Page 29 - Electric Drives and Electromechanical Systems

Chapter 1 Electromechanical systems 21

underactuated design, in an attempt to replicate the complexities of the human hand
with its 20 degrees of freedom.
The location and method of transmission of power is crucial to the successful
operation of any end effector, in particular the end effector size should be compact and
consistent with the size of the manipulator. Both fully and under-actuated dexterous
artificial hands have been developed using electric, pneumatic or hydraulic actuators.
The use of electrically powered actuators has, however, been the most widely used, due
to both its convenience and its simplicity compared to the other approaches. The use of
electrically powered actuator systems ensures that the joint has good stiffness and
bandwidth. One drawback with this approach is the relatively low power to weight/
volume ratio which can lead to a bulky solution: however, the developments in magnetic
materials and advanced motor design have (and will continue to) reduced this problem.
In many designs the actuators are mounted outside the hand with power transmission
being achieved by tendons. Pneumatic actuators exhibit relatively low actuation band-
width and stiffness and consequently, continuous control is complex. Actuation solu-
tions developed using pneumatics (if the pump and distribution system are ignored)
offer low weight and compact actuators that provide considerable force. Hydraulic ac-
tuators can be classified somewhere in between pneumatics and electrically powered
actuators. With hydraulics the system stiffness is good due to the low compressibility of
the fluid. While pneumatic actuators can be used with gas pressures up to 5e10 MPa,
hydraulic actuators will work with up to 300 MPa. One approach that is being considered
at present is the development of artificial muscles; Klute et al. (2002) provide a detailed
overview of the biomechanics approach to aspects of muscles and joint actuation. In
addition, the paper presents details of a range of muscle designs, including those based
on pneumatic design which can provide 2000 N of force. This force equates to that
provided by the human’s triceps. The design consists of an inflatable bladder sheathed
double helical weave so that the actuator contracts lengthwise when it expands radially.
Other approaches to the design for artificial muscles have been based on technologies
including shape-memory alloy, pneumatics, electro-resistive gels and dielectric elasto-
mers which are discussed in Chapter 9, Related motors and actuators.
When considering conventional technologies, the resultant design may be bulky and
therefore the actuators must be placed somewhere behind the wrist to reduce system
inertia. In these systems power is always transmitted to the fingers by using tendons or
cables. Tendon transmission systems provide a low inertia and low friction approach for
low power systems. As the force transmitted increases considerable problems can be
experienced with cable wear, friction and side loads in the pulleys. One of the main
difficulties in controlling tendon systems is the that force is unidirectional - a tendon
cannot work in compression. The alternative approach to joint actuation is to use a solid
link which has a bi-directional force characteristic, thus it can both push and pull a
finger segment. The use of a solid link reduced the number of connections to an indi-
vidual finger segment. The disadvantage of this approach is a slower non-linear dynamic
response, and that ball screw or crank arrangement is required close to the point of