Control (management) - The Elements of Control

The Elements of Control

The four basic elements in a control system —

(1) the characteristic or condition to be controlled

(2) the sensor

(3) the comparator

(4) the activator

— occur in the same sequence and maintain a consistent relationship to each other in every system.

The first element is the characteristic or condition of the operating system which is to be measured. We select a specific characteristic because a correlation exists between it and how the system is performing. The characteristic can be the output of the system during any stage of processing or it may be a condition that is the result of the system. For example, it may be the heat energy produced by the furnace or the temperature in the room which has changed because of the heat generated by the furnace. In an elementary school system, the hours a teacher works or the gain in knowledge demonstrated by the students on a national examination are examples of characteristics that may be selected for measurement, or control.

The second element of control, the sensor, is a means for measuring the characteristic or condition. For example, in a home heating system this device would be the thermostat, and in a quality-control system this measurement might be performed by a visual inspection of the product.

The third element of control, the comparator, determines the need for correction by comparing what is occurring with what has been planned. Some deviation from plan is usual and expected, but when variations are beyond those considered acceptable, corrective action is required. It involves a sort of preventative action which indicates that good control is being achieved.

The fourth element of control, the activator, is the corrective action taken to return the system to expected output. The actual person, device, or method used to direct corrective inputs into the operating system may take a variety of forms. It may be a hydraulic controller positioned by a solenoid or electric motor in response to an electronic error signal, an employee directed to rework the parts that failed to pass quality inspection, or a school principal who decides to buy additional books to provide for an increased number of students. As long as a plan is performed within allowable limits, corrective action is not necessary; however, this seldom occurs in practice.

Information is the medium of control, because the flow of sensory data and later the flow of corrective information allow a characteristic or condition of the system to be controlled. To illustrate how information flow facilitates control, let us review the elements of control in the context of information.