TEMPERATURE COMPENSATION OF TRANSDUCERS

Abstract: The invention provides a method of adjusting the temperature compensation of a temperature-sensitive transducer having a sensor element and a temperature-sensitive element which senses the temperature of the sensor element, and, combined therewith, a signal conditioning circuit which includes the temperature-sensitive element and is capable of effecting such temperature compensation, the method including the steps of making required adjustments to the signal conditioning circuit at one temperature and at another different temperature of the sensor element, characterised in that for making adjustments at the higher of the said one and the said different temperatures the sensor element is heated to that higher temperature by supplying a heating current to a heater element within the transducer and in good thermal connection with the sensor element. If the sensor element and the temperature-sensitive element which senses its temperature are provided on a semiconductor diaphragm of the transducer, the heater element may also be provided on the diaphragm.

Description: TEMPERATURE COMPENSATION OF TRANSDUCERS This invention relates to the temperature compensation of transducers so as to correct for offset and sensitivity changes caused by change of temperature. Semiconductor transducers such as silicon pressure sensors and accelero eters often use a single crystal silicon diaphragm as the sensor element. These sensors are very popular because they are robust and cheap to produce. Their major drawback is that, for a given magnitude of the variable being sensed, they suffer a relatively large change in output in response to a change in temperature. One proposal for overcoming the drawback of temperature sensitivity of such a transducer, as described for example in USSR patent No. 1000804, is to provide the silicon sensor element not only with the desired strain gauge or other desired sensing device but also with a resistive heater element and with a temperature-sensitive device such as a thermistor and to operate the transducer while maintaining its silicon element at a constant elevated temperature, this being achieved by supplying the heater element with a heating current under the control of a temperature feedback signal derived from the thermistor or other temperature-sensitive device. However, this proposal has not generally found favour, for a variety of reasons. It is usual to use these sensors with a signal conditioning circuit which will correct for output-signal offset and sensitivity errors at room temperature and also compensate for any changes in sensitivity and offset caused by a change in temperature. The signal conditioning circuit can be placed on the silicon sensor element itself or it can be on a separate substrate. Sometimes the signal conditioning circuit also incorporates sensor element output amplification. A problem with silicon transducer elements is that there is a part-to-part variation In the temperature coefficients of offset and sensitivity even between transducers of the same type. The only way to accurately calibrate these transducers is to raise the temperature of each transducer and individually adjust components of the signal conditioning circuit to correct the resulting output errors. Using conventional methods this can be a slow process since the transducer would be placed in a climatic chamber and it can take an hour for the temperature of a transducer to stabilize in such a climatic chamber, before temperature compensation can be started. Furthermore, it is difficult to correct the output of the transducer whilst it is in such a chamber, because access to the transducer is restricted. In a mass production process, sensor-error correction is often affected by use of a laser or abrasive trimming machine which trims resistors and thermistors in the signal conditioning circuit to the values required for offset and sensitivity compensation. It is not easy to accommodate a laser trimming machine in a climatic chamber; accordingly, the trimming operation would be greatly facilitated if the sensor could be adequately heated without putting it in a climatic chamber. Also, it has been appreciated that, although heating the whole sensor would take a relatively long time, most of the temperature-dependent change in these transducers is caused by temperature changes in the silicon element itself and it is therefore only necessary to heat the element itself, which can be effected much more rapidly, and not the whole device. According to one aspect of the present invention there is provided a method of adjusting the temperature compensation of a temperature-sensitive transducer having a sensor element and a temperature-sensitive element which senses the temperature of the sensor element, and, combined therewith, a signal conditioning circuit which includes the temperature-sensitive element and is capable of effecting such temperature compensation, the method including the steps of making required adjustments to the signal conditioning circuit at one temperature and at another, different, temperature of the sensor element, characterised in that for making adjustments at the higher of the said one and the said different temperatures the sensor element is heated to that higher temperature by supplying a heating current to a heater element within the transducer and in good thermal connection with the sensor element. According to a further aspect of the invention there is provided a method of providing temperature compensation of a transducer having a sensor element which has a temperature- dependent sensitivity and/or offset and provided with a temperature-sensitive element which senses the temperature of the sensor element, and having combined with the transducer a signal conditioning circuit which includes the temperature-sensitive element and is capable of compensating for temperature-dependent changes in the sensitivity and/or offset of the sensor element, the method including the steps of adjusting the signal conditioning circuit to establish a desired sensitivity and/or offset of the sensor element at one temperature thereof and, at a different temperature thereof, adjusting the signal conditioning circuit to adjust the sensitivity and/or offset at the said different temperature to equal that at the one temperature, characterised in that for making adjustments at the higher of the said one and the said different temperatures the sensor element is heated to that higher temperature by supplying a heating current to a heater element within the transducer and in good thermal connection with the sensing element. In either of the above-indicated aspects of the invention, the sensor element and the temperature-sensitive element which senses its temperature will often be provided on a semiconductor (e.g. single-crystal silicon wafer) diaphragm of the transducer and the heater element will also be provided on the diaphragm. The invention will be more fully understood from the following description with reference to the accompanying drawing in which: Figure 1 shows, greatly magnified, a silicon-diaphragm pressure sensor element of a pressure transducer suitable for temperature compensation in accordance with the invention; Figure 2 is a diagrammatic perspective view of the pressure transducer which incorporates the pressure sensor element shown in Figure 1, and of a thick-film hybrid signal conditioning circuit on the substrate of which the pressure transducer is mounted; and Figure 3 is a circuit diagram of the signal conditioning circuit of which only certain circuit elements are indicated in Figure 2. The sensor element shown in Figure 1 and indicated generally by the reference numeral 11 is a square piece of single-crystal silicon wafer formed in known manner with a reduced-thickness diaphragm portion 12 and with an ion-implanted monolithic silicon piezoresistor 13 which constitutes a strain gauge, indicated generally as 14, having input terminals 15 and 16 connected to opposite ends of the resistor 13 and output terminals 17 and 18 connected to opposite sides of the midpoint of the resistor 13. In known manner, when a voltage is applied between the terminals IS and 16, an output voltage appears between the terminals 17 and 18, this output voltage being (apart from any offset and temperature dependence) proportional to difference in the pressures acting on opposite faces of the diaphragm 12. The sensor element 11 is mounted in a housing 19, typically of steel or plastic, of a pressure transducer indicated generally in Figure 2 by the reference numeral 20 and having a tubular port 21, for connection to a fluid whose pressure is to be measured, and eight electrical terminal pins 22 by means of which the transducer is mechanically mounted on a substrate 23 which also carries a thick-film hybrid circuit which constitutes a signal conditioning circuit for the transducer. The conditioning circuit may be as disclosed in our co-pending international patent application published as No. W0 91/07713, with a circuit as indicated generally by the reference numeral 24 in Figure 3, which also shows how the circuit 24 is electrically connected, via the transducer terminal pins 22, to the sensor element 1 of the transducer. Four of the pins 22 are connected within the transducer 20 to the strain-gauge terminals 15, 16, 17 and 18. Two further pins 22 are connected to terminals 25 and 26, formed on the silicon element 11, between which, as shown in Figure 1, is connected a temperature-sensing diode 27 formed directly in the silicon of the element 11 and connected as an amplifier feedback loop in the circuit 24 as shown in Figure 3. The silicon element 11 is also, for the purposes of the invention as will be described, formed with a resistor 28 and with terminals 29 and 30 therefor. The terminals 29 and 30 are connected, via the two remaining terminal pins 22, to two external terminals HI and H2 which are secured on the substrate 23. Two further such external terminals, Po+ and Po-, are connected respectively to the output terminals 17 and 18 of the strain gauge 14; and finally, external terminals Gnd and Vss provide the circuit 24 with a ground connection (to which the terminal 15 of the strain gauge 14 is also connected) and a connection for applying a stabilised supply voltage. It will not be necessary here to describe the operation of the circuit 24 in detail, but a brief outline will be appropriate. At a given temperature, and with a fixed voltage applied to its terminal 16, the transducer 14 provides at the terminals Po+ and Po- an output signal in the form of a voltage difference which to a satisfactory degree varies linearly with pressure applied through the tube 21 to one face of the diaphragm 12 of the transducer 20. A desired pressure sensitivity at some chosen reference temperature may be obtained by suitable adjustment of the value of a resistor referenced Rl in Figure 3. However, the pressure sensitivity of the strain gauge 14 is temperature-dependent, and, in order to maintain the relationship between the transducer output signal and applied pressure independent of the temperature of the strain gauge, it is required to vary the voltage applied to the transducer terminal 16 so that increased sensitivity is compensated by a reduction in the voltage applied to the terminal 16. The temperature of the silicon element 11 and thus of the strain gauge 14 is sensed by the diode 27, and the required rate of temperature dependence of the voltage applied to the terminal 16 is adjusted by appropriately adjusting the magnitude of a resistor referenced as R2 in Figure 3. No attempt is made to show the whole of the circuit 24 laid out on the substrate 23 in Figure 2, but the resistors Rl and R2 are shown, in locations when they are readily accessible for abrasive or laser trimming. As explained in the above-mentioned international patent application No. WO 91/07713, the signal conditioning circuit shown in Figure 3 is adjusted initially by equalising the two input signals to the amplifier for which the diode 27 constitutes a feedback loop to the inverting input terminal. This is done, at a particular temperature, by adjusting one or other of the two resistors to whose node the non-inverting input of that amplifier is connected, and the result is that the output signal from that amplifier is equal to each input signal. That output signal is shown as being applied to the inverting input of a second amplifier having the resistor R2 as a feedback loop, and at the same temperature the input signal to the non-inverting input of that second amplifier, from the node of a further pair of resistors, is also made equal by adjusting one or other of those resistors. That input signal is also applied to the non-inverting input of a third amplifier, to whose inverting input the output of the second amplifier is applied, and the result is to make the output from this third amplifier equal, at the given temperature, to the signal applied to the non-inverting input of the first amplifier, this being true independently of the value of resistor R2. The output of the third amplifier is applied to the inverting input of a fourth amplifier whose output is connected to provide the supply voltage to the terminal 16 of the transducer. In carrying out the method of the invention in connection with the illustrated transducer and signal conditioning circuit, therefore, the circuit is plugged into a test jig ( not shown) under a laser or abrasive trimmer (not shown ) and powered up via the external connections Vss and Gnd. With the transducer at, conveniently, ambient temperature, the trimmer is used to trim resistors connected to the non-inverting inputs of the first, second and third amplifiers to obtain, as described above, an input signal for the inverting input of the fourth amplifier which, at that temperature, is independent of the value of the resistor R2. At the same temperature, an output signal from the terminals Po+ and Po- is obtained for each of two pressures applied alternately via the tube 21, and the resistor Rl is trimmed until the difference between the two signals is appropriate to the ( known ) difference between the two pressures. That is, by adjustment of Rl the desired pressure sensitivity is achieved, at that temperature. A heating current is then passed, via the terminals HI and HI, through the heater element 28 to heat the silicon element 11 and thus the resistor 13 and diode 27 to a substantially higher temperature which, however need not be accurately known. After a period (which may be as short as 20 seconds) for the temperature to stabilise, output signals for the same two pressures are again obtained from the terminals Po+ and Po-, and the difference between them will be found to be different (usually less than before) because the pressure sensitivity has reduced with increased temperature. The laser trimmer is then used to trim the resistor R2, which has the effect of increasing the voltage applied to the terminal 16 and (since the transducer is ratiometric) to increase the sensitivity of the transducer. Trimming of R2 is continued until the difference between the output signals for the two pressures is again the same as before, indicating that the pressure sensitivity has been restored to its original value at low temperature. Trimming of R2 is then discontinued, the supply of current to the heater resistor 28 is interrupted, and on cooling again to the original temperature (or to any intermediate or not too distant temperature ) it is found that the pressure sensitivity now remains substantially as its original value, and constant over a wide range of temperatures. If trimming of the resistor R2 would provide adjustment in the wrong direction, the corresponding feedback resistor in the next amplifier stage is trimmed instead. Comparably, if trimming resistor Rl provides adjustment in the wrong direction, the resistor which is connected between it and the terminal Vss would be trimmed instead. The method of the invention may be applied also in a case where the signal conditioning circuit includes amplifier means for amplifying the transducer output signals appearing at the terminals 17 and 18 before providing them as amplified output signals at the terminal Po+ and Po-. Particularly in such a case, correction for any offset and temperature dependence of such offset may be applied, as well as amplification, to the output signals. Suitable circuits for combining offset correction with amplification are disclosed, for example, in our co-pending international patent application No. WO 91/07815. In cases where temperature-dependence of the transducer cannot be regarded as linear, it may be desirable to make calibrating adjustments at more than one elevated temperature. This can be effected of course, using difference currents applied to the heater element 28 in order to achieve different elevated temperatures. In the above-described example, an external signal conditioning circuit is used. In some cases, some or all of the signal conditioning circuit will be inside the transducer housing, possibly on the sensor element itself. If the signal conditioning circuit is on the sensor element, then laser trimming will be used to change the value of resistors on the chip and access to the chip must be provided during calibration. In this situation connection to the heater circuit may be by means of probes temporarily placed in contact with the sensor element, rather than via external terminals of the transducer. Construction of the heater element and temperature-sensitive element will usually use thin film or ion implantation techniques and the circuit itself can be resistive or it may contain semiconductor devices such as diodes. The transducer shown in the drawings is of the silicon- diaphragm type, but the method according to the invention may be applied in connection with other materials and other sensor-element types. The general requirement is for the sensor element itself to be small and to be a good heat conductor so that its temperature remains uniform, and for the sensor element mounting to be a relatively poor heat conductor so that undue power consumption in the heater circuit can be avoided. Application of a constant voltage across the heater circuit may result in a relatively slow stabilization of sensor element temperature because the temperature will rise asymptotically towards its final value. A much quicker way of stabilizing the sensor element temperature would be to profile the current with time. For example, a current of 10 mA may produce an eventual temperature rise of 40°C after several minutes. A quicker way of reaching this temperature is to apply 20 mA until the temperature reaches 40°C (which might take only a few seconds) and then to reduce the current to, say, 10 mA in order to maintain the temperature steady.

Claims: CLAIMS 1. A method of adjusting the temperature compensation of a temperature-sensiti e transducer having a sensor element and a temperature-sensitive element which senses the temperature of the sensor element, and, combined therewith, a signal conditioning circuit which includes the temperature-sensitive element and is capable of effecting such temperature compensation, the method including the steps of making required adjustments to the signal conditioning circuit at one temperature and at another different, temperature of the sensor element, characterised in that for making adjustments at the higher of the said one and the said different temperatures the sensor element is heated to that higher temperature by supplying a heating current to a heater element within the transducer and in good thermal connection with the sensor element. 2. A method of providing temperature compensation of a transducer having a sensor element which has a temperature- dependent sensitivity and/or offset and provided with a temperature-sensitive element which senses the temperature of the sensor element, and having combined with the transducer a signal conditioning circuit which includes the temperature-sensitive element and is capable of compensating for temperature-dependent changes in the sensitivity and/or offset of the sensor element, the method including the steps of adjusting the signal conditioning circuit to establish a desired sensitivity and/or offset of the sensor element at one temperature thereof and, at a different temperature thereof, adjusting the signal conditioning circuit to adjust the sensitivity and/or offset at the said different temperature to equal that at the one temperature, characterised in that for making adjustments at the higher of the said one and the said different temperatures the sensor element is heated to that higher temperature by supplying a heating current to a heater element within the transducer and in good thermal connection with the sensing element. 3. A method as claimed in claim 1 or claim 2 characterised in that the sensor element and the temperature-sensitive element which senses its temperature are provided on a semiconductor diaphragm of the transducer and the heater element is also provided on the diaphragm. 4. A method of adjusting the temperature compensation of a temperature-sensitive transducer, substantially as described herein with reference to the accompanying drawings.

Inventors: BINNS JOHN PHILIP LINCOLN, BINNS, JOHN, PHILIP, LINCOLN
Applicants: BRITISH TECH GROUP, BRITISH TECHNOLOGY GROUP LTD.
Application year: 1991
Main classification: G01K15
IPC codes: G01D3, G01L1, G01L9, G01L19, G01P1, G01P15