MYOELECTRIC ARM

DEVELOPMENT OF ORTHOTIC AND PROSTHETIC LIMBS

The word "orthosis" is derived from the Greek word "ortho," which means "to make straight." Orthoses (plural for orthosis) do "straighten" bones, but they also prevent deformities, enhance walking, assist with daily activities

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.

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

MYOELECTRIC ARM

WHY DO WE NEED A MYOELECTRIC ARM

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

EMG

Electromyography (EMG) is a medical technique for measuring muscle response to nervous stimulation.. An electromyograph detects the electrical potential generated by muscle cells when these cells contract.

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:

•The minimum EMG voltage required is 15 micro volts.

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

EMG ELECTRODE PLACEMENT

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.

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

BLOCK DIAGRAM OF MYOELECTRIC ARM

USER(EMG POTENTIAL)

SIGNAL ACQUISITION AND PROCESSING

CONTROL

ACTUATOR SENSOR

PROSTHESIS

OUTSIDE WORLD

CONSTRUCTION OF MYOELECTRIC ARM

MOTOR

STEPPER MOTOR VS DC MOTORS

THE GEAR BOX

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.

THE POTENTIOMETER

DC requires the addition of a potentiometer for position detection. Since the movement of the stepper motor is controlled by pulses, the exact position of the robotic arm can be determined by observing the number of pulses sent to the motors

GLOVES

SILICONE GLOVES

•Lifelike look and feel.

•Durability- about 4 years for myoelectric hands, depending on use and care. If used in a harsh environment.

•Flexible. Holds objects better than PVC.

•Color stability- the basic material is intrinsically colored.

•Resistant to stains like newsprint and ink.

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.

FULLY CONSTRUCTED MYOELECTRIC ARM

Interface with control system

Elbow and hand circuits.

Sensor hand