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DMC-1700/1800 Chapter 10 Theory of Operation • 187
Y POSITION
X POSITION
Y VELOCITY
X VELOCITY
TIME
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.
The analogy between adjusting the water temperature and closing the position loop carries further. We have all
learned the hard way, that the hot water faucet should be turned at the “right” rate. If you turn it too slowly, the
temperature response will be slow, causing discomfort. Such a slow reaction is called overdamped response.