Linear position sensor

Abstract

The simple and very popular linear position sensor comprises LED (or laser diode) as light source, rectangular photodiode and cover fixed on mechanical moving part which position is measured. Position of the cover defines the light energy that reaches the photodiode surface. The energy is transformed in sensor output voltage proportional to cover displacement by condition that photodiode illumination is absolutely uniform. In practice the linear sensor is volume limited therefore the sensor illumination uniformity and sensor's scale factor accordingly are not constant values. Non-compensated variable sensor scale factor causes control system destabilization where sensor provides cover linear position measurement. This paper proposes the mathematical model which involves approximated characteristics of the LED irradiance intensity. We apply this approximation for calculating voltage versus displacement characteristic of linear sensor consisting of two overlapped photodiodes. Using this characteristic we provided its linearization and stability of control closed position system, where non-linear sensor was main destabilized factor.