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A person wearing an artificial hand will want to control the movement of the fingers and thumb or generate grip force. They will need some device that could be an electronic, electromechanical or mechanical interface. The person may simply desire to open and close the hand. An electronic signal generated from some action by the person can be sent to the actuator(s) to cause the desired effect. A simple open-loop controller can be made to achieve such an action (Figure 4.1). For the position of the fingers to be controlled precisely requires a detailed mathematical model of all the parts of the system. The model parameters will need to be determined through measurements made on the components. Only then can precise control be achieved. However, most systems will have variable parameters such as friction and backlash in the finger mechanisms. A much better idea is to use negative feedback shown in Figure 4.2. The advantages of using a feedback control system have been extensively researched. For the artificial hand, there are the advantages of improved performance in terms of making precise finger movements and automatic grips. The person controlling an artificial hand can control posture and force generated in an open-loop way, but this would require continuous monitoring of the hand and have a cognitive burden. Adopting a hierarchical control structure frees the person of having to think all the time about state of the hand (Figure 4.3). The lower levels of control are automatic electronic systems, so that the person simply instructs the controller to instigate grip of an object and monitor any slipping that may occur. People who use an artificial hand would like to have some sensory feedback, i.e., be able to feel some property of an object such as surface texture. Here, sensors can estimate these physical properties and through the skin surface stimulate neurons to elicit a sense of feeling, i.e., haptic feedback.

Chapter Contents:

  • 4.1 Finger position
  • 4.2 Object slip
  • 4.3 Reference inputs
  • 4.4 Artificial prehension
  • 4.5 Summary
  • References

Inspec keywords: open loop systems; prosthetics; motion control; haptic interfaces; feedback

Other keywords: artificial hand; sensors; thumb; electronic signal; control posture; feedback control system; person; haptic feedback; hierarchical control structure; continuous monitoring; precise finger movements; fingers; automatic grips; open loop controller; surface texture; grip force; mathematical model; negative feedback; electromechanical interface; precise control; finger mechanisms; sensory feedback; movement control; electronic interface; skin surface stimulate neurons; automatic electronic systems

Subjects: Prosthetic and orthotic control systems; Interactive-input devices; Spatial variables control

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