Page 50 - Handbook of Biomechatronics

44 Reva E. Johnson and Jonathon W. Sensinger

and the permanent magnets are located on the stator. Compared with many
other motors, these motors have a high torque to weight ratio, because the
field strength of permanent magnets is very high. The current that can be
delivered to the coils is limited by sparking across the brushes; this in turn
limits the torque that the motors can produce. Some brushed motors have
hollow rotors (no iron core), termed coreless motors. These motors have
reduced inertia (enabling greater acceleration), but can also produce reduced
torque.
In contrast, brushless permanent-magnet motors use sensors such as
Hall sensors to determine when to reverse the polarity of current across
the coil (Fig. 5D). These actuators require more complicated and delicate
circuitry that handles commutation of the phases, but can produce greater
torque as there are no brushes. They are increasingly being preferred over
brushed motors in biomechatronic applications for this reason. There are
two variants—internal-rotor motors (which are more common) and
exterior-rotor motors (which are common in applications like remote-
control quad copters), and are increasingly being used in biomechatronic
applications (Lenzietal., 2016) for their superior torque-generating capa-
bilities (Sensinger et al., 2011).
It is worth noting that the term “mechatronic” was coined (and
trademarked) to describe the innovative concept of decoupling sensing from
actuation in brushed permanent-magnet motors, resulting in a brushless
motor.

3.1.2 Fluidic Actuators
Pneumatic artificial muscles convert pneumatic energy (pressurized gas) to
mechanical motion and force. The most common pneumatic artificial mus-
cles are called McKibben muscles, which are composed of tubes surrounded
by woven threads (Fig. 6). When inflated with pressurized air, the tubes
expand radially and contract axially, generating tensile forces. McKibben
muscles were originally designed to mimic natural muscle function for pros-
thetic devices (Chou and Hannaford, 1994). For a review, see Daerden and
Lefeber (2002).
A major advantage of McKibben muscles is that they have inherently
variable compliance, depending on the pressure of the gas. They are light-
weight and unaffected by magnetic fields, which makes them attractive
choices for imaging applications (e.g., inside MRI machines).