Important Practical Valve and Controller Issues

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Important Practical Valve and Controller Issues
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Proportional band
Proportional Band
Definition The of controller input change
required to cause a 100 controller output change
Note If you wish to turn the proportional gain
on a controller down you must turn PB UP !
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• Control equations
• http//www.manufacturing.net/ctl/article/CA186117
• Conventional PID equation
• Other forms may be used ie
• Check the manufacturers literature on the
  controller you use!

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Controller Action
Controller action
Definition The sign on Kc or PB.
If the action on a controller is backwards it
will attempt to drive the process in the wrong
direction. The process will be at either its
maximum or minimum levels
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Failure Modes
Valve Failure modes
Actuators in general and valves in particular may
be selected or designed to fail to maximum,
minimum, or current settings.
The selection of failure mode is a part of the
safety design considerations in a system.
Discuss practical examples
Students be prepared to answer a question on
failure mode for valves on an exam
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Reset windup
Reset windup
Definition The integral action that is
accumulated when the manipulated variable is at
its maximum and the controlled variable is not
forced to the setpoint.
Discuss and example
Correction for reset windup Most controllers
set the integral action to zero when the
manipulated variable is at its maximum and the
controlled variable is not forced to the setpoint.
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Reset time vs. Reset rate
Reset (integral) time (tI) vs. Reset rate (tRI)
From before a PID controller equation is
It can also be written
Note if a controller uses tRI, the effect of
adjustment of the integral setting will be
opposite of controllers using tI
It can also be written
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Fixing the setpoint derivative problem
For the control system we developed earlier
Notice that a step in qset will cause a step in e
and the derivative of e will become infinite
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The fix
We can eliminate the derivative of the setpoint
and still get the effect we desire from the
derivative action. So let
This equation along with filtering of
qout_measured is used in commercial controllers
with derivative action.