A ratiometric optical proximity sensor which measures the distance between an approaching object and the sensor within a range of 1 cm to about 10 cm is described. With the proposed method, the influence of the reflection coefficient and the orientation of the object is strongly reduced. The sensor can be used in robot grippers and in other applications where contactless measurement of short distances is required. The output of the optical proximity detector depends on the reflectivity of the target material. This dependence can be reduced by using two receivers at some distance apart along the optical axis of the system. The ratio between the two outputs is independent of the reflection coefficient of the target and of the intensity of the power source. The sensitivity of the sensor system and the range are fixed by the geometrical configuration and the radiant properties of the source and detectors. Major design parameters are the angles between the optical axes of the transmitter and receivers and the distances between these optical components. The ratiometric output is achieved by dividing the difference between the two receiver outputs by one of the outputs or their sum. To eliminate the influence of environmental light, it is advisable to use a modulated light source and synchronous detection of the receiver outputs.
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