GB2611299A

GB2611299A - Displacement or pressure sensing system for a touchpad or keyboard

Info

Publication number: GB2611299A
Application number: GB2113814.4A
Authority: GB
Inventors: Hastings Clark James
Original assignee: Sonuus Ltd
Current assignee: Sonuus Ltd
Priority date: 2021-09-28
Filing date: 2021-09-28
Publication date: 2023-04-05
Also published as: GB202113814D0; WO2023052090A1

Abstract

A displacement or pressure sensor comprising a transmitter comprising an open loop antenna 2 and having an input node 1 to receive an RF drive signal to drive the antenna to transmit an RF signal; a receiver 3 separated from the antenna, the receiver comprising an inductor to receive the RF signal and having at least one output node 5, 6, to output an output RF signal, wherein the receiver is configured to generate the output RF signal having at least one of a phase and amplitude dependent on a position of the antenna relative to a position of the inductor. The sensor may be configured to allow the separation between antenna 2 and the inductor to vary, wherein the dependence on the positions comprises dependence on separation. The sensor may be configured to allow at least one of antenna 2 and the inductor to move laterally, wherein the dependence on the positions comprises dependence on a lateral offset between the positions. Other aspects are a sensing system and a temperature compensation method. The sensors are suitable for a keyboard (Fig. 13), touch pad (Fig. 15) or touch screen eg for a computer or musical instrument.

Description

Displacement or Pressure Sensing System for a Touchpad or Keyboard FIELD The present invention generally relates to a displacement or pressure sensor, to a sensing system comprising a plurality of such sensors, and to a temperature compensation method for such a sensor or sensing system. Some embodiments are in particular suitable for a keyboard, touch pad or touch screen eg for a computer or musical instrument.

BACKGROUND

Pressure-sensing touchpads using capacitive sensing have various disadvantages, such as poor sensitivity when a user is wearing gloves (eg in a medical environment), and high sensitivity to environmental factors. Disadvantages similarly exist for resistive sensors. For example, a force-sensitive resistive matrix is expensive.

There therefore remains a need for improvements to displacement or pressure sensing systems, for example such system for touchpads and/or keyboards. Such improvements may relate to, inter alia, one or more of: low cost; simpler manufacture; fast scanning; reduced sensitivity to environmental factors; robustness; resolution; accuracy; low power; high speed, reduced thickness and/or weight, etc..

We refer to: UK Patent Application No 2003845.1 in the name of Sonuus Limited Computer Input Devices', published as GB2584763A on December 16, 2021.

UK Patent Application No. 2108161.7 in the name of Sonuus Limited: 'Position or movement sensing system', filed on June 8, 2021 (currently unpublished).

SUMMARY

According to a first aspect of the present invention, there is provided a displacement or pressure sensor comprising: a transmitter comprising an open loop antenna and having an input node to receive an RF drive signal to drive the antenna to transmit an RF signal; a receiver separated from the antenna, the receiver comprising an inductor to receive the RF signal and having at least one output node to output an output RF signal, wherein the receiver is configured to generate the output RF signal having at least one of a phase and amplitude dependent on a position of the antenna relative to a position of the inductor.

Such a sensor may enable detection or measurement of applied pressure, and/or distance, position, movement or velocity of, the transmitter relative to the receiver (or vice versa) when the sensor is actuated. Such actuation may involve pressure being applied to a transmitter and/or receiver side of the sensor eg by a user touch. Advantageously, the open loop antenna (ie single ended or monopole antenna; the antenna may for example comprise an open loop coil and/or a single-ended conductor such as a preferably linear wire or PCB trace) generally allows such detection or measurement to be performed even when the relative vertical or lateral change of position of the transmitter and receiver is very small, eg less than about 10mm, 1mm, 0.5mm or 0.1mm. Generally, only near-field emission may occur in an embodiment, eg due to reactive and/or evanescent coupling, the near-field range being very close to the antenna and thus allowing sensing of very small displacements. Merely as an example, in a prototype embodiment of the invention, an 8x8mm coil allows 0.2 -0.4 mm movement sensing. Correspondingly, the separation may be less than about 10mm, e.g., about less than about 1mm, 0.5mm or 0.1mm, so that the sensor may be very thin, for example when the transmitter and receiver are fabricated within respective PCBs or respective layers of a shared multilayer PCB. Flexibility of such PCB(s) may provide further advantages in practical applications such as wearables etc. The receiver inductor generally comprises at least one loop. Sensitivity of a sensor may be enhanced by increasing a number of loops or turns of the transmitter and/or receiver (some embodiments may have eg 1 or 5 turns for a receiver coil). Either or both of the transmitter antenna and receiver inductor may be planar.

Also advantageously, embodiments may result in lower power consumption and/or lower EMC emissions. This may be related to low quiescent current, in turn owing to lack of current flow in the open loop transmitter antenna. The EMC of a small antenna that does not radiate into the far field is generally low.

There may further be provided the sensor, configured to allow the separation between the antenna and the inductor to vary, wherein the dependence on the positions comprises a dependence on the separation. For example, the dependency may be on variable length of the shortest distance between the open loop antenna and receiver inductor.

There may further be provided the sensor, configured to allow at least one of the antenna and the inductor to move laterally, wherein the dependence on the positions comprises a dependence on a lateral offset between the positions (eg, an absolute offset between centre positions of the antenna and inductor, or a lateral change of position of one of the antenna or inductor relative to the other). A lateral movement may comprise movement parallel to a plane separating the transmitter and receiver, eg parallel to the antenna and/or inductor where either or both are planar. More specifically, the movement may be perpendicular to a shortest virtual straight line connecting the antenna to the inductor. An extent of the separation may be unchanged by the movement, for example if the antenna and inductor are planar and one of them merely slides over / under the other. In embodiments, the lateral movement may cause stretching of part of the receiver (eg of a flexible PCB comprising the receiver inductor and/or of a separator element) and elasticity of that part may allow the sensor to return to its initial state once touch / pressure is removed. Advantageously, detection or measurement of lateral movement (with or without similar detection/measurement of vertical movement) may for example allow detection/measurement of nuances of a pianist's touch on a piano key, or pitch bend or vibrato such as on stringed instrument fingerboard, eg violin, cello, viola, double bass, guitar or lute. More generally, lateral displacement or pressure detection may be used to enhance actuation or touch detection based primarily on the separation.

There may further be provided the sensor, wherein a length, L, of the antenna satisfies the equation L5A/2rr, where A is the wavelength of the RF drive signal. Generally, this may ensure that the coupling of energy from the said transmitter antenna to the receiver is primarily near-field (evanescent) coupling.

There may further be provided the sensor, comprising: an RF signal detector to detect an amplitude or phase, or a change of a said amplitude or phase, of the output RF signal, wherein the sensor is configured to determine, based on the detection,: the position, or a change thereof, of the antenna or the inductor; and/or a pressure, or change thereof, applied to the antenna and/or the receiver inductor. The result of the determination (detection and/or measurement) may comprise an analog or digital output indicating / representing a value of the determination result. A calibration of the sensor may be performed to assist subsequent measurement of an absolute value such as an absolute position or change thereof.

The RF signal detector comprises a phase sensitive detector preferably having a lock-in amplifier. Such a lock-in amplifier may enhance detection of very small signals and/or improve performance in noisy environment.

Alternatively, the RF signal detector may comprise an amplitude-sensitive demodulator preferably having a peak hold detector. Such a peak hold detector may be advantageous for example if the sensor is operated using a burst drive mode.

There may further be provided the sensor, wherein the receiver inductor is electrically open, preferably wherein the receiver inductor comprises an open loop coil. Thus, in some embodiments the receiver inductor in addition to the transmitter antenna may be open at one end, eg may comprise a floating loop or coil. For simpler or less costly manufacture, the receiver inductance and transmitter antenna may then be substantially identical, eg where a returning conductive trace is not required for a receiver loop/coil. Advantageously, this may allow the transmitter and receiver to be formed in substantially identical PCBs or a symmetrical multi-layer PCB.

There may further be provided the sensor, wherein the receiver inductor is electrically closed, preferably wherein the receiver inductor comprises a closed loop or double-ended coil. To provide a second output node, the loop(s) eg coil may have a returning conductive trace when fabricated in/on a PCB.

There may further be provided the sensor, comprising an electrically insulating and deformable separator element between the transmitter and the receiver. Preferably, the separator is elastically deformable. Thus either or both position(s) may change responsive to varying pressure on the sensor and return to an initial state when the pressure (eg by a human touch) is removed. This may be advantageous for pressure sensing, for example where a sensor is momentarily depressed by a finger and afterwards returns to its original state, Where a distinct solid, preferably deformable, separator element is not present, the separation may be provided by an air gap or by an intermediate material such as that of PCB(s) having the antenna and/or inductor.

There may further be provided the sensor, wherein the separator element allows lateral movement of the antenna and/or of the inductor. Such a separator element may be rigid.

Embodiments provide a sensing system comprising a plurality of sensors of any preceding claim, the system comprising at least one antenna element and at least one receiver element separated from and crossing at least one said antenna element, wherein: each said antenna element forms the open loop antenna of each of a plurality of the sensors; and/or each said receiver element forms the receiver inductor of each of a plurality of the sensors. The plurality of sensors may form a matrix or array, however the sensors are not necessarily regularly spaced or positioned. Generally, an antenna element may be elongate and may provide a low impedance path. The open loop antennae of the element may or may not be distinct, eg the antenna element may comprise one shared preferably linear trace lacking distinct coils, or may comprise distinct, single-ended coils. When the antenna and/or receiver element(s) are planar, eg linear, this may allow manufacture of the receiver and/or transmitter in a single layer thus potentially reducing costs.

There may be provided the sensing system, comprising a multiplexer to receive at least one selection signal to select a sensor, the multiplexer configured to couple a said RF drive signal to a said antenna element forming the open loop antenna of the selected sensor. Thus, drive circuitry may be shared by the transmitters of the multiplexed sensors. For example, the multiplexer may receive the RF drive signal directly from a shared drive signal generator, or via a wave shaper such as a filter or sine-wave generator (so that the transmitter elements do not receive eg digital square waves).

There may be provided the sensing system, comprising a multiplexer configured to couple to an RF signal detector a said output RF signal from a said receiver element forming the receiver of the selected sensor. The RF signal detector is generally configured to detect amplitude or phase, or change thereof, of the coupled RF signal. Such detector circuitry may be shared by the receivers of the multiplexed sensors, eg a single detector (and thus any analog to digital convertor (ADC) and/or digital processor circuitry to which a detection result is input) may be used for multiple sensors.

There may be provided the sensing system, wherein each said antenna element comprises a linear conductor such as a linear wire (eg copper tape) or PCB trace.

There may be provided the sensing system, wherein each said antenna element comprises serially connected, preferably multi-turn, coils that each form the open loop antenna of a respective said sensor.

There may be provided the sensing system, wherein each said receiver element comprises a single-or multi-turn coil that forms the receiver inductor of at least one sensor, preferably wherein turns of adjacent said coils are wound in opposite directions. In embodiments, a receiver inductor may comprise a single loop or a multi-turn coil; two adjacent coils may be formed of a figure-of-8 coil. Regardless, the opposite directions, generally provided by clockwise and anti-clockwise windings, may allow external electromagnetic (EM) fields of the coils to substantially (eg fully) cancel,

preferably in the far-field.

There may be provided the sensing system, comprising multiple said antenna elements, wherein each said receiver element is formed as a loop to surround a said open loop antenna of each of the antenna elements, wherein a spacing between opposite sides of the loop narrows between adjacent said open loop antennae. Such narrowing may allow the signal pickup region(s) to be localised and may reduce noise pickup. Each open loop antenna preferably had as a least one turn or loop, eg is a single-or multi-turn coil.

There may be provided the sensing system, comprising a shielding element such as a ground trace between adjacent said receivers or receiving elements, preferably wherein the sensing system comprises multiple said receiver elements each having oppositely wound coils and a said shielding element is disposed between adjacent said receiver elements. Such a ground trace may reduce (eg avoid) cross-talk between sensors, for example if the receiver inductors are packed very close together.

There may be provided the sensing system, wherein a plurality of the sensors are each configured to inductively couple from the open loop antenna of the sensor to the receiver inductor of the sensor a said RF signal having a respective said wavelength, wherein the respective wavelengths (and thus frequencies) of the sensors are different, preferably wherein said sensors that couple different wavelengths / frequencies are adjacent. In this regard, embodiments may have RF signal generator(s) configured to generate drive signals of different wavelength(s) and/or RF detectors(s) responsive to respective RF frequencies, eg having band pass filters and/or lock-in amplifiers on their inputs.

There may be provided the sensing system, comprising a multiplexer configured to receive at least one selection signal to select multiple said sensors, the multiplexer configured to simultaneously (eg overlapping in time) couple to the open loop antennae of the selected sensors a said RF drive signal, wherein wavelengths of the respective RF drive signals coupled to respective said open loop antennae are different. Similarly as above, embodiments may have RF signal generator(s) configured to generate drive signals of different wavelength(s) and/or SF detectors(s) responsive to respective SF frequencies, eg having band pass filters and/or lock-in amplifiers on their inputs.

There may be provided the sensing system, wherein at least one said antenna element comprises the open loop antenna of each of a plurality of the sensors, wherein the open loop antennae comprise open loop coils (single-or multi-turn) connected in parallel. Each open loop coil (or loop(s)) may be directly connected at one end to a preferably linear part of the antenna element, eg a preferably fatter, lower resistance track. The parallel coils may be preferred where multiplexing of SF drive signal is employed. Such multiplexing may mean that, if pressure is applied to one transmitter antenna eg coil, this may affect other such antennae and thus result in cross-talk within the output signals from the receivers. The effect is generally greater when the transmitter antenna are effectively connected in series within a single transmitter element. Consequently, a parallel arrangement may be preferred. An antenna element that lacks a closed loop may lend itself to such an arrangement. Such an antenna element additionally or alternatively lends itself to manufacture of the transmitters using a single layer, eg a single-layer PCB or flexi circuit. In this regard, it is noted that a conventional laptop keyboard may have 2 PCBs overlaid to give a connection when pressed, therefore it is desirable to replace these similarly with 2 PCBs (or otherwise a single multilayer PCB) potentially resulting in no extra costs to the manufacturer.

The sensing system may be configured to couple a said SF drive signal to a said antenna element, the sensing system comprising at least one voltage or current regulator to maintain the provided RE drive signal at a predetermined level. Thus, the drive signal may be maintained substantially constant, potentially thus reducing the above-mentioned cross-talk. In embodiments, the regulator may be implemented using an opamp with negative feedback.

There may be provided the sensing system, wherein at least one said receiver element comprises coils connected in parallel, each coil forming the receiver inductor of the receiver of a respective said sensor. Each coil may again be single-or multi-turn. The receiver inductors may be provided as coils connected in parallel via a part of the receiver element (eg a low impedance linear trace) to a receiver output node. The system may further comprise at least one signal sensing circuit coupled to a said antenna element forming the open loop antennae of a plurality of said sensors, the signal sensing circuit to detect variation of an SF drive signal present on the antenna element, wherein the sensing system is configured to compensate at least one output RF signal of a said sensor, preferably by applying an offset to a result of the determination by the RF signal detector. Additionally or alternatively, software algorithms may be used perform compensation.

Embodiments of the sensor or sensing system may comprise: a driver to generate a said RF drive signal to drive a said open loop antenna of a said sensor; a drive enable circuit to generate a control signal to enable and disable the driver; a sample-and-hold circuit for an amplitude signal indicating the amplitude of a said output RF signal at the receiver, wherein the sample-and-hold circuit is to hold a peak level of the amplitude signal, the peak level being a peak level during a period when the driver is enabled; and a measurement circuit to detect a relative position and/or movement between the open loop antenna and the receiver inductor of the sensor based on the held peak level, the detection based on measuring the held peak level when the driver is disabled by the drive enable circuit.

Embodiments may be or comprise a key, keypad or keyboard, optionally for a laptop computer or musical instrument such as an electronic piano, comprising the sensor or sensing system.

Embodiments may be or comprise a touch pad, optionally for a laptop computer or for a musical instrument, preferably for a fingerboard of a stringed instrument such as a violin or guitar, the touchpad comprising a sensor or sensing system of any preceding claim. The touch pad may for example be in the form of a computer touchpad to detect finger movement to guide a screen cursor or a touchpad for a fingerboard of a string instrument and preferably to detect vibrato.

Embodiments may be or comprise a touch screen, optionally for a computer, the touch screen comprising the sensor or sensing system of any preceding claim, preferably wherein at least one of the antenna and receiver inductor, eg loop(s) or coil, comprises ITO. Use of ITO, and/or for example very thin wires or PCB traces, may allow the antenna and/or receiver (preferably the entire sensor) to be substantially transparent or visually imperceptible. Use of single PCB layers may allow putting the transmitters on the front and the receivers on the back of the touch screen, whereas a multi-layer embodiment that is substantially transparent may be placed on the front of the touchscreen.

According to another aspect of the present invention, there is provided a temperature compensation method for the sensor or sensing system, the method comprising: measuring, while the transmitter antenna of a said sensor is not being driven, a quiescent output of an RE signal detector configured to perform measurements of a said SF output signal of a sensor; calculating a temperature-dependent offset in the quiescent output signal based on a difference between a first said measurement and a subsequent said measurement; and measuring a said SF output signal of the sensor when the transmitter antenna is being driven; and adjusting the measurement of the SF output signal when the transmitter antenna is being driven, based on the offset. In this regard, where a receiver or receiving element has a bias voltage, when a measurement of the quiescent output signal is performed while the corresponding SF drive signal is turned off, temperature-dependent variations in said bias voltage and of electronic components connected to said bias voltage cause variations in the quiescent output signal and thus temperature-compensation may be performed.According to a related aspect of the invention, there is provided a non-transitory data carrier carrying processor control code which when running on a processor, causes the processor to implement the above-described method. Thus, an aspect further provides processor control code to implement the above-described methods, for example on a general purpose computer system or on a digital signal processor (DSP). The code may be provided on a transitory or non-transitory carrier such as a disk, a microprocessor, CD-or DVD-ROM, programmed memory such as non-volatile memory (e.g. Flash) or read-only memory (Firmware). Code (and/or data) to implement embodiments of the invention may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code. As the skilled person will appreciate such code and/or data may be distributed between a plurality of coupled components in communication with one another.

Regarding terminology, the present specification refers primarily to 'displacement', which may correspond to: a position preferably relative to a reference, eg a position of a sensor transmitter relative to a sensor receiver, or vice versa; or a movement comprising a change of such a position, eg a distance (absolute or directional) moved by a sensor transmitter relative to a sensor receiver, or vice versa; and/or a velocity (eg speed) corresponding a rate of change of such a position.

Sensing of such a displacement may detect and/or measure the displacement. Thus, the sensing may merely indicate if a movement of a sensor transmitter and/or receiver has occurred, thus for example detecting actuation or touch of a key, touchpad or touch screen. Additionally or alternatively, the sensing may measure the displacement and thus output a measured value.

More generally, references herein to 'displacement' are interchangeable with eg 'pressure', 'distance', 'position', 'movement' or velocity, so that the term 'displacement sensor' is similarly interchangeable with 'pressure sensor', 'distance sensor', 'position sensor', 'movement sensor' or 'velocity sensor'. More specifically, the term 'displacement sensor' may be interchangeable with 'touch sensor'. Any such sensor may be configured to sense pressure, distance, position, movement or velocity, any of which may be along a vertical or lateral direction of a sensor. In this regard, 'vertical' is generally in a direction across a separation between a transmitter and a receiver of a sensor, and 'lateral' is generally parallel to a plane separating the transmitter and receiver, eg parallel to a planar receiver inductor and/or parallel to a planar transmitter antenna.

We further note that references herein to pressure sensing may relate to detection of eg touch and/or a measurement of an amount of pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 shows an electronic circuit of a displacement sensor according to embodiments.

Fig. 2A shows an example printed circuit design, on an enlarged scale, for a transmitter.

Fig. 2B shows an example printed circuit design, on an enlarged scale, for a receiver.

Fig. 3A shows an example configuration for measuring separation displacement.

Fig. 38 shows an example configuration for measuring lateral displacement.

Figs. 4A shows an electronic circuit for interfacing a displacement sensor receiver, wherein said receiver comprises a closed-loop coil, according to embodiments.

Figs. 4B shows an electronic circuit for interfacing a displacement sensor receiver, wherein said receiver comprises an open-loop coil, according to embodiments.

Fig. 5 shows a phase-sensitive-detector suitable for detecting the output of the displacement sensor receiver according to embodiments.

Fig. 6 shows an amplitude-sensitive detector circuit suitable for detecting the output of the displacement sensor receiver according to embodiments.

Fig. 7 shows a plurality of displacement sensors configured as a matrix according to an embodiment.

Fig. 8 shows an example of a simplified printed circuit design, on an enlarged scale, for a plurality of displacement sensors configured as a matrix.

Fig. 9 shows an example of a simplified printed circuit design, on an enlarged scale, for a plurality of transmitters wherein each of a plurality of transmitter elements comprises a plurality of antenna coils.

Fig. 10 shows example of a simplified printed circuit design, on an enlarged scale, for a plurality of receivers wherein each of a plurality of receiver elements comprises a plurality of receiver coils.

Fig. 11A shows an example of a simplified printed circuit design, on an enlarged scale, for a plurality of transmitters wherein each of a plurality of antenna elements comprises a plurality of antenna coils.

Fig. 11B shows an example of a simplified printed circuit design, on an enlarged scale, for a plurality of receivers wherein each of a plurality of receiver elements comprises a plurality of open-ended receiver coils.

Fig. 11C shows how the simplified printed circuit designs of Fig. 11A and Fig. 11B may be overlaid to form a displacement-sensing matrix.

Fig. 12 shows an example electronic circuit for a displacement-sensing matrix.

Fig. 13 shows a laptop keyboard comprising a displacement-sensing matrix according to an embodiment.

Figs. 14A and 14B each show a touchscreen comprising a displacement-sensing matrix according to an embodiment. In Fig. 14A, a display element is interposed between transmitter and receiver planes; In Fig. 14B a display element is positioned behind the transmitter and receiver planes.

Fig. 15 shows a laptop touchpad comprising a displacement-sensing matrix according to an embodiment.

Fig. 16 shows a block diagram of an inductive position sensing system according to an embodiment.

Fig. 17 shows a flowchart of a measurement process according to an embodiment.

Fig. 18 shows an example timing diagram of a measurement process of a single position sensor.

Fig. 19 shows an example timing diagram of a multiplexing scheme, corresponding to multiple measurements of a plurality of example sensors.

DETAILED DESCRIPTION

A simplified circuit diagram of a displacement sensor according to embodiments is shown in Fig. 1 wherein a transmitter comprising an input node 1 and an antenna 2 is coupled to a receiver comprising: at least one receiver coil 3; a primary output node 5; and a secondary output node 6 (any node mentioned herein may alternatively be referred to as a terminal). A drive signal at a frequency is applied to the said input node 1 and a corresponding signal is coupled via said antenna 2 to said receiver coil 3 whereafter an output signal is presented at said primary output node 5 and said secondary output node 6 which varies as the relative displacement 4 of said antenna 2 and said receiver coil 3 is varied. Said receiver coil 3 may be a closed loop, in which case both said primary output node 5 and secondary output node 6 are available, or an open loop, in which case only said primary output node 5 is available and the other end of said receiver coil remains disconnected.

In embodiments, antenna and receiver loop(s)/coil(s) may be formed of electrically conductive materials including but not limited to: metallic wire; electrically-conductive ink; and/or electrically-conductive traces on a rigid or flexible printed circuit board (PCB). For embodiments having transmitter(s) and receiver(s) in a flexible single layer, through-connections are generally not required so that sensors or sensing systems may be manufactured using roll-to-roll printing, potentially allowing very low costs.

In embodiments, the transmitter antenna is electrically connected to the transmitter input node at a single point and other points of said antenna remain disconnected.

Thus, in embodiments where the shape of said antenna forms a coil or loop, one end of said coil or loop (e.g., refer to 41 in Fig. 2A) remains disconnected; i.e., said antenna is formed as an open loop. This may have the advantage of substantially reducing the quiescent power required to drive the said antenna because there is no direct path for a drive current to flow in a circuit from a signal driver via said input node through said antenna and then return to said signal driver.

In embodiments, the length of the transmitter antenna is electrically short, i.e., said length of said antenna, L, satisfies the equation L A/ 27r, where A is the wavelength of the drive signal. This may ensure that the coupling of energy from the said transmitter antenna to the receiver is primarily near-field (evanescent) coupling. This has potentially two advantages: to substantially reduce the magnitude of unwanted electromagnetic radiation from said antenna; and/or to ensure that at small distances from said antenna the energy level surrounding said antenna varies significantly with distance thus permitting small displacements near said antenna to be detected.

Thus by employing a transmitter antenna that is both formed as an open loop and that is electrically short, the sensing of small displacements using a displacement sensor of an embodiment may be enhanced. For example, in embodiments, a displacement sensor comprising a transmitter antenna formed as an open loop of about 8mm diameter and a similarly-sized receiver coil may accurately sense displacement between said transmitter antenna and said receiver coil in the range eg about Omm to 0.4mm. Conversely, employing a transmitter antenna formed as a closed loop may generally produce a magnetic field that extends to a distance from said transmitter antenna of at least the radius of said closed loop and that does not vary significantly with distance from said closed loop when said distance from said closed loop is small.

Thus, sensing small displacements between a transmitter antenna and a receiver coil may be difficult, potentially impossible, when using a transmitter antenna formed as a closed loop.

An illustrative example of a suitable design for the transmitter is shown in Fig. 2A and for the receiver is shown in Fig. 2B in which the antenna and the receiver coil are formed as traces on a printed circuit board. Referring to Fig. 2A, said antenna 2 is formed preferably (ie optionally) as a spiral conductive trace on a single layer of a printed circuit board, connected at preferably only one point to the input node 1 via a connecting conductive trace 43; the other end 41 of the said spiral conductive trace of said antenna 2 remains disconnected. Referring to Fig. 2B, said receiver coil 3 is formed as an open-loop or a closed-loop eg multi-turn coil preferably comprising at least one spiral conductive trace on at least one layer of a printed circuit board. One end of said receiving coil may be connected via an additional conductive trace to the primary output node 5. In the case where said receiver coil 3 is formed as a closed-loop, a returning conductive trace 40 on another layer of said printed circuit board may connect the opposite end of said receiver coil to the secondary output node 6. Although the said spirals for said antenna and for said receiving coil are shown as being constructed on a single layer of a printed circuit board, in some embodiments it may be preferable to form these said spirals on a plurality of printed circuit board layers.

A displacement sensor according an embodiment can sense displacement because the output signal of the receiver varies as the relative displacement of the transmitter antenna and the receiver coil of said receiver varies due to variations in the coupling, caused by said relative displacement, between said transmitter antenna and said receiver coil. Said variation in coupling produces a variation in the magnitude and/or phase of the signal that is received by said receiver coil.

In some embodiments, as shown in Fig. 3A, the relative displacement of the transmitter antenna 2 (formed on a transmitter plane 8) and the receiver coil 3 (formed on a receiver plane 7) is the varying separation 10 between said transmitter plane 8 and said receiver plane 7. Said transmitter plane 8 and said receiver plane 7 are separated by a separator element 9 which allows said transmitter plane 8 to move relative to said receiver plane 7, and/or which allows said receiver plane 7 to move relative to said transmitter plane 8. Said separator element 9 may be deformable or may be rigid. In the case where said separator element 9 is rigid, it is preferably positioned adjacent to said transmitter antenna and said receiver coil. Pressure applied to said transmitter antenna 2 (or conversely to said receiver coil 3) causes movement or deformation (flexing) of said transmitter plane 8 (or conversely to said receiver plane 7). Thus variations in pressure may be measured by measuring the displacement caused by the application of said pressure to said sensor. (Some embodiments may have an air gap between the planes, rather than a distinct separator element. Other embodiments may have an air gap or distinct separator, for example if one or more of the transmitter and receiver of the/each sensor is embedded in a preferably flexible PCB, eg both are within the same multi-layer PCB).

In some embodiments, as shown in Fig. 3B, the relative displacement of the transmitter antenna 2 (formed on a transmitter plane 8) and the receiver coil 3 (formed on a receiver plane 7) is the lateral movement 11 of said transmitter plane 8 with respect to said receiver plane 7.

To measure the output of the displacement sensor, the receiver is generally interfaced to a detector.

Fig. 4A and Fig. 4B show electronic circuits which are used in some embodiments to interface the receiver to a detector which processes a preferably single-ended signal. As shown in Figs. 4A and 43, said receiver primary output node 5 is connected to a biasing voltage 14 via a biasing resistive element 44. In embodiments where the receiver coil of said receiver is formed as a closed loop, the secondary output node 6 of said receiver may be connected to a single-ended output node 13, as shown in Fig. 4A.

In embodiments where the receiver coil of said receiver is formed as an open loop, the primary output node 5 of said receiver may be connected to a single-ended output node 13. The biasing voltage may be any voltage that is compatible with the detector; this may in embodiments be a positive voltage, negative voltage or a zero voltage.

Some embodiments employ a phase-sensitive-detector to process the signal from the receiver. Such a phase-sensitive detector may comprise a lock-in amplifier. As shown in Fig. 5, a phase-sensitive detector may in an embodiment comprise: a signal multiplier 17 connected to said receiver output 13; a reference input 15 connected to a reference signal synchronous to the drive signal connected to the transmitter; a phase shifting element 16 to phase-shift said reference signal; a low-pass filter 18 to filter the output of said signal multiplier 17; and an output 19 from where said phase-sensitive detector output is obtained On embodiments any one or more of the elements of Fig. 5 may be omitted).

Another suitable detector configuration employed in some embodiments is an amplitude-sensitive demodulator. As shown in Fig. 6, such a demodulator may rectify the receiver output 13 using a diode 20 which forms a peak-hold circuit with a capacitive element 21. The output of said peak-hold circuit is presented at an output 19, optionally via a buffer or amplifier 23. During peak-hold a switch 22 is in the open (high-impedance) state, allowing said capacitive element 21 to charge. The peak-hold circuit is reset by closing said switch 22 to allow said capacitive element 21 to discharge. A new peak-hold cycle is then initiated by opening said switch 22. Thus, multiple peak-hold operations may occur sequentially. (In embodiments, any one or more of the elements of Fig. 6 may be omitted).

In many systems it is desirable to measure displacement at a plurality of locations; sequentially, simultaneously, or a combination of sequentially and simultaneously. Although this can be achieved by employing a plurality of displacement sensor systems, whereby each said sensor system comprises a displacement sensor with drive circuitry and detection circuitry, it is advantageous to share said drive circuitry and/or said detection circuitry between two or more displacement sensors by using a multiplexing scheme. Such a said multiplexing scheme, for convenience, may refer to shared drive circuitry as columns and shared detection circuitry as rows and such nomenclature shall be used hereinafter as a means of clarification.

The displacement sensor of an embodiment can be used advantageously in a multiplexing scheme to measure displacement at a plurality of locations. Particular advantages include, but are not limited to: low power; high speed; low cost; high precision displacement measurement; and/or robustness to external interference. In a sensing system implementing such a multiplexing scheme, an arrangement, eg matrix, of displacement sensors may be formed by overlap regions between a plurality of transmitter antennae (columns) each of which is connected to a respective input node and a plurality of receiver coils (rows) wherein each of said plurality of receiver coils is part of a receiver. Preferably, at any time at most one of said input nodes is active, hereinafter referred to as the active input node, whereby a drive signal is applied to said active input node thereby driving the respective antenna, known as the active antenna, connected to said active input node. Each of said receivers may be connected to a unique detector and displacement measurements performed simultaneously or each of said receivers may be connected to a shared detector using a switching scheme and displacement measurements performed eg sequentially. Thus it is possible to measure displacement at each said displacement sensor where said active antenna overlaps said receiver coils. Furthermore, displacement may be measured at all said displacement sensors in said matrix by sequentially activating each of said input nodes and measuring the output of each said detector.

In some embodiments of a sensing system, an arrangement eg matrix of displacement sensors may comprise: one antenna (column) and a plurality of receiver coils (rows); or a plurality of antennae (columns) and one receiver coil (row). Each such antenna may be referred to as an antenna element that in effect forms or comprises the open loop antenna(e) of one or more sensors. Each of the receiver coils may be referred to as a receiver element.

Fig. 7 shows a sensing system embodiment comprising (merely by way of example) a 4x5 matrix of displacement sensors involving 4 receiver elements each crossing 5 antenna elements. The sensors are formed at the overlap regions between a plurality of transmitter antennae 2 (columns) and a plurality of receiver coils 3 (rows) wherein each of said plurality of receiver coils is a receiver element formed as a single-turn loop and each of said transmitter antennae is an antenna element formed linearly. Thus, as shown in Fig. 7, each receiver coil 3 is a receiver element comprising a single-turn loop and each antenna element is an open linear element in effect providing the open loop antennae of multiple On this case 4) sensors. This embodiment may have the advantage of being simple to construct but also may have disadvantage(s): the amplitude of the signal coupled from each of said plurality of antennae to each of said plurality of receiver coils may be small; and/or each receiver coil of said receiver coils may be susceptible to undesirable external interference signals (pick-up).

A further sensing system embodiment which may overcome disadvantage(s) of the embodiment shown in Fig. 7 is shown in Fig. 8, again showing merely by way of example a 4x5 matrix. In such an embodiment: each of a plurality of transmitter antennae 2 is an antenna element having serially-connected coils to create a plurality of localised transmitting regions On this case each element having 4 coils); and/or each of a plurality of receiver elements is a receiver coil or loop 3 shaped to have a plurality of localised receiving regions whereby each receiving region of said plurality of receiving regions overlaps one said transmitting region of said plurality of transmitting regions of said plurality of transmitting antennae (ie antenna elements). Transmitting regions thus formed with coils increases the amplitude of the signal coupled from each of said plurality of antennae to each of said plurality of receiver coils. Furthermore, receiving regions thus formed by shaping reduce the susceptibility of each receiver coil of said plurality of receiver coils to undesirable external interference signals.

In some embodiments, a first receiver coil of a first receiver may be positioned adjacent to, and in close proximity to, a second receiver coil of a second receiver. Such said close proximity can permit interference (crosstalk) between said first receiver and said second receiver whereby excitation of the said first (or second) receiver coil couples a corresponding signal into the said second (or first) receiver coil which is detrimental to displacement sensing. Said interference can be mitigated in different ways: by introducing a shielding element (for example, a grounded trace on a printed circuit board) between said first receiver coil and said second receiver coil; and/or, by operating said first receiver at a substantially different frequency of operation than that of said second receiver, for example by employing band-pass filtering or phasesensitive-detection in the detectors connected to said first and said second receivers.

In sensing system embodiments where an arrangement eg matrix of displacement sensors comprises a plurality of receivers configured to operate at two or more frequencies of operation, a multiplexing scheme may apply drive signals to the transmitter antenna input nodes at two or more frequencies at or close to said frequencies of operation. Said drive signals may be applied sequentially, with at most one drive signal frequency active at a given time, or simultaneously with more than one drive signal frequency active at a given time, whereby the active displacement sensors of said matrix of displacement sensors are those corresponding to the overlap regions where a receiver coil that is part of a receiver overlaps a transmitter antenna being driven at a frequency equal to or close to frequency of operation of said receiver.

In some embodiments comprising an arrangement eg matrix of displacement sensors where at least one transmitter antenna is coupled to a plurality of receiver coils there may be interference (crosstalk) between displacement sensors which share at least part of the same transmitter antenna (for example as shown in any of Fig. 7 -9, 11A and 11C wherein at least a linear part of an antenna element is shared between sensors) whereby displacement at a first displacement sensor, corresponding to a first location on said transmitter antenna produces a change in output at a second displacement sensor, corresponding to a second location on said transmitter antenna, when no physical displacement has occurred at said second displacement sensor. Said interference occurs when a change in coupling between said transmitter antenna at said first location and the corresponding receiver coil overlapping this said first location results in a variation in the drive signal (current and/or voltage) present on said transmitter antenna. A configuration to mitigate said interference is shown in Fig. 9 wherein each antenna element has transmitter coils formed as parallel-connected coils 25 to create a plurality of localised transmitting regions. Said parallel-connected coils 25 are preferably connected via a low-impedance path 24 of the antenna element to a shared input node 1 of a transmitter. To further minimise said interference, the drive signal applied to said input node 1 may be regulated to maintain a constant signal amplitude, for example via feedback. A further advantage of this configuration compared to the configuration shown in Fig. 8 is that the transmitter antennae may be formed on a single layer of a printed circuit board, whereas the transmitter antennae shown in Fig. 8 may in embodiments use at least two layers of a printed circuit board to serially-connect the coils.

A further method to mitigate interference (crosstalk) between displacement sensors which are coupled to the same transmitter antenna is by configuring a matrix of displacement sensors to have at least one non-sensing receiver (i.e., a receiver that is not used for displacement sensing) coupled to said transmitter antenna. The non-sensing receiver may alternatively be referred to as a signal sensing circuit. Variations of the output signal of said non-sensing receiver are produced when the drive signal present on said transmitter antenna varies (current and/or voltage) due to changes in coupling between said transmitter antenna and other receivers as the displacement between said transmitter antenna and said other receivers varies. The said variations of the said output signal of said non-sensing receiver may be used to compensate the output of said displacement sensors, for example by: performing a first measurement of the output of said non-sensing receiver at a first time when the displacement between said transmitter antenna and said other receivers is at a reference value (for example during a calibration); performing a second measurement of the output of said non-sensing receiver at a second time when the displacement between said transmitter antenna and one or more of said other receivers has varied; calculating the difference between said first measurement and said second measurement; applying an offset to the output of said displacement sensors wherein said offset is proportional to, or inversely proportional to the said difference.

An additional or alternative method to mitigate interference (crosstalk) between a plurality of displacement sensors coupled to the same transmitter antenna is by using software algorithms to process the detected outputs from the said plurality of displacement sensors to calculate offsets required to compensate the said interference.

Receiver coils in embodiments may be susceptible to undesirable external interference signals (pick-up). A particularly effective configuration of said receiver coils that may be used to minimise said pick-up is to form each receiver coil of said receiver coils as a series of smaller coils whereby a first set of said smaller coils are wound in a direction opposite to a second set of said smaller coils such that the external electro-magnetic field coupled into said first set substantially cancels the external electro-magnetic field coupled into said second set.

Fig. 10 shows a plurality of receiver elements which, when overlaid with the transmitter elements shown in Fig. 9, forms a preferred embodiment of a matrix of displacement sensors suitable for multiplexing which reduces crosstalk between said displacement sensors and which reduces pick-up from external interference wherein: receiver elements ie receiver coils 3, are formed as a series of smaller coils would in opposition 27 to cancel undesirable external interference signals; and said receiver elements / coils are separated from adjacent receiver elements by shielding elements 26 to minimise crosstalk between adjacent receiver coils.

Similarly as mentioned above in relation to shared transmitter antennae, in some embodiments comprising a matrix of displacement sensors where at least one receiver coil is coupled to a plurality of transmitter antennae there may be interference (crosstalk) between displacement sensors which share the same receiver coil whereby displacement at a first displacement sensor, corresponding to a first location on said receiver coil produces a change in output at a second displacement sensor, corresponding to a second location on said receiver coil, when no physical displacement has occurred at said second displacement sensor. Said interference occurs when a change in coupling between said receiver coil at said first location and the corresponding transmitter antenna overlapping this said first location results in a variation in the output signal (current and/or voltage) present on said receiver coil. A configuration that may be used to mitigate said interference is to form said receiver element or coil as a plurality of parallel-connected coils, similar to the configuration of transmitter antenna shown in Fig. 9. Fig. 11B shows such a configuration wherein a plurality of receivers of each receiver element is formed as a plurality of parallel-connected coils 46 connected via a low-impedance path 45 of the receiver element to a shared primary output node 5 of said each receiver. Fig. 11A shows a corresponding array of transmitter elements which when overlaid on said receiver elements shown in Fig. 118 forms a matrix of displacement sensors, as shown in Fig. 11C. A further advantage of this configuration compared to the configuration shown in Fig. 10 is that the receiver element or coils may be formed on a single layer of a printed circuit board, whereas the receiver element or coils shown in Fig. 10 may in embodiments require at least two layers of a printed circuit board to serially-connect the coils. Note that for the configuration shown in Fig. 11 it is possible for a printed circuit board comprising the said transmitter to be identical to a printed circuit board comprising the said receiver (the printed circuit traces of Fig. 11B may be substantially identical to those in Fig. 11A, albeit rotated by 90 degrees). Thus such a configuration may have a further advantage of simplifying manufacture and/or reducing cost.

An embodiment of a multiplexing scheme to measure the displacement at a plurality of locations using displacement sensors according to an embodiment is shown in Fig. 12 wherein: a processor 28 outputs a drive-control signal 39 eg in the form of a square wave or pulses; said drive-control signal may be processed by a wave-shaper 29 (eg filter or square-to-sine wave generator) to reduce the amplitude of high-frequency signal components to produce a shaped drive-control signal 42; the preferably shaped drive-control signal 42 may be buffered with an amplifier 30 (eg opamp) and may be passed through a resistive element 32 to a first analogue multiplexer 34. The multiplexer 34 applies the drive signal eg sequentially to each antenna element Cie to each of a plurality of transmitter antennae 2), under control of said processor; a default feedback path 33 may in embodiments be provided to stabilise the drive signal to maintain a constant signal amplitude at the input to said first analogue multiplexer 34 to minimise interference between said displacement sensors which share the same transmitter antenna 2; receiver elements comprising receiver coils 3 overlapping said transmitter antennae 2 are coupled to respective or shared detector(s) 37 whereinafter each output signal of each said detector is input to an analogue-to-digital converter (ADC) (in embodiments, part of said processor); preferably, where the internal resistance of said first analogue multiplexer 34 is significant relative to the resistance of resistive element 32, to minimise interference between said displacement sensors which share the same transmitter antenna, the said drive signal is regulated by employing an alternative feedback path where the said drive signal applied to each said transmitter antenna is monitored by a second analogue multiplexer 35 and applied as negative feedback to the inverting input node 36 of said amplifier 30 and in this case the default feedback path 33 may not be not present. A high-impedance feedback resistive element 31 may ensure stability of the said amplifier 30 during switching of the said first analogue multiplexer 34 and/or during switching of said second analogue multiplexer.

(It is noted that each part of the circuitry surrounding the matrix in Fig. 12 is optional, eg wave shaper 29, amplifier 30, second multiplexer 34 etc.. As noted above, the second multiplexer may be desired if the first multiplexer has high resistance (eg 100 Ohms), which may result in a higher voltage drop across the first multiplexer that can be difficult to sufficiently compensate. Alternatively, a digital multiplexer could be used, receiving a square wave, however it may then be more difficult to control cross-talk.

Regardless, use of a regulator eg opamp 30 may be desirable if the drive signal received by an antenna element otherwise decreases at each 'stop' ie sensor along the element, for example due to current limiting by the resistor 32. The processor 28 could be replaced by eg two different processors for generating the drive signal and receiving the signals from the receivers, though tighter synchronisation using a single processor may be more desirable if burst control is used).

In implementations, the frequency of the drive signal used to drive the transmitter antenna may be selected as a fixed frequency from a suitable range of frequencies, or be a variable frequency within the range of said suitable range of frequencies. A lower frequency limit of said suitable range of frequencies may in embodiments be a frequency at which coupling between said transmitter antenna and the corresponding receiver coil is reduced to a level that displacement sensing becomes insensitive. An upper frequency limit of said suitable range of frequencies may be a frequency at which parasitic effects (eg resulting in resonances) become deleterious to displacement sensing. To permit fast multiplexing of displacement sensors in a matrix of displacement sensors, it is advantageous for the said drive signal to be at a high frequency. Conversely, to minimise unwanted high-frequency electro-magnetic radiation emissions, it is advantageous for the said drive signal to be at a low frequency. Scan speed, EMC emissions and/or power may affect selection of the frequency for an embodiment. As an example, the said lower frequency limit may be eg -100kHz, and/or the said upper frequency limit may be -20 or 30MHz. Such a broad range may mean that the driving frequency doesn't need to be tightly controlled, eg a low cost RC oscillator may be used; no processor is required to control it. Regarding scan speed, the drive frequency may limit how fast pulses can be sent to a transmitter or matrix (multiplexing can be performed at high speed). In embodiments, a scanning rate of eg 10kHz may be achieved On contrast, capacitive touch screens generally don't scan quickly eg only 10s or 100s of Hz). Even for a touch pad for human touch, a fast scanning speed may be advantageous for tracking where the user's finger/hand is moving quickly across the touch pad.

In implementations, small changes in displacement can be sensed with a small separation between the transmitter antenna(e) and the receiver coil(s). Consequently, a displacement sensor herein disclosed may be particularly well suited to several applications, including but not limited to a: pressure-sensing multi-touch touchpad; keypad whereby movement of a key of said keypad is coupled to one of a plurality of displacement sensors; pressure-sensing touchscreen; keyboard for computers, especially laptop computers, whereby key(s) on said keyboard are each configured to couple movement to at least one displacement sensor.

Fig. 13 shows an example laptop 50 comprising a keyboard 47 which employs a matrix of displacement sensors according to an embodiment to sense position and/or pressure of the keys of said keyboard wherein: transmitter elements in the form of transmitter antennae 2 are formed on a first flexible printed circuit board (plane) 8; receiver elements in the form of receiver coils 3 are formed on a second flexible printed circuit board (plane) 7 and separated from said first flexible printed circuit board by a separator element 9 preferably comprising apertures to allow said transmitter antennae 2 to deform towards said receiver coils 3 when a key is pressed. Each of said transmitter antennae is aligned with one of said receiver coils and further aligned with one of said keys on said keyboard. The said matrix of displacement sensors does not need to form a regularly spaced two-dimensional grid, but may follow the positions of the said keys on said keyboard.

Fig. 14 shows a touchscreen comprising a display element 51 and a matrix of displacement sensors according to an embodiment wherein said matrix of displacement sensors comprises a plurality of transmitters formed on a transparent transmitter plane 8 and a plurality of receivers formed on a receiver plane 7. Fig. 14A shows one configuration wherein said display element 51 is interposed between said transmitter plane 8 and said receiver plane 7. Fig. 14B shows an alternate configuration wherein said display element 51 is positioned behind said transmitter plane 8 and said receiver plane 7. In this said alternate configuration the receiver plane 7 is preferably transparent.

Fig. 15 shows a laptop 50 comprising a touchpad 48 which employs a matrix of displacement sensors 49 according to an embodiment to sense a plurality of touch positions and touch pressures whereby touch position and/or touch pressure may be determined by interpolation of displacement measurements output from said matrix of displacements sensors 49. The said matrix of displacement sensors may be formed as two single-layer flexible printed circuit boards, for example as show in Fig. 11C, with a separator element interposed between said flexible printed circuit boards.

Regarding other applications of embodiments, these may include inter alia musical instruments, game controllers, power tools, wearable electronic devices etc..

Furthermore, it is noted that embodiments may be flexible (eg using a flexi PCB) and may have irregular arrangements of sensors, eg may not comprise a 2D matrix with a regular grid of sensors. This may be advantageous for wrapping a sensing system around a 3D object such as a controller, potentially providing a 3D structure with pressure points to be touched. This may be of interest regarding eg wearables and/or prosthetics such as robotics hands that can pressure sense when they pick things up.

Temperature Compensation The following relates to embodiments of the displacement sensor described herein, operated in conjunction with a temperature compensation, as described in the above-mentioned GB2108161.7.

It is desirable for the performance of a displacement sensing system to be stable over a range of operating temperatures. Although the transmitter antenna and receiver loop(s)/coil used by embodiments of a displacement sensor as described herein may have excellent temperature stability, other electronic elements in, or characteristics of, the circuit can have properties that change with temperature which may cause a variation in the detected output signal of the said displacement sensor with variations in operating temperature. Such electronic elements/characteristics include but are not limited to: bias voltage 14 at the receiver; diode 20 in the detector circuit; gain of detector amplifier 23; analogue-to-digital converter 38; and/or voltage regulators.

Therefore a temperature compensation scheme may be advantageous to minimise variations in the output signals of a displacement sensor, or of a plurality of displacement sensors in an arrangement eg matrix of displacement sensors, caused by variations in operating temperature.

An example temperature compensation scheme comprises: performing measurements of the output signal of a displacement sensor detector while the transmitter antenna is not active such that changes in displacement between said transmitter antenna and the displacement sensor's receiver coil does not affect the said output signal of said displacement sensor detector; the first of the measurements may be performed during a calibration procedure; subsequent measurements may be performed periodically, eg within additional time slots of a time-division multiplexed scheme; calculating temperature-dependent offsets in the output signal by subtracting subsequent measurements from the first measurement; and adding the offsets to the measurement of the output signal when the transmitter antenna is being driven to measure displacement. Such a temperature compensation scheme may utilise one temperature-dependent offset respectively for: a single displacement sensor; each displacement sensor in a matrix of displacement sensors; each group of displacement sensors in a matrix of displacement sensors (for example a row in a multiplexing scheme); or for all displacement sensors in a matrix of displacement sensors.

In a related aspect there is provided a method of periodically compensating a response of a multiplexed matrix of displacement sensors. Each sensor may comprise a transmitter antenna, receiver loop/coil and detector. The method may comprise retrieving from storage a detected initial output signal of the sensor, Oto, at a first time, to, wherein at to the transmitter antenna is not being driven. The method may further comprise, preferably periodically, for at least one of the sensors, detecting a later output signal of the sensor, Oti, at a time after to when the transmitter antenna is not being driven. The method may then calculate an adjustment value, for example a difference between the initial output signal of the sensor and the later output signal of the sensor. The method may then further comprise compensating the response of the matrix of displacement sensors by adjusting an operational output of the sensor using the adjustment value. The operational output may be an output from the sensor when the transmitter antenna is being driven. The method may further comprise operating the sensor according to a time division multiplexed addressing scheme. The method may then use a "spare" time slot of the time division multiplexed addressing scheme, in which the sensor is not operational, for the detecting.

Advantageously, embodiments may provide good reliability, for example due to a non-contact configuration, eg lacking mechanical switches that may wear out. Additionally or alternatively, embodiments may be manufactured at low cost, similarly for example to a membrane keyboard. Further advantageously, an embodiment may detect a small displacement or pressure with precision, for example an embodiment may allow detection of pressure changes by measuring small displacements of a deformable material.

Burst Drive The following relates to embodiments of the displacement sensor described herein, operated in conjunction with a 'burst drive mode', as described in the above-mentioned GB2108161.7. This may allow improvements in position and/or movement sensing such as reduced electrical power requirements, reduced EMI emissions, greater accuracy, increased sensor output reproducibility, and/or increased sensor speed for example to allow faster scan rates.

In this regard, a displacement or pressure sensor or sensing system embodiment may comprise: a driver to generate a said RE drive signal to drive a said open loop antenna of a said sensor; a drive enable circuit to generate a control signal to enable and disable the driver; a sample-and-hold circuit for an amplitude signal indicating the amplitude of said output RF signal at the receiver, wherein the sample-and-hold circuit is to hold a peak level of the amplitude signal, the peak level being a peak level during a period when the driver is enabled. The sensor further comprises a measurement circuit to detect a relative position and/or movement between the open loop antenna and the at least one loop of the sensor based on the held peak level, the detection based on measuring the held peak level when the driver is disabled by the drive enable circuit.

In some examples, the sample-and-hold circuit may be configured to be effectively isolated from the driver (e.g. by grounding) during a detection by the measurement circuit. For example, the sample-and-hold circuit may have a threshold voltage such that, where an input to the sample-and-hold circuit is below said threshold voltage, the peak level held by the sample-and-hold circuit is unaffected by noise and/or other signals at the input. The sample-and-hold circuit may be implemented digitally or with analogue circuits. In either case, advantageously, it is not necessary that the driver remain active/enabled in order for the measurement circuit to detect a relative position and/or movement within the sensor. In this way, the sampling circuit may hold a peak amplitude from a point in time at which the transmitter was being driven. Thus, a relative position or movement may be detected/measured indirectly, after the driver has been disabled. This may allow for a reduction in power required for the sensing. Additionally or alternatively, the system may provide for a reduction in Electromagnetic Interference (EMI), since the sensor driver need not be active during the sensing.

The control signal of the driver may be configured or timed to repeatedly and/or periodically enable and disable the driver, preferably such that the driver does not drive the transmitter simultaneously with a measurement / detection by the measurement circuit.

In some implementations the measurement circuit may be to detect a relative velocity between the receiver loop(s) and the open loop antenna. In this regard, a plurality of successive relative positions may be detected in order to determine/calculate a relative velocity between the open loop antenna and the receiver loop(s).

In implementations, the sample-and-hold circuit comprises a peak detection input circuit, the peak detection input circuit configured to perform amplitude sensitive demodulation to generate the amplitude signal. Furthermore, the peak detection input circuit may comprise a diode configured to charge a capacitor to generate a voltage corresponding to the peak level. Advantageously, the diode can provide an additional protection from electrical or EMI noise.

Any of the above-disclosed implementations may further provide that the sample-and hold circuit is to receive an input signal dependent on the RF signal from the receiver loop(s), and to generate the amplitude signal on the basis of the input signal when the input signal is above a threshold voltage.

Such an arrangement provides the further advantage that, regardless of the state of the driver, where the sample-and-hold circuit is effectively isolated from the driver, EMI may be further reduced since the sample-and-hold circuit can be effectively grounded from any input signals.

In implementations, the driver may be configured to drive the transmitter with a predetermined number of RF pulses during the period when the driver is enabled. A drive signal of the driver may be a series of pulses, for example a pulse-train such as a square wave or a sinusoidal wave. Generally, a pulse-train may have any duty-cycle, however around 50% is preferable to provide a good balance between power consumption and efficiency.

Generally, the sample-and-hold circuit will reach a peak level and/or steady state in response to a certain number of pulses. Thus, the number of pulses may be chosen to be sufficient to achieve this steady state/maximum capacity. After achieving this steady state, the driver may be disabled. The driver may therefore be actively driven only for a period of time sufficient for the sampling circuit to reach a steady state. Thus, power consumption may be substantially reduced, since the driving signal need not be continuously active, and/or the period of time over which the drive signal is enabled may be controlled precisely.

A sensor according to any above-described implementation may be configured wherein: the driver has a counter to count the RF pulses; and the drive enable circuit is to disable the driver in response to the count reaching the predetermined number. This may result in an inherent synchronicity, wherein counting the number of RF pulses results in a reproducible drive signal, and therefore a generally reproducible response from either the open loop antenna or receiver loop(s). More accurate and/or reliable position and/or movement detections may then be obtained.

In implementations, the sample-and-hold circuit may be configured to be reset after the measurement and before a subsequent re-enabling of the driver by the drive enable circuit. This may allow a previously held peak level of the amplitude signal to be immediately reset, e.g. by discharge of a capacitive element, such that a further peak level can be sampled after a further enablement of the drive signal.

Alternatively, in embodiments without said reset function, the sample-and-hold circuit may allow a held peak level signal to decay (e.g. in an exponential fashion). The measurement circuit may perform a said detection during this decay period. In such an example, no reset switch is needed.

In implementations, the measurement circuit is to perform the measurement after elapse of a settling period during which the driver is disabled A settling or waiting period may allow any residual noise or RF frequency from, e.g., the driver, open loop antenna or receiver loop(s), to dissipate before a detection by the measurement circuit. Thus, reliability and/or accuracy of the measurement may be improved due to the reduction in errors otherwise introduced by EMI noise.

According to a further aspect, there is provided a system comprising a multiplexer to enable a plurality of said transmitters to be driven at same or different times, preferably the transmitters to be driven sequentially, and to enable each measurement circuit to perform a said measuring during a period when driving of each of the transmitters by a driver is disabled by a said drive enable circuit. The system may be configured to enable the measurements by respective measurement circuits to occur simultaneously during the period when driving of each of the transmitters by a driver is disabled.

Thus, a plurality of measurements may be performed simultaneously when all of the drivers are disabled. In embodiments, this may allow a fast scanning rate for multiple sensors controlled by a multiplexer. This may be beneficial in, e.g. a musical keyboard comprising e.g. 88 keys, and/or a computer keyboard comprising over 100 keys, where a rapid scan-rate is generally desired.

Further advantageously, because the sample-and-hold circuit may be configured to hold a peak level such that the driver need not be continuously active, multiple sensors may be driven in parallel, e.g., exactly in phase or only slightly out of phase. In other words, the detection by the measurement circuit for a plurality of sample-and-hold circuits can be obtained in the same measurement slot. This may contrast with measuring for one sensor per measurement cycle or time slot, e.g. driving an active circuit and subsequently performing a said detection by the measurement circuit one sensor at a time, which is generally less efficient.

The detection of relative position and/or movement can therefore be carried out simultaneously or sequentially for multiple sensors, each preferably during a period of time in which all driver(s) for the transmitters is/are disabled. The detection by the measurement circuit for multiple sensors may then be carried out within one measurement cycle, which may allow for a faster scanning rate in a multiplexing system.

In an example, a receiver part of a conductive finger-pad which flexes in response to a touch and the amount of flex is measured by said sensor. Furthermore, a plurality of active sensors responsive to the said flexing of said conductive finger-pad allows the position of said touch to be determined by, for example, interpolation of the measured position output of each of said plurality of active sensors.

Fig. 16 shows a block diagram of an improved sensing embodiment. A sample-andhold circuit 27a has been interposed between the amplitude or phase detector 5a (eg as shown in Fig. 5 or 6) and a measurement circuit 6a, and a controller 28a has been added to control each of: the sensor RE drive signal 4a; a sample-and-hold circuit 27a; and the measurement circuit 6a. The peak-hold circuit of Fig. 6 is suitable for use as a sample-and-hold 27a circuit in some examples, e.g. where amplitude-sensitive detection is employed and thus Fig. 6 performs the function of both the amplitude-sensitive detector and said sample-and-hold 27a circuit. In embodiments, the driver 4a generally drives the transmitter of a sensor with a predetermined number of RE pulses during the period when the driver is enabled.

Fig. 17 shows a flowchart of an example measurement process. It should be appreciated that the order of the steps of the flowchart is not limited to the order shown. For example, the order of steps S100, S102, S104, can be rearranged, or preferably performed in parallel. At steps S106 and S108, the pulses are counted in order to ensure a reproducible output signal. Preferably, as described below, the phase of the pulse-train signal is synchronous with the activation S102 of the drive signal, such that an integer number of pulses are provided each time. The drive signal is disabled at step S110, in response to reaching a pre-determined number of pulse counts (e.g., corresponding to the time taken, tdri", for outputting said number of pulses.) It should further be appreciated that step S112 in particular is optional, e.g., measurement could be performed immediately following disablement of the drive signal for increased scan rate. However, in some examples, it is preferable to allow a settling period where no drive signal is active in order to further reduce unwanted signals and/or EMI from the drive signal. A detection and/or measurement circuit measures the held peak signal at S114, after which the sample-and-hold circuit is reset at S116 (for example, by temporarily closing switch 22 in the circuit of Fig. 6).

Fig. 18 shows a timing diagram of an example measurement process, corresponding to the flowchart of Fig. 17. The process of Fig. 18 is initiated by enabling output of the sensor drive signal 29a and applying the signal 29a to the sensor as a series of pulses 32a. The sample-and-hold circuit is operable to sample and hold the peak-level of the drive signal for a period of time 37a, e.g. for the circuit shown in Fig. 6 the reset switch 22 is open to allow the capacitive element 21 to build charge. The detected output 33a from the sensor may approach a stable value during the active time, tdri", of said series 32a of pulses, and is subsequently peak-held by said sample-and-hold circuit (as seen in the plateau of detector output 33a). It will be understood that generally any oscillating RF drive signal is suitable (e.g. sinusoidal, pulse-train with asymmetric duty cycle, etc.), and that an inverted drive signal (relative to the pulses 32a) is also possible.

After the pre-determined number of said pulses, Ndrive, has been applied to said sensor, the output of said sensor drive signal 29a is disabled (e.g. by a drive enable circuit, or digital controller) preferably in such a way that the output voltage of said sensor is below Vth to ensure that any further fluctuations of the said output voltage of said sensor do not affect the voltage held by said sample-and-hold circuit. After an optional settling time (e.g. the interval between periods 34a and 35a), a measurement 30a is initiated to measure the said voltage held by said sample-and-hold circuit, performed over a period of time 35a. Finally, the said sample-and-hold circuit is reset 31a (for example, by closing a reset switch 22 in the case of the sample-and-hold circuit of Fig. 6) for a period of time 36a. Thereafter, the sampling and measurement process as shown in Fig. 18 may repeat for subsequent measurements. The time interval between periods 35a and 36a is also optional, and is merely shown for clarity. For example, to provide an advantageously fast scan rate, the sample-and-hold circuit may be immediately reset following a measurement 35a.

A circuit such as in Fig. 6 is preferable for use with the sampling and measurement process of Fig. 18. Advantageously, the held signal 33a of the sample-and-hold circuit may be stable. This stability can be attributed to the lack of drive signal and/or the lack of an RC time constant used in Fig. 6, which may allow for low EMI noise/errors in the detected output.

Furthermore, it is advantageous for the sensor drive signal 32a (e.g., the phase of a pulse-train signal) to be synchronous with the timing of the activation and deactivation 29a (e.g. the enabling and disabling of the drive signal by a drive enable circuit) of said sensor drive signal. In this manner, the form and phase of the sensor drive signal 32a is substantially similar each time the sensor is driven, preferably resulting in a reproducible output from said sensor and detector output 33a, which in turn may yield measurements from the sensing system with high reproducibility and/or low error.

Further, such a reproducible signal generally does not require averaging or filtering with a long time constant, allowing for faster sensor scan rates. In detail, for a given measurement performance, imposing this synchronicity allows t -drive to be smaller than when the sensor drive signal is not synchronous with the timing of the activation/deactivation of the sensor drive signal. Reducing the length of t _drive, may advantageously reduce the electrical power requirements of the said sensing system, and/or allow said measurements from the said sensing system to be made more frequently. Thus, an embodiment may allow any one or more of faster changes in position to be measured, faster scan rates for sensors, more accurate sensor velocity measurements, and/or a greater number of sensors to be effectively scanned in a multiplexing sensor system.

In embodiments, a time division multiplex system (e.g., controlled by a circuit or suitable digital controller) may be used to multiplex a plurality of the open loop antenna of sensors to determine the relative position or motion of a plurality of sensors. A musical keyboard comprising one sensor per key is an example implementation. In such an implementation, a subset of position sensors are enabled at any given time. Generally, multiplexing systems have the advantage of reducing cost, complexity, power consumption and electro-magnetic emissions where a large number of sensors are required.

Advantageously, sample-and-hold techniques and implementations of the present disclosure allow for faster multiplexing systems, e.g. where the driving and measurement of a plurality of sensor can effectively be performed in parallel, or much faster sequential succession, preferably without compromising on EMI emissions. For example, the circuit of Fig. 6, in particular the threshold voltage inherent in the diode 20, may provide for an effective electrical isolation between the sampling/detection circuits and any directly connected or adjacent drive signals/resonant sensors.

Fig. 19 shows a timing diagram of a multiplexing scheme according to an embodiment, in which the measurement process of a single sensor (e.g., such as the process of Fig. 18) is applied to a plurality of sensors. Advantageously, in the improved multiplexing scheme shown, each sensor of said plurality of sensors is measured at least once in a given time slot, N. This contrasts with multiplexing systems that measure at most one sensor per time slot.

Within each said time slot: a drive signal 32a is applied as a series of pulses to each said sensor of the plurality of sensors; the detected output 33a of each said sensor is measured at least once within a valid time period 30a; and the sample-and-hold circuit of the detector is reset 31a. Preferably, the timing may be configured such that only one of the said drive signals for any of the said sensors is active at a given time. This may provide the advantage of reducing the peak power required by the measurement system and thus reducing the peak level of unwanted electro-magnetic radiation. Within each said time slot there may be a period of time 44a wherein it is possible to measure all of said sensors of the said plurality of sensors simultaneously, or sequentially. Optionally, as with S112 of Fig. 17, there may be an optional settling time between the last-enabled sensor drive and measurement. However, such a settling time generally may not be necessary, as the sequential drive signal enablement shown in Fig. 19 may provide an inherent settling time for at least sensor 1 and sensor 2 as shown.

It would nevertheless be possible to enable (and subsequently disable) the plurality of drive signals simultaneously, e.g. to provide an even faster multiplexing scan rate.

Such a parallel driving scheme may still provide the advantage of reduced EMI, as the measurement period 44a occurs in the absence of any drive signal.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS: 1. A displacement or pressure sensor comprising: a transmitter comprising an open loop antenna and having an input node to receive an RF drive signal to drive the antenna to transmit an RF signal; a receiver separated from the antenna, the receiver comprising an inductor to receive the RF signal and having at least one output node to output an output RF signal, wherein the receiver is configured to generate the output RF signal having at least one of a phase and amplitude dependent on a position of the antenna relative to a position of the inductor.
2. The sensor according to claim 1, configured to allow the separation between the antenna and the inductor to vary, wherein the dependence on the positions comprises a dependence on the separation.
3. The sensor according to claim 1 or claim 2, configured to allow at least one of the antenna and the inductor to move laterally, wherein the dependence on the positions comprises a dependence on a lateral offset between the positions.
4. The sensor according to any preceding claim, wherein a length, L, of the antenna satisfies the equation L.A/27r, where A is the wavelength of the RF drive signal. 25
5. The sensor according to any preceding claim, comprising: an RF signal detector to detect an amplitude or phase, or a change of a said amplitude or phase, of the output RF signal, wherein the sensor is configured to determine, based on the detection,: the position, or a change thereof, of the antenna or the inductor; and/or a pressure, or change thereof, applied to the antenna and/or the inductor.
6. The sensor of claim 5, wherein the RF signal detector comprises a phase sensitive detector preferably having a lock-in amplifier.
7. The sensor of claim 5, wherein the RF signal detector comprises an amplitude-sensitive demodulator preferably having a peak hold detector.
8. The sensor of any preceding claim, wherein the inductor is electrically open, preferably wherein the inductor comprises an open loop coil.
9. The sensor of any one of claims 1 -7, wherein the inductor is electrically closed, preferably wherein the inductor comprises a closed loop coil.
10. The sensor of any preceding claim, comprising an electrically insulating and deformable separator element between the transmitter and the receiver, the separator preferably elastically deformable.
11. The sensor of any preceding claim, wherein the separator element allows lateral movement of the antenna and/or of the inductor.
12. A sensing system comprising a plurality of sensors of any preceding claim, the system comprising at least one antenna element and at least one receiver element separated from and crossing at least one said antenna element, wherein: each said antenna element forms the open loop antenna of each of a plurality of the sensors; and/or each said receiver element forms the inductor of each of a plurality of the sensors.
13. The sensing system of claim 12, comprising a multiplexer to receive at least one selection signal to select a sensor, the multiplexer configured to couple a said RE drive signal to a said antenna element forming the open loop antenna of the selected sensor.
14. The sensing system of claim 12 or 13, comprising a multiplexer configured to couple to an RE signal detector a said output RE signal from a said receiver element forming the receiver of the selected sensor.
15. The sensing system of any one of claims 12 -14, wherein each said antenna element consists of a linear conductor such as a linear wire or PCB trace.
16. The sensing system of any one of claims 12 -14, wherein each said antenna element comprises serially connected coils that each form the open loop antenna of a respective said sensor.
17. The sensing system of any one of claims 12 -16, wherein each said receiver element comprises a single-or multi-turn coil that forms the inductor of at least one sensor, preferably wherein turns of adjacent said coils are wound in opposite directions.
18. The sensing system of any one of claims 12 -17 and comprising multiple said antenna elements, wherein each said receiver element is formed as a loop to surround a said open loop antenna of each of the antenna elements, wherein a spacing between opposite sides of the loop narrows between adjacent said open loop antennae.
19. The sensing system of any one of claims 12 -18, comprising a shielding element such as a ground trace between adjacent said receivers or receiving elements, preferably wherein the sensing system is according to claim 17 and comprises multiple said receiver elements each having oppositely wound coils and a said shielding element is disposed between adjacent said receiver elements.
20. The sensing system of any one of claims 12-19, wherein a plurality of the sensors are each configured to inductively couple from the open loop antenna of the sensor to the inductor of the sensor a said RF signal having a respective said wavelength, wherein the respective wavelengths of the sensors are different, preferably wherein said sensors that couple different wavelengths are adjacent.
21. The sensing system of 20, comprising a multiplexer configured to receive at least one selection signal to select multiple said sensors, the multiplexer configured to simultaneously couple to the open loop antennae of the selected sensors a said RF drive signal, wherein wavelengths of the respective RF drive signals coupled to respective said open loop antennae are different.
22. The sensing system of any one of claims 12 -21, wherein at least one said antenna element comprises the open loop antenna of each of a plurality of the sensors, wherein the open loop antennae comprise open loop coils connected in parallel.
23. The sensing system of claim 22, configured to couple a said RF drive signal to a said antenna element, the sensing system comprising at least one voltage or current regulator to maintain the provided RF drive signal at a predetermined level.
24. The sensing system of any one of claims 12 -23, wherein at least one said receiver element comprises coils connected in parallel, each coil forming the inductor of the receiver of a respective said sensor.
25. The sensor or sensing system of any preceding claim, comprising: a driver to generate a said RF drive signal to drive a said open loop antenna of a said sensor; a drive enable circuit to generate a control signal to enable and disable the driver; a sample-and-hold circuit for an amplitude signal indicating the amplitude of a said output RF signal at the receiver, wherein the sample-and-hold circuit is to hold a peak level of the amplitude signal, the peak level being a peak level during a period when the driver is enabled; and a measurement circuit to detect a relative position and/or movement between the open loop antenna and the inductor of the sensor based on the held peak level, the detection based on measuring the held peak level when the driver is disabled by the drive enable circuit.
26. A key, keypad or keyboard, optionally for a laptop computer or musical instrument such as an electronic piano, comprising a sensor or sensing system of any preceding claim.
27. A touch pad, optionally for a laptop computer or for a musical instrument, preferably for a fingerboard of a stringed instrument such as a violin or guitar, the touchpad comprising a sensor or sensing system of any preceding claim.
28. A touch screen, optionally for a computer, the touch screen comprising the sensor or sensing system of any preceding claim, preferably wherein at least one of the antenna and receiver inductor comprises ITO.
29. A temperature compensation method for a sensor or sensing system of any of claims 1 -25, the method comprising: measuring, while the transmitter antenna of a said sensor is not being driven, a quiescent output of an RF signal detector configured to perform measurements of a said RF output signal of a sensor; calculating a temperature-dependent offset in the quiescent output signal based on a difference between a first said measurement and a subsequent said measurement; and measuring a said RF output signal of the sensor when the transmitter antenna is being driven; and adjusting the measurement of the RF output signal when the transmitter antenna is being driven, based on the offset.