UNIT 4: PROGRAMMABLE LOGIC CONTROLLERS (PLCs)
A. Fundamentals and Architecture
Definition: A Programmable Logic Controller (PLC) is a specialized industrial computer designed for robust, real-time control of electromechanical processes in harsh environments. It is programmed to monitor inputs, execute logic, and control outputs.
Core Functions:
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Input Scanning: Reads status of field devices (sensors, switches).
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Logic Execution: Runs user program (scan cycle) to make decisions.
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Output Updating: Drives actuators (motors, valves, indicators) based on logic.
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Communication: Exchanges data with other PLCs, HMIs, and SCADA systems.
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Self-Diagnosis: Performs internal checks and reports faults.
Key Applications in Industrial Automation:
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Assembly line control
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Material handling & conveyor systems
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Process control (chemical, pharmaceutical)
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Machine tool control
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Building automation (HVAC, lighting)
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Traffic signal control
Comparison with Conventional Relay Controllers:
| Feature | Conventional Relay Controller | Programmable Logic Controller (PLC) |
|---|---|---|
| Wiring | Hardwired, physical relays & contacts. Changes require rewiring. | Software-based. Logic change = program edit. |
| Flexibility | Very low. Difficult to modify. | Very high. Easy reprogramming for new tasks. |
| Reliability | Lower (mechanical contacts wear out). | Higher (solid-state, no moving parts in logic). |
| Troubleshooting | Difficult; requires tracing physical wires. | Easier; built-in diagnostics, online monitoring. |
| Space & Cost | Large for complex logic; high labor cost for changes. | Compact; lower long-term modification cost. |
| Speed | Slower (contact bounce, mechanical delay). | Fast (microsecond scan times). |
| Complex Functions | Limited to basic logic; timers/counters bulky. | Built-in timers, counters, math, data handling. |
Exam Tip: Be prepared to list at least 3 advantages (flexibility, reliability, ease of troubleshooting/ modification) and 2 disadvantages (higher initial cost, need for programming skills).
Block Diagram & Components:
[Power Supply] --> [CPU (Microprocessor/Microcontroller)] <--> [Memory (RAM/ROM/EEPROM)]
^ ^
| |
[Input Module] [Output Module]
| |
[Field Devices] [Actuators]
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Power Supply: Converts line AC/DC to regulated DC for system.
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CPU: "Brain." Executes control program, manages I/O, communicates.
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Memory: Stores user program (ladder logic), system data, I/O status.
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Input/Output (I/O) Modules:
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Digital (Discrete) I/O: On/Off signals (24V DC, 120V AC).
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Analog I/O: Continuous signals (4-20mA, 0-10V) for temperature, pressure.
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Specialty I/O: High-speed counters, motion control, temperature modules.
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Programming Device: PC or handheld programmer used to write/download program.
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Communication Interface: Ports for networks (Ethernet/IP, Modbus, Profibus).
B. PLC Programming
Standard IEC 61131-3 Programming Languages:
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Ladder Logic (LD): Graphical, resembles relay logic schematics. Most widely used. Uses "nets" or "rungs" with contacts and coils.
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Instruction List (IL): Low-level, text-based (like assembly). Less common now.
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Function Block Diagram (FBD): Graphical, blocks representing functions (e.g., timer, counter) with data flow.
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Structured Text (ST): High-level, text-based (like Pascal/C). Used for complex math/data handling.
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Sequential Function Chart (SFC): For step/transition-based sequential processes.
Ladder Logic Fundamentals:
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Scan Cycle: PLC reads inputs → executes program left-to-right, top-to-bottom → updates outputs. Cycle repeats continuously.
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Key Elements:
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Contacts: Represent inputs (normally open
--| |--, normally closed--|/|--). -
Coils: Represent outputs (
--( )--). -
Logic: Series = AND, Parallel = OR.
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Internal Relays (Bits): Software flags for internal logic, not physical outputs.
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Timers (TON, TOF): On-delay, Off-delay timers.
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Counters (CTU, CTD): Up, Down counters.
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Basic Programming Examples:
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Motor Start/Stop (Seal-in Circuit):
| I:0.0/Start | I:0.1/Stop | M:0.0 |
|-------------|------------|-------| | M:0.0 | | O:0.0 |
```
*`I:0.0` = Start PB (NO), `I:0.1` = Stop PB (NC), `M:0.0` = Internal Bit, `O:0.0` = Motor Contactor.*
**Logic:** Pressing Start energizes `M:0.0` and Motor `O:0.0`. `M:0.0` seals the circuit. Pressing Stop breaks the seal-in path.
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On-Delay Timer (TON):
| I:0.2 | T4:0/DN |
|-------|---------| | T4:0 | |
```
*`T4:0` = Timer with preset `PRE` (e.g., 5000 ms) and accumulated `ACC` value. `DN` = Done bit.*
**Logic:** When `I:0.2` goes ON, timer `T4:0` starts timing. After `PRE` time, `T4:0/DN` bit goes ON.
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Up Counter (CTU):
| I:0.3 | C5:0/DN |
|-------|---------| | C5:0 | |
```
*`C5:0` = Counter with preset `PRE`. `CU` = Count Up input (rising edge). `DN` = Done bit.*
**Logic:** Each rising edge of `I:0.3` increments `C5:0.ACC`. When `ACC >= PRE`, `C5:0/DN` goes ON.
PLC Operating Modes:
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PROGRAM Mode: PLC is stopped. Program can be edited, downloaded, or monitored. Outputs are disabled.
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RUN Mode: PLC executes the control program normally. All I/O are active.
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TEST Mode (or RUN/MONITOR): PLC runs program but allows forced I/O values for debugging/troubleshooting without affecting physical outputs (or with controlled override).
C. PLC Communication and System Integration
Standard Communication Protocols:
| Protocol | Type / Key Feature | Typical Application |
|---|---|---|
| Modbus (RTU/TCP) | Master-Slave, simple, widely supported. | Basic device-to-device or PLC-to-SCADA. |
| Profibus (DP/PA) | Deterministic, high-speed (DP) for factory, (PA) for process. | Factory automation, process control (Siemens ecosystem). |
| Ethernet/IP | Uses standard Ethernet hardware, CIP protocol. | Integration with IT networks, high-data applications. |
| Profinet | Real-time Ethernet (IRT), high performance. | High-speed motion control, synchronized systems. |
| DeviceNet | CAN-based, device-level network. | Connecting smart sensors/actuators to PLC. |
Interface with Power Electronics Control Systems:
PLCs interface with power electronics (drives, converters, inverters) via:
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Digital Outputs: For start/stop, fault reset, direction control.
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Analog Outputs: To send speed/torque reference signals (0-10V, 4-20mA) to drive controllers.
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Analog Inputs: To receive feedback (actual speed, current, temperature) from power systems.
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High-Speed Counters/Pulse Outputs: For precise positioning (servo/stepper drives).
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Communication Networks: Using protocols like Modbus RTU/TCP or Profibus DP to exchange setpoints, status, and parameters with intelligent drives (e.g., Siemens SINAMICS, Allen-Bradley PowerFlex).
Event-Driven Devices:
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Concept: Devices that initiate an action or interrupt the normal scan cycle based on a hardware event (e.g., high-speed counting, precise position capture, pulse measurement).
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Examples: High-Speed Counters (HSC), Interrupts, Pulse Train Outputs (PTO).
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Integration: PLC program configures these modules. When the event occurs (e.g., encoder pulse), the CPU suspends normal scan to execute a dedicated interrupt routine, ensuring timely response critical for motion control, precise timing, or safety interlocks.
Exam Tip: For "interface with power electronics," emphasize analog signals for references/feedback and digital for control/faults. For "event-driven," highlight interrupts for time-critical tasks that bypass the normal scan cycle.