WO2004020936A2

WO2004020936A2 - Multiturn absolute rotary position sensor with coarse detector for axial movement and inductive fine detector for rotary movement

Info

Publication number: WO2004020936A2
Application number: PCT/GB2003/003718
Authority: WO
Inventors: Mark Anthony Howard; Colin Sills; Darran Kreit; Bruce Macaulay
Original assignee: Tt Electronics Technology Limited
Priority date: 2002-08-27
Filing date: 2003-08-27
Publication date: 2004-03-11
Also published as: AU2003259372A8; EP1549912A2; WO2004020936A3; AU2003259372A1

Abstract

There is described a rotary position sensor for sensing the rotational movement of a shaft about an axis in which a follower is coupled to the shaft and moves over an associated range of linear movement in concert with rotation of the shaft about the axis, wherein each position within the range of rotary movement of the shaft corresponds to a single position within the range of linear movement of the follower. A first detector outputs a signal which is representative of a plurality of rotary positions of the shaft about the axis, and a second detector detects the linear position of the follower within the range of movement of the follower. A position calculator determines the position of the shaft within the range of rotary movement of the shaft using the positions detected by the first and second detectors. The first detector comprises a planar transmit aerial and a planer receive aerial having an electromagnetic coupling which varies in dependence upon the relative rotary position of the shaft and the mounting.

Description

SENSING APPARATUS AND METHOD

This invention relates to a method of sensing the position or the speed of an object, and an apparatus therefor. ■ The invention has particular, but not exclusive, relevance to a steering wheel assembly for an automobile in which the sensed object position information corresponds to the position of the steering wheel within its range of rotational movement.

In some new automobiles, the direct mechanical connection between the steering wheel and the wheels of an automobile has been replaced by an electrical connection. In such an arrangement, a rotary encoder detects rotary movement of the steering wheel and a control system within the car determines a corresponding control signal for a servo system which controls the steering of the wheels. In this way, if the movement of the steering wheel is likely to be dangerous (e.g. a sharp turn at high speed), the control system is able to ensure that the control signals applied to the servo system keep the steering action within safe levels.

Many types of rotary encoder are known which can determine the angular position of a rotatable object such as a steering wheel. For example, the inductive sensor described in UK patent application GB 2374424A can be used as a rotary encoder. A common problem with rotary encoders is that if the range of movement of the rotatable object is more than one full rotation, as is generally the case for a steering wheel for an automobile, then the position information from the rotary encoder corresponds to more than one position within the range of movement because the signal from the rotary encoder repeats with every revolution of the rotatable object .

According to an aspect of the present invention, there is provided an apparatus for monitoring rotational movement of a shaft about an axis in which a follower is coupled to the shaft and moves over an associated range of linear movement in concert with rotation of the shaft about the axis, wherein each position within the range of rotary movement of the shaft corresponds to a single position within the range of linear movement of the follower. A first detector outputs a signal which is representative of a plurality of rotary positions of the shaft about the axis, and a second detector detects the linear position of the follower within the range of movement of the follower. A position calculator determines the position of the shaft within the range of rotary movement of the shaft using the positions detected by the first and second detectors. In this way, the position detected by the second detector is used to remove the ambiguity caused by the rotary position of the shaft corresponding to more than one position within the range of rotary movement of the shaft because only one of the rotary positions within the range of movement of the shaft will agree with the position detected by the second detector.

Preferably, the first detector comprises an inductive sensor in which the electromagnetic coupling between a planar transmit aerial and a planar receive aerial varies in dependence on the position of an intermediate coupling element that rotates with the shaft. The use of planar aerials is advantageous because planar manufacturing techniques are well developed, allowing high resolution measurements to be achieved at relatively low cost. This high accuracy of measurement enables the second detector to have a comparatively low resolution because the signal from the second detector is only used to identify which of the possible positions corresponding to the signal detected by the first detector is correct.

Preferably, the second detector also comprises an inductive sensor which uses planar aerials which are parallel to the planar aerials of the first detector, because this leads to reduced manufacturing costs. In an embodiment, the first and second detectors are formed on a plurality of planar substrates, which are preferably joined together to form a multi-layer substrate for ease of manufacturing and reduction in size.

In an embodiment, the shaft is operable to rotate more than a full rotation about the axis .

In an embodiment, the follower is coupled to the shaft so that rotational movement of the shaft about the axis causes axial movement of the follower.

According to another aspect of the present invention, there is provided a position sensor comprising a plurality of parallel planar substrates, with each planar substrate having a respective aperture with the plurality of apertures aligned along a measurement direction. A sensor element is mounted to move relative to the plurality of planar substrates along the measurement direction. The position sensor also includes a transmit aerial and a receive aerial, wherein the electromagnetic coupling between the transmit aerial and the receive aerial varies in dependence upon the relative position of the sensor element and the plurality of planar substrates. At least one of the first and second aerials comprises a conductor having conductive track portions provided on at least two of the planar surfaces of the multi-layer planar substrate.

Various embodiments of the invention will now be described with reference to the attached Figures in which:

Figure 1 shows a perspective view of a steering wheel assembly, with a portion of a casing cut away to reveal a sensor arrangement; Figure 2 shows a sectional view through part of the steering wheel assembly illustrated in Figure 1;

Figure 3 schematically shows a plan view of an end surface of a multi-layer printed circuit board forming part of the sensor arrangement illustrated in Figure 1; Figure 4 is a perspective view of the multi-layer printed circuit board illustrated in Figure 3 showing a transmit aerial and a receive aerial formed on the multilayer printed circuit board;

Figure 5 schematically shows the variation along the axis of the steering wheel assembly illustrated in Figure 1 of an axial magnetic field strength component produced by the transmit aerial illustrated in Figure 4 ;

Figure 6 schematically shows the main components of signal generation and processing circuitry forming part of the steering wheel assembly illustrated in Figure 1; Figure 7 shows a push button assembly; Figures 8A and 8B schematically show components of an inductive position sensor using two printed circuit boards ; and Figure 9 schematically shows an alternative steering wheel assembly to the steering wheel assembly illustrated in Figure 1.

Figure 1 shows a steering wheel assembly 1 in which a steering wheel 3 is connected to an elongate cylindrical shaft 5 which is partially mounted within a sleeve 7. In this embodiment, the range of rotational movement of the shaft 5 relative to the sleeve 7 encompasses three full rotations. A casing 9 is fixed to the sleeve 7 and 5 surrounds a- sensor arrangement for detecting the rotational position of the steering wheel 3 relative to the sleeve 7. A portion of the casing 9 has been cut away in Figure 1 to reveal a disk-shaped member 11, which is fixed co-axially to the shaft 5 so as to rotate with

L0 the shaft 5, and a circular multi-layer PCB 13, which is fixed co-axially to the sleeve 7, which form part of the sensor arrangement. As shown in Figure 1, the single layer PCB 11 and the multi-layer PCB 13 are adjacent the position where the shaft 5 enters the sleeve 7.

15

Figure 2 shows a cross-sectional view, through a plane including the axis of the shaft 5, of the portion of the steering wheel assembly 1 adjacent the position where the shaft 5 enters the sleeve 7. As shown, the disk-shaped

20 member 11 is formed at one longitudinal end of a hollow cylinder 21 through which the shaft 5 passes in a tight fit so that the hollow cylinder 21 rotates with the shaft 5. The other longitudinal end of the cylinder 21 extends into the sleeve 7 and includes a spiral thread 25 which

25 runs around the outside cylindrical surface of the part of the cylinder 21 inserted within the sleeve 7. As shown in Figure 2 , the threaded region of the cylinder 21 is positioned inside the sleeve 7 adjacent the multilayer PCB 13.

30

A flange 27 is provided on the shaft 5 at a point which is inserted further into the sleeve 7 than the cylinder 21. The flange 27 engages a slot formed by two flanges 29a, 29b provided on the inner surface of the sleeve 7

35 to prevent axial movement of the shaft 5. The sleeve 7 also has a neck portion 31 which is adjacent to the end of the sleeve 7, so that the neck portion 31 is adjacent to the threaded region of the cylinder 21 and the multi-layer PCB 13. The neck portion 31 generally defines a cylindrical cavity between the shaft 5 and the sleeve 7 in which a hollow cylindrical metallic bush 33 is positioned. In this embodiment, the bush 33 is made of ferrite and has an axial length of 15mm.

A spline 35, which is aligned with the axis of the shaft 5, is formed on the inner surface of the neck portion 31 of the sleeve 5 and engages a corresponding groove formed in the outer cylindrical surface of the bush 33 to prevent rotational movement, while allowing axial movement, of the bush 33 as the shaft 5 rotates. The inner cylindrical surface of the bush 33 has a female threading which engages the spiral thread 25 provided on the cylinder 21, and therefore when the shaft 5 rotates the corresponding rotation of the threaded region of the cylinder 21 applies a force to the bush 33 causing axial movement of the bush 33, with the direction of the axial movement of the bush 33 being dependent on the direction of the rotation of movement of the shaft 5. In this way, the axial position of the bush 33 corresponds to the position of the steering wheel 3 within its rotational range of movement. In this embodiment the range of axial movement of the bush 33 is 4mm.

In this embodiment, the angular position of the shaft 5 is determined by measuring the location of a sensor element 41, which is fixed to an outer radial position on the disk member 11, using a first transmit aerial and a first receive aerial which are provided on the multilayer PCB 13. Further, a second transmit aerial and a second receive aerial are also formed on the multi-layer PCB 13, and are used to determine the axial position of the bush 33. A control unit 45 is provided on a printed circuit board 47 adjacent to the multi-layer printed circuit board 43, and is connected to the aerials on the multi-layer printed circuit board 13.

The aerials provided on the multi-layer PCB 13 will now be described in more detail with reference to Figures 3 to 5.

Figure 3 schematically shows a plan view of the end surface 51 of the multi-layer PCB 13 which faces the disk member 11. The first transmit aerial is formed by a first sine coil 53 and a first cosine coil 55, shown in chained lines in. Figure 3, and the first receive aerial is formed by a first sense coil 57. For illustrative purposes, the first sine coil 53 and the first cosine coil 55 are shown formed entirely on the end surface 51, whereas in practice the first sine coil 53 and the first cosine coil are formed by portions of conductive tracks on both planar surfaces of the layer of the multi-layer PCB 13 facing the disk member 11 connected through via holes in a manner which is well known- in the art.

As shown in Figure 3, the first sine coil 53 effectively forms a sequence of six current loops along a path which extends around the axis of the shaft 5. When a signal is applied to the first sine coil 53, current flows around adjacent loops of the first sine coil 53 in opposite directions. In this way, the first sine coil 53 periodically repeats three times. In response to a current I(t) being applied to the first sine coil 53, a magnetic field is generated having a magnetic field component B_x perpendicular to the surface 51 of the form:

where 0 is an angle about the axis of the shaft 5 measured from a reference radius 59.

5 The first cosine coil 55 is formed by a sequence of six . loops around the same path as the sine coil 53, but with the loops of the cosine coil 55 being offset from the loops of the sine coil 53 by one quarter of a period. When a signal is applied to the first cosine coil 55, L0 current flows around adjacent loops of the first cosine coil 55 in opposite directions. In this way, when a current Q(t) flows through the first cosine coil 55 a magnetic field is generated having a magnetic field component perpendicular to the surface 51 of the form:

15

B₂ oc Q(t) cos 3φ (2)

In this embodiment, the sensor element 41 includes a piece of printed circuit board having a coil of conductive track whose ends are connected to respective

20 terminals of a capacitor. The coil of conductive track has an associated inductance, and therefore in combination with the capacitor forms a resonant circuit. In this embodiment the resonant frequency of the resonant circuit is 2MHz.

25

The signal I(t) applied to the first sine coil 61 is generated by amplitude modulating an oscillating carrier signal having a carrier frequency f₀, which is substantially identical to the resonant frequency of the

30 sensor element 41, using a first modulation signal which oscillates at a modulation frequency f_l which in this embodiment is 3.9kHz. Similarly, the signal Q(t) applied to the first cosine coil 55 is generated by amplitude modulating the oscillating carrier signal having carrier frequency f₀ using a second modulation signal which oscillates at the modulation frequency f_lf with the second modulation signal being π/2 radians (90°) out of phase with the first modulation signal. In this way, an electro-motive force (EMF) is induced in the resonant circuit of the sensor element 41 which results in an induced resonant current I_res of the form:

I_res oc cos2rfot.cos(2;rfit -

- θ<ή ( ³ )

where θ_c is a fixed phase shift and 0_r is the angular position of the sensor element 41. measured from the reference radius 59.

The induced current I_res in the sensor element 41 in turn generates a magnetic field which induces a voltage within the first sense coil 55, resulting in a first sense signal S^t) being formed in the first sense coil 55 which is proportional to the resonant current I_rβs. In this way, the phase of the component of the first sense signal S^t) at the modulation frequency f_: is indicative of the angular position of the sensor element 41, and is therefore also indicative of the angular position of the shaft 5.

It will be appreciated from equation (3) that each phase reading of the first sense signal S_x(t) can correspond to three different possible angular positions of the shaft 5 as measured from the reference radius 59. Further, in this embodiment the steering wheel is rotatable through 6π radians (i.e. 1080°) and therefore each phase reading of the first sense signal S^t) corresponds to nine different possible positions of the shaft 5 within the range of rotational movement of the shaft 5. in order to determine which of the possible positions of the shaft 5 is correct, the axial position of the bush 33 is measured using an axial inductive sensor which includes the second transmit aerial and the second receive aerial.

The second transmit aerial is formed by a second sine coil 61, part of which is visible in Figure 3, and a second cosine coil 63, whose terminals 65a and 65b are shown in Figure 3, which are distributed over the layers of the multi-layer PCB 13. The second receive aerial is formed by a second sense coil 67, part of which is shown in Figure 3, which is also distributed over the layers of the multi-layer PCB 13. The second transmit aerial and the second receive aerial will now be described in more detail with reference to Figure 4 and 5.

Figure 4 shows a perspective view of the multi-layer PCB 13 including the second transmit aerial and the second receive aerial. In Figure 4 the conductive tracks forming the first transmit aerial and the first receive aerial have been omitted for ease of illustration.

As shown in Figure 4, in this embodiment the multi-layer PCB 13 has four circular layers 71a to 7Id. In this embodiment, the multi-layer PCB 13 is a four layer FR4 grade circuit board, with each layer having a thickness of 0.5mm. An aperture is formed through the centre of the multi-layer PCB 13 through which the shaft 5 passes. In this embodiment, the diameter of the aperture through the multi-layer PCB 13 is 25mm.

The second sine coil 61 is formed by a conductive track which starts at a first terminal 72a on the surface 51 of the first layer 71a and forms a first current loop 73a around the shaft 5 on the first layer 71. The conductive track then passes through a first via hole 75a to the surface between the second layer 71b and the third layer 71c, and forms a second current loop 73b around the shaft 5 on the surface between the second layer 71b and the third layer 71c. The direction of the second current loop 73b around the shaft 5 is opposite to the direction of the first current loop 73a around the shaft 5. The conductive track then passes through a second via hole

75b to the end surface 77 of the multi-layer PCB 13

■ opposing the end surface 51, and forms a third current loop 73c around the shaft 5. The direction of the third current loop 73c around the shaft 5 is the same as the direction of the first current loop 73a around the shaft 5. The conductive track then passes to a second terminal 72b on the surface 51 of- the first layer 71a through a third via hole 75c.

The second cosine coil 63 is formed by a conductive track which passes from the first terminal 65a on the surface 51 of the first layer 71a to the surface between the first layer 71a and the second layer 71b through a fourth via hole 75d, and forms a fourth current loop 73d around the shaft 5. The conductive track then passes to the surface between the third layer 71c and the fourth layer 7Id through a fifth via hole 75e, forms a fifth current loop 73e around the shaft 5, and then passes to the second terminal 65b on the surface 51 of the first layer 71a through a sixth via hole 75f. The fourth current loop 73d loops around the shaft 5 in the opposite direction to the first current loop 73a, and the fifth current loop 73e loops around the shaft 5 in the same direction as the first current loop 73a. As schematically shown in Figure 5, when a signal is applied to the second sine coil 61, a magnetic field is generated having an axial magnetic field component which varies along the axial direction z with maximum values at the axial positions of the first, second and third current loops 73a, 73b, 73c and minimum values at the axial positions of the fourth and fifth current loops 73d, 73e. The maximum values at the axial positions of the first and third current loops 73a, 73b have the same polarity, and have an opposite polarity to the maximum value at the second current loop. When a signal is applied to the second cosine coil 63, a magnetic field is generated having maximum values of opposite polarities at the axial positions of the fourth and fifth current loops 73d, 73e, and minimum values at the axial positions of the first, second and third current loops 73a, 73b, 73c. In particular, as shown in Figure 5, the. magnetic field components along the axial direction generated by the second sine coil 61 and the second cosine coil 63 vary sinusoidally, but a quarter of a cycle out of phase, along the axial direction z .

Returning to Figure 4, in this embodiment the second sense coil 67 is formed by a conductive track which starts at a terminal 79a on the end surface 51 of the first layer 71a, forms a sixth current loop 73f around the shaft 5, and then passes through a seventh via hole 75f to the end surface 77 of the fourth layer 7Id. The conductive track then forms a seventh current loop 73g around the shaft 5, which passes around the shaft 5 in the opposite direction to the sixth current loop 73f. The conductive track then passes to a terminal 79b on the end surface 51 of the first layer 71a through an eighth via hole 75h. With this arrangement, in the absence of the bush 33 the second sense coil 67 is balanced- with respect to both the second sine coil 61 and the second cosine coil 63. In other words, in the absence of the bush 33 the nett electromotive force induced in the second sense coil 67 by current flowing through the second sine coil 61 is substantially zero, and similarly the nett electromotive force induced in the second sense coil 67 by current flowing through the second cosine coil is substantially zero. However, with the bush 33 in place the nett electromotive forces induced in the second sense coil 67 by current flowing in the second sine coil 61 and the second cosine coil 63 respectively depends on the position of the bush 33.

In this embodiment, the signal applied to the second sine coil 61 is identical to the signal I(t) applied to the first sine coil 53, and the signal applied to the second cosine coil 63 is identical to the signal Q(t) applied to the first cosine coil 55. This results in a second sense signal S₂(t) being formed in the second sense coil of the form:

where L is the axial distance between the end surfaces of the multi-layer PCB 13. In this embodiment, the range of axial movement of the bush 33 corresponding to the full range of rotational movement of the shaft 5 is less than the distance L, and therefore each phase reading of the sense signal S₂(t) induced in the second sense coil corresponds to a single position of the bush 33. This axial position is then used to determine which of the nine possible rotary positions of the shaft 5 measured by the inductive rotary sensor is correct. The main components of the control apparatus 45 will now be described with reference to Figure 6.

As shown, a quadrature signal generator 101 outputs a quadrature pair of signals at the modulation frequency _ι to a modulator 103, which uses the quadrature pair of signals to modulate a carrier signal, at the carrier frequency f₀, generated by a signal generator 105. The resulting pair of modulated signals are respectively input to a pair of coil drivers 107a, 107b, which amplify the modulated signals to produce the in-phase signal I(t) and the quadrature signal Q(t).

The in-phase signal I(t) and the quadrature signal Q(t) are respectively input to first and second switches 109a, 109b of a switching network, which also includes a third switch 109c. The switching network has two possible configurations . In one configuration of the switching network, the first switch 109a directs the in-phase signal I(t) to the first sine coil 53, the second switch 109b directs the quadrature signal Q(t) to the first cosine coil 55, and the third switch 109c connects the first sense coil 57 to a demodulator 111. In the other configuration of the switching network, the first switch 109a directs the in-phase signal I(t) to the second sine coil 61, the second switch 109b directs the quadrature signal Q(t) to the second cosine coil 63, and. the third signal 109c connects the second sense coil 79 to the demodulator 111.

The demodulator 111 demodulates the received sense signal S(t), using a signal at the carrier frequency f₀ from the signal generator 105, to form a demodulated signal at the modulation frequency f^ The demodulated signal output by the demodulator 111 is input to a phase detector 113, which measures the phase of the demodulated signal, and outputs the phase measurement to a position calculator 115. The switching network is periodically switched between the two possible configurations, and the position calculator 115 calculates a position value which corresponds to the determined phase of the first sense signal S^t) from the inductive rotary sensor and the determined phase of the second sense signal S₂(t) from the inductive axial sensor. The calculated position value is then output to the central processing unit of the associated automobile which generates a control system for the servo system which steers the wheels .

SECOND EMBODIMENT

In the first embodiment, a linear position encoder is formed by a transmit aerial and a receive aerial which are formed on a multi-layer PCB and are used to detect the position of an intermediate coupling element (i.e. the bush) along an axis perpendicular to the surface of the multi-layer PCB. Although in the first embodiment the axial position encoder is used in combination with a rotary position encoder, there are many applications in which this type of axial position encoder is useful by itself.

A second embodiment will now be described with reference to Figure 7 in which the axial position encoder of the first embodiment is incorporated within a push button. As shown in Figure 7, the push button comprises a resiliently flexible membrane 201 mounted to one side of a planar substrate 203. The planar substrate 203 has a hole formed therethrough and the membrane 201 is arranged to arc above the hole. A multi-layer PCB 205 is mounted to the face of the substrate 203 opposite to the flexible membrane 201, and has a hole which is aligned with the centre of the hole through the planar substrate 203.

An elongate rod 207 is connected to the arced portion of 5 the membrane 201 such that it projects through the hole in the substrate 203 and the multi-layer PCB 205. The rod 207 includes a ferrite portion which is generally aligned with the hole through the PCB 205. The multi-layer PCB 205 has coils formed around the hole in L0 the same manner as described with reference to the axial position sensor of the first embodiment so that when a user presses the arced portion of the membrane 201, movement of the ferrite element 209 is measured.

L5 An advantage of this kind of push button is that the membrane 201 can form a hermetic seal isolating the processing electronics of the push button. This is particularly advantageous in situations where liquids and dirt may come into contact with the push button. For

20 example, this type of switch is particularly advantageous for electrical equipment within medical operating theatres.

MODIFICATIONS AND FURTHER EMBODIMENTS

25

The combined rotary and axial position sensor arrangement described in the first embodiment enables the absolute angular position of a steering wheel to be measured over more than one rotational cycle. Such a combined sensor

30 arrangement is useful in other situations where the absolute angle of a position of a body which rotates over more than one full- rotation is desired. For example, the

^■ combined sensor arrangement could be incorporated in a control valve in which a wheel is rotated over a

35 plurality of rotations. In the first embodiment, the sensor element 41 includes a resonant circuit which forms an intermediate coupling element between the first transmit aerial and the first receive aerial. Alternatively, the intermediate coupling element could be formed by a conductive loop or a magnetically permeable element. In a similar fashion, although a magnetically permeable element (i.e. the bush 33) forms an intermediate coupling element between the second transmit aerial to the second receive aerial, alternatively the intermediate coupling element could be formed by a conductive loop or a resonant circuit.

The push button described in the second embodiment may be used as a user interface on many different machines. In instances where the user interface may vibrate, for example on a washing machine or a treadmill, it is preferable to ensure in the mechanical design of the push button that the resonant vibration frequency of the push button does not match an expected frequency or vibration of the associated machine.

In the first and second embodiments, a position encoder using planar aerials is used to measure displacement along a path which intersects the planes of the aerials. The range of displacement is dependent upon the axial extent of the magnetic field generated by the sine coil and the cosine coil, which is of the order of the diameter of the windings of the sine coil and the cosine coil. The range of measurement can be increased by using more than one position sensor. Figures 8A and 8B show an embodiment in which two multi-layer PCBs 301a, 301b are spaced apart with their planes parallel, each multi-layer PCB 301 being identical to the multi-layer PCB described in the first embodiment. As shown, a rod 303 projects perpendicularly from a planar surface 305 through holes provided in the PCBs 301.

The rod 303 is generally insulating apart from two ferrite portions 305a, 305b which are spaced apart by a distance which is less than the distance between the two printed circuit boards 301. In particular, the two ferrite portions 305 are spaced so that when the first ferrite portion 305a is within the axial measurement range of the first PCB 301a, as shown in Figure 8A, the second ferrite portion 305b is outside the linear measurement range of the second PCB 301b. However, as the first ferrite portion 305a leaves the linear measurement range of the first. PCB 301a in the direction of the second PCB 301b, the second ferrite portion 305b enters the linear measurement range of the second printed circuit board 301b, as shown in Figure 8B. In this way, the effective linear measurement range can be almost doubled. Those skilled in the art will appreciate that by using either or both of additional magnetic portions and additional printed circuit boards, the linear measurement range could be still further increased.

In the first embodiment, the rotary position of a steering wheel is measured using an inductive sensing arrangement. In addition, inductive sensing can be used for non-contact detection of the state of switches provided on the steering wheel. This is advantageous because, for manufacturing reasons, it is desirable to minimise the number of electrical connections between the parts which rotate with the steering wheel and the stationary parts of the steering wheel column. An example of such a switch arrangement will now be described with reference to Figure 9.

Figure 9 shows part of a steering wheel assembly with the casing around the sensing arrangement removed. The steering wheel assembly includes the rotary position encoder and axial position encoder of the first embodiment, which will not be described again in detail. As shown, three switches 401a, 401b and 401c are formed on the steering wheel 403. In this embodiment, the first switch 401a controls a car horn, the- second switch 401b turns the radio on and off, and the third switch 401c turns the windscreen wipers on and off. Each switch 401 is connected in series with an inductor 405 and a capacitor 407 (only one inductor and one capacitor are shown in Figure 9 for ease of illustration) formed on a disk member 409 which rotates with the steering wheel 403.

When a switch is closed, a resonant circuit is formed, and this resonant circuit is detected by a transmit aerial and a receive aerial formed on a multi-layer PCB 411. In an embodiment, the resonant circuits formed by closing the switches 401 have respective different frequencies, so that by applying excitation signals of different frequencies to the transmit aerial and measuring the signals induced in the receive aerial, it is possible to determine which of the switches 401 are closed.

It will be appreciated that steering wheel rotary position sensing arrangement as described in the first embodiment and the switch sensing arrangement described with reference to Figure 9 could share common excitation signal generation circuitry and sense signal processing circuitry, with a multiplexer being provided to vary periodically which sensing arrangement is operational. Additionally, the signal generation circuitry and sense signal processing circuitry could also be multiplexed with inductive sensors for detecting movement of stalk controls, for example indicator controls, and for detecting the orientation of the steering wheel assembly, to allow automatic steering wheel adjustment between different drivers.

In the first embodiment, the shaft 5 of the steering wheel 3 is prevented from moving axially, and a bush is. coupled to the shaft 5 by a thread is prevented from rotating with the shaft 5 so that rotary movement of the shaft 5 causes axial movement of the bush 33. Alternatively, the steering wheel could be screwed directly into the sleeve 7, so that rotary movement of the steering wheel 3 causes relative axial movement between the shaft 5 and the sleeve 7. The axial position encoder is then used to measure the relative axial displacement between the shaft 5 and the sleeve 7. in this case, the sleeve 7 can be considered to be a follower which moves relative to the shaft 5 over a linear range of movement in response to rotary movement of the shaft 5.

If the shaft 5 of the steering wheel assembly is screwed into the sleeve 7, then as the steering wheel is rotated the distance of the sensor element 41 from the multilayer PCB 13 varies, resulting in a variation in the amplitude of the -signal induced in the first receive aerial. This variation in amplitude can be measured to determine which of the possible readings of the rotary encoder is correct, thereby removing the need for the second transmit aerial and the second receive aerial .

In the first embodiment, the receive aerial of the axial position encoder is formed by a current loops on the end faces of the multi-layer PCB. It will be appreciated the receive aerial could also include one or more current loops on the intermediate faces of the multi-layer PCB.

The axial position encoder of the first embodiment has a sine coil and a cosine coil formed by single current loops on respective surfaces of the multi-layer PCB. In alternative embodiments, the sine coil and the cosine coil could have . plural conductive loops formed on respective surfaces of the multi-layer PCB. Further, the multi-layer PCB could have more than four layers, in which case the number of conductive loops on each surface forming part of the sine coil and the cosine coil varies along the axial direction in order to generate axial magnetic field components which vary in accordance with the sine function and the cosine function respectively.

In the first embodiment, the signal generator within the control apparatus 45 generates an in-phase signal I(t) and a quadrature signal Q(t) comprising a carrier signal at a carrier frequency modulated by respective modulation signals at a modulation frequency which is significantly less than the carrier frequency. The signal processor within the control apparatus measures the phase of a component of a sense signal S(t) at the modulation frequency in order to determine a position value. This arrangement advantageously combines the increase in the magnitude . of the coupling between a transmit aerial and a receive aerial resulting from the use of a comparatively high carrier frequency with the straightforward signal processing techniques used to measure the phase of a signal at the lower modulation frequency. Further, the filtering effect of the coil drivers and the resonant coupling between the transmit aerial and the resonant circuit enable the use of comparatively low quality digitally-generated excitation signals .

In the first embodiment, a carrier signal at 2MHz is modulated by a modulator signal at 3.9kHz. Typically, the carrier signal may be in the range 500kHz to 10MHz and the modulation signal may be in the range 10kHz to 100kHz.

Other types of inductive rotary encoder could be used in place of the inductive rotary encoder described in the first embodiment. For example, instead of having two excitation windings in the transmit aerial and detecting the phase of a signal induced in a receive aerial formed by a single sense winding, the transmit aerial could be formed by a single excitation winding and the receive aerial could be formed by two sense windings, with the coupling between the excitation winding and the two sense windings varying with rotary position. An example of such a rotary encoder is described in International Patent Application WO 95/31696, whose content is incorporated herein by reference.

Similarly other forms of inductive axial sensor could be used to replace the inductive axial sensor described in the first embodiment. Preferably, a multi-layer PCB sensor, such as the PCB-LVDT position sensor described in International Patent Application WO 02/097374 whose content is hereby incorporated herein by reference, is used because such sensors are relatively cheap. Alternatively, other types of linear position encoder could be used, e.g. the linear position encoder described in WO 95/31696. Alternatively, a magneto-strictive sensor, a magneto-resistive sensor or a Hall effect sensor could be used as the axial sensor. In the described embodiments, the position of a movable body is measured. It will be appreciated that by periodically measuring the position of a movable object, the speed of movable object can also be determined.

The described sensors have many potential applications including as displacement measurement sensors and user interfaces in automobiles, domestic appliances, entertainment systems, audio and video equipment, IT equipment, computing equipment, fitness equipment, process control systems, valves, actuators, fork-lift trucks, heavy goods vehicles, defence vehicles, buses, medical systems, industrial equipment, building controls, lifts, telephones and mining equipment.

Claims

1. A rotary position sensor comprising: a shaft mounted in a mounting which allows a range of rotary movement of the shaft about an axis; a follower coupled to the shaft and operable to move over a range of linear movement in concert with rotation of the shaft about said axis, wherein each position within the range of rotary movement of the shaft corresponds to a single position within the range of linear movement of the follower; a first detector operable to generate a first detection signal indicative of a plurality of different rotary positions within the range of rotary movement of the shaft; a second detector operable to generate a second detection signal indicative of a single position of the follower within the range of linear movement of the follower; and a position calculator operable to determine the position of the shaft within the range of rotary movement of the shaft using the detection signals generated by the first and second detectors, wherein the first detector comprises a planar transmit aerial and a planar receive aerial having an electromagnetic coupling which varies in dependence upon the relative rotary position of the shaft and the mounting.

2. A rotary position sensor according to claim 1, wherein the range of rotary movement of the shaft is greater than one complete rotation about the axis .

3. A rotary position sensor according to claim .1 or claim 2, wherein the planar transmit aerial and the planar receive aerial are fixed relative to the mounting, and an intermediate coupling element is fixed relative to the shaft.

4. A rotary position sensor according to claim 3, wherein the intermediate coupling element comprises a resonant circuit.

5. A rotary position sensor according to any preceding claim, wherein the planar transmit aerial and the planar receive aerial are formed on a common planar substrate.

6. A rotary position sensor according to any preceding claim, wherein the second detector comprises a transmit aerial, a receive aerial and an intermediate coupling element mounted so that the electromagnetic coupling between the transmit aerial and the receive aerial of the second detector varies with movement of the follower.

7. A rotary position sensor according to claim 6, wherein the follower comprises the intermediate coupling element .

8. A rotary position sensor according to claim 6 or 7, wherein the first aerial and the second aerial of the second detector are formed on a plurality of planar substrates which are arranged in parallel with each other, with each planar substrate having a respective aperture aligned so that the direction of linear movement of the follower passes through the aperture.

9. A rotary position sensor according to claim 8, wherein at least one of the first and second aerials comprises a conductor having conductive track portions provided on at least two planar surfaces of the plurality of planar surfaces.

10. A rotary position sensor according to claim 8 or 9, wherein the plurality of planar substrates are integrated in a common laminar structure,

11. A rotary position sensor according to claim 10, wherein the laminar structure is a multi-layer printed circuit board.

12. A rotary position sensor according to claim 11, wherein the second detector is operable to detect the position of the follower relative to the shaft along a rectilinear direction.

13. A rotary position sensor according to any preceding claim, wherein the follower has a passage through which the shaft passes, and the surface of the passage of the follower and the surface of the shaft comprise respective ones of a conjugate pair of threaded portions.

14. A rotary position sensor according to claim 13, further comprising means for preventing rotation of the follower with the shaft so that, in response to rotary movement of the shaft, the shaft and the follower move relative to each other in an axial direction.

15. A rotary position sensor according to claim 14, wherein said preventing means comprises a spline formed on the mounting which engages a corresponding groove formed on the follower.

16. A rotary position sensor according to any preceding claim, in which the shaft has a cylindrical cross-section and the mounting comprises a sleeve, wherein the shaft is mounted in the sleeve so that the shaft is operable to rotate relative to the sleeve.

17. A rotary position sensor according to claim 13, wherein the -follower is fixed relative to the mounting so that, in response to rotation of the shaft about the axis, the shaft moves relative to the follower along the axial direction.

18. A rotary position sensor according to claim 13, wherein the a sensor element is fixed relative to the shaft, and the second detector is operable to detect the component of relative movement between the sensor element and the shaft along the axial direction.

19. A rotary position sensor according to any preceding claim, further comprising: a signal generator operable to generate an excitation signal; a signal processor operable to process a sense signal; and a multiplexer operable to connect the signal generator and the signal processor to the first and second detectors sequentially.

20. A rotary position sensor according to claim 19, further comprising additional detectors fixed with relation to the shaft, wherein the multiplexer is operable to connect the signal generator and the signal processor to the first and second detectors and the additional detectors sequentially.

21. A rotary position sensor comprising: a shaft mounted in a mounting which allows a range of rotary movement of the shaft about an axis, wherein sais shaft and said mounting are coupled to each other via a conjugate pair of screw threads so that relative rotational movement around the axis between the shaft and the mounting causes relative axial movement between the shaft and the mounting; a sensor element fixed relative to the shaft; a first detector operable to generate a first detection signal indicative of a plurality of different rotary positions of the sensor element within the range of rotary movement of the shaft; a second detector operable to generate a second detection signal indicative of the axial position of the sensor element; and a position calculator operable to determine the position of the shaft within the range of rotary movement of the shaft using the detection signals generated by the first and second detectors, wherein the first detector comprises a planar transmit aerial and a planar receive aerial having an electromagnetic coupling which varies in dependence upon the relative rotary position of the shaft and the mounting.

22. A rotary position sensor according to claim 21, wherein one of the planar transmit aerial and the planar receive aerial comprises a first set of windings and the other of the planar transmit aerial and the planar receive aerial comprises a second set of windings, wherein the first detector is operable to generate the first detection signal in dependence upon a comparison of the electromagnetic coupling between two or more pairs of windings, each pair of windings formed by one winding from the first set and one winding from the second set, and wherein the second detector is operable to generate the second detection signal in dependence upon the amplitude of the electromagnetic coupling between one or more pairs of windings.

23. A steering wheel assembly comprising a rotary position sensor as claimed in any preceding claim.

24. A steering wheel assembly comprising: a steering wheel having a shaft which defines an axis of rotation; a mounting for rotatably supporting the shaft; a follower coupled to the shaft and operable to move over a range of linear movement in concert with rotation of the steering wheel about said axis, wherein each position within the range of rotary movement of the steering wheel corresponds to a single position within the range of linear movement of the follower; a first detector operable to generate a first detection signal indicative of a plurality of different rotary positions within the range of rotary movement of the steering wheel; a second detector operable to generate a second detection signal indicative of a single position of the follower within the range of linear movement of the follower; and a position calculator operable to determine the position of the shaft within the range of rotary movement of the shaft using the detection signals generated by the first and second detectors, wherein the first detector comprises a planar transmit aerial and a planar receive aerial having an electromagnetic coupling which varies .in dependence upon the relative rotary position of the shaft and the mounting.

25. An automobile comprising a steering wheel assembly as claimed in claim 23 or 24.

26. A position sensor comprising: a plurality of planar substrates with each planar substrate having a respective aperture, wherein the plurality of planar substrates are arranged in parallel with the respective plurality of apertures. aligned along a measurement direction; a sensor element operable to move relative to the plurality of planar substrates along the measurement direction; and a first aerial and a second aerial, wherein the electromagnetic coupling between the first aerial and the second aerial varies in dependence upon the position of the sensor element relative to the plurality of planar substrates along the measurement direction, wherein at least one of the first and second aerials comprises a conductor having conductive track portions provided on at least two planar surfaces of the plurality of planar substrates.

27. A position sensor according to claim 26, wherein the plurality of planar substrates form respective layers of a laminar structure.

28. A position sensor according to claim 27, wherein the laminar structure is a multi-layer printed circuit board.

29. A position sensor according to any of claims 26 to 28, wherein said conductive track portions are connected via at least one hole through at least one of the plurality of planar substrates.

30. A position sensor according to any of claims 26 to 29, wherein the conductor is operable to generate a magnetic field having a magnetic field component parallel with the measurement direction which varies along the measurement direction.

31. A position sensor according to claim 30, wherein said magnetic field component varies sinusoidally along the measurement direction.

32. A position sensor according to any of claims 26 to 31, wherein said at least one of the first and second aerials further comprises a second conductor having conductive track portions on. at least two of the planar substrates .

33. A position sensor according to claim 32, wherein the second conductor is operable to generate a magnetic field having a magnetic field component parallel with the measurement direction which varies along the measurement direction.

34. A position sensor according to claim 33, wherein said magnetic field component of the second conductor varies sinusoidally along the measurement direction one quarter of a cycle out of phase with said magnetic field component of the first-mentioned conductor.

35. A push button comprising a position sensor according to any of claims 26 to 34.