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2.2 OPERATING PROCEDURE WITH FEEDFORWARD

Still considering the Taylor 1700R as a process controller example here . The addition of feedforward may change the operating procedure depending on what control modes are made available to the operator. The modes required for the application and made available are specified in the 'Allowed Control Modes' configuration for the controller.

Usually feedforward control is used with feedback control. If this is the only option required in addition to Manual, then the Allowed Control Modes entry in the data base memory is specified as M.B where B Feedforward-Feedback.

Whenever the operator pushes the AUTO/MODES pushbutton, the controller will transfer from Manual to Feedforward-Feedback control. When in this mode, two status indicators will be on: FF and AUT. AUT is the same as feedback control. Therefore, this combination, FF and AUT, indicates to the operator that the loop is being controlled by feedforward-feedback control .

Some applications require operating in other modes also. Feedforward or feedforward-feedback cannot be used in combination with cascade control., for example. The Allowed Control Modes specified during controller configuration determines what combinations of feedforward and feedback can be used by the operator. If the feedforward control application requires that the operator

pH Control System

More complicated feedforward calculations can be made in a 1700N Math Unit if required.

Reagent flow F2 cracking of the variations in influent flow Fl can be enhanced by adding flow control to F2 as shown in Figure 6(c). Valve characteristics no longer enter into the proper F2/Fl ratio consideration. The nominal design ratio F2/Fl should be for a value of one since the midrange on the multiplier in the feedforward calculation is 1. Full range is 0 to 2. Reagent flow F2 is now linear with the remote setpoint oil controller FC2. A cascade system is formed whereby controller pHC is the master and controller F2C is the slave. Feedforward input Fl is combined with the  output of pHC as shown in Figure 5. Usually the control modes required on controller pHC are Manual, Auto, and Feedforward-Feedback. Controller FC2 may require. Manual, Auto and Cascade .

Even though controller FCC is one instrument in a system that uses feedforward control it is not the one on which feedforward is applied. Therefore, the restriction that Cascade cannot be used in combination with either feedforward or feedforward-feedback mode does not apply.

Much simplified auto/cascade switching at the location of flow controller FC2 can be provided with the addition of a signal to be tracked and a track command as shown in Figure 7. 

Figure FO-1 shows the complete system as it would be installed with hard wiring between instruments. Hardwiring might be used, for example, if the instruments were physically separated by a distance greater than that allowed for communications over the Instrument Communications Network (ICN).

  

 

Process Example Template 6-1 applies to Figure FO-1 with its notations. Note that three pair of wires are required between controller pHC and flow controller FC2.

Figure FO-2 shows the same system described above except that the wiring between the two instruments is done with the ICN and requires only one  pair of wire .

Process Example Template 6-2 applies to Figure 9 with its notations.

 

 

Figure 4. Break Points Determine Where Gain Will Start to Change, Process, Deviation or Remote Input Adaptive Function .

 

Figure 5. Both Break Points Can Have Same Value, Process, Deviation or Remote Input Adaptive Function

2.3.1.2 Gain Factors

The value of the gain factor determines if the active gain or reset will increase or decrease, starting from the break points as shown in Figure 7.

If the gain factor is less than I (e.g., 0.875), the gain or reset will decrease. When the gain factor value is 1, the gain or reset will not change. If the gain factor value is greater than 1 (e.g., 2.625), the gain or reset will increase.

2.3 GENERAL DESCRIPTION OF ADAPTIVE GAIN OR RESET FUNCTIONS

2.3.1 Process, Deviation and Remote Input adaptive Gain or Reset functions

The parameters that define the adaptive algorithms for process, deviation and remote input functions are set up in the same manner. A large combination of break points, gain factors and active gain or reset limits can be used to provide many different adaptive algorithms. Because each adaptive algorithm is set up individually, the number of overall controller algorithms is very large. The adaptive functions can be on gain or reset, but not on both simultaneously.

2.3.1.1 Break Points

The location of the break points determines where the gain or reset will start to change as a function of-the adaptive variable as shown in Figure 4. The gain or reset value between the break points is the base gain or base reset and never changes. Both break points can be located at the same range value as shown in Figure 5, or they may be set at different values as shown in Figure 6. When the two break points are set at different values, Figure 6, BP1  must be at a lower range value than BP2. The active gain or reset between the two break points is equal to the base gain or reset (because the gain factor is always 1 between the two break points), while the active gain or reset beyond the break points changes as determined by the gain factors.

Figure 6. Break Points Can Have Different Values, Process, Deviation or Remote Input Adaptive Function

Figure 7. Gain Factor Values Determine if Gain or Reset Rat Will Increase or Decrease From Break Points, Process, Deviation or Remote Input Adaptive Function.

Both gain factors can increase, both can decrease, or either gain factor can increase while the other decreases active gain or reset. The combinations of gain factors and break points can be used to produce a large variety of algorithms.. Some of the possibilities are shown in Figure 8.

The gain factor values are multipliers. The base gain or reset is multiplied by the gain factor for the adaptive gain or reset function. The result is the value of active gain or reset at one, and only one, point on the range. This point, Figure 9, will be 10% downscale from BPI or 10% upscale from BOB, and it establishes the slope of the line in the algorithm.

 

 

 

 

 

 

 

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Figure 8 . Various combinations of Breakpoints and Gain Factors that can be used to produce a Large Variety of Algorithms , Process , Deviations or Remote Input Adaptive Functions .

 

 

 

 

Figure 9 . Gain Factors Establish Slope of Algorithm beyond Break Points , Process , Deviations or Remote Input Adaptive Function .

 

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