Page 618 - Mechatronics with Experiments

604 MECHATRONICS

Executive functions Power supply

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Trajectory generator Filter Amp Actuator
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Controller Drive

Sensor
FIGURE 8.1: Functional blocks of a closed loop motion control system: actuator, sensor,
amplifier, controller filters.

3. a housing (with end plates for rotary motors),
4. two bearings, one for each end, to support the rotor in the housing, including some
washers to allow axial play between the shaft and the housing.

In addition, brush-type motors have commutator and brush assembly to direct current into
the proper coil segment as a function of rotor position. Brushless motors have some type
of rotor position sensor for electronic commutation of the current (i.e., Hall effect sensors
or incremental encoders). Commutation means the distribution of current into appropriate
coils as a function of rotor position.
Traditionally AC induction motors have been used in constant speed applications,
whereas DC motors have been used in variable speed applications. With the advances in
solid-state power electronics and digital signal processors (DSP), an AC motor can be
controlled in such a way that it behaves like a DC motor. One way of accomplishing this is
the “field oriented vector control” algorithm used in the drive for current commutation.
In the following discussion, a magnetic pole refers to a north (N) or south (S) pole, a
magnetic pole pair refers to a N and a S pole. When we refer to a two pole motor, it means
it has one N and one S pole. Likewise, a four pole motor has two N and two S poles.
An electric motor is a power conversion device. It converts electrical power to mechan-
ical power. Input to the motor is in the form of voltage and current, and the output is
mechanical torque and speed. The key physical phenomenon in this conversion process is
different for different motors.
1. In the case of DC motors, there are two magnetic fields. In brush-type DC motors,
one of the magnetic fields is due to the current through the armature winding on the
rotor, and the other magnetic field is due to the permanent magnets in the stator (or
due to field excitation of the stator winding if electromagnets are used instead of
permanent magnets). In the case of brushless DC motors, the roles of rotor and stator
are swapped.
2. In the case of AC motors, the first magnetic field is setup by the excitation current
on the stator. This magnetic field in turn induces a voltage in the rotor conductors by
Faraday’s induction principle. The induced voltage at the rotor conductors results in
current which in turn sets up its own magnetic field, which is the second magnetic
field. The torque is produced by the interaction of the two magnetic fields. In the case
of a DC motor and AC induction motor (with field oriented vector control), the two
magnetic fields are always maintained at a 90 degree angle in order to maximize the