GB2107060A

GB2107060A - Temperature sensors

Info

Publication number: GB2107060A
Application number: GB08125210A
Authority: GB
Inventors: Martyn Neil Gilbert
Original assignee: Texas Instruments Ltd
Current assignee: Texas Instruments Ltd
Priority date: 1981-08-18
Filing date: 1981-08-18
Publication date: 1983-04-20
Also published as: GB2107060B

Abstract

A temperature sensor 1 is constructed as a capacitive element in which the dielectric material 4 between two electrodes 2 changes its dielectric constant abruptly at a specific temperature. The preferred dielectric material is de-ionised water whose dielectric constant changes abruptly on freezing, i.e. at 0 DEG C, and the preferred electrode material is aluminium with aluminium oxide layers 3 formed between the water and the metal, so as to prevent reaction of the water with the metal. The sensor can form a frequency determining capacitor in an oscillator circuit whose output is counted. Alternatively, the capacitance can be monitored by observing the rate of charging/discharging of the capacitor when coupled to charging/discharging current circuits. <IMAGE>

Description

SPECIFICATION Improvements in and relating to temperature sensors The invention relates to temperature sensors.

Known temperature sensors either have a temperature-dependent parameter which is monitored to provide an indication of temperature, or are constructed as switch contacts which close or open at a set temperature under the control of a bimetallic strip or the like.

A drawback of known temperature sensors is the need for initial calibration and for periodic readjustment.

It is an object of the present invention to provide a temperature sensor which, without calibration, is capable of indicating a specific temperature.

According to the invention a temperature sensor is constructed as a capacitive element in which the dielectric material changes its dielectric constant abruptly at a specific temperature.

Preferably, an insulating layer separates the capacitor electrodes fro the dielectric material.

This arrangement resists contamination of the dielectric material.

Advantageously, the capacitor electrodes are aluminium and the insulating layer is aluminium oxide.

Preferably, the dielectric material is de-ionised water.

Advantageously a temperature sensing circuit includes a temperature sensor according to the invention.

Advantageously, the temperature sensing circuit includes an oscillator the operating frequency of which is dependent on the capacitance of the temperature sensor, and means for detecting the oscillator frequency.

Advantageously, the oscillator is an astable multivibrator.

Preferably, the astable multivibrator includes a Schmitt trigger.

Advantageously, the means for detecting the oscillator frequency includes a counter which, in operation, reaches its terminal count only when the temperature sensor has its low value of capacitance.

Alternatively, the temperature sensing circuit includes means for altering the charge on the temperature sensor at a constant rate, an oscillator which, in operation, effects alternate charging and discharging of the temperature sensor by the means for altering the charge on the temperature sensor, and a threshold detector arranged to sense the voltage across the temperature sensor and to charge output state only when the voltage change is consistent with the low value of sensor capacitance.

Preferably, the oscillator includes current sources arranged in balanced pairs. This arrangement compensates for semiconductor temperature drift within the oscillator.

A temperature sensor according to the present invention, and circuits employing the temperature sensor, will now be described by way of example only and with reference to the accompanying drawings, in which: Fig. 1 shows a part cross-section through one form of a capacitive temperature sensor according to the invention, Fig. 2 shows the variation in effective permittivity of the dielectric material of capacitive temperature sensors, according to the invention, without and with an insulating layer on the electrodes.

Fig. 3 shows a first circuit employing a capacitive temperature sensor to detect a set temperature, and Fig. 4 shows a second circuit employing a capacitive temperature sensor to detect a set temperature.

Referring to Fig. 1, a capacitive element temperature sensor 1 includes a pair of parallel plate electrodes 2 provided with an insulating layer 3 and enclosing a volume of di-ionised water 4.

The plate electrodes 2 are large in comparison with their separation so as to minimise the stresses caused by the change in volume when the water freezes. The separation of the plate electrodes 2 is, in practice, made as small as possible in order to minimise thermal inertia, and this, of course, provides a small wafer-thin device.

The insulating layer 3 acts to reduce the capacitance of the element and, for this reason, it is desirable that it should be of minimum thickness. The thin insulating layer 3 may be provided by the use of aluminium electrodes 2 and the growth of aluminium oxide on the electrodes 2. The layer 3 of aluminium oxide should be thick enough to ensure that the electrodes 2 are covered fully. A method of producing the aluminium oxide layer is anodisation of the aluminium electrodes 2.

The fuction of the insulating layer 3 is the prevention of reaction between the electrodes 2 and the water 4. The insulating layer 3 must therefore provide both chemical and electrical isolation of the electrode 2 from the water 4. In addition, the insulating layer 3 should provide good thermal conduction so as not to degrade excessively the thermal inertia of the device.

The theory of operation of the temperature sensor is that its capacitance is directly proportional to the permittivity of its dielectric material and that de-ionised water undergoes an abrupt change in permittivity as it freezes, so that the capacitance of the device changes abruptly at about OOC.

It is possible to achieve a 10:1 change in capacitance of a device having the following dimensions: (i) Size of electrodes -- 7.4 cm2 (ii) Separation of electrodes -- 4 mm (iii) Thickness of insulating layer -- 0.025 mm Referring now to Fig. 2, it may be noted that in a temperature sensor constructed as a capacitive element the dielectric material shows an effective change in relative permittivity of from about 3.7 to about 0.2 between 20C and -1 OC when no insulating layer is provided on the electrodes, while the effective change in relative permittivity is from about 1.5 to about 0.1 between 20C and -1 0C with insulated electrodes of 1 mm thickness.It is to be understood that any change ratio in excess of 10 will provide adequate indication of the freezing of the water dielectric, and that both the change ratio and the effective permittivity values may be controlled by the choice of insulating layer thickness.

Reference is now made to Fig. 3 as a circuit suitable for use as a freezing condition indicator.

A capacitive temperature sensor 1, according to the invention, is provided as a frequencycontrolling component in an oscillator. A Schmitt trigger 5 and a resistor 6 constitute the remainder of the oscillator which drives a counter 8 by way of an AND gate 7. The counter 8 is reset at regular intervals by a timebase pulse. The system is so arranged that, above freezing, the counter 8 is reset by the timebase pulse before the full count is reached, and below freezing, when the oscillator frequency rises abruptly because of the fall in the capacitance of the sensor 1, the counter becomes full and changes its output level.

Fig. 4 shows alternative form of circuit suitable for use as a freezing condition indicator. In this alternative circuit, an oscillator consisting of a capacitor 9, current sources 10, 11, 12, resistors 13, 14, 15, comparators 16, 17, and a flip-flop 18, controls the charging and discharging of capacitive temperature sensor 1 by way of current sources 19, 20 and switches 21, 22. At temperatures below freezing, the voltage across the capacitive temperature sensor 1 does not reach a high enough value to trigger a comparator 24 by way of an amplifier 23, but on reduction of the capacitance of the capacitive element 1 at freezing, the rate of rise of the potential of the capacitive element 1 is increased and the comparators 24 are caused to switch, thereby providing an output pulse train indicating freezing conditions. The output pulse train may be used to actuate a warning device or may be used as an audible alarm.

The provision of the current sources in pairs (19 and 20), (10 and 11), ensures a measure of opposition to temperature induced stability drift.

Claims

1. A temperature sensor constructed as a capacitive element in which the dielectric material changes its dielectric constant abruptly at a specific temperature.

2. A temperature sensor as claimed in claim 1, wherein an insulating layer separates the capacitor electrodes from the dielectric material.

3. A temperature sensor as claimed in claim 2, wherein the capacitor electrodes are aluminium and the insulating layer is aluminium oxide.

4. A temperature sensor as claimed in claim 3, wherein the dielectric material is de-ionised water.

5. A temperature sensor substantially as herein described with reference to and as iilustrated by Fig. 1 of the accompanying drawings.

6. A temperature sensing circuit including a temperature sensor as claimed in any one of claims 1 to 5.

7. A temperature sensing circuit as claimed in claim 6, including an oscillator the operating frequency of which is dependent on the capacitance of the temperature sensor, and means for detecting the oscillator frequency.

8. A temperature sensing circuit as claimed in claim 7, wherein the oscillator is an astable multivibrator.

9. A temperature sensing circuit as claimed in claim 8, wherein the astable multivibrator includes a Schmitt trigger.

1 0. A temperature sensing circuit as claimed in any one of claims 7 to 9, wherein the means for detecting the oscillator frequency includes a counter which, in operation, reaches its terminal count only when the temperature sensor has its low value of capacitance.

11. A temperature sensing circuit as claimed in claim 6, including means for altering the charge on the temperature sensor at a constant rate, an oscillator which, in operation, effects alternate charging and discharging of the temperature sensor by the means for altering the charge on the temperature sensor, and a threshold detector arranged to sense the voltage across the temperature sensor and to charge output state only when the voltage change is consistent with the low value of sensor capacitance.

12. A temperature sensing circuit as claimed in claim 11 , wherein the oscillator includes current sources arranged in balanced pairs.

1 3. A temperature sensing circuit substantially as herein described with reference to and as illustrated by Fig. 3, or Fig. 4, of the accompanying drawings.