Improving accuracy of high precision displacement measurement system with optical pickup head by using differential astigmatism focus error detection

Measurement of the surface profiles of machined workpieces is becoming increasingly important to the progress of ultra-precision engineering. Optical non-contact probe is fit for measuring small, thin, soft parts, and it will not cause abrasion damage for the fragile surface of the workpiece. Astigmatic detection systems (ADS) based on the commercial digital versatile disk (DVD) optical pickup heads (OPH) are in widespread use for this purpose due to its well-established standards, high detection bandwidth, compact size, low cost and ease of use. Unfortunately, it has some serious drawbacks relating to issues concerning about dynamic characteristic, low stability and accuracy. In this paper, a high precision displacement measurement system was constructed with a modified DVD optical head by using differential astigmatism focus error detection. The proposed system is cost-effective to quickly accurately estimate and correct systematic errors of four quadrant detector (4QD). Moreover, based on replaceable micoscope objective and automatism testing system of 4QD, it can also detect the two-dimensional angular tilt of the object surface. Our experimental results demonstrated that the OPH showed about 8um measuring range and 20nm resolution.