An Implantable Two-degree-of Freedom Joint Angle Transducer

An implantable joint angle transducer suitable for use in humans has been developed. The transducer will sense two degrees of freedom for supplying command control information from shoulder or wrist movement and in feedback control of wrist movement. The transducer may also be suitable for providing joint angle information from the ankle, knee or hip. These applications are important in upper and lower extremity Functional Electrical Stimulation (FES) neuro-prosthetic systems. This paper presents the development and fabrication of the implantable two-degree-of-freedom joint angle transducer, and discusses its evaluation in bench tests. The transducer senses the relative movement between two opposing bones of a joint by using an array of three Hall Effect sensors and a permanent magnet surgically implanted in the bones. The electrical signals generated by the Hall Effect sensors are processed to produce a measure of the movement of the magnet relative to the sensors, which is related to the movement of the joint. The sensor array and the magnet are packaged in hermetic titanium capsules. A mathematical model was developed to describe the movement of a magnet and sensor array in a joint. This model will be used to predict the transfer characteristics of a transducer when the magnet and sensor array are surgically placed into different joints, with different geometric configurations.