1. What must the sensor detect?
Describe the target using real production limits:
• smallest and largest size;
• darkest and lightest colour;
• glossy, matte, transparent, translucent, or uneven surface;
• orientation and allowable position variation;
• speed, spacing, and maximum switching frequency;
• whether detection means presence, counting, edge position, distance, or another measurement.
A test using only an easy sample can hide problems that appear with the most difficult target variation.
2. Which sensing mode fits the installation?
Through-beam uses separate emitter and receiver units. It often provides strong signal margin for opaque objects but requires mounting and wiring on both sides.
Retro-reflective uses one sensor and a reflector. It simplifies wiring but depends on the reflector and may require polarized or transparent-object optics.
Diffuse-reflective detects light returned from the target. It supports one-sided installation, but target colour, surface, angle, and background can affect the result. Background-suppression models can help when a nearby background must be ignored.
Do not assume one mode is always more accurate or contamination-resistant. Compare the exact model specifications and test conditions.
3. What sensing distance is actually required?
Measure the minimum, nominal, and maximum distance across the complete machine cycle. Include mounting tolerances and target-position variation. Keep operating margin rather than designing at the catalogue limit.
For retro-reflective sensors, specify the reflector and distance. For diffuse sensors, verify which reference target was used for the published range. For distance-measuring sensors, separate range, resolution, repeatability, linearity, and absolute accuracy.
4. What background and neighbouring objects are present?
Record the distance, colour, reflectivity, and movement of objects behind or beside the target. A diffuse sensor may detect the background, while a shiny target may return light in an unexpected direction. Reflective machine panels can also affect a retro-reflective optical path.
Test the complete scene, including guards, rails, labels, packaging film, and product carriers.
5. How fast must the sensor respond?
Calculate the available detection time from object speed and the effective sensing zone. Then compare that requirement with sensor response time, switching frequency, filters, timers, PLC input delay, and scan time.
A “fast” sensor does not guarantee a reliable high-speed system if the target is too small for the beam or the controller and wiring add delay.
6. What electrical interface is required?
Confirm:
• supply voltage and current;
• PNP, NPN, push-pull, relay, analogue, or communication output;
• normally open/normally closed or Light-ON/Dark-ON logic;
• maximum output current and residual voltage;
• cable length, connector type, and pinout;
• PLC input common and input threshold;
• diagnostics, teach input, IO-Link, or remote adjustment requirements.
Match the sensor to the PLC input diagram. Do not select PNP or NPN from regional habit alone.
7. What environmental conditions apply?
List normal and worst-case conditions:
• operating and cleaning temperature;
• dust, oil mist, water, coolant, chemicals, and washdown method;
• sunlight, welding light, or nearby optical sensors;
• vibration, shock, and bracket movement;
• indoor or outdoor exposure;
• hygienic, corrosive, or hazardous-area requirements.
An enclosure rating describes specific ingress-test conditions; it does not by itself prove chemical compatibility, outdoor life, food-contact suitability, or performance through smoke, steam, dust, or fog. Check the complete datasheet and applicable certificates.
8. How will the sensor be mounted and maintained?
Check available space, alignment access, bracket rigidity, cable bend radius, connector clearance, lens-cleaning access, and protection from impact. A mechanically unstable installation cannot be fixed by increasing sensitivity.
Plan how maintenance personnel will verify alignment, signal margin, settings, and wiring after replacement. Record the final position and teach values.
9. What safety or regulatory requirements apply?
For ordinary process detection, select a standard industrial sensor with the required electrical and environmental ratings.
For personnel protection, guarding, access control, or a safety interlock, use devices and control architectures designed and certified for the required safety function. A standard photoelectric sensor must not be substituted for a safety light curtain, safety laser scanner, or safety interlock device.
For a hazardous location, do not choose from enclosure material or IP rating alone. Confirm the required protection concept, zone or division, gas/dust group, temperature class, certificate, and installation method with qualified personnel.
10. What evidence should be obtained before production approval?
Request the exact model datasheet, wiring diagram, dimensional drawing, connector details, applicable declarations or certificates, sample lead time, production lead time, warranty process, and change-control policy.
Validate a sample with:
• the hardest target and real background;
• minimum and maximum distance;
• normal production speed;
• expected lighting, vibration, and contamination;
• the final PLC input and cable route;
• repeated cycles, not a single demonstration.
Create acceptance criteria before testing and record the final result.
Quick RFQ checklist
Send the supplier the sensing task, target details, distance, background, speed, environment, supply voltage, output type, connector, quantity, destination country, and required delivery date. Attach drawings, photographs, and a short application video when available.
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