Page 42 - Handbook of Biomechatronics

36 Reva E. Johnson and Jonathon W. Sensinger

depends on the magnitude and time duration of current. The current that
flows through the body is a ratio of applied voltage to the body’s electrical
impedance. Thus, safe levels of voltage depend on how the actuator contacts
the body—for example, dry skin has very high impedance, whereas internal
body cavities have much lower impedances (Fish and Geddes, 2009).
Acceptable levels of current and voltage have been developed by regulatory
bodies such as the International Electrotechnical Commission (B. S.
Publication, 2016). These guidelines are especially important for actuators
such as electroactive polymers (EPAs) that often require high voltages for
operation. Some actuators, such as electric motors, generate significant
amounts of heat during operation. Heat dissipation should be considered
in the design of the actuator system.

2.2 Performance
How do I make this actuator perform as well or better than the equivalent human
system?
Whether humans or actuators move in free space or interact with the
environment, there is an interplay between forces and motions. Perfor-
mance can be thought of in terms of these forces and motions: can my fingers
produce 100N of force? Can I move my arm over my head? Can I rotate my
elbow 180degrees/s? Can I give a burst of acceleration fast enough to stand
up before I fall over? Any mechanical performance metric can be thought of
as a function of these four generalized parameters (force/torque, position,
velocity, acceleration).
Many activities for which we wish to evaluate performance require com-
binations of force and motion parameters. For example, can I rotate my
elbow 180degrees/s while holding a 10N weight? Can I do that same
activity when my elbow starts from rest fully extended, and stop before
my hand collides with my upper arm at 145degrees of flexion? The capa-
bilities of actuators also often depend on combinations of those four param-
eters. For example, the maximum torque that an electric motor can produce
is a function of its speed: it cannot produce high torques at high speeds, and it
cannot produce high speeds at high torques (Sensinger et al., 2011). Other
actuators, such as shape-memory alloys (SMAs) and human muscles, have
maximum forces that are dependent on their position or percent contrac-
tion. It accordingly makes sense to think of both the activities we wish to
perform, and the capabilities of actuators, in terms of force as a function
of motion (position, velocity, and acceleration).