PLC sortation control is the automation system that moves parcels through a warehouse and sends each one to the correct lane, chute, or loading area. It matters because modern logistics depends on sorting thousands of items per hour with high accuracy and low downtime. A programmable logic controller, or PLC, reads sensors, executes control logic, and commands motors, diverters, scanners, and alarms in real time.
This turns a moving conveyor line into a coordinated system that can identify, track, and route every parcel.
Key Facts
- Basic conveyor speed relation: v = d/t, where v is belt speed, d is distance traveled, and t is time.
- Throughput estimate: parcels per hour = 3600 / spacing time, if one parcel enters every fixed time interval.
- PLC input devices include photoeyes, limit switches, encoder pulses, barcode scanners, and emergency stop circuits.
- PLC output devices include motor starters, variable frequency drives, pneumatic diverter valves, stack lights, and alarms.
- Encoder tracking converts motion into position: distance = pulses / pulses per meter.
- Sortation accuracy depends on scan timing, conveyor speed, parcel spacing, sensor alignment, and correct destination logic.
Vocabulary
- PLC
- A programmable logic controller is an industrial computer that reads inputs, runs control logic, and turns outputs on or off to control machines.
- Photoeye
- A photoeye is an optical sensor that detects whether a parcel is present by sensing a light beam.
- Diverter
- A diverter is a mechanical device, such as a paddle, pop-up wheel, or sliding shoe, that moves a parcel from the main conveyor to a selected lane.
- Encoder
- An encoder is a sensor that sends pulses as a conveyor moves so the controller can track parcel position.
- Interlock
- An interlock is a control condition that prevents an action unless required safety or operating conditions are satisfied.
Common Mistakes to Avoid
- Ignoring sensor delay, because a photoeye or scanner may not report instantly and the parcel can move during that time.
- Using conveyor speed without checking units, because mixing meters per second, feet per minute, and pulses per meter gives incorrect timing and position values.
- Placing parcels too close together, because the PLC may not have enough time or distance to identify each parcel and actuate the correct diverter.
- Treating the PLC as only an on/off switch, because real sortation control also uses tracking, timers, counters, interlocks, fault logic, and communication with higher-level systems.
Practice Questions
- 1 A conveyor moves at 1.2 m/s. A barcode scanner is 3.6 m before a diverter. How many seconds after a parcel is scanned should the PLC command the diverter, assuming no delay correction is needed?
- 2 A sortation line releases one parcel every 0.75 s. Estimate the maximum throughput in parcels per hour.
- 3 A parcel is scanned correctly, but it reaches the wrong chute only when parcels are very close together. Explain two likely control or sensing causes and how the system could reduce the error.