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Method and Apparatus for Position Error Measurement of Optical Rotary Encoders

IP.com Disclosure Number: IPCOM000127560D
Publication Date: 2005-Sep-01
Document File: 6 page(s) / 131K

Publishing Venue

The IP.com Prior Art Database

Abstract

By feeding the digital encoder output signal into a frequency to voltage converter coupled to a dynamic signal analyzer, we record the spectral frequency at the "once around" interval of the encoder at a known driven speed. The voltage value read at this frequency can then be converted mathematically to velocity error thence to displacement error at the driven roll surface. This displacement error can then be converted to an arc minute value by knowing the physical dimension of the driven element. This method also allows for measurement of harmonic errors due to mounting configurations which traditional methods cannot measure and can achieve position error measurement down to +/- 1 arc minute

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Method and Apparatus for Position Error Measurement of Optical Rotary Encoders

Traditionally, position error measurement for optical incremental encoders has been measured using specialized equipment, which includes sophisticated hardware and data acquisition systems in order to achieve desired accuracy. This process is very slow and does not lend itself to encoder screening on a production or laboratory basis.

This invention can achieve position error measurement down to +/- 1 arc minute by utilizing standard measurement instrumentation combined with a unique rotational fixture that exhibits good motion quality.

By feeding the digital encoder output signal into a frequency to voltage converter coupled to a dynamic signal analyzer, we record the spectral frequency at the “once around” interval of the encoder at a known driven speed. The voltage value read at this frequency can then be converted mathematically to velocity error thence to displacement error at the driven roll surface.  This displacement error can then be converted to an arc minute value by knowing the physical dimension of the driven element.  This method also allows for measurement of harmonic errors due to mounting configurations which traditional methods cannot measure.

Encoder vendors measure the position error of encoders by attaching the encoder under test (EUT) to a high accuracy master encoder (in the million lines per revolution range).   The EUT is turned very slowly and its signal is compared to the master encoder signal.  This method is very time consuming and expensive.  The method described in this invention is fast and measures the position error directly as the rotating element would see it.  This fixture also allows for the measurement of harmonics that could occur from mounting vibration errors.

 

Definition of position error:  (also referred to as instrument error)

Position error is the sum of the code disc pattern errors, code disc eccentricity, bearing run outs and other mechanical imperfections within the encoder. It affects the electrical output signal by showing up as timing differences from lead edge of any one pulse to lead edge of all other pulses for a full revolution of the encoder. This error tends to vary slowly around each revolution.

Refer to figure 1 for a graphic representation.

Test Set up:

In order to measure encoder position error using the above methods, a test fixture needs to be utilized to drive the encoder rotationally. This fixture must have good rotational motion quality in the desired speed range for accurate test results. The motion quality of the test fixture is monitored using a digital rotary encoder in addition to the “EUT” (encoder under test). This encoder will be referred to as the surface encoder.  It has approximately twice the lines per revolution and two times better position error accuracy as the EUT.  Details for installation of the surface encoder are included in figure 2.

Mechanical Test Fixture:

The...