Nonlinear observer for electromagnetic position estimation using active current control

Abstract This paper proposes a novel electromagnetic position estimation system suitable for piston-cylinder actuators using active current control. The inexpensive non-contacting position measurement system is composed of an electromagnet on the stationary cylinder and a magnetic sensor on the piston rod. The measured amplitude of an alternating magnetic field is used to estimate piston position. The current supply to the electromagnet needs to be actively controlled so as to increase monotonically with piston position, allowing for sufficient magnetic sensitivity along the whole stroke length. Since the position itself is unknown, this poses a challenging coupled estimation-control problem. A nonlinear state observer is designed based on Lyapunov theory to ensure both asymptotically stable position estimation and current modulation. The desired current profile is optimized using a direct trapezoidal collocation method to achieve accurate position estimation even for regions with low magnetic sensitivity. The active position system is experimentally validated using a piston-cylinder setup with 20 cm stroke length. Experimental results show that the active position estimation system can achieve 1% measurement accuracy all through the stroke length while a system with constant current consuming the same average power shows nearly twice the estimation error. Through active control of the current supply, the electromagnetic position estimation system offers the potential to either improve measurement accuracy at locations with weak sensitivity, extend the overall position sensing range, or to save electrical energy cost.