Absolute incremental position encoder and method

• US 20040173735A1
• Filed: 03/05/2003
• Published: 09/09/2004
• Est. Priority Date: 03/05/2003
• Status: Active Grant

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Wrigglesworth, Walter, Williams, Darin
• Granted Patent: US 7,034,283 B2
• Time in Patent Office: Days
• US Class Current: 250/231.13
• CPC Class Codes: G01D 5/2495 Pseudo-random code
• CPC Class Codes: G01D 5/34792 with only digital scales or both digital and incremental scales

Abstract

A position encoder uses a track encoded with a pattern of bit-widths in accordance with a sequence. The sequence may be any sequence having unique subsequences, and may be a pseudo-random noise (PRN) sequence such that each N-bit subsequence occurs only once on the track. Sensors detect transitions between the bits and bit-widths as the track moves with respect to the sensors to provide in-phase and quadrature-phase pick-off signals. The pickoff signals are summed and absolute value thresholded. The absolute value thresholded sum signal is sampled when the quadrature pairs are in the “00” or “11” quadrants, and latched when the sum signal goes high to distinguish between wide and narrow bit widths. The latch is shifted into a shift data register for use in determining the position of the encoder track. In the case of a PRN sequence having a length of 2^N bits, the position may be an absolute position when the number of valid bits in the shift data register is at least N. The position may be an incremental position when the number of transitions detected is less than N.

Claims

1. A position encoder comprising:

a track encoded with a pattern comprised of a plurality of unique subsequences;

sensors to detect the pattern as the track moves; and

a processing element to detect a portion of at least one of the unique subsequences from the signals provided by the sensors for use in determining either an absolute or incremental position of the track.

2. The encoder of claim 1 wherein the track is encoded with bit-widths in accordance with the pattern, and wherein the sensors detect transitions between the bit-widths as the track moves with respect to the sensors to provide pick-off signals;

and the processing element combines the pick-off signals to generate the portion of at least one of the unique subsequences.

3. The encoder of claim 2 wherein each bit-width encoded on the track has either a first width or a second width determined by the pattern, the first width representing “

ones”

in the sequences, the second width representing “

zeroes”

in the sequences.

4. The encoder of claim 3 wherein sensors comprise first and second optical sensors positioned to have partially overlapping fields of view, and wherein the pattern on the track is a pattern of alternating dark and light portions having the bit-widths in accordance with the sequences.

5. The encoder of claim 3 wherein the sensors comprise magnetic field sensors, and wherein the pattern on the track is a pattern of alternating electric fields in accordance with the bit-widths representing the sequences.

6. The encoder of claim 4 wherein the first and second sensors are spaced apart approximately one-half their field of view to provide the partially overlapping fields of view, wherein track bit-widths are selected so that signal extremes are larger for wider bit-widths.

7. The encoder of claim 1 wherein the sensors comprise first and second sensors, the first sensor to provide a quadrature-phase pick-off signal, the second sensor to provide an in-phase pick-off signal,

wherein the processing element:

to generate quadrature pairs from the quadrature-phase and in-phase pick-off signals,

to sum the quadrature-phase and in-phase pick-off signals and absolute value threshold the sum to generate a quantized signal,

to sample the quantized signal when the quadrature pairs indicate “

00”

or “

11”

quadrants,

to latch a bit when the sampled quantized signal is high when the quadrature pairs indicate “

00”

or “

11”

quadrants,

to shift the latched bit into a data-shift register when a transition through either the “

00”

or “

11”

quadrant occurs, and

to determine a position of the track from bits shifted into the data-shift register, the bits shifted into the data-shift register corresponding with a portion of at least one of the unique subsequences.

8. The encoder of claim 7 wherein the pattern is a pseudo-random noise (PRN) sequence having a length of 2^N bits, and wherein the position is an absolute position of the track when a number of latched bits is at least N, and the position is an incremental position of the track when the number of latched bits is less than N.

9. The encoder of claim 8 further comprising a memory to store a table with the plurality of unique subsequences, the processing element to compare the bits in the data-shift register with the unique subsequences in the table to determine the position of the track at a last-detected transition.

10. The encoder of claim 8 further comprising a code generator to generate a sequence comprised of the unique subsequences, the processing element to compare subsequences of the generated sequence with the bits in the data-shift register to determine the position of the track at a last-detected transition.

11. The encoder of claim 8 wherein the processing element shifts the latched bit into a first one of the data shift registers for positive rotational movement of the track, and shifts the latched bit into a second one of the data shift registers for negative rotational movement of the track,

and wherein the encoder further comprising a set of tag registers initialized with zeros, wherein the processing element shifts a tag bit into one of the tag registers for latched bits shifted into the shift data registers, the processing element using the tag bits to determine whether the position is an incremental position or an absolute position.

12. A method of determining a position of a track encoded with a pattern comprised of a plurality of unique subsequences, the method comprising:

detecting transitions as the track moves with sensors;

combining outputs of the sensors to generate a least a portion of the unique subsequences; and

comparing the generated portion with the pattern to determine either an absolute or incremental position of the track.

13. The method of claim 12 wherein the track is encoded with bit-widths in accordance with the pattern, the method further comprising:

the sensors detecting transitions between the bit-widths as the track moves with respect to the sensors to provide pick-off signals; and

combining the pick-off signals to generate a portion of at least one of the unique subsequences for use in determining either the absolute or incremental position of the track.

14. The method of claim 13 wherein each bit-width encoded on the track has either a first width or a second width determined by the pattern, the first width representing “

ones”

in the sequences, the second width representing “

zeroes”

in the sequences.

15. The method of claim 13 wherein sensors comprise first and second optical sensors positioned to have partially overlapping fields of view, and wherein the pattern on the track is a pattern of alternating dark and light portions having the bit-widths in accordance with the sequences.

16. The method of claim 12 further comprising:

providing a quadrature-phase pick-off signal with a first sensor and an in-phase pick-off signal with a second sensor;

generating quadrature pairs from the quadrature-phase and in-phase pick-off signals;

summing the quadrature-phase and in-phase pick-off signals and absolute value thresholding the sum to generate a quantized signal;

sampling the quantized signal when the quadrature pairs indicate “

00”

or “

11”

quadrants;

latching a bit when the sampled quantized signal is high when the quadrature pairs indicate “

00”

or “

11”

quadrants;

shifting the latched bit into a data-shift register when a transition through either the “

00”

or “

11”

quadrant occurs; and

determining a position of the track from bits shifted into the data-shift register, the bits shifted into the data-shift register corresponding with a portion of at least one of the unique subsequences.

17. The method of claim 16 wherein the pattern is a pseudo-random noise (PRN) sequence having a length of 2^N bits, and wherein the position is an absolute position of the track when a number of latched bits is at least N, and the position is an incremental position of the track when the number of latched bits is less than N.

18. The method of claim 16 further comprising comparing the bits in the data-shift register with unique subsequences in a table to determine the position of the track at a last-detected transition.

19. The method of claim 16 further comprising:

generating a sequence corresponding with the unique subsequences of the pattern; and

comparing unique subsequences of the generated sequence with the bits in the data-shift register to determine the position of the track at a last-detected transition.

20. The method of claim 16 further comprising:

shifting the latched bit into a first one of the data shift registers for positive rotational movement of the track;

shifting the latched bit into a second one of the data shift registers for negative rotational movement of the track;

shifting a tag bit into a tag register each for each latch bit shifted into the shift data registers; and

using the tag bits to determine whether the position is an incremental position or an absolute position.

21. A system comprising:

first and second nested gimbals;

a first position encoder to determine an angular position of the first gimbal with respect to a base; and

a second position encoder to determine an angular position of the second gimbal with respect to the first gimbal,

wherein the position encoders have tracks encoded with a pattern of bit-widths in accordance with sequence comprised of a plurality of subsequences, have sensors to detect the pattern as the tracks move, and have a processing element to determine the positions of the tracks from sensor signals.

22. The system of claim 21 wherein the track of each the position encoder is encoded with bit-widths in accordance with the pattern, and wherein the sensors detect transitions between the bit-widths as the track moves with respect to the sensors to provide pick-off signals;

and the processing element combines the pick-off signals to generate the portion of at least one of the unique subsequences.

23. The system of claim 22 wherein each bit-width encoded on each track has either a first width or a second width determined by the pattern, the first width representing “

ones”

in the sequences, the second width representing “

zeroes”

in the sequences.

24. The system of claim 23 wherein each sensor comprises first and second optical sensors positioned to have partially overlapping fields of view, and wherein the pattern on the track is a pattern of alternating dark and light portions having the bit-widths in accordance with the sequences.

25. The system of claim 23 each sensor comprises magnetic field sensors, and wherein the pattern on the track is a pattern of alternating electric fields in accordance with the bit-widths representing the sequences.

26. The system of claim 24 wherein the first and second sensors of each encoder are spaced apart approximately one-half their field of view to provide the partially overlapping fields of view, wherein a width of a narrower of either the first or second width is at least equal to the field of view of one of the sensors.

27. The system of claim 21 wherein each of the sensors comprise first and second sensors, the first sensor to provide a quadrature-phase pick-off signal, the second sensor to provide an in-phase pick-off signal,

wherein the processing element of each encoder:

to generate quadrature pairs from the quadrature-phase and in-phase pick-off signals,

to sum the quadrature-phase and in-phase pick-off signals and absolute value threshold the sum to generate a quantized signal,

to sample the quantized signal when the quadrature pairs indicate “

00”

or “

11”

quadrants,

to latch a bit when the sampled quantized signal is high when the quadrature pairs indicate “

00”

or “

11”

quadrants,

to shift the latched bit into a data-shift register when a transition through either the “

00”

or “

11”

quadrant occurs, and

to determine a position of the track from bits shifted into the data-shift register, the bits shifted into the data-shift register corresponding with a portion of at least one of the unique subsequences.