Myoelectric Arm

ACKNOWLEDGEMENT: We take this opportunity to thank Ms. Hetul Gandhi and Mr. Nimit Shah who have guided us through out this seminar and guided us to prepare our presentation effectively. Also our special thanks go to all our friends and others who have directly or indirectly contributed to the success of this seminar.

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ABSTRACT The motto of new era in medical sciences is “Repair it if you can. Replace it 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. The myoelectric arm works under the influence of Electromyographic signals extracted from skin surface. These Electromyographic signals are utilized to run various motors which enables the user (patient) to grip and move is limbs in a much more effective manner. The myoelectric arms have become extremely popular after the wars which have left several efficient soldiers without their arms. They give them a psychological boon as they very closely resemble their natural arm. Hence it can be correctly said that technology never puts a full stop to the development and these developments are effectively utilized for the betterment of human life.

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INDEX: Sr No

CONTENTS

PAGE No.

1

Introduction

1

2

Why do we need myoelectric arm?

2

3

Brief history of development of orthotic and prosthetic arm

3

4

What is EMG?

6

5

Upper Extremity Prosthetic Devices

7

6

Block diagram of myoelectric arm

9

7

Construction of myoelectric arm

26

8

Cosmetic gloves for myoelectric arm

32

9

Control systems of myoelectric arm

35

10

How the surgery is performed

40

11

Who is appropriate for myoelectric arm?

41

12

Comparison between prosthetic arm and myoelectric arm

43

13

Future Enhancement

44

14

Conclusion

45

15

Bibliography

46

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INTRODUCTION Rehabilitation may be defined as reintegration of an individual with a disability in the society.

Rehabilitation engineering is the application of science and technology to

ameliorate the handicaps of individual with disabilities. Rehabilitation may be classified into two parts: (i)

Orthosis

(ii)

Prosthesis.

Orthosis: It is an appliance that aids an existing function of a limb. Prosthesis: It is a medical appliance that substitutes a limb both structurally and functionally. Rehabilitation of humans with disabilities requires effective usage of assistive systems for restoration of motor functions. Important features of an effective system are (i)

Reliability

(ii)

Minimum increase of energy rate and cost with respect to able bodied subjects performing the same task.

(iii)

Minimum disruption of normal activities when employing the assistive system;

(iv)

Cosmetics and practicability.

These requirements and availability of technology have led to development of externally powered prostheses that interface directly with the neuromuscular system. These devices may be battery operated, microprocessor based, or reliable biological sensor. Some basic requirements of prosthesis are: (i)

The prosthesis must support body weight of amputee like a normal limb

(ii)

Body is supported such that undesirable socket or stumps interface pressures and gait abnormalities due to painful socket/stumps contacts are prevented.

(iii)

Prosthesis should duplicate as nearly as possible the kinematics and dynamics of the normal gait. 4

WHY DO WE NEED A MYOELECTRIC ARM???? Advances in myoelectric technology in recent years have made these upper extremity prosthetic components far superior to body-powered equivalents. Myoelectric hand/arm components perform better than conventional prostheses in terms of function, weight, comfort, and cosmetics. Even small children can learn to create the signals controlling operation. The single exception may be manual laborers who transport heavy weights in wet, dirty, and otherwise physically-challenging work environments.  Myoelectric wearers don’t require the bulky, heavy harness of a body-powered prosthesis.  Improved grip force and proportionate control permit strong grasp as well as delicate handling.  Improved physiological control of gripping movement through operation of prehensor by closest muscle groups.  Many users report decreased phantom pain.  Improved cosmetic of myoelectric glove over hook terminal device.

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BRIEF HISTORY OF DEVELOPMENT OF ORTHOTIC AND PROSTHETIC LIMBS: We know from Egyptian stelae (2500 BCE) and from early Roman mosaics that prostheses and simple walking aids (orthoses) have been used during much of recorded history. People in parts of the world still use a head-high wooden stick to vault over on the side of their nonfunctional limb when they walk, much as some disabled Egyptians did thousands of years ago. Simple wooden canes must be nearly as old as human kind itself. Wooden peg legs have been effective aids to walking for thousands of years. Until the 20th Century, wood and leather were the favorite composite materials in O&P devices. Paintings of Brueghel from the 16th Century show clearly the plight of persons without limbs or with dysfunctional limbs as a result of polio or cerebral palsy. Most of their locomotory aids were fashioned from wood and leather, perhaps by themselves. Wars and conflicts have inevitably stimulated developments in O&P technology, and the armor makers of the medieval era were early O&P practitioners. The noble German knight, Götz von Berlichingen, remarked in Goethe’s play The Iron Hand, that his iron hand had served him better in the fight than ever did the original of flesh. Ambroise Paré (1510-1590), a French army surgeon can rightly be called the father of amputation surgery and prosthetics. He developed the ligature, which eliminated searing the residual limb to stop bleeding. He used site selection to try to produce limbs that were as useful as possible, and he designed prostheses and followed the outcome of his patients. Not all warriors wore prostheses. The next time you are in Trafalgar Square in London, observe the statue of Viscount Horatio Nelson. Nelson lost his arm above the elbow at Tenerife, lost sight in

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his right eye on Corsica, received a severe head injury at Alexandria, and watched his greatest naval victory at Trafalgar while propped up on deck with a fatal spinal cord injury. His only rehabilitation aid was the “Nelson knife”, now frequently called a rocker knife, which remains even today one of the best eating aids for persons with only one arm. The Napoleonic wars played their part in prosthetics development, mostly in France and in England. Lord Uxbridge, Wellington’s cavalry officer at Waterloo became the wearer an above knee prosthesis that became known as the Anglesea Leg after the island of Anglesea where Uxbridge resided after the war. It was a unique prosthesis that raised the toe as the knee was flexed in order to reduce stumbling. The concept is still used today. The Anglesea prosthesis, after some changes, was used widely in America by veterans of the Civil War. The enormous number of amputations resulting from the American Civil War established the prosthetics industry in the United States during the late 1800s. However, it was WWI that set the stage for modern prosthetics. Many of the early advances occurred in Germany. In Zürich, about 1915, a well-known German surgeon, Ferdinand Sauerbruch, worked with Aurel Stodola, a famous turbine engineer and professor of mechanics at the Polytechnic Institute of Zürich to produce a hand prosthesis that was controlled and powered directly from surgically prepared muscles of the residual limb. The surgical technique developed to achieve this biological control mechanism was called muscle tunnel cineplasty. Sauerbruch was one of the first surgeon/physicians to recommend

multidisciplinary

scientific

and

engineering

endeavors

in

the

prosthetics/rehabilitation field. After successfully developing the Sauerbruch hand he said, “Henceforth, surgeon, physiologist, and technologist will have to work together.” After WWI, American surgeons studied surgical and prosthetic rehabilitation methods in Europe, such as Sauerbruch’s tunnel cineplasty and Krukenberg’s surgical fashioning of the radius and ulna of the limb of long below elbow amputees into two large “fingers” that could be used effectively for gripping large objects.

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However, no research and development work was fostered in America after WWI. World War II mobilized research and development of prostheses all over the world. In America this burst of research activity was stimulated partially by veteran amputees who were languishing in hospitals and who were disappointed by the state of limb prosthetics in 1945. As a consequence of their lobbying, the surgeon general of the Army asked the National Research Council to call a meeting to select which prostheses would be best for the veterans of WWII. This meeting, held in Chicago during January of 1945, produced recommendations for scientific and engineering studies of limb prostheses. From this meeting the first federal grants were issued to promote the science and technology of prostheses and amputation. Early investigations included tours of O&P facilities in many countries. The early studies and the new research were dramatically successful, and the period from 1945 to 1975 was perhaps the most productive period ever in American orthotics and prosthetics. In fact, this period was productive for O&P technology worldwide. The O&P field is international in scope and this brief history has captured only a few happenings in a handful of countries.

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WHAT IS EMG? Electromyography (EMG) is a medical technique for measuring muscle response to nervous stimulation. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these cells contract.

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The above figure shows the normal EMG graph and the frequency spectrum of an EMG signal.

SIGNIFICANCE OF EMG IN MYOELECTRIC ARM: The potentials developed by the muscles are used in a myoelectric arm for its movement. There are two conditions to be satisfied: (i)

The minimum EMG voltage required is 15 micro volts.

(ii)

The scar through the EMG potentials are obtained should not break down under the weight of the prosthetic arm.

UPPER EXTRIMITY PROSTHETIC SYSTEM: PASSIVE OR COSMETIC DEVICES:

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These are the conventional type of prosthetic limbs which are just a psychological consolation to the patient. They are just for name sake. They can do nothing that a natural limb can do.

BODY POWERED DEVICES: They are mainly categorized in the field of orthosis. Here the prosthetic device is strapped to the shoulder or the trunk of patient which enables him some degree of freedom of movement. But this freedom of movement is restricted to only certain positions. Not all angle of movement is achievable in this method of rehabilitation.

MYOELECTRIC DEVICES: The myoelectric prosthesis or ‘myo’ was invented in 1948 by Reihold Reiner. This type of prosthesis uses sensors to detect, commonly, a threshold of electromyography activity to switch an electric motor in the artificial ‘hand’, and can also be used to switch powered wrist and elbow components. Electromyography (EMG) activity originates from the depolarization and repolarization of the individual muscle cell membranes during muscle activity. Using surface electrodes it is possible to measure these potential differences on surrounding skin. There are many permutations of this control scheme, however, a salient point is that commercial myoelectric prostheses do not operate in a ‘volitional’ manner. Rather, the amputee is taught to achieve the necessary degrees of muscular contraction corresponding to the threshold levels of electrical activity needed for operation. Additionally, the myoelectric control method only provides an ‘efferent’ signal to the prosthesis from the amputee. There is no ‘afferent’ signal returned to the amputee to inform what grip strength is being applied or what position the fingers are in; unlike the mechanical connection to the body of the body-powered device. The greatest benefits of

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myoelectric prostheses are their increased grip strength compared to body-powered devices, and that there is no necessity for the donning of elaborate control straps combined with the fact that the myo often has a more hand-like appearance.

THE ROBOTIC ARM: Robotic arm is basically a robot in the shape of an arm capable of performing almost all the task that can be done using a normal limb. But robotic arm is far too bulky to be attached to the human body. It can perform all the work like eating lunch with knife and fork or holding a wine glass and drinking wine. But it cannot be attached to the body so it is not portable. Recently interfacing between the brain and robotic arm is trying to be established and is termed as brain computer interface.

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BLOCK DIAGRAM OF MYOELECTRIC ARM: BLOCK DIAGRAM FOR WORKING OFMYOELECTRIC ARM: USER (EMG POTENTIAL)

SIGNAL ACQUISITION AND PROCESSING

CONTROL

ACTUATOR

SENSOR

PROSTHESIS

OUTSIDE WORLD

Figure: Block diagram of myoelectric arm.

EXTRACTION OF EMG SIGNAL: Electromyographic (EMG) signals, collected at the skin surface, have been used for the control of upper limb prosthetic devices since 1948; because they provide easy and noninvasive access to physiological processes that cause the contraction of the 13

muscles. At present, the process of EMG signals is the most common approach used for the control of active prosthetic hands.

ACQUISITION AND PROCESSING OF EMG SIGNAL The formal scheme for the acquisition and analysis of the EMG signal for the control of prosthetic devices is composed of several modules: • signal conditioning and preprocessing • feature extraction • pattern recognition • offline and online learning.

SIGNAL ACQUISITION AND PROCESSING

FEATURE EXTRACTION

OFFLINE AND ONLINE LEARNING

PATTERN RECOGNITION

The first module that is the signal acquisition and processing unit preprocesses the EMG signal in order to reduce noise artifacts and/or enhance spectral components that contain important information for data analysis. Moreover, it detects the onset of the movement and activates all the following modules. During the feature extraction phase, the

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measured EMG signal is processed in order to emphasize the relevant structures in the data, while rejecting noise and irrelevant data, producing the so-called “original feature. Sometimes a reduction of the dimensionality is needed to simplify the task of the classifier. In this case, a pattern recognition algorithm is used on the (reduced) feature L, and the measured signal is classified into the output space. The learning modules are used to adapt the device to the EMG signals generated by the users because of its time-variant characteristics.

EMG ACQUISITION AND PROCESSING: Precise detection of discrete motor events, such as the onset of voluntary muscle contractions, is a prerequisite for various psycho physiological approaches in sensorimotor system analysis. EMG signal for prosthetic applications is generally acquired by placing one or more differential electrodes on the skin of the user, depending on his/her level of amputation and on the data that should be extracted from the signal. A good acquisition of the EMG signal, in fact, is a prerequisite for good signal processing. In particular, the consortium defined some recommendations about electrode shape and size, interelectrode distance, electrode material, and sensor construction (where sensor is defined as the ensemble of electrodes, electrode construction, and integrated preamplifier, if any)., e use of one or more low-noise, high-input impedance amplifiers to acquire the EMG signal is suggested. After the acquisition, the signal is filtered, generally using a band-pass filter with high CMRR and gain in order to reduce motion artifacts (high-pass filter) and noise (lowpass filter). Generally, about 95% of the power spectrum of the EMG is accounted for by harmonics up to 400 Hz, and most of the remaining is electrode and equipment noise. A low-pass filter, or anti-aliasing, is usually applied to the signal. The cut-off. frequency varies from 250 to 2000 Hz, the most common choice being around 500 Hz.². A high-pass filter is also required to attenuate movement artifacts and the instability of the electrode– skin interface. In the literature, the lower cut-off. frequency varies from 0.1 to 100 Hz,

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but generally a value between 10 and 20 Hz is used. In simple on–o. devices a notch .filter at 50 or 60 Hz (depending on the frequency of the electric power supply) could be added. However, it is worth noting that this filter could also eliminate some important information present in the EMG signal and should not be used for multifunctional hands. At this stage the signal is sampled and converted into a digital stream of data.

EMG FEATURE EXTRACTION: Many EMG-based control systems are able to control a single Degree of Freedom in a prosthetic limb (hand open/close, wrist or elbow flexion/extension). These systems generally extract the EMG amplitude or rate of change by using two electrodes placed on two antagonist muscles (e.g., biceps and triceps brachii or flexor and extensor of the forearm, depending on the level of the amputation), is information is used to defined the state of the hand and to control its speed or strength in a constant or even proportional way. Starting from the late 1970s, the EMG signal was modeled as amplitude modulated Gaussian noise whose variance was related to the force developed by the muscle. As a consequence, most commercial myoprocessors used in prosthetic control are now based only on one dimension of the EMG signal—the variance or mean absolute value. Two independent measurements and control systems ensure that the hand switches to grip force mode when an object is gripped and the grip force is proportional to the muscle signal.

(a) Original EMG signal

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(b)

Rectified EMG signal

(c)

Low pass filtered EMG signal

(d) Threshold-based detection of movement Despite some promising results, this method turned out to be sensitive to changes in signal amplitude. All these systems have been successfully implemented, but they cannot provide sufficient information to effectively control more than one DoF. Generally, all commercial myoelectric control systems are based on the common assumption that the instantaneous value of the myoelectric signal contains no information. Users are trained to produce a constant level of activation of muscles, and the prostheses are tuned according to these values. The steady-state EMG signal, however, has very little temporal structure because of the active modification of recruitment and firing patterns needed to sustain a contraction. The parameters that could be extracted to quantify its amplitude (e.g., variance, mean absolute value) or its frequency characteristics. Starting from the 1990s, researchers found that there is useful information in the transient burst of myoelectric signal.

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ONLINE AND OFFLINE LEARNING: The online and offline learning are necessary for the adaptation of the device to the EMG potentials generated by the user or the patient. In the offline phase, the user has to generate the adequate EMG potentials that are necessary for the proper working of the myoelectric arm. If the user fails to do so, then he/she has to go to the rehabilitation centre and get his controller tuned. On the other hand, in online learning the controller is designed in such a way that it adjusts to the EMG potential generated by the patient and performs the controlling operation of the myoelectric arm.

CONTROL OF MULTIFUNCTIONAL PROSTHETIC HANDS USING EMG: Replicating the performance of the human hand is beyond current technical capabilities. In fact, the human hand is extremely complex: it has 22 degree of freedom in rotation and movement, controlled by about 38 muscles in the hand. Commercial hand prostheses have a limited number of degree of freedoms (one or two for finger movements and thumb opposition), and thus they have low grasping functionality. In fact, they do not allow adequate encirclement of objects, compared to the adaptability of the human hand. ,

The main advantage of current prosthetic hand devices is that they can generate large

grasping forces and are simple to implement and control, in particular by using EMG signal. Electromyographic signal is a simple and easily obtained source of information on what the users of prosthesis would like to do with their artificial hands. Surface electrodes are easy to use and manage, and they do not require surgery. Moreover, there are no harnesses that could limit the movement of the forearm. It is possible to control an active device with just one differential electrode placed on the residual limb, even in infants.

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ACTUATORS AND SENSORS: The function of actuators is to trigger the myoelectric arm in accordance with the EMG which carries the information of the brain. The sensors do the vice versa. They sense the activity of the outside world and inform the brain.

ELECTRODE AND AMPLIFIER DESIGN The design of the electrode unit is the most critical aspect of the electronics apparatus which will be used to obtain the signal. The fidelity of the EMG signal detected by the electrode influences all subsequent treatment of the signal. It is very difficult (almost impossible) to improve the fidelity and signal-to-noise ratio of the signal beyond this point. Therefore, it is important to devise an electrode unit that provides minimal distortion and highest signal-to-noise ratio. The following characteristics are important for achieving this requirement.

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Electrode

Amplifier

Muscle fiber

Filter

To the DC motor.

A-D Converters

Windowing

Extracting characteristics

To the stepper motor.

Figure: ACTUAL BLOCK DIAGRAM OF MYOELECTRIC ARM.

• Differential amplification – In order to eliminate the potentially much greater noise signal from power line sources, a differential detecting configuration is employed. The differential amplification technique is shown schematically in Figure. The premise is simple. The signal is detected at two sites, electronics circuitry subtracts the two signals and then amplifies the difference. As a result, any signal that is "common" to both detection sites will be removed and signals that are different at the two sites will have a "differential"

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that will be amplified. Any signal that originates far away from the detection sites will appear as a common signal, whereas signals in the immediate vicinity of the detection surfaces will be different and consequently will be amplified. Thus, relatively distant power lines noise signals will be removed and relatively local EMG signals will be amplified. This explanation requires the availability of a highly accurate "subtractor". In practice, even with the wondrous electronics of today, it is very difficult to subtract signals perfectly. The accuracy with which the differential amplifier can subtract the signals is measured by the Common Mode Rejection Ratio (CMRR). A perfect subtractor would have a CMRR of infinity. A CMRR of 32,000 or 90 dB is generally sufficient to suppress extraneous electrical noises. Current technology allows for a CMRR of 120 dB, but there are at least three reasons for not pushing the CMRR to the limit: 1) Such devices are expensive. 2) They are difficult to maintain electrically stable, and 3) the extraneous noise signals may not arrive at the two detection surfaces in phase, and hence they are not common mode signals in the absolute sense.

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• Input impedance – The source impedance at the junction of the skin and detection surface may range from several thousand ohms to several megohms for dry skin. In order to prevent attenuation and distortion of the detected signal due to the effects of input loading, the input impedance of the differential amplifier should be as large as possible, without causing ancillary complications to the workings of the differential amplifier. Present day electronics devices easily provide input impedances of the order of 1012 ohms in parallel with 5 picofarads. In addition to the magnitude of the input impedance, the balance between the impedances of the two detection sites is also of great importance. This consideration requires careful circuit design.

• Active electrode design – The requirement for a high input impedance introduces a problem known as capacitance coupling at the input of the differential amplifier. A small capacitance between the wires leading to the input of the differential amplifier and the power When an electrode is placed on the skin, the detection surfaces come in contact with the electrolytes in the skin. A chemical reaction takes place which requires some time to stabilize, typically in the order of a few seconds if the electrode is correctly designed. But, more importantly, the chemical reaction should remain stable during the recording session and should not change significantly if the electrical characteristics of the skin change from sweating or humidity changes. Even with the above considerations, the EMG signal will be contaminated by some noise. The signal to noise ratio can be increased by judicious filtering between 20-500 Hz with a roll-off of 12 dB/oct. (Strict design characteristics could consider 400 Hz as the upper bandwidth cut-off. The 500 Hz value allows for a safety margin in the design of the circuitry.) This filtering is generally accomplished at the amplifier stage located outside the active electrode. television signal strength to increase when one places ones hand near the antenna input, but does not touch it. The solution is to place the differential amplifier as close as possible to the detection surfaces of the electrode. This solution has become known as the "active electrode". One other advantage of this configuration is that the

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output impedance of the differential amplifier can be made to be very low, on the order of 10 ohms. Therefore, any movement of the cable from the output of the electrode will not generate significant or even notable noise signals in the cable which feeds into the subsequent amplifier line will introduce a power line noise signal into the amplifier. This phenomenon is similar to that which causes a television signal strength to increase when one places ones hand near the antenna input, but does not touch it. The solution is to place the differential amplifier as close as possible to the detection surfaces of the electrode. This solution has become known as the "active electrode". One other advantage of this configuration is that the output impedance of the differential amplifier can be made to be very low, on the order of 10 ohms. Therefore, any movement of the cable from the output of the electrode will not generate significant or even notable noise signals in the cable which feeds into the subsequent amplifier.

• Filtering – Even with the above considerations, the EMG signal will be contaminated by some noise. The signal to noise ratio can be increased by judicious filtering between 20-500 Hz with a roll-off of 12 dB/oct. (Strict design characteristics could consider 400 Hz as the upper bandwidth cut-off. The 500 Hz value allows for a safety margin in the design of the circuitry.) This filtering is generally accomplished at the amplifier stage located outside the active electrode.

• Electrode stability – When an electrode is placed on the skin, the detection surfaces come in contact with the electrolytes in the skin. A chemical reaction takes place which requires some time to stabilize, typically in the order of a few seconds if the electrode is correctly designed. But, more importantly, the chemical reaction should remain stable during the recording session and should not change significantly if the electrical characteristics of the skin change from sweating or humidity changes.

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• Preferred method of use – Given the high performance and small size of modern day electronics, it is possible to design active electrodes that satisfy the above requirements without requiring any abrasive skin preparation and removal of hair.

ELECTRODE GEOMETRY Throughout the history of electromyography, the shape and the layout of the detection surface of the electrode have not received much attention. Most likely because past users of electromyography have been interested only in the qualitative aspects of the EMG signal. The advent of new processing techniques for extracting quantitative information from the EMG signal requires greater focus on the configuration of the electrode. The major (but not all) points to consider are: 1.) the signal to noise ratio of the detected signal, 2.) the bandwidth of the signal, 3.) the muscle sample size, and 4.) the susceptibility to crosstalk.

• Signal-to-noise ratio – The signal-to-noise ratio is a function of complicated interactions between the electrolytes in the skin and the metal of the detection surfaces of the electrode. This is an involved subject that is beyond the scope of this short treatise. Suffice it to say that there are several approaches for reducing the noise, such as using large surface areas for the detection surfaces, employing conductive electrolytes to improve the contact with the skin, and removing dead (less conductive) dermis from the surface of the skin. Through trial and error we have found that detection surfaces made of pure (>99.5%) silver in the form of bars 1 cm in length and 1 mm in width provide a sufficiently good medium for the detection surface. The amplitude of the EMG signal is directly proportional to the distance between the detection surfaces. Hence, this distance should be maximized. But, increasing this distance introduces undesirable characteristics to the electrode design. As the electrode becomes larger, it becomes unwieldy and cannot be used to detect EMG signals from relatively small (in width as well as in length) muscles such as those found

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in the hand, forearm and the leg. Additionally, as the distance increases the filtering characteristics of the differential amplification decreases in bandwidth. Thus, a compromise is necessary. We have found by calculations and by heuristics that an interdetection surface spacing of 1 cm provides an acceptable compromise.

• Bandwidth – The bandwidth of the EMG signal is affected by the inter-detection surface spacing and the conduction velocity of the action potentials along the muscle fibers. The differential configuration possesses a spatial filtering feature that can be expresses as a band pass filter in the spectral frequency region of the EMG signal. For an average conduction velocity of 4.0 m/s and an inter-detection surface distance of 1.0 cm, the pass frequency is 200 Hz and the null point is at 400 Hz. This bandwidth captures the full frequency spectrum of the EMG signal and suppresses noise at higher frequencies.

• Muscle sample size – The muscle sample size need not be large because the muscle fibers of motor units are distributed throughout most of the muscle cross-section. Therefore, it is not necessary to cover a large portion of the muscle with the detection surface of the electrode to obtain a representative sample of the EMG signal for a particular set of active motor units.

• Cross-talk susceptibility – The susceptibility to cross-talk is an often overlooked design aspect of EMG electrodes. The greater the width and length of the detection surfaces and the greater the interdetection surface distance the closer the electrode will be to adjacent muscles. Thus, larger electrodes are more susceptible to detecting signals from adjacent (lateral and below) muscles. In situations where this issue is of concern, it is advisable to reduce the size of the electrode.

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EMG ELECTRODE PLACEMENT: • Location and orientation of the electrode – The electrode should be placed between a motor point and the tendon insertion or between two motor points, and along the longitudinal midline of the muscle. The longitudinal axis of the electrode (which passes through both detection surfaces) should be aligned parallel to the length of the muscle fibers. Figure provides a schematic representation of the preferred electrode location.

• NOT on or near the tendon of the muscle – As the muscle fibers approach the fibers of the tendon, the muscle fibers become thinner and fewer in number, reducing the amplitude of the EMG signal. Also in this region the physical dimension of the muscle is considerably reduced rendering it difficult to properly locate the electrode, and making the detection of the signal susceptible to crosstalk because of the likely proximity of agonistic muscles.

• NOT on the motor point –

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During the past one-half century it has been taught that for the purpose of detecting a surface EMG signal the electrode should be located on a motor point of the muscle. The motor point is that point on the muscle where the introduction of minimal electrical current causes a perceptible twitch of the surface muscle fibers. This point usually, but not always, corresponds to that part of the innervation zone in the muscle having the greatest neural density, depending on the anisotropy of the muscle in this region. Presumably, the motor points have been used as landmarks because they were identifiable and provided a fixed anatomical landmark. Unfortunately from the point of view of signal stability, a motor point provides the worst location for detecting an EMG signal. In the region of a motor point, the action potentials travel caudally and rostrally along the muscle fibers, thus the positive and negative phases of the action potentials (detected by the differential configuration) will add and subtract with minor phase differences causing the resulting EMG signal to have higher frequency components. In the time domain, the signal appears as more jagged and with more sharp peaks. The loss of stability occurs from the fact that a minor displacement ( 0.1 mm) will affect in an unpredictable fashion the amount of change in the frequency characteristics of the signal. A note of caution about the motor points and innervation zones. Most muscles have multiple innervation zones throughout the muscle. They can be identified by applying electrical stimulation to the skin above the surface of the muscle or by other more technically complicated surface mapping techniques. If neither procedure is convenient, then place the electrode in the middle of the muscle between the origin and insertion point.

• NOT at the outside edges of the muscle – In this region, the electrode is susceptible to detecting crosstalk signals from adjacent muscles. It is good practice to avoid this situation. For some applications, crosstalk signals may be undesirable.

• Orientation of the electrode with respect to the muscle fibers – The longitudinal axis of the electrode (which passes through both detection surfaces) should be aligned parallel to the length of the muscle fibers. When so arranged, both detection

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surfaces will intersect most of the same muscle fibers. Hence, the spectral characteristics of the EMG signal will reflect the properties of a fixed set of muscle fibers in the region of the electrode. Also, the frequency spectrum of the EMG signal will be independent of any trigonometric factor that would provide an erroneous estimate of the conduction velocity. The resultant value of the conduction velocity affects the EMG signal by altering the temporal characteristics of the EMG signal, and consequently its frequency spectrum.

REFERENCE ELECTRODE PLACEMENT The reference electrode (at times called the ground electrode) is necessary for providing a common reference to the differential input of the preamplifier in the electrode. For this purpose, the reference electrode should be placed as far away as possible and on electrically neutral tissue (say over a bony prominence). Often this arrangement is inconvenient because the separation of the detecting electrode and reference electrode leads requires two wires between the electrodes and the amplifier. It is imperative that the reference electrode make very good electrical contact with the skin. For this reason, the electrode should be large (2 cm x 2 cm). If smaller, the material must be highly conductive and should have strong adhesive properties that will secure it to the skin with considerable mechanical stability. Electrically conductive gels are particularly good for this purpose. Often, power line interference noise may be reduced and eliminated by judicious placement of the ground electrode.

ELECTRICAL SAFETY CONCERNS The failure of any electrical instrumentation making direct or indirect galvanic contact with the skin can cause a potentially harmful fault current to pass through the skin of the subject. This concern is less relevant in devices that are powered exclusively by low voltage (3-15 V) batteries. To ensure safety, the subject should be electrically isolated from any electrical connection (to the power line or ground) associated with the power source. This isolation is generally achieved in one of two ways: either through the use of optical isolators or through the use of isolation transformers. Both approaches are satisfactory, but both require careful consideration for not distorting the EMG signal. This is especially true when a transformer is used. This isolation provides the added benefit of reducing the amount of radiated power line noise at the electrode detection surfaces.

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EMG SIGNAL PROCESSING For several decades it has been commonly accepted that the preferred manner for processing the EMG signal was to calculate the Integrated Rectified signal. This was done by rectifying (rendering the signal to have excursions of one polarity) the EMG signal, integrating the signal over a specified interval of time and subsequently forming a time series of the integrated values. This approach became widespread and it was possible to make these calculations somewhat accurately and inexpensively with the limited electronics technology of earlier decades. The advances made in electronics devices during the past decades have made it possible to conveniently and accurately calculate the root-mean-squared (rms) and the average rectified (avr) value of the EMG signal. The avr value is similar to the integrated rectified value, if the calculations are made correctly and accurately. Both these variables provide a measurement of the area under the signal but do not have a specific physical meaning. On the other hand, the rms value is a measure of the power of the signal, thus it has a clear physical meaning. For this reason, the rms value is preferred for most applications

THE ANALOG TO DIGITAL CONVERTERS: They convert the analog signal into coded digital pulsed which are fed into the stepper motors.

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CONSTRUCTION OF MYOELECTRIC ARM: Various motions like pronation & supination are achieved with the help of various motors and gears. These motors may be either stepper motor or DC motors. Let’s see how these are selected.

MOTOR CHOICE: STEPPER MOTOR VS DC MOTORS: When considering which type of motor should be used, the most important criteria was the size of the motor, the torque produced by the motor, as well as the requirement for variable speed control. The motor of choice would need to be mounted onto the robotic arm. A large motor would be both cumbersome and impractical. The load, which comprised of the lower arm and the hand of the robotic arm, weighed a total mass of approximately 220 grams. The motor chosen would need to be able to output torque capable of lifting such a load. As mentioned earlier, as a visual aid, the robotic arm is to flex and extend according to the RMS value of the EMG signal being read and processed from the

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amputated arm. As such, an increase in the RMS value causes the arm to flex at a faster rate and vice versa. For this reason, it was imperative that the chosen motor have the capability of variable speed.

STEPPER MOTORS: Stepper motors do as their name suggests, they "step" a little bit at a time. The stepper motor is controlled by a series of pulses. Though the stepper motor operates using pulses, it cannot simply respond to a clock signal. Instead, several windings need to be energized in the correct sequence before the motor's shaft will rotate. Reversing the order of the sequence will cause the motor to rotate in the opposite direction. If the control signals are not sent in the correct order, the motor will not turn properly. A circuit, called a translator, is responsible for converting step and direction signals into winding energization patterns. The stepper motor control systems also includes a driver, as shown in Figure, to handle the current drawn by the motor's windings.

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FIGURE: Translator and Driver of Stepper Motor As mentioned above, the stepper motor operates by a sequence of pulses. Therefore, by varying the speed at which the sequence of pulses is sent to the motor, the speed of rotation is easily varied. Increasing the speed of the sequence increases the rotational speed of the shaft. The opposite is true for decreasing the speed of the sequence. With that established, the stepper motor met the criterion of variable speed. The stepper motor is particularly suited for precise positioning because by its very structure it steps a predetermined angle. Taking 360 degrees and dividing by the number of steps the motor will make for one rotation will give the step angle. In terms of torque, the stepper motor produces the highest levels of torque at low speeds and lower levels of torque at high speeds. Stepper motors also have another characteristic of holding torque, which is not present in DC motors. Holding torque 32

allows a stepper motor to hold its position firmly when not turning. This can be useful for applications where the motor will be starting and stopping, while the force acting against the motor remains present. This eliminates the need for a mechanical brake mechanism. The stepper motors acquired were that of an appropriate size. Unfortunately, through testing, it was determined that the stepper motors in the appropriate size range were unable to produce the required amount of torque to lift a load of 220 grams. Though stepper motors of a larger size were considered, it was established that motors which would output the required torque would be impractical in size. Therefore, the DC motor was found to be the most suitable motor for the required application.

THE DC MOTORS: DC motors function using direct current power supply. The operation of the DC motor can be observed below in Figure. To allow the rotor to spin without twisting the wires, the ends of the coil is connected to a set of contacts called the commutator, which rubs against a set of conductors called the brushes. The brushes make electrical

33

contact with the commutator as it spins, and are connected to the positive and negative leads of the power source, allowing electricity to flow through the loop. The electricity flowing through the loop creates a magnetic field that interacts with the magnetic field of the permanent magnet to make the loop spin.

DC motors have a gradual acceleration and deceleration curve which causes slow stabilization. The addition of gearing to the motor will to reduce this problem, but overshoot is still present and will throw off the anticipated stop position. A potentiometer, which acts as a feedback mechanism, is also attached to determine the

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exact positioning of the motor. In addition, a control circuit, which compares the position of the motor with the desired position, moves the motor accordingly. DC

motors

differ

from

stepper

motors

in

their

torque-speed

relationship. DC motors generally do not produce high torque levels at low speeds without the aid of a gearing mechanism. However, the DC motor is capable of producing quite high levels of torque at higher speeds. With the need for variable speed, it was established that a gear box was required for gear reduction.

THE GEAR BOX: With the rps of the DC motor determined, it was decided that gearbox would be required to provide gear reduction for the motor driving the robotic arm. With a gear reduction, the output speed can be reduced while the torque is increased. When constructing the gear box, the desired output rps was decided to be approximately one revolution per second at maximum voltage of +15 V. To decrease the speed to one rps, a gear reduction in the range of 1:90 was needed. In order to accomplish this, spur gears were utilized. Spurs gears have straight teeth and are mounted on parallel shafts. In addition, a worm gear was included to provide the

35

appropriate level of holding torque. Worm gears are often used when large gear reductions are required. Worm gears possess a uniqueness unlike any other gears as the worm can easily rotate the gear, but the gear cannot rotate the worm. This is because the angle on the worm is of such a shallow depth that when the gear attempts to spin it, the friction between the gear and the worm holds the worm in place. Typical metal spur gears and worm gear can be seen in Figure

Examples of Spur Gears (left) and Worm Gear (right)

THE POTENTIOMETER: One major advantage of the stepper motor over the DC motor is the latter requires the addition of a potentiometer for position detection. Since the movement of the stepper

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motor is controlled by pulses, the exact position of the robotic arm can be determined by observing the number of pulses sent to the motor5. With the DC motor chosen, it was determined that a potentiometer would need to be affixed to the robotic arm to provide feedback on positioning. This positioning detection is imperative for the protection of the gears being used by the motor. 15 volts is applied into one terminal while the terminal at the other end of the resistor is connected to ground. The resulting voltage output is read through the middle terminal. Voltages were read at various positions of the potentiometer. At the fully flexed position, the voltage at the output of the potentiometer was found to be -2.8 V. At the fully extended position, the output was -4.3 V. Therefore, at any angles that produced an output voltage between the range of -2.8 and -4.3 V were within the region of safe operation. However, any output voltages lower than -2.8 V and higher than -4.3 V, required a breaking algorithm to stop the motor from running any further.

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Model of Modified Myoelectric Arm with Potentiometer

COSMETIC GLOVES FOR MYOELECTRIC ARM: 38

CUSTOM MADE GLOVES: They are usually made of silicone, and are sculpted in the presence of the patient to match the shape and size of the intact hand as closely as possible. The prosthesis is painted with details including half moons of the fingernails, age spots, freckles, hair, veins, etc. The patient is sometimes required to stay near the facility where the glove is fabricated for several days while the prosthesis is being made. At the high end of cost.

STANDARD GLOVES: They are made from existing molds to fit standard hands to save costs. They may be made of silicone or PVC. They are usually not customized specifically to the patient, but some manufacturers add basic “generic” details like veins and shading to give the glove a more realistic look.

MAERIALS: SILICONE GLOVES:  Lifelike look and feel.  Durability- about 4 years for myoelectric hands, depending on use and care. If used in a harsh environment, for example, an industrial setting, silicone gloves will tear easier than PVC.  Flexible. Holds objects better than PVC.  Color stability- the basic material is intrinsically colored.  Resistant to stains like newsprint and ink.  Some gloves may fit directly over metal parts, so no inner hand shell is needed.  Also can serve as a high-quality protective cover for the vulnerable hand mechanism

and electronics of a myoelectric prosthesis.

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PVC GLOVES: - Cheaper than silicone. - Stain easily from ballpoint ink and newsprint. - Durability- Typically will need to be replaced every 4-6 months due to discolorations, but they are much more resistant to tearing than silicone.

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Interface with control system

Sensor hand

Elbow and hand circuits. Figure: Fully constructed myoelectric arm

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THE CONTROLSYSTEM: SYSTEM OVERVIEW: The control system designed was named AmpuProTM. A flowchart of this system is depicted in Figure. It is made up of three main sections: input signal processing, motor control algorithms and motor driver sections. The input signal processing contains a real-time data retrieval section, and history buffer section. These are used in combination to capture and measure EMG signals from the subject’s bicep muscles and produce a stream of steady RMS values. This array of RMS values is then used to determine the next movement of the arm. Subsequently, it is converted into the motor control output voltage by the motor device drivers and is amplified to control the movement of the arm. Using the proper selection of threshold and fitness levels, any type of workout may be achieved for the subject.

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Control System Flow Chart

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A MICROPROCESSOR-BASED MULTIFUNCTION MYOELECTRIC CONTROL SYSTEM INTRODUCTION: Myoelectric prostheses are well accepted by below elbow amputees but less well by those with higher level amputations. The primary limitation at present lies in the control system. Although these systems have been successful for single device control (hand or elbow), the extension to the control of more than one device (either simultaneously or sequentially) has been difficult. It is the control systems which now limit the performance and, at times, the acceptance of the prosthetic fitting. For the high-level amputee (aboveelbow or higher) and especially for the bilateral amputee, the need for improved control systems for multifunction prostheses is critical. Over the past several years the Institute has been developing a myoelectric control system which is easy to operate, yet provides control of many independent prosthetic limb functions.

SYSTEM DESIGN: The control scheme developed in this research [1,2] uses information collected from the amputee to train a pattern classifier in the control system to recognize the contraction patterns specific to each amputee. The basic operation of the control system is illustrated in Figure. The classifier uses features extracted from the first 200ms of myoelectric activity following the initiation of a contraction to determine the intent of the amputee. The classifier matches this feature set with the features sets obtained from the amputee during the initial system calibration. The closest match is used to select which device (hand/elbow/wrist) is to be controlled. Control of this device continues until the signal level returns to a predetermined low level. As shown in Figure 2, the controller can operate in two modes: (a) PC-interface (training & configuration) and (b) Prosthetic control ( normal operation). The first mode is used to train the control system to recognize the myoelectric control inputs for each

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individual and requires a host computer for off-line processing. This control system-toPC host interface serves several functions: Configure Control Parameters: specify myoelectric control parameters. Collect Patterns: collect an ensemble of MES patterns, extract features, and store for subsequent training of the ANN. Train the ANN: from the features of the collected MES patterns, train the ANN to obtain a set of weights. Download Weights: send the ANN weights to the control unit, and store permanently in nonvolatile memory. Virtual Arm: simulate the interactive control of a prosthesis by controlling a threedimensional “virtual arm” on the PC screen.

Figure: Block diagram myoelectric arm.

of

Microprocessor

based

multifunction

45

Figure: Control System Operation.

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HOW IS THE SURGERY PERFORMED? Myoelectric arm is intended to transmit the instructions to the brain via unused nerves to the point outside the body. This is done by surgically moving the nerves that once led to arm and transplanting them to the chest. The nerves take around six months to take the root. Once they took root, sensors are placed over the nerve endings to amplify the electrical signals still coming from the brain. In case the amputation is below the elbow the grafting of nerves is not necessary. If the patient thinks to close his hand, then the nerve that used to close his hand made a little slip of his chest muscle contract. This slip can be detected because every time the muscle contracts, it emits a certain action potential. Therefore, two little antennas are fitted over that muscle, which now could tell when it contracted and then tell the artificial hand to close. So when he thinks "close hand," muscle contracts, artificial hand closes. The muscle is used as a biological amplifier of his nerve signal. Like the switches on his mechanical arm, the antennas are inside the supporting vest. The device senses the movement and translates it into mechanical motion of the pincer and the elbow.

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WHO IS APPRORIATE FOR MYOELECTRIC ARM? (i)

EVALUATION OF CANDIDATE:

The Myoelectric Arm 3 offers several control choices including EMG (myoelectric control) by one or two muscles, as well as 3 harness-mounted sensors (linear potentiometer, force sensor and touch pads [1 or 2]). Evaluation of the muscular as well as the remnant shoulder anatomy is one of the first steps in evaluation for an electric prosthesis. For myoelectric control, at least one muscle EMG signal is required, measuring 5 microvolts for hand control and 15 microvolts for the elbow. Careful probing with a sensitive EMG tester is necessary to identify all potential control sites, since fitting considerations may rule out some sites. The evaluator should be thoroughly familiar with the anatomy of the arm and shoulder so that the remnant musculature can be accurately identified and the patient properly instructed to contract the muscle being tested.

(ii) MOTIVATION AND PSYCHOLOGICAL ADJUSTMENTS: It almost goes without saying that the success of any prosthesis depends a great deal upon the patient's own motivation. Motivation is difficult to assess, and many clinics feel that merely participating in the fitting steps, such as muscle training, demonstrates adequate motivation. Others require consistent use of a conventional prosthesis before a myoelectric limb will be fit. That strategy, indeed, resolves many questions about the patient's motivation (and provides a backup prosthesis as well), but risks denying a myoelectric limb to some who could use it, but cannot or prefer not to use a conventional prosthesis.

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Realistic orientation (with all appropriate optimism) is also important, and if the patient is not aware of the details of the fitting process, then he should be informed of both the capabilities and the limitations of the prosthesis. If available, experienced prosthesis users who are willing to talk with a new amputee can give a realistic point of view and can offer inspiration to a new amputee who may have doubts about his future. Counseling or therapy to aid amputees in their adjustment may be helpful, especially if the amputee does not communicate well with the other fitting team members. If the clinic team or fitting center does not have training in psychology or counseling, adjustment problems can be particularly baffling. Typically; we may see the technical aspects of the fitting progressing, but the patient is not making the expected progress in his rehabilitation. The patient may need professional help in working through their adjustment problems, or perhaps just more time to solve their problems themselves or with their families. In any case, the fitting process may be blocked indefinitely until the patient is psychologically ready to progress.

(iii) SKIN CONDITIONS: Myoelectric signals can be obtained even through scar tissue. However, care must be taken to ensure that the scar tissue does not break down either under the weight of the prosthesis or from pressure under the electrodes (Low profile electrodes are supplied with the Utah Arm for use in such cases.)

(iv) SIZE AND STRENGTH OF AN INDIVIDUAL: The Myoelectric arm has been fitted to patients as young as 13 years old and may be suitable for some even younger, but the forearm length cannot be shortened beyond 8 inches (20.32 cm). The finished weight of a Utah Arm is about three to four pounds, depending on the terminal device and wrist used, and the individual should be capable of supporting this amount of weight.

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Neuromas and phantom limb pain may be complications in some patients and should be carefully evaluated if they are exacerbated with the use of the prosthesis. Higher level amputees, e.g., shoulder disarticulation, may experience muscle cramping in the trapezius due to weight or suspension forces. Muscle strengthening with exercise and/or redesign of the socket may make the weight more tolerable. The questions that arise in the evaluation of a patient often cannot be answered by physical examination alone. For some patients, a trial fitting is the only way to assess questions such as the comfort of wearing a prosthesis day-to day, or the motivation to utilize the prosthesis regularly. In short, the trial fitting can be used to verify the tendency of a patient for the Utah Arm or for a particular TD. Also, if the patient proves to be inappropriate for the first components tried, other components may be substituted, usually using the original trial socket with several different components. (V) THE TRIAL FITTING AS AN EVALUATION DEVICE: Trial fittings typically involve the fitting of a temporary socket, which should be substantial enough to be used for several months. (We presently fabricate above-elbow temporary sockets with vacuum-formed plastic like Surlyn, with a reinforcing outer layer of fiberglass casting tape.) The trial period also includes the initial training period and should be continued long enough to allow the patient to adapt to using a prosthesis in daily life, which in our experience suggests a 1- to 6- month trial period.

(vi) FUNDING DEVICES A candidate for a myoelectric prosthesis must also have adequate funding to pay several times the cost of a body-powered prosthesis. Experience shows, however, that health insurance, or other third-party payers, will fund a myoelectric prosthesis if properly prescribed and justified to the insurer. We recommend that if the patient's policy includes prosthetic benefits, the physician's prescription and a detailed letter of justification be submitted to the insurer, along with the estimate for the costs of the prosthesis. 50

We have found that insurers appreciate the "success guaranteed" nature of the trial fitting process. A fitting center or clinic can perform such a trial fitting (using rental of "spare" components) and apply most of the trial fitting costs towards the purchase, even if the trial indicates a different prosthesis is necessary.

COMPARISON BETWEEN CONVENTIONAL PROSTHETIC ARM AND MYOELECTRIC ARM:

(i)

The low grasping capabilities, because current prosthetic hands have no more than two active DoFs (and act like a simple gripper).

(ii)

The noncosmetic and unnatural appearance of the grasping movement resulting from the low number of DoFs. On the other hand, cosmetic devices have no active functionality and can be used only as a passive support.

(iii)

The lack of sensory information given to the user. , ere is no feedback except visual from the outside, so the user has to judge by sight when to stop moving the hand.

(iv)

The lack of a “natural,” intuitive, nonfatiguing command interface, to enable practical long-term use of a multifunctional prosthetic hand.

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FUTURE ENHANCEMENT: The future of myoelectric arm is the High Speed sensor hand of Otto Bock health care, which adjusts to the grip automatically and is the claimed to be the first to open and close quickly to allow patients to catch and throw a ball. DARPA (Defense Advanced Research Projects Agency) is seeking to advance the bionic arm extensively to the point that a person with a bionic arm can play musical instruments and type on a computer. They have proposed a program called Prosthesis 2007. By the year 2007, this program plans to have a working bionic arm that functions like a real human arm. It will have a full range of motion like the human arm. Fingers, hands, arms, shoulders will all move more naturally. They plan to develop something that can pick up neural signals directly from the nerves without grafting the nerves onto the chest muscles and then taking the EMG readings. Instead a device will read neural signals. The arm translates it and then would send out directions to the arm. This would improve the response time. As a result the bionic arm would operate more naturally. Here are a few dramatic improvements they believe the current bionic. It needs to improve upon: neural sensing •

control systems

•

power storage & distribution

•

neural control

•

sensory feedback

•

transmission design

•

signal processing

•

information science

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They plan to have a working bionic arm that function like the human arm control directly by the human brain directly with the neural sensory from the brain by the year 2007. Then for the next two years after, they plan to perfect their bionic arm prototype. After fours from today, a bionic will hopefully be out on the market for amputees. These are the goals of DARPA.

CONCLUSION: Technology never puts a full stop. It is always finding new ways to improve the existing devices. In order to make the life of people with a malfunctioning limb or without a limb comfortable to certain extent, the myoelectric arm was developed from earlier prosthetic devices. Hence it is totally justified to conclude that the myoelectric arm have surely made the life of amputee easier and convenient.

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BIBILIOGRAPHY: (1)

Rehabilitation

engineering,

science

and

technology-

Charles.J.Robinson. (2)

The biomedical engineering handbook- Second edition

(3)

Reader’s digest February 2006

(4)

Website www.bionicarm.com

(5)

Website www.neuroscience.com

(6)

Website

www.google.co.in/myoelectric

arm/

transcarpal

myoelectric prosthesis (7)

Website www.google.co.in/ scholars/ myoelectric arm.

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