Open loop vs closed loop

The terms open-loop control and closed-loop control are often not clearly distinguished. Therefore, the difference between open-loop control and closed-loop control is demonstrated in the following example of a room heating system. In the case of open-loop control of the room temperature according to Figure 1.1 the outdoor

Figure 1.1: Open-loop control of a room heating system

temperature will be measured by a temperature sensor and fed into a control device. In the case of changes in the outdoor temperature ( disturbance ) the control device adjusts the heating flow according to the characteristic of Figure 1.2 using the motor M and the valve V. The slope of this characteristic can be tuned at the control device. If the room temperature is changed by opening a window ( disturbance ) this will not influence the position of the valve, because only the outdoor temperature will influence the heating flow. This control principle will not compensate the effects of all disturbances.

Figure 1.2: Characteristic of a heating control device for three different tuning sets (1, 2, 3)

In the case of closed-loop control of the room temperature as shown in Figure 1.3 the room temperature is measured and compared with the set-point value , (e.g.  ). If the room temperature deviates from the given set-point value, a controller (C) alters the heat flow . All changes of the room temperature , e.g. caused by opening the window or by solar radiation, are detected by the controller and removed.

Figure 1.3: Closed-loop control of a room heating system

The block diagrams of the open-loop and the closed-loop temperature control systems are shown in Figures 1.4 and 1.5, and from these the difference between open- and closed-loop control is readily apparent.

Figure 1.4: Block diagram of the open-loop control of the heating system

Figure 1.5: Block diagram of the closed-loop control of the heating system

The order of events to organise a closed-loop control is characterised by the following steps:

• Measurement of the controlled variable ,
• Calculation of the control error (comparison of the controlled variable with the set-point value ),
• Processing of the control error such that by changing the manipulated variable the control error is reduced or removed.

Comparing open-loop control with closed-loop control the following differences are seen:

Closed-loop control

• shows a closed-loop action (closed control loop);

• can counteract against disturbances (negative feedback);

• can become unstable, i.e. the controlled variable does not fade away, but grows (theoretically) to an infinite value.

Open-loop control

• shows an open-loop action (controlled chain);

• can only counteract against disturbances, for which it has been designed; other disturbances cannot be removed;

• cannot become unstable - as long as the controlled object is stable.

Summarising these properties we can define:

Systems in which the output quantity has no effect upon the process input quantity are called open-loop control systems.

Systems in which the output has an effect upon the process input quantity in such a manner as to maintain the desired output value are called closed-loop control systems.