Lect04
- 1. 4-1 Dr. D. J. Jackson Lecture 4-1Electrical & Computer Engineering Programmable Logic Controllers Basic Ladder Logic Programming Dr. D. J. Jackson Lecture 4-2Electrical & Computer Engineering Outline • Boolean statements and ladder logic equivalents – Logical AND – Logical OR – Logical NOT • Commonly used ladder logic sequences – Start-stop-seal circuits – Basic interlocks • Properly formatted outputs
- 2. 4-2 Dr. D. J. Jackson Lecture 4-3Electrical & Computer Engineering Boolean logic control programs • Boolean logic control programs examine and control on and off states – Boolean here is used interchangeably with the word “discrete” • Each control program (ladder diagram sequence) can contain one or more conditionals • Example – If (a part is on the conveyor) AND (there is not a box in the chute) THEN (turn the conveyor motor on) • In terms of sensors and actuators this becomes – If (sensor_A is ON) AND (sensor_B is NOT ON) THEN (turn actuator_C ON) Dr. D. J. Jackson Lecture 4-4Electrical & Computer Engineering Conveyor motor control system sensor_A sensor_B actuator_C
- 3. 4-3 Dr. D. J. Jackson Lecture 4-5Electrical & Computer Engineering Logical AND ladder diagram • The logical AND function is constructed by series combinations of digital (discrete) inputs – Two (or more) series components I:1/0 AND I:1/1 I:1/0 AND I:1/1 AND I:1/2 Dr. D. J. Jackson Lecture 4-6Electrical & Computer Engineering Logical OR ladder diagram • The logical OR function is constructed by parallel combinations of digital (discrete) inputs – Two (or more) parallel components I:1/0 OR I:1/1 I:1/0 OR I:1/1 OR I:1/2
- 4. 4-4 Dr. D. J. Jackson Lecture 4-7Electrical & Computer Engineering Logical NOT • The logical NOT function is constructed by referencing the input signal with a normally closed contact (XIO instruction) Dr. D. J. Jackson Lecture 4-8Electrical & Computer Engineering Complex Boolean expressions • More complex Boolean expressions can be formulated with various serial-parallel combinations of XIC and XIO instructions – NAND, NOR, XOR, XNOR
- 5. 4-5 Dr. D. J. Jackson Lecture 4-9Electrical & Computer Engineering Start-stop-seal circuits • For PLC systems without latch and unlatch instructions, a circuit is needed that will allow a process to start, continue to run after a start button is released, and stop under control of another button – A circuit that implements this functionality is commonly referred to as a start-stop-seal circuit • A feedback path (i.e. a contact) that references the output is normally used to seal around the start contact Dr. D. J. Jackson Lecture 4-10Electrical & Computer Engineering Start-stop-seal ladder diagram Initial state START pushbutton pressed START pushbutton released STOP pushbutton pressed
- 6. 4-6 Dr. D. J. Jackson Lecture 4-11Electrical & Computer Engineering Start-stop-seal variations • In practice several start and/or several stop buttons can be used in a process • Start buttons (with XIC instructions) can be used – In series if it is required that ALL be pressed before a process starts – In parallel if pressing ANY start button is to start a process • Stop buttons (with XIO instructions) are normally used in series if pressing ANY stop button is to stop a process Dr. D. J. Jackson Lecture 4-12Electrical & Computer Engineering Start-stop-seal circuit example
- 7. 4-7 Dr. D. J. Jackson Lecture 4-13Electrical & Computer Engineering Interlock circuits • Interlocks can prohibit output(s) from energizing under a certain condition • Example: O:2/0 should not energize if O:2/1 is energized (and vice versa) Dr. D. J. Jackson Lecture 4-14Electrical & Computer Engineering Formatting considerations • Ladder logic rungs should be formatted so the reader can easily infer the meaning of the intended logic • One mechanism to help this is the grouping of related signals within an area on a given rung of logic • For example: – Group signals together that have some common intent • Start signals • Stop signals • Emergency stop signals (E-stop) • Interlocks – Controls that might have greater importance (i.e. E-stop) might be located on the left hand side of the rung if possible
- 8. 4-8 Dr. D. J. Jackson Lecture 4-15Electrical & Computer Engineering Formatting considerations E-stop conditions Normal Stop Start Interlocks (if any) Outputs This is also a good example of instruction and rung documentation. Dr. D. J. Jackson Lecture 4-16Electrical & Computer Engineering Properly formatted outputs • An output energize instruction (OTE) referencing a specific output bit should appear only once in a ladder logic program
- 9. 4-9 Dr. D. J. Jackson Lecture 4-17Electrical & Computer Engineering Properly formatted outputs • Only one output energize instruction (OTE) should appear in a rung of ladder logic Dr. D. J. Jackson Lecture 4-18Electrical & Computer Engineering Properly formatted outputs • If more than one output is to be controlled by a certain rung of ladder logic, the output energize (OTE) instructions can be placed in parallel