Interferometric Distance Sensors

Abstract

Optical interferometry is a well-known technique successfully used to perform displacementmeasurements in the industrial framework. In general, commercial devices use incrementalinterferometry: to measure a displacement it is necessary to fix a reference position and tocarefully displace a suitable reflector (or some other optical component of the interferometer)from that position to the final one. This must be done avoiding any optical misalignment thatcould result in a loss of the interference signal and therefore of the measure. An incrementalinterferometric displacement measurement always implies a precise device to move the reflector,and a counter to keep trace of the number of periods of interference fringes that the photodetectorhas detected during the movement. The main advantage of this measurement technique is a verylow measurement uncertainty (down to some fractions of λ in commercial meters) coupled to anextended range (up to tens of meters). In general, therefore, an interferometric system is the rightchoice when both high accuracy and a long measuring range are required.A number of industrial needs cannot be successfully met by incremental interferometers,especially in the fields of dimensional gauging of surfaces, robot monitoring and characterization,autonomous vehicle controls, etc.. In fact in these fields the quantity to be measured is distanceand not displacement. Taking as an example the problem of industrial robot calibration, the idealmeasuring tool should be able to freely follow the robot end-effector, to measure the actualposition along an arbitrary trajectory. An absolute (i.e. non-incremental) distance meter pointingto the robot manipulator could accomplish this task. Interferometric sensors have great potentialin performing non-incremental measurement at a very high degree of accuracyThe following paragraphs describe the available interferometric techniques to perform nonincrementalmeasurements outlining the principle of operation and the applicability to industrialcases. At the end a comparison of the investigated techniques is given.