Sensor using electro active curved helix and double helix

Various sensors use an electro-active device (11) electrically connected to a detector circuit. The electro-active device (11) comprises an electro-active structure in the form of a continuous electro-active member (12) curving in a helix around a minor axis (13) which is in itself curved for example in a helix around a major axis (14). On activation by relative displacement of the ends (16) of the device (11), the electro-active structure twists around the minor axis due to the fact that the minor axis (13) is curved. The continuous member (12) has a bender construction of a plurality of layers (21) and (22) including at least one layer of electro-active material so that concomitantly with the twisting the continuous member (12) bends generating an electrical signal detected by the detector circuit. The electro-active device (11) is advantageous as a sensing element in a sensor because it has a large displacement, high sensitivity and low compliance.

The present invention relates sensors using an electro-active device, that is devices employing an electro-active structure arranged to sense relative displacement of the ends of the structure.

Electro-active materials are materials which deform in response to applied electrical conditions or, vice versa, have electrical properties which change in response to applied deformation. The best known and most developed type of electro-active material is piezoelectric material, but other types of electro-active material include electrostrictive material and piezoresistive material. Many devices which make use of electro-active properties are known.

Simple piezoelectric acceleration sensors comprise a block of piezoelectric material. However, as the piezoelectric effect is small, of the order 10−10m/V, the sensitivity of the sensors is very low and the level of the output electrical signal is very low. This is one of the main drawbacks of this type of sensor, because the low signal level makes it susceptible to noise. Typically, special charge amplifiers and special cabling are required to minimise the noise and ensure that the generated signal detected by a detector circuit has a sufficiently low signal-to-noise ratio. This has caused many manufacturers to integrate the acceleration sensor and electronics into a single silicon device using modern micro-machining technology and such devices are in widespread use for applications such as car air bags. As the displacement of the block of piezoelectric material is small, a mechanical actuator is needed, typically a basic cantilever, but this limits the sensitivity of the sensor as a whole.

More complicated electro-active structures have been developed to achieve larger displacements and have to an extent found application as displacement sensors. However such electro-active structures suffer from the following problems.

A known electro-active structure is the bender construction, for example a bimorph bender construction. With a bender construction, the electro-active structure comprises a plurality of layers at least one of which is of electro-active material. On mechanical activation by bending the bender perpendicular to the layers, the layers deform with a differential change in length between the layers for example one layer expanding and another layer contracting due to the layers being constrained by being coupled to one another. Accordingly a voltage is output on relative displacement of the ends of the structure. However, the relative displacement of the bender does not follow a linear path in space. As the structure bends and the degree of curvature increases, the relative displacement of the ends follows a curve in space. Furthermore, for a relatively large displacement, it is necessary to increase the length of the structure which therefore becomes inconvenient. For example, to achieve a displacement of the order of 0.1 mm with a bimorph bender construction, a structure of length around 5 cm is typically needed.

For use in a sensor it would be desirable for an electro-active device to be capable of undergoing a relatively large displacement which is linear in space with a relatively low compliance to improve sensitivity.

According to a first aspect of the present invention, there is provided a sensor comprising an electro-active device electrically connected to a detector circuit, the electro-active device comprising an electro-active structure which extends along a curved minor axis and arranged, on mechanical activation by relative displacement of the ends of the structure, for the structure concomitantly to twist around the minor axis to generate an electrical signal, the detector circuit being arranged to detect the generated electrical signal.

Concomitantly with the relative displacement between the ends of the device the twist of the structure around the minor axis occurs because of the fact that the device extends along a curved minor axis. The electro-active device uses the physical principal that twisting of a curved object causes displacement perpendicular to the local curve, and vice versa displacement of the ends of a curved object causes twisting along its length. The displacement is equivalent to a change in the orientation of the minor axis of the structure relative to its original orientation.

The device uses an electro-active structure which twists on activation. Considering any given small section of the structure along the curved minor axis it is easy to visualise how twist of that given section rotates adjacent sections and hence relatively displaces them in opposite directions perpendicular to the local curve of the given section, because the adjacent sections extend at an angle to the given section as a result of the curve of the minor axis. Therefore twisting of the given section is concomitant with a relative displacement of the adjacent sections perpendicular to the plane of the curve. The degree of twisting is proportional to the degree of curvature in the given section and the magnitude of the relative displacement. For minor axes which extend along a regular curve around a major axis, such as along an arc of a circle or a helix, relative displacement of the ends in the direction parallel to the major axis of the structure produces displacement in each section in the same direction. Therefore, the electro-active device in accordance to the present invention can sense displacement which is linear in space.

The range of displacement which may be sensed is proportional to the length of the structure along the minor axis, because each section of the structure along the minor axis contributes to the overall displacement. Therefore the sensor may be arranged to sense any desired degree of displacement by suitable design of the device, in particular by selection of the length of the structure along the minor axis and of the type of structure which controls the magnitude of the twisting-field response. As a result of the structure extending along a minor axis which is curved, a relatively compact device may be produced.

Also the device has a low compliance because an applied force or displacement is shared over all the sections of the device. This provides the device with a high sensitivity. A major advantage of the electro-active device as a sensor is its sensitivity. Because the device is mechanically compliant it produces a relatively large response signal and provides a good signal to noise ratio. Additionally, the precise value of this compliance can be engineered to match a given application by varying the device's geometric parameters.

In general, the curve along which the minor axis extends may be of any shape.

One possibility is for the curve along which the minor axis extends to be planar, for example as the arc of a circle or a spiral. In this case, the displacement on activation occurs perpendicular to the plane of the curve. The thickness of the device in the direction in which relative displacement occurs is merely the thickness of the electro-active structure so a relatively thin device may be produced.

Another possibility is for the curve along which the minor axis extends to be a helix. In this case, each helical turn of the structure contributes towards displacement in the direction along the geometrical major axis around which the helix is formed. Therefore a large degree of displacement may be achieved proportional to the number of helical turns, therefore producing relatively high displacement for a relatively compact device.

The electro-active structure is preferably arranged with portions arranged to be mechanically activated by being bent around the minor axis concomitantly with twisting of the structure around the minor axis. As a result, the electro-active portion may have any construction which bends on activation. Use of an electro-active structure which bends on activation is advantageous, because it allows selection of structure in which all the electro-active material of the structure is activated thereby increasing the efficiency of the device. Also, use of an electro-active structure which bends on activation provides for simplicity of design. The preferred construction is the known bender construction comprising a plurality of layers including at least one layer of electro-active material, preferably a bimorph bender construction having two layers. Such a construction is well known and understood as applied to a straight bender and particularly easy to manufacture. The same benefits are obtained when the bender construction is applied to the portions of the present invention. However, any other construction which provides bending on activation may be used.

Preferably, the electro-active structure comprises a continuous electro-active member curving around the minor axis, said electro-active portions being adjacent finite portions of the continuous member.

This structure is particularly easy to manufacture, for example by winding a deformable continuous electro-active member into shape.

Preferably wherein the continuous electro-active member curves in a helix around the minor axis.

By using a continuous electro-active member which curves in a helix around the minor axis a number of advantages are achieved. Firstly, it is easy to provide a structure which is regular along the length of the minor axis and hence provide the same degree of twisting along the entire length of the minor axis. Secondly, the helix is easy to manufacture, for example by winding a deformable continuous member into shape or by making a helical cut in a tubular electro-active member. Thirdly, the device is compact as the helical turns of the member around the minor axis may be packed closely together.

However the electro-active structure may alternatively comprise a continuous electro-active member having a different shape which provides for bending around the minor axis concomitantly with twisting around the minor axis. For example it may comprise a continuous member having the shape of a flat member twisted around the minor axis. Furthermore, instead of comprising a continuous electro-active member, the electro-active structure may comprise a plurality of electro-active portions coupled together.

According to the present invention various sensors use an electro-active device. For clarity, the electro-active device will first be described, followed by the various sensors.

In the following description, the electro-active devices are described with reference to minor and major axes which are imaginary, but are nonetheless useful for visualising and defining the devices.

A first electro-active device1in accordance with the present invention is illustrated inFIG. 1. The device1comprises a structure consisting of a continuous electro-active member2curving in a helix around a minor axis3so that the structure extends along the minor axis3. The minor axis3is curved, extending in a curve which is an arc of a circle around a geometrical major axis4perpendicular to the plane of the minor axis3, i.e out of the plane of the paper inFIG. 1. As the minor curve3is planar, the thickness of the device parallel to the major axis4is merely the thickness of the helical structure of the electro-active member2.

A second electro-active device11in accordance with the present invention is illustrated inFIG. 2. The device2comprises a structure consisting of a continuous electro-active member12to curving in a helix around a minor axis13so that the structure extends along the minor axis13. The minor axis13is curved, extending in a curve which is a helix around a geometrical major axis14. The electro-active device11is illustrated inFIG. 2with a minor axis which extends along of a helix of three turns merely for illustration, any number of turns being possible.

FIG. 3illustrates a portion20of either the continuous member2of the first device1ofFIG. 1or the continuous member12of the second device11ofFIG. 2. The construction of the portion20being the same for both the first device1and the second device2the electro-active portion20is a finite portion of the continuous member2or12and hence the electro-active member2or12may be considered as a plurality of adjacent portions20as illustrated inFIG. 3disposed successively along the minor axis3or13. Hence, the portion20extends along part of a helical curve around the minor axis3or13as shown inFIG. 3.

FIG. 3illustrates the construction of the electro-active portion20. This construction is preferably uniform along the entire length of the minor axis3or13in order to provide uniform properties on activation. Alternatively, the device1or11may be designed with some variation along the length of the minor axis3or13, either in the construction of the continuous member2or20or in the shape of the curve of the continuous member2or20around the minor axis3or13.

The electro-active portion20has a bimorph bender construction comprising two layers21,22of electro-active material extending along the length of the portion20. The layers21,22of electro-active material both face the minor axis3or13. The electro-active layers21or22preferably extend, across the width of the portion20, parallel to the minor axis3or13, although there may be some distortion of the electro-active portion20of the continuous member2or12due to the nature of the curve around the minor axis3or13. Alternatively, the layers21or22may extend, across the width of the portion20, at an angle to the minor axis3or13so that one edge along the electro-active portion20is closer to the minor axis3or13than the opposite edge.

The material of the electro-active layers21or22is preferably piezoelectric material. The piezoelectric material may be any suitable material, for example a piezoelectric ceramic such as lead zirconate titanate (PZT) or a piezoelectric polymer such as polyvinylidenefluoride (PVDF). However, the material of the electro-active layers21,22may be any other type of electro-active material, for example piezoresistive material, in which the electrical resistance changes as the material is deformed or strained, or electrostrictive material, which constricts on application of an electric field.

The electro-active portion20further comprises electrodes23to25extending parallel to the layers21,22of piezoelectric material. Outer electrodes23,24are provided outside the electro-active layers21,22on opposite sides of the electric-active portion20. A centre electrode25is provided between the electro-active layers21and22. The electrodes23to25are used to apply poling voltages and to operate electro-active portion20in a bending mode.

On mechanical activation by bending of the portion20voltages are developed on the electrodes23to25and conversely on electrical activation, activation voltages are applied to the electrodes23to25. On bending, the electro-active layers21and22undergo a differential change in length concomitant with bending of the portion20due to the constraint of the layers being coupled together at their interface formed by the centre electrode25. One of the electro-active layers21or22expands and the other one of the electro-active layers21and22contracts

The relative direction and magnitude of the activation and poling voltages may be selected in the same manner as for known linear electro-active devices having a bender construction. For example, poling voltages of sufficient magnitude to pole the electro-active layers21and22may be applied in opposite directions across the electro-active layers21and22by grounding the centre electrode25and applying poling voltages of the same polarity to both the outer electrodes23,24. In this case, on mechanical activation of the electro-active portion20the activation voltages are developed in the same direction across the electro-active layers21and22producing voltages of opposite polarity on the two outer electrodes23and24with respect of the centre electrode25.

On activation the electro-active portion20bends around the minor axis3or13, either towards or away from the minor axis3,13depending on the polarity of the activation voltages. On mechanical activation, the activation voltages developed at the electrodes23to25are fed to a circuit26. On electrical activation the activation voltages are applied from a circuit26through external terminals27electrically connected to the electrodes23to25in the manner known for known straight piezoelectric devices having a bender construction.

Electrical connection to the electrodes23to25may be made in the same way as is known for known straight devices having a bender construction, in principle at any point along the length of the device of which the portion20forms part but preferably at the end. The preferred technique is to provide the electrodes with fingers (not shown) extending at the end of the device at different lateral positions across the width of the device as known for straight devices having a bender construction.

It will be appreciated that other bender constructions could equally be applied to the portion20, for example a unimorph bender construction comprising a layer of electro-active material and an inactive layer or a multimorph bender construction comprising a plurality of layers of electro-active material.

Whilst the bender construction illustrated inFIG. 3is preferred for simplicity and ease of manufacture, it will be appreciated that the continuous numbers2or12could in fact have any construction which bends around the minor axis3or13on activation. For example, the continuous members could be electro-active elements of the type described in the application being filed simultaneously with this application entitled “Electro-Active Elements and Devices” in which the elements have two pairs of electrodes extending along the length of the member for bending across the width on activation.

On activation, the electro-active portions20of the continuous member2or12bend around the minor axis3or13. As a result of the continuous electro-active member2or12curving around the minor axis3or13, in particular in a helix, such bending is concomitant with twisting of the continuous member2or12around the minor axis3or13. This may be visualised as the turns of the continuous member2or12as the bending tightening or loosening causing a twist of the structure of the member2or12along the minor axis3or13. The twist of the continuous member2or12occurs along the entire length of the minor axis3or13causing a relative rotation of the ends of the structure labelled5and6in the first device1ofFIGS. 1 and 15and16in the second device11ofFIG. 2.

It will be appreciated that the continuous member2or12could curve around the minor axis3or13in curves other than a helix to produce such twisting, for example by having the shape as though formed by twisting a flat member round the minor axis. It will also be appreciated that other structures other than a continuous member could be applied to produce twisting around the minor axis. For example the electro-active structure could consist of a plurality of electro-active portion disposed successively along the minor axis and coupled together so that the bending of each individual portion twists the adjacent portion around the minor axis causing twisting of the structure as a whole. Alternatively the electro-active structure could be a device of the type described in the application being filed simultaneously with this application entitled “Piezoelectric Devices” which comprises a plurality of electro-active torsional actuators which may comprise electro-active elements activated in shear mode.

Considering the first device1ofFIG. 1, the twisting of the continuous member2around the minor axis3is concomitant with relative displacement of the ends of the device5and6perpendicular to the curve of the minor axis3, that is parallel to the major axis4. The relative displacement of the ends5and6derives from the twisting of the continuous member2around the minor axis3in combination with the curve of the minor axis3. It is an inevitable result that twisting of a curved object causes relative displacement of the ends of that object perpendicular to the local curve of the object.

In a similar manner, on activation of the second device11ofFIG. 2, the twisting of the continuous member12around the minor axis13is concomitant with displacement of the ends15and16of the device parallel to the major axis14. Again, this relative displacement derives from the rotation of the continuous member12around the minor axis13in combination with the curve of the minor axis13. In this case, the relative displacement caused by any given small section of the structure along the minor axis13causes relative displacement of the ends of that section perpendicular to the local curve of the minor axis13. The overall displacement of the ends15,16of the device11is the sum of the displacements of all the sections which results in an overall relative displacement parallel to the major axis14.

Therefor for both the first and second devices1and11relative displacement of the ends of the structure ends5and6of the device is concomitant with twisting of the structure along its length which is concomitant with mechanical activation of each portion20causing an electrical signal to be generated on the electrodes23to25.

The exact construction and dimensions of the member2or12and the form of the electro-active structure may be freely varied to produce the desired response. A suitable member2or12has a 0.5 mm thickness tape wound as a 4 mm diameter minor helix around the minor axis3or13. When this forms the first device1in which the minor curve extends around about three quarters of a circle of 30 mm diameter the observed displacement is about ±6 mm. Similarly if this structure was used to form the second device11in which the minor curve extends along a20turn helix of diameter 30 mm, this would produce displacement of around ±120 mm.

In general, the minor axis, along which the structure of devices in accordance with the present invention extend, may follow any curve and the resultant displacement of the ends of the structure will be the sum of the displacement caused by each section of the structure along the curve. However, curves which are regular such as the curve of the minor axis of the first and second devices1and11are preferred so that all sections of the device caused relative displacement in a common direction and also because design and manufacture are thereby simplified.

The first and second devices1and11may be electrically activated to create mechanical displacement between the ends5and6or15and16, or may be mechanically activated in which case relative displacement of the ends5and6or15and16causes an electrical voltage to be developed across the electrodes23to25. In the case of electrical activation, the ends5and6or15and16of the electro-active device1or11are coupled to further elements to be relatively displaced similarly in the case of mechanical activation the ends5and6or15and16are coupled to elements which drive deformation of the device1or11.

Manufacture of the electro-active devices1and11will now be described.

The preferred method of manufacture is to initially form the electro-active structure extending along a straight minor axis and subsequently to bend the straight electro-active structure so that the minor axis along which it extends becomes curved.

To form the continuous member2or12as an electro-active structure along a straight minor axis there are two preferred techniques.

The first preferred technique is to initially form the continuous member2or12as a straight member and subsequently to deform it to curve around the straight minor axis. The bender construction of the continuous member2or12is in itself known and the continuous member2or12may be formed by applying any of the known techniques for manufacturing a device having a bender construction. For example, the continuous member12may be initially manufactured by co-extrusion of the layers21and22of plasticised material or by co-calendering of the layers21and22. Alternatively, the continuous member2or12may be made through lamination of thin layers21and22. These thinner layers may be made by any suitable route, such as high shear mixing of a ceramic powder, polymer and solvent mixer followed by co-extrusion and calendering. Alternatively, techniques such as tape casting or the process called the Solutech process known in the field of ceramics nay be used.

The electrodes may be formed as an integral part of the manufacture of the continuous member2or12, for example by being in co-extruded or co-calendered. Further electrodes, which may be activation layers23to25or may be terminal electrodes to allow access to the electrodes23to25, may be applied by printing, by electro-less plating, through fired-on silver past or by any other appropriate technique.

The second preferred technique is to initially manufacture the continuous member as a cylinder or other tube with a multi-layered bender construction of electro-active layers21and22and electrodes23to25and subsequently to cut the member along the helical line to leave the continuous member2or12extending in a helix around the axis of the cylinder or tube which then constitutes the minor axis.

Subsequently the straight structure is bent to curve the minor axis along which the structure extends.

To deform the member and structure, there must exist in the initially formed member a sufficient degree of flexibility. Suitably deformable electro-active materials are known, typically including constituent polymers which enhance the deformability. With such materials after shaping, the constituent polymers are burnt out, typically at up to 600° C. and the material is then densified through further sintering at higher temperature, typically 1000° C. to 1200° C. In this case, the electro-active structure is initially formed with enlarged dimensions to allow for linear shrinkage which occurs during sintering, typically of around 12 to 25%.

The curving of the straight member and the bending of the structure may be performed around formers. The formers are subsequently removed either physically or by destruction of the former for example by melting, burning or dissolving.

A number of sensors will be described using the electro-active devices of the type described above. The electro-active structures of the electro-active devices are arranged to generate an electrical signal on mechanical activation by relative displacement of the ends of the structure. The sensors further comprise a detector circuit connected to the electro-active device to detect the generated electrical signal, and hence the relative displacement. The electro-active devices are arranged to be relatively displaced by a system to be sensed and hence provide for the conversion of a motion into an electrical signal allowing the motion, or another property, to be measured.

In the figures illustrating the sensors, the electro-active device is shown as having a structure extending along a minor axis which curves in a helix, as in the second device11ofFIG. 2, but this is merely for illustration and the electro-active device may have any of the types of structure described above.

The simplest form of sensor using the electro-active device described above is a displacement sensor for measuring the displacement of a movable object. In this case, the moving object is arranged to displace one end of the structure of the electro-active device with respect to the other end which is held fixed. For such a displacement sensor, the electro-active device is designed to have a mechanical stiffness sufficiently low that it does not significantly influence the displacement of the system being measured. In such an application, the advantage of using the electro-active device of the type described above is that the electro-active device provides a large displacement capability and has a relatively low compliance.

FIG. 4illustrates such a displacement sensor40comprising an electro-active device41electrically connected to a detector circuit42. One end43of the electro-active device41is coupled to an object44whose displacement is to be sensed. The other end45of the electro-active device41is coupled to a support46relative to which the object44is moveable. As the object44moves in the direction A the ends43and45of the electro-active device41are relatively displaced, thereby mechanically activating the electro-active device41which generates an electrical signal. The detector circuit42detects the generated electrical signal. The electrical signal is a voltage, so the detector circuit42is a voltage detector and is arranged to have a high input impedance.

FIG. 5illustrates a fluid level sensor50which is another form of displacement sensor. Except as described below, the fluid level sensor50is identical to the displacement sensor40, so a description thereof will not be repeated. The free end43of the electro-active device41is coupled to an element51comprising a float52and a weight53. In use, the float52floats in a fluid54whose level is to be measured. As the level of the fluid54changes, the float52is displaced, which displacement is measured by the sensor50. The weight53is provided to ballast the float downwards to float at the correct level in the fluid54.

It will be appreciated that various modifications to the fluid level sensor50are possible. For example, the float54may be arranged on a guide to guide movement of the float54. Also, a mechanical limit may be provided to limit the movement of the float54from passing the maximum relative displacement of the end of the structure of the electro-active device41.

The electro-active device may also be used as a force sensor. In this case, the electro-active device is again coupled between two relatively moveable elements, one of which is to receive the applied force and the other of which is a support which is held fixed. The force sensor is arranged to mechanically resist the applied force so that the ends of the electro-active device are relatively displaced by an amount corresponding to the magnitude of the applied force. Depending on the magnitude of the force to be measured, this can be achieved by selecting an electro-active device with an appropriate stiffness. Alternatively, a further mechanical arrangement may be provided in parallel with the electro-active device, for example a resilient biassing means such as a spring. The advantage of using the electro-active device of the present invention as such a force sensor is that the relatively low compliance of the electro-active device significantly improves the sensitivity of the force sensor. Also, a high resolution, large range, linear sensor is produced due to the linear characteristic of the electro-active device. A force sensor may for example be used to measure weight, and thus mass.

FIG. 6illustrates a pressure sensor60which is a form of such a force sensor. The pressure sensor60comprises a support61defining a pressure chamber62. A piston63is provided with one end64in the pressure chamber62. The piston63is mounted by a bearing65to the support61to be reciprocally movable along the pressure chamber62. The pressure sensor60further comprises an electro-active device66, a first end67of which is coupled to the piston63and the second end68of which is coupled to the support61. The electro-active device66is electrically coupled to a detector circuit69which detects the electrical signal generated by the electro-active device66and hence provides a measurement of the displacement and force applied to the electro-active device66. In use, a fluid whose pressure is to be measured is introduced into the pressure chamber62. This applies a force to the piston63dependent on the pressure of the fluid, which relatively displaces the ends67and68of the electro-active device66, causing an electrical signal to be generated which is detected by a detector circuit69.

FIG. 7illustrates a pressure sensor70which is another form of force sensor. The pressure sensor70comprises a support71defining a pressure chamber72. A piston73is in the form of a flexible diaphragm coupled around its circumference to the support71forms one end of the pressure chamber72. The pressure sensor70further comprises an electro-active device76, a first end77of which is coupled to the piston73and the second end78of which is coupled to the support71. The electro-active device76is electrically coupled to a detector circuit79which detects the electrical signal generated by the electro-active device76and hence provides a measurement of the displacement and force applied to the electro-active device76. In use, a fluid whose pressure is to be measured is introduced into the pressure chamber72. This applies a force to the piston73dependent on the pressure of the fluid, which relatively displaces the ends77and78of the electro-active device76, causing an electrical signal to be generated which is detected by a detector circuit79.

A velocity sensor may be produced by arranging a plurality of pressure sensors, for example the pressure sensors60or70illustrated inFIGS. 6 and 7, in a Pitot tube. The difference between the two measured pressures gives the fluid velocity. Such an arrangement using a Pitot tube with other types of pressure sensor is in itself conventional.

The electro-active device may also be used to detect sound so that the sensor constitutes a microphone. In this case, one end of the electro-active device is coupled to a sound detecting element adapted to be moved by sound waves incident thereon, the design of the sound detecting element being the same as for conventional microphones and dependent on the frequency and magnitude of the sound waves to be detected.

Such a microphone may have the same arrangement as for known microphones using an electromagnetic coil with the electromagnetic coil replaced by the electro-active device. As compared to using an electromagnetic coil, the microphone in accordance with the present invention achieves a number of advantages. The use of the electro-active device allows the microphone to have a large dynamic range, a large frequency range and a high sensitivity because the level of the generated electrical signal is high. In particular, the frequency response of the electro-active device enables such a microphone to have a frequency response which covers the full audio range and some sub-audio frequencies. The low level frequency response achievable also means that the microphone can be used as a non-contact displacement meter. Another advantage is that the mechanical design of the microphone can be tuned to have a good impedance match with the air impedance, this being difficult with an electromagnetic microphone. This property, and the large strains of the electro-active device, enable the microphone to produce a significantly larger electrical signal than is produced by traditional microphones. Such a larger signal amplitude improves the signal-to-noise ratio. A further advantage is that the electro-active device has a significantly reduced weight as compared to the corresponding electromagnetic coil arrangement.

As an example,FIG. 8illustrates such a microphone80in which an electro-active device81is coupled at one end82to a cone83which forms a sound detecting element. The other end84of the electro-active device81is coupled to a support84. The cone83is also supported by the support84by a flexible seal85disposed around the circumference of the cone83. The arrangement of the microphone80is conventional except for the electro-active device81. In use, sound waves incident on the cone83cause vibrational movement of the cone83which drives vibrational displacement of the ends82and84of the electro-active device81. The electro-active device81is coupled to a detector circuit86which detects the electrical signal output by the electro-active device81which corresponds to the sound. The detector circuit86has conventional circuitry for detecting electrical signals corresponding to sound signals.

The electro-active device may also be used to form an acceleration sensor by coupling one of the ends of the electro-active device to a mass. The inertia of the mass will limit its movement when the other end of the electro-active device is displaced, so that the generated output signal is representative of the acceleration of the other end of the electro-active device. Thus the electro-active device may be used to detect acceleration of the other end of the electro-active device or an element coupled thereto.

An acceleration sensor in accordance with the present invention provides a number of advantages as compared to known acceleration sensors formed from a simple piezoelectric crystal. Most significantly, an acceleration sensor in accordance with the present invention allows for much more sensitive detection. Also, the electro-active device generates significantly larger signal amplitudes which reduces the signal-to-noise ratio and hence reduces the signal conditioning needed to maintain a low noise electrical signal. Furthermore, the use of the electro-active device allows the acceleration sensor to operate at low frequencies, close to DC.

The generated electrical signal provides a measurement of acceleration. As with known acceleration sensors, the detector circuit may be arranged to integrate this signal to obtain a measurement of velocity and displacement, so that the sensor constitutes a velocity or displacement sensor. Thus the acceleration sensor may be used to produce a position sensor for a number of applications. By arranging a plurality of such sensors to detect acceleration in orthogonal directions, a 2-D or 3-D position sensor may be produced, thereby providing the potential use as a 2-D or 3-D “mouse” as an input device for a computer system. This would have the considerable advantage over conventional “mice” of requiring no exposed parts such as a ball and no wearing surfaces. Similarly, the 3-D “mouse” could be used as an input device for a virtual reality system.

FIG. 9illustrates such an acceleration sensor90comprising an electro-active device91coupled at one end92to a mass93and at the other end94to a support95. The mass93and the support95are relatively moveable, so the inertia of the mass93limits its movement when the support95is displaced. Accordingly, the generated output signal is representative of the acceleration of the support95. The electro-active device91is electrically connected to a detector circuit96for detecting the generated electrical signal.

The detector circuit in all the sensors in accordance with the present invention may be a high impedance voltage detector. As an alternative the detector circuit may be an integrating current detector circuit. This extends the performance of the sensors from AC to DC, as follows. The electro-active devices of the type described above are inherently AC-coupled due to their electrical nature which is a very large resistance in parallel with a moderate capacitance (typically hundreds of nano-farads). The resistance derives from the resistance of the electro-active material and the capacitance derives from the electrodes disposed across the electro-active material. Using an integrating current detector circuit which measures the movement of charge of the electro-active device over a long period of time, it is possible to extend the performance of the sensor to DC.

As an example,FIG. 10illustrates an integrating current detector circuit100.FIG. 10illustrates the effective circuit of an electro-active device101as a resistance102in parallel with a capacitance103. The electro-active device101is electrically connected across the terminals104of the detector circuit100. The detector circuit100comprises a low resistance sensing element105which may be a low impedance resistor or a special sensor such as a SQUID (Superconducting QUantum Interference Device) that measures current by the magnetic field induced by moving charges. As the electro-active device101is displaced the charge thus generated passes through the low resistance sensing element105. The detector circuit100further includes a circuit106arranged to integrate or count, the passage of charge through the low resistance sensing element105. The circuit106includes instrumentation electronics which may be either analogue or digital, provided that it has sufficient sensitivity to measure small current.

The electro-active device may also be used to detect rotation of a rotary element, so that the sensor constitutes a rotary encoder. As an example,FIG. 11illustrates such a rotary encoder110comprising a rotary element111rotatable about a shaft112and thereby defining an axis of rotation. The circumference113of the rotary element111is formed with a plurality of teeth114. Thus the teeth114, provide the circumference with a varying radius. Any features of varying radius other than the teeth114may be used.

The rotary encoder110further comprises an electro-active device115of the type described above electrically connected to a detector circuit120. The electro-active device115is coupled at one end116to a member117which engages the circumference of the rotary element111. The other end118of the electro-active device115is coupled to a support119which is fixed relative to the axis of rotation of the rotary element111about the shaft112. As a result of the member117engaging the circumference of the rotary element111, as the rotary element111rotates the passage of the teeth114past the member117displaces the member117. This in turn relatively displaces the ends116and118of the electro-active device115generating an electrical signal which is detected by the detector circuit120. Consequently, the detector circuit120detects the rotation of the rotary element, in a similar manner to conventional rotary encoders.

The teeth114are provided with a different profile in opposite directions around the axis of rotation, in particular by having different slopes on either side of each tooth114. As a result the form of the electrical signal generated by the electro-active device115differs depending on the direction of rotation of the rotary element111. This allows the detector circuit120to detect the direction of rotation from the form of the output signal.

It will be appreciated that other variations in the shape of the features on the rotary element111may be made in the same manner as is conventional for known rotary encoders, for example by varying the spacing or width of the features to determine the absolute rotary position of the rotary element111.

The electro-active device of the type described above such as that illustrated inFIGS. 1 to 3may also be employed as a gyroscope by modifying the construction of the electro-active structure so that it provides for (a) electrical activation of the electro-active device by application of an activating electrical signal to drive mechanical displacement of the device and, independently therefrom, (b) generation of an electrical signal by a mechanical activation. In use, the sensor is electrically activated by an alternating electrical signal and the resulting displacement of the electro-active device is detected. If the electro-active device is stationary, the activating and generated electrical signals will correspond. However, when the external accelerations of the electro-active device change, the mechanical displacement of the electro-active device will vary depending on the relative orientation of the external acceleration and the electro-active device. The mechanical displacement of the electro-active device will vary in accordance with the change in orientation of the device, causing a corresponding change in the output electrical signal. The difference between the activating electrical signal and the generated signal provides a measure of the change in orientation of the sensor in a similar manner to conventional gyroscopes. Thus the sensor constitutes a gyroscope because it detects changes in the orientation of the system and is suitable for use in stability and control systems. By providing plural electro-active devices orientated in orthogonal directions a multi-dimensional gyroscope may be constructed.

FIG. 12illustrates the construction of a portion124of an electro-active device for providing the independent (a) electrical activation and (b) detection of mechanical displacement. The construction is preferably uniform along the entire length of the minor axis3or13and is an alternative to the construction of the electro-active portion20illustrated inFIG. 3. The portion124is a portion of either the continuous member2of the first device1ofFIG. 1or the continuous member12of the second device11ofFIG. 2, so the portion124extends along part of a helical curve around the minor axis3or13as shown inFIG. 12.

In a first part121of the electro-active portion124, the electro-active portion124has a bimorph bender construction which is identical to the bimorph bender construction of the electro-active portion20ofFIG. 3, so a description thereof will not be repeated. However, there is a difference in how this first part121of the portion124is activated in use. In particular, instead of being mechanically activated, this part121is electrically activated by application of an activating electrical signal of a predetermined frequency from a circuit123. The activating electrical signal causes the portion124to bend around the minor axis3or13and concomitantly the structure of the electro-active device twists around the minor axis3or13causing a relative displacement of the ends of the structure occurs.

In addition, a second part122of the electro-active portion124is able to independently detect the mechanical displacement generated when the first part121is electrically activated. The second part122, like the first part121, has an identical construction to the electro-active portion20illustrated inFIG. 3. The layers of the first and second parts121and122extend parallel to one another. The second part122is mechanically activated by the mechanical displacement of the first part121, thereby causing generation of an electrical signal corresponding to the actual mechanical displacement. The electrodes of the second part121are electrically connected to the circuit123which acts as a detector circuit to detect the generated electrical signal. The circuit123also compares the generated electrical signal with the activating electrical signal applied to the electrodes of the first part121to derive a measure of the change of orientation of the electro-active device in the same manner as for known gyroscopes. Thus when the construction of the portion124is used, the electro-active device constitutes a gyroscope.