Logistics and warehouse systems use automated controls to move packages quickly, safely, and accurately. Conveyors, photoelectric sensors, barcode scanners, diverters, motors, stack lights, and emergency stops must all work together in the correct sequence. Two common ways to control these devices are relay logic and programmable logic controllers, called PLCs.
Understanding the difference matters because the control method affects speed, reliability, troubleshooting, and future upgrades.
Relay logic uses electromechanical relays wired together so that contacts open and close to control loads. A PLC uses input modules, a processor, and output modules to read sensors, run a stored program, and command devices such as motor starters or solenoids. In a warehouse conveyor, both systems can start a belt when a box is detected, stop during an emergency, or activate a diverter at the right time.
PLC systems usually handle complex timing, data, diagnostics, and changes more easily, while relay systems can be simple and easy to understand for small fixed tasks.
Key Facts
- Relay logic controls outputs by physically wiring relay coils and contacts in series and parallel.
- A PLC control loop is input scan, logic scan, output update, then repeat.
- Basic control idea: Output ON = required input conditions are TRUE and safety conditions are TRUE.
- Ohm's law for control circuits: V = IR.
- Electrical power for a load can be estimated with P = VI.
- A normally closed emergency stop contact opens when pressed, breaking the control circuit and stopping motion.
Vocabulary
- PLC
- A programmable logic controller is an industrial computer that reads inputs, executes control logic, and switches outputs.
- Relay logic
- Relay logic is a control method that uses wired relay coils and contacts to create decision-making circuits.
- Input device
- An input device is a sensor or switch, such as a photoeye or emergency stop, that sends a signal to the control system.
- Output device
- An output device is a controlled actuator or indicator, such as a motor starter, diverter solenoid, or stack light.
- Ladder logic
- Ladder logic is a PLC programming style that represents control instructions in rungs similar to relay wiring diagrams.
Common Mistakes to Avoid
- Confusing a relay contact with a relay coil. The coil creates the magnetic action, while the contact is the switch that changes state.
- Assuming a PLC output powers every field device directly. Many outputs only provide a control signal and still need a relay, contactor, or motor drive for larger loads.
- Ignoring normally closed safety contacts in emergency stop circuits. A safe design often uses closed contacts that open on fault so a broken wire can also stop the machine.
- Treating PLC logic changes as only a software task. Any change must still be checked against wiring, device ratings, safety rules, and the real motion of the conveyor.
Practice Questions
- 1 A 24 V DC diverter solenoid draws 0.75 A when energized. Calculate its power using P = VI.
- 2 A PLC scans its program every 20 ms. If a sensor signal stays ON for 0.30 s, how many full PLC scans occur while the signal is ON?
- 3 A warehouse conveyor must add barcode routing, fault logging, and easy future changes. Explain whether relay logic or a PLC is the better choice and give two reasons.