Biomedical Engineering Reference
In-Depth Information
FIGURE 3-49
Maxon micro-drive
servo motor.
3.2.6.2 Professional Servos
Hobby servos are obviously not really suitable for biomechatronic applications where
good reliability and long life are essential. In these applications, high-quality motors such
as those made by Maxon and Faulhaber should be used. In the past, a servo system would
be assembled using a motor-gearbox combination with a separate encoder and power
electronics, but now integrated systems such as the micro-drive system are starting to
appear.
The micro-drive system, shown in Figure 3-49, consists ofa6mmBLDC motor,
magnetic encoder, and backlash-free harmonic drive gearhead. Supplying a constant 1.2 W,
the motor can turn at up to 100,000 rpm and weighs only 2.8 g.
The backlash-free gearhead has a diameter of 8 mm and consists of a 1:160 reduction
ratio harmonic drive and high-resolution 100-count incremental encoder. The positioning
unit's shaft is hollow, and the drillhole diameter is 0.65 mm.
3.2.7 AC Motors
There are many different types of alternating current motors of which the most common
are induction motors, hysteresis motors, and synchronous motors. This section outlines
the operation of induction motors because they are probably encountered most often in
the biomechatronics field.
3.2.7.1 Induction Motors
In an induction motor, the stator's magnetic field induces an alternating current into the
rotor squirrel-cage conductors, which constitute a shorted transformer secondary winding.
This induced rotor current in turn creates a magnetic field. The rotating stator magnetic
field interacts with this rotor field, which attempts to align with it. The result is rotation
of the squirrel-cage rotor. If there were no load torque, no bearing, windage, or other
losses, the rotor would rotate at the synchronous speed. However, the slip between the
rotor and the synchronous speed stator field causes the magnetic flux to cut through the
