Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 1 Motor and Motion Control Systems 29
mutation. The stator is formed as an ironless sleeve of copper coils
bonded together in polymer resin and fiberglass to form a rigid structure
similar to cup-type rotors. However, it is fastened inside the steel laminations
within the motor housing.
This method of construction permits a range of values for starting current
and specific speed (rpm/V) depending on wire gauge and the number
of turns. Various terminal resistances can be obtained, permitting the
user to select the optimum motor for a specific application. The Halleffect
sensors and a small magnet disk that is magnetized widthwise are
mounted on a disk-shaped partition within the motor housing.
Position Sensing in Brushless Motors
Both magnetic sensors and resolvers can sense rotor position in brushless
motors. The diagram in Figure 1-25 shows how three magnetic sensors
can sense rotor position in a three-phase electronically commutated
brushless DC motor. In this example the magnetic sensors are located
inside the end-bell of the motor. This inexpensive version is adequate for
simple controls.
In the alternate design shown in Figure 1-26, a resolver on the end cap
of the motor is used to sense rotor position when greater positioning
accuracy is required. The high-resolution signals from the resolver can
Figure 1-25 A magnetic sensor
as a rotor position indicator: stationary
brushless motor winding
(1), permanent-magnet motor
rotor (2), three-phase electronically
commutated field (3), three
magnetic sensors (4), and the
electronic circuit board (5).