This document discusses feedforward control in a process instrumentation and control system. It describes a three-tank system used to control the concentration of a solute stream. Feedforward control is introduced to improve quality of control for frequent load disturbances. A feedforward controller uses information about disturbances detected at the inlet stream to adjust the control valve and prevent changes in the outlet composition from the setpoint. The document provides equations to model the feedforward control system and discusses using constant terms for the feedforward controller transfer function.

1. ICE401: PROCESS INSTRUMENTATION
AND CONTROL
Class 34
Advanced Control Strategies –
Feedforward Control
Dr. S. Meenatchisundaram
Email: meenasundar@gmail.com
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015

2. Feedforward Control:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• If a particular load disturbance occurs frequently in a control
process, the quality of control can often be improved by the
addition of feedforward control.
• Consider the composition control system shown in Fig. in which
a concentrated stream of control reagent containing water and
solute is used to control the concentration of the stream leaving a
three-tank system.
• The stream to be processed passes through a preconditioning
stirred tank where composition fluctuations are smoothed out
before the outlet stream is mixed with control reagent.
• A three-tank system has been chosen for ease of computation in a
numerical example that follows.

3. Feedforward Control:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015

4. Feedforward Control:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• The values of Kc and tI were chosen by trial and error to give
the response to a step change in set point shown in curve II of
Fig; this response, which has a decay ratio of about 1/4, was
obtained with Kc 2.84 and tI 5.0.
• The Ziegler-Nichols settings (Kc 3.65 and tI 3.0) give a set point
response shown as curve I of Fig, which is too oscillatory.

5. Feedforward Control:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015

6. Feedforward Control:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• The response of the system to a step change in Ci of 10 units,
shown as curve I in of Fig. shown in previous slide.
• Note that the response is oscillatory and has a long tail.
• This response illustrates the fact that the feedback control system
does not begin to respond until the load disturbance has worked its
way through the forward loop and reaches the measuring element,
with the result that the composition can move far from the set point
during the transient.
• If the change in load disturbance Ci can be detected as soon as it
occurs in the inlet stream, this information can be fed forward to a
second controller that adjusts the control valve in such a way as to
prevent any change in the outlet composition from the set point.

7. Feedforward Control:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• A controller that uses information fed forward from the source of
the load disturbance is called a feedforward controller.
• The block diagram that includes the feedforward controller Gf
as well as the feedback controller Gc is shown in Fig. in next slide.

8. Analysis of Feedforward Control:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• The response of C to a change in Ci and R can be written as
Where, E(s)= R(s) – C(s)
1( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )i f p i c pC s G s C s G s G s C s G s G s E s= + +

9. Feedforward Control:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• To determine the transfer function of Gf (s) that will prevent any
change in the control variable C from its set point R, which is 0,
we solve Eq. for Gf (s) with C=0 and R=0. The result is
• For the example under consideration in Fig.
• Rather than use the Gf (s) of above Eq. in the feedforward
controller, one can try using only the constant term of Gf (s), that
is, -1, -0.5, etc., and the results are shown in previous plot.
( ) ( )f pG s G s= −
1
( )
5 1
fG s
s
−
=
+

10. References:
• Process Systems Analysis And Control, by Donald R Coughnowr,
Third Edition, Tata Mcgraw Hill.
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015