermally-induced shifts in this output signal. To ensure that circuit adjustments made at the set temperature do not affect the circuit conditions at the datum temperature, the voltage reference amplifier 8 is adjusted so that the signal zero is equal to the sense voltage at the datum temperature. This measure obviates the need for additional null circuitry on line 9. The output of the transducer bridge 1 must be conditioned to give a single-ended voltage which must also be made equal to the signal zero at the datum temperature. The differential amplifier 7 fulfils this function. However, adjustment of the gain of this amplifier alters its input impedance. To ensure that this does not adversely affect the transducer bridge, the bridge output is buffered by the amplifiers 5 and 6. The individual bridge arm resistances can vary in such a way as to make the bridge output voltage between lines 3 and 4 either positive or negative with increasing temperature. Hence, the conditioning circuit must have the ability to sum or difference the sense voltage with transducer signal. At the set temperature, the transducer output will, due to thermal offset shift, be different from its value at the datum temperature. To effect an accurate compensation the sense voltage must be scaled and its polarity must be adjusted to match this difference. Both these requirements are met by the potentiometer R₂. By adjusting R₂ by the appropriate amount and in the appropriate direction away from its mid-point, it is possible to accurately match the sense voltage with the transducer's thermal drift. Thereafter, the output of the sensing amplifier 10 is automatically compensated.

In an alternative embodiment, the manually adjustable resistors R₂ and R₄ are replaced with digitally-addressable resistor ladder networks. In this case, a computer may be programmed to select appropriate network values in order to facilitate the calibration procedure.

Figure 2 shows, schematically, a second embodiment of the invention whereby temperature compensation is achieved by digital rather than analogue means. The output from a transducer bridge 1, which is powered by a constant voltage source 2, is fed into an anti-aliasing filter 13 and subsequently converted into a digital signal by an analogue to digital converter (ADC) 14. The voltage across a temperature-stable sense resistor R₁ is also digitised by the ADC 14. Calibration data, which is held in a memory 15, is fed into a signal processor 16 together with the digitised transducer bridge signal and sense resistor signal. The signal processor 16 computes a temperature-compensated value of transducer bridge pressure in accordance with an equation detailed below. This value is converted back to analogue form by a digital to analogue converter 17.

Figure 3 shows, typically, the response of the transducer bridge 1 from which calibration data is deduced. Curves A and B show applied pressure versus measured pressure for two values of transducer bridge temperature T₁ and T₂ respectively. The relative displacement of the two curves and their different gradients are manifestations of the thermally-induced zero balance offset and sensitivity variations respectively. From the two curves, the following values are deduced and stored in the memory 15: viz x, y, Ex, y₁, y₂, y₃, y₄, x₁ and x₂. Ex is the known applied pressure datum for zero transducer output at temperature T. and all other values are as defined by Figure 3.

The signal processor 16 converts the digitised measured transducer voltage into pressure units to give a value Ym. It also converts the corresponding digitised sense resistor signal to pressure units and applies a further empirically derived correction which compensates for small thermal hysteresis effects to give a value Vs. The signal processor 16 then computes a temperature-compensated value of pressure, Ycomp, in accordance with the following equation:

By employing this digital technique, compensation for zero balance offset and for sensitivity variations is achieved automatically, the only additional quantity requiring measurement being the sense resistor voltage. The signal processor 16 calculates a value of Ycomp for every digitised value of transducer bridge output and sense resistor voltage which is sampled by the ADC 14. Thus, the apparatus shown in Figure 2 provides a real-time correction of analogue data.

Claims

1. A signal conditioning circuit for use In combination with a transducer of the resistance bridge type (1) whose output is offset by an amount dependent on ambient temperature; the circuit comprising means (R₁) for producing a signal dependent on transducer temperature and characterised by means (8) for generating a reference voltage; means (7) for comparing the transducer output with the reference voltage to produce a transducer difference signal; and means (10) for summing the temperature-dependent signal and transducer difference signal in order to compensate for the temperature-dependent offset in the transducer output.

2. A signal conditioning circuit as claimed in Claim 1 in which the means for producing a signal dependent on transducer temperature comprises a resistor (R₁) connected in series between the transducer (1) and a transducer voltage supply (2).

3. A signal conditioning circuit as claimed in either preceding claim in which the means for generating a reference voltage is an operational amplifier (8).

4. A signal conditioning circuit as claimed in any preceding claim in which the means for comparing the transducer output with the reference voltage is a differential amplifier (7).

5. A signal conditioning circuit as claimed in any preceding claim in which impedance buffers (5, 6) are connected between the transducer (1) and the differential amplifier (7) and are comprised of operational amplifiers.

6. A signal conditioning circuit as claimed in any preceding claim in which the means for summing the temperature-dependent signal and the transducer difference signal comprise a potentiometer (R₂) and a summing amplifier (10).

7. A signal conditioning circuit as claimed in any of Claims 1 to 5 in which the means for summing the temperature-dependent signal and the transducer difference signal comprise a digitally-addressable resistor ladder network and a summing amplifier (10).

8. A signal conditioning circuit as claimed in any preceding claim in which the temperature dependence of the transducer output sensitivity is compensated by transducer output gain control means comprising an operational amplifier (12) whose gain is varied by a temperature-sensitive resistor (R₃) mounted in close proximity to the transducer (1).

9. A signal conditioning circuit suitable for use in combination with a transducer of the resistance bridge type, comprising a resistor (R₁) connected in series with a transducer bridge voltage supply (2) the voltage across the resistor thus varying as a function of the temperature of the transducer, means for converting the output of the transducer (1) and the voltge across the resistor into digital signals to produce a transducer output value and a resistor output value respectively; a digital memory (15) for storing calibration data; and a signal processor (16) for receiving the transducer output value, the resistor output value and the calibration data and for computing a temperature-compensated transducer output value by applying correction terms involving the resistor output value and the calibration data to the transducer output value and characterised in that the calibration data is deduced from previously measured values of the transducer output when subjected to different values of applied presure and at different ambient temperatures.

10. A signal conditioning circuit as claimed in Claim 9 in which the temperature-compensated transducer output value Ycomp is computed from an equation of the form:

where Y_m is the transducer output value, V_s is a value related to the resistor output value, and the calibration data, x₁, x₂ and y₁, y₃, the known and measured pressures at the same temperature, Ex, the intercept of the line joining x₁ and :x₂ , and x and y, the intersection of similar lines at different temperatures derived from measured values y₂ and y₄ are stored in the digital memory (15).