Feed Forward Control - Electrical Engineering (EE) PDF Download

Feedforward control

A feedback controller responds only after it detects a deviation in the value of the controlled output from its desired set point. On the other hand, a feedforward controller detects the disturbance directly and takes an appropriate control action in order to eliminate its effect on the process output.

Consider the distillation column shown in Fig (V.1) The control objective is to keep the distillate concentration at a desired set point despite any changes in the inlet feed stream.
Feed Forward Control - Electrical Engineering (EE)

Feed Forward Control - Electrical Engineering (EE)

Fig V.1: Feedback and Feedforward control configuration of a distillation column
 

Fig.V.1(a) shows the conventional feedback loop, which measures the distillate concentration and after comparing it with the desired setpoint, increases or decreases the reflux ratio. A feedforward control system uses a different approach. It measures the changes in the inlet feed stream (disturbance) and adjusts the reflux ratio appropriately. Fig V.1(b) shows the feedforward control configuration.
Feed Forward Control - Electrical Engineering (EE)

Fig V.2 shows the general form of a feedforward control system. It directly measures the disturbance to the process and anticipates its effect on the process output. Eventually it alters the manipulated input in such a way that the impact of the disturbance on the process output gets eliminated. In other words, where the feedback control action starts after the disturbance is “felt” through the changes in process output, the feedforward control action starts immediately after the disturbance is “measured” directly. Hence, feedback controller acts in a compensatory manner whereas the feedforward controller acts in an anticipatory manner.

V.I.I Design of feedforward controller

Let us consider the block diagram of a process shown in Fig V.3. The Fig V.3(a) presents the open-loop diagram of the process. The process and disturbance transfer functions are represented by Gand Gd respectively. The controlled output, manipulated input and the disturbance variable are indicated as  Feed Forward Control - Electrical Engineering (EE)  and  Feed Forward Control - Electrical Engineering (EE)  respectively.
Feed Forward Control - Electrical Engineering (EE)

Feed Forward Control - Electrical Engineering (EE)

Feed Forward Control - Electrical Engineering (EE) 
(c) Process diagram with feedforward controller, sensor and valve 
 Fig V.3: The schematic of a feedforward controller mechanism

 

The process output is represented by
Feed Forward Control - Electrical Engineering (EE)                       V.1

The control objective is to maintain  Feed Forward Control - Electrical Engineering (EE)  at the desired setpoint  Feed Forward Control - Electrical Engineering (EE) . Hence the eq (V.1) can be rewritten as
Feed Forward Control - Electrical Engineering (EE)                      V.2

The eq. (V.2) can be rearranged in the following manner:
Feed Forward Control - Electrical Engineering (EE)               V. 3

The eq. V. (3) can be schematically represented by Fig 3(b).

For the sake of simplicity, measuring element and final control element were not considered as parts of the feedforward control configuration as shown in Fig V.3(b). In a more generalized case, when such elements are added in the controller configuration, the resulting control structure takes the form of Fig V.3(c). A generalized form of controller equation can be written as
Feed Forward Control - Electrical Engineering (EE)

and Feed Forward Control - Electrical Engineering (EE)               (V 5)

In case of regulatory problem (disturbance rejection) i.e. when  Feed Forward Control - Electrical Engineering (EE) , the controller should be able to reject the effect of disturbance and ensure no deviation in the output, i.e. Feed Forward Control - Electrical Engineering (EE)  . In other words,
Feed Forward Control - Electrical Engineering (EE)  (V6)

or

Feed Forward Control - Electrical Engineering (EE)  (V7)

In case of servo problem (setpoint tracking), i.e. when  Feed Forward Control - Electrical Engineering (EE) , the controller should be able to ensure that output tracks the setpoint, i.e. Feed Forward Control - Electrical Engineering (EE)  . In other words,

GpGcGfGSP =1             (V 8)

or

Feed Forward Control - Electrical Engineering (EE)            (V9)

Example of design of feedforward controller

Consider an overflow type continuous stirred tank heater shown in Fig V.4. The fluid inside the tank is heated with steam whose flow rate is Fst and supplying heat at a rate of Q to the fluid. Temperatures of the inlet and outlet streams are Ti and respectively. is the volume of liquid which is practically constant in an overflow type reactor. A control valve in the steam line indicates that the steam flow rate can be manipulated in order to keep the liquid temperature at a desired setpoint. Temperature of the inlet stream flow is the source of disturbance (change in Ti ) to the process.
Feed Forward Control - Electrical Engineering (EE)Feed Forward Control - Electrical Engineering (EE)

Fig V.4: Feedforward control configuration of an overflow type continuous stirred tank heater

A simple energy balance exercise will yield the model equation of the above process as:

Feed Forward Control - Electrical Engineering (EE)           V.10

All the variables are assumed to be in the deviation form. Hence, taking Laplace transform on both sides we obtain:
Feed Forward Control - Electrical Engineering (EE)                     V.11

Feed Forward Control - Electrical Engineering (EE)                V.12

Feed Forward Control - Electrical Engineering (EE)                 V.13

Feed Forward Control - Electrical Engineering (EE)             V.14

The feedforward controller is meant for ensuring Feed Forward Control - Electrical Engineering (EE). Hence,

Feed Forward Control - Electrical Engineering (EE)                        V.15

Feed Forward Control - Electrical Engineering (EE)                             V.16

Hence, one needs to set Fst in such a way that Q amount of heat as given in eq.(V.16) is transferred to the process. Fig V.4(b) represents the feedforward structure of the controller. 

Remarks:

•  The feedforward controller ideally does not get any feedback from the process output. Hence, it solely works on the merit of the model(s). The better a model represents the behavior of a process,the better would be the performance of a feedforward controller designed on the basis of that model. Perfect control necessitates perfect knowledge of process and disturbance models and this is practically impossible. This inturn is the main drawback of a feedforward controller.

•  The feedforward control configuration can be developed for more than one disturbance in multi-controller configuration. Any controller in that configuration would act according to the disturbance for which it is designed.

•  External characteristics of a feedforward loop are same as that of a feedback loop. The primary measurement (disturbance in case of feedforward control and process output in case of feedback control) is compared to a setpoint and the result of the comparison is used as the actuating signal for the controller. Except the controller, all other hardware elements of the feedforward control configuration such as sensor, transducer, transmitter, valves are same as that of an equivalent feedback control configuration.

•  Feedforward controller cannot be expressed in the feedback form such as P, PI and PID controllers. It is regarded as a special purpose computing machine .

•  Let us consider a system where process delay is higher than disturbance delay, eg. Feed Forward Control - Electrical Engineering (EE) and  Feed Forward Control - Electrical Engineering (EE) ; in such case, Feed Forward Control - Electrical Engineering (EE)  . That means one needs to know the future values of disturbance in order to decide present control action. This is physically unrealizable controller.

 

V.I.3 Combination of Feedforward-Feedback Controller

The following table provides a comparative assessment of feedforward and feedback controllers.

Table V.1: Merits and demerits of feedforward and feedback controllers
MeritsDemerits
Feedforward controllers
Takes corrective action before the process “feels” the disturbanceRequires measurement of all disturbances affecting the system
Good for sluggish systems and/or system with large deadtimeSensitive to variation in process parameters
Does not affect the stability of the processRequires a “near perfect” model of the process
Feedback controllers
Does not require disturbance measurementActs to take corrective action after the process “feels” the disturbance
Insensitive to mild errors in modelingBad for sluggish systems and/or system with large deadtime
Insensitive to mild changes in process parametersMay affect the stability of the process

 

Let us now explore how a combination of feedforward and feedback controller would perform when they are designed to act simultaneously. The schematic of a feedforward-feedback controller is shown in Fig V.5.

Feed Forward Control - Electrical Engineering (EE)

Without losing the generality we shall ignore the transfer functions of the measuring element and the final control element.

Now the closed loop transfer function of feedforward-feedback controller can be derived in the following manner: 

Feed Forward Control - Electrical Engineering (EE)          V.17

Rearranging the above we get,

Feed Forward Control - Electrical Engineering (EE)                          V.18

It is observed that the stability of the closed loop response is determined by the roots of the characteristic equation: Feed Forward Control - Electrical Engineering (EE)  . Hence, the stability characteristics of a process does not change with the addition of a feedforward loop.

The following numerical example demonstrates the efficacy of a feedforward-feedback controller. Consider a process having process and disturbance transfer functions as

Feed Forward Control - Electrical Engineering (EE)                V.19

A feedback PID controller,with  Feed Forward Control - Electrical Engineering (EE) , is used to control the process for disturbance rejection purpose. A feedforward controller has also been designed for the process however it has been assumed that the time constant of the process has been measured erroneously as 2.1 instead of 2. A first order filter with time constant 0.1 has been augmented to the transfer function of Gsp in order to make it causal.

A Simulink code (Fig V.6) has been generated to simulate the process under the three types of controllers as said above. 
Feed Forward Control - Electrical Engineering (EE)

The performance of three controllers, viz., feedforward, feedback and feedforward-feedback, are presented in Fig V.7

Feed Forward Control - Electrical Engineering (EE)

It is clearly observed that the performance of feedforward-feedback controller is far better than the other two individual controllers.

 

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FAQs on Feed Forward Control - Electrical Engineering (EE)

1. What is feed forward control?
Ans. Feed forward control is a control technique used in systems engineering to anticipate and compensate for disturbances or changes in a system before they affect the system's output. It involves measuring the disturbances and using that information to adjust the control input in order to minimize the impact on the system's performance.
2. How does feed forward control work?
Ans. Feed forward control works by measuring the disturbances or changes in a system and using that information to adjust the control input. This is done by having a model of the system that relates the disturbances to the system's output. The measured disturbances are then fed into the model, which calculates the required control input adjustments to compensate for the disturbances. These adjustments are then applied to the system's control input to ensure that the system's output remains stable and within the desired range.
3. What are the advantages of feed forward control?
Ans. Feed forward control offers several advantages. Firstly, it allows for proactive control by compensating for disturbances before they affect the system's output. This can result in faster response times and improved performance. Secondly, it can reduce the reliance on feedback control, which can be limited by system dynamics and measurement noise. Additionally, feed forward control can improve system stability by reducing the effects of disturbances on the system's output.
4. What are some applications of feed forward control?
Ans. Feed forward control is commonly used in various fields and industries. Some examples include temperature control in industrial processes, where feed forward control can compensate for changes in ambient temperature to maintain a stable process temperature. It is also used in robotics to anticipate and compensate for external forces or disturbances on the robot's movements. Feed forward control is also applied in aerospace engineering to adjust control inputs based on anticipated changes in aerodynamic forces.
5. Are there any limitations or challenges with feed forward control?
Ans. Yes, there are limitations and challenges with feed forward control. One major challenge is accurately modeling the system and its disturbances. Inaccurate models can lead to ineffective control adjustments and reduced performance. Additionally, feed forward control is typically unable to compensate for unmeasured or unpredictable disturbances. Therefore, it may still be necessary to combine feed forward control with feedback control techniques to ensure robust and reliable system performance.
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