Moving towards two-way control

Abstract

An important advantage of body-powered prostheses, compared to other types of prostheses, is the presence of proprioceptive feedback. Proprioception makes it easier to perform goal directed movements. Body-powered prostheses (for transradial upper-limb amputees) used nowadays make use of one way control, which means that only the opening or the closing is controlled voluntary. By using one way control, proprioceptive feedback is only available for the movement that is controlled voluntary. To gain a broader feedback, it would be better to use two-way control. This means that the opening and the closing can be controlled voluntary. Two-way control can make the control of the prosthesis more natural and intuitive. To design such a prosthesis in the future, the inputs and the outputs must be known. The goal of this study is to investigate possible movements generating the inputs (operating forces and displacements) for a two-way controlled prosthesis. The selection of suitable movements for control is done in two phases. First, the body parts useful for control in a cosmetically acceptable way are determined. Second, all movements produced by these body parts are analysed by using three criteria: 1. The control movement is no ‘daily movement’, 2. The movement can be performed in every posture needed to function normally in daily life, 3. The movement does not influence the position of the artificial hand. Only the movements fulfilling all criteria were considered suitable for control, resulting in the following movements: elevation of the shoulder, protraction of the shoulder, flexion of the trunk, rotation of the trunk and flexion of the toes. Twenty subjects without arm defects (10 male/10 female, 25±2 years old) were requested to perform the five movements left and right. Per movement the maximum displacement and maximum forces at 0, 25, 50 & 75% of the maximum displacement were measured. Subjective perceptions of the participants regarding easiness, comfort and intuition were recorded in questionnaires. For all movements, the operating force decreased with increasing displacement. The highest forces and displacements were found for shoulder elevation (left: 98.57±29.48N/80.5±22.6mm; right: 103.59±32.11N/85.5±19.9mm), whereas the lowest force and displacement values were found for toe flexion (left: 22.15±6.47N/17.5±5.0mm; right: 22.39±7.26N/17.5±5.0mm). All movements were easy and comfortable to perform. Flexion of the trunk was experienced less intuitive than other movements. Of the five movements investigated, only shoulder protraction and elevation are considered suitable when using a Bowden cable. By designing a two-way system in the future, it is recommended to investigate the use of a wireless system. By using such a system, no hindering cables are necessary, which makes the prosthesis less elaborate and restricting. In the case of a wireless system, all investigated movements are suitable.