Sequencer - Programmable Logic controllers

The drum sequencer is a form of counter that is used for sequential control. It replaces the mechanical drum sequencer that was used to control machines that have a stepped sequence of repeatable operations. One form of the mechanical drum sequencer consisted of a drum from which a number of pegs protruded When the cylinder rotated, contacts aligned with the pegs were closed when the peg impacted them and opened when the peg had passed. Thus for the arrangement shown in Figure, as the drum rotates, in the first step the peg for output 1 is activated, in step 2 the peg for the third output, in step 3 the peg for the second output, and so on. Different outputs could be controlled by pegs located at different distances along the drum. Another form consisted of a series of cams on the same shaft, the profile of the cam being used to switch contacts on and off.

The PLC sequencer consists of a master counter that has a range of preset counts corresponding to the various steps; so as it progresses through the count, when each preset count is reached it can be used to control outputs. Each step in the count sequence relates to a certain output or group of outputs. The outputs are internal relays, which are in turn used to control the external output devices.

Suppose we want output 1 to be switched on 5 s after the start and remain on until the time reaches 10 s, output 2 to be switched on at 10 s and remain on until 20 s, output 3 to be switched on at 15 s and remain on until 25 s, and so on. We can represent these requirements by a time sequence diagram, shown in Figure demonstrating the required time sequence.

We can transform the timing diagram into a drum sequence requirement. Taking each drum sequence step to take 5 s gives the requirement diagram shown in Table Thus atstep 1 we require output 1 to be switched on and to remain on until step 2. At step 2 we require output 2 to be switched on and remain on until step 4. At step 3 we require output 3 to be switched on and remain on until step 5. At step 5 we require output 4 to be switched on and remain on until step 6.

With a PLC such as a Toshiba, the sequencer is set up by switching on the Step Sequence Initialize (STIZ) function block R500. This sets up the program for step 1 and R501. This relay then switches on output Y020. The next step is the switching on of R502.

This switches on the output Y021 and a on-delay timer so that R503 is not switched on until the timer has timed out. Then R503 switches on Y022 as well as the next step in the sequence.

With the Allen-Bradley form of PLC the sequencer is programmed using a sequence of binary words in the form of the outputs required, such as those listed in Table.Thus we would have the following binary word sequence put into the program using the programming device. We have seven steps and four outputs.

The term sequencer output (SQO) is used by Allen-Bradley for the output instruction that uses a file or an array to control various output devices. As an illustration, Figure shows a basic Allen-Bradley ladder program using such a sequencer. The timer is started by an input to I:012/1 and has a preset time of 30 s. It is reset by its DN bit. The DN bit also increments the SQO instruction to the next output word. Thus the sequencer is incremented every 30 s. The location of the data for the words is given by FILE, which gives the starting address for the registers in which the binary data for each step is stored. Sometimes the sequencer is not required to operate on the entire word, so MASK gives the bit pattern that masks off certain bits so they are not controlled by the sequencer. Thus we could have a MASK word of: 1110011011111110 and so, because bits 1, 9, 12, and 13 are 0, these bits in the sequencer words are not changed.

SOURCE is the address of the input word or file for an SQC, and DESTINATION is the address of the output word or file. CONTROL is the address that contains parameters with control information. LENGTH is the number of steps of the sequencer file. POSITION is the step in the sequencer file from/to which the instruction moves data.


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