Magnetic Sensing for Proprioception of Rolling Contact Joints

Rolling contact joints are an advantageous building block for soft-rigid hybrid robots due to their out-of-plane compliance, low resistance to bending, and wide range of motion. Typically, proprioception in these cable-driven mechanisms is achieved through torque and position sensors at the servomotor actuator. However, this indirect measurement of joint position can be inaccurate when the system is non-ideal (e.g., the cables are extensible, friction is not negligible, or the linkage encounters an external disturbance). Here, we introduce a magnetic sensor integrated into rolling contact joints to measure joint position and force. We then explore the design space of this magnetic sensor by varying the orientation of the magnets and the stiffness of the magnetoelastomer. A multilayer perceptron is used to relate the joint position and force to the magnetometer readings, providing the flexibility to use this sensor with various joint geometries and sensing modalities. We find that the accuracy of this sensing and modeling approach is coupled with the configuration of the magnetic elements, and that our system can predict the joint angle, twist, and contact forces with errors as low as 1.6%, 0.1%, and 8.8%, respectively. This work describes a method for enabling proprioception in rolling contact joints, and, more broadly, compliant joints for rigid-soft hybrid robots.