User manual

PID Loop Operation
(DL450 Only)
Maintenance
8--5
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
The PID controller controls a continuous feedback loop that keeps the process
output (control variable) flowing normally by taking corrective action whenever there
is a deviation from the desired value (setpoint) of the process variable (PV) such as,
rate of flow, temperature, voltage, etc. An “error“ occurs when an operator manually
changes the setpoint or when an event (valve opened, closed, etc.) or a disturbance
(cold water, wind, etc.) changes the load, thus causing a change in the process
variable.
The PID controller receives signals from sensors and computes corrective action to
the actuator from a computation based on the error (Proportional), the sum of all
previous errors (Integral) and the rate of change of the error (Derivative).
We can look at the PID controller in more simple terms. Take the cruise control on an
automobile as an example. Let’s say that we are cruising on an interstate highway in
a car equipped with cruise control. Thedriver decides to engage the cruise control by
turning it ON, then he manually brings the car to the desired cruising speed, say 70
miles per hour. Once the cruise speed is reached, the SET button is pushed fixing
the speed at 70 mph, the setpoint. Now, the car is cruising at a steady 70 mph until it
comes to a hill to go up. As the car goes up the hill, it tends to slow down. The speed
sensor senses this and causes the throttle to increase the fuel to the engine. The
vehicle speeds up to maintain 70 mph without jerking the car and it reaches the top at
the set speed. When the car levels out after reaching the top of thehill it will speed up.
The speed sensor senses this and signals the throttle to provide less fuel to the
engine, thus, the engine slows down allowing the car to maintain the 70 mph speed.
How does this application apply to PID control? Lets look at the function of P, I and D
terms:
S Proportional -- is commonly referred to as Proportional Gain. The
proportional term is the corrective action which is proportional to the
error, that is, the change of the manipulated variable is equal to the
proportional gain multiplied by the error (the activating signal). In
mathematical terms:
Proportional action = proportional gain X error
Error = Setpoint (SP) -- Process Variable (PV)
Applying this to the cruise control, the speed was s et at 70 mph which is
the Setpoint. The speed sensor senses the actual speed of the car and
sends this signal to the cruise controller as the Process Variable (PV).
When the car is on a level highway, the speed is maintained at 70 mph,
thus, no error since the error would be SP -- PV = 0. When the car goes
up the hill, the speed sensor detected a slow down of the car, SP--PV =
error. The proportional gain would cause the output of the speed
controller to bring the car back to the setpoint of 70 mph. This would be
the Controlled Output.
S Integral -- this term is often referred to as Reset action. It provides
additional compensation to the control output, which causes a change in
proportion to the value of the error over a period of time. In other words,
the reset term is the integral sum of the error values over a period of
time.
S Derivative -- this term is referred to as rate. The Rate action adds
compensation to the control output, which causes a change in
proportion to the rate of change of error. Its job is to anticipate the
probable growth of the error and generate a contribution to the output in
advance.