FEEDFORWARD (SEBORG – CHAPTER 15)

FEEDFORWARDAdvantages Disadvantages

FEEDBACKCorrective action occurs as soon as the CV deviates from the set point, regardless of the source and type of disturbance

No corrective action is taken until a deviation in the controlled variable occurs. Thus, the perfect control, is theoretically impossible.

Feedback control requires minimal knowledge about the process to controlled.

Does not provide predictive control action to compensate for the effects of known or measurable disturbances

The PID controller is both versatile and robust. If the process conditions change, retuning the controller usually produces satisfactory control

Does not satisfied processes with large time constant and/or large time delays.

Some situations, the CV cannot be measured online, consequently feedback control is not feasible

FEEDFORWARD

Act before the disturbance upset the system Disturbances must be measured online,

Good for slow system or with significant dead time

The quality of the control depends on the model

Does not introduce instability for closed loop response

Ideal feedforward may not be physically realizable.

FEEDBACK- FEED FORWARD SYSTEM

In practical – feed forward will be combined with feedback system.

Based on the block diagram , the closed loop transfer function for disturbance change is derived as

For perfect control ,CV must remains at set point despite the changes in D. This condition can be satisfied by setting the numerator of the above equation equal to zero and solving for Gf, gives us

HOW TO TUNE FF ? For disturbance transmitter and valve

where Kt & Kv denote a steady state gain Suppose,

From

Rearrange all the transfer function;

This controller is consider a lead-lag unit, with a gain of

Example:

Then

This is unrealizable, because negative time delay. Approximate by increasing

TUNING FEEDFORWARD

Step 1 – Adjust Kf Step 2 – Determine initial values for 1 and 2

Step 3 – Fine tune 1 and 2

RATIO CONTROL(SEBORG – CHAPTER 15, SMITH CORRIPIO – CHAPTER 10 & LUYBEN-

CHAPTER 8 )

A type of feed forward control The objective is to maintain the ratio of two

variables at a specified value One variable is manipulated to keep it as ratio of

another. Implemented in two basic schemes.

multiplier

divider

Required to blend 2 liquid streams, A and B in some ratio

R=FB/FA

•Measuring the flow in steam A

•Multiplying it with the desired ratio to obtain the required flow rate in stream B (set point to the flow controller of stream B)

•FBset = R x FA

•If the flow of stream A varies, the set point of the controller of stream B will vary.

Multiplier : set the ratio

The flow of stream B must vary, as the flow rate of stream A is varies

(Wild flow)

•Measuring both stream

•Divide them in FY16 to obtain the actual ratio flowing through the system

•R=FB/FA

•The calculated ratio is the sent to the controller which manipulated the flow B to maintain set point.

•The set point to this controller is the required ratio.

Divider

TYPICAL APPLICATION OF RATIO CONTROL

1. To maintain a ratio of feed flow rate and the steam in the reboiler of a distillation column

2. Maintaining the stoichiometric ratio of reactants to a reactor

3. Maintaining the reflux ratio in distillation column4. Maintaining the ratio of fuel and air to a furnace at the

optimum combustions5. Hold the ratio of two blended streams, in order to

maintain the compositions of the blending.6. Hold the ratio of purge stream to the recycle streams7. Maintaining the ratio of the liquid flow rate to vapor flow

rate in an absorber, in order to achieve desired composition in the exit vapor stream.

TYPICAL APPLICATION OF RATIO CONTROL

TYPICAL APPLICATION OF RATIO CONTROL

EXAMPLE OF RATIO CONTROL

• A control system for blending two liquid streams, A and B.

• Stream A is a wild stream, is used to adjust the flow of stream B so that the desired ratio is maintained.

• The signal for stream A is multiplied by the desired ratio Kr, to provide signal that is the set point to the flow control loop for stream B.

• The parameter Kr, is adjusted to desired ratio.

Kr = ratio For incompressible fluid, no lag between the change in valve position and the flow rate. Hence = 1

Gm , represents the dynamic lag of the flow-measuring for stream A.

The control action is normally PI controller.

FB,FF, FF-FB & RATIO CONTROL OF BLENDINGSYSTEM(LUYBEN - CHAPTER 8)

FB FF

Fig 8.7 a Fig 8.7 b

FF-FB RATIO

FB,FF, FF-FB & RATIO CONTROL OF BLENDINGSYSTEM(LUYBEN - CHAPTER 8)

Fig 8.7 dFig 8.7 e

REFERRING TO THE EXAMPLE OF RATIO CONTROL,SIMULATE THE EXAMPLE OF RATIO CONTROL, BYCHOOSING YOUR OWN KR VALUE & TUNE YOUR PID

ACCORDINGLY.

EXERCISE : 1

EXERCISE 2

With respect to Fig. 8.7 c & d, design the control block diagram that can be simulated using a SIMULINK