Feedforward control--concepts + applications

Goals

• Describe the concept and strategy of feedforward control

• Develop and then clearly describe the tuning procedures for feedforward control.

Application and definition of feedforward control

If, within a process control's feedback system, large and random changes to either the PV or lag time of the process occur, the feedback action becomes very ineffective in trying to correct these excessive variances. These variances usually drive the process well outside its area of operation, and the feedback controller has little chance of making an accurate or rapid correction back to the SP term. The result of this is that the accuracy and standard of the process becomes unacceptable. Feedforward control is used to detect and correct these disturbances before they have a chance to enter and upset the closed or feedback loop characteristics.

It must be remembered that feedforward control does not take the process variable into account; it reacts to sensed or measurement of known or suspected process disturbances, making it a compensating and matching control to make the impact of the disturbance and feedback control equal.

The difference between feedforward and feedback control can be considered as:

• Feedforward is primarily designed and used to prevent errors (process disturbances) entering or disturbing a control loop within a process system.

• Feedback is used to correct errors, caused by process disturbances, that are detected within a closed loop control system. These errors can be foreseen and corrected by feedforward control, prior to them upsetting the control loop parameters.

It’s this second factor alone that makes feedforward a very attractive concept.

Unfortunately, for it to operate safely and efficiently, a sound knowledge is required both of the process and the nature of all relevant disturbances.

Manual feedforward control

Feedforward is a totally different concept to feedback control. A manual example of feedforward control. Here, as a disturbance enters the process, it’s detected and measured by the process operator. Based on their knowledge of the process, the operator then changes the manipulated variable by an amount that will minimize the effect of the measured disturbance on the system.

+=+=+=+ Manual feedforward control: Process; Process disturbance; Controlled variable; Manipulated variable (flow rate)

This form of feedforward control relies heavily on the operator and their knowledge of the operation of the process. However, if the operator makes a mistake or is unable to anticipate a disturbance then the controlled variable will deviate from its desired value and if feedforward is the only control, an uncorrected error will exist.

Automatic feedforward control

+=+=+=+ the concept of automatic feedforward control. Disturbances that are about to enter a process are detected and measured. Feedforward controllers then change the value of their manipulated variables (outputs) based on these measurements as compared with their individual setpoint values.

Process; Controlled quantities; Disturbances; Sensed (measured) values of disturbances; Setpoints; Feedforward controllers; Needed values of manipulated quantities

+=+=+=+ Automatic feedforward control.

Feedforward controllers must be capable of making a whole range of calculations, from simple on-off action to very sophisticated equations. These calculations have to take into account all the exact effects that the disturbances will have on controlled variables.

Feedforward control, although a very attractive concept, places a high requirement on both the systems designer and the operator to both mathematically analyze and understand the effect of disturbances on the process in question.

As a result feedforward control is usually reserved for the more important or critical loops with a plant. Pure feedforward control is rarely encountered, it’s more common to find it embedded within a feedback loop where it assists the feedback controller function by minimizing the impact of excessive process disturbances.

In (combined feedback and feedforward control), we will be examining the concepts and applications of feedforward control when combined with a cascaded feedback system. It’s important to remember that feedforward is primarily designed to reduce or eliminate the effect of changes in both process reaction times and the magnitude of any measured process variable change.

Examples of feedforward controllers

As discussed above, feedforward controllers can be required to carry out simple (on-off) control up to high order mathematical calculations. Due to the vast variances of requirements for feedforward controllers they can be considered as functional control blocks. They can range, as stated, from simple on-off control to lead/lag (derivative and integral functions) and timing blocks. Their range of functionality is virtually unlimited as most systems allow them to be 'programmed' in as software-based math functions.

The four basic requirements for the composition of a feedforward controller are:

1. A recognizable input (derived from the measured disturbance)

2. A setpoint or point of origin and control

3. A math function operating on the input and setpoint values

4. An output which is the result of this math function.

In essence, the controller's action can be described purely by the mathematical function it performs.

Time matching as feedforward control

+=+=+=+ an application of feedforward control where the time taken for a process to react in one direction (heating) is different to the time taken for the process to return back to its original state (cooling). If the reaction curve (dynamic behavior of reaction) of the process disturbance is not equal to the control action, it has to be made equal. We normally use lead/lag compensators as tools to obtain equal dynamic behavior.

They compensate for the different speeds of reaction. The block diagram shows this principle of compensation.

A problem of special importance is the drifting away of the PV. We can be as careful as we want with our evaluation of the disturbances, but we never reach the situation of absolute perfect compensation. There are always factors not accounted for. This causes a drifting of the PV which has to be corrected manually from time to time, or an additional feedback control has to be added. +=+=+=+ a carefully designed example, taking into account all major and measurable variables possible.

The feedforward control uses mass flow calculation for the inlet flow and uses a fuel flow controller to avoid the influence of fuel flow pressure changes.

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Lag-compensator solves timing problem in feed-flow-related changes of temperature:

If the reaction curve for feed-flow related changes of temperature is faster than the reaction curve for fuel-flow changes, a lead-compensator would be required (1) Increase in feed-flow decreases temperature

(3) There is a timing problem in compensating feed-flow and fuel-flow related changes of temperature Increase in fuel-flow increases temperature with lag-compensator increase in fuel-flow increases temperature Without lag-compensator

+=+=+=+ Time matching of feedforward control Disturbance Feed-flow FF-control f(disturb) f (control) f (process) Process

Fuel-flow; Feedforward controller; Objective : T2 =Outlet temperature The objective is to keep the PV constant despite disturbances. To achieve this, the blocks FF-control and f (control) must change the PV by the same magnitude and timing but in opposite direction to that which the disturbance would do without control. Then the feedforward control principle of compensating the disturbance is fulfilled.

+=+=+=+ Block diagram of feedforward control

+=+=+=+ Feedforward control for feed heater

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