Galil Motion Control DMC-42 0 Series, Руководство пользователя

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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 over-damped response.
The results may be worse if we turn the faucet too fast. The overreaction results in temperature oscillations.
When the response of the system oscillates, we say that the system is unstable. Clearly, unstable responses are bad
when we want a constant level.
What causes the oscillations? The basic cause for the instability is a combination of delayed reaction and high gain.
In the case of the temperature control, the delay is due to the water flowing in the pipes. When the human
reaction is too strong, the response becomes unstable.
Servo systems also become unstable if their gain is too high. The delay in servo systems is between the application
of the current and its effect on the position. Note that the current must be applied long enough to cause a
significant effect on the velocity, and the velocity change must last long enough to cause a position change. This
delay, when coupled with high gain, causes instability.
This motion controller includes a special filter which is designed to help the stability and accuracy. Typically, such a
filter produces, in addition to the proportional gain, damping and integrator. The combination of the three
functions is referred to as a PID filter.
The filter parameters are represented by the three constants KP, KI and KD, which correspond to the proportional,
integral and derivative term respectively.
The damping element of the filter acts as a predictor, thereby reducing the delay associated with the motor
response.
The integrator function, represented by the parameter KI, improves the system accuracy. With the KI parameter,
the motor does not stop until it reaches the desired position exactly, regardless of the level of friction or opposing
torque.
The integrator also reduces the system stability. Therefore, it can be used only when the loop is stable and has a
high gain.
The output of the filter is applied to a digital-to-analog converter (DAC). The resulting output signal in the range
b10 and -10 Volts is then applied to the amplifier and the motor.
The motor position, whether rotary or linear is measured by a sensor. The resulting signal, called position
feedback, is returned to the controller for closing the loop.
Chapter 10 Theory of Operation ▫ 157 DMC-42x0 User Manual