Page 49 - Handbook of Biomechatronics

Actuator Technologies 43

Fig. 5 Electromagnetic actuators: (A) illustration of solenoid coil, and photograph of
packaged solenoid; (B) illustration of a stepper motor with four electromagnets, and
photograph of stepper motor; (C) illustration of brushed DC motor, and photograph
of DC motor rotor; and (D) illustration and photo of a brushless DC motor. ((A) and
(B) Reproduced with permission from Adafruit (www.adafruit.com); (C) From Wikipedia;
(D) From Wikimedia Commons.)

motors can spin continuously. Electric motors are ubiquitous in our lives,
and there are cascading levels through which they may be grouped. At
the highest level, these include direct-current (DC) motors and alternating
current (AC) motors. There are a variety of subsets for each (e.g., for AC
there are single phase vs polyphaser vs universal, induction vs synchronous;
squirrel cage vs wound rotor; for DC, there are permanent magnet, series
wound, shunt wound, and compound wound) (Alciatore and Histand,
2003). The majority of biomechatronic actuators use permanent magnet
DC motors, and there are three subsets worth exploring.
Stepper motors are a type of permanent magnet DC motor that can
rotate in both directions, move in precise angular increments, sustain a hold-
ing torque at zero speed, and be controlled with digital circuits (Alciatore
and Histand, 2003)(Fig. 5B). However, they typically cannot produce high
torques, high speeds, or high frequencies, and are accordingly only used for a
subset of biomechatronic actuators.
Brushed permanent-magnet motors use electrical brushes to switch the
direction of current in the coils (Fig. 5C). The coils are located on the rotor,