TECHNICAL NOTE
MOTION APPLICATION ENGINEERING GROUP
Yaskawa Electric America - 2121 Norman Drive South – Waukegan IL 60085
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11/23/2005
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eng/05.055/MCD
Sequencing Techniques
Machinery is required to repeat processes accurately and consistently with robust control, without
intervention other than user clearing mechanical jams and user interface to prompt their actions.
When developing sequences using a machine controller, it is important to program for consistent
execution of tasks, with built in recovery functionality to handle sequence exceptions.
State Machine programming vs Step Sequencing
Step Sequencing is defined as motion or logic sequences programmed in steps using individual
bits of words to initiate and monitor the steps of the sequence. Another common practice is state
programming where states are defined by actions that take place. Transition conditions
determine the sequence, and the state is determined by an integer that is updated when a new
state is entered. Both methods are useful, below is a more detailed description of Step
Sequencing.
Step Sequencing Technique:
Most controller programmers use two different types of sequencing by bits.
1) Retained Step Sequencing: each step of the sequence is set true, and remains true
until the end of the sequence, or the sequence is aborted when all of the bits are
reset.
2) Toggle Step Sequencing: In each step of the sequence, a bit is set. Then there is a
corresponding bit representing that the step is completed, which toggles or resets the
initial step bit. When the sequence completes, then all the “step completed” bits are
reset. Some favor this method because steps that are completed should have no
interaction with active steps.
Regardless of the method selected, its recommended to use self-latching bits with the unlatching
bits between the branch and coil, not in the branch, as shown below.
DWG L10.01