How does it work? Who controls it?
Prosthetics is a large concept that needs to be discussed in parts.
Upper limb prosthesis:
An upper limb prosthesis that can actively move is an upper limb myoelectric prosthesis. The control system is composed of two parts: "myoelectric signal sensor" and "control circuit". Myoelectric signal: Surface electromyographic signal (sEMG) is a non-stationary weak signal that is superimposed on the skin surface by action potential sequences (Motor Unit Action Potential Trains, MUAPT) generated by motor units recruited when muscles are excited.
After the myoelectric signal is obtained through the sensor, it is processed by the control circuit, which controls the action system to make various actions. At this stage, the main action is to open and close the tiger's mouth.
The control circuit part mainly plays two roles. First, determine the intention. The electromyographic signal is unstable. Sometimes the antagonist muscles will send out higher electrical signals at the same time, sometimes the electrical signals are not sustained enough, and sometimes they will be interfered by the electromagnetic environment, etc. These all require judgment and adjustment by the control circuit. Second, determine the action. Modern myoelectric prostheses have more and more functions. With certain myoelectric signals, sometimes it is necessary to control the circuit to judge the type, duration, speed and intensity of the action, etc.
PS: Most of the upper limb prostheses currently in clinical use are myoelectric prostheses. Voice control and brain wave are not practical so far. There are also products that use mechanical switches (cable switches, touch switches) to replace myoelectric signal sensors, but they are rare and are mainly used in situations where myoelectric signals cannot be obtained. At the same time, there is currently no myoelectric prosthesis that can provide tactile feedback.
Lower limb prostheses:
The lower limb prostheses that need to be controlled are mainly artificial knee joints, which are needed by patients with thigh amputation and hip dislocation amputation. There are currently five main ways to control artificial knee joints:
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Manual control. It is rare and is generally used for patients with poor control ability. The form is a manual lock to control whether the knee joint can flex.
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Mechanical friction control. Commonly found in low-end products. Knee joint swing is controlled by mechanical friction.
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Air pressure control. The knee joint swing is controlled through the damping of the pneumatic cylinder.
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Hydraulic control. The swing of the knee joint is controlled through the damping of the hydraulic cylinder. Note that it is rare for pneumatic and hydraulic pressure to appear on the artificial knee joint at the same time.
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Electromagnetic control. Ossur®rheo knee&power knee uses magnetorheological material to electromagnetically control the swing of the knee joint.
Note that the control of smart prostheses also uses the above five control methods. For example, the ottobock cleg uses hydraulic control.
PS: Currently, except for the ossur® power knee, there is no self-powered knee joint. The other knee joints rely on the strength of the amputee’s residual limb and the strength of the spring to achieve flexion and extension in the swing phase.
Control method:
At present, most knee joints have preset and adjustable control methods, such as pneumatic and hydraulic knee joints. The flexion and extension posture of the prosthetic limb during the swing phase is controlled by adjusting the valve. Or support period control. Smart knee joint uses sensors and microcontroller to control walking posture. Commonly used sensors include angle sensors, pressure sensors, and acceleration sensors.
There have been attempts to use other control methods, but they are not currently in clinical use.
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