OPTICAL ENCODER HAVING CONTAMINATION AND DEFECT RESISTANT SIGNAL PROCESSING

• US 20110273725A1
• Filed: 05/05/2010
• Published: 11/10/2011
• Est. Priority Date: 05/05/2010
• Status: Active Grant

• Current Assignee: Mitutoyo Corporation
• Original Assignee: Mitutoyo Corporation
• Inventors: Milvich, Michelle Mary
• Granted Patent: US 8,493,572 B2
• Time in Patent Office: Days
• US Class Current: 356/616
• CPC Class Codes: G01D 5/24476 Signal processing G01D5/2448 - G01D5/24495 take precedence
• CPC Class Codes: G01D 5/34715 Scale reading or illumination devices

Abstract

An encoder configuration comprises an illumination portion, a scale comprising a scale track, and a signal processing electronics. The signal processing electronics may include a detector comprising a first set of three detector sub-portions that provide a first set of signals comprising three respective sub-portion signal subsets that have nominally the same signal characteristics when the scale track is not contaminated or defective. The processing electronics analyze the first set of signals and identify a least-similar sub-portion signal subset that has a corresponding signal characteristic value that is least similar to comparable signal characteristic values associated with more-similar sub-portion signal subsets of the first set of signals. Position measurements are based on valid signals including a plurality of the more-similar sub-portion signal subsets and not including the least-similar sub-portion signal subset if it is significantly different. Accurate measurement may be provided despite significant scale contamination or defects.

Claims

1. An encoder configuration usable to determine a relative position between a scale element and a detector along a measuring axis direction with reduced influence from the effects of scale track defects and contaminants, the encoder configuration comprising:

an illumination portion;

a scale element including at least a first scale track including a first scale track pattern extending along the measuring axis direction and arranged to receive light from the illumination portion and output a first periodic spatially modulated light pattern corresponding to the first scale track pattern along a first light path; and

a signal processing electronics comprising a first detector portion and a valid signal selection portion, the first detector portion configured to receive the first periodic spatially modulated light pattern along the first light path, and to spatially filter the received first periodic spatially modulated light pattern and output signals that depend on the relative position,

wherein;

the first detector portion comprises a first set of at least three respective sub-portions, the at least three respective sub-portions producing a first set of signals comprising at least three respective sub-portion signal subsets that have nominally the same signal characteristics when the first scale track is not contaminated or defective; and

the signal processing electronics is configured to;

input the first set of signals to the valid signal selection portion;

analyze the first set of signals and identify a least-similar sub-portion signal subset that has a corresponding signal characteristic value that is least similar to comparable signal characteristic values associated with other more-similar sub-portion signal subsets of the first set of signals; and

perform operations to determine a first scale track position-indicating signal based on a set of valid signals, the valid set of signals including;

a plurality of the more-similar sub-portion signal subsets of the first set of signals; and

not including the least-similar sub-portion signal subset if the signal characteristic value of the least-similar sub-portion signal subset is significantly different than the comparable signal characteristic values associated with the other more-similar sub-portion signal subsets.

2. The encoder configuration of claim 1, wherein the signal processing electronics is configured to determine that the signal characteristic value of the least-similar sub-portion signal subset is significantly different than the comparable signal characteristic values of the more-similar sub-portion signal subsets when the signal characteristic value of the least-similar sub-portion signal subset falls outside of an allowed difference range defined based on at least one of the signal characteristic values associated with the more-similar sub-portion signal subsets.

3. The encoder configuration of claim 2, wherein the allowed difference range is defined to coincide with the range of the signal characteristic values associated with the more-similar sub-portion signal subsets.

4. The encoder configuration of claim 2, wherein the signal processing electronics is configured to:

define the allowed difference range such that it is larger than the range of the signal characteristic values associated with the more-similar sub-portion signal subsets;

determine that the signal characteristic value of the least-similar sub-portion signal subset is not significantly different than the comparable signal characteristic values of the more-similar sub-portion signal subsets when the signal characteristic value of the least-similar sub-portion signal subset does not fall outside of the allowed difference range; and

perform operations to determine the first scale track position-indicating signal based on a plurality of the more-similar sub-portion signal subsets and based on the least-similar sub-portion signal subset, if the signal characteristic value of the least-similar sub-portion signal subset is not significantly different than the comparable signal characteristic values associated with other more-similar sub-portion signal subsets.

5. The encoder configuration of claim 4, wherein a lower limit of the allowed difference range is a first defined difference amount less than one of (a) the lowest of the signal characteristic values associated with the more-similar sub-portion signal subsets, (b) an average of at least two signal characteristic values associated with the more-similar sub-portion signal subsets, and (c) the highest of the signal characteristic values associated with the more-similar sub-portion signal subsets.

6. The encoder configuration of claim 5, wherein the first defined difference amount is proportional to one of (a) the lowest of the signal characteristic values associated with the more-similar sub-portion signal subsets, (b) an average of at least two signal characteristic values associated with the more-similar sub-portion signal subsets, (c) the highest of the signal characteristic values associated with the more-similar sub-portion signal subsets, (d) a difference between two signal characteristic values associated with the more-similar sub-portion signal subsets, and (e) a measure of distribution associated with the signal characteristic values of the more-similar sub-portion signal subsets.

7. The encoder configuration of claim 4, wherein an upper limit of the allowed difference range is a second defined difference amount more than one of (a) the lowest of the signal characteristic values associated with the more-similar sub-portion signal subsets, (b) an average of at least two signal characteristic values associated with the more-similar sub-portion signal subsets, and (c) the highest the signal characteristic values associated with the more-similar sub-portion signal subsets.

8. The encoder configuration of claim 7, wherein the second defined difference amount is proportional to one of (a) the lowest of the signal characteristic values associated with the more-similar sub-portion signal subsets, (b) an average of at least two signal characteristic values associated with the more-similar sub-portion signal subsets, (c) the highest of the signal characteristic values associated with the more-similar sub-portion signal subsets, (d) a difference between two signal characteristic values associated with the more-similar sub-portion signal subsets, and (e) a measure of distribution associated with the signal characteristic values of the more-similar signals.

9. The encoder configuration of claim 1, wherein the first set of at least three respective sub-portions each comprise a first similar detector element set and are positioned apart by an integer number of spatial periods of the first spatially modulated light pattern along the measuring axis direction.

10. The encoder configuration of claim 9, wherein the similar detector element set comprises one of a single detector element, a set of non-adjacent detector elements having the same spatial phase, a set of non-adjacent detector elements including a plurality of spatial phases, and a set of adjacent detector elements including a plurality of spatial phases.

11. The encoder configuration of claim 10, wherein the set of adjacent detector elements including a plurality of spatial phases are evenly spaced over an integer number of spatial periods of the first spatially modulated light pattern.

12. The encoder configuration of claim 11, wherein the valid signal selection portion comprises at least one signal combiner, the similar detector element set comprises the set of adjacent detector elements including a plurality of spatial phases, and each respective sub-portion signal subset is input to a signal combiner which outputs a respective combined signal that provides the signal characteristic value that corresponds to that respective sub-portion signal subset.

13. The encoder configuration of claim 10, wherein:

the similar detector element set of the first set of at least three respective sub-portions comprises one of a single detector element having a first spatial phase, and a set of non-adjacent detector elements each having the first spatial phase;

the detector portion further comprises a second set and a third set of at least three respective sub-portions, the second set of at least three respective sub-portions producing a second set of signals comprising at least three respective sub-portion signal subsets that have nominally the same signal characteristics when the first scale track is not contaminated or defective, the third set of at least three respective sub-portions producing a third set of signals comprising at least three respective sub-portion signal subsets that have nominally the same position-indicating signal characteristics when the first scale track is not contaminated or defective;

the second set of at least three respective sub-portions each comprise a second similar detector element set and are positioned apart by an integer number of spatial periods of the first spatially modulated light pattern along the measuring axis direction, and the third set of at least three respective sub-portions each comprise a third similar detector element set and are positioned apart by an integer number of spatial periods of the first spatially modulated light pattern along the measuring axis direction;

the second similar detector element set comprises one of a single detector element having a second spatial phase, and a set of non-adjacent detector elements each having the second spatial phase, and the third similar detector element set comprises one of a single detector element having a third spatial phase, and a set of non-adjacent detector elements each having the third spatial phase; and

the signal processing electronics is configured to;

input the second set of signals to the valid signal selection portion and input the third set of signals to the valid signal selection portion;

analyze the second set of signals and identify a least-similar sub-portion signal subset that has a signal characteristic value that is least similar to comparable signal characteristic values associated with other more-similar sub-portion signal subsets of the second set of signals;

analyze the third set of signals and identify a least-similar sub-portion signal subset that has a signal characteristic value that is least similar to comparable signal characteristic values associated with other more-similar sub-portion signal subsets of the third set of signals; and

perform operations to determine a first scale track position-indicating signal based on a set of valid signals including;

the plurality of the more-similar sub-portion signal subsets of the first set of signals, a plurality of the more-similar sub-portion signal subsets of the second set of signals and a plurality of the more-similar sub-portion signal subsets of the third set of signals, and

not including the least-similar sub-portion signal subset of the first set of signals if its signal characteristic value is significantly different than the comparable signal characteristic values associated with the other more-similar sub-portion signal subsets of the first set of signals, not including the least-similar sub-portion signal subset of the second set of signals if its signal characteristic value is significantly different than the comparable signal characteristic values associated with the other more-similar sub-portion signal subsets of the second set of signals, and not including the least-similar sub-portion signal subset of the third set of signals if its signal characteristic value is significantly different than the comparable signal characteristic values associated with the other more-similar sub-portion signal subsets of the third set of signals.

14. The encoder configuration of claim 1, wherein:

the encoder configuration is an absolute encoder configuration;

the scale element includes a fine scale track having a fine wavelength and at least one absolute scale track having a wavelength longer than the fine wavelength; and

the first scale track is one of the at least one absolute scale track.

15. A method for determining a relative position between a scale element and a detector along a measuring axis direction with reduced influence from the effects of scale track defects and contaminants, the method comprising:

outputting light from an illumination portion;

receiving the light from the illumination portion with a scale element including at least a first scale track including a first scale track pattern extending along the measuring axis direction, and outputting a first periodic spatially modulated light pattern corresponding to the first scale track pattern along a first light path;

receiving the first periodic spatially modulated light pattern along the first light path with a signal processing electronics comprising a first detector portion and a valid signal selection portion, and spatially filtering the received first periodic spatially modulated light pattern and outputting signals that depend on the relative position, wherein the first detector portion comprises a first set of at least three respective sub-portions, the at least three respective sub-portions operate to produce a first set of signals comprising at least three respective sub-portion signal subsets that have nominally the same signal characteristics when the first scale track is not contaminated or defective; and

processing the first set of signals with the signal processing electronics, comprising;

inputting the first set of signals to the valid signal selection portion;

analyzing the first set of signals and identify a least-similar sub-portion signal subset that has a corresponding signal characteristic value that is least similar to comparable signal characteristic values associated with other more-similar sub-portion signal subsets of the first set of signals; and

performing operations to determine a first scale track position-indicating signal based on a set of valid signals, the valid set of signals including;

a plurality of the more-similar sub-portion signal subsets of the first set of signals; and

not including the least-similar sub-portion signal subset if the signal characteristic value of the least-similar sub-portion signal subset is significantly different than the comparable signal characteristic values associated with the other more-similar sub-portion signal subsets.

16. The method of claim 15, wherein processing the first set of signals with the signal processing electronics comprises determining that the signal characteristic value of the least-similar sub-portion signal subset is significantly different than the comparable signal characteristic values associated with the other more-similar sub-portion signal subsets when the signal characteristic value of the least-similar sub-portion signal subset falls outside of an allowed difference range defined based on at least one of the signal characteristic values associated with the more-similar sub-portion signal subsets.

17. The method of claim 16, wherein the allowed difference range is defined to coincide with the range of the signal characteristic values associated with the more-similar sub-portion signal subsets.

18. The method of claim 16, wherein processing the first set of signals with the signal processing electronics comprises:

defining the allowed difference range such that it is larger than the range of the signal characteristic values associated with the more-similar sub-portion signal subsets;

determining that the signal characteristic value of the least-similar sub-portion signal subset is not significantly different than the comparable signal characteristic values of the more-similar sub-portion signal subsets when the signal characteristic value of the least-similar sub-portion signal subset does not fall outside of the allowed difference range; and

performing operations to determine a first scale track position-indicating signal based on a set of valid signals, the valid set of signals including;

a plurality of the more-similar sub-portion signal subsets of the first set of signals; and

including the least-similar sub-portion signal subset if the signal characteristic value of the least-similar sub-portion signal subset is not significantly different than the comparable signal characteristic values associated with the other more-similar sub-portion signal subsets.

19. The method of claim 18, wherein defining the allowed difference range includes defining a lower limit of the allowed difference range that is a first defined difference amount less than one of (a) the lowest of the signal characteristic values associated with the more-similar sub-portion signal subsets, (b) an average of at least two signal characteristic values associated with the more-similar sub-portion signal subsets, and (c) the highest of the signal characteristic values associated with the more-similar sub-portion signal subsets.

20. The encoder configuration of claim 19, wherein the first defined difference amount is proportional to one of (a) the lowest of the signal characteristic values associated with the more-similar sub-portion signal subsets, (b) an average of at least two signal characteristic values associated with the more-similar sub-portion signal subsets, (c) the highest of the signal characteristic values associated with the more-similar sub-portion signal subsets, (d) a difference between two signal characteristic values associated with the more-similar sub-portion signal subsets, and (e) a measure of distribution associated with the signal characteristic values of the more-similar sub-portion signal subsets.