Operation Of Closed-Loop Systems - Galil Motion Control DMC-21 5 Series User Manual

This program corresponds to the velocity profiles shown in Figure 10.2. Note that the profiled positions show
where the motors must be at any instant of time.
Finally, it remains up to the servo system to verify that the motor follows the profiled position by closing the servo
loop.
The following section explains the operation of the servo system. First, it is explained qualitatively, and then the
explanation is repeated using analytical tools for those who are more theoretically inclined.

Operation of Closed-Loop Systems

To understand the operation of a servo system, it can be compared 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 the motor position needs to be commanded to move 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. The hot water
faucet should be turned at the "right" rate. If it is turned it too slowly, the temperature response will be slow,
causing discomfort. Such a slow reaction is called over-damped response.
Chapter 10 Theory of Operation ▫ 125
X VELOCITY
Y VELOCITY
X POSITION
Y POSITION
Figure 10.2: Velocity and Position Profiles
TIME
DMC-21x5 User Manual 1.0a1