Telegraphy – Systems – Position coordinate determination for writing
Patent
1995-03-17
1997-10-07
Kuntz, Curtis
Telegraphy
Systems
Position coordinate determination for writing
345157, 250 55C, 235454, G08C 2100
Patent
active
056751290
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method and apparatus for determining the track of an element in terms of the components of its displacement parallel to a predetermined axis, the element being movable relative to a pattern having pattern features which considered in directions parallel to said predetermined axis form a series exhibiting a windowing property.
BACKGROUND ART
It is known to detect linear position by providing a linear pattern that encodes a pseudo-random binary sequence (PRBS, also known as an "m-sequence") and then using the window property of such a sequence to effect position detection by detecting a window-length subsequence of the pattern. An example of such a system is described in the article "New Pseudorandom/Natural Code Conversion Method" by E. M. Petriu appearing in Electronic Letters 27th October 1988 Vol. 24 No. 22.
By "window property" is meant the property that for a PRBS of length (2.sup.k -1), where k is an integer, any subsequence of length k will be unique and therefore uniquely locatable in the sequence; this and other properties of PRBSs are described, for example, in the article "Pseudo-Random Sequences and Arrays" by F. Jessie MacWilliams and Neil J. A. Sloane appearing in the Proceedings of the IEEE, Vol. 64, No. 12 December 1976.
Our co-pending International Application No. PCT/GB92/00594 describes two-dimensional position sensing apparatus which in the preferred implementations use patterns that encode two orthogonal binary windowing sequences, the apparatus being effective to decode and separate out the portions of these sequences sensed by a sensing head as it is moved over the pattern. Two-dimensional position determination is thus reduced to effecting position determination along two orthogonal sequences as separate processes (although, of course, it is still necessary to coordinate the results of these processes to produce a two dimensional position track). For each sequence a tracking process is used that involves effecting an initial position fix using the windowing property of the sequence. However, because the initial track of the sensing head may include reversals, it will generally require a substantial track length before a window length sequence free of reversals is obtained from which an absolute position fix can be derived. Once an absolute position has been established along one of the sequences, subsequent position tracking for that sequence is effected by an incremental tracking technique coupled with a process for dealing with limited track reversals.
In the tracking process disclosed in the aforesaid International Application, the initial track of the sensing head across the pattern occurring prior to the first absolute position fix, is not recoverable. In practice, this has proven to be a disadvantage. It is an object of the present invention to overcome this disadvantage by providing for initial track recovery.
DISCLOSURE OF THE INVENTION
According to one aspect of the present invention, there is provided a method of determining the track of an element in terms of the components of its displacement parallel to a predetermined axis, the element being movable relative to a pattern having pattern features which considered in directions parallel to said predetermined axis form a series exhibiting a windowing property, the method involving: element in moving relative to the pattern, and storing a representation of said succession of features, axis by comparing window-length portions of the stored representation of said succession of features with a stored representation of said series, the detection of a match at any particular position along the latter serving to determine the location of the element along said series, and backtracking process to seek to recover the initial track of the element over at least the pattern features of said succession that precede those of the window-length portion used in determining the initial location of the element.
Preferably, the said series of pattern features is an encoding of a binary
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patent: 5477012 (1995-12-01), Sekendur
"New Pseudorandom/Natural Code Conversion Method" by E.M. Petriu appearing in Electronic Letters 27th Oct. 1988 vol. 24 No. 22.
"Pseudo-Random Sequences and Arrays" by Jessie MacWilliams and Neil J.A. Sloane sppearing in the Proceedings of the IEEE, vol. 64, No. 12 Dec. 1976.
"Developments in the Measuring of the Absolute Position of Automated Guided Vehicle", by E.M. Petriu and Frans C.A. Groen.
Proceedings of the IEEE, "PseudoRandom Sequences and Arrays", Dec. 1976, MacWilliams et al, pp. 1715-1730.
Burns John
Lloyd Sheelagh Anne
Hewlett--Packard Company
Kuntz Curtis
Loomis Paul
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