NEW CONCEPTS IN EXTERNAL POWERED ARM COMPONENTS

Abstract

An electrically powered joint can be of considerable support for many unilateral or bilateral above elbow amputees in every day activities. The DynamicArm (Photo 1) represents the next evolutionary step, a milestone in powered elbow prosthetics. It has been designed around previously proven components and also adds new developments that will change the way the amputee interacts with the prosthesis. Multiple control strategies and inputs running in parallel as well as hardware that allows for a more natural operation with much less effort will heighten the level of expectation from every patient wearing this design. This is already the case based on the response of the patients that have been fitted so far. Patients who have had many years experience with powered systems or those who have had experience with hybrid systems report the decrease in mental and physical energy within moments of being fit. Repositioning exercises, switching between components and actively controlling each component seem to be done with ease. Some of the advantages for the amputee are rapid movement and pre-positioning. This allows the patient to lift higher weights and position the elbow without complex control sequences. Because pre-positioning is more easily facilitated the patient will chose to move the component rather than make up for some controllability deficit with a compensation movement of the trunk. High loads can also be held in position without fear that something will be damaged if it is overloaded thanks to the already known locking mechanism of the 12K50 ErgoArm. Built in force sensors protect the system from unlocking in undesirable conditions. The control characteristics follow those of previously designed components like the DMC+ and the SensorHand Speed. Otto Bock electronics follow the logic of „First Signal Wins“ control strategy but a new improvement allows for fast changing between opening and closing the hand or changing from flexing to extending or from supination into pronation. This gives the advantage of being able to fit the patient who co-contracts but still allows for high performance controls. Signal inputs are now digitized by the locking microprocessor and from there are not as susceptible to Electromagnetic Interference. Power consumption Photo 1: DynamicArm system has been reduced because when the system is in free swing, the mechanics are completely de-coupled from the movement of the Automatic Forearm Balance mechanism. New possibilities of control include being able to raise and lower loads at various speeds and repositioning without needing to produce an “unlock” signal. The wrist rotator is controlled by the main electronics which allows for an ultra sensitive control and the possibility to run the wrist motor proportionally and at incredibly low speed allowing for very precise positioning. The terminal devices as before can be controlled with proportional speed and grip force based on the strength of the muscle signal which is supported by having the electronics combined with the hand mechanics. Simultaneous control of the hand and the elbow with using a variety of input devices is now possible. And now for the first time new input devices and new switching methods allow for more flexibility for the amputee to control the prosthesis.