Feedforward and ratio control

In class activities

Activities

1. Consider a boiler drum as shown in the figure below

Figure 1: Boiler drum

1. Identify controlled variable, manipulated variable and disturbance

2. Draw a conventional feedback control to maintain liquid level in the boiler.

3. What are the drawbacks of this system?

Solution

Controlled variable: Boiler drum liquid level.

Manipulated variable: Feedwater flow, via the feedwater control valve or pump speed.

Major disturbances: Steam demand, furnace heat input, drum pressure changes, feedwater pressure or temperature changes, and blowdown flow.

Figure 2: Boiler drum simple feedback control

Drawbacks of this simple system:

• Poor rejection of steam flow disturbances. A sudden increase in steam demand lowers level before the controller reacts.

• Shrink and swell effects. Pressure and boiling changes cause apparent level changes that mislead the controller.

2. Consider the operation of heat exchanger to demonstrate the application of feedforward control strategy. Identify all disturbances and the target disturbance to be remove using the feedforward control strategy

Figure 3: Heat exchanger

Solution

Objective: keep outlet temperature at setpoint by manipulating steam flow.

Controlled variable: Outlet temperature, \(T_{out}\).

Manipulated variable: Steam flow to the exchanger.

Important disturbances:

• Feed (inlet) flow rate, \(F_{in}\).
• Feed (inlet) temperature, \(T_{in}\).
• Product draw‑off flow changes (level control actions).
• Steam supply pressure/temperature to the valve.
• Exchanger fouling or changes in overall \(UA\).
• Condensate backpressure or trap performance.
• Pump/recirculation rate variations.
• Ambient heat losses and property changes (\(C_p\), \(\rho\)).

Target disturbance for feedforward:

• The dominant, measurable load changes: inlet flow \(F_{in}\) and inlet temperature \(T_{in}\). Use them to compute the needed steam flow setpoint

Figure 4: Heat exchanger feedback feedforward control

3. For the blending system shown in Figure 5

Figure 5: Blending system

1. Identify controlled variable

2. Manipulated variable

3. Disturbance

4. Propose a feedforward strategy to control outlet composition

5. Propose a strategy to control composition when inlet flowrate \(w_2\) is fluctuating

6. propose a feedforward feedback control strategy

Solution

1. Identify controlled variable

• Outlet composition x

2. Manipulated variable

• the flow of stream 2, w2

3. Disturbance

• Stream‑1 flow and composition, w1 and x1
• Stream‑2 composition x2 (and sometimes w2 if it is not the MV)
• Density or temperature changes that affect analyzer or flowmeters

4. Propose a feedforward strategy to control outlet composition

Figure 6: Blending system feedforward strategy

5. Propose a strategy to control composition when inlet flowrate \(w_2\) is fluctuating

Figure 7: Blending system feedback strategy

6. propose a feedforward feedback control strategy

Figure 8: Blending system feedforward-feedback strategy

4. The process, and disturbance transfer functions for problem 1 are given by

   \[G_p(s)= \frac{0.1 e^{-s}}{2.5s + 1} \tag{1}\] \[G_d(s)=\frac{0.5 e^{-s}}{2.5s + 1} \tag{2}\]

   use PID tuner app to tune a PID controller for this process.

5. Propose a feedforward control system for Figure 1. Explain how a feedforward control can improve disturbance rejection or regulatory control performance.

6. Calculate ideal feedforward compensator for problem 1 and implement pure feedforward control scheme in simulink.

7. Draw a combined feedforward-feedback PID for Figure 1. Implement feedforward-feedback system in simulink.

8. Consider the following process and disturbance transfer functions:

   \[ \text { Process: } G_p(s)=\frac{2 \exp (-4 s)}{15 s+1} \tag{3}\]

   \[ \text{Disturbance: }G_d(s)=\frac{5 \exp (-2 s)}{30 s+1} \tag{4}\]

   Do the following:

   1. For the process given above, can an idealized feedforward controller be used to reject the disturbance.

   2. Obtain a feedforward controller and PID controller for the process above using the unified feedforward-feedback control method.

9. Ratio control for ammonia synthesis reactor.

10. Furnace ratio control Figure 9

    Draw the schematic of a ratio control strategy. What is the target disturbance? What is the manipulated variable? What is the controlled variable?

Figure 9: Furnace

Solution

Figure 10: Furnace ratio control

11. Wastewater pH neutralization Figure 11

    Devise a ratio control strategy to provide desired flow rate of NaOH solution to maintain the pH of the effluent.

Figure 11: Wastewater treatment tank

Solution

Figure 12: Wastewater treatment tank ratio control