Modern warehouses move thousands of packages per hour using conveyors, photoelectric sensors, barcode scanners, motors, diverters, and robotic equipment. A programmable logic controller, or PLC, coordinates these devices so that each action happens in the right order and at the right time. PLC timers are essential because real machines need delays, pulse lengths, settling time, and safety intervals rather than instant switching.
Understanding PLC timers helps students connect digital logic to real logistics systems such as sorting lines, merge conveyors, and palletizers.
A PLC timer works by counting time while certain input conditions are true, then changing a timer bit or output when a preset value is reached. For example, a sensor may detect a box, start a timer, and trigger a pneumatic pusher exactly 0.8 s later when the box reaches a sorting lane. Common timer types include on-delay timers, off-delay timers, and retentive timers, each used for a different control behavior.
In warehouse automation, these timers reduce jams, synchronize motion, and make equipment respond predictably even when packages arrive at different intervals.
Key Facts
- TON on-delay timer: output turns on after the input has been true for the preset time.
- TOF off-delay timer: output stays on for the preset time after the input turns false.
- RTO retentive timer: accumulated time is saved when the input turns false until a reset instruction clears it.
- Elapsed time = timer counts x time base, such as 350 counts x 0.01 s = 3.50 s.
- Conveyor travel time can be estimated with t = d / v, where d is distance and v is belt speed.
- A timer is usually scanned once per PLC cycle, so scan time affects how precisely short timing events can be measured.
Vocabulary
- PLC
- A programmable logic controller is an industrial computer that reads inputs, runs control logic, and switches outputs to operate machines.
- Timer preset
- The timer preset is the target time value that must be reached before the timer changes its done bit or output state.
- Accumulated time
- Accumulated time is the amount of time a timer has already counted during its operation.
- Photoelectric sensor
- A photoelectric sensor detects objects by using a beam of light that is reflected or interrupted.
- Scan cycle
- The scan cycle is the repeated process in which a PLC reads inputs, executes logic, updates outputs, and performs internal tasks.
Common Mistakes to Avoid
- Using an on-delay timer when an off-delay timer is needed. This is wrong because a TON delays turning an output on, while a TOF delays turning it off.
- Ignoring conveyor speed when setting a diverter delay. This is wrong because the package position depends on t = d / v, so a fixed delay may fail if the belt speed changes.
- Forgetting to reset a retentive timer. This is wrong because an RTO keeps its accumulated value and may activate immediately during the next cycle if it was already near the preset.
- Assuming timer accuracy is better than the PLC scan time. This is wrong because very short pulses can be missed or measured with error if they occur between scans.
Practice Questions
- 1 A package sensor is 1.2 m before a sorting gate, and the conveyor speed is 0.60 m/s. Calculate the timer delay needed to trigger the gate when the package reaches it.
- 2 A PLC timer has a time base of 0.01 s and a preset of 250 counts. Calculate the delay time in seconds, then state whether it is long enough for a 3.0 s labeling delay.
- 3 A warehouse conveyor motor should keep running for 5 s after the last box leaves a sensor so the belt clears the zone. Identify the best timer type and explain why it matches the required behavior.