| Description |
Above elbow prosthesis control has trended toward increasing the number of control channels in the human-prosthetic system, to provide simultaneous joint control. Several methods have had varying success, such as Targeted-Muscle-Reinnervation (TMR) and Electromyograph (EMG) pattern recognition. While the number of control channels is increased, the fundamental control loop is still based on amputees placing the prosthetic end effector through visual feedback. In most clinical uses prosthetic joints are driven with a standard proportional EMG antagonistic muscle controller (S). The S controller can be difficult for the amputee as nonintuitive muscle contractions are needed to overcome internal joint and induced external torques, in particular from gravity. To address these issues, two new controllers, which use gravity and friction compensation techniques, have been developed to share the control of the prosthetic elbow joint and reduce control effort on prosthetic users. The new controllers were tested against the S proportional control by having 10 test subjects reach to 6 targets in their user workspace utilizing a Utah Arm 2 testbed. Motion capture cameras recorded the reaching motions. The controllers were compared using quantitative metrics which define the approach, time to target and smoothness (jerk), and holding, steady state error and variance, stages of a reaching motion. A qualitative metric was also used which surveys a test subject's effort in performing a reach. It was found that when considering the new controllers using the combined data for all test subjects at all targets they outperformed the S controller, except in smoothness. It was also found that the new controllers statistically performed best over the S controller at target locations where the humerus was in flexion at approximately 45o, except in smoothness. Smoothness is predicted to be more influenced by the joint friction in the elbow joint. Only one friction compensation method was tested. Further studies on friction affects by varying joint impedance is suggested. Considering these findings, including gravity compensation in the control for active prosthetic elbow joints is found to improve the control over the standard proportional control, as captured in the majority of the physical metrics and in test subject ratings. |
