The four-stage operation, repeated again and again, serves to maintain control (e.g., of the position of an automobile in a lane). Taking a more general view, consider the four stages of a cycle or loop of control (Fig. 10-1). The first stage is the collection or gathering of information — data acquisition, as it is called. This involves a reduction to symbols and the communication of the symbols. The second or processing stage involves sorting, summarizing, organizing, and transforming the symbols. The third or comparison stage involves contrasting the results of the processing with previously obtained „standards“ or „measures“ to obtain an indication of the extent of the correspondence. The fourth or selection stage involves converting or translating the results of the comparison stage into a choice of an appropriate course of action. It also involves communicating the symbols designating this choice of the action selected to the point where the action is to be taken.
Two major types of control are found: closed-loop, and open-loop. The distinguishing feature of the closed loop of control is that the information gathered in the collection stage is on the same situation that the selected action affects. (…) In the examples above, the information gathered was about the position of the car in the lane, and the action selected regulated the position of the car in the lane. The distinguishing feature of the open loop of control is that the information gathered in the collection stage is not about the same situation that the selected action affects. The control exercised by most (fixed-cycle) traffic lights is an example of an open loop control; a change in the color of traffic lights is not directly affected by the amount, timing, or direction of the traffic.
To put the matter another way, the main feature that distinguishes the closed loop from the open loop is feedback. Consider the classic thermostat example. When the temperature of the house is low, the thermostat turns the furnace on; then the increase in the temperature in the house is sensed by (feeds back to) the thermostat, which, after the temperature reaches a certain point, will act to turn the furnace off. All closed loops of control employ feedback — that is, use information about the thing controlled to control it further. The control loop is associated with the theory of servomechanisms, or „servo“ theory, as it is often called. Many applications of servomechanisms are made in regulating or controlling machines and processes.