Improving feedback in body powered protheses

Improving feedback in body powered protheses

J. Herder

, M. Munneke

1 Delft University of Technology, Faculty of Mechanical Engineering, Measurement and Control

2 Free University of Amsterdam, Faculty of Human Movement Sciences, Currently at Leiden Academic Hospital AZL

Abstract

This paper concentrates on the control of hand prostheses. It is stated that body powered voluntary closing devices possess the best control potential. Feedback paths are identified and the observation is made that feedback is strongly influenced by the design of the prosthesis. Finally, the operating force feedback path is investigated in more detail. Psychophy sical measurement methods point out (1) that operating forces should not be lower than 2.5N, and (2) that perforated self-adjusting shells should be used for good sensitivity in the information transmission through the interface.

Introduction

The Wilmer group is dedicated to applying principles of control engineering in mechanical design. Functional structures are conceived of from a system theory perspective, an approach that facilitates theoretical achievements such as feedback and feedforward to be implemented in a mechanical configuration.

The design of hand prostheses is one of the main application fields. Over the years, the Wilmer group assessed a set of basic demands for rehabilitation aids, that may be summarised as the triple-C criteria: cosmetics, comfort and control [1]. In this paper, the control aspect of hand prostheses will be emphasised.

Body power

Worldwide, most research aims to implement servo systems (myo-electric control) in order to restore motoric function [2], However, servo systems require the availability of energy sources (external energy), and in spite of much research [3], no satisfactory artificial information feedback system has been realised. Feedback, essential for controllability of any system, is intrinsically present in body powered devices, which use muscle groups of the prosthesis user for control and energy supply.

Commonly, the shoulder girdle is used to supply operating forces. However, the burden of the shoulder harness makes people prefer the concept of elbow control [4], where elbow moment drives the prosthesis [5].

Most current body powered prostheses are voluntary opening: muscle action is required to open the prosthesis while a spring delivers the closing force. Disadvantages are that voluntary opening control works opposite to human physiology, and the closing spring must be stretched each motion cycle [6]. Voluntary closing hand prostheses are designed such that they close on muscle action. In figure 1, its principle is sketched. As the user's muscles generate pinching force, their propriocepsis supports prosthesis control [7], physiologically appropriateness is guaranteed [6,8], and pinching force is easier dosable compared to voluntary opening prostheses [9].

Figure 1 Principle of an elbow controlled voluntary closing hand prosthesis

Voluntary closing control

In goal-directed movements, the central nervous system conducts muscle action and reads the current status of the motoric system from sensors within muscles, tendons, around joints, and in tissue (propriocepsis) [10]. The prosthesis is driven by muscle action, which is accompanied by the generation of sensory information in the driving musculature, and at the locations of the human-prosthesis interface. Figure 2 shows the block diagram of this process. Except for vision of the hand opening, there is no direct feedback of the state of the variables hand opening and pinching force. However, if the prosthesis is designed such that a clear relationship exists between hand opening and elbow flexion, and between pinching force and operating and/or muscle force, then also the other feedback paths in figure 2 support prosthesis control. This phenomenon is called extended physiological proprioception [11]. Extended physiological proprioception is obstructed when the relationship between input and output of the prosthetic hand is disturbed by friction and by parasite spring forces from the cosmetic covering. Research projects were started to eliminate these influences [12,13].

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Methods

Being convinced from the benefit patients would have of a properly functioning voluntary closing hand, the Wilmer group started a research project to solve several practical problems [14], and to determine prosthesis geometry for optimal feedback.

Explorative research has been executed on optimising the relationship between hand opening and elbow flexion angle [15], but no reports were found on the prosthesis geometry for optimal force feedback in voluntary closing devices.

In order to make full use of proprioceptive capacity, the forces presented to the prosthesis user must match the highest sensitivity range of the sensors in the human body. In a well designed prosthesis, the muscle force, the operating force on the upper arm, and the reaction forces at the stump all contain information on the exerted pinching force (see figure 2). Of these, the operating force, exerted on the upper arm of the user, is not influenced by the weight of the prosthesis. For that reason, as a first step, the prosthesis design parameters were assessed for optimal feedback of the operation force.

Psychophysical measurement methods were used to assess (1) the sensitivity of the upper arm as a function of operating force level, and (2) the difference in sensitivity function when operating levers are used with or without perforated shells at the contact with the user's upper arm.

Psychophysical measurements

The science of psychophysics investigates the relationship between the perception of stimuli and their physical characteristics. This relation is called the psychometric function [16], which is dependent on the kind of stimulus, its location, its intensity, and its duration [10 p331/3]. Methods for quantifying the psychometric function are based on the comparison of two stimuli, occurring sequentially or at different locations. Indirect methods use one stimulus as a standard, while the other must be reproduced, or judged qualitatively with respect to the standard [16]. In direct methods, one stimulus is given a value, while the other stimulus must be rated with respect to this value, or a stimulus of another value must be reproduced [17]. For the purpose of this study,

ingredients of several indirect methods were composed to a simple measurement procedure. The experimental is furnished with two operating levers, one at each upper arm (figure 3a). Operating forces are applied left and right simultaneously. Several dozens of stimuli are applied in pairs: the operating force on the right side is constant each time, while the left side is varied in arbitrary

Order. The experimental is to Figure 3 (a) Experimental set-up. (b) operating lever m«»

say whether the variable force and without perforated shell. (left) feels larger, equal, or

Uncertainty Range A I

smaller than the constant référence (right). This procédure is repeated for four différent référence force levels: 2.5N, 5N, ION and 15N. Additionally, the same experiment was executed to assess the influence of a

perforated shell that can be mounted on the operating lever (figure 3b). Seven healthy young men were subject to the experiments. Of each référence force level, the lower and upper transition points were assessed (see figure 4). The force range between the transition points was defined as the uncertainty range: the expérimental gave différent judgements to the same variable stimulus. 000 +++ 000 or +++ +++ 000 TH JND(-) 1 nommai MD(+) ++++++ i » Intensity of stimulus T - Transition poml

Figure 4 Perception of stimulus I; + = higher, o

= equal, - = lower than référence.

Results

Many of the expérimentais showed a systematic error: forces on one arm were judged differently than the same forces on the other arm. As variations of operating force are more important to note than the absolute force level, the systematic error is not taken into account. Consequently, the relative sensitivity is defined as the uncertainty range over the average of the upper and lower transition point: AI/(T(+)-T(")) (instead of the uncertainty range over the

référence force level: AI/In o m i n a l).

Figure 5a shows the relationship between référence intensity and relative sensitivity for the seven expérimentais. It was found that an operating force level of 2.5N possesses a significantly lower relative sensitivity than operating forces of 5N and larger. For the latter range, the relative sensitivity amounts roughly 0.4 or 40%. Secondly, furnishing the operating levers with perforated self-adjusting shells improves relative sensitivity considerably at low operating forces (figure 5b).

, . ,M, Intensity [N]

Intensity (NI '

Figure 5 (a) Relative sensitivity as a function of référence stimulus intensity for seven healthy young men, operating lever without perforated shell. (b) Influence of perforated shell.

Conclusion

From a control engineering perspective, hand prostheses should be voluntary closing because that control concept possesses most feedback paths. Prostheses must be designed such that full use is made of this control potential. In this study, the operating force feedback was investigated, and it was found that operating forces should be 5N or higher to match the sensitivity of the human upper arm. If the contact area is enlarged by a self-adjusting perforated shell, sensitivity for low operating forces improves.

Future research will include feedback through reaction forces, and combination of force feedback and movement feedback. When feedback is optimised and practical problems are solved, a hand prosthesis with superior control qualities is expected to result.

References

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International Congress of the World Federation of Occupational Therapists, London, p781/3.

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Scandinavica, supplementum 178, Munskgaard, Copenhagen.

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11th International Congress of the World Federation of Occupational Therapists, London, pl046/8.

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