This paper puts forward a linear variable stiffness joint (VSJ) based on the electromagnetic principle. The VSJ is constituted by an annular permanent magnet (PM) and coaxial cylindrical coil. The output force and stiffness are linearly proportional to the coil current. In consequence, the stiffness adjustment motor and mechanisms required by many common designs of VSJs are eliminated. A physical prototype of the electromagnetic VSJ is manufactured and tested. The results indicate that the prototype can achieve linear force-deflection characteristics and rapid stiffness variation response. Using an Arduino and H-bridge driver board, the electromagnetic compliance control system is developed in order to realize the precise control of the interaction force. The static force control error is no more than ±0.5 N, and the settling time can be controlled within only 40 ms. At last, an experiment of squeezing the raw egg is conducted. The experiment intuitively exhibits the performance of electromagnetic compliance in stable force control and keeping safe robot-environment interaction.

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