Successfully reported this slideshow.
Upcoming SlideShare
×

# PID loop optimisation: the benefits of using PROFIBUS PA instrumentation - Peter Thomas

976 views

Published on

To gain a basic understanding of the principles of PID Loop Optimisation.

To understand why “Loop Tuning” is often not the solution to achieving (or restoring) stability in a process control loop.

To understand how PROFIBUS PA instrumentation can assist in Loop Optimisation.

• Full Name
Comment goes here.

Are you sure you want to Yes No

### PID loop optimisation: the benefits of using PROFIBUS PA instrumentation - Peter Thomas

1. 1. PID Loop Optimization & the benefits of using PROFIBUS PA instrumentation Peter Thomas Control Specialists Ltd www.controlspecialists.co.uk Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation – Apr 2014
2. 2. 2 Objectives • To gain a basic understanding of the principles of PID Loop Optimisation. • To understand why “Loop Tuning” is often not the solution to achieving (or restoring) stability in a process control loop. • To understand how PROFIBUS PA instrumentation can assist in Loop Optimisation. Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
3. 3. 3 Loop Optimization or Loop Tuning? • PID Loop Optimisation describes a series of activities that lead (or should lead) to a reduction in variance in a process control loop. • “Loop Tuning” specifically refers to the adjustment of three parameters in a process controller (P)roportional, (I)ntegral and (D)erivative to reduce variance. • Variance usually shows itself in the form of the process variable (PV) oscillating around the desired value (i.e. the Setpoint SP). • A commonly held view is that “Loop Tuning” is the answer to all control loop variance issues. Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
4. 4. Measure Decide Action Noise Variability Tuning Controller Design Hysteresis Stiction Non-Linearity Operator Changes Loop not In Normal Disturbances Valve at Limit Interactions Typical Control Loop Issues Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
5. 5. 5 Common Loop Tuning Methods • Gut Feel / “Seat of the Pants” Tuning. • Copying PID values from another controller. • Model-Based Tuning Rules (Ziegler-Nicholls, Cohen-Coon, Lambda). • Auto-Tuning Controllers. • Software-based Loop Optimisation Software (ExperTune PID Loop Optimizer, Protuner) Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
6. 6. 6 Common Loop Tuning Methods (1) Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014 Method Comments Gut Feel / “Seat of the Pants” tuning. Success is very dependent upon the experience of the person doing the work. The objective is usually to get the loop responding as required with no importance being placed on the actual PID values that have achieved this. This approach often leads to hiding loop-related issues. No consistency in the approach and unlikely to look for problems outside of the controller itself.
7. 7. 7 Common Loop Tuning Methods (2) Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014 Method Comments Model-Based Tuning Rules (Ziegler- Nichols, Cohen-Coon, Lambda). Ziegler-Nichols is the most well known of these. All assume that the open loop response of the loop approximates to a first-order. No use on Level Loops Some are better than others when deadtime is an issue, i.e. it is not always obvious which is the most appropriate to use.
8. 8. 8 Common Loop Tuning Methods (3) Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014 Method Comments AutoTuning Controllers Quick and simple way to tuning a loop. They accept the loop response as found with no regard to the causes of the instability. Can hide loop-related issues by continually retuning to overcome them.
9. 9. 9 Common Loop Tuning Methods (4) Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014 Method Comments Software-Based Loop Optimisation Sotware Tools (ExperTune / Protuner) They promote the idea that loop optimisation is the optimisation of the entire loop, not just the parameters of the process controller. They uncover loop related problems that can make tuning difficult and/or impossible. The PID values they calculate are at least equal to, and often better, than the values you are likely to get from the most appropriate model- based tuning rule. Require OPC Access to the PV and OP of each loop.
10. 10. Control Valve Problems • Oversized • Non-Linear Response • Hysteresis / Deadband • Stiction • Poor Rangeability • Actuator Problems Control valves are estimated to be the cause of 30% of control problems Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
11. 11. Inherent Characteristics A plot of valve opening verses flow Assuming constant pressure drop across valve On/Off applications Most Common Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
12. 12. What is Valve Hysteresis? A failure of the valve to reposition itself correctly following a reversal in the control signal. Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
13. 13. Causes of Hysteresis • Undersized actuator • Seal friction in rotary valves • Friction in plug valves • Inadequate air supply • Loose or worn actuator linkages • Loose linkages to positioner • Improper calibration of positioner Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
14. 14. Valve Stiction (Stick – Slip) Time (s) Stick Slip Position(%) Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
15. 15. 15 Demonstration Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014 We will use PID Loop Optimizer and Loop Simulator from ExperTune to demonstrate the following: • Tuning a simple flow loop. • The effect of over-filtering the process variable. • The effect of valve oversizing. • The effect of incorrect valve characterisation. • The effect of valve Hysteresis / Stiction.
16. 16. 16 Demonstration Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014 Demonstration
17. 17. 17 PROFIBUS PA Transmitter Benefits • Transmitter range easily adjusted. • Allows you to linearize the signal – e.g. level in a cylindrical vessel. • Actions to be taken on specific types of PV fault can be programmed into the PLC by monitoring the status byte of the PV. • PV Filter can be monitored at the SCADA and easily changed Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
18. 18. 18 PROFIBUS PA Valve Positioners • Can be easily tuned – The positioner can be considered as the secondary loop of a Cascade Pair. Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
19. 19. 19 PROFIBUS PA Valve Positioners Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014 • Valve Characteristic easily changed.
20. 20. 20 PROFIBUS PA Valve Positioners Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014 • Can provide a time-delayed alarm when the actual position differs from the demanded – identification of Hysteresis / Stiction. • Able to advise on the number of valve reversals – valve working too hard. • Able to accumulate total valve travel – opportunity to reduce costs due to consequential use of compressed air.
21. 21. 21 Summary • Loop Tuning is only part of the overall process of optimising a PID Loop. • Transmitter filtering and ranging can affect the performance of a process control loop. • Control Valves are a common source of control loop problems. • PROFIBUS PA instrumentation can dramatically reduce the time taken to optimise a process control loop and allows loop-related issues to be identified as a cause of variance. Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
22. 22. 22 Questions? Peter Thomas Control Specialists Ltd peter.thomas@controlspecialists.co.uk LinkedIn – http://www.linkedin.com/company/2198880 www.con Tel +44(0)1925 824003 Mob +44(0)7971 405871 Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014