Position encoder using a pseudo-random coding sequence

Coded data generation or conversion – Digital pattern reading type converter – Magnetic – inductive or capacitive

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341 11, 341 9, H03M 100

Patent

active

051736934

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the decoding of random or pseudo-random sequences and is applicable, for example, to absolute position encoders.
It is possible to use a maximal length binary sequence (called an m-sequence) in an absolute position encoder. An m-sequence is a sequence of 2.sup.n -1 bits so arranged that all of the n-tuple groups of successive bits are unique. If the sequence is considered to be circular, there are then 2.sup.n -1 unique n-tuples. The bit pattern of each n-tuple thus defines its position in the m-sequence. It is thus possible to make an absolute encoder in which the limit of physical resolution is one bit of the m-sequence. This is particularly useful for optical and high precision encoders where the physical resolution precludes the use of more than a single optical track. Unfortunately, one feature of the m-sequence is that it is pseudo-random and there is no simple relationship between any n-tuple and its position in the sequence. In practice it is possible to determine the position of any given n-tuple by, for example, either: known reference point, usually all 1's, is found; or is written.
Both of these techniques can be used if the coefficients of the generating polynomial are known, as they are in an encoder application. However, the first introduces a processing delay between an n-tuple being presented and an absolute position being available and although the second approach is faster it does require storage memory. Both approaches become unattractive when long sequences such as for n>20 are used when, for example, approach (1) could require stepping through upto 2.sup.n -1 n-tuples to find the match and approach (2) requires a look-up table of >1M.times.20 bits.
According to one aspect, the invention involves interleaving fragments, e.g. two fragments, of the same maximal length sequence, where the fragments have lengths with no common factors.
This allows the above two approaches to be used more economically in, for example, absolute encoders.
According to a second aspect of the invention there is provided a device having a sequence of bits characterised in that said sequence of bits comprises a plurality of different interleaved fragments of a single maximal length sequence, said fragments having lengths with no common factors such that there exists integer x whereby any x-tuple of x successive bits of the sequence of bits defines an absolute location in the sequence.
For example, one may use two fragments only, with a maximal length sequence of length of 2.sup.X/2 -1.
According to a third aspect of the invention there is provided a device having a sequence of machine readable bits characterised in that, for a given integer x, in any x-tuple of x successive bits of said sequence, different bits of the x-tuple form a set of y-tuples together uniquely defining the position of the x-tuple in the sequence, each x-tuple being unique but each y-tuple occurring more than once in different x-tuples.
This may be easily and generally achieved where the fragments differ in length by 1. In one example, to replace a sequence of order n (where n is even) one would generate a sequence of order (n/2+1) (i.e. length 2 .sup.(n/2+1) -1) and break this into fragments of length 2.sup.n/2 and 2 .sup.n/2 -1.
According to a fourth aspect of the invention there is provided a method of determining an absolute location characterised by the steps of: lengths with no common factors; an integer x for which any x-tuple of x successive bits of the new sequence defines an absolute location in the new sequence;
According to a fifth aspect of the invention there is provided a method of preparing a coding sequence characterised by the steps of: lengths with no common factors; and
For a better understanding of the present invention and as to how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:
FIG. 1 shows an interleaved pseudo random code;
FIG. 2 shows a circuit arrangement for decoding the code of FIG. 1;
FIG. 3 shows an encod

REFERENCES:
patent: 3882482 (1975-05-01), Green et al.
patent: 4572952 (1986-02-01), March
patent: 4628298 (1986-12-01), Hafle et al.
patent: 4631519 (1986-12-01), Johnston
patent: 4780600 (1988-10-01), Johnston
patent: 4906992 (1990-03-01), Wingate et al.
patent: 4914437 (1990-04-01), Kibrick et al.
patent: 4947166 (1990-08-01), Wingate et al.
patent: 4965503 (1990-10-01), Watanabe et al.
New Pseudorandom Encoding Technique for Shaft Encoders With Any Desired Resolution, Electronic Letters, vol. 23, No. 10, May 7, 1987 (Hitchin, Herts, GB) pp. 507-509.

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