Feedforward control is based on principle of pre-emptive load counter-action i.e. if all major loads (i.e. disturbances) on a process variable are monitored, and their effects on that process variable are well-understood, a control system programmed to take the appropriate action based on load changes will shield the process variable (PV) from any adverse effect.
Cascade control reduces the effect of disturbances occurring early in the forward loop but generally cannot deal with load/demand disturbances which occur close to or affect directly the process variable as there is no intermediate variable or accessible control point.
Disturbances directly affecting the process variable must produce an error before the controller can react. Certainly therefore, the output signal will suffer, with the speed of recovery being determined by the loop response. Plants that are difficult to control tend to have low gains and long integral times for stability and therefore have slow response. Such plants are prone to error from disturbances.
Generally, a closed loop system can be considered to behave as a second order system with a natural frequency wn and a damping factor. At frequencies above wn, the closed loop gain falls off rapidly (at 12 dB/octave). Disturbances occurring at a frequency much above 2wn will be uncorrected. If the closed loop damping factor is less than unity (i.e. representing an undamped system), the effect of disturbances with frequency components around the natural frequency wn of the system can be magnified.
Figure (a) below shows a system being influenced by a disturbance. Cascade control cannot be applied to this system because there is no intermediate variable between the point of entry and the process variable.
If the disturbance can be measured, and its effect known (or even approximated), a correcting signal can be added to the controller output signal to compensate for the disturbance as illustrated in Figure (b) in what is known as Feedforward Control.
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The correcting signal, arriving by blocks H, F and P1 should ideally exactly cancel the original disturbance; both in the steady state and dynamically under changing conditions.
The transfer functions of the transducer H and plant P1 are fixed with F a compensator block designed to match H and P1.
Generally the compensator block transfer function will be
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The feedforward compensation doesn’t have to match exactly the plant characteristics; even a rough model will give substantial improvement (through a perfect model will give a perfect control). Generally a simple compensator will be enough.
Bottom Line
Cascade control typically can deal with supply disturbances and feedforward with load or demand disturbances. These two control systems precisely complement each other, so it is very common to find a system where Feedforward modifies the setpoint for the inner cascade loop.
