PID Advances in Industrial Control


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Plenary presentation given by Terry Blevins at the IFAC PID'12 conference in Brescia, Italy on March 30th, 2012.

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PID Advances in Industrial Control

  1. 1. PID Advances in Industrial Control Terry Blevins Principal Technologist DeltaV Future Architecture Team Austin, TXSlide 1
  2. 2. 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 2Slide 2
  3. 3. 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. 3Slide 3
  4. 4. 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 Intranet Access Electronic Marshalling Wireless 4Slide 4
  5. 5. Electronic Marshalling Electronic marshalling has simplified field wiring 5Slide 5
  6. 6. 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. 6Slide 6
  7. 7. PID is the Basis For Continuous Process Control PID is the dominant technology for feedback control Single loop and multi-loop PID Advanced control e.g. MPC, Fuzzy 7Slide 7 >95% of control application Logic < 5% of control application
  8. 8. 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 8Slide 8
  9. 9. 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. 9Slide 9
  10. 10. 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. 10Slide 10
  11. 11. 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. 11Slide 11
  12. 12. Control System Manufacturer’s Focus Customer Issues e.g. Performance Monitoring New Technology Control System Product e.g. Wireless Research and Changes Device Development Standards e.g. IEC61804, S88 12Slide 12
  13. 13. PERFORMANCE MONITORING Measurement status and control mode as defined by IEC61804 are key to performance monitoring 13Slide 13
  14. 14. 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 14Slide 14
  15. 15. 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 15Slide 15
  16. 16. 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. 16Slide 16
  17. 17. 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 17Slide 17
  18. 18. 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. 18Slide 18
  19. 19. 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 PI Control 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 19Slide 19
  20. 20. Example - Boiler Outlet Steam Temperature  If steam generation exceeds the attemperator capacity the boiler outlet steam temperature will exceed the outlet Standard PID PID w/Variable Pre-load setpoint with the spray valve fully open.  When boiler firing rate is reduced, the spray value should be cut back as the 50% Drop in outlet temperature steam generation SP Overshoot drops. 20Slide 20
  21. 21. 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. 21Slide 21
  22. 22. Conventional Approach – Over Sampling of Measurement Process Output 63% of Change O Time Constant ( ) Deadtime (TD ) Process Input I Control Execution New Measurement Available 22Slide 22
  23. 23. 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 23Slide 23
  24. 24. *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. 24Slide 24
  25. 25. 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. 25Slide 25
  26. 26. PIDPlus Using Wireless Transmitter vs. Conventional PID and Wired Transmitter Lambda Tuning ʎ = 1.0 Communication Resolution = 1% Communication Refresh = 10sec Control Setpoint PIDPlus Measurement PID PIDPlus Control Output PID Unmeasured Disturbance 26Slide 26
  27. 27. 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. 27Slide 27
  28. 28. PIDPlus - Modified Derivative Action 28Slide 28
  29. 29. 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 29Slide 29
  30. 30. Wireless Communication Loss – During Setpoint Change Setpoint PIDPlus Control Measurement PID PIDPlus Control Output PID Communication Loss 30Slide 30
  31. 31. Wireless Communication Loss – During Process Disturbance PIDPlus Setpoint Control Measurement PID PIDPlus Control Output PID Communication Loss 31Slide 31
  32. 32. 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. 32Slide 32
  33. 33. Broadley James Bioreactor Setup VSD VSD Media 37 oC VSD Inoculums TC TT 41-7 41-7 VSD Glutamine VSD VSD Bicarbonate Glucose Heater 7.0 pH AY AC AT AT Splitter AC 41-1 41-1 41-1 41-4s1 41-4s1 0.002 g/L pH Glucose 2.0 g/L AC AT AT AC 41-2 41-2 41-4s2 41-4s2 DO Glutamine 2.0 g/L AT AT AT 41-5x1 41-5x2 Bioreactor 41-6 Viable Dead Product Cells Cells CO2 MFC LT 41-14 AY Level Splitter VSD 41-2 O2 MFC Drain 33Slide 33 Air MFC
  34. 34. Wireless Temperature Loop Test Results 34Slide 34
  35. 35. Wireless pH Loop Test Results 35Slide 35
  36. 36. 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 36Slide 36
  37. 37. Steam Flow To Stripper Heater 37Slide 37
  38. 38. Column Pressure Control 38Slide 38
  39. 39. PC215 On-line Column Pressure Control  The same Wired Measurement dynamic control Used in Control response was observed for SP changes  Original plant PID tuning was used for both wired and wireless control GAIN=2.5 RESET=4 Wireless Measurement RATE=1 Used in Control Answers Questions 2a & 2b 39Slide 39
  40. 40. 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 40Slide 40
  41. 41. 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. 41Slide 41