9.7 CONTROLLING CAM SPEED—MOTORS
type of driver device is needed to provide the input torque to the camshaft. A
motor or engine* is the logical choice. Motors come in a wide variety of types.
The most common energy source for a motor is electricity, but compressed air
and pressurized hydraulic fluid are also used to power air and hydraulic
motors. Gasoline or diesel engines are another possibility.
conventionally driven industrial machine will use open-loop electric motors,
often 3-phase AC, shunt-wound DC, or permanent magnetDC motors that allow some
degree of speed control via circuitry that converts the readily available AC
power from the line to DC and controls current to the motor for speed control.
The torque-speed characteristic of these motors varies depending on the type of
windings and field-armature connections.
designers are increasingly using servomotors to power automated assembly
equipment. Though the expense is high compared to other types of motors, the
added cost is sometimes justified on the basis of the superior control and
programming flexibility that results. The design of servo systems is well
beyond the scope of this book and requires specialized knowledge not typically
possessed by the machine designer. Servo engineers usually work as a team with
machine design engineers to accomplish this task. We will limit our discussion
to an overview of some of the differences, advantages, and disadvantages of
using servomotors in lieu of conventional power sources. First, some
definitions, terminology, and descriptions.
* The terms motor and engine are often used
interchangeably, but they do not mean the same thing. Their difference is
largely semantic, but the “purist” reserves the term
for electrical, hydraulic, and
pneumatic motors and the term
for thermodynamic devices such as steam engines and
internal combustion engines. Thus, your automobile is powered by an engine, but
its windshield wipers and window lifts are run by motors.
† Other battery types have different cell voltages.
Carbon-zinc batteries are 1.5 V/cell, alkaline batteries are 1.3 or 1.55V cell,
and nickel-cadmium batteries are 1.2 V/cell.