04 pid tuning

Reliance Technology Group – APC / RTO For Internal Circulation1

Process Engineering Process Engineering GroupGroup

PID TuningPID Tuning

Umesh C. J.Umesh C. J.

0404thth April, 2012 April, 2012

April 27, 2023 Reliance Technology Group – APC / RTO For Internal Circulation2

Scope

Control Systems and PID Introduction

Problems in Control valves and Controllers

Guidelines for selection of Controller type

PID Tuning Methods

Guide lines for tuning


Control System

Device or set of devices to manage, command, direct

or regulate the behavior of other devices or systems.

logic or sequential controls

feedback or linear controls

fuzzy logic


PID Controller

A control loop feedback mechanism (controller)

Minimizes the difference between a measured process

variable and a desired setpoint.

Involves three separate parameters; the proportional, the

integral and derivative values.

PID controller is backbone for APC


PID Controller

Process and instrument knowledge is required for

successful process control

No amount of Advanced Control which relies on the use of

poor field instrumentation can be expected to yield

worthwhile benefits.

Valve size and type, actuator, positioner, and measuring

element selection are key to successful closed loop control


Types of Control


Definitions Inherent Characteristic of control valve

The relationship between the flow rate and the closure member (disk) travel as it is moved from the closed position to rated travel with constant pressure drop across the valve.


Continued…

Installed Characteristic of control valveThe relationship between the flow rate and the closure member (disk) travel as it is moved from the closed position to rated travel with varying process conditions

LinearityThe closeness to which a curve relating to two variables approximates a straight line

Time ConstantTime interval measured from the first detectable response of the system to a small step input until the system output reaches 63% of its final steady-state value

Response TimeMeasured by a parameter that includes both dead time and time constant


Continued… Process Gain

The ratio of the change in the controlled process variable to a corresponding change in the output of the controller

Sizing (Valve)Systematic procedure to ensure the correct valve capacity for a set of specified process conditions

Closed LoopThe interconnection of process control components such that information regarding the process variable is continuously fed back to the controller set point to provide continuous, automatic corrections to the process variable

Open LoopThe condition where the interconnection of process control components is interrupted such that information from the process variable is no longer fed back to the controller set point so that corrections to the process variable are no longer provided


Problems in Control Valves & Controllers

Positioner overshoot

Weak Actuator

Stiction

Backlash

Valve Size

Non-Linearity


Control Valve Schematic


Valve Actuator and Positioner

Weak Actuator

If the controller output does not seem to manipulate the

process variable, it could be because the actuator is too

weak

Strong Positioner

Controller output moves smoothly but the process variable is

oscillatory. Reason could be the aggressive tuning of

Positioner, causing overshoot and oscillations


Sticky Valve

Check gland packing and bend stem Put a strong actuator and a good positioner Use Derivative action in PID to keep the valve

moving


High Process Gain

Process Gain Too High

Re-range the transmitter or reduce the valve Cv


Backlash or Hysteresis


Non-Linearity

Controller output behaves differently depending on

which part of the output range is used

Process or the valve should be characterized

Inherent characteristics should be changed


Characterization in pH Control

Filters Instrument Noise

Measurement fluctuations can be avoided using instrument filters Process Noise

Regular variations in process parameters due to some unmeasured disturbances or unstable regulatory controllers can be avoided using process filtersEx. Reflux ratio when product is on LIC, Filter averages the draw off

Moving average filter is used for regular variations like in valve outputs first order filter is used for irregular variations

Advantages of adding Filters Smoothens the recorded process variable measurement signals More aggressive tuning can be used Saves the life of control valves by reducing movements

Drawbacks of Filters Controller does not act on the true response of the process, as Filter adds lag

time to the process variable signal Filters can be applied on CVs not on DVs.


Healthy Control Valve

What is a healthy Control Valve ? Practical Ideal

Process Gain 0.5 < Kp < 3 1

Linearity 0.5 < Kp max/Kp min< 3 1

Hysteresis < 3 % 0

or Backlash

Stiction << 1 % 0

Note: Kp= ∆PV/ ∆CO, ∆PV and ∆CO are % changes in process variable (PV)and controller output (CO) respectively


PID Controller

PID loop can achieve good performance with proper choice

of PID parameters. Improper parameters can perform poorly

or even make the loop unstable

PID algorithm influences the dynamic behavior of a closed

loop system, so PID parameters must be determined

simultaneously

PID parameters used in the calculation must be tuned

according to the nature of the system


P- Proportional Mode Change in controller output proportional to error

-CO = Kc*E + C-Where CO= controller OP-E = error-Kc = controller gain-C= valve position at zero error

Proportional band = 100/Kc -% change in error required to move valve full scale

Proportional only control results in offset-Never reaches SP

Action is instantaneous (proportional kick)-As soon as error is produced, control action is taken

EFFECT OF P,I,D MODES


I - Integral Mode

Used to Remove OffsetRate of change of controller output is proportional to error

-Kc = gain as used in proportional only control -Ti = reset/integral time (mins) -1/Ti sometimes used (repeats/min)As long as error exists, integral action will work to eliminate the error

t

edtTiKc

0

CONTINUED …

D - Derivative Mode

Change in controller output proportional to rate of change of error

CO = Kc.Td.(dE/dt)

Where Td = derivative time

Derivative action amplifies measurement noise

Very effective if deadtime present and often under-rated

But problems if noisy signal

Sensitive to tuning errors

Provides an additional “pulse” to push the control in the right

direction

Continued …


Selection of Controller Type (Guidelines)

Proportional Controller

CO = Kc . e(t) + COss

Proportional controller can be used if:

Acceptable offset can be achieved with moderate values of Kc

The process has an integrating action for which the proportional

controller does not exhibit offset


Continued …

PI Controller

CO = Kc e(t) + (Kc / τI ).(∫e(t).dt) + COss

A PI controller should be used when

Proportional control alone cannot provide sufficiently small

steady state errors (offsets)

The speed of the closed-loop remains satisfactory despite the

slowdown caused by the integral control mode when the system

response is fast (Flow loops) PI is often used for control.


Continued …

PID Controller

CO = Kc e(t) + (Kc / τI ).(∫e(t).dt) + (Kc D).de(t)/dt) + COss

PID controller should be used To increase the speed of the closed-loop response and retain

robustness Addition of derivative action with its stabilizing effect allows the use of

higher gains, which produces faster responses without excessive oscillations

Derivative action is recommended for control where the processes are sluggish Therefore recommended for temperature and composition control.


Classification of PID Algorithms

Ideal PID Algorithm

Series (Interactive) PID Algorithm

Parallel (Non-Interactive) PID Algorithm

There are more than 300 variations in the PID algorithm There are more than 300 variations in the PID algorithm implemented in various DCS systemsimplemented in various DCS systems

ssc COdt

de(t)De(t)dt

I1e(t)KCO

ssc COdtdD1e(t)dt

I1e(t)KCO

ssc COdt

de(t)De(t)dt

I1e(t)KCO


Feedback Control

ym

Controllerysp Final

Control Element

Measuring device

dymce

Control mechanism

Process+-


PID Tuning Methods

Ziegler-Nichols Tuning Method Closed-loop tuning method

Applicable to PID controllers with an ideal algorithm

Using proportional control only and with the feedback

loop closed, introduce a set point change and vary the

proportional gain until the system oscillates

continuously. This controller gain is ultimate gain-Ku


Continued …

Ziegler-Nichols Method

Control

Action

Gain Reset Pre-Act

P Ku/2 0 0

PI Ku/2.2 1.2/T 0

PID Ku/1.7 2/T T/8


Continued …

Dahlin (Lambda) Tuning Method

an open-loop tuning method

Set the control loop to MANUAL mode and give a step

in controller output

Note open loop time constant ‘t’ (min)

The Dahlin tuning parameter results in a process that

does not overshoot the set point.


Continued …

Dahlin (Lambda) Tuning MethodGain, Kc = Q(1-L)/(Kp.L)

Reset = L/T(1-L)

Pre-Act = 0

Where

• Kp = Process Gain = ∆PV/∆CO

• Q = 1 - exp(-T/t’)

• t’ = Desired closed loop time constant

• T = Time took to reach steady state

• L = 1 - exp(-T/t)

• t = Process time constant


Trial and error tuning no matter what technique used to determine initial tuning constants, further adjustment may be necessary

Advantages

simple, logical, requires patience and understanding

deals with all components in process

Disadvantages

Can be very time consuming

Requires experience in tuning


CONTINUED … Proportional action

Start with a low value

Increase gradually

Reduce if PV oscillates excessively if MV has large overshoot or if you see proportional “kick-back”

kick-back is a phenomenon where the PV moves towards the SP but then turns back away from it and is due to the proportional action being too high

Proportional affects both integral and derivative actions !


Integral ActionStart with high value (low integral action)

Increase integral action if approach to setpoint is slow

Too much integral can also cause oscillation

Use more integral action and less proportional action to stop kickback

CONTINUED …


CONTINUED …

Derivative actionSome people are afraid of it

Do not be afraid to use it!

But do not use it all the time either

Use it when you have deadtime and no noise

Introduce it slowly and carefully

Re-adjust proportional and integral action as necessary

(deadtime/4) is a reasonable start

Do not use too much as it can lead to instability if over-used.


GUIDE LINES FOR TUNING

Four types of PIDflow : No dead time , less time constantPressure : No dead time , Big time constantTemp : Significant Dead time , time constantLevel : Integrating


Guide for flow controllers

Integral action dominates tuning

Set Ti small (order of a few seconds)

Set Kc small

Usually set Td = 0

CONTINUED …


Guide for pressure controllers

Liquid pressures behave like flows

Vapor pressures, use PI may have some dynamics

derivative action rarely needed unless deadtime in system

CONTINUED …


Guide for temperature controllers

Significant dynamics so may need full PID

Can try with PI first then add D slowly

not normally noisy

often hardest of basic controller types to tune because derivative is

needed (full PID)

CONTINUED …


Guide for level controller

Gain dominates tuning

Use only a little Integral (minutes rather than seconds)

Almost never use derivative

May need to filter, often noisy

CONTINUED …


Analyzers and composition controllers

Often deadtime is significant

Can sometimes use PID if deadtime not too great

Be careful if discontinuous

If deadtime > 10min, consider use of a deadtime elimination

algorithm

CONTINUED …


Cascade Loops - Requirement


Cascade Loops - Requirement

A failure of the control valve to reposition itself correctly following a reversal in the control signal – level will cycle !!


Cascade Loops - Importance


Cascade Loops - Importance

Disturbances to the inner loop are compensated for before

they upset the outer loop

Prevents non-linearities in the inner loop from reaching the

outer loop


Cascade Loop Tuning

Inner Loop Tuning - put slave into Local Auto or Manual and

tune the slave controller as a normal PID loop.

Outer Loop Tuning - put slave into Cascade and tune master

controller as a normal PID loop.

Adjust outer loop tuning values to ensure that the Relative

Response Time of outer loop is 3-5 times slower than the

inner loop.

PID Tuning

April 27, 2023 Reliance Technology Group – APC / RTO 48

1. Break TAC & Cascades wherever required2. Re-range transmitters where required 3. Provide filters to reduce instrument noise in control loops where required 4. Provide filters to reduce process noise in control variables where required5. Identify new instruments required6. Identify sticky control valves (wrong size or type)7. Correct weak Actuator 8. Tune Positioner 9. Tune PID controllers10.Avoid Overrides & Split range operations during step test & operations

No amount of Advanced Control which relies on the use of poor field instrumentation and regulatory control (PID) can be expected to yield worthwhile benefits

Question or Suggestions

04 pid tuning

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