Robotic Prosthesis Control

Robotic prosthesis control is a method for controlling a prosthesis in such a way that the controlled robotic prosthesis restores a biologically accurate gait to a person with a loss of limb. This is special branch of control that has an emphasis on the interaction between humans and robotics.

Impedance of control is an approach used to control the dynamic interactions between the environment and a manipulator. This works by treating the environment as an admittance and the manipulator as the impedance. 

The relationship this imposes for robotic prosthesis the relationship in between force production in response to the motion imposed by the environment.

This translates into the torque required at each joint during a single stride, represented as a series of passive impedance functions piece wise connected over a gait cycle. Impedance control doesn’t regulate force or position independently, instead it regulates the relationship between force and position and velocity.

Electromyography (EMG) is a technique used for evaluating and recording the electrical activity produced by skeletal muscles. 

Advanced pattern recognition algorithms can take these recordings and decode the unique EMG signal patterns generated by muscles during specific movements. The patterns can be used to determine the intent of the user and provide control for a prosthetic limb. 

For lower limb robotic prosthesis it is important to be able to determine if the user wants to walk on level ground, up a slope, or up stairs. Currently this is where myoelectric control comes intro play.

The speed-adaption mechanism is a mechanism used to determine the required torque from the joints at different moving speeds. 

During the stance phase it has been seen that quasistiffness, which is the derivative of the torque angle relationship with respect the angle, changes constantly as a function of walking speed. This means that over the stance phase, depending on the speed the subject is moving, there is a derivable torque angle relationship that can be used to control a lower limb prosthesis. 

Commercial solutions exploit superficial EMG signals to control the prosthesis. Furthermore, researchers are investigating alternative solutions that exploit different biological sources:

  • implanted electrodes (neural, intramuscular and epymisial electrodes) to record neural or muscle activity;
  • pressure sensor matrices to detect force changes during muscle contraction;
  • the myokinetic approach to measure muscle deformation.

Myokinetic control represents an alternative to standard myoelectric control. It aims at measuring muscle deformation during contraction instead of muscle electrical activity.

A novel approach recently emerged in 2017 which is based on sensing the magnetic field of permanent magnets directly implanted into residual muscles. Localizing the position of the magnet is equivalent to measuring the contraction/elongation of the muscle it is implanted in as the magnet moves with it.

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