Page 48 - Handbook of Biomechatronics

42 Reva E. Johnson and Jonathon W. Sensinger

their effort-flow relationships into self-induction machines, slip-driven
machines, linear effort-controlled machines, linear flow-controlled
machines, and concave effort-flow machines (muscle-like machines)
(Hannaford and Winters, 1990). Hollerbach et al. categorized actuators into
macro-motion (electromagnetic, hydraulic, and pneumatic), micro-motion
(piezoelectric and magnetostrictive), and muscle (nature’s easily scalable
actuator) (Hollerbach et al., 1991). In our discussion of actuators, we sepa-
rate the technology used for the core actuator technologies (the motors) and
that used for the structure of how the actuator interacts with the overall sys-
tem (the transmissions). An example of an emerging core technology, or
motor, is that of EPAs: materials that convert electrical energy to mechanical
deformation. An example of novel transmission design is that of variable
impedance strategies—many of which use traditional electric motors. In
an attempt to minimize confusion and cleanly categorize technologies,
we separate the motors and transmissions in the further sections.
3.1 Motors

The motor is the subsystem that converts one type of energy (electrical, flu-
idic, thermal, and chemical) to mechanical energy. In this section, we cat-
egorize motors according to the type of input energy.

3.1.1 Electromagnetic Actuators
Electromagnetic actuators take advantage of Lorentz’s force law, which
states that when a current-carrying conductor is moved in a magnetic field,
a force is produced in a direction perpendicular to the current and magnetic
field directions (Alciatore and Histand, 2003). The magnetic field may either
be produced by permanent magnets, or by another energized coil.
There are many types of electromagnetic actuators, although some are
used more than others in biomechatronic actuators. Solenoids and relays
are simple devices with a stationary iron core and coil (Fig. 5A), and a mov-
able armature core attached to the stationary core through a spring. These
are rarely used for biomechatronic actuators because they cannot produce
large forces or high frequencies. Voice coils are a similar concept that has
a stationary iron core and permanent magnet, and a movable coil. Voice coils
cannot produce large forces either, but can produce high frequencies, and
are thus used for some biomechatronic actuators such as tactors (e.g.,
Schultz et al., 2009).
Electric motors have a stationary housing, called the stator, and a part that
rotates, termed the rotor. In contrast to solenoids and voice coils, electric