Control Engineering Basics
Which of the following correctly describes sequence control?
- AControl that continuously measures the controlled variable and drives the error from the setpoint toward zero with PID calculations, used to keep a temperature or speed constant
- BControl that advances through its stages one by one according to a predetermined order and conditions, used in washing machines, traffic lights, factory conveyors and so on
- CControl that measures a disturbance and applies in advance a manipulated variable that cancels its effect, with no notion of dividing control into stages or deciding their order
Show answer and explanation
Answer: B. Control that advances through its stages one by one according to a predetermined order and conditions, used in washing machines, traffic lights, factory conveyors and so on
Sequence control advances operations according to order, conditions and time, as in "when the start button is pressed, fill with water; when the water reaches the level, start washing; after a set time, drain". It is logic control using on/off signals from sensors and switches, and its purpose differs from that of feedback control, which keeps bringing the controlled variable toward the setpoint. In factory machinery it is often implemented with a PLC (programmable logic controller) and described with ladder diagrams. Real equipment combines the two, for example by using feedback control within each stage.
- A. This describes feedback control (PID control).
- B. Correct. Sequence control advances through a fixed procedure step by step.
- C. This describes feedforward control.