Pid loop optimisation and the benefits of using profibus pa instrumentation - peter thomas

PID Loop Optimisation
& the benefits of using
PROFIBUS PA
instrumentation
Peter Thomas
Control Specialists Ltd
www.controlspecialists.
co.uk
Copyright Control Specialists Ltd 2013 - PID Loop Optimisation & the benefits of using PROFIBUS PA instrumentation V1.1

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.

3
Loop Optimisation 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.

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

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)

6
Common Loop Tuning Methods (1)
www.controlspecialists.co.ukCopyright Control Specialists Ltd 2013 - PID Loop Optimisation & the benefits of using PROFIBUS PA instrumentation V1.1
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
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
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.

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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
Demonstration
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
Demonstration

12
PROFIBUS PA Transmitter Benefits
• Transmitter range easily adjusted.
• Allows you to linearise 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

13
PROFIBUS PA Valve Positioners
• Can be easily tuned – The positioner can be
considered as the secondary loop of a Cascade
Pair.

14
• Valve Characteristic easily changed.

15
• 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.

17
Questions?
Peter Thomas
Control Specialists Ltd
www.controlspecialists.co.uk
Tel +44(0)1925 824003
Mob +44(0)7971 405871

Pid loop optimisation and the benefits of using profibus pa instrumentation - peter thomas

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Pid loop optimisation and the benefits of using profibus pa instrumentation - peter thomas