Control
Systems Intro - Components of a Digital Control Loop
When you implement a control system using a digital computer as part of
the control loop, there is a generic structure to the control loop that
results. Most control loops look like the one below.
In this system, there
is an input - the desired response - and an output - the actual response.
Let's follow the loop around from the output.
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First, the output of the
system is measured with a sensor, producing a measured
response. In many sensors, the measured
response is actually a voltage proportional to the actual response.
-
The measured response
is converted to digital from in an A/D.
Once that conversion takes place, the signal is in the digital part of
the loop - shown in blue.
-
Inside the digital part
of the loop, the measured response - now in digital form - is compared
to the input - the desired response to form an error.
-
The controller
looks at the error and uses that information to generate a control signal
- also called the control effort. That might be done using an ON-OFF
control method, proportional
control or some other control algorithm.
-
Once the control signal/control
effort has been computed, the control effort is applied to the actuator
by converting the digital signal to an analog voltage using a D/A.
-
The actuator
is a device that takes the control effort and applies it to the system
being controlled - the plant.
The actuator might be a power amplifier (if, for example, you are trying
to control the speed of a large motor, like on a subway car) or a hydraulic
system (if, for example, you are trying to control the position of an aileron
on an airplane).
-
The plant is the thing
you are trying to control.
That's it. That is the basic structure of a digital control loop.