Page 272 - Programmable Logic Controllers, Fifth Edition - Mobile version

12.1 Mechanical Sequencers
Sequencer instructions are designed to operate much
like the mechanical rotating cam limit switch shown in
Figure 12-1. These mechanical type sequencers are often
referred to as drum switches, rotary switches, stepper
switches, or cam switches. They are often used to control Switch assembly Enclosure Symbol
machinery that has a repetitive cycle of operation. Figure 12-1 Rotating cam limit switch.
Figure 12-2 illustrates the operation of a cam-operated Source: Images Courtesy of Rockwell Automation, Inc.
sequencer switch. An electric motor is used to drive the
cams. A series of leaf-spring mounted contacts interacts
with the cam so that in different degrees of rotation of the Contacts
cam, various contacts are closed and opened to energize
and de-energize various electrical devices. As the cams
rotate, load devices connected to the contacts can change
from an on to an off state, from an off to an on state, or
remain at the same state.
Figure 12-3 illustrates a typical mechanical drum-
operated sequencer switch. The switch consists of a series
of normally open contact blocks that are operated by pegs
located on the motor-driven drum. The operation of this
sequencer can be summarized as follows:
• Pegs are placed at specific locations around the Motor
circumference of the drum to operate the contact Cam
blocks. Figure 12-2 Mechanical cam-operated sequencer.
• When the drum is rotated, contacts that align with
the pegs will close, whereas the contacts where
there are no pegs will remain open. • Each location where there was a peg is represented
• The presence of a peg can be interpreted as logic 1, by a 1 (on), and the positions where there were no
or on, and the absence of a peg as logic 0, or off. pegs are each represented by a 0 (off ).
• The equivalent sequencer data table illustrates the Sequencer switches are useful whenever a repeatable
logic state for the first four steps of the drum cylinder. operating pattern is required. One example is the timed

Equivalent sequencer data table
0 1 0 1 0 1 1 0 0 0 1 0 1 0 1 0 4
1 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 3
0 1 1 1 0 0 1 0 1 0 1 0 1 0 1 0 2
1 1 1 1 0 0 1 1 0 0 1 0 1 0 1 1 1

NO
switch Steps
1
2
Motor 3
4

Peg
Figure 12-3 Mechanical drum-operated sequencer switch.

Sequencer and Shift Register Instructions Chapter 12 253

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