Abstract: Aiming at the complicated installation process and difficult maintenance of the displacement sensor built into the hydraulic cylinder, an external magnetic induction displacement sensor was designed using tunneling magneto resistive(TMR)elements.Firstly, the composition, working principle and signal circuit of the sensor were introduced. Then, based on Ansoft Maxwell electromagnetic simulation platform, the sensor model was established. The magnetic field state of the permanent magnet ring and the output signal of the magneto resistive elements were analyzed. The method of calculating the displacement of the permanent magnet ring by superposition of effective linear working areas of multiple magneto resistive elements was put forward,and the influence of the hydraulic cylinder on the calculation method was studied. Finally, the performance indicators of the sensor were tested by the experimental device. The results show that under different cylinder permeability and thickness, the length of displacement interval corresponding to the effective linear working area of magnetoresistance element is equal, and the sensor can measure hydraulic cylinders with different cylinder materials and thicknesses. It is found that the linearity of the actual output curve of the magneto resistive element is 2.39%, and the accuracy is 0.16%, which meets the linearity requirement of 4% when the measurement error is less than 1mm, the sensor resolution is 0.0083%, and the repeatability is good. The feasibility of the design is verified.

Key words:  electro hydraulic servo system; hydraulic pressure controlled system; hydraulic cylinder displacement detection; external displacement sensor; Ansoft Maxwell simulation;linear output