Lecture 1 Flashcards
Describe Feedback Control
The process element being measured is connected to a sensor. The sensor is then physically connected to a transmitter. The controller then receives a signal. The controller compares the measured value to the value desired. A decision is made on any changes to make.
What are the 3 basic components of all control systems?
Sensor-Transmitter : often called primary and secondary elements
Controller : the “brain” of the system
Final Control Element : often a control valve but not always. Other common final control elements are variable-speed pumps, conveyors, electric motors and electric heaters.
What are the 3 basic OPERATIONS that components must do in every control system?
Measurement, Decision, Action.
Present in every control system - have to be in a closed loop.
Describe a disturbance.
Any variable that caused the controlled variable to deviate from the set point.
Describe manual control.
The condition in which the controller is disconnected from the process.
Describe closed-loop control.
The controller is connected to the process, comparing set point to controlled variable + taking corrective action.
What is regulatory control?
Systems designed to compensate for controlled variable deviating from set point due to disturbances.
What are the 3 types of signals used in the process industries?
- Pneumatic signal, or air pressure, ranges normally between 3-15 psi.
- The electrical signal ranges normally between 4 and 20mA.
- The digital, or discrete signal (zeros and ones).
Why is it often necessary to change the units of a signal?
Signal is not in engineering units but rather mA, psig or volt (proportional to measurement depending on calibration). Often necessary to change one type of signal into another - usually does by a transducer/converter.
If looking at feedback control graphically, why does it show an oscillatory response?
It is essentially a trial and error operation until the controlled variable remains at the set point.
Give the advantage and disadvantage of FEEDBACK control.
Advantage: very simple technique that compensates for all disturbances.
Disadvantage: Can only compensate for disturbance after the deviation from the set point has taken place.
Describe FEEDFORWARD control
Objective: measure the disturbances and compensate before the controlled variable deviates from the set point.
Describe the disadvantage of FEEDFORWARD control.
Only prepared for certain disturbances (decided in the beginning). If other disturbance occur, the result will be permanent deviation from set point. To avoid this: some feedback control used.
Feedforward control now controls major disturbances and feedback control compensates for all other disturbances.
Describe the disadvantage of advanced control.
Very expensive compared to feedback control in terms of hardware, computing power and manpower necessary to design, implement and maintain them.
Describe the transfer function.
The transfer function completely defines the steady-state and dynamic characteristics of a process described by a linear differential equation. Its terms determine whether the process is stable or unstable and whether its response to a nonoscillatory input is oscillatory.
