Page 58 - Handbook of Biomechatronics

52 Reva E. Johnson and Jonathon W. Sensinger

teeth that are parallel to the axis of rotation, enabling them to run more
smoothly and quietly, but at the cost of introducing thrust and decreasing
efficiency. Bevel and worm gears are sometimes used in biomechatronic
applications, and hypoid gears, in which the shaft axes do not intersect, have
recently been considered by some groups.
Planetary gears are similar to spur gears, but use a set of three or more
planets that revolve around a central sun gear (Fig. 11B). They are also in
contact with a fixed annulus. Compared with spur gears, planetary gears
have a higher strength-to-weight ratio, and multiple stages can be stacked
within the same annulus. Some groups have used friction planetary gears,
in which none of the components are toothed. Friction planetary gears have
great potential as they do not have static friction or backlash, but they require
pretensioning that reduces the lifespan of the components. They have been
considered for use in some biomechatronic actuators (e.g., Sup et al., 2008).
Many high-torque biomechatronic actuators use harmonic drives, in
which an elliptical input cam deforms a flexible wave generator, which is
in contact with a rigid annulus (Fig. 11C). The wave generator has two less
teeth than the rigid annulus, such that for every cycle of rotation of the input,
the wave generator shifts two teeth with respect to the annulus. Thus, very
high gear ratios may be achieved in a compact package. Examples include
the LTI Boston Elbow and Sensinger and Weir (2008). Harmonic drives
are often favored in robotics because they do not have backlash. However,
they have substantial inertia, since they require a large, heavy, elliptical cam
on the input side of the gear ratio (which is then reflected by the square of the
large gear ratio). They also introduce torque ripple, due to the elliptical
nature of the input cam.
Cycloid drives (Sensinger, 2010b) are topologically equivalent to har-
monic drives, but use an offset input rather than an elliptical input
(Sensinger, 2013)(Fig. 11D). They are less common in biomechatronic
applications, although easier to manufacture (Lenzi et al., 2016). Cycloid
drives have less reflected inertia than harmonic drives, and better efficiency
(since they can operate on rolling, vs sliding contact). However, they have
backlash, and their gear ratio fluctuates depending on the position of the
input shaft (Sensinger and Lipsey, 2012) (Of historical note, the involute
tooth profile used in most modern gear teeth was invented to achieve a con-
stant gear ratio, in contrast to the cycloidal tooth profile previously used).

3.2.3 Other Transmissions
Some biomechatronic actuators use differential gear transmissions, including
wolfram transmissions, Ikona transmissions, and differential cycloids.