Fundamentals of Automatic Control and Loop Systems

What is Automatic Control?

Automatic control is the process of maintaining a desired value in a quantity or condition by measuring the existing value, comparing it with the desired value, and using the difference to proceed to reduce it. Consequently, automatic control requires a closed loop of action and reaction that functions without human intervention.

Understanding Open Loop Systems

An open loop is a type of control where there is no feedback. The operator has access to the final control element, and it is they who govern the percentage of opening or closing the valve to control the output of the product. Open loop control is one in which the system variables influence the output. In this type of system, the person in charge of taking measurements and control actions intermittently closes the loop.

Understanding Closed Loop Systems

A closed loop is a type of control in which there is continuous feedback. This means that the operator only sets the set point. The controller closes the loop, working in a local automatic or remote automatic way. If it is remote, it takes the reference from another process or driver to fix the set point and act on the final element of control. Three criteria must be met: stability, continuity, and accuracy.

Key Control System Definitions

• Off-set: The difference between the variable and the ordered measure.
• SAMA: American Standard which stands for the Scientific Apparatus Makers Association.
• Calibration Adjustment: This belongs to one or both categories: adjusting for zero error and angular error adjustment.
• Zero Adjustment (Offset Shift): A linear displacement adjustment where the linear range is moved to obtain a correct reading of zero when the instrument has no process input.
• Angular Adjustment (Span Shift): Presents a correct reading at any arbitrary point on the scale (e.g., 0 on the scale).
• Span: The algebraic difference between the upper and lower measurement range of the instrument.
• Discrete Control: Conditions for True (V) or False (F), such as a valve being on or off.
• CAR (Cascade/Remote): The set point is not delivered by the operator; it is provided by another parameter of the process or the output of another controller.
• CAL (Local): The set point is set directly by the operator.
• ML (Manual): The operator has direct access to the final control elements.