01-02-2012, 03:54 PM
Myoelectrically
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1: INTRODUCTION
The motto of new era in medical sciences is “Repair it if you can. Replace it if you can’t.” This motto serves right when we peep into the history of the prosthetic arm. Latest in the field of development is the myoelectric arm, which gives additional gripping facilities along with various degrees of freedom of movements.
Myoelectrically controlled arm prostheses are externally powered prostheses, which mean that they are not driven by the muscle strength of the patient, but with the aid of electric power. It is basically fabricating electronic components on to the prosthetic sockets so that the prosthetic arm can be moved on receiving the EMG signals directly from the body. Human body generates around 5-10 microvolts of voltage signal. This is picked up by the sensors placed on the conjunction of the amputated arm and the prosthetic socket. The sensors convert the voltage signal into an appropriate current value.
This current is then fed into the amplifier which amplifies the current value. This is then supplied to the motor drivers which drives the motor on/off. This arrangement allows the user to simply instruct the prostheses to open, close, hold, squeeze, pinch or release an object, so that there is limited conscious effort from the user but the prostheses maintains a natural co-ordination of the fingers. Most people prefer this type of control because non-electric prostheses are often laborious to operate, whereas simply flexing a muscle can control myoelectrically powered prostheses. They eliminate the need for the tight harness amputees have to wear if they choose a non-electric prosthesis. Since electric prostheses do not have to utilize a control cable or harness, cosmetic skin made of silicon or latex can be applied to the prosthesis, greatly enhancing the cosmetic restoration. Basically improved shaping and a better understanding of soft tissue mechanics are needed to create better input data for electrically- and computer-controlled prostheses.
PROSTHETIC ARM
Prosthetic arm was first implemented by Reinhold Reiter, a physics student at Munich University in May 1945. The report of Reiter‘s work, in German medical newspaper, described a myoelectric prosthetic arm designed for the amputee factory worker. A prototype was demonstrated at the Honnover export fair in 1945. The research leading to this device was supported by the Bavarian Red Cross and private source. Pudlusky was Reiter‘s business manager for the project. Development of the system was terminated due to the lack of funds after the German currency reform in 1948[1]. The idea behind the control system was to amplify the myoelectric signal from a contracting muscle in order to control a wooden hand, which was modified to be actuated by an electric solenoid. Reiter used single muscle site in the residual limb. Control of opening and closing motion was derived from using two different rhythms of contraction. This scheme of using the signal from a single muscle to control two motions was later to be known as three state controls. Reiter’s work was not alone in being overlooked in the early development of myoelectric control. The myoelectric signal has been used to monitor lookout alertness as early as 1947 and by 1957 to control respirators for polio victims. Indeed, it has been investigated as a possible control source for prosthesis as early as 1949, with encouraging results. In the late 1950s and early 1960s, research again started in myoelectric control system. This work occurred independently and almost simultaneously in the USSR, the United Kingdom, the USA, Europe and Canada. It was aided greatly by the availability of transistors, without which a truly portable myoelectric prosthesis was not practical.
ARTIFICIAL ARM
Artificial arm is a boon for those persons who have lost their hand due to some mishap. One of the main requirements of artificial arm is that it should be as near as possible to the natural arm. There are various designs of artificial arm. These designs fall under the categories of mechanical, electrical and myoelectric arm. Mechanical devices are functional prostheses that use some motion of the body to provide the force necessary to control the prosthetic component. Electrical arms operate the hand by a motor driven by micro-switches and relays. Myoelectric arm is stimulated by muscle signal available from the stump of amputee. Powered hand prostheses are used to replace the function of a lost natural hand. Most of the commercial prosthetic hands in clinical use are controlled by myoelectric signal and are referred to as myoelectric hands. One of the important organs of the body is the hand. We perform 90% of our daily work by hand like eating, lifting, gripping an object, writing, typing, driving etc. Many persons, especially industrial workers carrying out different type of machining tasks in the factory, lose their arm due to accidents. Their lost arm is to be replaced by an artificial arm which fulfils all the criteria of a normal arm so that the amputee may lead the life of a normal person and feels rehabilitated. Efforts to develop mechanical artificial arms have been initiated since long and prosthesis of many such arms has been implemented successfully on many amputees. In the last decade, many electronic arms have been introduced in the market. With the advent of new signal processing techniques, advanced features like operation with muscle signal and proportional grip force have been added to the capabilities of these arms offering the user a wide choice to select a model according to user requirements.
TYPE OF ARTIFICIAL ARMS
There are mainly four types of artificial arms that have been reported in the literature. They may be categorized as:
MECHANICAL ARM
These devices are functional prostheses that use some motion of the body to exert the force needed to control the prosthetic component. Particularly noteworthy is the Bowden cable for use in the prosthetics field. A Bowden cable consists of an inner core cable that is free to move within a sleeve cable which is fixed in place at either end.
These devices require a harness, to be worn about the shoulders, to which one or more Bowden cables are attached. The conventional below-elbow, body-powered prosthesis has a single control cable that runs from the harness to a terminal device. Terminal device opening and closing is then controlled by shoulder shrug and/or flexion of the residual upper arm. An above-elbow amputee has additional control cable, which is used to switch control of the harness from terminal device opening to elbow flexion by unlocking the elbow. Body-powered prostheses are the most common kind of prosthesis used all over the world, due to the intimate connection of the control cable, which is provided between input and output. It helps the user of a body-powered limb to feel closely connected to the operation of the prosthesis. These prostheses are also lightweight, durable and of relatively low cost. However, body-powered prostheses have a number of shortcomings. The major issues are the uncomfortable harness mechanism, the somewhat ungainly control motions, particularly in the case of above elbow prostheses, restricted range of motion and limited load-lifting capacity.
ELECTRICAL ARM
These are externally powered devices and receive their power from an external electric source to the body. These are relatively new (last 15 to 20 years) addition to the armamentarium of prosthetic devices.
Touch Switches: A pair of touch switches remains in contact with antagonistic wrist muscles flexors and extensors. The wrist flexors activate the =CLOSE ‘switch while extensors operate the =OPEN ‘touch switch.
Control Circuit: Each microswitch is connected to a Flip-flop which is configured to operate in set-reset mode. It is =SET ‘by the micro switch and =RESET ‘by the limit switches provided at the extremities of hand positions.
