Closed and Open Control Loops

A closed control loop exists where a process variable is measured, compared to a setpoint, and action is taken to correct any deviation from setpoint. An open control loop exists where the process variable is not compared, and action is taken not in response to feedback on the condition of the process variable, but is instead taken without regard to process variable conditions. For example, a water valve may be opened to add cooling water to a process to prevent the process fluid from getting too hot, based on a pre-set time interval, regardless of the actual temperature of the process fluid.

Open-loop and closed-loop control

Having defined the term "controlled system" it only remains to give definitions of closed-loop control as contained in standards. First it is useful to fully understand the difference between open-loop control and closed-loop control.

Open-loop control

German standard DIN 19 226 defines open-loop control as a process taking place in a system where by one or more variables in the form of input variables exert influence on other variables in the form of output variables by reason of the laws which characterize the system.
The distinguishing feature of open-loop control is the open nature of its action, that is, the output variable does not have any influence on the input variable.

Example

Volumetric flow is set by adjusting a control valve. At constant applied pressure, the volumetric flow is directly influenced by the position of the control valve. This relationship between control valve setting and volumetric flow can be determined either by means of physical equation or by experiment. This results in the definition of a system consisting of the "valve" with the output variable "volumetric flow" and the input variable "control valve setting" (see Picture 1).

Picture 1: Open-loop control of volumetric flow setting


This system can be controlled by adjusting the control valve. This allows the desired volumetric flow to be set. However, if the applied pressure fluctuates, the volumetric flow will also fluctuate. In this open system, adjustment must be made manually. If this adjustment is to take place automatically, the system must have closed-loop control.

Closed-loop control

DIN 19 226 defines closed-loop control as a process where the controlled variable is continuously monitored and compared with the reference variable. Depending on the result of this comparison, the input variable for the system is influenced to adjust the output variable to the desired value despite any disturbing influences. This feedback results in a closed-loop action. This theoretical definition can be clarified using the example of volumetric flow control.

Deviation

Example: The volumetric flow (the output variable) is to be maintained at the predetermined value of the reference variable. First a measurement is made and this measurement is converted into an electrical signal. This signal is passed to the controller and compared with the desired value. Comparison takes place by subtracting the measured value from the desired value. The result is the deviation.

Manipulating element

In order to automatically control the control valve with the aid of the deviation, an electrical actuating motor or proportional solenoid is required. This allows adjustment of the controlled variable. This part is called the manipulating element (see Picture 2).

Picture 2: Closed-loop control of volumetric flow


The controller now passes a signal to the manipulating element dependent on the deviation. If there is a large negative deviation, that is the measured value of the volumetric flow is greater than the desired value (reference variable) the valve is closed further. If there is a large positive deviation, that is the measured value is smaller than the desired value, the valve is opened further.

Setting of the output variable is normally not ideal:

>> If the intervention is too fast and too great, influence at the input end of the system is too large. This results in great fluctuations at the output;
>> If influence is slow and small, the output variable will only approximate to the desired value.

In addition, different types of systems (control system) require different control strategy. Systems that respond slowly must be adjusted carefully and with forethought. This describes some of the control engineering problems faced by the closed-loop control engineer.

Design of a closed-loop control requires the following steps:

>> Determine manipulated variable (thus defining the controlled system);
>> Determine the behaviour of the controlled system;
>> Determine control strategy for the controlled system (behaviour of the "controller" system);
>> Select suitable measuring and manipulating elements.