Operation Of Closed-Loop Systems - Galil Motion Control DMC-1600 Series User Manual

X VELOCITY
Y VELOCITY
X POSITION
Y POSITION
Figure 10.3 - Velocity and Position Profiles

Operation of Closed-Loop Systems

To understand the operation of a servo system, we may compare it to a familiar closed-loop
operation, adjusting the water temperature in the shower. One control objective is to keep the
temperature at a comfortable level, say 90 degrees F. To achieve that, our skin serves as a
temperature sensor and reports to the brain (controller). The brain compares the actual
temperature, which is called the feedback signal, with the desired level of 90 degrees F. The
difference between the two levels is called the error signal. If the feedback temperature is too low,
the error is positive, and it triggers an action which raises the water temperature until the
temperature error is reduced sufficiently.
The closing of the servo loop is very similar. Suppose that we want the motor position to be at 90
degrees. The motor position is measured by a position sensor, often an encoder, and the position
feedback is sent to the controller. Like the brain, the controller determines the position error,
which is the difference between the commanded position of 90 degrees and the position feedback.
The controller then outputs a signal that is proportional to the position error. This signal produces
a proportional current in the motor, which causes a motion until the error is reduced. Once the
error becomes small, the resulting current will be too small to overcome the friction, causing the
motor to stop.
166 • Chapter 10 Theory of Operation
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DMC-1600