Chapter 1, Problem 1.1E

Interpretation Introduction

Interpretation:

The given statements are to be stated as true or false along with the appropriate explanation.

Concept introduction:

Two types of the control system to control the dynamics of a process are:

1. Feedback control system.

2. Feedforward control system.

The feedback control system considers the output of the system and then compares it with the desired value to adjust the measured value to provide accurate results. Whereas in the feedforward control system, disturbances are taken into account beforehand and they are rectified to control it from deviating from the desired set point.

Answer to Problem 1.1E

(a) True

(b) True

(c) True

(d) False

(e) True

Explanation of Solution

(a)

In feedback control systems, controlled variables are measured and are compared with the desired values which are then controlled accordingly by adjusting the manipulated variables.In the feedforward control systems, disturbance variables are measured and controlled before it deviates from the set point.

Therefore, in both the control systems, measured variables are required. Thus, the given statement is true.

(b)

In feedforward control systems, necessary corrective measures are taken before the controlled variables are affected by the disturbances so that the controlled variables have not deviated from the desired set point.

Therefore, in this system, the measured variable is the one to be controlled. Thus, the given statement is true.

(c)

When the process model which is used to control the system is designed perfectly, the feedback control can theoretically provide perfect control on the measured variable. The controlled variable is measured and compared with the desired value, which is then manipulated to control the system, accordingly, providing the perfect control for the process.

Thus, the given statement is true.

(e)

As compared to the feedforward control system, the feedback system rectifies the disturbance in the measured value of the controlled variable regardless of the source of error and provides correct value as the set point or the desired value. It is more accurate than the feedforward system where only known sources of errors can be solved. Thus, the given statement that the feedback control system is superior to the feedforward control system is true.

Conclusion

(d)

In the feedforward control system, necessary corrective measures are taken before the controlled variables are affected by the disturbances so that the controlled variables have not deviated from the desired set point. This system has several disadvantages such as:

1. The measurement of the disturbance variables should be done accurately.

2. The disturbances which are not measured are not rectified.

3. A process model is necessary for this control system.

Thus, perfect control of the process cannot be achieved for the imperfect process model having an unknown type of disturbances.

Therefore, the given statement is false.