Control Using Two Manipulated Parameters

1
Control Using Two Manipulated Parameters
Terry Blevins (Principal Technologist) and Greg
McMillan (Principal Consultant)
2
Presenters

• Terry Blevins
• Greg McMillan

3
Introduction

• Overview Typical Examples
• Split-Range Control
• Concept, variations in implementation
• Setup in field vs. Splitter Block and IO for each
  valve.
• Using Splitter Block, Example.
• Valve Position Control
• Concept and typical implementation
• Setup of I-only control in implementation
• Impact of mode/status, Example.
• Combining Split Range and Valve Position Control
• How to implement in DeltaV
• Example
• Utilizing Predict/PredictPro for Control Using
  Two Manipulated Parameters
• Advantage if process has large deadtime,
  difference in dynamics
• Setup of MPC and MPC-Pro Blocks
• Example Applications
• Summary
• References

4
Control Using Two Manipulated Parameters

• Under specified problem that has multiple
  solutions for unlimited operation.
• Extra degree of freedom is used to achieve unique
  solution that satisfied specific control
  objective.
• Most common techniques are split range, valve
  position control.
• Combination of these technique and MPC offer new
  capability to address this class of problems

One(1) Controlled Parameter
Two(2) Manipulated Parameters
Controller
Process
SP
Unmeasured Disturbance
5
Split Range Traditional Implementation

• Sequencing of valve accomplished through
  calibration of positioner, selection of actuator
  (A/O or A/C)
• Pro Less expensive installation (1 pair of
  wires to field and 1 I/P)
• Con You are not using the best technology for
  valve performance (e.g. digital positioners).
• Con -Difficult to initially calibrate and
  continuously improve to get best gap and most
  constant gain.
• Con -Individual valves not accessible for trouble
  shooting loop and actuator/valve problem.
• Con The actuator, pneumatic positioner, and I/P
  performance shift with time and field conditions
• Con I/P failure disables 2 valves
• Con - Replacements in the night may not have the
  special settings

Temperature Example
IP 101
4-20ma
3-15PSI
A/C
TT 101
Cooling
Process
A/O
Heating
6
Split Range Traditional Implementation
pH Example

• Sequencing of fine and coarse valve requires
  pressure switch, two solenoid valves and
  associated wiring and tubing
• Con Complex installation
• Con You are not using the best technology for
  valve performance (e.g. digital positioners).
• Con -Difficult to initially calibrate and
  continuously improve to get best gap and most
  constant gain.
• Con -Individual valves not accessible for trouble
  shooting loop and actuator/valve problem.
• Con The switch, actuator, pneumatic positioner,
  and I/P performance shift with time and field
  conditions
• Con I/P failure disables 2 valves
• Con - Replacements in the night may not have the
  special settings

IP 102
4-20ma
PS 102
3-15PSI
pH
A/O
AT 102
Process
Fine Valve
A/O
Coarse Valve
100
Valve Position ( of Span)
Fine Valve Coarse Valve
0
3
15
I/P Output ( PSI )
7
Split Range DeltaV Implementation

• Splitter bock is used to implement split range
  control.
• When using traditional valves, split range
  control may implemented in DeltaV Controller
  using two(2) current outputs
• Split range control may be partially or fully
  assigned to fieldbus devices.

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Split Range Control in DeltaV
9
Splitter Block Calculation
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IN_ARRAY Parameter

• The SP range associated with each output is
  defined by IN_ARRAY.
• SP range of outputs may be defined to overlap
• The SP upper end of range must be greater that
  lower end of range for each output

SP range associated with OUT1
SP range associated with OUT2
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OUT_ARRAY Parameter

• When SP is outside defined range, then the value
  at the end of range is used to determine the
  output.
• LOCKVAL determines if OUT1 value is held if SP is
  greater that the upper end of range defined for
  OUT1.
• No restrictions are placed on the output range.

OUT1 Range for associated SP range
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Splitter Block
13
Heating-Cooing Example
PID
SPLT
AO
AI
IP103A
TIC103
FY103
TT103
AO
IP103B
FY 103
IP 103A
IP 103B
TT 103
COOLER
HEATER
14
Split Range Output (FY103)
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Steam Header Example
AI
PID
SPLT
AO
PT104
IP104A
PIC104
FY104
AO
IP104B
1475 Header
Boiler
FY 104
IP 104A
IP 104B
Turbo Generator
PT 104
400 Header
16
Split Range Output (FY104) - Capacity
100
Valve Position ( of Span)
Valve 104A Valve 104B
0
0
100
PIC104 Output ( of Span)
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Basic Neutralizer Example
AI
PID
SPLT
AO
AT105
IP105A
AIC105
FY105
AO
IP105B
IP 105A
Reagent
Coarse Valve
Fine Valve
IP 105B
FY 105
AT 105
Discharge
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pH Nonlinearity and Sensitivity
pH
8
6
Reagent Flow Influent Flow
19
Split Range Output Valve Sequencing
100
HYSTVAL
Valve Position ( of Span)
Fine Valve (IP105B) Coarse Valve (IP105A)
0
0
100
AIC105 Output ( of Span)
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Calculating Splitter SP Ranges

• A 1 change in controller output to the splitter
  should have the same impact on control parameter
  when operating with either valve.
• When manipulating the same or similar material
  e.g. steam flow to header, then the range may be
  calculated based on valve rating.
• Tests may be performed to determine impact of
  each valve on the controlled parameter.

Example Steam flow to Header, splitter
interfacing directly to PRVs, no overlap Valve 1
rating 50kph Valve2 rating 150kph Desired
Splitter Span valve 1 100(50/(15050))
25 SP range for valve 1 0-25 SP range for
valve 2 25-100
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Testing Process to Determine Splitter SP Ranges
Example Slaker feed temperature controlled using
heating and cooling valves

• With the process at steady state and AOs in Auto
  mode, determine the magnitude of change in the
  controlled parameter for a 1 percent change in
  each valve.
• Calculate the splitter SP span and range for each
  output based on the observed response

Controlled Temperature
1.1degF
2.2degF
1
Heating
Cooling
1
Time
Desired Splitter Span cooling valve
100(1.1/(1.12.2)) 33 SP range for cooling
valve 0-33 SP range for heating valve 33-100
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Example Split Range
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Response to SP Change Split Range Output To
Large Valve/Small Valve
SP
Small Valve
PID OUT
PV
Large Valve
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Split Range Strengths and Weaknesses

• Pro - Process operation in simplified since two
  actuators are treated as one control manipulated
  parameter.
• Pro immediate change in target actuator
  position can be achieved over the entire
  operating range independent of the size of change
  in the splitter SP
• Con To achieve stable control over the entire
  operating range, Controller tuning must be
  established based on the slower responding
  manipulated parameter.
• Con- Does not take advantage of difference in
  resolution of actuator e.g. fine vs. coarse
  valve.
• Valve position control may be used in place of
  split range control when there are differences in
  dynamic response or resolution in actuators.

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Valve Position Control Traditional
Implementation
pH Example

• PID control is implemented using the actuator
  with finer resolution or fastest impact on
  controlled parameter
• The actuator with coarse resolution or slower
  impact on the controlled parameter is adjusted by
  an I-only controller to maintain the long term
  output of the PID controller at a given target
• I-Only controller must be disabled when the PID
  controller is not in an Automatic mode.

IP 106A
Fine Valve
Mode
IP 106B
AT 106
Target Valve Position
I-Only Controller
Process
A/O
Coarse Valve
pH
Fine Valve
Target Valve Position
Coarse Valve
Time
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Valve Position Control DeltaV Implementation

• I-Only control is achieved by configuration of
  the PID Block STRUCTURE, GAIN and RESET
  parameters.
• It is possible to implement valve position
  control in the DeltaV controller or for this
  function to be distributed to fieldbus devices.

Traditional field devices
Fieldbus devices
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Valve Position Control in DeltaV

• Actuator with fastest impact or highest
  resolution is used to maintain the controlled
  parameter at setpoint.
• The OUT of the PID used for control is wired to
  IN on the PID block used for I-Only regulation of
  slower responding or coarse resolution.

PID configured for I-Only control
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Configuring PID for I-Only Control

• The STRUCTURE parameter should be configured for
  I action on Error, D action on PV
• The GAIN should be set to 1 to allow normal
  tuning of RESET (even though proportional action
  is not implemented.
• RESET should be set significantly slower than
  that the product of the PID gain and reset time
  used for control e.g. 5X slower

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Precise Flow Using Big/Small Valve
AI
PID
AO
IP107A
FT107
FIC107
I-Only
AO
ZC107
IP107B
IP 107A
FT 107
IP 107B
30
Neutralizer Using Valve Position Control
AI
PID
AO
IP108A
AT108
AIC108
I-Only
AO
ZC108
IP108B
IP 108B
Reagent
Coarse Valve
Fine Valve
IP 108A
AT 108
Discharge
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Example -Boiler BTU Demand
SUM
AI
PID
AO
IP109A
FT109B
FY109
FIC109
AI
I-Only
AO
FT109A
ZC109
IP109B
BTU Demand
FY 109
IP 109A
FT 109B
HI BTU Fuel
Boiler
IP 109B
FT 109A
Low BTU Waste Fuel
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Example Reformer Air Demand
AI
PID
AO
IP110
FT110
FIC110
I-Only
AO
ZC110
SC110
Total Air Demand
IP 110
SC 110
Secondary Reformer
FT 110
Air Machine
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Example Valve Position Control
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Response to SP Change - Valve Position Control
with Large Valve/Small Valve

• Target position for fine valve is 30.
• When the fine valve saturates, then response is
  limited to be reset of the I-Only control

PV
SP
Coarse Valve
Limited
Fine Valve
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Valve Position Control Strengths and Weaknesses

• Pro Immediate control response is based on
  actuator with finest resolution and/or faster
  impact on controlled parameter.
• Pro Actuator with coarse resolution or slower
  impact on controlled parameter is automatically
  adjusted to maintain the output of the controller
  output long term at a specified operating point.
  
• Con The controller output may become limited in
  response to a large disturbance or setpoint
  change. For this case, the dynamic response
  becomes limited by the slower tuning of the
  I-only controller.
• Con Since stick-slip or resolution limits are a
  of stroke, the big valve will go into a slow
  limit cycle
• The features of split range control and valve
  position control may be combined to provide
  immediate response to large changes in demand
  while retaining the features of valve position
  control for normal changes.

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Combining the Best Features of Split Range and
Valve Position Control

• A composite Block can be created that combines
  the features of split range and valve position
  control
• Support for BKCAL_IN and BKCAL_OUT can be
  implemented to provide bumpless transfer

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Composite Algorithm
NORMAL
SP
OUT_1
CAS_IN
-
FILTER_TC
Filter
T
-
OUT_2
MODE
RANGE SPAN
Scaling
BKCAL_IN1
BKCAL_OUT
Balance Calculation
BKCAL_IN2
38
Composite Implementation

• Parameters that must be configure are FILTER_TC,
  SPAN (of SP), RANGE (of OUT1), and NORMAL
  (desired position of
• The FILTER_TC should be configured similar to
  the reset time of the I-Only Controller that
  would be used for valve position control.

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Demo Composite Combining Valve Position and
Split Range Control
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Example Response to SP Change

• For small changes in SP or load disturbance, the
  response is similar to that provided by valve
  position control
• For large changes in SP or load disturbance, the
  immediate response is similar to split range
  control

Large change
Small change
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Composite for Valve Position/Split Range Control
Strengths and Weaknesses

• Pro All the advantage of valve position control
  without the dynamic limitations on large setpoint
  change or load disturbance.
• Con If there is a significant delay in the
  control parameter response to changes in the two
  valves, then this limits the response that can be
  achieved using PID for the control .
• Model Predictive control automatically
  compensates for process dynamic and may be
  configured to provide the best features of valve
  position and split range control and can also
  address operating constraints.

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Example of Different Dynamic Response Waste
Fuel Boiler Control

• Objective Maximize use of bark, only use gas
  when required to maintain Steam SP.
• Steam response to change in bark is much slower
  than for a change in gas.
• Bark alone may not be sufficient to address a
  sudden increase in steam demand.

Steam SP
20
Steam Flow
Desired Response to unmeasured disturbance
Lo Cost Slow Waste Bark
Hi Cost Fast Fuel Gas
43
Example of Different Dynamic Response Bleach
Plant Control

• Objective Maintain KAPPA target though the
  addition of Chemical 1 and Chemical 2. Minimize
  the use of Chemical 2.
• Desired operation is for Chemical 2 to be used
  for short term correction in KAPPA to replace
  Chemical 2 with Chemical 1 in the longer term.

44
Utilizing MPC for Control

• Both Predict and PredictPro can be configured and
  tuned for maintaining the critical controlled
  variable (CV), such as steam or composition, at
  its target and maximizing the low cost slow MV
  set point as an optimization variable.

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MPC Guidelines for This Application

• The best load and set point response for the
  critical CV is obtained with a short term
  tradeoff in efficiency by reducing the penalty on
  error (PE) for the optimization variable.
• When riding the low cost MV maximum set point,
  this PE lets both the slow and fast MV to move to
  improve the load and set point response of the
  critical CV.
• When riding the high cost MV low set point limit,
  it does not slow down the response of the other
  MV to upsets and set point changes to the
  critical CV. Only the response of the
  optimization variable is slowed down. This is
  consistent with the general theme that
  disturbance rejection must be fast while
  optimization can be slow.
• For coarse and fine valve control, the small
  valve is a low cost (low stick-slip) fast MV and
  the big valve is a high cost (high stick-slip)
  slow MV. The optimization variable is fine valve
  set point with a strategy of keeping it within
  limits (mid range throttle position). The PE for
  the optimization variable is reduced rather then
  the PM increased for the coarse valve so that
  both are available for load disturbance
  rejection.
• For the following examples, the slow MV has a
  lower cost, so its optimization strategy is
  maximization.

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DeltaV Predict Configuration

• MPC block should be configured for two control
  and two manipulate parameters.
• The controlled measurement is wired to CNTRL1

47
DeltaV Predict Configuration (Cont)

• CNTRL2 is configured as an optimized parameter -
  Maximize (not wired)

48
Control Generation - DeltaV Predict

• In Predict, the Penalty on Error (PE) is
  significantly decreased on the Controller
  Generation screen as shown in this example.
• The PE was lowered form 1.0 to 0.1 to make the
  optimization of the slow MV much less important
  than the control of the critical PV at its target

49
MPC Response to Disturbance and Set Point Changes

• In this example, low cost MV initially is riding
  its maximum set point, which leaves the fast cost
  MV free to respond
• Later, the maximum for the low cost MV has been
  increased to the point where it is no longer
  achievable, which drives the high cost MV to its
  low set point limit.

50
DeltaV PredictPro Configuration

• When configuring the MPC-Pro block, selects
  Target in the optimize column for the critical
  PV, and Maximize for the low cost MV.
• Browse to specify the RCAS_IN of the low cost
  slow MV (FC1-2) to specify the measurement
  associated with the low cost slow MV..

51
Control Parameter - MPC-Pro Block
52
Control Generation - DeltaV PredictPro

• The Penalty on Error (PE) is significantly
  decreased on the Controller Generation screen
• In this example, the PE was lowered form 1.0 to
  0.1 to make the optimization of the slow MV much
  less important than the control of the critical
  PV at its target.

53
MPC-Pro Response to Disturbance and Set Point
Changes

• In this example, the low cost MV initially is
  riding its maximum set point, which leaves the
  fast cost MV free to respond
• Later, the maximum for the low cost MV has been
  increased so it is no longer achievable, which
  drives the high cost MV to its low set point
  limit.

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Summary

• Split range control allows fully dynamic response
  to major setpoint of load disturbance changes.
  Valve position control may be used to takes
  advantage of any difference in control response
  or resolution in the manipulated parameters. A
  composite block has been demonstrated that
  combines the best features of split range and
  valve position control.
• DeltaV Predict and PredictPro and the associated
  MPC and MPC-Pro blocks may be effective used to
  address control using two manipulated parameters.
  Improved performance over PID is expected if the
  process has significant dead time or the
  manipulated variables have significantly
  different dynamics. Also, using this approach
  allow operating constraints and feedforward to be
  easily incorporated into the control strategy.
• Please direct questions or comments on this
  presentation to Terry Blevins (Terry.Blevins_at_Emers
  onProcess.com) or Greg McMillan
  (Greg.McMillan_at_EmersonProcess.com ).

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Where To Get More Information

• Effectively Addressing Control Applications,
  Terry Blevins, Emerson Exchange, 2004.
• Addressing Multi-variable Process Control
  Applications, Dirk Thiele, Willy Wojsznis, Pete
  Sharpe, Emerson Exchange, 2004
• Advanced Control Unleashed, Plant Performance
  Management for Optimum Benefit. Terry Blevins,
  Gregory McMillan, Willy Wojsznis, Mike Brown, ISA
  Publication, ISBN 1-55617-815-8, 2003.