Page 61 - Handbook of Biomechatronics

Actuator Technologies 55

4 PURPOSES OF BIOMECHATRONIC ACTUATORS
4.1 Biological Function Replacement

One purpose for biomechatronic systems is to replace a missing function or
component of the body. There are high expectations for these systems,
because the body’s intact actuators—muscles—are strong, lightweight, flex-
ible, and closely integrated with the neural control system. There is not yet
any artificial actuator that approaches the capabilities of natural muscle. Fur-
thermore, the body’s natural communication systems are missing, and rec-
ognition of the user’s movement intentions remains a huge challenge.
Historically, the actuators used to drive prostheses were body-powered,
meaning the user acts as the motor and inputs power to a transmission. For
upper-limb amputees, body-powered prostheses typically feature a Bowden
cable transmission (Weir and Sensinger, 2009). For lower-limb amputees,
passive spring-based prostheses offer energy storage and return, while
linkage-based prostheses such as polycentric knees and multi-axis feet offer
stability over uneven terrain. An advantage of body-powered actuators is
that because the user acts as the motor and transmits force through a mechan-
ical transmission, they receive direct sensory feedback—a phenomenon
called extended physiological proprioception (Doubler and Childress,
1984). Because of this direct sensory feedback, low cost, and high durability,
body-powered prostheses remain a commonly used type of prosthesis.
For powered upper-limb prostheses, size and weight are especially crit-
ical for the user’s comfort (Biddiss et al., 2007). Most commercially available
prosthetic hands are actuated by DC motors with geared transmissions such
as worm gears and lead screws (Belter et al., 2013). To decrease weight and
size further, researchers are designing custom exterior rotor motors, har-
monic drives, and cycloid drives (Lenzi et al., 2016). There are also research
efforts focusing on compliant and underactuated devices.
The main advantage of powered lower-limb prostheses is that providing
net positive power enables more functionality, such as sit-to-stand move-
ments and stair climbing. The actuators of lower-limb prostheses must sus-
tain body-weight loading, and typically feature conventional electric motors
with gear train transmissions. For reviews on the actuators of lower-limb
prostheses, see Pieringer et al. (2017) and Windrich et al. (2016).
Other applications of biological function replacement are less focused on
large-scale movements but still include actuator technologies. For example,
biomechatronic devices are being developed to replace organs or tissues,
such as heart valves.