Control Systems

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Control Systems

• Wonderware-In touch
• Operator Interface screens
• Presentation

2
Agenda

• What is a control loop and how do they work?

• Basic troubleshooting of control loops

• Screen shots of the skid

• Questions?

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OverviewWhat is a Control Loop

• What is a control loop?
• Used to automatically control processes with
  minimum operator intervention.
• For example Flow rates, Pressure, Level, etc.
• The operator interfaces with two variables
• Process variable (pv) - the real time value of
  what your trying to control.
• Set point variable (sp) - the desired value for
  that variable.

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Control loop basics

• The PID loop tries to automate what an operator
  with a gauge and a control knob would do.
• The operator uses the knob to adjust the output
  of the process until the input measurement
  stabilizes at the desired value on the gauge.
• The position of the needle on the gauge is a
  "process variable".
• The desired value on the gauge is called a
  "setpoint."
• The distance between the gauge's needle and the
  setpoint is the "error".

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Define PID?

• Proportional-Integral-Derivative
• In a PID loop, correction is calculated from the
  error in three ways
• cancel out the current error directly
  (Proportional)
• the amount of time the error has continued
  uncorrected (Integral)
• anticipate the future error from the rate of
  change of the error over time (Derivative).

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Process variables and Setpoints Example

• Citric injection
• The pv is the actual pH
• The sp is the where we want the pH to be
• The P (or Kp) makes an adjustment to correct for
  the error by increasing the flow rate.
• The D (or Kd) anticipates that the adjustment
  will cause an over shoot and decreases the flow
  rate to compensate.
• The I (or Ki) adds up the error over time and
  subtracts it from the P to lesson the chance of
  overshooting the adjustment

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Process variables and Setpoints

• Ideally the process variable closely follows the
  set point variable with minimum deviation
• Changes in the process can/will cause upsets
• This will cause the process to drift from steady
  state
• The process should return to steady state within
  a reasonable amount of time

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Basic Troubleshooting of Control Loops

• If the process variable does not closely follow
  the set point variable
• The loop could require retuning
• Sometimes needed if the process changes
• Retuning should only be performed by the
  engineering staff and should not normally be
  required
• Faulty equipment (pumps, control valves)
• Faulty equipment should be repaired or replaced.
  Retuning should not be used as a band aid

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Basic Troubleshooting of Control Loops

• Control of flow rates
• Higher flow rates mean higher pressure
• Main flow (vanilla) rates can be increased by
  adjusting the flow rate set point
• This will adjust the flow rate as long as the
  pressure
• in the skid stays below the pressure set point
• If higher flow rates are required, the pressure
  set point must be adjusted

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PID Control Screen
This screen allow access to PID parameters. This
is where you can change the Control loop between
AUTO and Manual and Local/Remote control. Check
setpoints and process variables and look at a
real-time snapshot of the loop variables.
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The Vanilla Tank Screen
This is a common screen to all the finishing
lines. This screen shows the levels in the
vanilla base tanks and the Pressure PV and
Setpoints, and allows you to control the vanilla
pumps and control valves. There are Vanilla Tank
PID screens where the setpoints can be changed if
needed.
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Vanilla Tank PID Screen
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pH Meter Pop-Up Screens
These screens allow control of the pH meters.
Selection of which pH meter will control. A
meter can also be locked out from being used if
it is found to be faulty or in need of
calibration. A supervisor must unlock the meter
before it can be put back in service. The meter
must be up before it can be flushed.
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Motor Pop-Up Screens
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On/Off Valve Pop-Up Screens.
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VFD Motor Pop-Up Screens
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Proportional Valve Pop-Up Screen
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Questions?