WO2007003913A2

WO2007003913A2 - Position sensing apparatus and method

Info

Publication number: WO2007003913A2
Application number: PCT/GB2006/002436
Authority: WO
Inventors: David Alun James
Original assignee: Sensopad Ltd
Current assignee: Sensopad Ltd
Priority date: 2005-06-30
Filing date: 2006-06-30
Publication date: 2007-01-11
Anticipated expiration: 2007-12-30
Also published as: WO2007003913A3; GB0513414D0

Abstract

There is discussed a position encoder comprising first and second members which are movable relative to each other. A transmit aerial and a receive aerial are fixed relative to the first member, and a layer of magnetisable material is positioned between the transmit aerial and the receive aerial. A magnetic field generator is fixed relative to the second member, and the magnetic field generator generates a magnetic field which causes the layer of magnetisable material to have a positionally varying magnetisation state such that the electromagnetic coupling between the transmit aerial and the receive aerial varies with the position of the magnetic field generator. An excitation circuit applies an excitation signal to the transmit aerial in order to generate an oscillating magnetic field, and a processing circuit processes a sense signal induced in the receive aerial to determine a value indicative of the relative position between the first and second members. In an embodiment, the first member is a pen device and the second member forms part of a tablet apparatus so as to forms a pen- input system.

Description

POSITION SENSING APPARATUS AND METHOD

This application claims the right to priority based on British patent application number 0513414.3 filed on 30 June 2005, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.

This invention relates to a method of sensing the position or speed of an object, and an apparatus therefore.

There are many known types of position sensor. Of these, inductive sensors have attracted interest as a low-cost position sensor which allows non-contact position determination with good resolution. In an inductive position sensor, the electromagnetic coupling between a transmit aerial and a receive aerial varies with movement between • two members so that the relative position of the two members can be detected. In some embodiments, the transmit aerial and the receive aerial are both fixed to the same member and an intermediate coupling element is fixed on the other member, whereas in other embodiments the transmit aerial is fixed to one member and the receive aerial is fixed to the other member.

US patent numbers 4,639,667, 4,774,465 and 5,204,621 and International patent applications WO 97/14935 and WO 2005/085763 (the whole contents of all these patent documents being hereby incorporated herein in their entirety by reference) discuss position sensors in which the position of a magnet is detected by virtue of the magnetic field produced by the magnet locally saturating a magnetic film. The present invention relates to this type of magnet position sensor.

Various embodiments of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 schematically shows an exploded perspective view of a position sensor according to a first embodiment of the invention;

Figure 2A schematically shows the layout of a sine coil which forms part of the position sensor illustrated in Figure 1;

Figure 2B schematically shows the layout of a cosine coil which forms part of the position sensor illustrated in Figure 1; Figure 2C schematically shows the layout of a sense coil which forms part of the position sensor illustrated in Figure 1;

Figure 3 schematically shows the main electrical components of the position sensor illustrated in Figure 1;

Figure 4 schematically shows the magnetic flux lines generated by a magnet forming part of the position sensor illustrated in Figure 1, in which the poles of the magnet are aligned parallel with a printed circuit board on which the sine coil, cosine coil and sense coil are formed;

Figure 5 shows a graph illustrating the variation in relative permeability of a permeable layer forming part of the position sensor illustrated in Figure 1 as a result of the presence of a magnet aligned parallel with the printed circuit board as illustrated in Figure 4;

Figure 6 schematically shows the magnetic flux lines generated by a magnet forming part of the position sensor illustrated in Figure 1, in which the poles of the magnet are aligned perpendicular to the printed circuit board on which the sine coil, cosine coil and sense coil are formed;

Figure 7 shows a graph illustrating the variation in relative permeability of a permeable layer forming part of the position sensor illustrated in Figure 1 as a result of the presence of a magnet aligned perpendicular to the printed circuit board as illustrated in Figure 6; Figure 8 shows a graph illustrating the variation in magnetic field strength with position as a result of a magnet aligned perpendicular to the printed circuit board as illustrated in Figure 6;

Figure 9 schematically shows a side view through a pen input system forming a second embodiment of the invention;

Figures 1OA to 1OE respectively show the layout of five different coils which are formed on a printed circuit board forming part of the pen input system illustrated in Figure 9;

Figure 11 schematically shows a side view through a first alternative position sensor to the position sensor illustrated in Figure 1;

Figure 12 schematically shows a side view through a second alternative position sensor to the position sensor illustrated in Figure 1;

Figure 13 schematically shows a side view through a third alternative position sensor to the position sensor illustrated in Figure 1; Figure 14 schematically shows a side view through a fourth alternative position sensor to the position sensor illustrated in Figure 1;

Figure 15 schematically shows a side view through a fifth alternative position sensor to the position sensor illustrated in Figure 1; Figure 16 schematically shows a side view through a sixth alternative position sensor to the position sensor illustrated in Figure 1;

Figure 17 schematically shows a side view through a seventh alternative position sensor to the position sensor illustrated in Figure 1;

Figure 18 schematically shows a side view through an eighth alternative position sensor to the position sensor illustrated in Figure 1; Figure 19 schematically shows a side view through a ninth alternative position sensor to the position sensor illustrated in Figure 1;

Figure 20 schematically shows a side view through a tenth alternative position sensor to the position sensor illustrated in Figure 1;

Figure 21 schematically shows a side view through a tenth alternative position sensor to the position sensor illustrated in Figure 1;

Figure 22 schematically shows a side view through an eleventh alternative position sensor to the position sensor illustrated in Figure 1;

Figure 23 schematically shows a side view through a twelfth alternative position sensor to the position sensor illustrated in Figure 1; Figure 24 schematically shows a side view through the tip of an alternative pen input device for the pen input system illustrated in Figure 9; and

Figure 25 schematically shows a side view through the tip of a second alternative pen input device for the pen input system illustrated in Figure 9.

FIRST EMBODIMENT

Overview Referring to Figure 1, in the first embodiment of the invention the position of a magnet 1 along a measurement direction (the x-direction in Figure 1) is determined by a signal generation and processing unit 3 which supplies excitation signals to a transmit aerial formed on a printed circuit board (PCB) 5 and processes resultant sense signals induced in a receive aerial which is also formed on the PCB 5.

A layer of permeable material 7 is sandwiched between the PCB 5 and a layer of metal 9 (shown in exploded view in Figure 1 for ease of illustration) . In this embodiment, the permeable material 7 is a 0.05mm thick layer of MuMetal, which is a nickel-iron alloy composed of 77% nickel, 15% iron, plus copper and molybdenum. MuMetal is a high permeability, magnetically soft alloy whose magnetism saturates in the presence of a comparatively low magnetic field, such that the relative permeability varies from a maximum unsaturated value in the region of eighty thousand to a minimum saturated value of approximately one. In this embodiment_r the metal 9 is a 5mm thick layer of aluminium.

In this embodiment, when the signal generation and processing unit 3 applies oscillating excitation signals to the transmit aerial, an oscillating excitation magnetic field is generated. In the absence of the magnet 1, the magnetic flux of the excitation magnetic field on the side of the PCB 5 adjacent the permeable layer 7 is predominantly confined to the permeable layer 7. The excitation magnetic field induces eddy currents in the permeable layer 7 (as MuMetal is conductive) , and these eddy currents generate an oscillating secondary magnetic field which opposes the excitation magnetic field. As will be explained in more detail hereafter, the transmit aerial and the receive aerial are designed such that in the absence of the magnet 1, the transmit aerial and the receive aerial are balanced so that substantially no signal is induced in the receive aerial by the combination of the excitation magnetic field and the secondary magnetic field.

When the magnet 1 is placed adjacent the metal layer 9, the static magnetic field associated with the magnet saturates a localised portion of the permeable layer 7 adjacent to the magnet 1.- This saturation results in: i) an increase in the skin depth of the permeable layer 7 in the locally saturated region; and ii) magnetic flux leaking out of permeable layer 7 at the saturated region. This leads to a signal being induced in the receive aerial which varies in dependence on the position of the magnet 1. The inventor's present understanding of the physical mechanisms involved is as follows.

The increase in the skin depth is due to the skin depth being inversely proportional to the square root of the permeability. Accordingly, a decrease in permeability from eighty thousand to one gives an approximately three hundredfold increase in the skin depth. This is significant because the increase in skin depth leads to a localised decrease in the sheet resistance of the permeable layer 7, and accordingly a localised increase in the magnitude of eddy currents formed in the saturated portion of the permeable material 7 which leads to a localised enhancement of the secondary magnetic field. As the localised enhancement of the secondary magnetic field is not balanced with respect to the receive aerial, a signal is induced, in the receive aerial which is dependent on the position of the saturated portion of the permeable material 7, and hence is dependent on the position of the magnet 1.

In addition, the leakage of magnetic flux from the saturated portion of the permeable layer 7 has the effect of locally changing the coupling between the receive aerial and the excitation magnetic field, in particular a localised change in the electromagnetic coupling between the receive aerial and the excitation magnetic field at the saturated portion of the permeable material 7. This results in a signal being generated in the receive aerial which is dependent on the position of the saturated portion of the permeable material 7, and hence the position of the magnet 1.

Therefore, for a transmit aerial and a receive aerial which are balanced relative to each other, in the absence of the magnet 1 a null signal is induced in the receive aerial when excitation signals are supplied to the transmit aerial . When the static magnetic field associated with the presence of the magnet 1 causes local saturation of the permeable material 7, the change in coupling between the excitation magnetic field and the receive aerial at the saturated portion and the increased eddy currents at the saturated portion lead to signals being induced in the receive aerial indicative of the position of the magnet 1.

The various components of the position sensor will now be described in more detail. The Transmit Aerial and the Receive Aerial

In this embodiment, the transmit aerial is formed by a sine coil 15 and a cosine coil 17.

As shown in Figure 2A, the sine coil 15 is formed by a conductive track which generally extends around the periphery of the PCB 5 apart from a cross-over point halfway along the PCB 5 in the measurement direction, at which the conductive track on each widthwise edge of the PCB 5 crosses to the corresponding opposing widthwise edge of the PCB 5. In this way, effectively a first current loop 21a and a second current loop 21b are formed. When a signal is applied to the sine coil 15, current flows around the first current loop 21a and the second current loop 21b in opposite directions, and therefore the current flowing around the first current loop 21a generates a magnetic field which has an opposite polarity to the magnetic field generated by current flowing around the second current loop 21b. This results in the sinusoidal variation of the field strength of the component of the first magnetic field B₁ resolved perpendicular to the PCB 5.

In particular, the lay-out of the sine coil 15 is such that the field strength of the component of the first magnetic field B₁ resolved perpendicular to the PCB 5 which is generated by current flowing through the sine coil 15 varies along the measurement direction from approximately zero at the point where x equals 0 , to a maximum value at x equals L/4 (the position A as shown in Figure 2A) , then back to zero at x equals L/2 (the position C as shown in Figure 2A) , then to a maximum value (having opposite polarity to the maximum value at position A) at x equals 3L/4, and then back to zero at x equals L. Thus the sine coil 15 generates a magnetic field component perpendicular to the PCB 5 which varies according to one period of the sine function.

As shown in Figure 2B, the cosine coil 17 is formed by a conductive track which generally extends around the periphery of the PCB 5 apart from two cross-over points, located one-quarter and three-quarters of the way along the PCB 5 in the measurement direction respectively. In this way, three loops 23a, 23b and 23c are formed of which the outer loops 23a and 23c are half the size of the inner loop 23b. When a signal is applied to the cosine coil 17, current flows in one direction around the outer loops 23a and 23c and in the opposite direction around the inner loop 23b. In this way, the magnetic field generated by the current flowing around the inner loop 23b has an opposite polarity to the magnetic field generated by the current flowing around the outer loops 23a and 23c. This results in a sinusoidal variation of the field strength of the component of the second magnetic field B₂ resolved perpendicular to the PCB 5.

In particular, the lay-out of the cosine coil 17 is such that the field strength of the component of the second magnetic field B₂ resolved perpendicular to the

PCB 5 which is generated by current flowing through the cosine coil 17 varies along the measurement direction from a maximum value at x equals 0, to zero at x equals L/4 (the position A as shown in Figure

2B) , then back to a maximum value (having opposite polarity to the maximum value at x equals 0) at x equals L/2 (the position C as shown in Figure 2B) , and then back to zero at x equals 3L/4 , and then back to a maximum value (having the same polarity as the maximum value at x equals 0) at x equals L. Thus, the cosine coil 17 generates a magnetic field component perpendicular to the PCB 5 which varies according to one period of the cosine function.

In this embodiment, the receive aerial is formed by a sense coil 19. As shown in Figure 2C, the sense coil 21 is formed by a conductive track which generally extends around the periphery of the PCB 5 forming a single loop.

The layout of the sine coil 15 and the sense coil 19 is such that, in the absence of the magnet 1, the electric current induced in the sense coil 19 by current flowing around the first current loop 21a is substantially cancelled out by the electric current induced in the sense coil 19 by current flowing around the second current loop 21b. Similarly, for the cosine coil 17 in the absence of the magnet 1 the current induced in the sense coil 19 by the outer loops 23a, 23c is cancelled out by the current induced in the sense coil 19 by the inner loop 23b.

The Signal Generation and Processing Unit The main components of the signal generation and processing apparatus 3 will now be described with reference to Figure 3.

As shown, a quadrature signal generator 31 outputs a quadrature pair of signals at a modulation frequency f_x to a modulator 33, which uses the quadrature pair of signals to modulate a carrier signal, at a carrier frequency f₀, generated by a signal generator 35. In this embodiment, the modulation frequency f_x is 3.9 kHz and the carrier frequency f₀ is 2 MHz. The pair of modulated signals are respectively input to a pair of coil drivers 37a, 37b, which amplify the modulated signals to produce an in-phase signal I(t) and a quadrature signal Q(t) .

The in-phase signal I(t) and the quadrature signal Q(t) are respectively applied to the sine winding 15 and the cosine winding 17. As discussed previously, in the absence of the magnet 1, signals flowing through the cosine winding 17 and the sine winding 15 induce a negligible signal in the sense winding 19. However, the static magnetic field associated with the magnet 1 causes localised saturation of the permeable layer I₁ resulting in a signal being induced in the sense winding 19 which varies with the position of the magnet 1 along the measurement direction. In particular, an electro-motive force (EMF) is induced in the sense winding 19 which results in an induced sense signal S(t) of the form:

(1)

where L is the period of the sinusoidal variation in the magnetic field strength component along the measurement axis produced by the cosine winding 17 and the sine winding 15. The sense signal S(t) therefore corresponds to a signal at the modulation frequency £₁ having a phase which varies linearly with the position of the magnet 1 modulated by a signal at the carrier frequency f₀

The sense signal S (t) is input to a demodulator 39 which demodulates the received sense signal S (t) , using a signal at the carrier frequency f₀ from the signal generator 35, to form a demodulated signal at the modulation frequency f₁. The demodulated signal output by the demodulator 39 is input to a phase detector 41, which measures the phase of the demodulated signal, and outputs the phase measurement to a position calculator 43.

The position calculator 43 determines the position of the magnet 1 from the phase measurement output by the phase detector 41. In particular, in this embodiment the position is substantially proportional to the measured phase, and therefore the phase calculator simply multiplies the measured phase by a calibration factor.

In this embodiment, the circuitry within the signal generation and processing unit 3 is a modified version of the circuitry used in the position sensor described in WO 03/038379, the modification being that the synchronous detection at a quarter cycle phase shift from the carrier frequency is not used as in this embodiment the sense signal is in phase (or in antiphase) with the excitation signals applied to the sine coil 15 and the cosine coil 17.

The Orientation of the Magnet

As discussed above, the magnet 1 generates a static magnetic field. The magnetic flux lines associated with the magnet 1 follow paths extending between the north and south poles of the magnet. It will be appreciated that given the small thickness of the permeable layer 7, only magnetic field components in the plane of the permeable layer 7 affect the magnetisation of the permeable layer. Figure 4 schematically shows flux lines associated with the static magnetic field generated when the magnet 1 is configured with its magnetic axis aligned parallel with the plane of the permeable layer 7. As shown, the magnetic flux lines bunch within the permeable layer 7 apart from in a saturated region in which the magnetic flux lines leak out of the permeable layer 7.

Figure 5 is a graph showing the variation of the relative permeability of the permeable layer 7 along the measurement direction when the magnet 1 is positioned with its magnetic axis parallel with the plane of the permeable layer 7. As shown, away from the region adjacent to the magnet 1 (from about 10 to 25 in Figure 5) the relative permeability is generally about seventy-five thousand. However, in the region adjacent to the magnet 1 the relative permeability drops to approximately one due to magnetic saturation.

Figure 6 schematically shows flux lines associated with the static magnetic field generated when the magnet 1 is configured with its magnetic axis aligned perpendicular to the plane of the permeable layer 7, while Figure 6 is a graph showing the variation in relative permeability of the permeable layer 7 along the measurement direction. As shown, in a portion directly adjacent the magnet 1 the magnetic flux lines are generally perpendicular to the plane of the permeable layer 7 and accordingly have a negligible effect on the magnetisation of the permeable layer 7. This can be seen in Figure 8, which is a graph showing how the in-plane magnetic field component and the out- of-plane (i.e. perpendicular to the plane of the permeable layer) magnetic field component vary with position along the measurement direction.

As the static magnetic field of the magnet 1 has little effect on the magnetisation of region of the permeable layer directly adjacent the magnet 1, the relative permeability of the permeable layer 1 is substantially unchanged from its unsaturated value in that region. However, the region surrounding the unsaturated region adjacent the magnet 1, saturation does occur leading to the relative permeability in that saturated region approaching one .

Although when the magnetic axis is aligned perpendicular to the permeable layer 1 the region adjacent the magnet 1 is unsaturated, the signal generation and processing unit still calculates the correct position of the magnet 1 because the receive aerial effectively integrates the signal induced as a result of the saturated region. Accordingly, whether the orientation of the magnetic axis of the magnet 1 is parallel with or perpendicular to the plane of the permeable layer 7 is not critical.

SECOND EMBODIMENT

In the first embodiment, the position of a magnet along a rectilinear measurement direction is detected. A second embodiment will now be described with reference to Figures 9 and 10 in which the magnet is housed within a stylus, and the aerials and associated circuitry are modified to allow two-dimensional position detection, in order to form a pen-input device .

As shown in Figure 9, the pen device 49 has a magnet 51 mounted within a pen housing 53 adjacent the pen tip 55. In this embodiment, a resonant circuit 54 is also mounted within the pen housing 53 on the other side of the magnet 51 to the pen tip 55. The resonant frequency of the resonant circuit 54 is selected from a predetermined set of resonant frequencies, with each of the set of resonant frequencies being associated with a respective different type of pen. For example, a first resonant frequency may be associated with a black pen, a second resonant frequency may be associated with a red pen and so on. Further, a selected resonant frequency may be associated with an eraser device .

A tablet 57 includes a PCB 59 mounted between a fascia panel 61, which in this embodiment is a 5mm plastic sheet, and a layer of permeable material 63, in this embodiment a 0.05mm thick layer of MuMetal . A transmit aerial and a receive aerial are formed on the PCB 59, and are connected to a signal generation and processing unit 65.

In this embodiment, the permeable layer 63 is on the far side of the PCB 59 from the magnet 51. In this way, the magnet flux lines on the pen side of the PCB 59 are substantially unaffected by the permeable layer 63 , and accordingly couple with the resonant circuit 54. However, the static magnetic field associated with the magnet 51 passes through the PCB 51, and accordingly can still saturate the permeable layer 63.

In order to achieve the two-dimensional position measurement, a sine coil and a cosine coil pair is aligned in each of two orthogonal measurement directions, which will hereafter be referred to as the X direction and the Y direction. Figures 1OA to 1OE show the conductive tracks which form the transmit aerial and the receive aerial. In particular, Figure 1OA and 1OB respectively show the sine coil 71 and the cosine coil 73 for the X direction, Figure 1OC and 1OD respectively show the sine coil 75 and the cosine coil 77 for the Y direction, and Figure 1OE shows the sense coil 79.

The signal generation and processing unit 65 is a modified version of the signal generation and processing unit described in WO 03/038379, the modifications being that:

1. the excitation signal generation circuitry is operable to alternately excite i) the sine coil 71 and the cosine coil 73 in the X direction, and ii) the sine coil 75 and the cosine coil 77 in the Y direction_/ and

2. the excitation signal generation circuitry is operable to vary the carrier frequency.

In use, in order to sense the position of the magnet 51, and hence the pen tip 55, in the X direction, the signal generation and processing unit 65 applies an in-phase signal I(t) (or anti-phase signal) to the sine coil 71 for the X direction and a quadrature signal Q(t) to the cosine coil 73 for the X direction, with the carrier frequency not matching any of the set of resonant frequencies possibly associated with the resonant circuit 54. As in the first embodiment, the static magnetic field associated with the magnet 51 causes local saturation of the permeable layer 63, resulting in a signal being induced in the sense coil 79 which is indicative of the position of the pen tip in the X-direction. Similarly, in order to sense the position of the pen tip 55 in the Y direction, the signal generation and processing unit 65 applies the same in-phase signal I(t) (or anti-phase signal) to the sine coil 75 for the Y direction and the same quadrature signal Q(t) to the cosine coil 77 for the Y direction and measures the phase of the signal induced in the sense coil 79. In this embodiment, the position of the sensor element in the X direction and the position of the sensor element in the Y direction are separately measured.

In this embodiment, separately from the position measurement the signal generation and processing circuitry 65 intermittently applies excitation signals with a carrier frequency at each of the predetermined set of resonant frequencies . When the carrier frequency matches the resonant frequency of the resonant circuit 54, a strong signal is induced in the sense coil 79. In this way, the signal generation and processing unit 65 is able to identify the type of pen device .

Using a combination of a magnet 51 and a resonant circuit 54 is advantageous. Although the resonant circuit 54 could be used for position measurement, it is preferred to use the magnet 51 because the position detection is then less sensitive to the orientation of the pen device 49. However, the magnet 51 does not allow any identification of the type of pen device 49, and accordingly the use of the resonant circuit 54 advantageously also allows identification of the type of pen device. FURTHER EMBODIMENTS

In the first embodiment the permeable layer is formed on the PCB between the magnet and the PCB, whereas in the second embodiment, the permeable layer is formed on the far side of the PCB from the magnet . As illustrated in Figure 11, a first alternative position sensor has a permeable layer 7a, 7b formed on each side of the PCB 5 on which the transmit aerial and receive aerial is formed. This can be particularly advantageous when the position sensor is used in a context requiring strict controls on electromagnetic emissions as the magnetic field generated by the transmit aerial is largely confined within the permeable layers 7a, 7b.

In the first and second embodiments, the transmit aerial and the receive aerial are formed on the same PCB. As illustrated in Figure 12, a second alternative position sensor has the transmit aerial formed on a first PCB 5a and the receive aerial formed on a second PCB 5b, with a permeable layer 7 being sandwiched between the first and second PCBs 5a, 5b. In this way, in the absence of the magnet 1 the permeable layer 7 substantially isolates the receive aerial from the transmit aerial. However, when the static magnetic field of the magnet 1 saturates a localised portion of the permeable layer 7 , this localised portion effectively forms a window through which the magnetic field generated by the transmit aerial is able to couple with the receive aerial . In this way, a signal is induced in the receive aerial which is indicative of the position of the localised saturated portion and hence is indicative of the position of the magnet 1. While the second alternative position sensor has been described as having two separate PCBs, it will be appreciated that it is possible to use a single PCB structure having a permeable layer embedded therein.

As illustrated in Figure 13 , a third alternative position sensor is a modified version of the second alternative position sensor illustrated in Figure 12 in which a second permeable layer has been added on the side of the first PCB 5a (i.e. the PCB on which the transmit aerial is formed) away from the second PCB 5b. In this way, both sides of the transmit aerial are covered by permeable material thereby reducing electromagnetic emissions in the same way as for the first alternative position sensor.

In the first embodiment, the permeable layer is positioned between a metal layer and the PCB. In a fourth alternative position sensor, as shown in Figure 14, a second metal layer 9b is formed on the side of the PCB opposite the permeable layer 7, so that the transmit and receive aerials are effectively surrounded by metal. This is advantageous in many applications. For example, in the aerospace industry in addition to providing extra strength surrounding the aerials and permeable material with metal both reduces electromagnetic emissions and improves immunity to external electromagnetic fields . For some medical devices, an additional advantage is that the metal casing may be biocompatible (for example stainless steel) .

In the second embodiment, a fascia plate separates the PCB from the pen device, with a permeable layer being positioned the other side of the PCB. In a similar fashion, as illustrated in Figure 15 a fifth alternative position sensor has a metal layer 9 between the PCB 5 and the magnet 1, with the permeable layer 7 being formed on the other side of the PCB 5 to the metal layer 9. As illustrated in Figure 16, in a sixth alternative position sensor a second metal layer 9b is added so that the permeable layer 7 is between the PCB 5 and the second metal layer 9b in order to reduce electromagnetic emissions.

In the second and third alternative position sensors illustrated in Figures 12 and 13 respectively, the transmit aerial is formed on a first printed circuit board 5a and the receive aerial is formed on a second printed circuit board 5b, with the first and second printed circuit boards 5a, 5b being separated by a permeable layer 7. Around this basic three-layer structure, a metal layer 9 could be placed on one side (as in the seventh alternative position sensor illustrated in Figure 17) or on both sides (as shown in the ninth alternative position sensor illustrated in Figure 19) . Further, an additional permeable layer could be positioned so that the transmit aerial is effectively surrounded by permeable material (as shown in the eighth and ninth alternative position sensors respectively illustrated in figures 18 and 19) .

A tenth alternative position sensor is illustrated in Figure 20. The tenth alternative position sensor has a first PCB 5a carrying the transmit aerial sandwiched between second and third PCBs 5b, 5c which respectively carry first and second receive aerials (which are identical to each other) . A first permeable layer 7a is provided between the magnet 1 and the second PCB 5b so that when a portion of the first permeable layer saturates, a signal is induced in the first receive aerial indicative of the position of the magnet 1. A second permeable layer 7b is provided adjacent the third PCB 5c so that the third PCB 5c is sandwiched between the first PCB 5a and the second permeable layer 7b.

For the tenth alternative position sensor, any background magnetic fields have substantially the same effect on the first permeable layer 7a and the second permeable layer 7b. Accordingly, any signals induced in the first and second receive aerials by virtue of background magnetic fields will produce substantially identical signals in the first receive aerial and the second receive aerial . The outputs of the first and second receive aerials are input to a difference circuit which generates a signal corresponding to the difference between the output signals. Accordingly, signals induced in the first and second receive aerials by a background magnetic field substantially cancel out. When the magnet 1 is placed adjacent to the first permeable layer 7a, there will be a significant difference between the static magnetic field lines generated by the magnet 1 at the first permeable layer 7a and the second permeable layer 7b. Accordingly, the signals induced in the first and second receive aerials by the magnet 1 are not identical. In this way, the difference circuit 101 outputs to the signal processing circuitry a signal which corresponds to the position of the magnet 1 with reduced error due to background magnetic fields.

The use of two receive aerials and a difference circuit is also possible for embodiments where a permeable layer is sandwiched between the transmit aerial and the receive aerial, as in the second alternative position sensor illustrated in Figure 12. For example, Figure 21 shows an eleventh alternative position sensor in which a transmit aerial is formed on a first PCB 5a, and first and second receive aerials are respectively formed on second and third PCBs 5b, 5c. A first permeable layer 7a is sandwiched between the first and second PCBs 5a, 5b to separate the transmit aerial and the first receive aerial, and a second permeable layer 7b is sandwiched between the first and third PCBs 5a, 5c to separate the transmit aerial and the second receive aerial . The outputs of the first and second receive aerials are input to a difference circuit 101 so that, in the same manner as the tenth alternative position sensor, any signals induced in the first and second receive aerials by background magnetic fields will substantially cancel out, while signals induced by the magnet 1 will not due to the difference between the static magnetic field produced by the magnet 1 at the first and second permeable layers 7a, 7b.

A similar effect to using two receive aerials and a difference circuit may be achieved by using two transmit aerials which produce opposite magnetic fields. Accordingly, Figure 22 shows a twelfth alternative position sensor in which a receive aerial is formed on a first PCB 5a, and first and second transmit aerials (which in response to receiving identical excitation signals produce opposite magnetic fields) are respectively formed on second and third

PCBs 5b, 5c. A first permeable layer 7a is sandwiched between the first and second PCBs 5a, 5b to separate the first transmit aerial and the receive aerial, and a second permeable layer 7b is sandwiched between the first and third PCBs 5a, 5c to separate the second transmit aerial and the receive aerial . As any background magnetic field will affect the first and second permeable layers 7a, 7b in substantially the same manner, such background magnetic fields cause the first and second transmit aerials to induce signals which are substantially equal in magnitude but opposite in polarity in the receive aerial, thereby generating a substantially null signal. However, the magnet field associated with the magnet 1 at the first and second permeable layers 7a, 7b is significantly different, resulting in a signal being induced in the receive aerial by the first and second transmit aerials which is indicative of the position of the magnet 1.

A similar effect is observable in a system, as in the thirteenth alternative position sensor illustrated in Figure 23, in which a first PCB 5a carrying a receive aerial is sandwiched between second and third PCBs 5b, 5c carrying first and second transmit aerials, with permeable layers 7a, 7b being positioned on either side of the three PCBs 5.

In the second embodiment, a magnet is positioned in the tip of a pen device. While the position detection system is relatively insensitive to the position of the magnet, this insensitivity can be improved by modifying the magnetic field produced by the magnet from a simple dipole field.

As shown in Figure 24, in the tip of an alternative pen input device for the second embodiment, a steel ball bearing 123 is positioned in the tip 125 of the alternative pen input device, and a regular dipolar magnet 121 is fixed to the other side of the ball bearing 123 from the stylus tip 125. The magnetic axis of the magnet 121 is radially aligned relative to the ball bearing 123 so that magnetic flux enters the ball bearing 123. The ball bearing 123 modifies the magnetic flux distribution so that the magnetic flux lines pass normally through the surface of the ball bearing 123. In this way, the magnetic field distribution around the surface of the ball bearing 123 is made more homogeneous than would be the case in the absence of the ball bearing 123. In this way, the sensitivity to the orientation of the pen input device is reduced.

In the second embodiment, the pen device includes a resonant circuit having a resonant frequency selected from a set of resonant frequencies to identify the type of the pen device . In a second alternative pen input device, as schematically shown in Figure 25, in addition to the static magnet, the pen input device has a resonant circuit whose resonant frequency is indicative of the pressure applied to the tip of the pen input device. As shown in Figure 25, the tip of the housing 131 of the second alternative pen input device has an opening through which a steel ball bearing 123 protrudes. A dipolar magnet 121 is fixed to the ball bearing 123 in the same manner as for the first alternative pen input device. The ball bearing 123 and magnet 121 assembly is fixed to the pen housing 131 via two spring members 133a, 133b which bias the ball bearing in a direction out of the opening .

The resonant circuit is formed by a planar inductor (not shown) and a capacitor formed by a first capacitor plate 135a and a second capacitor plate 135b. The first capacitor plate 135a is fixed relative to the ball bearing 123 and magnet 121 assembly while the second capacitor plate 135b is fixed relative to the pen housing 131. During use, as the user pushes the ball bearing 123 into the tablet surface with pressure, the ball bearing 123 moves relative to the pen housing 131 leading to relative movement between the first capacitor plate 135a and the second capacitor plate 135b. The resultant change in capacitance leads to a change in the resonant frequency of the resonant circuit. In an embodiment, the signal generation and processing unit intermittently scans the carrier frequency to identify the resonant frequency of the resonant circuit, to give a reading which is representative of the pressure applied to the pen input device.

In another alternative pen input device, the pen input device includes a switch which changes the capacitance of the resonant circuit, leading to a change in the resonant frequency of the resonant circuit . In this way, for example, with the switch in one position the pen input device could act as a coloured pen and with the switch in another position the pen input device could act as an eraser.

In another alternative embodiment, an elongate pen device could have at one end a first tip with a magnet and a resonant circuit having a first resonant frequency, and at the other end a second tip with a magnet and a resonant circuit having a second resonant frequency different from the first resonant frequency. In this way, one end of the pen device could act as a pen and the other end of the pen device could act as an eraser.

MODIFICATIONS

It will be appreciated that any of the position encoder systems described in the 'Further Embodiments' section above could be used in a pen-input system such as discussed in the second embodiment.

While the use of the resonant circuit in the pen device of the second embodiment is preferred, it is not essential.

In the embodiments described above, the permeable layers (i.e. magnetisable layers) are formed of MuMetal . However there are many other types of suitable material which are readily available, for example: Metglas, Hipernom, HyMu-80, Permalloy, Supermalloy, Hyperco-50, Vanadium Permedur, Kerafol- f96, "Co-netic" alloys, Finemet, and Vitroperm. Alternatively it could just be a nickel film or a nickel electro-plating on a substrate. Any permeable material which can be locally saturated by a magnetic field giving a significant local change in relative permeability is suitable.

As discussed in the first embodiment, as MuMetal is conductive eddy currents play a significant role in the signal which is induced in the receive aerial. This will apply for any conductive permeable material . However, non-conductive permeable materials can also be used because the magnetic flux leakage at the locally saturated portion induces a signal in the receive aerial. In the described embodiments, a permanent magnet forms a magnetic dipole and generates a corresponding static magnetic field. It will be appreciated that other types of magnetic field generator could be used. For example, an electromagnet in which DC current flowing in a loop forms a magnetic dipole could be used. Further, a magnetic quadrupole or other type of multipolar magnet could be used to increase the gradient of the generated magnetic fields and reduce the far-field effect of the magnetic field generator. In addition, in the case of an electromagnet the field does not need to be completely static. In an embodiment, a slowly varying magnetic field is generated and the signalling processing includes phase sensitive detection at the frequency of the slowly varying magnetic field. In this way, any effects caused by steady-state background magnetic fields is substantially filtered out.

In the described embodiments, the sine coil 15 and the cosine coil 17 are arranged so that their relative contributions to the total magnetic field component perpendicular to the PCB 5 vary in accordance with position along the measurement direction. In particular, the sine and cosine coils have an alternate twisted loop structure. However, it would be apparent to a person skilled in the art that an enormous variety of different excitation winding geometries could be employed to form transmit aerials which achieve the objective of causing the relative proportions of the first and second transmit signals appearing in the ultimately detected combined signal to depend upon the position of the magnet in the measurement direction. The position sensor described in the first embodiment could be adapted to measure a linear position along a curved line, for example a circle (i.e. a rotary position sensor) by varying the layout of the sine coil and the cosine coil in a manner which would be apparent to persons skilled in the art. The position sensor could also be used to detect speed by periodically detecting the position of the sensor element as the sensor element moves along the measurement path, and then calculating the rate of change of position.

While in the described embodiments, the excitation windings are formed by conductive tracks on a printed circuit board, they could also be provided on a different planar substrate or, if sufficiently rigid, could even be free standing. Further, it is not essential that the excitation windings are planar because, for example, cylindrical windings could also be used with the magnet moving along the cylindrical axis of the cylindrical winding.

In the previous embodiments , a quadrature pair of modulation signals are applied to carrier signals to generate first and second excitation signals which are applied to the sine coil 15 and cosine coil 17 respectively. However, the use of a quadrature pair of modulation signals is not essential because it is merely required that the information carrying components of the excitation signals are distinct in some way so that the relative contributions from the first and second excitation signals can be derived by processing the combined signal. For example, the modulation signals could have the same frequency and a phase which differs by an amount other than 90 degrees . Alternatively, the modulation signals could have slightly different frequencies thus giving rise to a continuously varying phase difference between the two signals .

In the described embodiments, the excitation signal generating circuitry and the sense signal processing circuitry is based on that used in the position sensor described in GB 2374424A which uses a variation of an LVPT sensor in which the excitation signal comprises a high frequency carrier signal modulated by a low frequency, and the sense signal processor demodulates the sense signal to leave a signal at the modulation frequency having a phase which varies with the position of a sensor element. Alternatively, a more conventional LVPT arrangement could be used. In an embodiment, a quadrature pair of signals at a single excitation frequency are respectively applied to the sine and cosine windings of a transmit aerial as described in the first embodiment.

In the described embodiments, a transmit aerial is formed by two excitation windings and a receive aerial is formed by a single sensor winding. It will be appreciated that many other arrangements of transmit aerial and receive aerial in which the electromagnetic coupling between the transmit aerial and the receive aerial varies along a measurement path in dependence upon the magnetisation state of a magnetisable layer could be used. For example, the transmit aerial could be formed by a single excitation winding and the receive aerial could be formed by a pair of sensor windings. In an embodiment, the transmit and receive aerials and signal generation and processing techniques described in WO 95/31696 are utilised.

Although one specific layout of the windings is described, it will be appreciated that many different winding layouts are possible which achieve the same effects .

In the second embodiment, the resonant frequency of a resonant circuit incorporated within a pen device is measured by varying the oscillation frequency of an excitation signal applied to the transmit aerial in a frequency range including the resonant frequency, and monitoring the magnitude of the sense signal induced in the receive aerial. It will be appreciated that as no position measurement is taking place, only one of the excitation coils (i.e. the sine coils in the X and/or Y directions and the cosine coils in the X and/or Y directions in the second embodiment) need be energised.

Claims

1. A position encoder comprising: first and second members which. are movable relative to each other; a transmit aerial and a receive aerial which are fixed relative to the first member; a layer of magnetisable material positioned between the transmit aerial and the receive aerial; a magnetic field generator which is fixed relative to the second member, said magnetic field generator being operable to generate a magnetic field which causes the layer of magnetisable material to have a positionally varying magnetisation state such that the electromagnetic coupling between the transmit aerial and the receive aerial varies with the position of the magnetic field generator; an excitation circuit operable to apply an excitation signal to the transmit aerial in order to generate an oscillating magnetic field; and a processing circuit operable to process a sense signal induced in the receive aerial to determine a value indicative of the relative position between the first and second members .

2. A position encoder according to claim 1, wherein the magnetic field generator is operable to generate a static magnetic field.

3. A position encoder according to claim 1 or 2, further comprising a second layer of permeable material provided on the other side of the transmit aerial from the first-mentioned layer of permeable material .

4. A position encoder comprising: first and second members which are movable relative to each other,- a transmit aerial, a first receive aerial and a second receive aerial which are fixed relative to the first member; a first layer of magnetisable material positioned adjacent the first receive aerial such that the electromagnetic coupling between the transmit aerial and the first receive aerial is dependent on the magnetisation state of the magnetisable material of said first layer; a second layer of magnetisable material positioned adjacent the second receive aerial such that the electromagnetic coupling between the transmit aerial and the second receive aerial is dependent on the magnetisation state of the magnetisable material of said second layer; a magnetic field generator which is fixed relative to the second member, said magnetic field generator being operable to generate a magnetic field which causes the magnetisation of the first layer of magnetisable material to vary according to a first distribution and the magnetisation of the second layer of magnetisable material to vary according to a second distribution which is different from the first distribution; an excitation circuit operable to apply an excitation signal to the transmit aerial in order to generate an oscillating magnetic field; a differencing circuit operable to subtract a first sense signal induced in the first receive aerial from a second sense signal induced in the second receive aerial to produce a difference signal; and a processing circuit operable to process the difference signal to determine a value indicative of the relative position between the first and second members .

5. A position encoder according to claim 4, wherein the first layer of magnetisable material is positioned between the transmit aerial and the first receive aerial, and wherein the second layer of magnetisable material is positioned between the transmit aerial and the receive aerial.

6. A position encoder according to claim 4, wherein the first receive aerial is positioned between the transmit aerial and the first layer of magnetisable material, and wherein the second receive aerial is positioned between the transmit aerial and the second layer of magnetisable material .

7. A position encoder comprising: first and second members which are movable relative to each other; a first transmit aerial, a second transmit aerial and a receive aerial which are fixed relative to the first member; a first layer of magnetisable material positioned adjacent the receive aerial such that the electromagnetic coupling between the first transmit aerial and the receive aerial is dependent on the magnetisation state of the magnetisable material of said first layer; a second layer of magnetisable material positioned adjacent the receive aerial such that the electromagnetic coupling between the second transmit aerial and the receive aerial is dependent on the magnetisation state of the magnetisable material of said second layer; a magnetic field generator which is fixed relative to the second member, said magnetic field generator being operable to generate a magnetic field which causes the magnetisation of the first layer of magnetisable material to vary according to a first distribution and the magnetisation of the second layer of magnetisable material to vary according to a second distribution which is different from the first distribution; an excitation circuit operable to apply an excitation signal to the first and second transmit aerials in order to generate an oscillating magnetic field; a processing circuit operable to process a sense signal induced in the receive aerial to determine a value indicative of the relative position between the first and second members .

8. A position encoder according to claim 7, wherein the excitation circuit is arranged to apply identical excitation signals to the first and second transmit aerials, and wherein in response to the applied excitation signal the first transmit aerial is operable to generate an electromagnetic field having a first field distribution and the second transmit aerial is operable to generate an electromagnetic field having a second field distribution which is the opposite of the first field distribution.

9. A position encoder according to any preceding claim, further comprising a metal layer provided between the magnetic field generator and the one or more permeable layers .

10. A position encoder according to any preceding claim, wherein the or each transmit aerial and the or each receive aerial are encased in metal .

11. A position encoder according to any preceding claim, wherein the excitation circuit is operable to generate an excitation signal comprising a periodic carrier signal having a first frequency modulated by a periodic modulation signal having a second frequency, the first frequency being greater than the second frequency.

12. A position encoder according to claim 11, wherein the processing circuit comprises a demodulator operable to demodulate the signal received by the processing circuit to obtain a demodulated signal at the second frequency.

13. A position encoder according to claim 12, wherein the processing circuit further comprises a phase detector operable to detect the phase of the demodulated signal at the second frequency.

14. A pen-input system comprising: a pen device having a pen tip, wherein the pen device has a magnetic field generator in the vicinity of the pen tip; and tablet apparatus having: a transmit aerial ; a receive aerial; a layer of magnetisable material; an excitation circuit operable to apply an oscillating excitation signal to the transmit aerial in order to generate an oscillating magnetic field; and a processing circuit operable to process a sense signal induced in the receive aerial, wherein the magnetic field generator is operable to generate a magnetic field which causes the layer of magnetisable material to have a positionally varying magnetisation state such that the electromagnetic coupling between the transmit aerial and the receive aerial varies with the position of the magnetic field generator, and wherein the processing circuit is operable to process the sense signal induced in the receive aerial to determine a value indicative of the position of the pen device.

15. A pen-input system according to claim 14, wherein in use the layer of magnetisable material is provided on the opposite side of the transmit aerial from the pen device.

16. A pen-input system according to claim 14 or 15, wherein the pen device further has a resonant circuit having an associated resonant frequency, wherein the excitation circuit is operable to vary the oscillation frequency of the excitation signal in a frequency range including said resonant frequency, and wherein the processing circuit is operable to process the sense signal induced in the receive aerial as the oscillation frequency of the excitation signal is varied to identify the resonant frequency, and to output a signal which corresponds to the identified resonant frequency.

17. A pen-input system according to claim 16, comprising a plurality of pen devices having respective resonant frequencies .

18. A pen-input system according" to claim 17, wherein the resonant frequency of each pen device corresponds to a colour associated with that pen device .

19. A pen-input system according to claim 16, wherein the pen device further comprises a switch for changing the resonant frequency associated with the pen device.

20. A pen-input system according to claim 16, wherein the resonant frequency of the pen device varies in accordance with the amount of pressure applied to the pen tip.

21. A pen-input system according to claim 20, wherein the pen device has a pen housing and a pen tip portion resiliently mounted relative to the pen housing so that in response to pressure applied to the pen tip portion, the pen tip portion moves relative to the pen housing, and wherein the resonant circuit comprises a capacitor having a first capacitor body fixed relative to the pen tip portion and a second capacitor body fixed relative to the pen housing so that in response to movement of the pen tip portion relative to the pen housing, the capacitance of the capacitor varies, thereby varying the resonant frequency of the resonant circuit.

22. A pen-input system according to any of claims 16 to 21, wherein the magnetic field generator comprises a dipolar magnet having one pole adjacent a permeable spherical body.

23. A pen-input system according to claim 22, wherein the permeable spherical body is positioned between the pen tip and the dipolar magnet .

24. A pen-input system according to claim 22 or 23, wherein the permeable spherical body is a steel ball.

25. A pen-input system according to any of claims 14 to 24, wherein the layer of magnetisable material is positioned between the transmit aerial and the receive aerial.

26. A pen-input system according to claim 25, further comprising a second layer of magnetisable material on the other side of the transmit aerial from the first- mentioned layer of magnetisable material .

27. A pen-input system according to any of claims 14 to 24, wherein the receive aerial comprises a first receive aerial and a second receive aerial respectively provided on either side of the transmit aerial with a first layer of magnetisable material provided adjacent the first receive aerial and a second layer of magnetisable material provided adjacent the second receive aerial, and wherein the pen-input system further comprises a differencing circuit operable to subtract a signal induced in the first receive aerial from a signal induced in the second receive aerial .

28. A pen-input system according to claim 27, wherein the first layer of magnetisable material is positioned between the transmit aerial and the first receive aerial, and the second layer of magnetisable material is positioned between the transmit aerial and the second receive aerial .

29. A pen-input system according to claim 27, wherein the first receive aerial is positioned between the transmit aerial and the first layer of magnetisable material, and wherein the second receive aerial is positioned between the transmit aerial and the second layer of magnetisable material .

30. A pen-input system according to any of claims 14 to 24, wherein the transmit aerial comprises a first transmit aerial and a second transmit aerial respectively provided on opposing sides of the receive aerial .

31. A pen-input system according to claim 30, wherein the excitation circuit is arranged to apply identical excitation signals to the first and second transmit aerials, and wherein in response to the applied excitation signal the first transmit aerial is operable to generate an electromagnetic field having a first field distribution and the second transmit aerial is operable to generate an electromagnetic field having a second field distribution which is the opposite of the first field distribution.

32. A pen-input system according to any of claims 14 to 31, wherein the excitation circuit is operable to generate an excitation signal comprising a periodic carrier signal having a first frequency modulated by a periodic modulation signal having a second frequency, the first frequency being greater than the second frequency.

33. A pen-input system according to claim 32, wherein the processing circuit comprises a demodulator operable to demodulate the signal received by the processing circuit to obtain a demodulated signal at the second frequency.

34. A position encoder according to claim 33, wherein the processing circuit further comprises a phase detector operable to detect the phase of the demodulated signal at the second frequency.

35. A pen device having the pen device features of any of claims 14 to 34.

36. A tablet apparatus having the tablet apparatus features of any of claims 14 to 34.