Page 232 - Electric Drives and Electromechanical Systems

228 Electric Drives and Electromechanical Systems

in Chapters 5e8. As discussed by Webb (2001) a considerable amount of work is being
undertaken at the boundary of robotics and biology. This has resulted in the develop-
ment of a new field of robotics e the soft robot. The definition of a soft robot a systems
that are capable of autonomous behaviour, and that are primarily composed of materials
with a Young’s modulus that is in the range of soft biological materials (Rus and Tolley,
2015). This definition effectively prevents the use of conventional motors and trans-
mission systems in the soft robot field, and the development of a wide range of un-
conventional actuators, including shape memory alloys and fluidic elastomer actuators
In addition some actuators are based on chemical or similar properties such as elec-
troactive polymers (Boyraz et al., 2018).
A fluidic elastic actuator can be based either on hydraulic or pneumatic technologies,
though the basic principle is the same, a flexible shell or structure converts the potential
energy, delivered by a pressurized fluid, into a mechanical force, that alters the position
of a joint. In a pneumatic actuator, the design is based on a inflatable tube surrounded
by a braided mesh. The characteristics of the actuator depend on the design of this mesh,
permitting the actuator to either expand or contact as the internal pressure is increased.
Typical pressures within this type of actuator are 350e700 KPa. An alternative is to use
a low-pressure fluid (approximately 60e80 kPa) powering an actuator manufacture from
a relatively soft rubber. As discussed in the literature, if the actuator is fabricated to
include a number of inflatable spaces of different sizes it is possible to easily obtain a
bending motion (Marchese et al., 2015).

9.1 Voice coils

Voice coils or solenoids are ideally suited for short linear (typically movements of less
than 50 mm) closed-loop servo applications and both operate on similar principles.
Typical positioning applications include direct drives on pick and place equipment,
medical equipment, and mirror tilt and focusing actuators. In addition, voice coils can
also be used in applications where precise force control is required, due to the linear
force versus current characteristics.
Voice coil actuators are direct drive, limited motion devices that utilise a permanent
magnet field and coil winding (conductor) to produce a force proportional to the current
applied to the coil. A voice coil is wound in such a way that no commutation is required,
hence a simple linear amplifier can be used to control the actuator’s position. The result
is a much simpler and more reliable system. Voice coils have several significant ad-
vantages including small size, very low electrical and mechanical time constants, and
low moving mass that allows for high accelerations, though this depends on the load
being moved.
The electromechanical conversion mechanism of a voice coil actuator is governed by
the Lorentz force principle; which states that if current-carrying conductor is placed in a
magnetic field, a force will result. The magnitude of the force is determined by the