GB2298486A - SAW displacement transducer having a moveable electrode - Google Patents

Abstract

The present invention describes a device for measuring relative linear movement. It can therefore be employed as a strain gauge, extensometer or displacement transducer. It is based on a surface acoustic wave (SAW) delay line with a moveable electrode structure, whose movement with respect to the main part of the device results in a phase change in the output signal. The moveable electrode may be located either at one end of the substrate, or in the centre of the substrate where it communicates with two fixed electrodes at either end (Fig 4, not shown). The moveable electrode may be used as an input, an output, or both (Fig 6, not shown).

Description

DISPLACEMENT TRANSDUCER The present invention ' relates to a transducer responsive to relative linear movement and suitable for use, for example, as a strain gauge, extensometer or displacement transducer.

The maximum operating temperature for conventional resistive and capacitive strain gauges is determined mainly by the reduction in insulation resistance of the insulating materials in both the gauge and cables as temperature increases. As a result conventional transducers cannot be used at all outside a limited temperature range and even within their operational range accuracy may be impaired by temperature fluctuations.

According to the present invention, there is provided a transducer comprising a surface acoustic wave device including a piezo-electric substrate and an electrode structure, in which the electrode structure and substrate are movable with respect to each other.

This present invention employs SAW delay line principles so that small relative linear movements produce phase changes in a high frequency electrical signal.

Lowered resistance of insulation materials simply results in loss of amplitude, but as long as some signal remains there is no loss of accuracy. Strain gauges or displacement transducers based on this invention will be able to operate satisfactorily at temperatures above the practical limits for resistive and capacitive strain gauges. Until recently, the piezo-electric material with the highest operating temperature was quartz, but a new material, Gallium Orthophosphate, has become available which is usable up to 8500C. However, the invention is not restricted to high temperature applications and could be used throughout the operating range of any piezo-electric material.

The SAW device consists of a substrate of piezo-electric material with at least one movable electrode structure located close to the surface. The electrode structure has the interdigitated form commonly used in SAW devices. In one implementation an alternating voltage of appropriate frequency is applied to the movable electrode structure which produces acoustic waves in the surface of the piezo-electric material. These travel to a further electrode structure and induce an alternating voltage of the same frequency as the input but delayed by the time required to travel across the surface. A change in the relative spacing between the electrode structures results in a change in this delay, which can be viewed as a change in phase of the output signal.The phase change can be detected by comparison of the output from a SAW device with a movable electrode with that from an adjacent similar reference device with fixed electrodes. Alternatively the movable electrode structure can be placed at the centre of the piezo-electric substrate with a further electrode structure at each end. In this case movement of the moveable electrode causes phase changes of opposite sign at the further output electrodes. Both of the above methods result in a temperature compensated output.

The further electrode structures may be fixed to the piezo-electric substrate, or may be movable with respect to the substrate.

Alternatively, only a single movable electrode structure is provided. In this case, a short burst of alternating voltage of appropriate frequency is applied to the movable electrode structure, and this generates an acoustic wave in the surface of the piezo-electric material. This wave travels to the end of the substrate and is reflected to the electrode structure where it induces an alternating current of the same frequency but delayed by the time taken to travel across the surface and be reflected back. Accordingly, a change in the position of the electrode structure with respect to the substrate will cause a change of phase of the output signal.

Examples of the present invention will now be described in further detail with reference to the accompanying drawings, in whidh: Figure 1 shows a basic Surface Acoustic Wave (SAW) delay line device; Figure 2 shows the first embodiment of the invention using a SAW device with one movable electrode structure as a displacement transducer; Figure 3 shows a further embodiment of the present invention, in which two movable electrode structures are provided; Figure 4 shows a possible variation of the basic SAW delay line, with a centrally placed input electrode structure and two output electrode structures; Figure 5 shows another embodiment of the invention, consisting of a SAW device as in Figure 4, but with a movable central electrode structure; and Figure 6 shows a further embodiment of the present invention, in which a single electrode structure is provided.

Figure 1 shows a basic Surface Acoustic Wave (SAW) delay line device. The principle of operation is well known. An A.C. electrical signal of suitable frequency is applied to the interdigitated electrode structure labelled "input", which is formed on the surface of a piezo-electric material. The electric field generates a surface acoustic wave which travels along the surface of the piezo-electric material. There is a similar electrode structure at the far end at which the SAW induces an electrical output, which in general has the same frequency as the input, but has been delayed by passage across the surface the device.

SAW delay lines are common components in many electronic circuits, and their operating principles and design constraints are well understood.

Figure 2 shows the first embodiment of the novel idea to use such a device as a displacement transducer. In this case, one of the electrode structures is located close to the surface of the piezo-electric substrate but not on it.

In the Figure it is shown on a separate block, which can be moved with respect to the substrate. This movable electrode structure could be either the input or output, or, as shown in Figure 3, both electrode structures could be movable. The SAW is generated by the electric field produced by the voltage applied between the interdigitated electrodes. There is no requirement for the electrodes to be on the surface of the piezo-electric material. The spacing between the electrodes and the piezo-electric should be as close as possible. There will be a loss in efficiency of SAW generation which will increase as this spacing increases. Such losses are less important if the movable electrode structure is the input where the voltage applied can be increased to compensate and accordingly it is preferred for the input electrode to be movable.If the movable input electrode structure is moved parallel to the surface of the piezo-electric material so that it is closer to or further away from the output, then the result will be a variation in the delay of the signal appearing at the output, which can be viewed as a change in its phase. When used as a strain gauge, a suitable mechanical structure would be used in which the substrate is attached at one point of an object and the movable electrode at another point. Strain occurring in the object would cause relative movement of the movable electrode, resulting in the phase change described above.

The delay time of such a device will also depend on other factors such as temperature. A further embodiment of the invention is a temperature compensated strain gauge or displacement transducer which uses two SAW devices. One "active" device is as shown in Figure 2 or 3, and the other "reference" device has similar dimensions but fixed electrodes. Movement of the movable electrode structure on the active device would result in a difference in phase compared with the reference device. The devices would be located closely adjacent so as to be at the same temperature. They would be equally affected by temperature changes so the phase difference between them would not change with temperature. In a further embodiment both SAW devices would have movable electrodes as shown in Figure 2 or 3.One device would be the active device as described above and the other reference device would be provided with means of adjusting the movable electrode structure for zeroing purposes. In both of these embodiments, the two SAW devices would be fed with the same A.C. voltage.

Movement of the electrode structure on the active device would result in a change in phase between the two outputs, which would be a directly proportional to the distance moved.

Figure 4 shows a variation of the basic SAW delay line. In this case the input is at the centre of the piezo-electric substrate, and applying an A.C. voltage to it causes SAWs to propagate away from it in both directions. The two electrode structures labelled A and B each behave as outputs and produce delayed signals. If the path lengths are the same, the delay will be the same, and these signals will be in phase. Figure 5 shows a further embodiment of the invention with a centrally placed movable electrode structure. It can be seen that movement of this electrode structure towards A, for instance, will decrease the path length to A and increase that to B. If a suitable A.C. voltage is applied across the electrodes, such movement will result in a change in phase between the two outputs which is directly proportional to the distance moved by the input block.Changes of temperature will affect both halves of the piezo-electric material equally, and if the movement of the central electrode structure is small compared with the path length to the output electrodes, the device will be self compensating for temperature changes.

In such devices using phase change, there is only a unique correlation between phase and distance provided that the distance moved is less than one wavelength. This limitation may be overcome by, for example, making the measurements at two unrelated frequencies, when the repeat distance would be increased to the wavelength of the beat frequency between the two frequencies used.

Figure 6 shows a further embodiment of the present invention. According to this embodiment, a single electrode structure is provided which is movable with respect to a piezo-electric substrate. An alternative voltage applied to the electrodes will generate a SAW which propagates along the surface of the substrate. At a distance from the area of generation there is a structure which is capable of reflecting the SAW. This structure may be a perpendicularly cut end, but other types of reflector could be used. The SAW is thus reflected to return to the area of generation where it induces an alternating voltage.

The delay time of the output with respect to the input will vary depending on the relative movement between the electrode structure and the substrate. The alternating voltage is supplied to the electrode structure as a series of short bursts at a period such that the reflected signal is detected between the bursts.

Claims (9)

1. A transducer comprising à surface acoustic wave device including a fixed electrode structure located on the surface of a piezo-electric substrate and a further electrode structure which is movable relative to the substrate.

2. A device as in claim 1 with two points of attachment one connected mechanically to the movable electrode structure and the other connected mechanically to the piezo-electric substrate so that the device can be used to measure changes in the separation of the two points of attachment.

3. A device as in claim 2 with a second similar device equipped with either fixed or adjustable electrode structures so that movement of the attachment points results in a change in the phase difference between the outputs of the two devices.

4. A device as in claim 1 which has three electrode structures, the central one being a movable input with two output electrode structures spaced on each side of it, so that movement of the central electrode results in a change in the phase difference between the two outputs.

5. A device as in claim 4 with two points of attachment one connected mechanically to the movable electrode structure and the other connected mechanically to the piezo-electric substrate so that the device can be used to measure changes in the separation of the two points of attachment.

6. A device as described in claims 2, 3 or 5 in which the two attachment points are mounted on to two separate points on a body so as to measure the strain which occurs in that body.

7. A device consisting of a piezo-electric substrate with two electrode structures so that it behaves as a surface acoustic wave delay line, with the essential requirement that one or more of the electrode structures is movable with respect to the piezo-electric substrate so that the path length between the structures and thus the delay is variable.

8. A transducer comprising a surface acoustic wave device including a piezo-electric substrate and an electrode structure, in which the electrode structure and substrate are movable with respect to each other.

9. A device substantially as described with reference to Figures 2, 4, 5 or 6 of the accompanying drawings.