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Actuator Technologies 51

3.2.2 Rotary Transmissions
Rotary transmissions are typically concentric with the motor, although in
some designs they are not. Rotary transmissions tend to be more compact
than linear transmissions, but often have greater reflected inertia.
The simplest form of a rotary transmission is simply a train of spur gears
linked to each other (Fig. 11A). Helical gears are similar but do not have

Fig. 11 Rotary transmissions: (A) spur gear train, driven by a DC motor, (B) planetary
gear train, (C) diagram showing the basic operation of harmonic drives, (D) cycloid drive,
and (E) Capstan drive on a Phantom haptic device from Geomagic, Inc. (formerly
SensAble Technologies Corp.). ((A) www.adafruit.com; (C) Reproduced with permission
from Tjahjowidodo, T., Al-Bender, F., Van Brussel, H., 2013. Theoretical modelling and exper-
imental identification of nonlinear torsional behaviour in harmonic drives. Mechatronics
23 (5), 497–504; (D) Illustration by Petteri Aimonen; (E) Reproduced with permission from
Baser, O., Ilhan Konukseven, E., 2010. Theoretical and experimental determination of cap-
stan drive slip error. Mech. Mach. Theory 45 (6), 815–827.)