Optical angle sensor

ABSTRACT: The optical angle sensor can statically measure the rotation angle of any apparatus. At the core of the sensor is an optical medium with an embossed structure which diffracts light according to Fresnel’s formula. The measurement is conducted by at least two photodiodes (measuring probe and reference diode). Through a stacked construction the sensor can measure up to more than 270 degrees.

BACKGROUND: There are different measurement principles for angle sensors: Rotary encoders mostly operate according to the light barrier principle. Static sensors either use inductive or capacitive measuring or electrical resistance measuring (potentiometer). Disadvantages of static sensors are the required referencing, frequent voltage fluctuations and abrasion of the metering elements. Thus at least one more sensor is necessary to create a redundant system.

TECHNOLOGY: The optical sensor principle uses the angle-dependent transmission of a light ray with defined wavelength through an optical interface between materials showing different refractive indexes. In a slice (e.g. a silicon wafer) a depression in the surface is created (e.g. through chemical etching) which gets beamed at by laser diodes from different directions. Depending on the wafer’s rotation angle the light is refracted multiple times which allows the exact determination of angular positioning through photodiodes.
In an attached graph the sensor’s transfer function is shown. Through embedding multiple photodiodes in a stacked or integrated array the angle measuring range is significantly extended from approx. 30 degrees (for a single photodiode) to more than 270 degrees.