Using a CCD to make a high-accuracy absolute linear position encoder

Abstract

The field of melrology always seeks increasingly accurate, high resolution absolute measurement devices having a fast frequency response. This has driven the field of linear position measurement to accept compromises to achieve the one most desired feature for a particular task at the expense of the other features. These compromises usually take the form of trading speed and/or absolute position information for accuracy with incremental position information; or trading accuracy and/or speed for absolute position information . To try and eliminate these compromises, an absolute linear position encoder was built using a linear C.C.D., a small high-intensity area infra-red light source, a new simple absolute scale, and a digital signal processor (for image processing, temperature compensation, and calibration compensation). The prototype encoder has a measurement distance of 500 mm, and a measurement resolution of 1.0 im at measurement update rate of 1 kHz or 0.1 jtm at 15.6 Hz. The mechanical backlash or short term repeatability of the device was found to be a total of 1 pm. This device was then calibrated and checked with respect to an Hewlett Packard 5528A laser interferometer. The encoder proved to be accurate to within 5.0 tm over the 500 mm length or to have more than 16 bits of accuracy. It is clear that this new type of absolute linear position measurement technology eliminates many of the above mentioned compromises of the past by combining relatively high speed, high resolution, high accuracy, temperature compensation, and absolute position information in one measurement system.