Control - 1 - Intro

Question 1

Definition: Control, cascade

Answer 1

Control in which the output of one controller → set point for another controller.

Question 2

Definition: Control, feedforward

Answer 2

Control in which information concerning one or more conditions that can disturb the controlled variable is converted, outside of any feedback loop, into corrective action to minimize deviations of the controlled variable.

Question 3

Definition: Controller, automatic controller

Answer 3

A device which operates automatically to regulate a controlled variable.

Question 4

Definition: Disturbance

Answer 4

An undesired change that takes place in a process that tends to adversely affect the value of a controlled variable

Question 5

Definition: Final controlling element, (actuator)

Answer 5

The element that directly changes the value of the manipulated variable.

Question 6

Definition: Sensor (transducer)

Answer 6

An element or device which receives information in the form of one quantity and converts it to information in the form of the same or another quantity. (e.g. converts temperature to an electrical signal).

Question 7

Definition: Set point

Answer 7

An input variable which sets the desired value of the controlled variable. It may be manually set, automatically set or programmed. Set point is expressed in the same units as the controlled variable

Question 8

Definition: Signal, error

Answer 8

The signal resulting form subtraction of a particular return signal from its corresponding input signal

Question 9

Definition: Signal, feedback

Answer 9

The return signal which results from a measurement of the directly controlled variable.

Question 10

Definition: Signal, output

Answer 10

A signal delivered by a device, element or system. Hence controller output is the signal delivered by the controller.

Question 11

Definition:Variable, controlled

Answer 11

In a control loop, the variable the value of which is sensed to originate a feedback signal.

Question 12

Definition: Variable, measured (process variable)

Answer 12

A quantity, property or condition which is measured. Common measured variables are temperature, pressure, rate of flow, thickness and speed.

Question 13

Feedback control system block diagram

Answer 13

Question 14

Cascade control (master-slave control) system block diagram

Answer 14

Bottom shows the block diagram.

Question 15

Feedforward control system block diagram

Answer 15

Question 16

Definition: Control, feedback

Answer 16

Control in which a measured variable is compared to its desired value to produce an actuating error signal which is acted upon in such a way as to reduce the magnitude of the error.

Question 17

Why is feedback control more accurate than feedforward control?

What advantage does feedforward have

Answer 17

• Feedforward
  
  • In principle could compensate perfectly for the disturbance by applying an equal and opposite movement of the manipulated variable.
  • In practice the response of the system is not perfectly known so the compensation is only approximate.
  • The response of a feedforward system is faster than feedback, and the initial deviation is larger for a feedack system.
• Feedback
  
  • A feedback controller drives the controller error to zero.
    
    • That is, it ensures the set point and controller variable become equal to one another in the steady state.
  • Therefore feedback control is more accurate than feedforward control.

Question 18

Feedback+feedforward vs. feedback plots

Answer 18

Question 19

Why isn’t the feedback of the valve taken after the tank?

Answer 19

After the tank there is a level measurement so th dimension would a ‘level’.

The output of the level control and flow control is all ‘flow’. Which is why the feedback element of the valve is in this of the system.

Remember you need to keep track of the dimensions that are going around in a system.