Automatic encoder calibration

• US 5,138,564 A
• Filed: 07/31/1990
• Issued: 08/11/1992
• Est. Priority Date: 07/31/1990
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: de Jong, Joannes N. M., Wolf, Barry
• Primary Examiner(s): Teska, Kevin J.
• Application Number:
  US07/560,813
• Time in Patent Office: 742 Days
• Field of Search: 364/571.01, 364/571.02, 364/571.03, 364/571.04, 364/571.05, 364/571.06, 364/571.07, 364/571.08, 364/565, 364/566, 364/575, 371/30, 250/231.14, 250/211 R
• US Class Current: 702/96
• CPC Class Codes: G01D 18/002 Automatic recalibration G01D18/008 takes precedence
• CPC Class Codes: G01P 21/02 of speedometers
• CPC Class Codes: H03M 1/1047 using an auxiliary digital/analogue converter for adding the correction values to the analogue signal H03M1/1052 takes precedence
• CPC Class Codes: H03M 1/308 with additional pattern means for determining the absolute position, e.g. reference marks

Abstract

A method and apparatus provides on-line calibration of low cost encoders by interfacing an encoder with a microprocessor and driving a shaft to which the encoder is attached at a slow constant speed and at a fast constant speed. Data is collected by the microprocessor for the slow and fast speeds and a mathematical analysis is performed which determines the torque disturbances occurring at each point in the rotational cycle of the shaft and which further determines the actual location of each encoder line on the encoder in the rotational cycle of the shaft. By knowing the actual location of each encoder line and the number of counts between encoder lines for a given speed, a high accuracy encoder can be realized at a minimal cost.

Claims

1. A method of controlling velocity variations of a rotating shaft having on encoder mounted on the shaft, the method comprising the steps of:

rotating the shaft at firs and second predetermined constant speeds to generate first and second pulse trains, respectively, from detection of line spacings on the encoder;

determining a profile of torque disturbances from the first pulse train when rotating the shaft at the first predetermined constant speed, the torque disturbance profile corresponding to angular positions of the shaft subject to torque drag;

determining a profile of line spacings from the second pulse train when rotating the shaft at the second predetermined constant speed, the line spacings profile corresponding to an actual number of line spacings to be sensed for a predetermined time interval;

rotating the shaft at a desired speed to obtain a pulse train of actual encoder pulses from the detection of the line spacings on the encoder;

comparing the pulse train of actual encoder pulses to the torque disturbance profile to generate a torque disturbance removal signal;

comparing the pulse train of actual encoder pulses to the line spacings profile to generate a set point correction signal; and

varying the desired speed in response to both the set point correction signal and the torque disturbance removal signal.

2. The method of claim 1, wherein the first predetermined constant speed is relatively slower than the second predetermined constant speed.

3. An apparatus for controlling velocity variations of a rotating shaft, comprising:

means for rotating the shaft at a plurality of rotational speeds;

encoder means mounted on the shaft and having a plurality of line spacings for outputting a pulse train representative of the rotational speed of the shaft;

means for determining a profile of torque disturbances from a first pulse train generated by rotating the shaft at a first predetermined constant rotational speed, the torque disturbance profile corresponding to angular positions of the shaft subject to torque drag;

means for determining a profile of line spacings from a second pulse train generated by rotating the shaft at a second predetermined constant rotational speed, the line spacings profile corresponding to an actual number of lines to be sensed for a predetermined time interval;

means for comparing a pulse train of actual encoder pulses generated by rotating the shaft at a desired rotational speed to the torque disturbance profile to generate a torque disturbance removal signal, and to the line spacings profile to generate a set point correction signal; and

controller means for varying the desired speed in response to both the set point correction signal and the torque disturbance removal signal.

4. The apparatus of claim 3, wherein the first predetermined constant speed is relatively slower than the second predetermined constant speed.