Non-Intrusive Piston Position Measurement System Using Magnetic Field Measurements

This paper proposes a novel sensing system 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. Through 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 the 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 vary from engine to engine and are cumbersome to calibrate for each engine. This challenge is addressed by utilizing two magnetic field sensors with known longitudinal separation between them. A number of estimation methods are proposed that identify and update magnetic field parameters in real time without requiring an additional linear variable differential transformer (LVDT) sensor for calibration. The advantages and performance of these estimation methods are compared. Experimental results from a free piston engine setup show that the proposed sensor can provide 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, pneumatic cylinders, and in many other position measurement applications.