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
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PID loop optimisation: the benefits of using PROFIBUS PA instrumentation - Peter Thomas
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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
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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
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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
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
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Common Loop Tuning Methods
• Gut Feel / “Seat of the Pants” Tuning. • Copying PID values from another controller. • Model-Based Tuning Rules (Ziegler-Nicholls,
Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
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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.
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Common Loop Tuning Methods (2)
Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
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.
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
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
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
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
Valve Stiction (Stick – Slip)
Time (s)
Stick
Slip Posi
tion
(%
)
Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
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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.
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Demonstration
Copyright Control Specialists Ltd 2014 - PID Loop Optimisation - The benefits of using PROFIBUS PA instrumentation - Apr 2014
Demonstration
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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
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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
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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.
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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.
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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