In case of sequence control systems the control process is divided into single steps, which are processed one after another corresponding to a plan/program stored in the controlling system. Certain commands are assigned to the single steps, which are given to the control object by means of the controlling system, either time-dependent or process status-dependent, when the time conditions determined for one step are fulfilled or agreed events in the process field have occurred. Due to these circumstances there is a difference between time-controlled and process-controlled sequence control systems. Time-controlled sequence control systems are used, when it is sure that either all processes in the controlled object are always in the same time-function or that the criteria decisive for the process can be measured hardly or not at all. They can be found e. g. in starters for motors that always have to start under the same load conditions, in automatic systems for controlling washing machines, centrifuges, forming machines, injection moulding machines and contact welding machines, as well as in all systems realizing processes regarding cleaning, warm-up, porosity, annealing, baking, boiling and drying. Process-controlled sequence control systems realize - due to their operating principle - a certain adaptation of the program sequence to accidental irregularities during the process. They are used in fields where it is no problem to detect certain events in the controlled object '...', '...' or'...' and to transmit them back to the controlling system. In industrial practice there are many different combinations of time- and process-controlled sequence control systems.
