Control - 1 - Intro Flashcards
Definition: Control, cascade
Control in which the output of one controller → set point for another controller.
Definition: Control, feedforward
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.
Definition: Controller, automatic controller
A device which operates automatically to regulate a controlled variable.
Definition: Disturbance
An undesired change that takes place in a process that tends to adversely affect the value of a controlled variable
Definition: Final controlling element, (actuator)
The element that directly changes the value of the manipulated variable.
Definition: Sensor (transducer)
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).
Definition: Set point
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
Definition: Signal, error
The signal resulting form subtraction of a particular return signal from its corresponding input signal
Definition: Signal, feedback
The return signal which results from a measurement of the directly controlled variable.
Definition: Signal, output
A signal delivered by a device, element or system. Hence controller output is the signal delivered by the controller.
Definition:Variable, controlled
In a control loop, the variable the value of which is sensed to originate a feedback signal.
Definition: Variable, measured (process variable)
A quantity, property or condition which is measured. Common measured variables are temperature, pressure, rate of flow, thickness and speed.
Feedback control system block diagram
Cascade control (master-slave control) system block diagram
Bottom shows the block diagram.
Feedforward control system block diagram
Definition: Control, feedback
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.
Why is feedback control more accurate than feedforward control?
What advantage does feedforward have
- 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.
- A feedback controller drives the controller error to zero.
Feedback+feedforward vs. feedback plots
Why isn’t the feedback of the valve taken after the tank?
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.
