DESIGN OF UPPER LIMB PROSTHESES: A NEW SUBJECT-ORIENTED APPROACH MARCO TRONCOSSI1,VINCENZO PARENTI-CASTELLI1 and ANGELO DAVALLI2 1 DIEM – Department of Mechanical Engineering, University of Bologna, Viale del Risorgimento 2, 40136 Bologna, Italy [email protected] 2 I.N.A.I.L. Prosthetic Center, Vigorso di Budrio (Bologna), Italy The challenge to develop innovative prostheses for upper limb amputees is the basis of this study. The whole project is intended to provide high-level bilateral amputees with devices which can give them back a sufficient quality of life, since current prostheses are rather limited. High mobility, advanced control and good “wearability” are the main features required of artificial arms. The method presented here provides the specifications needed to guide the mechanical design of the prosthetic system, defining its architecture as a trade-off solution between contrasting requirements like, for instance, functionality and simplicity. The approach is subject-oriented, that is the process is based upon the specific needs of patients undergoing prosthetic rehabilitation and the expected result is the systematic determination of a limited number of prosthesis architectures suitable to a few corresponding classes of amputee profiles. Thus, the mechanical design of the prosthetic system is based on these indications. Keywords: Upper limb amputees; Externally powered prostheses; Subject-oriented design; Serial robot architecture; Limited degrees of freedom robots; Performance indices. 1. Introduction Nowadays, the most advanced prosthetic device for upper limb amputees is the myoelectric prosthesis: here, electromechanical joints actuate the artificial arm segments and are directly activated by the amputee by means of electromyographic (EMG) signals, to be measured on the skin by sensors (myoelectric electrodes) and processed by a programmable control circuit (primary control scheme); rechargeable batteries power all these components. Some passive joints and/or locking mechanisms are sometimes included in the system and are useful to give the prosthetic limb an optimal pre-determined configuration when performing certain tasks. They are configured with non-myoelectric inputs before or after the direct control of the active joints (secondary control scheme). The generation of independent EMG signals (which are due to unrelated contraction of distinct bundles of muscles) implies a sequential control of the articulations, i.e. only one joint at a time can be activated. The good qualities of this prosthesis are a sufficient functionality, good performances (in terms of speed and forces) and a pleasant appearance. The critical aspects are the weight and the volume of the structure, and the complicated control (union of the primary and the secondary control schemes); therefore, in order to provide the amputee with a comfortable, humanlike and easy to control prosthesis taking into account different. For very high level amputees (bilateral. the practitioner has to choose appropriate arm components and control schemes that best suit the patient’s level of amputation. should be flexible and efficient and.e.A. the geometry and the topology of the artificial arm. the degrees of freedom (DoF) and the .(otherwise not accepted). range of motion. in order to solve this lack and to improve the quality of life of this amputee population. at the same time. the procedure foresees a given patient as the input and provides his/her corresponding optimal prosthesis architecture as the output. offer the mechanical design specifications of the new components to be introduced in the system. that is the algorithm associates a given patient with the architecture of the artificial arm which best satisfies his/her personal requirements. from a functional viewpoint) are available on the market: the prono-supination unit and the elbow. only two electromechanical active joints (the most meaningful. “Architecture” is intended as the geometry and the topology of a robotic arm model.e. or even of other parts of the body. the I. In order to draw up the project guidelines. applied to many patient profiles. an advanced control and a good “wearability” (i. thus limiting the flexibility and functionality of the artificial arm. the approach is subject-oriented. In particular. Prosthetic Center is supervising a project whose aim is the development of new electrically powered prostheses with a great mobility. Thus. above all). and is outlined here. providing him/her with the prosthesis which best matches his/her specific requirements. a method which determines the optimal answer to contrasting demands has been set up. The application of the procedure to many patient profiles is expected to provide a limited number of significant architectures which can satisfactorily match the needs of different amputees. vocational and avocational goals 1. of course. with a light and simple structure. To succeed in the prosthetic rehabilitation.N. These features are in conflict with respect to each other so that a trade-off has to be found. i. humanlike appearance…).L. user friendliness. contrasting aspects. We believe that even during the design process the relative importance to be given to each factor must strictly depend on the patient’s profile. easy to control and comfortable to wear.e. to execute many motor tasks. must be selected taking into account the personal needs and expectations of the patient. tissue and musculature conditions. Unfortunately. As regards the human arm.e. The approach followed to tackle the problem aims at determining a limited number of prosthesis architectures which best suit all the requirements of corresponding classes of high-level amputees. Even the prosthesis architecture. the possibility of choice is nowadays rather limited: besides the terminal device (with only one degree of freedom for grasping) and a number of passive joints and locking mechanisms. the dexterity of a prosthesis is very poor and the amputee has to resort to compensatory movements of the residual limb. size. learning ability. who have an extremely restricted residual movement ability. not all the physiological joint movements can be replicated. An original procedure which systematically collects and processes a great deal of information relative to a given subject has been proposed 2 . i. tolerable weight. current prostheses could be inadequate to guarantee the functionality needed to reach a satisfactory level of autonomy. The arrangement of the active joints in the robotic arm models and other information detectable from the procedure generate the design specifications for the development of the most appropriate prostheses. i. that is the artificial arms should have a humanlike appearance. the higher the level of amputation the greater the encountered difficulties. Methods The most important factors in designing upper limb prostheses are functionality and “wearability”.I. the number of active joints and their arrangements. The results of the method. 2. the responses are codified with proper labels in order to systematically portray a well-defined patient profile. easiness of control…). Step 2. properly combined in an overall index (i. “Sub-optimal” is intended as an artificial arm model with good responses (even if not the best) to different amputee requirements. because they execute the tasks with an error which increases as the number of active joints decreases. on one side. running automatically once the proper amputee data are given. Therefore. an architecture characterized by high value of the overall index (even if not the maximum) for several patient profiles. defining a first approximation of the size of their actuators. Finally. The procedure is composed of three sequential steps. The models with less than six DoF (limited robots) correspond to simpler robot architectures and thus are appreciated from the wearability viewpoint. An algorithm determines which upper limb activities are most significant for the patient from the viewpoint of reaching a satisfactory functional autonomy in everyday living.e. This is the starting point of the mechanical design of innovative prosthetic devices. that collects all the information necessary to classify the amputee’s needs: different aspects are investigated in order to define a personalized level of life quality to be reached after the prosthetic rehabilitation process (by means of proper devices). The structure simplification of the limited robots and the corresponding worsening of their global functionality have to be evaluated with respect to the quality of life assigned to the given patient. univocally determine the optimal prosthesis architecture. the procedure is intended to be applied to a huge number of patient profiles (theoretically.g. simplicity of the structure. i. to follow the corresponding reference trajectories. because it would be a too expensive process. A database collects. their performances are poorer than those of the 6 DoF models. the procedure processes appropriate information of a given patient and designates his/her optimal prosthesis architecture. i. 2. considering all the possible combinations of the identificationform labels) in order to define a limited selection of optimal or “sub-optimal” prosthesis architectures suitable to match different amputee requirements. as depicted from the previous characterization algorithm (in Step 1). the artificial arm model which can best satisfy his/her personal needs. 2. Step 3. functionality of the device. the robotic arm with the simplest and lightest structure which can best satisfy the patient’s personal needs. on the other side.2 Architecture selection It is not feasible to design an ad-hoc prosthesis for each patient.e. several kinematic models of anthropomorphic robots (with one up to six revolute joints differently arranged). upper limb activities of daily living and the corresponding reference trajectories which model them (normally requiring six DoF for positioning and orienting tasks). From a feasibility viewpoint the choice of a . Step 1.e. Kinematic simulations check the robotic model ability to satisfactorily perform the selected activities. i. weighted by the above mentioned subject-dependent parameters). and.1 The Procedure As described in Ref. The answer to each question in the form is to be selected from a number of pre-defined options. on the other. the artificial arm models have to be evaluated: their performances and the complexity of their architecture are measured by purpose-built indices which. The input of the procedure is an identification-form of the patient. 2.e.e. The algorithm also determines the values of parameters involved in the final selection of the best architecture (see Step 3) by weighting the relative relevance of the different factors which contribute to define the amputee’s quality of life (e. A further kinetostatic analysis calculates the torque and the power required at all the actuated joints when performing the tasks. i.corresponding range of motion of articulations to be adopted in the forthcoming design are defined. . the design of new components to be introduced in prosthetic systems is required. 2004. 13th Workshop on Robotics in Alpe-Adria-Danube Region. A possible arm architecture has been drafted as example. sub-optimal architecture is more sensible than aiming to provide each subject with the best prosthetic device designed on his/her individual demands.M. properly used. 3 Practical application The results of the presented approach can guide the practitioners to choose the appropriate solution for patients to be fitted with a new prosthesis. The method relies upon a procedure which. on the basis of a systematical process. determines a limited selection of prosthesis architectures suitable to fit the needs of corresponding classes of patient profiles. 1). Proceedings of RAAD’04. Miguelez J. improving both the mobility and the control schemes of the present devices. the arrangement of the joints in the arm models determines the kind of motion which the new articulations must actuate. defining which new active articulations must be designed. In particular. Qualitative and quantitative outcomes provide the input for the design of new electromechanical articulations to be considered in a prosthetic system. Discussion The basis for this study comes from the challenge to develop an innovative upper limb prosthesis for very high-level amputees. 1: Conceptual block schemes of the approach. 134-139. This paper outlined the approach followed for the determination of the mechanical design specifications.. the results of the kinematic and kinetostatic simulations respectively define the range of motion of the joints and the mechanical characteristics (peak and RMS values of torque and power) of the corresponding electromechanical actuators. Brno. in this way. . A Procedure for the Determination of the Optimal Upper Limb Prosthesis Architecture. Thus. Thus. 36-38 2.. Step 1: Procedure Step 2: Patient’s profile characterization Kinematic and kinetostatic simulations Patient 1 Patient 2 … … Patient n MECHANICAL DESIGN of new articulations Architecture 1 Example Step 3: Arm models evaluation … Architecture m m<<n Range of motion Torque Power Joint not available on the market Fig. 4.. the fundamentals mechanical design specifications are determined. Czech Republic.June 2-5. The present method is useful for this aim too (Fig. References 1. Sacchetti R.versatile. It might happen that the architecture indicated for a given patient (presumably a high-level amputee) corresponds to no devices available on the market. Troncossi M. JPO 14 (1). Parenti Castelli V. 2002. Critical Factors in Electrically Powered Upper-Extremity Prosthetics.