On a labelling machine, the applicator must stop exactly when one label is ready to be dispensed. A fork sensor, also called a slot or label sensor, makes this possible. The label web runs through the slot of the fork, and the sensor sees the change between the label and the gap on the backing paper.
How a fork sensor works
A fork sensor has the light emitter on one arm and the receiver on the other. Because both sit in one rigid housing, there is no alignment to do. The label web passes between them. The backing paper alone lets more light through than the paper plus a label, so the sensor can detect the gap even on thin, closely spaced labels.
Datalogic fork sensors use infrared light. Infrared passes through paper and most printing evenly, so the colour or print on the label does not disturb detection.
SR21-IR, SR23 and SR31 compared
| Model | Fork width | Resolution / min. label | Speed | Output | Best for |
|---|---|---|---|---|---|
| SR21-IR | 2 mm | 0.5 mm | 25 kHz, 20 µs | PNP and NPN | Paper labels on high-speed labellers |
| SR23-5-IR-PH / SR23-5-IR-NH | 5 mm | 0.5 … 2 mm label/gap | 12 kHz, 40 µs | PNP (PH) or NPN (NH) | Thicker webs and booklet labels |
| SR31 | 30 mm | 2 mm | 10 kHz, 50 µs | NPN and PNP | Small parts, edges and larger objects |
Which one should you choose?
- SR21-IR is the standard choice for self-adhesive paper labels. Its narrow 2 mm slot, 0.5 mm resolution and 25 kHz switching keep up with fast applicators. It has an M8 4-pole connector and both PNP and NPN outputs, so it suits most controllers.
- SR23 has a wider 5 mm fork, which helps with thicker web materials and multi-layer booklet labels. Choose the PH version for PNP inputs and the NH version for NPN inputs.
- SR31 has a 30 mm fork. It is used less for labels and more for detecting small parts, tabs, holes and edges on assembly and feeding machines.
Transparent labels need a different sensor
Clear plastic labels on a clear liner give very little change in infrared light. For these, an ultrasonic or capacitive label sensor is normally used. Tell us your label material and we will check the right solution.
Set-up tips
- Teach the sensor with the backing paper and a label in the slot, as described in the datasheet.
- Keep the web flat and centred in the fork; a fluttering web causes false signals.
- Clean the slot regularly. Label adhesive and paper dust collect on the optics over time.
- Choose a model with a connector so a spare can be fitted in minutes.
Get the right sensor from VXT Sensor
VXT Sensor, Noida, supplies genuine Datalogic sensors across India. Send us your machine details, the object or mark you want to detect and the distance, and we will suggest the right model, share the datasheet and confirm price and delivery. Contact VXT Sensor or browse the full Datalogic product range.
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