Control configuration in digital control
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This document discusses cascade, ratio, and feedforward control techniques. Cascade control reduces the effect of specific disturbances by using a faster slave loop. Ratio control reduces the effect of feed flow rate changes. Feedforward control compensates for measured disturbances before they affect the process. The document provides examples of how each control technique can be applied and their advantages and limitations.
Control configuration in digital control
- 1. To Cascade, Ratio, and Feed forward Control
- 2. Develop the skills necessary to function as an industrial process control engineer. Skills Tuning loops Control loop design Control loop troubleshooting Command of the terminology Fundamental understanding Process dynamics Feedback control
- 3. Each of these techniques offers advantages with respect to disturbance rejection: Cascade reduces the effect of specific types of disturbances. Ratio reduces the effect of feed flow rates changes Feedforward control is a general methodology for compensating for measured disturbances.
- 4. -6 -3 0 3 6 0 10 20 30 40 50 Time (seconds) T'(K) FB-only Compensating for disturbances
- 6. Without a cascade level controller, changes in downstream pressure will disturb the tank level. With cascade level controller, changes in downstream pressure will be absorbed by the flow controller before they can significantly affect tank level because the flow controller responds faster to this disturbance than the tank level process.
- 7. Secondary loop should reduce the effect of one or more disturbances. Secondary loop must be at least 3 times faster than master loop. The CV for the secondary loop should have a direct effect on the CV for the primary loop. The secondary loop should be tuned tightly.
- 9. Without cascade, changes in the cooling water temperature will create a significant upset for the reactor temperature. With cascade, changes in the cooling water temperature will be absorbed by the slave loop before they can significantly affect the reactor temperature.
- 10. FT AC AT TCTT FC RSP RSP This approach works because the flow control loop is much faster than the temperature control loop which is much faster than the composition control loop.
- 11. TT PT Condensate Steam Feed Draw schematic: A temperature controller on the outlet stream is cascaded to a pressure controller on the steam which is cascaded to a control valve on the condensate.
- 13. Useful when the manipulated variable scales directly with the feed rate to the process. Dynamic compensation is required when the controlled variable responds dynamically different to feed rate changes than it does to a changes in the manipulated variable.
- 14. Time ImpurityConcentration w/ ratio control w/o ratio control
- 16. The flow rate of base scales directly with the flow rate of the acidic wastewater. The output of the pH controller is the ratio of NaOH flow rate to acid wastewater flow rate; therefore, the product of the controller output and the measured acid wastewater flow rate become the setpoint for the flow controller on the NaOH addition.
- 19. FT FT TT Flue Gas Process Fluid Fuel Draw schematic: For a control system that adjusts the ratio of fuel flow to the flow rate of the process fluid to control the outlet temperature of the process fluid. Use a flow controller on the fuel.
- 21. Make-up Water To Steam Users LT LC Make-up Water To Steam Users LT FT FF To Steam Users LT FT FF LC + Make-up Water
- 22. Feedback-only must absorb the variations in steam usage by feedback action only. Feedforward-only handle variation in steam usage but small errors in metering will eventually empty or fill the tank. Combined feedforward and feedback has best features of both controllers.
- 25. Time cff ld/lg = ½ ld/lg = 1 ld/lg = 2
- 26. ffff KsG )( A static feedforward controller make a correction that is directly proportional to the disturbance change. A static feedforward controller is used when the process responds in a similar fashion to a change in the disturbance and the manipulated variable.
- 28. Q ToTi 0 2 4 6 8 10 Time (minutes) To 2 Ti 10ºC 10ºC 0 2 4 6 8 10 Time (minutes) 10ºC 10 kW Q To
- 29. -4 0 4 8 12 0 2 4 6 8 10 Time T'(ºC) 6.5 ºC When the inlet temperature drops by 20ºC, Q is immediately increased by 20 kW. Deviations from setpoint result from dynamic mismatch
- 30. -30 -15 0 15 30 0 2 4 6 8 10 Time (minutes) T'(ºC) Ti effect Net result FF Effect FF correction is mirror image of disturbance effect. Net effect is no change in controlled variable.
- 31. Since the Q affects the process slower than Ti , initially overcompensation in Q is required followed by cutting back on Q to 20 kW.
- 32. -4 0 4 8 12 0 2 4 6 8 10 Time (minutes) T'(C) w/o DC w/ DC
- 33. 0 5 10 15 20 Time (minutes) Q w/o DC w/ DC
- 34. Time cff ld/lg = ½ ld/lg = 1 ld/lg = 2
- 35. Make initial estimates of lead/lag parameters based on process knowledge. Under open loop conditions, adjust Kff until steady- state deviation from setpoint is minimized. Time y
- 36. Analyzing the dynamic mismatch, adjust ff. Time y
- 37. Finally, adjust (ld - lg) until approximately equal areas above and below the setpoint result. Time y
- 38. Can effectively eliminate disturbances for fast responding processes. But it waits until the disturbance upsets the process before taking corrective action. Can become unstable due to nonlinearity and disturbance upsets.
- 39. Compensates for d’s before process is affected Most effective for slow processes and for processes with significant deadtime. Can improve reliability of the feedback controller by reducing the deviation from setpoint. Since it is a linear controller, its performance will deteriorate with nonlinearity.
- 42. -6 -3 0 3 6 0 10 20 30 40 50 Time (seconds) T'(K) FB-only FF+FB FF-only
- 43. FB-only returns to setpoint quickly but has large deviation from setpoint. FF-only reduces the deviation from setpoint but is slow to return to setpoint. FF+FB reduces deviation from setpoint and provides fast return to setpoint.
- 44. TT PT Condensate Steam Feed TT Draw schematic: For a combined feedforward and feedback controller in which the inlet feed temperature is the feedforward variable and the outlet temperature is the feedback variable. The combined controller output is the setpoint for a steam pressure controller.
- 46. Cascade can effectively remove certain disturbances if the slave loop is at least 3 times faster than the master loop. Ratio control is effective for processes that scale with the feed rate. Feedforward can be effective for measured disturbances for slow responding processes as long as the process nonlinearity is not too great.