Control system and control loop


Thread Starter


dear all...
can u explain to me what is different between control system and control loop. we are doing the syllabus for diploma instrumentation. what subject to be learned in the two topics(troubleshoot control system and troubleshoot control loop). another question, what type of explanation in verify control logic of a control loop.
Control loop is a subset of a control system. In a control system there may be many control loops.
A control loop essentially tries to achieve control over one process parameter by the feedback control system. This can be achieved even by a dedicated single loop controller, or by a centralised control system. Control system handles many such control loops as per the program and also handles discrete event driven control logic. Control system also monitors the status of each control loop and sets an alarm if there is any abnormal behaviour of teh control loop.
Kind regards


Heavner, Lou [FRS/AUS]

When defining control loop, the emphasis should be on the word loop. Some process input is manipulated and the process response is measured. The response is compared to a target value called a setpoint and the manipulated
variable is adjusted to drive the controlled (measured) variable to the setpoint value. The controller, process, manipulated variable and
controlled variable can be graphically represented as a feedback loop. PID is the most common, but not the only algorithm. There are potential variants including constructs with multiple process disturbances measured and compensated (feed forward), multiple manipulated variables to extend control rangeability (split range), cascaded loops which may be considered
as multiple loops or as one large complex loop, etc.

A control system might refer to hardware and software used to execute loops. But there are other kinds of control structures besides "analog" feedback loops. There is discrete logic for started and stopping motors and
switching devices between discrete states. Discrete logic is also deployed as interlocks and permissives. There is sequencing logic which can be very simple to quite complex multi-product, multi-stream batch logic. Control systems provide additional functionality like process history, reporting, alarming, optimization, etc.

A feedback control loop is usually evaluated by examining the response to changes in loop setpoint or process loads. Feedback loop response is typically oscillatory although it may be so heavily damped that there is never any overshoot. Each process control application will have a performance requirement which will generally want to get the process back to
sepoint as quickly as possible without exceeding allowable overshoot or oscillation. Therefore some useful criteria for comparing or evaluating
different controllers are things like totalizing integrated absolute error, integrated time x absolute error and integrated square error in response to a step change to the process. You could probably imagine others.


Lou Heavner