Automating Manufacturing Systems - Process Control and ...

continuous actuators - 24.27
a)
(ans.
Motor Type
Cost
Torque
Speed
Applications
AC/Inuction
DC Brushed
DC Brushless
Stepper
Shunt
Series
low
low/med
high
low/med
med
med
med
med
med
low
med
high
limited
variable
variable
low
varies
varies
consumer applications/large power
short life
high precision
positioning
large break away torques
b) Deadband correction allows the motor to break free of the statis friction. Once moving freely
the torque required to ‘stick’ the motor is determined by the lower kinetic friction. Generally this
means that the motor can move slightly slower than the static friction minimum speed, but not the
kinetic friction minimum speed.
c) Calibration is a process where instrumentation outputs are related to inputs. These results are
then used later to relate measurement equipment outputs with actual phenomenon. For example,
in the laboratory, tachometers are calibrated by turning them at a steady speed. The speed is measured
with a strobe tachometer and the voltage output is also recorded. These are then used to
make a graph relating voltage and speed. Later the strobe tachometer is not used and the voltage
output of the tach. is used to calculate the speed.
24.8 ASSIGNMENT PROBLEMS
1. A stepper motor is to be used to actuate one joint of a robot arm in a light duty pick and place
application. The step angle of the motor is 10 degrees. For each pulse received from the pulse
train source the motor rotates through a distance of one step angle.
a) What is the resolution of the stepper motor?
b) Relate this value to the definitions of control resolution, spatial resolution, and
accuracy, as discussed in class.
c) For the stepper motor, a pulse train is to be generated by a motion controller.
How many pulses are required to rotate the motor through three complete revolutions?
If it is desired to rotate the motor at a speed of 25 rev/min, what pulse
rate must be generated by the robot controller?
2. Describe the voltage ripple that would occur when using a permanent magnet DC motor as a
tachometer. Hint: consider the use of the commutator to switch the polarity of the coil.
3. Compare the advantages/disadvantages of DC permanent magnet motors and AC induction
motors.
www.PAControl.com