Myoelectric forearm prostheses: State of the art from a user-centered perspective

User acceptance of myoelectric forearm prostheses is currently low. Awkward control, lack of feedback, and difficult training are cited as primary reasons. Recently, researchers have focused on exploiting the new possibilities offered by advancements in prosthetic technology. Alternatively, researchers could focus on prosthesis acceptance by developing functional requirements based on activities users are likely to perform. In this article, we describe the process of determining such requirements and then the application of these requirements to evaluating the state of the art in myoelectric forearm prosthesis research. As part of a needs assessment, a workshop was organized involving clinicians (representing end users), academics, and engineers. The resulting needs included an increased number of functions, lower reaction and execution times, and intuitiveness of both control and feedback systems. Reviewing the state of the art of research in the main prosthetic subsystems (electromyographic [EMG] sensing, control, and feedback) showed that modern research prototypes only partly fulfill the requirements. We found that focus should be on validating EMG-sensing results with patients, improving simultaneous control of wrist movements and grasps, deriving optimal parameters for force and position feedback, and taking into account the psychophysical aspects of feedback, such as intensity perception and spatial acuity

Development of an underactuated hand prosthesis with compliant control

The main subjects of this thesis are the mechanical design and control of a new hand prosthesis prototype: the UT Hand I.\ud The functionality of modern hand prostheses is mainly limited by the size and weight of their actuators. The UT Hand I features a minimal actuation system, where all finger joints are connected to a single electric motor. The ability to control these joints separately is preserved by miniature locking mechanisms which fit inside the fingers and palm. Combined with a separately movable thumb, the hand allows the user to perform various grasps relevant to daily living. To provide the user and the control system with feedback during grasping, the hand contains position sensors in the finger joints; additionally, each fingertip contains a set of four force sensors coated in rubber.\ud The control of the UT Hand I is based on ‘myoelectric’ signals: electrical impulses produced by the activation of the user’s remaining forearm muscles. A special interface provides a selection of grasp types for precision and power grasping; the user can control which of these grasps to use (and when) by flexing the appropriate muscles. A control system has been developed to automatically close the fingers and thumb around an object, and hold it with a force controllable by the user. The system also controls the energy applied by the motors, in order to maintain a stable grasp and ensure safe interaction with objects and people at all times.\ud The completed UT Hand I system demonstrates several innovative mechanical design and control techniques, which improve the functionality and controllability of modern myoelectric hand prostheses

Evaluation of pneumatic cylinder actuators for hand prostheses

DC motors are currently the preferred actuation method for externally powered hand prostheses. However, they are often heavy and large, which limits the number of actuators that can be integrated into the prosthesis. Alternative actuation methods are being researched, but have not yet found wide application. In this paper, a thin-walled pneumatic cylinder actuator is implemented to move a single-DOF prosthetic hand. Its performance is compared to that of a commercially available DC motor. Both systems are evaluated on speed, responsiveness, and energy capacity. Other properties such as size and mass are also taken into account. While the pneumatic cylinder is capable of high speeds and forces while remaining lightweight, quiet and small, it can prove difficult to control. Improvements to the cylinder design and valve system are recommended, in order to develop the potential of pneumatic cylinder actuators in modern multifunctional hand prostheses

Development of underactuated prosthetic fingers with joint locking and electromyographic control

Modern hand prostheses possess a large number of degrees of freedom. These degrees of freedom cannot simply be actuated by a single motor each, since their combined size and weight would exceed the limitations of an anthropomorphic prosthesis. Some hand prostheses try to remedy this by way of underactuation of the fingers or addition of entirely passive fingers, but this reduces the hand's ability to execute different grasp types. We present a joint locking system, allowing certain degrees of freedom to be fixed during actuation of an underactuated finger. These locks are actuated by miniature solenoids, and allow the fingers to support a variety of grasp types. In this paper, these locks are implemented in a two-fingered prosthesis prototype, which is able to perform several grasping motions important for prosthesis users. This prototype is controlled by pre-recorded electromyographic signals, which control different grasp types and their opening/closing. Various grasping experiments show that the prototype is able to execute three essential grasp types for daily living with a single main actuator, and can be intuitively controlled by means of six different electromyographic signals. This prototype demonstrates new joint locking mechanisms and control systems that can provide an anthropomorphic, myoelectric hand prosthesis with minimal actuation and intuitive control

Development of prosthesis grasp control systems on a robotic testbed

Modern myoelectric hand prostheses continue to increase in functionality, while their control is constrained by the limits of myoelectric input. This paper covers the development and testing of grasp control systems for multifunctional myoelectric prosthetic hands. The functionality of modern hand prostheses is often focused on the task of grasping, which can be divided into high-level grasp planning and low-level finger control. Initially, models can used to test these control systems, but for proper evaluation actual implementation on a physical system is required. The University of Bologna (UB) Hand IV prototype is an anthropomorphic, tendon-driven robotic hand, which makes it well-suited to represent the structure of modern prostheses. One of the main control systems tested in this paper is based on the intrinsically passive controller (IPC), the structure of which offers guaranteed passivity and stability. After several grasping tests, the systems are evaluated on compliant behavior, grasping ability, and dynamic appearance. IPC proves to be a powerful approach to interaction control, without the associated sensor requirements which could be difficult to meet in modern hand prostheses

Design of joint locks for underactuated fingers

Modern multifunctional hand prostheses have many degrees of freedom, but strong limitations on weight and size. The actuators commonly used in these systems are relatively large and heavy, so their number should be kept as low as possible. This is often accomplished by underactuation, which causes a natural motion of the fingers when grasping an object but reduces the ability to execute a variety of grasps. To remedy this, a series of locking mechanisms can be implemented to fix the position of one or more joints. This paper focuses on the development of such a joint locking system that could be used in anthropomorphic prosthetic fingers. Two lock concepts are implemented in a single-joint test setup and evaluated. A gear-based concept is tested, though its actuation requirements prove too high for viable implementation in a prosthesis. A mechanism based on friction amplification is shown to exhibit self-locking properties, which allows for a minimal lock actuation force while withstanding joint torques of over 2 Nm. The friction amplification mechanism is found suitable for prosthesis use, and will be developed further for implementation in a future prosthesis prototype