Electromagnetic Position Measurement System Immune to Ferromagnetic Disturbances

This paper develops an electromagnet-based position measurement system for industrial actuators which offers significant advantages compared with traditional position measurement systems. These advantages include low cost, non-contacting operation, easy installation, and robustness to magnetic disturbances. In the first embodiment of the sensor, the electromagnet is located on the stationary actuator housing while the sensor is mounted on the actuator's moving piston rod. This requires power to be supplied to the electronics on a moving object, and therefore a second embodiment that eliminates this disadvantage is developed. The second embodiment locates both the electromagnet and the powered sensor on the stationary housing. A mu-metal film of high magnetic permeability is located on the moving piston. Due to its high magnetic permeability, a significant fraction of the magnetic field measured by the sensor is coupled through the piston of the actuator. Hence, as the piston position changes, the magnetic field measured by the sensor changes. This leads to an excellent position measurement system with high sensitivity. The experimental results show that the second position sensor embodiment can provide 1% accuracy in position measurement, can reject the influence of disturbances from magnetic objects and can measure positions for stroke lengths up to 20 cm. Longer stroke lengths can be handled using additional daisy-chained sensors.