High-resolution laser rangefinder based on a phase-shift measurement method

The laser range-finder presented here has been designed as a part of a non-destructive testing system. The whole conception of the system is detailed. The method is based on phase-shift measurement, but using electronic heterodynage to provide a good resolution. An RF card is the heart of the system, including two VCO and two active mixers, providing the modulation signal and both reference and phase shifted signal at intermediate frequency of 125 kHz. The emitter is a laser diode at 12 mW average output power, the receiver uses an avalanche photodiode. We also find an amplification board with an 80 dB dynamic range AGC, and the frequencies control board. The phase shift is estimated by direct counting with more than 10 bits resolution, that means about 0.4 mm for a 60 cm range without phase ambiguity. The amplification level, obtained from the AGC loop, gives an estimation on the measurement quality and on the target surface. A PC computer controls the range-finder through an interface specially developed. It is based on two PLD, one for the general sequencement and one for the averaging of the data. The obtained data are stored in a 2Kb FIFO memory waiting a data request for the PC or generating an interrupt if it is full. The interface is easy to drive from the PC with the choice for averaging value, and amplification criterion. The imaging should be provided by scanning mirrors. Measurement of the target distance and estimation of the target surface conducts to laser imaging. Altogether the system is rather compact due to the disposition of the cards and also rather low-cost.