ABSTRACT
Many industrial processes require control actions to take place in certain, predefined sequences. Batch processes are perhaps the most striking example of this, where materials for making a batch must be loaded into the process vessels, parameters such as temperature and pressure controlled during the batch processing, and then discharge of the product monitored and controlled. Before the advent of reliable PLCs, this form of sequence control was usually accomplished by an electromechanical device known as a drum sequencer. This device worked on the principle of a rotating cylinder (drum) equipped with tabs to actuate switches as the drum rotated into certain positions. If the drum rotated at a constant speed (turned by a clock motor), those switches would actuate according to a timed schedule. During rotation, stationary wipers sense the presence of tabs (present = on, absent = off). This interaction makes or breaks electrical contact with the wipers, generating electrical outputs from the drum sequencer. The outputs are wired to devices on a machine for ON–OFF control. Drum sequencers usually have a finite number of positions within one rotation, called steps. Each step represents some process step. At power up, the drum sequencer resets to a particular step. The drum rotates from one step to the next based on a timer, or on some external event. During special conditions, a machine operator can manually force the drum to advance to the next step by using a jog control on the drum’s drive mechanism. The contact closure of each wiper generates a unique ON–OFF pattern called a sequence, designed for controlling a specific machine. Because the drum is circular, it automatically repeats the sequence once per rotation. Applications vary greatly, and a particular drum may rotate once per second, or as slowly as once per week. In this chapter, the implementation of the drum sequencer instruction with up to sixteen steps and sixteen digital outputs on each step is explained in detail. Six examples are considered to show the application of the drum sequencer instruction presented in this chapter.
