Encoder, CCD, Linear
Sunday, September 03, 2006
A Linear Encoder has been designed and developed that uses a Charged Coupled Device (CCD) with linear imaging to scan a backlit stainless steel scale. The Encoder has a resolution of 1.0 micrometer at a position update rate of 1 KHz (or 0.2 micrometer at 15.6 Hz), a short-term position repeatability of < 1.0 micrometer and an accuracy of ±5.0 micrometers / meter of measurement length within a 6.0 degree C temperature range
The Encoder's mechanical design allows a <1.0-micrometer of backlash. This is accomplished by using a 5-point semi-kinematic rolling mount for the CCD and light source combination (the encoder head). Three bearing points run on the stainless steel scale and two bearing points run on the aluminum housing. The bearing points constrict motion to only one degree of freedom along the linear position measurement axis. The bearing points are kept in contact with the running surfaces by means of a 3-spring suspension system between the encoder head and the traveler head.
The Encoder’s key design features are as follows: (1) every pair of slits on the scale is distinguishable because the slit spacing varies directly with position. (2) There is always at least one complete slit pair visible to the CCD at a given time. (3) The light source illuminates the scale and the light through the slit to the CCD is a Gaussian distribution. (4) Because of slight nonlinearities in the slit cutting, the CCD and the light source, the encoder should be calibrated with a Laser Interferometer. (5) The encoder head has a built-in temperature sensor for scale expansion compensation effects. (6) The stainless steel scale material has a C.T.E. of 0.3 ppm / degree F. This will allow accuracies of close to plus or minus 2.0 micrometer / meter over an 11oC temperature range.
Source: Sensor, June 1990, by S.E. Goodnick, Parker Hannifin Compumotor, Rohnert Park, CA
Reference: Encoder Identifier, ‘PhotoTrak’
Industrial Products
posted by JD52 @ 12:43 AM,
