Combined feedback + feedforward control

Goals

• Indicate the concept and strategy of combined feedback and feedforward control

• Demonstrate how to develop tuning procedures for combined feedback and feedforward control.

The feedforward concept

The prev. section highlighted some concepts of feedforward control and showed that one problem it gives us is drifting of the PV from the systems SP value. This is caused solely because the PV is not taken into account in feedforward control, if it was it would become a feedback (closed loop) controlled system. Examination of the feedforward concept shows us that it’s normally used to minimize the impact of disturbances on a process. This is achieved by detecting and measuring a process disturbance and changing a related manipulated variable before the disturbance has an adverse effect on the process itself. It’s important to remember that process disturbances constitute anything from unexpected changes in either: magnitude of pressure, flow, temperature and any other physical quantity associated with the process or changes in time, of any of the process responses. This latter variable, time, is very often overlooked as a quantity that may need correcting in a process environment. This is where we use feedforward to equalize the difference in heating and cooling times of a feed heater system.

This should make the process responses both in magnitude and time, the same, irrespective of the direction taken by the PV value. If this is achieved, tuning of the system is made easier, with the result the control is more stable and accurate.

The feedback concept

section 5 explains the concepts of closed loop control and stability as related to feedback systems. In general terms it's accepted that a feedback system operates more accurately and efficiently if both process disturbances and time delays (lag times) are kept to the minimum. It then becomes apparent that feedforward control can be used to achieve this requirement being made by the feedback control. If we then combine both an accurately con figured feedforward system with a well tuned feedback system the result should become almost an optimally operating control system.

Combining feedback and feedforward control

+=+=+=+a concept where we combine both control methods into our feed heater control system.

+=+=+=+ Combined feedback and feedforward control 'Controller output modes, operating equations and cascade control' and 'Concepts and applications of feedforward control' cover most of the aspects which have to be considered when using feedback and feedforward control. Here we will concentrate on the impact of the summer block and some tuning aspects of the combined control system.

Feedback-feedforward summer

We notice that we have only one manipulated variable, the fuel flow, but two control concepts combined. The mass flow feedforward control, equating as and the feedback cascaded control would appear to compete for the use of the one manipulated variable, the fuel flow.

However, if we remember the concept of compensation which governs feedforward control, the output of the feedforward control (from the lead/lag block) would usually be passed directly onto the SP of our fuel flow controller. To this value coming from the feedforward control, we have to add the output value of the primary controller (temperature controller, TC). It’s important to remember that these are incremental and not absolute calculations.

Initialization of a combined feedback and feedforward control system

As we have a combination of feedforward, feedback and cascade control the method of initialization is important.

The value for the initialization of the OP of the primary controller (TC) is calculated from the sum of:

• The value of the feedforward signal from the lead/lag block and (+)

• The SP value of the secondary controller (FC). As fluctuations occur to the inlet temperature and the inlet flow rate varies (F1 and T1 variables), and depending on which direction (up or down) they occur, the output of the lead / lag block will vary in accordance with the functional algorithm and lead/lag time constants which comprise the feedforward control. The magnitude and rate of change of this signal has to be compatible with any of the F1 and T1 variances so that they have minimal or no influence on the outlet temperature T2.

The summing block should be considered as part of the output block of the primary controller (TC).

Tuning aspects

Since strong feedback control has a tendency toward instability, it should be avoided if possible. Therefore, if the feedforward control is already doing the major part of the required control and feedback control is just there to eliminate drift of PV, proceed as follows:

• Tune the secondary controller used by feedback and feedforward control

• Tune the feedforward control

• Tune feedback control using the formulas developed by Ziegler and Nichols

• Evaluate the speed of drift of PV. If the drift of the process variable is insignificant, reduce K of the primary controller using process knowledge and personal judgment. Remember that in this case, the feedback control is just a supplement to feedforward control and must not introduce any form of oscillations or instability.

Exercise: Combined feedforward and feedback control Will give practical experience of combined feedback and feedforward control.

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