Self-mixing instrument for simultaneous distance and speed measurement

- An optical method for simultaneous measurement of distance and speed is proposed.
- The high measurement rate allows discarding unreliable measurements in real-time.
- The accuracy is about 4·10−4 in distance and 5·10−3 in velocity measurement.
- The system operates reliably also in difficult industrial conditions.

A novel instrument based on Self-mixing interferometry is proposed to simultaneously measure absolute distance and velocity. The measurement method is designed for working directly on each kind of surface, in industrial environment, overcoming also problems due to speckle pattern effect. The laser pump current is modulated at quite high frequency (40 kHz) and the estimation of the induced fringes frequency allows an almost instantaneous measurement (measurement time equal to 25 µs). A real time digital elaboration processes the measurement data and discards unreliable measurements. The simultaneous measurement reaches a relative standard deviation of about 4·10−4 in absolute distance, and 5·10−3 in velocity measurement. Three different laser sources are tested and compared. The instrument shows good performances also in harsh environment, for example measuring the movement of an opaque iron tube rotating under a running water flow.