Page 59 - Handbook of Biomechatronics

Actuator Technologies 53

However, the differential typology typically comes at the cost of substantial
reduction in efficiency (Sensinger, 2013).
For biomechatronic actuators that do not need large transmission ratios,
but that cannot afford backlash or stiction, capstan transmissions are often the
design of choice (e.g., Fig. 11E and Brown et al., 2012). These designs cou-
ple the motion of one pulley to that of another through pinned cabling.
They can produce high forces even in the absence of pretension between
the cables because the cables are pinned. They can only be used for small
gear ratios and for limited range of motion.
Some biomechatronic actuators use chains or belts and pulleys for their
transmission (e.g., Lawson et al., 2014), although for most designs these are
not compact enough and introduce substantial vibration at higher speeds.
Some biomechatronic actuators use linkages themselves—particularly in
parallel configurations, as a form of transmissions. Manipulandums, com-
monly used in assessing human motor control, are one such example.

3.2.4 Variable and Low Impedance
Variable and low-impedance actuators are increasingly important for both
industrial and research applications. They enable safer interaction with
humans and a more robust interaction with unknown environments.
The mechanical impedance of an object refers to the ratio of force an
object exerts relative to the frequency-dependent displacement of the
object. Impedance is the generalization of related concepts including stiff-
ness, viscosity, and inertia. Ratios that are constant across frequencies
(and thus only depend on displacement) can be expressed solely as stiffness.
Ratios that rise at 20dB/decade can be expressed solely as viscosity. Ratios
that rise at 40dB/decade can be expressed solely as inertias. Objects that have
multiple springs and inertias will typically “look” like a spring, or an inertia,
in various ranges of the frequency spectrum (Fig. 4).
Motors typically have a large output impedance that looks like an inertia
at high frequencies. This large output impedance is often caused by the
reflected inertia generated by a transmission, and leads to high forces during
impacts (high frequencies), which in turn can cause damage either to the
mechanism or the person. However, if a spring or damper is placed at the
end of the actuator, it acts as the “weakest link,” saturating the impedance
seen at the output.
The intentional introduction of compliance within an otherwise-rigid
electromechanical actuator has conventionally been avoided, because it
reduces high-frequency/high-magnitude force generation and creates the
potential for sensor/actuator de-colocation in some control strategies.