GB2066956A - Temperature sensitive apparatus - Google Patents

Abstract

A surface acoustic wave oscillator 2, 3 and 4 is exposed to infrared radiation which is focussed on it by a system which scans a field of view. The frequency of the oscillator changes with temperature so that, as it is scanned, the output contains all the necessary information to build up a thermal picture of the scene being viewed. The output 6 from the oscillator is mixed at 7 with the output 6A from an identical oscillator not exposed to the thermal radiation. The output of the mixer passes through a filter 10 which passes only those components of the signal representing the difference between signals 6 and 6A. The output from the filter 10 is amplified at 11 and then used to drive a video display. <IMAGE>

Description

SPECIFICATION Temperature sensitive apparatus This invention relates to temperature sensitive apparatus either for indicating the temperature of the subject or the temperature distribution over a scene under observation, or for performing a control function dependent on the temperature.

The invention provides temperature sensitive apparatus comprising means for exposing a surface acoustic wave device to heat from a subject whose temperature is to be sensed, the surface acoustic wave device being temperature sensitive and therefore producing, when in operation, an output which is indicative of the temperature of the subject, the output of the surface acoustic wave device being connected to means for indicating the temperature of the said subject, or for performing a control function dependent on temperature.

There are at least two ways in which it is envisaged that the invention may be implemented. In the first, the apparatus includes a surface acoustic wave oscillator which is scanned by a thermal image of a subject or scene under observation. The frequency of the oscillator varies with temperature so that its output can be used to perform a control function or to indicate the thermal distribution of the subject or scene, e.g. by producing a visible image on a display screen.

The other possibility envisaged is for the apparatus to include a transducer for transmitting surface acoustic waves repetitively along a path of propagation. A plurality of receiving electrodes are spaced along this path so as to receive each surface acoustic wave successively. A thermal image of at ieast part of the subject or scene is focussed on the aforementioned path. Therefore, as the surface acoustic wave travels along the path it produces an output signal whose frequency varies according to variations in the intensity of the thermal image on the path. The output signal can be used to perform a control function or to drive a display system producing a visible image illustrating the thermal distribution of the subject or scene.

Further features of the invention will appear from the following description of illustrative embodiments of the invention given with reference to the accompanying schematic drawings in which: Figure 1 is a sketch of a first embodiment of the invention for use in a thermal imager; Figure 2 is a sketch of a second embodiment of the invention, also for use in a thermal imager; Figure 3 is a vertical cross-section through the line Ill-Ill of Fig. 2; Figure 4 illustrates an alternative form of transducer for use in the embodiment of Fig. 1 or Fig. 2; and Figure 5 illustrates another alternative form of transducer.

Referring firstly to Fig. 1 the apparatus includes a piezoelectric member 1 which can, for example, be formed of quartz. Formed on the member 1 are two transducers 2 and 3 each formed by a plurality of interdigitated electrodes as illustrated. In operation the transducer 2 transmits surface acoustic waves through the member 1 to the transducer 3.

The latter receives signals from the transducer 2 after a delay which is equal to the time taken for the surface acoustic waves to travel from the centre point of transducer 2 to the centre point of transducer 3. This delay is not constant. It varies with temperature since the piezoelectric material expends and contracts.

The output from the transducer 3 is fed through an amplifier 4 back to the transducer 2 thereby forming an oscillator whose frequency depends on temperature. The piezoelectric member 1 and the electrodes on it are encapsulated within a casing (not shown in Fig. 1). The casing is provided with an infrared transparent window which is indicated in broken lines at 5 on Fig. 1. An optical focussing and scanning system, as is well known in thermal imagers, is used to focus different parts of a field of view sequentially through the window 5 onto the piezoelectric member 1. The signal at 6 therefore varies in frequency as the field of view is scanned and contains all the necessary information to enable a visible image of the thermal distribution within the field to be produced.

The signal at 6 is mixed, in a mixer 7, with a similar signal 6A produced by an oscillator which is similar to the oscillator previously described, the component parts being indicated by the same reference numerals with the addition of the suffix A. This last mentioned oscillator is not exposed to thermal radiation as is the first oscillator. It is shielded by the encapsulation of the member 1. The output 6A therefore does not vary with the scanning action of the field of view.

Long term variations in the respective frequencies of the two oscillators will occur, e.g.

as the part of the piezoelectric member 1 under the window 5 warms up during operation. Such long term variations are represented by a low frequency component of the output of the mixer 7. This low frequency component is passed by a filter 8 which controls a phase shifter 9 in such a way as to adjust the frequency of the second oscillator to keep it the same as the mean frequency over a set time period of the first oscillator.

The output of the mixer 7 will contain components equal to the sum of the signals at 6 and 6A and the difference of these signals.

Only the difference component is required and so a filter 10 is included to pass this frequency alone. The output from bandpass filter 10 is amplified at 11 and is then passed to a video display system.

Referring now to Fig. 2, this shows an alternative way in which the invention can be embodied. In this embodiment a whole linear section (instead of a point as in Fig. 1) of a scene is focussed onto a transducer 1 2 containing a very large number of interdigitated electrodes, only a few being shown in the drawing. The image on the transducer 1 2 spacially modulates the spacing between each pair of electrodes so that the time taken for a signal to pass between two electrodes at a particular position depends on the temperature at that position. Impulses are generated by a circuit 1 3 and are fed to a second transducer 14 which consists of only two electrodes.A surface acoustic wave transmitted by the transducer 14 passes sequentially past each pair of electrodes of the transducer 1 2 thereby producing at the line 1 5 a series of signals whose frequency varies in accordance with the variation of temperature along the transducer 1 2 and therefore in accordance with the variation in temperature of the field of view along the particular line being inspected at the time. The focussing system is designed to inspect different lines at different times so as to cover the whole field to be viewed.

The signal at 1 5 is amplified by an amplifier 1 6 and the component of the signal derived from the temperature variations is extracted by a mixer 1 7. The mixer 1 7 is supplied with a local oscillator signal by a voltage controlled oscillator 1 8 whose frequency is controlled by a control signal from a low pass filter 1 9. In this way the local oscillator frequency is kept the same as the current average frequency (over a given time period) of the signals from the transducer 1 2.

The output 20 from the mixer 1 7 is passed through a bandpass filter 21 and amplifier 22 which provides the same function as the filter 10 and amplifier 11 of Fig. 1.

Each signal from the transducer 1 4 also passes to a pair of electrodes 23 which provide a trigger signal to impulse generator 1 3 resulting in the latter feeding an impulse to the transducer 14. The electrodes 23 are positioned so that a new surface acoustic wave reaches the transducer 1 2 immediately after the previous surface acoustic wave has finished traversing the transducer 1 2. Thus a continuous output is provided at 1 5.

It is preferable that the transducer 2 and 3 of Fig. 1 or the transducer 1 2 of Fig. 2 have a low heat capacity in order to enable them to respond to fast temperature changes as the field of view is scanned. Fig. 3 shows a suitable arrangement comprising a base plate 24 carrying a piezoelectric member 25. The latter has an area 26 of reduced thickness and it is this area which is exposed to the thermal radiation, the latter entering through a transparent window 28 formed in a cap 29 fixed to the base 24. In Fig. 3 the interdigitated electrodes of transducers 12 and 23 are shown respectively at 1 2A and 23A. These are linked to connector pins 30 by means of spring clips 31 attached thereto. The pins 30 are mounted by insulating material 32 in bores in the base 24. The earthed electrodes (not shown in Fig. 3) are connected by a similar arrangement to a thid pin (also not shown).

As an alternative to the piezoelectric member 25 the arrangement shown in Fig. 4 can be used. This comprises an insulating base 33 on which is deposited the electrodes as shown at 34. On top of the electrodes 34 piezoelectric material 35 is deposited by some suitable method. In this way it is possible to use a very thin piezoelectric film having a particularly low heat capacity and therefore resulting in a particularly sensitive heat sensor.

Fig. 5 shows another alternative arrangement comprising two supports 36 across which a thin sheet of piezoelectric material 37 is stretched or otherwise mounted. The electrodes 38 are deposited on this piezoelectric member 37.

Claims (12)

1. Temperature sensitive apparatus comprising means for exposing a surface acoustic wave device to heat from a subject whose temperature is to be sensed, the surface acoustic wave device being temperature sensitive and therefore producing, when in operation, an output which is indicative of the temperature of the subject, the output of the surface acoustic wave device being connected to means for indicating the temperature of the said subject, or for performing a control function dependent on temperature.

2. Apparatus according to Claim 1 in which the means for exposing is directional so that the sensor senses the temperature of a subject in a particular direction or range of directions relative thereto.

3. Apparatus according to Claim 2 in which the means for exposing includes means for focussing heat in the form of electrnmag- netic radiation onto the surface acoustic wave device.

4. Apparatus according to Claim 2 in which the means for exposing includes apparatus for fixing the surface acoustic wave device in heat conducting relationship to the subject whose temperature is to be sensed.

5. Apparatus according to Claim 2 or 3 including means whereby the sensor scans an area under observation.

6. Apparatus according to any preceding Claim in which the surface acoustic wave device is an oscillator whose frequency is temperature dependent.

7. Apparatus according to Claim 3 in which the surface acoustic wave device in cludes a transducer for transmitting a surface acoustic wave along a path of propagation; and a plurality of receiving electrodes spaced along a section of the path so as to receive successively the said surface acoustic wave; the said means for focussing being arranged to focus an image of a subject onto the said section of the path thereby producing an output in the form of a succession of signals whose frequency varies according to variations in the intensity of the image along the said path section.

8. Apparatus according to Claim 7 including means for indicating the temperature of the said subject the said means being adapted to indicate different temperatures of different parts of the subject.

9. Apparatus according to Claim 8 including means for successively exposing the said path to different parts of the image.

1 0. Apparatus according to Claim 8 in which the means for indicating the temperature of the subject is a two dimensional visual display adapted to show different temperatures of different parts of the subject.

11. Apparatus according to Claim 1 and substantially as described with reference to Fig. 1.

12. Apparatus according to Claim 1 and substantially as described with reference to Fig. 2.