Generally speaking there are two orthogonal signals used in single-frequency laser interferometer for
differentiating direction and electronic subdivision. However there usually exist three errors with the
interferential signals: zero offsets error, unequal amplitude error and quadrature phase shift error. These three
errors have a serious impact on subdivision precision. Based on Heydemann error compensation algorithm, it
is proposed to achieve compensation of the three errors. Due to complicated operation of the Heydemann
mode, a improved arithmetic is advanced to decrease the calculating time effectively in accordance with the
special characteristic that only one item of data will be changed in each fitting algorithm operation. Then a
real-time and dynamic compensatory circuit is designed. Taking microchip MSP430 as the core of hardware
system, two input signals with the three errors are turned into digital quantity by the AD7862. After data
processing in line with improved arithmetic, two ideal signals without errors are output by the AD7225. At the
same time two original signals are turned into relevant square wave and imported to the differentiating
direction circuit. The impulse exported from the distinguishing direction circuit is counted by the timer of the
microchip. According to the number of the pulse and the soft subdivision the final result is showed by LED.
The arithmetic and the circuit are adopted to test the capability of a laser interferometer with 8 times optical
path difference and the measuring accuracy of 12-14nm is achieved.
The methods of how to improve the measuring accuracy of the autocollimator used over long outside distance are introduced in this paper. Pulse diode laser is adopted as a light source and CCD is used as signal receiver. By the optical design of common-path and the signal processing such as smoothing filtering, interpolating image center, eliminating anomaly data and identifying false points, a higher stability of 2.5” and a better repeatability of 5.6” are achieved in the positioning measurement over ten meters. The principle of auto-collimation, the optical system design, the signal processing and the experimental results are expatiated in this paper.