The four basic elements in a control system are: • the
characteristic or condition to be controlled • the
sensor • the
comparator • the activator They occur in the same sequence and maintain consistent relationships with each other in every system.
Controlled characteristic or condition The primary requirement of a control system is that it maintains the level and kind of output necessary to achieve the system's objectives. It is usually impractical to control every feature and condition associated with the system's output. Therefore, the choice of the controlled item (and appropriate information about it) is extremely important. There should be a direct correlation between the controlled item and the system's operation. In other words, control of the selected characteristic should have a direct relationship to the goal or objective of the system.
Sensor After the characteristic is sensed, or measured, information pertinent to control is fed back. Exactly what information needs to be transmitted and also the language that will best facilitate the communication process and reduce the possibility of distortion in transmission must be carefully considered. Information that is to be compared with the standard, or plan, should be expressed in the same terms or language as in the original plan to facilitate decision making. Using machine methods (computers) may require extensive translation of the information. Since optimal languages for computation and for human review are not always the same, the relative ease of translation may be a significant factor in selecting the units of measurement or the language unit in the sensing element. In many instances, the measurement may be sampled rather than providing a complete and continuous feedback of information about the operation. A sampling procedure suggests measuring some segment or portion of the operation that will represent the total.
Comparison with standard In a
social system, the norms of acceptable behavior become the standard against which so-called
deviant behavior may be judged. Regulations and laws provide a more formal collection of information for society.
Social norms change, but very slowly. In contrast, the standards outlined by a formal law can be changed from one day to the next through revision, discontinuation, or replacement by another. Information about deviant behavior becomes the basis for controlling
social activity. Output information is compared with the standard or norm and significant deviations are noted. In an industrial example, frequency distribution (a tabulation of the number of times a given characteristic occurs within the sample of products being checked) may be used to show the average quality, the spread, and the comparison of output with a standard. If there is a significant difference between output and plan that cannot be corrected, the system is "out of control." This means that the objectives of the system are not feasible in relation to the capabilities of the present design. Either the objectives must be re-evaluated or the system redesigned to add new capacity or capability. For example,
drug trafficking has been increasing in some cities at an alarming rate. The citizens must decide whether to revise the police system so as to regain control, or whether to modify the law to reflect a different norm of acceptable behavior.
Implementor The activator unit responds to the
information received from the comparator and initiates corrective action. If the system is a machine-to-machine system, the corrective inputs (decision rules) are designed into the network. When the control relates to a man-to-machine or man-to-man system, however, the individual(s) in charge must evaluate (1) the accuracy of the feedback information, (2) the significance of the variation, and (3) what corrective inputs will restore the system to a reasonable degree of stability. Once the decision has been made to direct new inputs into the
system, the actual process may be relatively easy. A small amount of energy can change the operation of jet airplanes, automatic steel mills, and hydroelectric power plants. The pilot presses a button, and the landing gear of the airplane goes up or down; the operator of a steel mill pushes a lever, and a ribbon of white-hot steel races through the plant; a worker at a control board directs the flow of electrical energy throughout a regional
network of stations and substations. It takes but a small amount of control energy to release or stop large quantities of input. The comparator may be located far from the operating system, although at least some of the elements must be in close proximity to operations. For example, the measurement (the sensory element) is usually at the point of operations. The measurement information can be transmitted to a distant point for comparison with the standard (comparator), and when deviations occur, the correcting input can be released from the distant point. However, the input (activator) will be located at the operating system. This ability to control from afar means that aircraft can be flown by remote control, dangerous manufacturing processes can be operated from a safe distance, and national organizations can be directed from centralized headquarters in Dublin, Ireland. - Kenard E. White ==Process==