Position encoder

Details Position encoder Position encoder

1996-12-12

1998-09-29

Horabik, Michael

Communications: electrical

Continuously variable indicating

With particular transmitter

336129, 32420717, G08C 2100

Patent

active

058150919

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to linear, rotary and radius position encoders generally. The invention has particular although not exclusive relevance to non-contact linear, rotary and radius position encoders. The invention is particularly suited for use in systems where there may be electromagnetic interference.


DISCUSSION OF THE PRIOR ART

Many types of non-contact linear position sensors have been proposed. A system similar to the present invention is the "inductosyn" type system described in U.S. Pat. No. 4,005,396. In particular, U.S. Pat No. 4,005,396 discloses a system that employs a plurality of square wave windings mounted on the surface of a stationary element, and a coil connected to an a.c. power source mounted on a movable element. The square wave windings each comprise a plurality of "high" and "low" parts and have different periods. The system determines the position of the movable element relative to the stationary element by utilising the variation in mutual inductance between the coil and the plurality of square wave windings. More specifically, when the power source energises the coil, a large current is induced in a square wave winding if the coil is adjacent a high part thereof. Only a small current is induced in a winding if the coil is adjacent a low part thereof. Therefore, the position of the movable element along the length of the stationary element can be determined from the signals on the windings.
However, this system has a number of disadvantages. Firstly, the system is not "balanced", i.e. it is not immune to electromagnetic interference. The present inventors have realised that this is because the ground connection is through a base plate or the like and there is no symmetry in the system. Indeed U.S. Pat. No. 4,005,396 acknowledges the interference problem and tries to solve it by providing an additional winding designed to detect electromagnetic interference. The signal from this extra winding is then used to adjust the signals from the square wave windings. However such a solution requires an additional "interference" winding which increases the system complexity and therefore manufacturing cost.
Secondly, the system is sensitive to variations in the power level of the power source and is relatively sensitive to changes in the separation between the movable element and the stationary element. The present inventors have realised that this is because the system relies on amplitude modulating the signal from the power source and no ratiometric type calculation is performed. However, this document offers one solution to the separation problem which involves constraining the movement of the movable element by providing guide rails along which the movable element runs. Although this solution may work in some applications, it will not work for all applications. For example it will not work in a lift system, since it is impractical to constrain the lift so that there is no significant lateral movement within the lift shaft.
Thirdly, the use of square wave windings give rise to the problem that if measurements are made close to the windings (i.e. at a spacing of typically less than 1/8 of the period of windings then the shape of the winding as perceived by the pick up device gives rise to harmonics e.g. third, fifth, seventh etc harmonics which distort the measured results which are obtained.
The present applicant has already proposed a rotary position encoder in WO94/25829 which employs an excitation coil, two receive coils and a resonant circuit mounted on the rotatable element. The arrangement is such that in response to an excitation current in the excitation coil, the resonant circuit produces signals in the receive coils whose amplitudes depend upon the orientation of the rotatable element. Therefore, by suitable processing the orientation of the rotatable member can be determined. However, the disclosure of this reference does not extend beyond rotary positioning encoding.


SUMMARY OF THE INVENTION

In one aspect the invention provides an apparatu

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