Slide 1

PID Advances in Industrial Control PID Advances in Industrial Control

Terry Blevins

Principal Technologist

DeltaV Future Architecture Team

Austin, TX

2 Slide 2

Agenda Agenda

State of Industrial Process Control as seen by one manufacturer of distributed control systems

Advances over the last 10 years

– Performance Monitoring

– On-Demand and Adaptive Tuning

Current Areas of Research and Development

– PID Recovery From Process Saturation

– PID Control Using Wireless Devices

Conclusion, Future

3 Slide 3

Advances in Control System Design Advances in Control System Design

Technology has allowed the cost and footprint of the control system to be reduced while allowing control functionality to be increased.

4 Slide 4

Fieldbus Impact Fieldbus Impact A variety of fieldbuses and field device have been introduced that simplify field wiring and allow control to be distributed to the device level

Wireless

Intranet

Access

Electronic

Marshalling

5 Slide 5

Electronic Marshalling Electronic Marshalling

Electronic marshalling has simplified field wiring

6 Slide 6

The impact of standards The impact of standards

IEC61804 defines function blocks for process control

Supports distribution of control to field devices.

Defines the use of measurement status indicate measure quality and mode to determine the source of setpoint and block output.

Electronic Device Description Language(EDDL) enables a control system to work with any manufacturer’s device.

HART, Foundation Fieldbus, and Profibus devices are consistent with this standard.

7 Slide 7

PID is the Basis For Continuous Process Control PID is the Basis For Continuous Process Control

PID is the dominant technology

for feedback control

Single loop and multi-loop PID

>95% of control application

Advanced control e.g. MPC, Fuzzy

Logic < 5% of control application

8 Slide 8

Example of Operator Interface to Control Example of Operator Interface to Control

The latest UI technology allows the plant operator to do more.

Graphic displays and historic trends allow the operator to view and interact with discrete and continuous control

Web technology allows remote access to measurement, control, and calculations

9 Slide 9

Issues Confronting The Process Industry Issues Confronting The Process Industry

In many cases the staffing at the plant level has been reduced. Plant support at the corporate level may have been severely downsized or eliminated.

Process engineers and instrumentation technicians may not have sufficient training to fully understand control setup and tune PID loops for best performance.

Resources at the plant level may only be sufficient to address issues that disrupt production.

The impact of control utilization/performance may not be monitored, documented and/or effectively communicated to plant management.

This is a global issue – Not specific to the control system manufacturer, commonly encountered in many major plants located in the US, Europe, Asia Pacific, Middle East.

10 Slide 10

Example – Pulp and Paper Example – Pulp and Paper

After plant management saw the results of this survey, a team was formed to address control issues in a timely fashion.

The reduction in variability led to significant improvements in plant throughput and product quality.

Two years later the plant set a new production record.

11 Slide 11

Example - Petrochemical Complex Example - Petrochemical Complex

Once plant management became aware of the low control utilization, manpower and funding were provided to investigate and correct the measurement and control issues

Work to improve control performance should begin with an assessment of control utilization.

12 Slide 12

Control System Manufacturer’s Focus Control System Manufacturer’s Focus

Control System

Research and

Development

Customer Issues

e.g. Performance

Monitoring

New Technology

e.g. Wireless

Device

Standards

e.g. IEC61804,

S88

Product

Changes

13 Slide 13

PERFORMANCE MONITORING PERFORMANCE MONITORING

Measurement status and control mode as defined by IEC61804 are key to performance monitoring

14 Slide 14

Performance Monitoring - Example Performance Monitoring - Example Explorer tree allows easy

navigation of control hierarchy

Overview display summarizes performance for System, Area, Units and Modules

Abnormal Control Conditions indicated for Problem Loops:

– Control Service Status:

• Incorrect mode

• Limited control output

• Bad/Uncertain input

– Control Performance Status:

• Standard Deviation

• Variability Index

• Oscillation Index

• Tuning Index

Device and Valve Diagnostics

15 Slide 15

Performance Reports - Example Performance Reports - Example

Communication of Results to Plant Management

Standard Reports

Quickly identify control problems

Track performance, Schedule for automatic generation

Customized Reports

Add production KPI‟s

16 Slide 16

On-Demand Tuning - Example On-Demand Tuning - Example

One of the most effective on-demand tuning technologies is relay oscillation as originally developed by Åstrӧm and Hägglund.

Allows tuning to be quickly established when commissioning control.

Tuning rules such as modified Ziegler Nichols tuning may be used to determine the PID tuning.

17 Slide 17

Adaptive Tuning Adaptive Tuning

Allows tuning the process model to be automatically established based on:

Normal setpoint changes made by the operator when the PID is in an automatic

PID output changes when the PID is in a manual mode.

Model switching with re-centering and interpolation may be used for process model identification – see Intelligent PID Product Design

18 Slide 18

Adaptive Control - Implementation Adaptive Control - Implementation

To allow the data used in adaptive control to be collected without communication skew or jitter, adaptive control is implemented directly in the PID

Enables adaptive control to be utilized even in high speed control applications.

19 Slide 19

Recovery From Process Saturation Recovery From Process Saturation

The recovery of the PID from process saturation is critical in many continuous and batch applications.

By utilizing a variable preload when the PID output is limited for an extended period of time (process saturation), it is possible to minimize setpoint overshoot on recovery from saturation.

See conference paper Improving PID Recovery from Limit Conditions.

PI Control with Variable Pre-load

PI Control

20 Slide 20

Example - Boiler Outlet Steam Temperature Example - Boiler Outlet Steam Temperature

If steam generation exceeds the attemperator capacity the boiler outlet steam temperature will exceed the outlet setpoint with the spray valve fully open.

When boiler firing rate is reduced, the spray value should be cut back as the outlet temperature drops.

Standard PID PID w/Variable Pre-load

SP Overshoot

50% Drop in

steam generation

21 Slide 21

PID Modifications for Wireless Control PID Modifications for Wireless Control

The Challenge – Control Using Wireless

Transmitter power consumption is minimized by reducing the number of times the measurement value is communicated.

Conventional PID execution synchronizes the measurement value with control action, by over-sampling the measurement by a factor of 2-10X.

The rule of thumb to minimize control variation is to have feedback control executed 4X to 10X times faster than the process response time (process time constant plus process delay).

The conventional PID design (i.e., difference equation and z-transform) assumes that a new measurement value is available at each execution and that control is executed on a periodic basis.

22 Slide 22

Conventional Approach – Over Sampling of Measurement Conventional Approach – Over Sampling of Measurement

Control Execution

63% of Change

Process Output

Process Input

Deadtime (TD )

O

I

New Measurement Available

Time Constant ( )

23 Slide 23

Conventional PID - Impact of Wireless Conventional PID - Impact of Wireless

The underlying assumption in traditional control design is that the PID is executed on a periodic basis.

When the measurement is not updated on a periodic basis, the calculated reset action may not be appropriate.

If control action is only executed when a new measurement is communicated, this could result in a delayed control response to setpoint changes and feedforward action on measured disturbances.

Conventional PID Design

24 Slide 24

*WirelessHART Solution *WirelessHART Solution

Window communication is the preferred method of communications for control applications. A new value will be communicated only if:

the magnitude of the difference between the new measurement value and the last communicated measurement value is greater that a specified trigger value

or if the time since the last communication exceeds a maximum update period.

Thus, the measurement is communicated only as often as required to allow control action to correct for unmeasured disturbances or response to setpoint changes.

For Windowed mode you must specify an update period, a maximum update period, and a trigger value.

*HART 7 specification that has been adopted as an international standard, IEC 62591Ed. 1.0.

25 Slide 25

PID Modification for Wireless Control PID Modification for Wireless Control

To provide the best control for a non-periodic measurement, the PID must be modified to reflect the reset contribution for the expected process response since the last measurement update.

Control execution is set faster than measurement update. This permits immediate action on setpoint change and update in faceplate.

26 Slide 26

PIDPlus Using Wireless Transmitter vs. Conventional PID and Wired Transmitter PIDPlus Using Wireless Transmitter vs. Conventional PID and Wired Transmitter

Control

Measurement

Control Output

Unmeasured

Disturbance

Setpoint PIDPlus

PIDPlus

PID

PID

Lambda Tuning ʎ = 1.0 Communication Resolution = 1%

Communication Refresh = 10sec

27 Slide 27

CONTROL PERFORMANCE DIFFERENCE CONTROL PERFORMANCE DIFFERENCE

Communications transmissions are reduced by over 96 % when window communication is utilized.

The impact of non-periodic measurement updates on control performance as measured by Integral of Absolute Error (IAE) is minimized through the PID modifications for wireless communication.

28 Slide 28

PIDPlus - Modified Derivative Action PIDPlus - Modified Derivative Action

29 Slide 29

PID Performance for Lost Communications PID Performance for Lost Communications

The Conventional PID provides poor dynamic response when wireless communications are lost.

The PID modified for wireless control provides improved dynamic response under these conditions

30 Slide 30

Wireless Communication Loss – During Setpoint Change Wireless Communication Loss – During Setpoint Change

Communication Loss

PID

PIDPlus

PIDPlus

PID

Control

Measurement

Control Output

Setpoint

31 Slide 31

Wireless Communication Loss – During Process Disturbance Wireless Communication Loss – During Process Disturbance

Communication Loss

PIDPlus

Setpoint Control

Measurement

Control Output

PID

PIDPlus

PID

32 Slide 32

Installation at Broadley James Installation at Broadley James

Portable Hyclone 100 liter disposable bioreactor

Rosemount WirelessHART gateway and transmitters for measurement and control of pH and temperature. Pressure monitored

BioNet is based on the DeltaV Control system.

33 Slide 33

Broadley James Bioreactor Setup Broadley James Bioreactor Setup

Bioreactor

VSD

VSD

TC

41-7

AT

41-4s2

AT

41-4s1

AT

41-2

AT

41-1

TT

41-7

AT

41-6

LT

41-14

Glucose

Glutamine

pH

DO

Product

Heater

VSD

VSD

VSD

AC

41-4s1

AC

41-4s2

Media

Glucose

Glutamine

VSD

Inoculums

VSD

Bicarbonate

AY

41-1

AC

41-1Splitter

AC

41-2

AY

41-2Splitter

CO2

O2

Air

Level

Drain

0.002 g/L

7.0 pH

2.0 g/L

2.0 g/L

37 oC

MFC

MFC

MFC

AT

41-5x2

Viable

Cells

AT

41-5x1

Dead

Cells

Bioreactor

VSDVSD

VSD

TC

41-7

TC

41-7

AT

41-4s2

AT

41-4s1

AT

41-2

AT

41-1

TT

41-7

AT

41-6

LT

41-14

Glucose

Glutamine

pH

DO

Product

Heater

VSD

VSD

VSD

AC

41-4s1

AC

41-4s1

AC

41-4s2

AC

41-4s2

Media

Glucose

Glutamine

VSD

Inoculums

VSD

Bicarbonate

AY

41-1

AC

41-1

AC

41-1Splitter

AC

41-2

AC

41-2

AY

41-2Splitter

CO2

O2

Air

Level

Drain

0.002 g/L

7.0 pH

2.0 g/L

2.0 g/L

37 oC

MFC

MFC

MFC

AT

41-5x2

Viable

Cells

AT

41-5x1

Dead

Cells

34 Slide 34

Wireless Temperature Loop Test Results Wireless Temperature Loop Test Results

35 Slide 35

Wireless pH Loop Test Results Wireless pH Loop Test Results

36 Slide 36

Separations Research Program, University of Texas at Austin Separations Research Program, University of Texas at Austin

The Separations Research Program was established at the J.J. Pickle Research Campus in 1984

This cooperative industry/university program performs fundamental research of interest to chemical, biotechnological, petroleum refining, gas processing, pharmaceutical, and food companies.

CO2 removal from stack gas is a focus project for which WirelessHART transmitters were installed for pressure and steam flow control

37 Slide 37

Steam Flow To Stripper Heater Steam Flow To Stripper Heater

38 Slide 38

Column Pressure Control Column Pressure Control

39 Slide 39

PC215 On-line Column Pressure Control PC215 On-line Column Pressure Control

The same dynamic control response was observed for SP changes

Original plant PID tuning was used for both wired and wireless control

GAIN=2.5

RESET=4

RATE=1

Wired Measurement

Used in Control

Wireless Measurement

Used in Control

Answers Questions

2a & 2b

40 Slide 40

Control Performance – Wired vs Wireless Control Performance – Wired vs Wireless Comparable control

as measured by IAE was achieved using WirelessHART Measurements and PIDPlus vs. control with wired measurements and PID.

The number of measurement samples with WirelessHART vs Wired transmitter was reduced by a factor of 10X for flow control and 6X for pressure control – accounting for differences in test duration.

Test #1 Test #2

41 Slide 41

Conclusion Conclusion Control system manufacturers’ research and development

focuses on the need to address customer control issues, incorporate standards, and adopt new technology.

A method for improving the recovery of the PID from process saturation is of interest to the process industry.

The use of non-periodic measurement updates is a requirement when PID control is done utilizing wireless transmitters.

Recent development of PID modifications have been demonstrated that improve recovery from process saturation and to allow non-periodic measurement updates from wireless devices to be used in control. Further research into the performance provided by these modifications would be of interest.

Based on the achieved results it seems very probable that in the next few years PID control will get smarter and continue to be the main workhorse of the process industry control.