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The Feedback Control Loop |
The system measures the process, compares it
to a setpoint, and then manipulates the output in the direction which
should move the process toward the setpoint.
Valve
Linearity
Valves are usually non-linear. That is, the flow through the valve is
not the same as the valve position. Several types of valves exist:
Linear
Same gain regardless of valve position
Equal Percentage
Low gain when valve is nearly closed
High gain when valve is nearly open
Quick Opening
High gain when valve is nearly closed
Low gain when valve is nearly open
As we will see
later, the gain of the process, including the valve, is very important
to the tuning of the loop.
If the controller is tuned for one process gain, it may not work for
other process gains.
Valve Linearity:
Installed characteristics
The flow vs. percent open curve changes due to the head loss in the
piping
At low flow, the head loss through the pipes is less,
leaving a larger differential pressure across the valve.At
high flow, the head loss through the pipe is more, leaving a
smaller differential pressure across the valve.
The effect is to increase the non-linearity of most valves.
Fail Open Valves
Valves are usually either: Fail Closed, air to open or
Fail Open, air to close
Regardless of the way the valve operates, the operator is interested in
the knowing and adjusting the position of the valve, not the value of
the signal. "Up is always open"
All controllers have some means of indicating the controller output in
terms of the valve position. When the operator increases the output as
indicated on the controller, the valve opens.
Indication Inversion
The output indication is inverted.
The controller action takes the valve action into acount.
The flow loop is direct acting.
Most analog controllers work like this.
Signal Inversion
 The output signal is
inverted.
The controller action ignores the valve action.
The flow loop is reverse acting
Some distributed control systems work like this.
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