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A control system provides an aggregate driven quantity demand signal for controlling an actuatable component. The system has a summing junction which generates the aggregate driven quantity demand signal by summing a first output signal, which converges on a steady state driven quantity requirement

A control system provides an aggregate driven quantity demand signal for controlling an actuatable component. The system has a summing junction which generates the aggregate driven quantity demand signal by summing a first output signal, which converges on a steady state driven quantity requirement value, and a change in driven quantity demand signal. The system further has a feedback loop which generates the first output signal in response to the aggregate driven quantity demand signal. The system also has a first variable gain which tunes the change in driven quantity demand signal in response to a reference demand signal. The feedback loop includes a second variable gain which tunes the rate at which the first output signal converges on the steady state driven quantity requirement value. Typically, the reference demand signal corresponds to a desired value or desired change in value of a parameter which is itself varied or controlled by actuation of the actuatable component.

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1. A control system which provides an aggregate driven quantity demand signal for controlling an actuatable component, the system comprising: a summing device which generates the aggregate driven quantity demand signal by summing a first output signal, which converges on a steady state driven quanti

1. A control system which provides an aggregate driven quantity demand signal for controlling an actuatable component, the system comprising: a summing device which generates the aggregate driven quantity demand signal by summing a first output signal, which converges on a steady state driven quantity requirement value, and a change in driven quantity demand signal,a feedback loop which generates the first output signal in response to the aggregate driven quantity demand signal, anda first variable gain unit which tunes the change in driven quantity demand signal in response to a reference demand signal;wherein the feedback loop includes a second variable gain unit which tunes a rate at which the first output signal converges on the steady state driven quantity requirement value. 2. A control system according to claim 1, wherein the feedback loop includes an inverse static process module which determines steady state driven quantity values as a function of a steady state reference characteristic. 3. A control system according to claim 1, the control system being a fluid flow engine-casing cooling air control system, wherein: the aggregate driven quantity demand signal is an aggregate valve position demand signal,the actuatable component is a cooling air flow metering valve which regulates a cooling air flow to an engine casing,the steady state driven quantity requirement value is a steady state valve position requirement value,the change in driven quantity demand signal is a change in valve position demand signal andthe reference demand signal is a cooling air flow demand signal. 4. A control system according to claim 1, wherein the feedback loop includes an integrator device which, preliminary to the generation of the first output signal by the feedback loop, integrates an estimate of a rate of change of a reference characteristic to provide an estimate of the reference characteristic. 5. A control system according to claim 4, wherein the feedback loop receives a second output signal which converges on a value of the rate of change of the reference characteristic with a driven quantity increment, the feedback loop combining the second output signal with the aggregate driven quantity demand signal to provide the estimated rate of change of the reference characteristic; and the feedback loop has a subsidiary loop which generates the second output signal in response to the estimate of the reference characteristic, the second variable gain unit also tuning the rate at which the second output signal converges on the rate of change of reference characteristic with the driven quantity increment. 6. A control system according to claim 5, wherein the feedback loop includes a transient process module in the subsidiary loop which determines the rate of change of the reference characteristic with the driven quantity increment as a function of the reference characteristic. 7. A control system according to claim 4, wherein the second variable gain unit acts on the estimated rate of change of the reference characteristic (i) before it enters the integrator device, or (ii) after it is provided by the integrator device. 8. A control system according to claim 5, wherein the second variable gain unit acts on the second output signal generated by the subsidiary loop before the second output signal is combined with the aggregate driven quantity signal. 9. A control system according to claim 2, wherein the second variable gain unit acts on the steady state driven quantity requirement determined by the inverse static process module to modulate the value of the first output signal. 10. A control system according to claim 1, wherein the second variable gain unit varies as a function of a modelled or actual value of a reference characteristic. 11. A control system according to claim 1, wherein the summing device, the feedback loop and the first variable gain unit are provided by one or more processors. 12. A control system according to claim 1, further comprising an actuatable component which is controlled by an aggregate valve position demand signal. 13. A control system according to claim 1, the control system being an engine fuel control system, wherein: the aggregate driven quantity demand signal is an aggregate fuel flow demand signal,the actuatable component is a fuel flow metering valve which regulates a fuel flow to the engine,the steady state driven quantity requirement value is a steady state fuel flow requirement value,the change in driven quantity demand signal is an overfuelling demand signal, andthe reference demand signal is a thrust or speed demand signal,wherein the engine fuel control system has a selector control system which includes the first variable gain unit, such that the selector control system generates the overfuelling demand signal in response to the thrust or speed demand signal. 14. A control system according to claim 13, wherein the feedback loop includes an engine model which has an inverse static process module which determines steady state fuel flow requirement as a function of steady state engine speed. 15. A control system according to claim 13 further comprising a fuel flow metering valve which is controlled by the aggregate fuel flow demand signal. 16. A control system according to claim 15, the control system fitted to an engine, wherein the fuel flow metering valve regulates a fuel flow to the engine. 17. A control system according to claim 3, further comprising a cooling air flow metering valve which is controlled by the aggregate valve position demand signal. 18. A control system according to claim 3, wherein the feedback loop includes an inverse static process module which determines steady state driven quantity values as a function of a steady state reference characteristic which is the fluid flow rate. 19. A control system according to claim 17, the valve regulating cooling air flow to a casing of the engine.