KEYBOARD SENSOR SYSTEMS AND METHODS

Abstract

A sensing system for a keyboard. Each key sensor comprises passive and active resonant circuits. The passive resonant circuit has a resonant frequency and the active resonant circuit excites the passive resonant circuit at the resonant frequency. A sensor driver drives the active resonant circuit with an RF drive signal, a multiplexing system multiplexes the drive signal such that simultaneously driven key sensors are separated by at least (k-1) keys, and a detector detects a level of RF signal from a driven key sensor for sensing a position and/or velocity of a key.

Claims

1. A sensing system comprising a set of sensors for a keyboard of a keyboard instrument, wherein the keyboard has a plurality of keys; wherein each sensor of the set of sensors comprises a passive resonant circuit for mounting on a moving part of a key and an active resonant circuit for mounting in a reference position, the passive resonant circuit having a resonant frequency, the active resonant circuit exciting the passive resonant circuit at the resonant frequency, each sensor of the set of sensors further having a detector to detect a variation of a resonant signal in the active resonant circuit corresponding to a variation of a relative position of the active and passive resonant circuits to detect a position and/or velocity of the key; and wherein the set of sensors comprises sensors having two or more different resonant frequencies arranged such that sensors having the same resonant frequency are non-adjacent.

2. The sensing system as claimed in claim 1, wherein the active resonant circuit comprises one or more coils with windings in opposite senses, wherein the windings in opposite senses are configured to generate magnetic fields in opposite senses to cancel one another.

3. The sensing system as claimed in claim 1, wherein the active resonant circuit comprises a pair of laterally adjacent pancake coils.

4. The sensing system as claimed in claim 1, further comprising a temperature-compensation system to temperature-compensate a detected resonant signal in the active resonant circuit, wherein the temperature-compensation system is configured to apply an off-resonance drive signal to at least one of the active resonant circuits, to measure a level of the off-resonance drive signal from the at least one detector, and to compensate a detected level of the resonant signal responsive to the level of the off-resonance drive signal.

5. The sensing system as claimed in claim 1, wherein each sensor of the set of sensors further comprises a deformable element configured to limit motion of one or both of the passive resonant circuit and the active resonant circuit for pressure sensing.

6. A polyphonic aftertouch keyboard comprising the sensing system of claim 1, each key of the plurality of keys having a deformable end-stop, such that an after-touch position corresponds to movement of a key beyond an end-stop position defined by the deformable end-stop, wherein identification of the aftertouch position for the key enables polyphonic aftertouch.

7. The sensing system as claimed in claim 1, wherein sensors having a first resonant frequency are interleaved with sensors having a second, different resonant frequency.

8. The sensing system as claimed in claim 1, further comprising a multiplexing system and/or controller configured to control selection of sensors of the set of sensors such that adjacent keyboard sensors are selected at different times.

9. The sensing system as claimed in claim 8, wherein the multiplexing system and/or controller is further configured to damp the active resonant circuits of unselected sensors.

10. The sensing system as claimed in claim 8, wherein the multiplexing system and/or controller is configured to perform time division multiplex operation of the sensors, wherein each resonant frequency defines a group of sensors having the resonant frequency, wherein the time division multiplexing defines a plurality of n time slots, and wherein successive keyboard sensors of each group are allocated successive time slots.

11. The sensing system as claimed in claim 10, wherein the multiplexing system and/or controller is configured to multiplex an RF drive signal used to drive active resonant circuits of the set of sensors such that one of the sensors is driven in each of a set of time slots.

12. The sensing system as claimed in claim 11, further comprising a temperature-compensation system configured to temperature-compensate a level of a resonant signal, in a first active resonant circuit of a first sensor, that is detected by a first detector of the first sensor, wherein the temperature-compensation system is configured to apply an off-resonance drive signal to the first active resonant circuit, to measure a level of the off-resonance drive signal from the first detector, and to compensate the detected level of the resonant signal responsive to the level of the off-resonance drive signal, and wherein the temperature-compensation system is configured to apply the off-resonance drive signal during an additional time slot to the set of time slots.

13. The sensing system as claimed in claim 10, wherein there are N resonant frequencies and N groups of sensors, wherein sensors of the groups of sensors are interleaved on the keyboard.

14. The sensing system as claimed in claim 13, wherein the multiplexing system and/or controller is configured such that keyboard sensors in the same group and activated in the same time slot have (n×N)-1 sensors between them.

15. The sensing system as claimed in claim 1, further comprising a processor configured to process the variation of the resonant signal in the active resonant circuit of each sensor to determine the motion of each key of the keyboard over a succession of time intervals as a depressed key moves between released and depressed positions, wherein the motion of each key comprises a position and a velocity of the key as the key moves between released and depressed positions.

16. The sensing system as claimed in claim 15, wherein the processor is configured to process the variation of the resonant signal in the active resonant circuit of each sensor to determine the velocity of a key, as the key moves between depressed and released positions, from changes in position of the key determined at successive time intervals, wherein the changes are filtered dependent upon key velocity.

17. The sensing system as claimed in claim 1, further comprising a processor configured to process the variation of the resonant signal to determine a key press and key release event for each key.

18. The sensing system as claimed in claim 15, wherein the processor is further configured to distinguish between at least three different key positions, a first, note-off position, a second, note-on position, and a third, aftertouch position, wherein the aftertouch position is beyond the note-on position and corresponds to additional pressure applied to the key after depression.

19. A method of sensing the positions of a plurality of keys of a keyboard instrument, the method comprising: providing each key with a sensor comprising a passive resonant circuit for mounting on a moving part of a key and an active resonant circuit for mounting in a reference position, the passive resonant circuit having a resonant frequency, the active resonant circuit exciting the passive resonant circuit at the resonant frequency, each sensor further having a detector to detect variation of a resonant signal in the active resonant circuit with relative position of the active and passive resonant circuits to detect a position and/or velocity of the key; and arranging the sensors to operate at two or more different resonant frequencies arranged such that keyboard sensors having the same resonant frequency are non-adjacent; and/or reducing interference between sensors by configuring one or more coils of at least the active resonant circuits to have windings in opposite senses.

20. The method as claimed in claim 19, further comprising providing polyphonic aftertouch by distinguishing between at least three different key positions, a first, note-off position, a second, note-on position, and a third, aftertouch position, wherein the aftertouch position is beyond the note-on position and corresponds to additional pressure applied to the key after depression and movement of a key beyond an end-stop position.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] These and other aspects of the invention will now be further described, with reference to the accompanying drawings, in which:

[0050] FIG. 1 shows an active tuned resonant circuit for use with example implementations of the system.

[0051] FIG. 2 shows a passive tuned resonant circuit for use with example implementations of the system.

[0052] FIG. 3A shows an example printed circuit design, on an enlarged scale, for an active tuned resonant circuit for use with example implementations of the system.

[0053] FIG. 3B shows an example printed circuit design, on an enlarged scale, for a passive tuned resonant circuit for use with example implementations of the system.

[0054] FIG. 4 shows a cross section view of a key of a musical keyboard wherein the position of a key is determined using an example implementation of the system.

[0055] FIG. 5 shows an example of a simple read-out electronic circuit comprising a synchronous demodulator which may be used by example implementations of the system.

[0056] FIG. 6 shows the relative positioning of a series of active tuned resonant circuits and a series of corresponding passive tuned resonant circuits positioned to minimise interference between adjacent tuned resonant circuits according to example implementations of the system.

[0057] FIG. 7 shows a timing diagram of a time division multiplex circuit used to multiplex a plurality of active tuned resonant circuits to determine the position of a plurality of keys on a musical keyboard according to an example implementation of the system.

[0058] FIG. 8 shows a time division multiplex circuit diagram of a preferred embodiment used to multiplex a plurality of active tuned resonant circuits to determine the position of a plurality of keys on a musical keyboard according to example implementations of the system.

[0059] FIG. 9 shows the sensor output versus key displacement of a key on a musical keyboard according to example implementations of the system.

[0060] FIG. 10 shows an example of the measured position and measured velocity of a key on a musical keyboard as it is depressed according to example implementations of the system.

[0061] FIG. 11 shows an example calibration procedure used to calibrate the detected position of a key on a musical keyboard according to example implementations of the system.

[0062] FIG. 12 shows an example algorithm used to detect note-on events, note-off events, expression events, and pressure events for a key on a musical keyboard according to example implementations of the system.

[0063] FIGS. 13A and 13B shows examples of sensor resonant circuits with coils having windings in opposite senses, according to example implementations of the system.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0064] A preferred embodiment comprises a musical keyboard with a plurality of moveable keys wherein each moveable key FIG. 4 comprises: a moveable top member 15 that is rotated about a pivot point 17 and which resists movement by means of a spring 16 or other mechanical linkage; a fixed bottom member 14; a deformable end-stop 18 which limits movement of said top member; and a position sensor comprising an active tuned resonant circuit 10 inductively coupled to an electrically reactive element 11, henceforth referred to as the target, providing a signal which varies as the mutual separation of said active tuned resonant circuit and said target is varied, drive electronics connected to said active tuned resonant circuit and read-out electronics connected to said active tuned resonant circuit.

[0065] The active tuned resonant circuit FIG. 1 comprises an input resistive element 4, a coil 1, two capacitive elements 2 and 3, an output resistive element 5, a means of connecting 6 drive electronics to said input resistive element and a means of connecting 7 read-out electronics to said output resistive element. Said input resistive element may be omitted, but it is preferred because: it limits the current supplied to said active tuned resonant circuit from said drive electronics which reduces the operating current and thus reduces both power consumption and electro-magnetic emissions from said active tuned resonant circuit; and it increases the sensitivity of proximity detection when said read-out electronics are connected to said active tuned resonant circuit. Said output resistive element may be omitted, but it is also preferred because said input and output resistive elements reduce the effect of connecting wires on the impedance of said active tuned resonant circuit thus allowing all the position sensors to be essentially the same regardless of the length of connections to the drive electronics and to the read-out electronics.

[0066] Referring to FIG. 2, the reactive element preferably comprises a passive tuned resonant circuit which comprises a coil 8 and a capacitive element 9 wherein said coil and said capacitive element are connected to form a closed resonant LC circuit. It is not necessary for the size nor for the value of inductance of the coils 1 and 8 to be substantially similar. The value of the capacitance of said capacitive element 9 is preferably chosen to tune the frequency of resonance of said passive tuned resonant circuit to match the frequency of resonance of the active tuned resonant circuit FIG. 1. When said passive and active circuits are thus tuned, it is possible to operate a plurality of position sensors where proximally located said position sensors are tuned to substantially different frequencies of resonance thereby minimising the interaction between said proximally located position sensors. Furthermore when said passive and active circuits are thus tuned the signal amplitude at 7 in FIG. 1 decreases as the distance between said passive and active circuits decreases because more energy is coupled to and dissipated by said passive tuned resonant circuit. Such variation in said signal amplitude is preferred because measuring variations in signal amplitude is faster than measuring variations in frequency of resonance as would be implemented in the case where said active tuned resonant circuit was detuned by proximity to said reactive element.

[0067] In the case where the moveable top member 15 of a key comprises an electrically conductive material, and air gap or spacer 13 comprised of a non-conductive material is interposed between said electrically reactive element 11 and said top member. Similarly, in cases there the fixed bottom member 14 comprises an electrically conductive material, and air gap or spacer 12 comprised of a non-conductive material is interposed between the active tuned resonant circuit 10 and said fixed bottom member.

[0068] The drive electronics comprise a means of generating an oscillating voltage drive waveform at a frequency equal to or close to the frequency of resonance of the active tuned resonant circuit. Typically, but by way of non-limiting example, this waveform is a square waveform generated by the output of a microcontroller timer or a digital or analogue timing circuit.

[0069] The read-out electronics comprise a means of generating a voltage proportional to the amplitude of the signal at the read-out point 7. Typically, but by way of non-limiting example, this comprises a synchronous demodulator circuit FIG. 5 wherein the signal from said read-out point is connected to 20 and demodulated by an analogue switch 22 controlled by the oscillating voltage drive waveform connected to 19 whose phase is optionally adjusted by a phase shifting element 21 and a low-frequency (or dc) voltage is presented at 25 by a low-pass filter comprising a resistive element 23 and a capacitive element 24. Alternative read-out electronic circuits may comprise phase-sensitive rectifiers, phase-insensitive rectifiers, non-synchronous demodulators and peak detectors as understood by those trained in the art.

[0070] The coils 1 and 8 used in the active tuned resonant circuit and the passive tuned resonant circuit respectively can be of any type. However using planar spiral coils formed by tracks on a printed circuit board has three main advantages: they are inexpensive, they can be made with highly reproducible values of inductance and the printed circuit board can also be used to mount the other components, namely the capacitive elements 2, 3 and 9, and the resistive elements 4 and 5. It is therefore possible to design a plurality of coils whose inductance values are closely matched.

[0071] Referring to FIG. 3A, a typical active tuned resonant circuit may be formed on a printed circuit board comprising a single electrically conductive layer or a plurality of electrically conductive layers whereon: the coil 1 is formed of a continuous spiral track whereby electrical continuity of said track is maintained by electrical connection through connecting vias 53 to a connecting wire or to another spiral track on another conductive layer or to a plurality of spiral tracks on a plurality of conductive layers of said printed circuit board; capacitive elements 2 and 3 and resistive elements 4 and 5 are proximally located; and connection points 6 and 7 are provided for drive electronics and read-out electronics, respectively.

[0072] Similarly, referring to FIG. 3B, a typical passive tuned resonant circuit may be formed on a printed circuit board comprising a single electrically conductive layer or a plurality of electrically conductive layers whereon: the coil 8 is formed of a continuous spiral track whereby electrical continuity of said track is maintained by electrical connection through connecting vias 54 to a connecting wire or to another spiral track on another conductive layer or to a plurality of spiral tracks on a plurality of conductive layers of said printed circuit board; and the capacitive element 9 is proximally located.

[0073] The inventors have found that the electro-magnetic emissions from an active tuned resonant circuit, and the susceptibility to electro-magnetic interference signals of said active tuned resonant circuit can be substantially reduced when the inductive coil of said active tuned resonant circuit is formed from a plurality of electrically connected primary smaller coils wherein the winding direction of said primary smaller coils is chosen such that the sum of the electro-magnetic far field radiated from said primary smaller coils is substantially zero. A particularly suitable, but by way of non-limiting example, of said inductive coil 1 is shown in FIG. 13A, wherein two primary smaller coils are wired in series with opposing winding directions 58 to form a figure-of-eight coil. In such an arrangement the electro-magnetic far field radiated from the first half of said figure-of-eight coil 56 is equal in magnitude but with opposite polarity to the electro-magnetic far field radiated from the second half of said figure-of-eight coil 57, thus said electro-magnetic far field radiated from said figure-of-eight coil is substantially zero.

[0074] In such an arrangement, a passive tuned resonant circuit as shown in FIG. 3B may be ineffective unless the inductive coil of said passive tuned resonant circuit is primarily inductively coupled to only one half 56 or 57 of the figure-of-eight coil of the active tuned resonant circuit. To maximise the output signal of the position sensor, it is preferable for said inductive coil of said passive tuned resonant circuit to be similarly formed of a figure-of-eight inductive coil, as shown in FIG. 13B, comprising two secondary smaller coils wired in series with opposing winding directions 58 wherein each said secondary smaller coil is primarily inductively coupled to a different primary smaller coil of said figure-of-eight coil of said active tuned resonant circuit.

[0075] The inventors have found that although a first passive tuned resonant circuit tuned to a first frequency of resonance of a first active tuned resonant circuit does not substantially affect the output of an adjacent second active tuned resonant circuit tuned to a substantially different second frequency of resonance, when a corresponding second passive tuned resonant circuit tuned to said second frequency of resonance is proximally located, movement of said first passive tuned resonant circuit may affect the output of said second active tuned resonant circuit due to mutual coupling between said first and second passive tuned resonant circuits. Such undesirable interaction can be minimised by offsetting the positions of physically adjacent passive tuned resonant circuits, as shown in FIG. 6 wherein: active tuned resonant circuits 26 and passive tuned resonant circuits 28 are tuned to a first frequency of resonance and active tuned resonant circuits 27 and passive tuned resonant circuits 29 are tuned to a second frequency of resonance.

[0076] In a further preferred embodiment the position sensors on the moveable keys of the musical keyboard are controlled by a time-division multiplexing scheme whereby a subset of position sensors are enabled at any given time. For a typical musical keyboard with a large number of keys such as 16 or more, such a scheme has the advantage of reducing cost, complexity, power consumption and electro-magnetic emissions.

[0077] In the case where a first position sensor operating at first frequency of resonance and a second position sensor operating at a substantially different second frequency of resonance are proximally located said position sensors can interact in such a way that the output of said first position sensor and the output of said second position sensor contains interference components which vary with a frequency of variation equal to the frequency difference of said first frequency of resonance and said second frequency of resonance. Synchronous demodulation of the output of said position sensors substantially removes said interference components when the cut-off frequency of the reconstruction low-pass filter is substantially lower than said frequency difference. However, the time response of said low-pass filter can limit the speed of response of said position sensors which is undesirable. Therefore, a mechanism to minimise this interference is desired. Using a time-division multiplexing scheme where physically adjacent sensors are not driven at the same time avoids this problem.

[0078] In practice it has been found that synchronous demodulation is not necessary for good performance.

[0079] Referring to FIG. 7, one illustrative example of such a mechanism is shown as a timing diagram for a subset of position sensors wherein position sensors 30 operating at a first frequency of resonance F1 are adjacent to position sensors 31 operating at a second frequency of resonance F2. In each time slot only one position sensor operating at a first frequency of resonance is enabled and only one position sensor operating at a second frequency of resonance is enabled. Furthermore, physically adjacent position sensors are never enabled at the same time, minimising said interference components. A plurality of said subsets of position sensors can be operated simultaneously.

[0080] In broad terms, in the multiplexing of FIG. 7 the keys illustrated shaded black and the keys illustrated shaded white each form a group of keys. The sensors in one group of keys may have a different resonant frequency to the sensors in another group of keys. With a group, say of the black keys, there are 8 time slots and every 8.sup.th key is activated (driven) simultaneously. The skilled person will understand that this approach could be adapted for k time slots, driving every kth key simultaneously (that is simultaneously driven keys have k-1 inactive keys between them). Keys in simultaneously active groups, e.g. black and white keys, may be (physically) separated as far as possible.

[0081] Some implementations of the system do not employ different groups of keys with different resonant frequencies. Instead all the sensors may have substantially the same resonant frequency. Use of such an approach is facilitated by the coil design described later with reference to FIG. 13. Thus there may be k time slots and every kth key may be active (driven) simultaneously—that is simultaneously driven keys may have k-1 inactive keys between them.

[0082] An example time-division multiplexed scheme is shown for a subset of position sensors operating at a single frequency of resonance in FIG. 8. In the system of FIG. 8 a processor 35 generates a drive waveform 36 whose frequency matches the frequency of resonance of said position sensors' active tuned resonant circuits; said processor generates selector signals 37 to select which position sensor is to be enabled; said position sensors' outputs 7 are coupled to an analogue multiplexer 34; said analogue multiplexer's output is coupled to an analogue-to-digital converter within said processor via a low-pass filter comprising a capacitive element 24 and resistive element within said analogue multiplexer; and an output 55 from said processor is used to send information regarding the position and velocity of said position sensors. A further advantage to using said analogue multiplexer to couple said position sensors' outputs to said analogue-to-digital converter is that said analogue multiplexer can perform the function of the analogue switch 22 used for synchronous demodulation whereby the output of said analogue multiplexer can be synchronously enabled and disabled via an enable input 39 coupled to said drive waveform 36. In the case where a plurality of position sensors are operated at substantially different frequencies of resonance said time-division multiplexed scheme can be replicated as necessary. A suitable processor is an ARM Cortex-M0.

[0083] FIG. 8 shows just one demultiplexer/multiplexer but if there are multiple resonant frequencies one demultiplexer/multiplexer may be employed for each of the resonant frequencies used. For example a second demultiplexer/multiplexer may be used where alternate resonant frequencies are mapped to alternate keys of the keyboard.

[0084] Decreased sensitivity to detuning of the position sensor's active tuned resonant circuit or passive tuned resonant circuit, for example, caused by variations of component tolerance, may be facilitated by coupling the output of the (optional) synchronous demodulator circuit to a peak detection circuit comprising a diode 40 a capacitive element 24 and optionally a resistive element 41 or a switching element 42 (to reset the charge on capacitive element 24). In the case where a switching element is used said switching element may reset the detected peak level synchronously with the selector signals used to control the multiplexers.

[0085] The signal from the detector (read-out circuitry) may be input to an analogue-to-digital converter 38, for example integrated into an analogue input of processor 35.

[0086] In the case where a disabled position sensor's active tuned resonant circuit is not being driven, said active tuned resonant circuit acts as a tuned antenna. This has the negative effect whereby moving the target corresponding to said disabled position sensor can effect a measurable variation in the output of a similarly-tuned position sensor even if said similarly-tuned position sensor is not physically adjacent to said disabled position sensor and the motion of said target is constrained to be within its normal limits above said disabled position sensor, according to FIG. 4 and FIG. 6. Said negative effect can be reduced by changing the frequency of resonance of said disabled position sensor's active tuned resonant circuit for the duration of the disablement, for example by changing the capacitance, resistance or inductance of said active tuned resonant circuit by electronic switching. Most simply this can be done by driving said disabled sensor with a direct-current, or low-frequency signal, to prevent resonance. Referring to FIG. 8, a particularly advantageous way to achieve this in a time-division multiplexed scheme is to use a digital demultiplexer 33 to drive the inputs 6 of the active tuned resonant circuits whereby enabled position sensors' active tuned resonant circuits are driven by a waveform 36 at the frequency of resonance of said active tuned resonant circuits and disabled position sensors' active tuned resonant circuits are driven by a direct-current signal corresponding to logic-high or logic-low of said digital demultiplexer.

[0087] It is important for the performance of a musical keyboard to be stable over a range of operating temperatures. Although the tuned resonant circuits used by a position sensor as described herein have excellent temperature stability, particularly when the tuned resonant circuits are formed on a printed circuit board and the capacitive elements of the tuned resonant circuits comprise temperature-stable dielectrics (Class 1 dielectrics), other electronic elements in the circuit can have properties that change with temperature which may cause a variation in the output signal of the position sensor with variations in operating temperature. Such electronic elements include but are not limited to: diode 40, digital demultiplexer 33, analogue multiplexer 34, resistive elements 4, 5 and 41, tracks on printed circuit boards, and voltage regulators. Therefore a temperature compensation scheme can be useful to minimise variations in the output signals of a plurality of position sensors on a musical keyboard caused by variations in operating temperature.

[0088] A particularly suitable, but by way of non-limiting example, temperature compensation scheme comprises: performing measurements of the output signal of a position sensor while driving said position sensor's active tuned resonant circuit with a direct-current, or low-frequency signal such that said position sensor's passive tuned resonant circuit has no effect on the output signal of said position sensor; the first of said measurements is performed during a calibration procedure; the subsequent said measurements may be performed periodically, for example within additional time slots of a time-division multiplexed scheme; calculating temperature-dependent offsets in said output signal by subtracting subsequent said measurements from said first measurement; and adding said offsets to the measurement of said output signal when said active tuned resonant circuit is being driven at a frequency equal to or close to the frequency of resonance of said active tuned resonant circuit to measure position. Such a temperature compensation scheme may utilise one temperature-dependent offset for: each position sensor in a musical keyboard; each group of position sensors in a musical keyboard; or for all position sensors in a musical keyboard.

[0089] A musical keyboard with moveable keys utilising a multiplexing scheme as hereinabove described allows fast and accurate measurement of the position of said keys. For example it is possible to multiplex the example shown in FIG. 8 wherein the frequency of update of selector signals 37 is at least 32,000 Hz thus allowing the position of each moveable key in a subset of 8 moveable keys to be determined at a frequency of 4,000 Hz. This example can be replicated and run in parallel for other subsets of moveable keys, thus allowing a full-size piano keyboard with 88 keys to have the position of the keys determined at a rate of at least 352,000 keys/second. The inventors hereof have found that positions of said keys should ideally be determined at least 250 times per second, corresponding to a rate of at least 22,000 keys/second for 88 keys, to allow suitably accurate timing of note-on events and note-off events and to determine the key velocity associated with said events. Clearly implementations of the described system easily exceed these targets.

[0090] Referring to FIG. 9, when a moveable key on a musical keyboard according to example implementations of the system is depressed there are three primary positions of said key: the resting position Kmax 43 when said key is at rest; the point Kzero 44 when the moveable top member 13 makes a first contact with the deformable end-stop 18; and the point of maximum depression Kmin 45, corresponding to the point of maximum pressure being applied to said key by a typical musician wherein the deformable end-stop 18 may be considered to be maximally deformed. For a plurality of such moveable keys, due to mechanical variation and due to electronic component tolerance, it is unlikely that the output signal of the position sensor of a first key at any one of said primary positions of said first key will be identical to the output signal of the position sensor of a second key at the same primary position of said second key. Therefore a calibration procedure is necessary to ensure that the position of any moveable keys is known relative to the respective primary positions of said moveable keys. Such a calibration procedure is shown in FIG. 11.

[0091] In the case where the position of a moveable key is between primary positions Kmax and Kzero, the calibrated position K of said key as a percentage of depression between Kmax and Kzero can thus be calculated from the measured position Ko of said key using the following equation: K=100%×(Ko-Kzero)/(Kmax-Kzero).

[0092] In the case where the position of a moveable key is between primary positions Kzero and Kmin, the calibrated position Kpress of said key as a percentage of depression between Kzero and Kmin, 50 in FIG. 9, can thus be calculated from the measured position of said key Ko using the following equation: Kpress=100%×(Ko-Kmin)/(Kzero-Kmin). In such a case Kpress may be considered to be the amount of pressure being applied to said key, corresponding to the range of depression 50 of said key.

[0093] In some embodiments the calculation of Kpress may include an offset, Kpoff, whereby Kpress is zero until the position of the key Ko lies between (Kzero-Kpoff) and Kmin; thence Kpress=100%×(Ko-Kmin)/(Kzero-Kpoff-Kmin). Said offset creates a dead-zone wherein variation in position of said key results in no variation of calibrated position K of said key and in no variation of Kpress. This facilitates implementation of an aftertouch threshold.

[0094] On a typical musical keyboard it is desirable for each moveable key on said keyboard to issue a note-on event when the depression of said key is beyond a secondary position Kon and to issue a note-off event when the depression of said key is returned to another secondary position Koff In some cases Kon may equal Koff, but it is preferred for Kon and Koff to be unequal. Referring to FIG. 9, preferably secondary position Kon 48 is chosen to be near the primary position Kzero 44. Similarly, the secondary position Koff 47 is chosen to be near the said secondary position Kon.

[0095] In some embodiments it is possible after issuing a first note-on event to issue a second note-on event, and optionally issue a note-off event preceding said second note-on event, when the depression of said moveable key has returned to a position before secondary position Kon but has not returned to secondary position Koff and then the depression of a moveable key varies to a position beyond secondary position Kon. This facilitates re-triggering.

[0096] In some embodiments the secondary position Koff 46 of each moveable key is chosen to be near the primary position Kmax 43. Such an arrangement allows the position of said key to be used to issue expression events prior to issuing a note-off event wherein the measured position Ko of said key between Koff and Kzero can be used to calculate a calibrated expression value Kexp=100%×(Ko-Kzero)/(Koff-Kzero), corresponding to the range of depression 49 of said key.

[0097] By way of non-limiting example, one particular algorithm shown in FIG. 12 may be used for each moveable key on a musical keyboard according to an example implementation of the system wherein the measured position Ko of said moveable key, when calibrated using primary positions Kmax, Kzero and Kmin and thence using secondary positions Kon and Koff, may be used to issue: note-on events, note-off events, expression events and pressure events for each moveable key on said musical keyboard.

[0098] A particular advantage of deriving the secondary positions Kon and Koff of a moveable key on a musical keyboard from the primary positions Kmax and Kzero of said moveable key is that said secondary positions can be modified easily by simple numerical calculations, allowing the response of said musical keyboard to be changed. Moreover such modification can be different for each individual key on a musical keyboard with a plurality of moveable keys, allowing a large range of responses to be achieved on said musical keyboard without requiring any mechanical changes to the musical keyboard.

[0099] To provide further expressive control of a musical sound production system it is common for a musical keyboard to send velocity information relating to note-on events and also possibly related to note-off events. Such velocity information can be determined by measuring the separation in time between two known points of key depression, or conversely measuring the change in said key depression at two known points in time.

[0100] In embodiments the velocity (speed and direction) of a moveable key is determined from a plurality of positions of said key at a plurality of corresponding times using averaging, filtering, or similar methods. An example is described in detail below. Such a method of calculating said velocity has several advantages over other methods: it does not assume a linear velocity profile as is used for a two-point measurement method but allows changes in velocity throughout the range of depression of said key to be detected thus measured values of velocity are more representative of the true velocity of said key thus making the response of said key more consistent; higher resolution and precision of velocity can be determined because a larger number of statistically significant data points are used; and it allows predictions of the future position of said key to be calculated allowing, for example, the future time at which said key's position equals secondary positions Kon and Koff to be estimated, thus permitting note-on or note-off events to be issued in advance of the corresponding physical event thus compensating for latency in a musical sound production system.

[0101] One example filtering procedure is as follows:

[0102] deltaV=deltaPos (i.e. the change in position between fixed time steps) alpha=k*abs(deltaV)

[0103] The filtering coefficient, alpha, depends on magnitude of deltaV; alpha is limited to sensible values to avoid overflow/underflow.

[0104] velocity=alpha*deltaV+(1-alpha)*last_velocity

[0105] Such a method, which may be implemented in the digital domain, can provide improved resolution because of the filtering, which is especially important for a very slowly moving key, without significantly compromising the time response for a fast-moving key. Modifying the filtering and/or a maximum permitted velocity value can modify the feel of an instrument, for example to give it a harder or softer response.

[0106] To illustrate such benefits of such a method, FIG. 10 shows the calibrated position 51 of a moveable key and the corresponding calibrated velocity 52 of said key wherein the depression of said key reaches primary point Kzero 44 within 7 ms of the start of depression of said key. The plot of FIG. 10 approximates a velocity calculated directly from differentiated position but when the position moves slowly the velocity filtering is heavier so the velocity lags a little. Such a method can yield substantially more information regarding the velocity of a moveable key on a musical keyboard than other methods.

[0107] Movement detection systems for musical keyboards have been described as well as sensing systems and methods for keyboard instruments. However the techniques described are not limited to musical keyboards and may also be used, for example for computer keyboards.

[0108] For example in some implementations the above described techniques may be employed in a laptop keyboard. In this case one or both of the passive and active resonant circuits may be mounted on a flexible PCB. For example the passive resonant circuits may be mounted beneath the keys, on a flexible PCB and the active resonant circuits may be mounted on an underlying rigid PCB. The ability to sense position may be used to sense pressure applied to a key, for example if some resilient material is provided between the active and passive resonant circuits. In some implementations, for example a laptop, computer, or other keyboard, where the keys are arranged in a 2D pattern on a flat or curved surface, the multiplexing may be arranged, for example in a generally corresponding manner to that described above, so that no key is driven at the same time as an adjacent key in two dimensions. For example in a rectangular 2D grid alternate keys in each of two dimensions in a surface defined by the keyboard may be active in alternate time slots (i.e. two sets of non-adjacent keys may be identified); this may be extended to key layouts defined by hexagonal and other grids where sets of non-adjacent keys may similarly be identified. Keys which are adjacent to one another in a surface defined by the keyboard may be inactive and/or damped when a target key is read. However, as previously described, the multiplexing may be arranged to read multiple keys of the keyboard simultaneously. The described techniques can be advantageous for computer and other keyboards because they can be fabricated inexpensively and because response times can be very quick, for example <1 ms.

[0109] In another implementation, the above described techniques may be employed to sense pressure, a sensor further comprising a deformable element, for example a block or layer of rubber, below and/or between of one or both of the passive resonant circuit and the active resonant circuit. Such an arrangement may be employed, for example, as a sensor for an electronic drum pad.

[0110] Further aspects of the invention are defined in the following clauses C1-C15:

[0111] C1. A sensing system for a keyboard, the sensing system comprising a plurality of key sensors, wherein each of the plurality of key sensors comprises: a passive resonant circuit, and an active resonant circuit, the passive resonant circuit having a resonant frequency, the active resonant circuit being configured to excite the passive resonant circuit at the resonant frequency; the sensing system further comprising: at least one sensor driver to drive the active resonant circuits with an RF drive signal; a multiplexing system to multiplex the RF drive signal such that simultaneously driven key sensors are separated by at least (k-1) keys, where (k-1) is an integer equal to or greater than 1; and at least one detector to detect a level of an RF signal from a driven key sensor for sensing a position and/or velocity of a key associated with the key sensor.

[0112] C2. The sensing system of C1, wherein the multiplexing system is further configured to damp the active resonant circuits of key sensors which are not driven.

[0113] C3. The sensing system of C1, wherein the active resonant circuit comprises one or more coils with windings in opposite senses, wherein the windings in opposite senses are configured to generate magnetic fields in opposite senses to cancel one another.

[0114] C4. The sensing system of C1, wherein the active resonant circuit comprises a pair of laterally adjacent pancake coils.

[0115] C5. The sensing system of C1, further comprising a temperature-compensation system to temperature-compensate the detected level of the RF signal, wherein the temperature-compensation system is configured to apply an off-resonance drive signal to at least one of the active resonant circuits, to measure a level of the off-resonance drive signal from the at least one detector, and to compensate the detected level of the RF signal responsive to the level of the off-resonance drive signal.

[0116] C6. The sensing system of C5, wherein the multiplexing system is configured to multiplex the RF drive signal such that one of the key sensors is driven in each of a set of time slots, and wherein the temperature-compensation system is configured to apply the off-resonance drive signal during an additional time slot to the set of time slots.

[0117] C7. The sensing system of C1, wherein each of the plurality of key sensors further comprises a deformable element configured to limit motion of one or both of the passive resonant circuit and the active resonant circuit for pressure sensing.

[0118] C8. The sensing system of C1, further comprising a substrate supporting the active resonant circuits for the sensors in a sequence corresponding to a sequence of keys of the keyboard.

[0119] C9. A keyboard, for a keyboard instrument, comprising the sensing system of claim 1.

[0120] C10. A polyphonic aftertouch keyboard comprising the sensing system of C8, each key of the plurality of keys having a deformable end-stop, such that an after-touch position corresponds to movement of a key beyond an end-stop position defined by the deformable end-stop, wherein identification of the aftertouch position for the key enables polyphonic aftertouch.

[0121] C11. A method of compensating a response of a keyboard, the keyboard comprising keys each having a sensor comprising an active resonant circuit, a passive tuned resonant circuit and a detector, the method comprising: retrieving from storage a detected initial output signal of the sensor, O.sub.t0, at a first time, t.sub.0, wherein at to said active resonant circuit is being driven at a frequency below a resonant frequency of said active resonant circuit; and periodically, for at least one of the sensors: detecting a later output signal of the sensor, O.sub.t1, at a time after t.sub.0; calculating an adjustment value, wherein the adjustment value is a difference between the initial output signal of the sensor and the later output signal of the sensor; and compensating the response of the keyboard by adjusting an operational output of the sensor using the adjustment value, where the operational output is an output from the sensor when the active resonant circuit is being driven at the resonant frequency of the active resonant circuit.

[0122] C12. The method of C11, further comprising operating the sensor according to a time division multiplexed addressing scheme, and using a time slot of the time division multiplexed addressing scheme in which the sensor is not operational for the detecting.

[0123] C13. A sensing system for a keyboard, the sensing system comprising: a plurality of key sensors, wherein each of the plurality of key sensors comprises: a passive resonant circuit, and an active resonant circuit, the passive resonant circuit having a resonant frequency, the active resonant circuit being configured to excite the passive resonant circuit at the resonant frequency; the sensing system further comprising: at least one sensor driver to drive the active resonant circuits with an RF drive signal; and at least one detector to detect a level of an RF signal from a driven key sensor for sensing a position and/or velocity of a key associated with the key sensor, wherein positions of physically adjacent passive resonant circuits of adjacent key sensors from among the plurality of key sensors are offset.

[0124] C14. The sensing system of C13, wherein the at least one sensor driver drives the active resonant circuits with the RF drive signal at the resonant frequency.

[0125] C15. The sensing system of Cl, further comprising a processor configured to process the detected level of the RF signal from the driven key sensor to determine a motion of the key associated with the key sensor over a succession of time intervals as the key moves between released and depressed positions, wherein the motion of the key comprises the position and the velocity of the key as the key moves between the released and depressed positions.

[0126] 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.