Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

22 Chapter 1 Motor and Motion Control Systems
Figure 1-17 Cutaway view of a
fractional horsepower permanent-magnet
DC servomotor.
than earlier generation comparably rated DC motors with alnico (aluminum–nickel–cobalt
or AlNiCo) magnets. Moreover, integrated circuitry
and microprocessors have increased the reliability and costeffectiveness
of digital motion controllers and motor drivers or
amplifiers while permitting them to be packaged in smaller and lighter
cases, thus reducing the size and weight of complete, integrated motioncontrol
systems.
Brush-Type PM DC Servomotors
The design feature that distinguishes the brush-type PM DC servomotor, as
shown in Figure 1-17, from other brush-type DC motors is the use of a permanent-magnet
field to replace the wound field. As previously stated, this
eliminates both the need for separate field excitation and the electrical
losses that typically occur in field windings.
Permanent-magnet DC motors, like all other mechanically commutated
DC motors, are energized through brushes and a multisegment commutator.
While all DC motors operate on the same principles, only PM DC motors
have the linear speed–torque curves shown in Figure 1-18, making them
ideal for closed-loop and variable-speed servomotor applications. These
linear characteristics conveniently describe the full range of motor perform-