SELF-COMPENSATING ANGULAR ENCODER

• US 20110282612A1
• Filed: 05/16/2011
• Published: 11/17/2011
• Est. Priority Date: 05/17/2010
• Status: Active Grant

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Cramer, Peter G.
• Granted Patent: US 9,234,773 B2
• Time in Patent Office: Days
• US Class Current: 702/94
• CPC Class Codes: G01D 18/001 Calibrating encoders
• CPC Class Codes: G01D 5/24485 using other sensors
• CPC Class Codes: G01D 5/3473 Circular or rotary encoders
• CPC Class Codes: G01D 5/34738 Axles; Driving or coupling means

Abstract

A method for self-compensating a method includes the steps of rotating the patterned element relative to the read heads, the rotation about the axis to a plurality of first angles covering a range of at least 360 degrees; obtaining, at each of the plurality of first angles, first angular readings for the m measure read heads and for the reference read head; calculating a first array for each of the m measure read heads, each first array including, for each of the plurality of first angles, a difference in the first angular readings of the measure read head and the first angular reading of the reference read head; calculating, for each of the m measure read heads, at least one first spectral component based at least in part on the first array; calculating, for each of the m measure read heads, at least one second spectral component, the second spectral component based, at least in part, on the at least one first spectral component and on estimates of the second angles of the m measure read heads; and recording, for each of the m measure read heads, the at least one second spectral component.

Claims

1. A method for self-compensating an angular encoder system, the angular encoder system including

a structure,

a patterned element configured to rotate by a first angle relative to the structure, the rotation performed about an axis,

read heads including one reference read head and a number m of measure read heads, the number m being greater than or equal to two, the read heads fixedly disposed on the structure and substantially located on a plane and positioned about a central point at the intersection of the axis and the plane, each read head disposed on the structure at a second angle about the central point;

the measure read heads arranged in such a way that separation between the second angles of adjacent measure read heads is substantially equal to 360 degrees divided by the number m, the reference read head arranged in such a way that separation between the second angle of the reference read head and the second angle of any of the m measure read heads is not equal to zero, with steps comprising;

rotating the patterned element relative to the read heads, the rotation about the axis to a plurality of first angles covering a range of at least 360 degrees;

obtaining, at each of the plurality of first angles, first angular readings for the m measure read heads and for the reference read head;

calculating a first array for each of the m measure read heads, each first array including, for each of the plurality of first angles, a difference in the first angular readings of the measure read head and the first angular reading of the reference read head;

calculating, for each of the m measure read heads, at least one first spectral component based at least in part on the first array;

calculating, for each of the m measure read heads, at least one second spectral component, the second spectral component based, at least in part, on the at least one first spectral component and on estimates of the second angles of the m measure read heads; and

recording, for each of the m measure read heads, the at least one second spectral component.

2. The method for self-compensating the angular encoder system of claim 1, further comprising the steps of:

rotating the patterned element about the axis to a third angle relative to the structure;

obtaining, at the third angle, a second angular reading for each of the m measure read heads;

calculating a first average equal to an average of the second angular readings over the m measure read heads;

calculating a second error at the third angle, the second error based at least in part on an average of m first errors, each evaluated at a different fourth angle equal to an estimate of one of the second angles plus the first average, each of the first errors based at least in part on the second spectral components, one of the second angles, and the first average;

calculating a fourth angle, which is a self-compensated estimate of the third angle, based at least in part on the first average and the second error; and

recording the fourth angle.

3. The method for self-compensating the angular encoder system of claim 1, wherein the step of calculating at least one second spectral component further comprises the step of improving the estimate of the second angle by adjusting the values of the second angles to minimize a residual error, the residual error based at least in part on the second spectral components, the first angles, and the estimates of the second angles.

4. The method of self-compensating the angular encoder system of claim 1, further comprising the steps of:

selecting a plurality of fifth angles;

calculating, for each fifth angle of the plurality of fifth angles, a first error based at least in part on the fifth angle, the second spectral components, and the estimate of the second angles;

calculating, for each fifth angle of the plurality of fifth angles, a second error, each second error based at least in part on an average of m first errors, each of the m first errors evaluated at a different fourth angle equal to the fifth angle plus the estimate of one of the second angles; and

recording the second errors in an array.

5. The method for self-compensating the angular encoder system of claim 4, further comprising the steps of:

rotating the patterned element about the axis to a third angle relative to the structure;

obtaining, at the third angle, a second angular reading for each of the m measure read heads;

calculating a first average equal to the average of the second angular readings of the m measure read heads;

calculating a first error based at least in part on at least one of the second angular readings and the array of second errors; and

calculating a fourth angle, which is a self-compensated estimate of the third angle, based at least in part on the first average and the array of second errors; and

recording the fourth angle.

6. The method of self-compensating the angular encoder system of claim 5, further including the steps of:

placing a reference angular encoder on the axis;

obtaining, at each of a plurality of sixth angles, sixth angular readings from the reference encoder and first angular readings of the angular encoder system;

obtaining, at each of the plurality of sixth angles, a difference in the angular readings of the angular encoder system and the reference encoder;

calculating third spectral components of the difference in the angular readings; and

supplementing the second spectral components with the third spectral components.

7. The method of self-compensating the angular encoder system of claim 2, further including the steps of:

placing a reference angular encoder on the axis;

obtaining, at each of a plurality of sixth angles, sixth angular readings from the reference encoder and first angular readings of the angular encoder system;

obtaining, at each of the plurality of sixth angles, a difference in the angular readings of the angular encoder system and the reference encoder;

calculating third spectral components of the difference in the angular readings; and

supplementing the second spectral components with the third spectral components.