Indexed optical encoder, method for indexing an optical encoder, and method for dynamically adjusting gain and offset in an optical encoder

• US 8,476,579 B2
• Filed: 10/19/2012
• Issued: 07/02/2013
• Est. Priority Date: 08/28/2008
• Status: Active Grant

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: York, Frederick
• Primary Examiner(s): LEE, JOHN R
• Application Number:
  US13/656,110
• Publication Number:
  US 20130116959A1
• Time in Patent Office: 256 Days
• Field of Search: 25023114-23118, 250/237.G, 324/161, 324/166, 324/175, 341/2, 341/3, 702/85, 702/94, 702/95, 702/104, 702/145, 702/147, 702/148, 702/150, 702/151
• US Class Current: 250/231.14
• CPC Class Codes: G01D 18/00 Testing or calibrating apparatus or arrangements provided for in groups G01D1/00 - G01D15/00
• CPC Class Codes: G01D 5/24476 Signal processing G01D5/2448 - G01D5/24495 take precedence
• CPC Class Codes: G01D 5/2457 Incremental encoders having reference marks
• CPC Class Codes: G01D 5/34 the beams of light being detected by photocells
• CPC Class Codes: G01D 5/34707 Scales; Discs, e.g. fixation, fabrication, compensation
• CPC Class Codes: G01D 5/3473 Circular or rotary encoders
• CPC Class Codes: G06F 17/18 for evaluating statistical data , e.g. average values, frequency distributions, probability functions, regression analysis forecasting specially adapted for a specific administrative, business or logistic context G06Q10/04

Abstract

An optical encoder may include an encoder disk, an illumination system, and a detector to detect light diffracted from the encoder disk. The encoder disk may include a signal track comprising a diffraction grating, and an index track comprising a reflective index mark, wherein a width of the index mark is larger than a pitch of the diffraction grating. An indexing method may include providing an encoder disk, providing an illumination system to direct light to the encoder disk, providing a detector structured to detect light diffracted from the encoder disk, calculating an estimated count of quadrature states from a rising edge of an index pulse to a middle of the index interval, and calculating the quadrature state at an approximate center of the index pulse. A dynamic parameter correction method may include calculating a target gain and offset and correcting values based on the target gain and offset.

Claims

1. A method of dynamically adjusting gain and offset in an optical encoder, the method comprising:

providing an encoder disk comprising a diffraction grating;

illuminating the encoder disk with light;

providing a detector structured to detect light diffracted from the diffraction grating and output a first fine count channel;

calculating a first target gain and first target offset for the first fine count channel; and

applying a correction to data sampled from the first fine count channel based on the first target gain and first target offset;

wherein calculating the first target gain and the first target offset includes;

determining a minimum value and maximum value in a set of data from the first fine count channel;

calculating a moving average minimum based on the minimum value and minimum values from a plurality of prior sets of data from the first fine count channel;

calculating a moving average maximum based on the maximum value and maximum values from the plurality of prior sets of data from the first fine count channel;

calculating the first target gain according to the equation G_cal=(ave_max_A−

ave_min_A)/2, where G_cal is the first target gain, ave_max_A is the moving average maximum, and ave_min_A is the moving average minimum; and

calculating the first target offset according to the equation OFFSET_cal=(ave_max_A+ave_min_A)/2, where OFFSET_cal is the first target offset.

2. The method of claim 1, wherein applying a correction to data sampled from the first fine count channel comprises:

calculating a gain G_A of a data sample according to the equation G_A=(max_A−

min_A)/2, wherein max_A is a maximum of the data sample and min_A is the minimum of the data sample;

calculating an offset OFFSET_A of the data sample according to the equation OFFSET_A=(max_A+min_A)/2;

calculating a gain correction factor GF_A according to the equation GF_A=G_cal/G_A, where G_cal is the first target gain;

calculating an offset correction factor Δ

OFF_A according to the equation Δ

OFF_A=OFFSET_cal−

OFFSET_A, where OFFSET_cal is the first target offset; and

correcting fine counts according to the equation CFC_A=(GF_A*FC_A)+Δ

OFF_A, where FC_A is raw fine count data from the first fine count channel and CFC_A is the corrected fine count data.

3. The method of claim 1, wherein the detector is structured to output a second fine count channel, and the method further comprises:

calculating a second target gain and second target offset for the second fine count channel; and

applying a correction to data sampled from the second fine count channel based on the second target gain and second target offset.

4. The method of claim 3, wherein the detector is structured to output a coarse count, and the method further comprises applying a correction to the course count based on the correction to data sampled from the first fine count channel and data sampled from the second fine count channel.

5. The method of claim 4, wherein applying a correction to the coarse count comprises:

defining a quantization operator Q(x) as;

Q(x)=0 for x<

0, and

Q(x)=1 for x≧

0;

calculating a compensated state pair {A′

, B′

} according to the equation;

{A′

, B′

}={Q(CFC_A−

Δ

OFF_A), Q(CFC_B,−

Δ

OFF_B)};

where CFC_A is corrected data sampled from the first fine count channel, CFC_B is corrected data sampled from the second fine count channel, Δ

OFF_A is an offset correction value for the first fine count channel, and Δ

OFF_B is an offset correction value for the second fine count channel;

comparing the coarse count to the compensated state pair;

adjusting the coarse count to match the compensated state pair.

6. The method of claim 1, wherein a plurality of detectors are provided, each of the plurality of detectors being structured to detect light diffracted from the diffraction grating and output a first fine count channel.

7. The method of claim 1, wherein the determining a minimum and maximum value in a set of data from the first fine count channel comprises:

determining the three highest values and the three lowest values in the set of data;

rejecting the minimum and maximum value if any of the three highest values differ from a median of the three highest values by greater than a predetermined amount or if any of the three lowest values differ from a median of the three lowest values by greater than a predetermined amount.

8. The method of claim 1, wherein the determining a minimum and maximum value in a set of data from the first fine count channel comprises:

rejecting the minimum and the maximum if the minimum differs from the moving average minimum by more than a predetermined amount or if the maximum differs from the moving average maximum by more than a predetermined amount.