PLC Training System
Industrial Control with Allen-Bradley Micro 800
CS 304: Robotic Agents
What is a PLC?
Programmable Logic Controller
- Industrial computer designed for factory automation
- Controls machines, processes, and assembly lines
- Reads inputs (sensors, switches) → Makes decisions → Controls outputs (motors, lights)
- Replaces traditional relay-based control systems
Why PLCs? Rugged, reliable, easy to reprogram, real-time control
Our Training System: LearnLab
Complete PLC training platform with industrial components
5 Major Components:
- PLC Unit - Allen-Bradley Micro 800 Series
- Motor Starter - Controls motor power
- Contactors & Ice Cube Relays - Switching devices
- Indicator Lamps & Limit Switches - Visual feedback & sensing
- Terminal Strips, User Controls & Power - Wiring infrastructure
Each component teaches real industrial automation skills
Component 1: PLC Unit
Allen-Bradley Micro 800 Series
Key Features:
- Compact design for small to medium applications
- Digital I/O: 20 input points, 12 output points
- Programming: Ladder logic via Connected Components Workbench
- Communication: Ethernet, USB, serial ports
- Real-time processing: Scan time ~1-10 ms
Brain of the system - reads inputs, executes logic, controls outputs
Component 2: Motor Starter
Purpose: Safely start and stop electric motors
Components:
- Contactor: Heavy-duty switch for motor power
- Overload relay: Protects motor from overcurrent
- Disconnect switch: Manual power isolation
Why Needed?
- Motors draw high inrush current
- Need overload protection
- Must meet safety codes
PLC Control
- PLC output energizes contactor coil
- Contactor closes, motor runs
- PLC can start/stop based on logic
Component 3: Contactors
Electromagnetically operated switch for high-power loads
How it works:
- Control side: Low-voltage coil (24V DC) from PLC output
- Power side: High-voltage contacts (120V/240V AC) for load
- When coil energized → electromagnetic field → contacts close → load powered
- When coil de-energized → spring opens contacts → load off
Key advantage: PLC's weak signal controls powerful equipment safely
Component 4: Ice Cube Relay
Small electromechanical relay (gets name from cube shape)
Characteristics:
- Plug-in design: Easy to replace without rewiring
- Multiple contacts: SPDT, DPDT, 3PDT, 4PDT configurations
- Ratings: Typically 10A at 250V AC
- LED indicator: Shows when coil is energized
Use case: Interface between PLC and multiple loads, or invert signals
Component 5: Indicator Lamps
Visual feedback for system status
Types on LearnLab:
- Panel mount LEDs: Red, green, yellow, white
- 120V AC rated: Can be driven directly by PLC outputs
- Common uses:
- Green = System running / Motor ON
- Red = Fault / Emergency stop
- Yellow = Warning / Manual mode
Critical for operators: Immediate visual feedback without looking at HMI
Component 6: Limit Switches
Mechanical position sensors for detecting object presence or position
How they work:
- Physical actuator: Roller, lever, or plunger
- When actuator pressed → internal contacts change state
- Types: Normally Open (NO) or Normally Closed (NC)
- Connected to PLC digital inputs
Applications: End-of-travel detection, door closed sensing, part presence verification
Component 7: Terminal Strips
Organized wiring connection points
Purpose:
- Central wiring hub: All field devices connect here first
- Easy troubleshooting: Can disconnect/test individual circuits
- Color-coded: Red = power, Blue = neutral, Green = ground
- Labeled: Each terminal has unique identifier
Best practice: Never wire directly to PLC - always go through terminals
Component 8: User Controls
Manual input devices for operator interaction
Common controls on LearnLab:
- Push buttons:
- Momentary (spring return) - Start/Stop
- Maintained (stays pressed) - Mode select
- Selector switches: 2 or 3 position (Auto/Off/Manual)
- Emergency stop: Mushroom-head, twist-to-release
- Key switches: Prevent unauthorized operation
Safety rule: Emergency stop must be hardwired, not PLC-controlled
Component 9: Power System
Multiple voltage levels for different components
Power distribution:
| Voltage |
Purpose |
| 120V AC |
Input power, indicator lamps, relay coils |
| 24V DC |
PLC power supply, digital I/O, sensors |
| 240V AC (optional) |
Large motors, heaters |
Safety: Circuit breakers, fuses, and proper grounding throughout
How It All Works Together
Signal flow in PLC system:
- Input: Operator presses START button → 24V signal to PLC input
- Processing: PLC scans ladder logic → determines output state
- Output: PLC energizes output → Contactor coil activates
- Power: Contactor contacts close → Motor receives 120V AC
- Feedback: Green lamp illuminates, limit switch confirms position
- Monitoring: PLC continuously scans, reacts to STOP or faults
Result: Safe, predictable, and repeatable industrial control
Why Learn PLCs in Robotics?
- Industry standard: 90%+ of factories use PLCs
- Deterministic control: Predictable, safety-critical systems
- Different paradigm: Ladder logic vs. Python/C++
- Integration: PLCs often coordinate with robots (like FANUC)
- Career skills: High demand, well-paid positions
Course perspective: Compare PLC (deterministic) with ROS (probabilistic)
What's Next?
Your PLC Learning Journey:
- Week 5: Hands-on training at Bessemer facility
- Safety first: Tour, lockout/tagout procedures
- Project 2: Program ladder logic for real industrial tasks
- Week 6-7: Integrate PLC with FANUC robot arm
Before next class: Read PLC Guidebook Part II
Key Takeaways
- PLCs are industrial computers for automation
- LearnLab system has 9 integrated components
- Allen-Bradley Micro 800 is our PLC unit
- Contactors & relays interface low-power PLC to high-power loads
- Safety components (E-stop, overloads) are critical
- Terminal strips organize wiring professionally
- System uses multiple voltage levels (120V AC, 24V DC)
Questions?
Next Session: Hands-on PLC training @ Bessemer
Bring: Laptop, safety awareness, curiosity!
Read before next class: PLC Guidebook Part II
Get ready to program real industrial automation!