Data Acquisition

1111.1 Open and closed loop controlBy definition, a data acquisition and control system is not only required to acquire datafrom a system or process, but also to act on it. In an industrial environment the methodsand techniques used to calculate and perform the appropriate actions at any given time,are often extremely critical. Large or incorrect control actions can adversely affect theperformance of the system, and can indeed prove to be extremely costly. One well usedmethod of controlling a system or process, in which the current state of the system is fedback to the controller (i.e. the PC), is closed loop control. This method, and the use of aPID control algorithm to implement it, are discussed in the following section.Control systems are classified as open loop or closed loop systems. The distinction isdetermined by the control action, which is the mechanism responsible for activating thesystem to produce the output. An open loop control system is one in which the controlaction is independent of the output. In this type of control system, there is no feedbackfrom the process on the results of a given control action taking place.Two important features of open loop control are:• Their ability to perform accurately is determined by their inherent accuracyand their calibration. Calibration is the re-establishing of an input–outputrelationship, to obtain the desired system accuracy.• They are not generally troubled with problems of instability.A closed loop control system is one in which the control action is dependent on theoutput. In this type of control system there is continuous feedback from the process on theresults of a given control action taking place.The most important features of a closed loop control system are:• Increased accuracy.