Preventing grasped objects from slipping out of a prosthetic hand would be useful for prosthesis users who cannot directly feel the grip force applied to grasped objects, and necessary grip force may change when the object is rotated or transported. In an open-access study published online in June in the Journal of Healthcare Engineering, researchers explored a human-inspired “grasp reflex controller” for prosthetic hands to prevent grasped objects from slipping when they are rotated. They found that the controller improved a powered prosthetic hand’s ability to maintain a precision grip on objects that were subjected to wrist pronation and supination.
The slip prevention controller was evaluated with six objects in benchtop tests and by 12 individuals (four women and eight men) without amputations during experiments that replicated realistic tasks of daily life.
An analysis of variance showed highly significant improvement in the number of successfully completed cycles for both the benchtop and human tests when the slip prevention reflex was active. An object sorting task, which was designed to serve as a cognitive distraction for the human subjects while controlling the prosthetic hand, had a significant impact on many of the performance metrics. Benchtop testing used six different objects with a range of mechanical characteristics. Human tests showed far fewer drop and break failures for each object and person with hand orientation feedback (HOF), the study found. In a sorting task, the object was broken and dropped much less frequently with the use of HOF while still sorting at about the same speed.
Each person was allowed roughly 15 minutes to become familiar with EMG control while the EMG hardware was calibrated for each individual. Each subject sat in a chair facing the prosthetic hand with the EMG pre-amplifiers strapped to the forearm of his or her nondominant hand; dominant hands were kept free for the sorting task that consisted of separating a mix of four types of nuts and bolts (50 pieces total) into unique containers. All subjects were timed as they completed the sorting task three times prior to EMG experimentation.
The subjects participated in four sets of experiments using a Motion Control Hand. Each test was repeated for three trials, and each trial consisted of ten possible pronation/supination cycles. The total number of completed cycles depended on the subject’s success rate. The first two tests performed by all subjects were either EMG control without HOF or EMG control with HOF. The third and fourth tests repeated the first two; however, the subjects simultaneously performed the nuts and bolts sorting task.
