The Datalogic SR21 Fork Sensor is a photoelectric label sensor for label positioning and print register-mark detection in packaging, labelling and print-and-apply machinery. With a maximum response time of 20 µs and switching frequency up to 25 kHz, it provides fast detection signals as material passes through its 2 mm fork slot.
The SR21 combines the light emitter and receiver in one housing, so they do not require separate optical alignment. Labels or film pass between them, and the sensor switches its output when the transmitted light crosses the taught threshold.
This arrangement suits roll-fed machinery where a label edge or printed registration mark provides a timing reference for feeding, printing, cutting or application. Material contrast, web guidance and controller response determine whether the complete installation meets the required positioning tolerance.
The SR21-IR, part number 953151070, uses infrared LED emission to distinguish labels from their backing material or detect holes in a continuous surface. Suitability depends on a sufficient difference in transmitted light between the two conditions.
The SR21-RG, part number 953151080, uses red/green LED emission for colour register-mark detection on transparent film. A register mark is a printed reference that the machine uses to coordinate operations such as cutting or sealing.
Detecting a printed mark on clear film does not establish suitability for an unprinted transparent label on transparent backing. For that requirement, consider the SRX3 Ultrasonic Fork Sensor, subject to checks on material, label gap and operating speed.
The SR21 is specified for a maximum 20 µs response time, switching frequency up to 25 kHz and a minimum detectable object width of 0.5 mm. The width specification describes the detection target; it is not a finished-label placement tolerance.
Illustrative timing check: a 2 mm gap travelling at 2 m/s passes the sensing point in 1 ms. Compare that interval with the sensor response, controller input filtering and control-cycle timing. An input that filters out short pulses can miss a gap even when the sensor detects it.
Teach-in establishes the switching threshold—the received light level at which the output changes—from the actual production material. Static setup acquires the label and backing separately, while dynamic setup learns their changing light levels as the material moves through the slot.
For static label setup, the manufacturer recommends teaching the most transparent label area and the most opaque backing area. This checks the conditions with the smallest optical difference. Repeat the setup when label stock, backing or printed marks change, and verify switching across representative samples.
The yellow LED indicates output activation; the green/red indicator shows readiness or an error. An unsuccessful teach caused by insufficient contrast requires a material and sensing-method review.
For high-speed operation, check the controller input response and the effect of cable capacitance and electrical loading on signal edges. The sensor supplies a detection signal; the machine controller performs the timing and actuator control.
The Datalogic SR23 Fork Sensor is the closest alternative when booklet or multilayer labels need more clearance. Its 5 mm slot accommodates a thicker material path, while SR21 offers the shorter specified response time.
| Characteristic | SR21 | SR23 |
|---|---|---|
| Slot width | 2 mm | 5 mm |
| Nominal slot depth | 50 mm | 50 mm |
| Maximum response time | 20 µs | 40 µs |
| Maximum switching frequency | 25 kHz | 12 kHz |
| Optical models | Infrared for labels; red/green for register marks | Infrared for labels, including booklet and multilayer formats |
| Output configuration | NPN and PNP on separate pins | PNP or NPN, depending on model |
These are sensor specifications, not comparative label-placement results. Check the material, mounting arrangement, electrical connection and controller timing before selecting either model as a replacement.
MarvelTech can review the detection target, material path, mounting space and controller requirements to help select the SR21 model. Where the material cannot pass through a narrow fork, a different format from the industrial sensors range may suit the installation.
The quotation should identify the sensor model and quantity, connection cable and any required mounting accessories. Specify whether application assessment, installation or commissioning support is required, and request confirmation of availability, lead time and applicable warranty terms.
Send MarvelTech the following details so the proposed configuration can be assessed against your machine:
For a replacement, include the existing sensor’s complete model and part number. Request a quote for SR21-IR or SR21-RG, or ask MarvelTech to recommend the model from your application details.
SR21-IR uses infrared light for label and hole detection, while SR21-RG uses red/green light for printed register marks on transparent film. Choose according to the feature that provides the machine’s timing reference: a label-to-backing transition, a hole or a printed mark. The order numbers are 953151070 for SR21-IR and 953151080 for SR21-RG.
Suitability depends on whether the label and backing produce a sufficient difference in transmitted light. SR21-RG’s ability to detect printed marks on transparent film does not establish suitability for unprinted clear labels. For clear-on-clear label stock, consider the SRX3 ultrasonic fork sensor, then check its material, minimum-gap and speed requirements against the application.
The Datalogic SR23 fork sensor is designed for booklet and multilayer labels, with a 5 mm slot offering more clearance than SR21’s 2 mm slot. SR21 has a shorter specified maximum response time: 20 µs versus 40 µs. Base selection on material thickness, optical contrast and timing requirements, and check mounting and wiring compatibility before replacing either sensor.
Compare the shortest label, gap or mark passage time with the complete detection and control system. SR21 is specified for up to 25 kHz switching and a maximum 20 µs response time. Divide the feature width along the travel direction by web speed to calculate its passage time, then account for controller input filtering, scan timing and electrical loading.
The 0.5 mm specification describes the minimum detectable object width, rather than finished-label placement accuracy. Placement also depends on material contrast, web movement, controller timing and the dispensing mechanism. For a positioning requirement, specify the permitted placement error and validate the complete machine using representative label stock at production speed.
Use the AUTO-SET push-button to teach the switching threshold from the actual material; a PC configuration package is not required. Static teaching acquires the label and backing separately, while dynamic teaching samples them as they move through the slot. For static label setup, the manufacturer recommends the most transparent label area and most opaque backing area. Verify switching after each material change.
Both SR21 models require 10–30 V DC and provide separate NPN and PNP outputs through an M8 four-pin connector. Maximum output current is 100 mA, with configurable light/dark operation. Match the output to the PLC input circuit and verify the connector pin assignment, supply capacity and input response before reusing an existing cable or replacing another sensor.
The moving material must pass through a 2 mm slot; nominal fork depth is 50 mm. Allow clearance for the combined label and backing thickness, splices and movement of the web. Position the detection track at the sensing point shown in the mounting drawing, and leave access for the teach button, connector and cable bend.
The SR21 has IP65 protection and a specified operating range of −20°C to +60°C. Its enclosure rating does not establish suitability for immersion, high-pressure washdown or aggressive cleaning chemicals. Check the cleaning process and connector protection before selection, and provide access to remove dust or adhesive deposits from the optical surfaces when detection becomes inconsistent.
Provide label and backing samples or details of the printed mark, together with minimum feature width, gap size, total material thickness and maximum web speed. Include the required positioning tolerance, PLC input type, mounting photographs, cable requirements and quantity. For a replacement, supply the complete existing model number. Ask the quotation to identify accessories, any requested commissioning support and delivery lead time.
The Datalogic SRX3 Ultrasonic Fork Sensor detects transparent, opaque and metallic-ink labels in packaging, labelling and print-and-apply machinery, including clear labels on clear backing. Its ultrasonic sensing method distinguishes labels from their backing without relying on optical contrast and supports single- and double-film detection after application-specific setup.
See DetailsThe Datalogic SRF Fork Sensor is a photoelectric fork sensor for small-part detection, edge positioning and object counting in packaging machinery, automotive assembly and automated production lines. Four fork widths, from 30 to 120 mm, and separate LED and laser models allow selection according to the available clearance, target size and switching-speed requirement.
See DetailsThe Datalogic SR23 Fork Sensor is a photoelectric label sensor for label-edge and gap detection in labelling machines, packaging lines and print-and-apply systems. Its 5 mm fork slot accommodates booklet and multilayer label formats, while a switching frequency of up to 12 kHz supports fast label-feed control.
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