Chapter - Structured Logic Design

sequentialproblem

simple/small

steps with

complex/large

single process

STATE DIAGRAM

EQUATIONSBLOCK LOGIC

shorterdevelopmenttime

performanceis important

multiple

SFC/GRAFSET

PETRI NET

processes

buffered (waiting)state triggers

no waiting withsingle states

SEQUENCE BITS

FLOW CHART

some deviations

very clear steps

Program Design with Sequential Bits

1. Understand the process.2. Write the steps of operation in sequence and give each step a number.3. For each step assign a bit.4. Write the ladder logic to turn the bits on/off as the process moves through its states.5. Write the ladder logic to perform machine functions for each step.6. If the process is repetitive, have the last step go back to the first.

Design Case:

Design a control system for a coffee maker that will followthe steps below.

1. The coffee maker is idle2. The start button is pushed, and the water heater is on for 5 minutes. The pot heater is also turned on.3. After that the water heater is turned off, but the pot heater is kept on for an additional 1 hour and 55 minutes.4. The coffee maker returns to an idles state.

Design with Timing Diagrams

1. Understand the process.2. Identify the outputs that are time dependent.3. Draw a timing diagram for the outputs. 4. Assign a timer for each time when an output turns on or off.5. Write the ladder logic to examine the timer values and turn outputs on or off.

door 1

door 2

2s 10s 14s

Design Example