Biomedical Engineering Reference
In-Depth Information
FIGURE 10-41
Block diagram
showing control
structure of an active
prosthetic limb.
Finally, these hands are ideal tools for investigations into rehabilitation and assistive
technology where safety, flexibility, and compliance are essential.
10.9.4 Control of Prosthetic Arms and Hands
Control of a prosthetic limb can be described by the block diagram shown in Figure 10-41.
The input indicated in black allows the user control over basic motions required of the
limb. The feedback paths indicated in gray allow the prosthesis to autonomously fine-tune
its function based on feedback from local sensors. This functionality is more subtle than
voluntary control because it usually goes unnoticed when controlling a natural limb. For
instance, when we pick up a glass of water, we do not think about how hard we should
grasp. Instead, our nervous system automatically takes care of that for us so that we can
hold the glass without breaking or dropping it. The dashed gray arrows in Figure 10-41
indicate a path that allows users to be made aware of the sensory feedback provided by
the hand.
Voluntary control inputs from the body can include outputs from devices that measure
gross body motions captured by a harness or cineplasty. These devices include switches
and potentiometers, among others. An alternative source of inputs, which is becoming
increasingly popular, is myoelectric signals. Simulated sensory outputs can be provided
by force (haptic) feedback or through vibrotactile transducers.
10.9.4.1 Microswitches, Force Sensors, and Linear Potentiometers
Harnesses such as those described earlier in this chapter can be designed to operate
switches, to extend linear potentiometers, or to apply a force to a strain gauge. The outputs
of these transducers are typically read into microcontrollers and then are used to control
the servo systems, which actuate the prosthesis. In Figure 10-42, a servo pro force sensor
from Motion Control is used to provide commands to control the position of an elbow joint.
Relatively simple transducers are often still used to control state-of-the-art prostheses
because they provide a reliable interface that requires little training to operate. For example,
the Luke arm can be controlled by the toes using joystick-like controllers embedded into
special shoes.
