Brush servo motors are well developed an
are inexpensive to produce.
Smooth rotation at low speeds:
Brush motors are available which are
specially designed for low speed smoothness with a large number of commutator
segments. Brushed motors are the smoothest of the three discussed motor
technologies.
Low cost drive:
A DC brush drive can be made very
economically since only a single bridge circuit is required.
No power used at standstill:
With no static loads on the motor, no
current is required to hold position.
High peak torque available:
In intermittent duty applications,
particularly when positioning mainly-inertial loads, the motor can be
overdriven beyond its continuous rating.
Flat speed-torque curve:
Gives optimum performance with easily
generated linear acceleration ramps.
Wide variety of types available:
Brush motors are produced in many styles
including very low inertia types for high dynamic applications.
High speed attainable:
Brush servos are typically good for
speeds up to 5000 rpm.
Brush servo Disadvantages:
Brush Maintenance:
Not necessarily a problem if the motor
is easily accessible, but a nuisance if the motor is not. Brushes also create
dust as they wear; therefore limiting their use in clean rooms, and other
environments where brush dust is not acceptable.
Problems in hazardous environments or a vacuum
Arcing at the brushes is fundamental to
their operation.
Commutator limitations:
Arduous duty cycles promote wear, and
the mechanical commutation limits top speed. Very short repetitive moves, less
than one revolution of the motor, may wear part of the commutator.
Poor thermal performance:
All the heat is generated in the rotor,
from which the thermal path to the outer casing is very inefficient.
Can be demagnetized:
Excessive current can result in partial
demagnetization of the motor.
Increased Installed cost:
The installed cost of a servo system is
higher than that of a stepper due to the requirement for feedback components.
Brushless servo benefits:
Maintenance free:
The lack of a commutator and brush
system eliminates the need for periodic maintenance.
Good thermal performance:
All the heat is generated in the stator
where it can be efficiently coupled to the outside casing.
Very high speeds possible:
There is no mechanical commutator to
impose a speed limit, small motors are typically rated at up to 12,000 rpm.
Virtually no environment constraints:
Due to the absence of brush gear, a
brushless servo can be used in almost any environment. For high temperature
operation, the use of a resolver feedback avoids any electronics buried in the
motor.
Brushless servo drawbacks:
Higher motor cost:
This is largely due to the use of rare
earth magnets
Drive more complex and costly:
Six state, or trapezoidal drives, are
not much more expensive than DC brush drives, but the higher performance sine
wave drive can cost several times that of the DC brush drive.
