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Topic 5

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  1. Topic 5 Enhanced Regulatory Control Strategies

  2. In the previous lecture… • Plant test • Interacting loops

  3. Topic 1 Introduction To Process Control Topic 2 Introduction To Process Dynamics Topic 3 Plant Testing And Data Analysis Topic 7 Process Control Hardware Systems Topic 8 Control Valves Topic 9 Process Control Troubleshooting What We Will Cover Topic 4 Controller Actions And Tuning Topic 5 Enhanced Regulatory Control Strategies

  4. In This Lecture… • Cascade Control • What is cascade control • Advantages of cascade control • Features of cascade control • Testing & tuning cascade control loops

  5. Furnace Example

  6. Controlling FOT with FG

  7. Controlling FOT with FG • In the diagram, the CV is controlled using the fuel gas valve opening as an MV • From our plant test, we obtain values for Kp • Kp tells us how much to open our valve by if we want the FOT to increase by a certain amount • Using this Kp value together with θandτ, we calculate PID tuning parameters

  8. Controlling FOT with FG • Take a situation where TIC is in AUTO and the PV is at SP. • Suddenly, the FG source pressure increases. • What do you think will happen to the FG flow? • What do you think will happen to the FOT?

  9. Controlling FOT with FG • If the FG source pressure increases, for a constant valve opening, the FG flow will increase • FOT increases due to the increase in FG flow. This happens after a while, because of deadtime and lag • When will the controller start to take action? • The controller starts to reduce the valve opening only after the FOT starts to increase • Can we prevent the FOT from increasing by reducing the valve opening early? • Yes, control the FG flow rate instead. • If the FG flow rate increases due to increased pressure, the flow controller will close the valve to keep the flow rate to SP. • Because flow dynamics is much faster than FOT dynamics, FG flow rate will be brought back to SP before FOT gets disturbed

  10. Controlling FOT with FG

  11. Controlling FOT with FG • Now, the control valve is the MV for controlling the FG flow rate • To control the FOT, the flow SP will need to be adjusted either manually or automatically • To do this automatically, we use a FOT controller and we link its OP to the SP of the flow controller • This is called a Cascade Controller

  12. FOT Cascade Control Master or Primary controller Slave or Secondary controller

  13. Controlling FOT with FG • The FOT PID controller calculates what FG flow rate is required to maintain FOT to SP • The flow PID controller calculates what valve opening is required to maintain flow rate to SP • Because the flow dynamics are faster than the FOT dynamics • Disturbances to the flow rate are detected much faster than disturbances to the FOT • Disturbances to the flow rate are rejected much faster than disturbances to the FOT • Flow controller can take action to restore flow rate to SP before it disturbs the FOT • FOT controller to control valve, on the other hand, requires the FOT to be disturbed before it can take action • Cascade control allows the FOT to be isolated from disturbances to the FG flow

  14. Different dynamics at different P • Consider the single-loop case again: CV-FOT, MV-Valve opening • At a normal pressures, • Increasing the valve opening results in a corresponding increase in FOT • SS Gain Kp obtained during plant test • At higher pressures, • Increasing the valve opening results in a greater increase in FOT due to the larger increase in FG flow • Higher SS Gain, Kp, at higher pressures • Ideally, PID tuning should be changed because of different dynamics • Cascade control allows the primary controller to be isolated from changes in dynamics in the secondary loop

  15. Cascade Control

  16. Features Of Cascade Control • At least two controllers are involved • A primary (master) controller • A secondary (slave) controller • More complex cascade controls have one or more secondary loops and one ultimate primary and one ultimate secondary • The primary controller is the one whose PV is the important one • In the Furnace example, the FOT controller is the primary controller because we care about the FOT, not the FG flow rate

  17. Features Of Cascade Control • The OP of the primary controller becomes the SP of the secondary controller • They are configured such that OP (in %) is changed into SP (in other engineering units like kg/hr etc…) • E.g. span of secondary is 100~300 kg/hr • Pri.OP = 0%  Sec.SP = 100 kg/hr • Pri.OP = 100%  Sec.SP = 300 kg/hr • Pri.OP = 50%  Sec.SP = 200 kg/hr

  18. Features of Cascade Control • Important! • Cascade control will only work if the secondary loop dynamics is much faster than the primary loop • Secondary loop has shorter deadtime and shorter lag • This means secondary loop is tuned much faster, i.e. smaller τI • This allows the secondary to bring its CV to SP before the primary demands a new SP change • If primary is faster than secondary, • Primary controller will adjust the secondary’s SP faster than the secondary can bring its PV to SP • Primary controller does not know the secondary has not reached SP yet • All it knows is that its own PV has not reached SP and it will adjust the secondary’s SP by larger amounts subsequently

  19. Advantages Of Cascade Control • 2 advantages: • Disturbances arising within the secondary loop are corrected by the secondary controller, often before they can influence the primary variable • SS Gain variations in the secondary part of the process are overcome within its own loop

  20. Testing Cascade Control Loops

  21. Testing Cascade Control Loops • Remember that there are two loops to test • Secondary loop (fuel gas flow) • Primary loop (furnace outlet temperature) • Which one to test first? • The secondary must work well first, before the primary can do its job • Imagine a good boss with loads of vision and insight instructing a lazy employee! No matter how good the instructions are, will work get done? • Slacker students will never do well even if the lecturer is excellent! Good thing that all present here are dilligent...

  22. Tuning Cascade Control Loops Tune primary after tuning secondary Tune secondary first

  23. Testing The Secondary Loop • Always start with the secondary loop first • Put it in manual • Step it a few times • Identify its dynamics • Tune the flow controller • Test it to ensure that it works • Test it as per single-loop controller (Topic 3)

  24. Testing The Primary Loop • Primary loop in what mode? • Manual • Secondary loop in what mode? • Cascade • Step the primary’s OP and observe its CV response

  25. Tuning Of Cascade Loops • Important reminder: • It is critical that the secondary loop must be working properly on its own if it is to act as a secondary loop • The secondary loop must be a much faster loop compared to the primary loop

  26. In This Lecture… • Cascade Control • What is cascade control • Advantages of cascade control • Testing cascade control loops • Tuning cascade control loops

  27. In The Next Lecture… • Feedforward Control • Measured Vs Unmeasured Loads • Purpose of feedforward control • Feedforward gain • Deadtime compensation