Self-compensating angular encoder

• US 9,234,773 B2
• Filed: 05/16/2011
• Issued: 01/12/2016
• Est. Priority Date: 05/17/2010
• Status: Active Grant

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Cramer, Peter G.
• Primary Examiner(s): Satanovsky, Alexander
• Assistant Examiner(s): RABOVIANSKI, IVAN I
• Application Number:
  US13/108,277
• Publication Number:
  US 20110282612A1
• Time in Patent Office: 1,702 Days
• Field of Search: 702/92, 702/94, 702/140, 702/154, 702/166, 73/17.5, 738/611.8, 738/626.9, 738/623.81
• US Class Current: 1/1
• 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 with steps comprising:

providing the angular encoder system that includes an angular encoder and a processor, the angular encoder including a structure, a patterned element, and read heads, the patterned element configured to rotate by a first angle relative to the structure, the rotation performed about an axis, the 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 located on a plane and positioned about a central point at an intersection of the axis and the plane, each read head in optical communication with the patterned element, each read head disposed on the structure at a first radius and 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 equal to 360 degrees divided by the number m, the reference read head arranged between two adjacent measure read heads, the processor electrically connected to the read heads;

selecting a desired integer M of first spectral components for which the angular encoder is to be self-compensated for modes j=1, 2, 3 . . . M;

determining a cotangent factor for a collection of angles h_k for each of the measure read heads k=1, 2, . . . , m, the cotangent factor being a maximum of values cot(jh_k/2) evaluated at the collection of angles h_k over the modes j=1, 2, 3 . . . M;

selecting for each of the measure read heads k=1, 2, . . . , m an angle β

_k=h_k for which the cotangent factor is a minimum;

placing the reference read head at a position to obtain the angles β

_k relative to the measure read heads;

rotating the patterned element relative to the read heads by N first angles covering a range of at least 360 degrees, wherein M≦

N/2;

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

calculating a first array d_k for each of the measure read heads k, each first array d_k including, for each of the N 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 measure read head k, at least one first spectral component F_k based at least in part on the first array d_k;

storing the m first spectral component F_k;

determining by the processor an angle φ

of rotation of the patterned element relative to the structure based at least in part on a summation of values that depend on measurements of the m measure read heads at the angle φ and

on the stored m first spectral components F_k; and

storing the determined angle φ

.

2. A method for self-compensating an angular encoder system with steps comprising:

providing the angular encoder system that includes an angular encoder, a motor, and a processor, the angular encoder including a structure, a patterned element, and read heads, the patterned element configured to rotate by an element angle relative to the structure, the rotation performed about an axis, the 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 located on a plane and positioned about a central point at an intersection of the axis and the plane, each read head in optical communication with the patterned element, each read head disposed on the structure at a first radius and a read-head angle about the central point, the measure read heads arranged in such a way that separation between the read-head angles of adjacent measure read heads is equal to 360 degrees divided by the number m, the reference read head arranged between two adjacent measure read heads, the motor configured to step the patterned element relative to the read heads by a desired angular step size, the processor electrically connected to the read heads;

in a first instance, performing a first encoder self-compensation procedure;

determining a first temperature;

rotating with the motor the patterned element relative to the read heads by N first compensation angles covering a range of at least 360 degrees;

obtaining, at each of the N first compensation angles, first angle self-compensation readings for the m measure read heads and for the reference read head;

determining by the processor first compensation parameters, based at least in part on the obtained first angle self-compensation readings and the first temperature;

in a second instance, making a second angle measurement;

obtaining second angle readings for the m measure read heads;

determining with the processor a second angle of rotation of the patterned element relative to the structure, the determined second angle of rotation based at least in part on the first compensation parameters and on the second angle readings;

in a third instance performing a second encoder self-compensation procedure;

determining a second temperature;

rotating with the motor the patterned element relative to the read heads by M second compensation angles covering a range of at least 360 degrees;

obtaining, at each of the M second compensation angles, third angle self-compensation readings for the m measure read heads and for the reference read head;

determining by the processor second compensation parameters, based at least in part on the obtained third angle self-compensation readings and the second temperature;

in a fourth instance, making a fourth angle measurement;

obtaining fourth angle readings for the m measure read heads;

determining with the processor a fourth angle of rotation of the patterned element relative to the structure, the determined fourth angle of rotation based at least in part on the second compensation parameters and on the fourth angle readings; and

storing the fourth angle of rotation.

3. The method of claim 2, further comprising:

determining a difference between the first compensation parameters and the second compensation parameters; and

determining a problem with the encoder, based at least in part on the determined difference between the first compensation parameters and the second compensation parameters.

4. The method of claim 3, further comprising:

determining a difference between the first temperature and the second temperature; and

the step of determining a problem with the encoder is further based on the determined difference between the first temperature and the second temperature.