Sequential Applications Notes

Sequential Applications

Sequential Function Charts (SFCs)

Provides a diagrammatic representation of control sequences in a program.
Suited for sequential applications.
Describes operation as a function chart, then translates to ladder logic.
Offers a "recipe" or basic instructions for creating a control program for sequential applications.

Process Narrative to SFC: Carton Sealer Control

Objective: Implement a program for a station that folds and seals a corrugated cardboard box.
System Description:
- Open boxes (pre-filled) are moved into the station via a conveyor motor.
- A photoelectric eye (PE1) detects the box's presence.
- Upon box detection:
  - The conveyor stops.
  - Two pneumatic rams (FRONT and BACK) extend to fold end flaps.
  - A glue sprayer activates for 1 second.
  - Two pneumatic rams (LEFT and RIGHT) extend to fold side flaps.
  - After a 5-second wait, all rams retract.
  - The conveyor restarts, moving the sealed box out.
- The cycle repeats upon detecting a new box.
- A gap between boxes ensures the photoelectric eye senses when a box has exited the station.
Components:
- Single-action linear pneumatic cylinders for rams, controlled by individual outputs.
- Rams extend as long as power is applied or until a mechanical stop is reached.
- Rams retract when power is removed (estimated 2 seconds retraction time).
- Limit switches on each ram detect full extension.
Start/Stop Logic:
- The first press of the start switch initiates the continuous operation cycle, assuming no box is present initially.
- Subsequent presses of the start switch have no effect while running.
- The stop switch pauses operation only when a box is in the station.
SFC Steps (Initial SFC):
- INITIAL: System start.
- STEP01: Wait for box (Conveyor motor ON) until PE1 is activated.
- STEP02: Push down end flaps (FRONT and BACK rams EXTEND).
- STEP03: Glue (Glue sprayer activates for 1 second).
- STEP04: Push down side flaps (LEFT and RIGHT rams EXTEND).
- STEP05: Dry (Wait for 5 seconds).
- STEP06: Retract rams.
- STEP07: Move box out (Conveyor motor ON) until PE1 is deactivated.

SFC to Ladder Logic Conversion

Translate the Sequential Function Chart (SFC) into a Ladder Logic program.
Utilize switches:
- STOP PB (Stop Push Button)
- PE1 (Photoelectric Eye)
- START PB (Start Push Button)
- RESET PB (Reset Push Button)
- FRONT ELS AND BACK ELS (Front End Limit Switch AND Back End Limit Switch)
- LEFT ELS AND RIGHT ELS (Left End Limit Switch AND Right End Limit Switch)
Implement timers:
- GLUE TIMER
- DRY TIMER
- RETRACT TIMER
Example Ladder Logic Elements and Steps:
- Step 1 - Wait for Box:
  - Transition Condition: PE1 (Photoelectric Eye)
- Step 2 - Push End Flaps Down:
  - Action: FRONT EXT, BACK EXT
  - Transition Condition: FRONT ELS AND BACK ELS
- Step 3 - Glue:
  - Action: GLUE SPRAY, GLUE TIMER
  - Transition Condition: GLUE TIMER.ON
- Step 4 - Push Side Flaps Down:
  - Action: FRONT EXT, BACK EXT, RIGHT EXT, LEFT EXT
  - Transition Condition: RIGHT ELS AND LEFT ELS
- Step 5 - Dry:
  - Action: FRONT EXT, BACK EXT, RIGHT EXT, LEFT EXT, DRY TIMER
  - Transition Condition: DRY TIMER.DN (Dry Timer Done)
- Step 6 - Retract Rams:
  - Action: RETRACT TIMER
  - Transition Condition: RETRACT TIMER.DN (Retract Timer Done)
- Step 7 - Move Box Out:
  - Action: CONV_MOTOR
  - Transition Condition: PE1

General Form of SFC

Three major parts:
- Steps of the sequential operation.
- Transition conditions to move to the next step.
- Actions of each step.

SFCs: Initial Step and Sequencing

The initial step is indicated by a double-line rectangle.
Sequencing can either end or repeat.

SFC Rules

Actions are outputs that are ON when a step is active.
Outputs not explicitly listed are OFF.
Transition conditions are physical inputs turning ON/OFF or the completion of a defined time period.
Physical outputs are never transition conditions.
Physical inputs are never actions.
Process: Process Narrative -> Sequential Function Chart -> Ladder Logic Program
$\overline{PROX} = NOT(PROX) = /PROX$

Convert Process Narrative to SFC

Identify the steps and transition conditions.
Steps and transition conditions often alternate.
It may be easiest to first identify the transition conditions, then the steps.
Steps span a length of time (long or short, defined or undefined).
Transition conditions are physical inputs or internal coils (timer or counter done) turning ON or OFF; they represent the change and do not take time.
Add step actions.
Some steps may not have any actions.
Do not need to specify turning outputs OFF – everything not listed is OFF.

Convert SFC to Ladder Logic

Ladder Logic program is broken into the following sections:
- START/STOP/PAUSE of overall operation.
- First start.
- Permissives.
- Lockouts.

Convert SFC to Ladder Logic (cont.)

Transitions between steps:
CURRENT STEP --> TRANSITION_CONDITION --> NEXT STEP
- Step actions (example):
  - STEP 1 --> RUN --> MOTOR

Convert SFC to Ladder Logic (cont.)

Don’t use Latch and Unlatch for real outputs – only for the internal step bits.
For the step actions, start with the outputs, then determine for which step(s) each output is on.
May need a reset.

Branching

Two kinds of branching:
- Simultaneous Divergence or AND Branching
  - When a transition out of a step causes more than one step to be activated simultaneously.
  - Uses double horizontal lines to start and end the branch.
  - May feature one common transition above or below the branch.
- Exclusive Divergence or OR Branching
  - Selection of one sequence from a set of sequences.
  - Uses a single horizontal line to start and end the branch.
  - Requires multiple, mutually exclusive transitions below the branch.
  - May feature multiple transitions above the close.