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Programmable Logic ControllersIntroduction
Part 1History, Definitions, & Standards
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PLC Industry Today
• PLC evolution began in 1968 when Dick Morley conceived the PLC concept
• Modicon - First PLC company, installed first PLC at General Motors and Landis Company in 1970
• PLCs – 6.5 billion dollar annual business
– Annual growth at about 20 percent per year
– Over 110 PLC vendors worldwide
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PLC Definitions
• PLCs
• Special-purpose industrial computers
• Specialized electronic devices
• Based on one or more microprocessors
• Used to control industrial machinery and automation systems
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General Characteristics
• PLCs are industrial computers that operate in harsh environments on the factory floor.
• PLC control applications vary from on/off control of a pump to control of a conveyor system sorting boxes based upon destination zip codes.
• PLCs use five standard programming languages
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Programming Languages
• Ladder Diagrams (LD) - Not the ladder logic languages provided by PLC vendors like Allen Bradley.
• Function Block Diagrams (FBD)
• Structured Text (ST)
• Instruction List (IL)
• Sequential Function Charts (SFC)
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Language Preferences
• U.S. production uses more ladder logic
• For process and motion control in the U.S. FBD is often used
• For sequential machine control the SFC language is often used
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PLC versus PC• Programmable Logic Controllers are called PLCs
• Personal Computers are called PCs.
• PLCs/PC similarities
• Architecture
• Motherboard
• Processor
• Solid state memory
• Expansion slots
• PLC/PC differences: PLCs -
• Have no disk drives
• No monitors but often use human machine interface (HMI) flat screen display
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PLC Architecture
• PLC’s CPU is a microprocessor(s)
• Memory and Input/Output (I/O) chips linked with parallel address, data, and control buses
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Processor Architecture
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Early Machine Control
• Initially, relays were used to control the sequence of operations in machines
• These systems were called relay ladder logic (RLL)
• RLL were the control standard for industry
• PLCs eliminated much of the relay logic used for sequential control applications.
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Electromagnetic Relay
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Standard Relay Components
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Relay Configurations
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Single Pole Double Throw Relay Operation• Single Pole Double Throw (SPDT) type relay
• SPDT relay has:
• One common contact (single pole)
• Two positions (NC and NO), which are called throws
• Throws can also be thought of as potential paths for current flow
• The armature has insulators used to isolate electrical switching contacts from the rest of the relay components
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Standard Relay SymbolsSPDT
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Electronic Symbol Control Symbol
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Control Schematic Symbol
• NO contact • Symbol has two parallel lines to
indicate normally open (NO) contacts
• NC contact • Symbol has same two parallel
lines with a diagonal line across them to indicate normally closed (NC) contacts
• The electromagnetic coil is indicated by a circle with a C
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Control Symbol
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Poles and ThrowsSPST
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(a)
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Machine Control SystemsTank Liquid Level Control
• Tank filled with an electrically operated valve and emptied by a motor-driven pump
• Control must satisfy the following logic:
• Pump operates only when the valve is open
• Valve can open at any time
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(a) Process Tank
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Machine Control SystemsElectronic Schematic
Operation:
1. Manually closing S1causes relay A to be energized
2. A1 and A2 close NO contacts
3. Input valve is energized -tank fills
4. A2 - no immediate action as it is waiting for switch S2 to be closed
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(a) Process Tank
(b) Electronic schematic
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Machine Control SystemsControl Ladder Diagram
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(c) Single phase pump motor control(b) Control drawing