WO2005003687A2

WO2005003687A2 - Multiturn absolute rotary position sensor with coarse detector for axial movement and inductive fine detector for rotary movement

Info

Publication number: WO2005003687A2
Application number: PCT/GB2004/002928
Authority: WO
Inventors: Darran Kreit; Colin Sills; Mark Howard
Original assignee: Scientific Generics Limited
Priority date: 2003-07-04
Filing date: 2004-07-05
Publication date: 2005-01-13
Also published as: GB0315738D0; WO2005003687A3

Abstract

There is described a position sensor in which a first Aerial and a second aerial are provided on plural planar substrates which are arranged in parallel with each other. A sensor element is operable to move relative to the plurality of planar substrates along a measurement direction transverse to the planar substrates. By appropriately distributing conductive tracks on at least two planar surfaces of the Plurality of planar substrates, the orthogonal position of a sensor element relative to the planar substrates may be detected.

Description

POSITION ENCODER

This invention relates to inductive position encoders.

Inductive position encoders are already known from disclosures by the same authors. In particular, inductive position encoders are known which measure the^' position of an electrical intermediate device relative to a planar arrangement of transmit and receive windings. This relative position is typically given by the EID's position along a plane generally parallel to a cycle of sinusoidally and cosinusoidally wound transmit windings. In such position encoders the transmit windings are typically energised with a pulse width modulated alternating electrical signal. The EID's position is calculated electronically from the phase of the signal received from via the receive windings .

Such position encoders work well in measuring the position of the EID along the windings but are generally incapable of measuring the orthogonal distance of the EID from the plane of the transmit and receive windings . This invention enables measurement of the EID's orthogonal position relative to the plane of transmit and receive windings .

This invention inductively measures the position of an EID's orthogonal position relative to the plane of transmit and receive windings . When combined with a position encoder of the previously disclosed design, the combined sensor can measure the position of the EID along the winding pattern as well as its orthogonal position. This combined technique is useful, for example, in measuring the absolute position of multi-turn rotary encoders where the EID rotates within a screw thread. With this invention the sensor is able to measure axial distance travelled and rotation angle. Therefore, absolute angular position can be measured as well as the number of rotational cycles . Such an arrangement is useful in steering wheel encoders for example.

Various illustrative embodiments of the invention will now be described with reference to the attached figures in which: Figure 1 schematically shows a cross-sectional view of a position encoder forming a preferred embodiment of the invention; Figure 2 schematically shows excitation windings forming part of the position encoder illustrated in Figure 1 ; and Figure 3 schematically shows a cross-sectional view of a position encoder forming another embodiment of the invention.

PREFERRED EMBODIMENT A diagram showing a section through the centre line of the preferred embodiment is shown in Figure 1.

In a preferred embodiment the EID [6] is a passive LC resonant circuit. Preferably the resonant circuit is made by a capacitor in series with an inductance formed by tracks on a printed circuit board (PCB) .

Preferably the transmit [2,3] and receive windings are formed as tracks on various layers of a printed circuit board which may be positioned around a hole through the PCB [1] . The transmit windings are arranged such that a generally sinusoidal field is formed by the first winding [2] and a generally cosinusoidal field formed by the second winding [3] . The field formed by the energised transmit windings is an alternating magnetic field [5] extending along the axis of the windings. The field [5] typically extends further than the windings physical position by a distance substantially equal to the radius of the windings. The EID's position is measurable within the limits of the field.

The receive winding [4] is also formed as tracks on the same PCB as the transmit windings .

Preferably the windings are embodied as etched copper tracks on an insulating substrate. FR4 type circuit board with plated through holes between the various types of winding are ideally suited to such a construction due to their ease and inexpensive methods of production.

The arrangement of transmit windings is shown in Figure 2. The diagram shows a simplified arrangement for purposes of clarity since the windings preferably comprise multiple turns typically between 1 and 100.

The transmit windings are energised with a pulse width modulated excitation signal where the frequency of the pulse width modulation is substantially lower than the frequency of the excitation signal. Typically frequencies in the range 10-100 kHz are used for the carrier signal compared to 500kHz - 10MHz for the excitation signal.

The excitation signal is substantially equal to the resonant frequency of the C resonant EID. The resonant frequency of the EID is preferably in the range of 500kHz to 10MHz.

The electronics used to generate the excitation signals, process the receive signals and calculate position is preferably the same as described in WO 03/038379 by the same authors.

FURTHER MODIFICATIONS & EMBODIMENTS

A sensor as described above can be combined with the previously disclosed position sensors so that both position along and orthogonal to the windings can be measured. One arrangement of such a combination is shown in Figure 3.

Figure 3 shows one arrangement of such a combination with a second resonant circuit [6b] rotating around and displacing orthogonal to the windings. The rotational sensors two transmit windings [7,8] surround the first position sensor with its receive winding [9] then surrounding the transmit windings.

The windings do not necessarily need to be embodied as tracks on a PCB but may alternatively be wound wire structures or produced by conductive printed ink tracks on an insulating substrate such as Mylar.

Alternatively, rather than an LC resonant circuit the EID may be a permeable element such as a ferrite component or alternatively a metallic component such as a copper, aluminium or steel cylinder. Fasteners such as screws or dowels may also be used as inexpensive and readily available components.

Claims

1. A position sensor comprising: a plurality of planar substrates arranged in parallel ; a sensor element operable to move relative to the plurality of planar substrates along a measurement direction transverse to the planes of the planar substrates; 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 .

2. A position sensor according to claim 1, wherein the plurality of planar substrates form respective layers of a laminar structure .

3. A position sensor according to claim 2, wherein the laminar structure is a multi-layer printed circuit board.

4. A position sensor according to claim 1 or 2, wherein at least one of the first aerial and the second aerial comprises a conductive printed ink track on a substrate .

5. A position sensor according to claim 4, wherein the substrate is Mylar.

6. A push button comprising a position sensor according to any preceding claim.

7. 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.