GB2267967A

GB2267967A - Apparatus for temperature measurement

Info

Publication number: GB2267967A
Application number: GB9212846A
Authority: GB
Inventors: Roger Mulhern Gooding
Original assignee: STATUS INSTR Ltd
Current assignee: STATUS INSTR Ltd
Priority date: 1992-06-17
Filing date: 1992-06-17
Publication date: 1993-12-22
Anticipated expiration: 2012-06-17
Also published as: GB2267967B; GB9212846D0

Abstract

In a method of, and apparatus for, temperature measurement, there is provided a reference resistance 12 in the form of a precision, high stability resister (such as a wire wound resister) connected in series with a resistive temperature sensor 10. The resistors are connected to a current source which passes a current through them, while an integrator 16 monitors the voltage across each resister, by being ramped up by the voltage across one resister and ramped down by the voltage across the other resister at a rate related to the magnitudes of those voltages. This process is repeated, and the ramping up and down durations are stored and accumulated for subsequent analysis by data processing means 20, which compares the accumulated ramping up durations and down durations in order to determine the ratio of resistance value of the sensor to that of the reference resister. The process may be repeated with a reverse polarity current to obtain a further ratio, which may be combined with the first obtained ratio to compensate for errors arising within the apparatus. <IMAGE>

Description

Title Apparatus for Temperature Measurement Field of the invention This invention relates to apparatus for temperature measurement, especially in industrial applications where a high level of accuracy and/or stability is required.

Background to the invention There are many known techniques for temperature measurement using electronic circuitry to amplify, linearise and compensate the signals from a variety of temperature sensing devices such as thermocouples, thermistors and semiconductor sensors. However, in order to achieve an acceptable level of performance for industrial applications, all the known forms of apparatus for temperature measurement require to be calibrated at regular intervals and, where necessary, corrective adjustments made in order to maintain the required level of accuracy.

There have been many different proposals made to overcome or minimise the need for regular adjustment, mostly relying on the use of high quality components together with a sensor with good long term stablity. In most cases, however, there may remain some changes in performance with time and/or ambient temperature conditions, due to the need in most cases to compensate for both "zero" drift and "span" change.

Currently, most known forms of apparatus for high accuracy temperature measurement are based on the platinum resistance sensor. As the temperature-resistance characteristic of platinum resistance sensors is known and is highly stable, it is only necessary accurately to determine the resistance of the sensor in order to determine its temperature.

Commonly, the sensor is incorporated into a N5heatstone Bridge arrangement, the bridge output voltage being proportional to the temperature. Alternatively the resistance can be calculated from the voltage generated across the sensor when a known current is passed. This requires both a stable current source and a calibrated voltage measuring device, usually incorporating a voltage reference device and some form of "auto-zero" compensation for an analog to digital converter. Low cost microprocessors do not normally include facilities for handling analog signals with sufficient accuracy or resolution.

The invention According to the invention, there is provided temperature measurement apparatus comprising a reference resistance in the form of a precision, high stability resistor connected in series with a resistive temperature sensor, a current source for producing a current which is passed through the series-connected resistors, an integrator which is ramped up by the voltage across one resistor and ramped down by the voltage across the other resistor in one cycle of operation, means for accumulating and storing the rampingup durations and the ramping-down durations during a large plurality of cycles, and means for comparing the accumulated ramping-up durations with the accumulated ramping-down durations in order to determine the ratio of the resistance value of the sensor to the known resistance value of the high stability resistor.

The current may be such that a series of small ramps is obtained, enabling high resolution to be achieved with low power supply voltages used by portable equipment.

The invention thus removes the need for periodic calibration of the temperature measurement apparatus and also simplifies the measurement circuitry whilst at the same time enabling utilisation of a low cost four bit microprocessor. The above-defined measurement technique allows temperature to be measured using a platinum resistance sensor and relies on one single high precision component for compensation of errors in the measuring circuitry. This component is the precision, high stability, preferably wire wound resistor, chosen for its initial accuracy and long term stability.

The invention as thus far defined represents the ideal case, and in practice the ratio obtained may be imperfect due to errors in the circuitry.

However, according to a further feature of the invention, by reversing the polarity of the input signals, a further ratio can be measured which is the inverse of the first, again with error contributions. The two ratios are now combined mathematically, eliminating all significant error terms and leaving a pure ratio with respect to the known value resistor, which may therefore be used to calculate the measured temperature precisely.

In such a case, the accuracy of the temperature measurement will be, in effect, determined by the accuracy of the sensor itself and that of the high stability resistor.

Preferably, the current source includes a signal selector for producing cyclically generated positive and negative currents.

Due to the nature of the error compensation, standard commercial grade signal switching devices may be used to direct the input signals to the integrator. This assists in keeping the costs of manufacture as low as possible.

Description of embodiment Temperature measurement apparatus in accordance with the invention is illustrated by way of example in the accompanying drawings, in which: Figure 1 is a block diagram of the apparatus; and Figures 2 to 4 are circuitry diagrams appertaining to the blocks of Figure 1.

Referring to Figure 1, sensor 10 is a platinum resistance sensor complying with BS1904/DIN43760. Connected in series with this sensor 10 is a reference resistance 12 in the form of a precision, high stability resistor, typically a 0.01% tolerance Econister (Trade Mark) manufactured by General Resistance.

Signal selector 14 samples, on a multiple cycle basis, a first series of voltages resulting from the current through the series connected resistive devices 10 and 12.

After a given number of cycles, the polarity of the current is reversed, and a second series of cycles of reverse polarity is obtained.

Integrator 16 is ramped up and down repeatedly by the voltages generated at resistive devices 10 and 12 during the first series of pulses. Additionally, the accumulated ramping-up durations and accumulated rampingdown durations are stored as D1 and D2 by a microprocessor 20, for example NEC uPD75P316AGF-3B9, which is a relatively low cost four-bit microprocessor. Similarly, during the second series of pulses, the accumulated ramping-up durations and the accumulated ramping-down durations are stored as D 1 and D 2.

Connected between the integrator 16 and the microprocessor 20 is a comparator 18 which is operable to determine whether the signal from the integrator has reached a predetermined upper or lower threshold level, depending on whether the integrator 16 is being ramped up or ramped down. When such a threshold is reached, the comparator 18 sends an appropriate trigger signal to the microprocessor 20 which causes the signal selector 14 to cause the integrator 16 to begin the next phase in the ramp up/ramp down sequence.

1 1 Microprocessor 20 derives the ratios D1/D2 and D 1/D 2.

The latter ratio is the inverse of the first ratio, and includes the same error contributions which may be present due to errors in the various components of the measurement circuitry.

Microprocessor 20 therefore also combines the two ratios 1 1 D1/D2 and D 1/D 2 mathematically in order completely to eliminate the aforesaid errors, leaving a pure ratio representing the exact value of the resistance of the sensor 10 with respect to the known value of the resistance of the reference resistor 12.

The microprocessor 20, processes the final ratio in order to control an LCD display device 22 on which the accurately measured temperature is displayed. Display device 22 may be of type TR320 manufactured by Varitronix, which is a three 1/2 digit LCD.

Further details of the circuitry involved will be clear to a person skilled in the art from Figures 2 to 4.

Figure 2 shows the battery supply circuitry for the sensor 10 and the reference resistor 12.

Figure 3 shows on the left-hand side the switching control circuitry 14 for generating the multiple cycles and in the centre the integrator 16 circuitry which derives signal values for the ramp durations. The right-hand side of Figure 3 shows the comparator 18. The circuitry of Figure 3 is preferably implemented as a single pcb.

Figure 4 shows the microprocessor 20 and the display device 22.

Various modifications of the above-described and illustrated embodiment are possible within the scope of the invention hereinbefore defined.

Claims

1. Temperature measurement apparatus comprising a reference resistance in the form of a precision, high stability resistor connected in series with a resistive temperature sensor, a current source for producing a current which is passed through the. series-connected resistors, an integrator which is ramped up by the voltage across one resistor and ramped down by the voltage across the other resistor in one cycle of operation, means for accumulating and storing the ramping-up durations and the ramping-down durations during a large plurality of cycles, and data processing means for comparing the accumulated ramping-up durations with the accumulated ramping-down durations in order to determine the ratio of the resistance value of the sensor to the known resistance value of the high stability resistor.

2. Apparatus according to claim 1 in which the data processing means is a four bit microprocessor.

3. Apparatus according to claim 1 or claim 2 in which the resistive temperature sensor comprises a platinum resistance sensor.

4. Apparatus according to any of the preceding claims in which the precision, high stability resistor comprises a wire wound resistor.

5. Apparatus according to any of the preceding claims, further comprising means for reversing the polarity of the current, and in which the data processing means is operable to determine a further ratio which is the general inverse of the first, so that the two ratios can be combined mathematically so as to eliminate error terms in the circuitry of the apparatus.

6. Apparatus according to claim 5 in which the current source includes a signal selector for producing cyclically generated positive and negative currents.

7. Apparatus substantially as defined herein with reference to, and as illustrated in, the accompanying drawings.

8. A method of temperature measurement comprising the steps of:a) passing an electric current through a high stability resistor connected in series with a resistive temperature sensor; b) monitoring the voltage across each resistor in turn by means of an integrator which is ramped up by the voltage across one resistor and ramped down by the voltage across the other resistor, the rate of ramping up and ramping down being related to the magnitudes of the respective voltages; c) storing the times taken for the integrator to ramp up and ramp down between given thresholds; d) repeating steps b) and c) above one or more times to provide a plurality of ramping up and ramping down durations; and e) comparing the accumulated ramping up durations with the accumulated ramping down durations in order to determine the ratio of the resistance value of the sensor to the known resistance value of the high stability resistor.

9. A method according to claim 8 in which the current, and ramps generated by the integrator, are sufficiently small to enable the method to be performed by a portable apparatus.

10. A method according to claim 8 or claim 9 including the further steps of reversing the polarity of the current and repeating steps (a)-(e) of claim 8 to obtain a further ratio, and then combining the two ratios so as to eliminate errors in the circuitry of the apparatus used to perform the method.

11. A method as substantially described herein with reference to the accompanying drawings.