A programmable logic controller, or PLC, is the industrial computer that coordinates many warehouse machines such as conveyors, scanners, sensors, diverters, lifts, and motor drives. It does not usually control these devices continuously in the human sense, but instead repeats a fast scan cycle many times per second. Understanding this cycle helps technicians predict when inputs are read, when logic is solved, and when outputs change.
In logistics systems, a few milliseconds can determine whether a carton is diverted correctly or misses its lane.
Key Facts
- A basic PLC scan cycle is input scan, program execution, output update, then communication and diagnostics.
- Scan time is the time for one complete cycle, often measured in milliseconds.
- Approximate response time can be as high as input delay + scan time + output delay.
- Frequency of scans can be estimated by f = 1/T, where T is scan time in seconds.
- If scan time = 10 ms, the PLC completes about 100 scans per second.
- Fast warehouse events may require high speed inputs, interrupts, or faster task scheduling.
Vocabulary
- PLC
- A programmable logic controller is a rugged industrial computer used to monitor inputs and control outputs in machines and processes.
- Scan cycle
- A scan cycle is the repeated sequence in which a PLC reads inputs, runs logic, updates outputs, and performs communication or diagnostics.
- Input image table
- The input image table is the PLC memory area that stores the input states read at the beginning of the scan.
- Output image table
- The output image table is the PLC memory area that stores the output commands that will be sent to devices at the output update step.
- Photoelectric sensor
- A photoelectric sensor detects objects by using a light beam and is commonly used to locate cartons, totes, and pallets on conveyors.
Common Mistakes to Avoid
- Assuming outputs change the instant an input turns on, which is wrong because most PLCs first read inputs, then solve logic, then update outputs later in the scan.
- Ignoring scan time when tracking fast cartons, which is wrong because a short sensor pulse may be missed if it occurs between input scans or is shorter than the required detection time.
- Using only a normal input for a high speed encoder signal, which is wrong because standard scan based logic may not count pulses accurately at high frequencies.
- Forgetting communication and HMI delays, which is wrong because a screen value may update more slowly than the actual PLC logic and can make troubleshooting misleading.
Practice Questions
- 1 A PLC has a scan time of 8 ms. How many complete scan cycles does it perform in 1 second?
- 2 A carton blocks a photoelectric sensor for 35 ms. The PLC scan time is 12 ms and the input module delay is 4 ms. Estimate whether the PLC is likely to detect the carton during at least one scan, and explain your calculation.
- 3 A conveyor diverter sometimes activates late even though the sensor and output are working. Explain how scan cycle timing, output update timing, and HMI display delay could each affect the technician's interpretation.