Feedforward control Subject

Ratio control and multiphcative feedforward control, in general, are subject to the same considerations. Ratio control can be of a steady-state or a dynamic form. It is often implemented using a setpoint as the load variable when the load variable has a controller associated with it and the controller is in auto mode. 

We do not yet have all the tools to deal quantitatively with feedforward controller design. When our Russian lessons have been learned (Laplace transforms), we will come back to this subject in Chap. 11. 

When processes are subject only to slow and small perturbations, conventional feedback PID controllers usually are adequate with set points and instrument characteristics fine-tuned in the field. As an example, two modes of control of a heat exchange process are shown in Figure 3.8 where the objective is to maintain constant outlet temperature by exchanging process heat with a heat transfer medium. Part (a) has a feedback controller which goes into action when a deviation from the preset temperature occurs and attempts to restore the set point. Inevitably some oscillation of the outlet temperature will be generated that will persist for some time and may never die down if perturbations of the inlet condition occur often enough. In the operation of the feedforward control of part (b), the flow rate and temperature of the process input are continually signalled to a computer which then finds the flow rate of heat transfer medium required to maintain constant process outlet temperature and adjusts the flow control valve appropriately. Temperature oscillation amplitude and duration will be much less in this mode. 

On many columns the decision is not clear cut. Here the approach should be to make a preliminary selection of one of the schemes, identify its limitations and attempt to enhance the scheme to deal with these. If this fails then switch to the alternative and enhance this one. For example we might have good reasons to select the material balance scheme but the column is subject to changes in feed rate. Installation of the feedforward scheme shown in Figure 12.60 will maintain a constant D/F ratio and so overcomes this limitation. While not quite as simple as drawn, a full description of feedforward control is presented later in this chapter. 

Perfect compensation is unattainable. For one thing, any process sufficiently problematic to warrant feedforward control can be expected to display some dead time in addition to capacity. This is true of the heat exchanger. But compensation for dead time is, at best, approxl mate. Second, the dynamics of most processes are subject to change. 

Feedforward is commonly applied to level control in a drum boiler. Because of the low time constant of the drum, level control is subject to rapid load changes. In addition, constant turbulence prevents the use of a narrow proportional band, because this would cause unacceptable variations in feedwater flow. The feedforward system simply manipulates feedwater flow to equal the rate of steam being withdrawn, since this rqiresents the load on drum level. The system is shown in Fig. 8.2. 

For control loops represented in the CCR (central control room), it is normal practice (as mentioned earlier) to furnish control stations. These may be analog pneumatic, analog electronic, or microprocessor based. In the last case, the station may be physically distinct, like an analog station, or may be represented on a CRT display as a faceplate. Each provides an indication of the process variable (flows, level, temperamre, etc.), the desired value (set point), and the valve loading signal (controller output, really). There is also a manual-automatic switch, which some vendors label hand-automatic. In the manual mode, the feedback controller is disconnected and there is a knob that enables the operator remotely to set the valve position. This may or may not be subject to restrictions imposed by feedforward compensation, overrides, and so on, depending on the design philosophy for a particular project. 

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