Page 27 - Electric Drives and Electromechanical Systems

Chapter 1 Electromechanical systems 19

where full strength is needed, is where the object is held in a clamp formed by the partly
flexed fingers and often a wide area of the palm. The hand conforms to the size and
shape of the object. All four fingers flex, with each finger accommodating a position so
that force can be applied, and the force applied to the object to perform a task or resist
motion. In a precision grasp, Fig. 1.9B, there is a greater control of the finger and thumb
position than in the power grasp. The precision grasp is carried out between the tip of
the thumb and that of one or more of the fingers. The object is held relatively lightly and
manipulated between the thumb and related finger or fingers.
The human hand consists of a palm, four fingers and a thumb. The internal structure
consists of nineteen major bones, twenty muscles within the hand, tendons from fore-
arm muscles, and a considerable number of ligaments. The muscles in the body of the
hand are smaller and less powerful than the forearm muscles and are used more for the
precise movements rather than the power grasps. A hand is covered with skin that
contains a wide range of sensors (e.g. temperature, tactile, vibration) and provides the
protective compliant covering.
The classification of movements of the hand in which work is involved can be placed
in two main areas: prehensile and non-prehensile. A prehensile movement is a
controlled action in which an object is held in a grasp or pinching action partly or wholly
in the working envelope of the hand, while a non-prehensile movement is one, which
may involve the whole hand, fingers, or a finger but in which no object is grasped or held.
The movement may be a pushing one such as pushing an object, or a finger-lifting action
such as playing the piano.
The dynamic specification of the human hand can be summarised as:
Typical forces in the range 285e534 N during a power grasp.
Typical forces in the range 55e133 N during a precision grasp.
o 1
Maximum joint velocity 600 s .
Maximum repetitive motion frequency, 5 Hz.
The development of dextrous hands or end effectors has been of considerable
importance to the academic robotic research community for many years, and while the
following examples are in no way exhaustive they do however present some of the
thinking that has gone into dextrous robotic systems.
A significant robotic end effector was the University of Southampton’s Whole Arm
Manipulator (Crowder, 1991). This manipulator was developed at for insertion into
a human sized rubber glove, for use in a conventional glove box. Due to this
design requirement, the manipulator has an anthropomorphic end effector with
four adaptive fingers and a prehensile thumb, Fig. 1.10. Due to size constraints the
degrees of freedom within the hand were limited to three.
The Stanford/JPL hand (sometimes termed the Salisbury hand) was designed as a
research tool in the control and design of articulated hands. In order to minimise
the weight and volume of the hand the motors are located on the forearm of the