ChE 433 DPCL

1
ChE 433 DPCL

• Model Based Control
• Smith Predictors

2
Model Based Control

• What can we do with a process model? Improve
  performance.
• 3 Methods
• Internal Model Control
• Model Free Adaptive Control
• Model based PID controllers

3
Internal Model Control

• Dynamic Matrix Control, DMC, forward projection
  of a process change is placed in an array and
  output changes are based on a least error squared
  value of the projected process variable.
  Multivariable handout
• Minimal Prototype Controller, where the
  controller output change is based on a projected
  change in process variable. This algorithm does
  not even use any elements from a conventional PID
  algorithm.

4
Model Free Adaptive Control

• Uses neural network to control the process. The
  output will move the process variable to the set
  point based on an internal network, not
  determined by the user.
• Some reasonable understanding of the process
  dynamics required. The process dynamics can
  change and the algorithm will learn the new
  conditions without being told to retrain itself.

5
MBC, Model Based Control

• Introduced to improve control response with
  dominant dead time processes
• Smith Predictor
• Concept If we know the process transfer
  function, we can place the transfer function in
  the feedback path and cancel the dead time effect.

6
Smith Predictor

• Smith Predictor describe how a model of the
  process is placed in the feed back path. The user
  believes that an exact calculation and
  representation is required to implement the
  technique.
• Consider the elements in the feedback path as
  compensation elements.

7
MBC Implementation

• The process model is divided into two sections,
  one that models the process first order time
  constants and a second that models the process
  dead time.
• The value of these terms are not precisely equal
  to the process model.

8
MBC Implementation

• The controllers compensated dead time should be
  smaller that the process dead time and the time
  constants should be slightly longer than the
  largest time constant.
• The compensated dead time approx. 25 percent
  shorter than the process dead time and the
  compensated lag 25 percent longer than the
  process time constants.

9
MBC Implementation

• It is not necessary for these compensating
  elements be precisely defined. The estimated
  values are usually sufficient. 85
• It is not necessary to know the exact process
  gain
• It is not necessary to have linear behavior the
  algorithm is configured to compensate for the
  model error.

10
MBC Implementation

• A standard PID algorithm with a remote set
  point, CAS_IN, can be used if the model
  compensating terms can be implemented in a
  separating computing function block external to
  the controller.
• Without an offset between set point and the
  algorithm output, and to correct for modeling
  error, a model correction term, MC is the ratio
  of the actual process variable to the output of
  the total process model, W.
• Model correction method should be implemented in
  any advanced model based control system

11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
Feed Forward to MBC

• The disturbance test was done without
  implementing any feed forward.
• Feed forward be implemented external to the
  predicted algorithm.
• Difficult to suppress the compensating action
  based on the feed forward signal, move the valve
  some amount and not allow the compensating
  algorithm to adjust for the change. The algorithm
  will correct for model errors as designed.

15
Potential Problems

• If dead time and time constants change
  significantly, the control loop will operate with
  choppy behaviour and not stabilize
• Nonlinearity can be compensated

16
Integral Delay Dead Time Compensation

• Add a delay before the integral function. Change
  in the error results in immediate change in the
  proportional action, reset or integral behavior
  will be delayed.
• Integral delay time should be equal to the
  process dead time. This prevents excessive
  integral action.

17
(No Transcript)
18
Problems Implementing Integral Delay

• Most commercially available controllers don't
  allow the user to configure the controllers
  internal elements. DeltaV does not. LabView does.
• Many do not offer delay or dead time function
  blocks. A requires the controller manufacture to
  use more dynamic memory, which increases the cost
• Use multiple first orders to simulate a dead time