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PID Control Theory
The P stands for proportional control, I for integral control and D for derivative control
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The basic function of a controller is to execute an algorithm (electronic controller) based on
the control engineer's input (tuning constants), the operators desired operating value
(setpoint) and the current plant process value
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In a basic
process control loop, the control engineer utilises the PID algorithms to achieve this
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In
order for control loops to work properly, the PID loop must be properly tuned
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While the basic algorithm has been unchanged for many years and is used in all distributed
control systems, the actual digital implementation of the algorithm has changed and differs
from one system to another
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In the diagram shown above the valve could be controlling the gas going to a heater, the
chilling of a cooler, the pressure in a pipe, the flow through a pipe, the level in a tank, or
any other process control system
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It looks at the absolute error and the rate of change of error
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When there is a "process upset", meaning, when the process variable or the setpoint quickly
changes - the PID controller has to quickly change the output to get the process variable
back equal to the setpoint
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Therefore the PID controller has to increase the cooling (output) to compensate for this rise
in temperature
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You want the output to be very steady (not changing)
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So there are these two contradictory goals
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Note that the output often goes past (over shoots) the steady-state output to get the
process back to the setpoint
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If someone opens the cooler, walks in, walks around to find
something, then walks back out, and then closes the cooler door -- the PID controller is
freaking out because the temperature may have raised 20 degrees! So it may crank the
cooling valve open to 50, 75, or even 100 percent -- to hurry up and cool the cooler back
down -- before slowly closing the cooling valve back down to 34 percent