Programmable Logic Controllers Introductionpeople.tamu.edu/~hsieh/ICIA/Alamo-PLC-Web/Intro_PLC_Part...
Transcript of Programmable Logic Controllers Introductionpeople.tamu.edu/~hsieh/ICIA/Alamo-PLC-Web/Intro_PLC_Part...
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Programmable Logic ControllersIntroduction
Part 4Principles of Operation
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Motor stirs liquid Automatically when:
o Temperature at set pointo Pressure at set point
Manually with pushbutton
Mixer Process Control Problem
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Mixer Process - Hardwired Control
Coil (M) energized when:• Pressure and temperature switches are closed• or• Manual pushbutton is pressed.
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Mixer Process PLC ControlSame input field devices (pressure switch, temperature switch, and pushbutton)
Field devices hardwired to input module.
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© 2011, The McGraw-Hill Companies, Inc.
Same output field device (MS) coil
MS coil hardwired to output module.
Mixer Process PLC Control
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Enter the logic into the CPU processor programSymbols represent instructionsNumbers represent instruction’s addressesAddresses let PLC know where devices are connected
Mixer Process PLC Control
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To Execute the Program• Controller is placed in RUN• Controller examines (scans) status of inputs• Executes user program• Changes outputs accordingly
Review program operation shown
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LogicPro simulation of the PLC program
InputModule Output
Module Ladder Logic Program
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Typical wiring for a fixed PLC controller
Review circuit & operation shown
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Programmable Logic ControllersIntroduction
Part 4Modifying the Operation
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One important PLC • Ease of changing program
Relay circuit change requires physical rewireReview operation of modified hardwired circuit
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PLC requires no rewiringI/O connections and addressing remain the sameRequires PLC ladder logic program change onlyReview PLC modified program
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PLCs versus Computers
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PLC architecture basically same as PC
PLC • Industrial environment• Wide ranges ambient
temperature• Varied humidity• Varied dust/dirt
PC• Office / Home environment• Controlled low temperatures• Controlled humidity• Controlled dust / dirt
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Properly installed PLC No / little electrical noise inherent in industrial locations
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PLC programmed in ladder logic or other easily learned languages
PLC comes with program languageDoes not require • Permanently attached keyboard• External drive• Monitor
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Computers• Complex computing machines• Execute several programs / tasks
simultaneously• Can run programs any order
PLC• Execute single program• Orderly / sequential execution
(scan cycle)• First to last instruction• Repeats cycle
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PLC control systemDesigned for ease of install and maintenance
Simplified troubleshooting• fault indicators• messages displayed programmer
I/O modules• Connect field device easily• Ease replacing field devices• Ease replacing modules
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PLC software written / runs on PC• Two broad categories:
• User accessible• HMI Monitoring
User Accessible• User program and document• User tools to program
• Ladder logic or another programming language• Document / explain program in detail
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Human machine interface (HMI)
Enables user to: View a process (graphical representation) Determine system run, trend values, alarms, etc…May not allow direct program access
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PLC Size and Application
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Categorizing PLCs include:• Functionality• Required number of I/O• Cost• Physical size
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I/O count most important size factor•Nano
• Smallest size• Less than 15 I/O points
•Micro• 15 to 128 I/O points
•Medium• 128 to 512 I/O points
• Large• over 512 I/O points
Matching PLC with application is key to selection
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Three major types PLC applications: • Single ended• Multitask• Control management
Single ended application• Involves one PLC controlling one process
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Multitask PLC application involves • One PLC controlling several processes• Adequate I/O significant factor in this type of installation
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Control management PLC application•Involves one PLC controlling several other PLC’s•Requires large PLC processor•Must communicate with other PLCs and computers•Control management PLC supervises several PLCs
• Downloads programs to other PLCs• Specifies what operation needs done
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Memory • Part of PLC controller• Stores data• Store instructions• Stores and runs control program
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Amount of memory required depends on:• Number of I/O points used• Size of control program• Data-collecting requirements• Supervisory functions required• Future expansion
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Instruction set • Particular PLC instructions supported
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Summary Review
• Process application & converting relay diagram to PLC program• Modifying relay circuit vs. PLC program change• PLC vs PC• PLC four considerations
• Functionality• I/O• Physical size• Cost
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Summary Review
• Three major types PLC applications• Single ended• Multi-task• Control management
• Four PLC sizes• Nano• Micro• Medium• Large
• Break (15 Minutes) • Lab Exercise Setup