Novel non-intrusive sensor for piston position measurement

This paper proposes a novel sensor for the non-intrusive real-time measurement of piston position inside a cylinder. The proposed sensor exploits the principle that any ferromagnetic object has an inherent magnetic field which varies as a function of position around the object. By modeling the magnetic field as a function of position and using sensors to measure magnetic field intensity, the position of the object can be estimated. This principle is used to measure piston position in a free piston engine without requiring any sensors inside the engine cylinder. The piston is approximated as a rectangular metallic object and the variation of the magnetic field around it is modeled. A challenge arises from the fact that the parameters of the model would vary from engine to engine and would be cumber-some to calibrate for each engine. This challenge is addressed by utilizing two magnetic field sensors with known longitudinal separation between them. An iterated least squares algorithm then provides adaptive parameter estimates and accurate position estimation. Experimental results from a free piston engine set up show that the proposed sensor can provide better than 0.4 mm accuracy in position estimates. The proposed sensing concept can be utilized for piston position measurement in multi-cylinder SI and diesel engines, hydraulic cylinders and in many other position measurement applications.