Controller Tuning

1. ICE401: PROCESS INSTRUMENTATION
AND CONTROL
Class 30
Controller Tuning
Dr. S. Meenatchisundaram
Email: meenasundar@gmail.com
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015

2. Open-Loop Transient Response Method:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• This method of finding controller settings was developed by Ziegler
and Nichols and is sometimes referred to as a process-reaction
method.
• The basic approach is to open the process control loop so that no
control action (feedback) occurs.
• This usually is done by disconnecting the controller output from the
final control element.
• All of the process parameters are held at their nominal values. This
method can be used only for systems with self-regulation.
• At some time, a transient disturbance is introduced by a small,
manual change of the controlling variable using the final control
elements.

3. Open-Loop Transient Response Method:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• This change should be as small as practical for making necessary
measurements.
• The controlled variable is measured (recorded) versus time at the
instant of and following the disturbance.

4. Open-Loop Transient Response Method:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• A typical open-loop controller response is shown in Figure, where
the disturbance is applied at t1.
• We have expressed the deviation as a percent of range as usual, and
we assume the final control element disturbance also is expressed
as a percentage change.
• A tangent line, shown as a dashed line, is drawn at the inflection
point of the curve.
• The inflection point is defined as that point on the curve where the
slope stops increasing and begins to decrease.
• Where the tangent line crosses the origin, we get
L = lag time in minutes
• As the time from disturbance application to the tangent line
intersection as shown in Figure.

5. Open-Loop Transient Response Method:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• Also from the graph we get T, the process reaction time, and
where
N = reaction rate in % / min
∆Cp = variable change in %
T = process reaction time in minutes
The quantities defined by Equations are used with the controlling
variable change to find the controller settings.
The following paragraphs give the stable control definitions for the
various modes as developed by Ziegler and Nichols.
pC
N
T
∆
=

6. Open-Loop Transient Response Method:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• Some corrections are developed by Cohen and Coon (when the
quarter-amplitude response criterion is indicated).
• In the latter case, a log ratio is used, defined by
where R = log ratio (unitless)
p
NL
R
C
=
∆
Mode Proportional
Gain
Integral Time
(Min)
Derivative
Time (Min)
P ∆P / NL --- ---
P + I 0.9 ∆P / NL 3.33 L ---
P + I + D 1.2 ∆P / NL 2 L 0.5 L

7. References:
• Process Control Instrumentation Technology, by Curtis D.
Johnson, Eighth Edition, Pearson Education Limited.
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015