GB2057805A - Piezo-electric coupling devices - Google Patents

Abstract

A remote signaling circuit includes a piezo-electric transducer 12 providing acoustic coupling between the input oscillator stage 11 and output stage 13 of the circuit. The input circuit includes a transistor oscillator (TR1) in which a portion of the transducer 12 provides both the frequency determining element and the positive feedback loop. The output circuit can comprise a switching circuit, as shown, or a direct current power supply. A number of input and output configurations of the transducer 12 are described, and thyristor switching arrangements for the output circuit. The transducer may be of disc form with input electrodes E51, 52 at its centre and C-shaped output electrodes E53-56; the disc is supported at its centre which is a vibrational mode. <IMAGE>

Description

SPECIFICATION Piezo-electric coupling devices This invention relates to piezo-electric coupling devices, and in particular to electro/ acoustic/electrical isolator arrangements in which a piezo-electric body is employed both as a transducer element and as part of a drive circuit providing acoustic signals for transmission via the body.

There are many remote control applications wherein an electrical input signal must be isolated from a circuit controllable via the signal. For efficient operation such an arrangement should be compact and rugged and should provide a high degree of electrical isolation between its input and output.

In recent years piezo-electric materials having a high degree of efficiency have become available, and such crystals are particularly suitable for the construction of transducer elements.

According to one aspect of the invention there is provided a remote signalling circuit, including an input oscillator stage, an output stage, and a piezo-electric transducer providing acoustic coupling between the input and output stages, and in which a portion of said transducer provides a frequency determining element and positive feedback loop for said input stage.

According to another aspect of the invention there is provided a remote signalling circuit including an input circuit coupling a bipolar transistor crystal oscillator in which the collector output and the base emitter circuit are coupled to respective first and second areas of a piezo-electric crystal transducer, and an output switching circuit coupled to a further area of the transducer, in which the portion of the transducer adjacent the first and second areas of the transducer provides the frequency determining element and positive feedback loop of the oscillator, and in which the transducer provides electrical isolation and acoustic coupling between the input and output circuits.

According to a further aspect of the invention there is provided a piezo-electrically coupled relay, including a housing having input and output terminals, a body of electrically insulating piezo-electric material mounted on its vibrational modes within the housing and provided with input and output electrodes, a transistor oscillator circuit coupled between the input terminals and the input electrodes, and an output switching circuit coupled between the output electrodes and the output terminals, wherein the piezo-electric body provides the frequency determining element and the positive feedback loop of the oscillator, and wherein, in response to acoustic signals induced in the piezo-electric body and received in the form of electrical signals by said output electrodes, said output switching circuit provides a change in the impedance presented by the output terminals.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a piezo-electrically isolated relay circuit; Figures la and Ib show modified input circuits for the relay circuit of Fig. 1; Figures 2a and 2b show alternative voltage multiplying output stages for use in the circuit of Fig. 1; Figure 3 shows a further relay circuit; Figure 4 shows a relay circuit for use with relatively high power loads; Figures 5a to 5d show various forms of piezo-electric transducer constructions; Figures 6a to 6c show various modes of coupling between the input and output of the transducer; Figure 7 shows a piezo-electrically isolated power supply circuit.

Figure 8 shows a signal coupler circuit; Figure 9 shows a linear coupler circuit.

Referring to Fig. 1, the circuit shown is employed for isolated switching of an AC or DC load. The circuit comprises an input oscillator stage 11 whereby acoustic signals are generated in a piezo-electric crystal transducer element 12, typically a PZT ceramic material, and transmitted via the element 1 2 to an output switching stage 1 3.

The input stage 11 is a transistor crystal oscillator in which a portion of the crystal 1 2 provides the necessary positive feedback and frequency determining element. The input circuit is switched on by the application of a direct voltage to the terminals T1 and T2, a capacitor C1 providing smoothing of this input voltage. Resistors R1 and R2 bias a npn (pnp) transistor TR 1 into its linear region. The collector output and the emitter of the transistor TR 1 are coupled to metallised electrode areas E2A and E2B respectively on the crystal surface. These electrode areas provide for the conversion of the collector voltage signal into crystal strain.This strain propagates as an acoustic signal through the electrically insulating crystal and is converted back to an electrical signal both at the output electrode E3A and E3B and the feedback electrodes E1A and E1B.

The electrodes E1A and E1B are connected across the base-emitter junction of the transistor TR1 to give positive feedback from the collector signal plus causing the input circuit to oscillate at a frequency determined by the dimensions and characteristics of the crystal 1 2. Electrode and crystal gemometries may be chosen so as to provide optimum acoustic coupling between the input electrodes E2A and E2B and the output electrodes E3A and E3B.

The output stage 1 3 may comprise a simple switch circuit in which the output signal from the crystal 12 is applied to the gate of a triac TR2, a load L through which an electrical current is to be switched being connected in the anode circuit of the triac. In a particularly advantageous embodiment the input and output circuit may be provided as film circuits one on each end of the transducer 1 2.

Fig. 1 a shows an alternative input stage in which an extra resistor R1A is connected in series with R2 so that the relay may operate over a wider input voltage range. The input voltage is fed from input 1 to common for relatively high voltages and from input 2 to common for relatively low voltages.

Fig. 1 b shows an input stage in which improved control sensitivity is obtained by providing a separate power supply for the oscillator circuit. The control input then only has to regulate and not power the oscillator supply.

Fig. 2a shows an optional voltage multiplying output stage for use in the circuit of Fig.

1. The output signal from the electrodes E3A and E3B is fed to a conventional diode/capacitor multiplier network, the output terminals 0 and P of the network then being coupled to the load L.

Fig. 2b shows an alternative voltage multiplier arrangement. In this arrangement a plurality of output electrodes E3A-E5A and E3B -E5B are provided on the output end of the transducer crystal. The electrodes are interconnected as shown to provide a summed output.

The relay circuit shown in Fig. 3 is particularly suitable for the isolated switching of mains powered loads. As before the circuit includes an input oscillator stage comprising transistor TR31 and resistors R31 and R32, the oscillator being coupled to the piezo-electric crystal 12 via electrodes E31 and E32 and a common earth electrode E33. Electroacoustic signals transmitted across the crystal are received by electrodes E34 and E37 of the output stage The output stage comprises a pair of thyristors TH3 1 and TH32 connected in anti-parallel and in series with the load to be switched.

The gate of each thyristor TH31 and TH32 is biased by a corresponding resistor R33 and R34 and is coupled to a respective output electrode E35 and E36.

The circuit is intended for switching alternating currents. When an electro-acoustic signal is transmitted across the crystal 12 the corresponding electrical signal applied to the gate of each thyristor TH31 and TH32 drives that thyristor into its conductive stage, the anti-parallel configuration of the thyristors allowing conduction of both half cycles where an alternating current is being switched or conduction of a direct current of either poiar- ity.

The relay circuit shown in Fig. 4 is intended for switching relatively high power and/or reactive loads such as electric motors. The input circuitry is identical to that of the relay circuit of Fig. 3 and need not therefore be described further. The output stage includes a pair of thyristors TH41 and TH42 connected in anti-parallel across the output terminals of the relay circuit. NPN transistors TR41 and TR42 are coupled one between the gate and cathode of each thyristor TH41 and TH42.

The transistors TR41 and TR42 are biaseed by the resistor chain comprising resistors R41, R42 and R43 such that one or other of the tranistors turns on when the load voltage at the output terminals exceeds a predetermined value. To ensure symmetrical switching of the transistors TR41 and TR42 their bias resistors R41 and R42 should be substantially equal in value. Reverse bias of the transistors is prevented by diodes D41 and D42.

When a transistor TR41 and TR42 is in its conductive state it shorts the corresponding thyristor gate to the cathode thus preventing switch on of that thyristor. In this way switch on of the thyristors can be effected only at relatively low values of the applied load voltage. I.E., when the load voltage is an alternating voltage, switching takes place adjacent the zero crossings of the voltage waveform.

The resistors R44 and R45 connected between the gate and cathode of the respective thyristors TH41 and TH42 provide a bypass for the small current arising from the anode/gate capacitance of the thyristor and thus prevent unwanted turn on by this current.

Fig. 5a shows one form of transducer construction. In this embodiment the transducer comprises two types of material. The end portions 51 of the transducer are made from a piezo-electric material, but the centre portion 52, through which the transducer vibrations are transmitted, is formed from a highly insulating material of low permittivity. This arrangement provides a high degree of electrical isolation between the input and the output ends of the transducer.

For even greater isolation an electrostatic and/or a magnetic screen may also be incorporated. Such a construction is illustrated in Fig. 5b. As before the end portions 51 of the transducer are formed from a piezo-electric material. The centre portion 53 of the transducer comprises an electrostatic and/or magnetic screening region, the end portions 51 being isolated from the centre portion 53 by low permittivity insulating regions 54 disposed therebetween.

Fig. Sc shows a further type of transducer construction in which the transducer is formed from a single body of piezo-electric material in which the centre portion 55 is unpolarised.

This provides the centre portion with a high resistivity and a low permittivity.

A preferred form of piezo-electric transducer construction is shown in Fig 5d. In this construction the transducer is disc-shaped and is provided with input electrodes E51, E52 of its centre and sets of C-shaped output electrodes E53 to E56 at or near its periphery. The disc is, in use, supported at its centre which point corresponds to a vibrational mode.

Various coupling modes between the input and output of the transducer may be employed. Such techniques are illustrated in Figs. 6a to 6c.

In the construction of Fig. 6a bulk wave acoustic coupling is employed. Input electrodes 61 and 62 are arranged to produce a strain across the thickness of the substrate 1 2. This strain is propagated through the bulk of the material and is detected as a thickness strain by the output electrodes 63 and 64.

Fig. 6b shows an alternative arrangement in which coupling is effected via surface acoustic waves. In this arrangement both the input electrodes 65 and 66 and the output electrodes 67 and 68 have a comb configuration.

The input electrodes set up strain waves substantially in the surface of the substrate 1 2.

To prevent distructive interference by waves reflected from the edges of the crystal one or more of the following techniques may be used: a. Place acoustic barriers 51 and 52 around the edges of the crystal. These barriers absorb the energy of the surface waves and so practically eliminate reflected waves.

b. Arrange for constructive interference by reflected waves from the edges of the crystal.

This would be achieved by careful control of the geometry of the crystal and electrodes.

c. Place specially shaped reflecting electrode patterns 49 and 50 on the surface of the substrate to cause constructive interference.

By selectively polarising the substrate 1 2 it is possible to improve the electrode layout.

Using the numbering of Fig. 1, electrodes E1A and E2B can now be connected to form one electrode.

Fig. 6c shows a further coupling method which employs a combination of the two previous coupling modes. In this construction the input electrodes 53 and 54 are of the conventional plate form and induce bulk waves in the transducer 1 2. The output electrodes 55 and 56 are of comb form and respond to the surface component of the bulk waves.

Fig. 7 shows the use of the transducer coupling technique described herein in the construction of a piezo-electrically isolated power supply. The circuit is designed for use with an alternating current supply which is rectified by diode D7 1, the rectified current being smoothed by capacitor C7 1. The smoothed direct current is then fed via resistor R71 to a crystal oscillator circuit comprising transistor TR71, resistor R72 and the trans ducercrystal 12.

Oscillations transmitted across the transducer 1 2 are received as an alternating current signal by output circuit 73 which comprises a rectifying D74A and B and an associated smoothing capacitor C73. In some applications a plurality of mutually isolated output circuits may be provided.

The circuits described herein may be advantageously mounted in a plastics housing provided with terminals for the circuit input and output. The relay circuit arrangements of Figs.

3 and 4 are particularly suitable for this application. The packaged circuit can then be plugged into a socket or soldered to a printed circuit board.

The circuit shown in Fig. 8 is intended for coupling signals between circuits which need to remain electrically isolated, e.g. in computer and telecommunication systems.

The positively biassed signal at the input terminals is converted to an AC modulated signal at E4 and E5. This modulated signal is demodulated by the rectifying circuit comprising D81, D82, C81 and R81. The time constant R81.C81 together with the crystal frequency determine the bandwidth of the system.

Further electrodes may be attached to the crystal to obtain additional isolated outputs.

The optional output device may be either a linear device such as a bipolar or FET transistor on a non-linear device such as an SCR.

Fig. 9 shows a linear coupler circuit in which the output voltage follows the applied input voltage. Resistors R92, R93, and transistor T9 1, form the conventional energisation circuit.

El and E2 are the output electrodes. D93, D94, R94 and C2 form the output rectifying filter. E6 and E7 are the feedback compensation electrodes, and D91, D92, C91 and R91 form the feedback rectifying filter. The feedback filter output is designed to be substantially the same as the coupler output.

ICI is a comparitor or amplifier. It adjusts the coupler drive to keep the feedback and hence output voltage substantially the same as the input voltage.

D93 and D94 and/or D91, D92 may be replaced by any convenient waveform amplitude detecting networks (e.g. peak detectors, rms detectors, etc.) R91 and C91 and/or R94 and C92 may be replaced by any convenient filter.

Claims (11)

1. A remote signalling or relay circuit, including an input oscillator stage, an output stage, and a piezo-electric transducer providing acoustic coupling between the input and output stages, and in which a portion of said transducer provides a frequency determining element and positive feedback loop for said input stage.

2. A remote signalling or relay circuit including an input circuit comprising a bipolar transistor oscillator in which the collector output and the base emitter circuit are coupled to respective first and second areas of a piezoelectric crystal transducer, and an output switching circuit coupled to a further area of the transducer, in which the portion of the transducer adjacent the first and second areas of the transducer provides the frequency determining element and positive feedback loop of the oscillator, and in which the transducer provides electrical"isolation and acoustic coupling between the input and output circuits.

3. A circuit as claimed in claim 1 or 2, and wherein the transducer includes a portion of non-piezo-electric material though which acoustic waves are transmitted.

4. A circuit as claimed in claim 1, 2 or 3, and wherein the transducer is of the surface acoustic wave type.

5. A remote signalling circuit substantially as described herein with reference to Fig. 1, 3, 4, 7, 8 or 9, together with any one of Figs.

1 a, 1 b, 2a, 2b, 5a to 5c; or 6a to 6d of the accompanying drawings.

6. A piezo-electrically coupled relay, including a housing having input and output terminals, a body of electrically insulating piezo-electric material mounted on its vibrational modes within the housing and provided with input and output electrodes, a transistor oscillator circuit coupled between the input terminals and the input electrodes, and an output switching circuit coupled between the output electrodes and the output terminals, wherein the piezo-electric body provides the frequency determining element and the positive feedback loop of the oscillator, and wherein, in response to acoustic signals induced in the piezo-electric body and received in the form of electrical signals by said output electrodes, said output switching circuit provides a change in the impedance presented by the output terminals.

7. A relay as claimed in claim 6, wherein the output switching circuit include a pair of thyristors coupled in anti-parallel configuration across the output terminals.

8. A relay as claimed in claim 7, wherein said thyristors are operated in a zero voltage switching mode.

9. A relay as claimed in claim 6, 7 or 8, wherein the piezo-electric body is disc shaped.

10. A piezo-electrically coupled relay substantially as described herein with reference to Fig. 3 or Fig. 4 of the accompanying drawings.

11. A telecommunications exchange provided with a plurality of relays as claimed in any one of claims 6 to 10.

1 2. A method of remote signalling substantially as described herein with reference to the accompanying drawings.