Sensor for Pallet Detection: How to Choose for Conveyors and Automated Pickup

Which sensor should you try first for pallet detection?

For a defined conveyor presence point, start with photoelectric sensing. Consider ultrasonic when recessed plastic pallets or unreliable optical returns make that point difficult. Use a distance sensor for a distance value, and a purpose-built perception system when a vehicle needs pallet position and fork-pocket location.

The deciding question is not just “Can it see this pallet?” It is “Will every permitted pallet produce the machine event we need?” A sensor can correctly see three separate blocks while the controller incorrectly counts three pallets.

An inductive proximity sensor is another option for a nearby metal carrier or intentional metal flag. It does not directly detect an ordinary wooden or plastic pallet. A detected flag also proves only the flag's presence unless the mechanism establishes its relationship to the pallet.

When should you use through-beam, retro-reflective or one-sided sensing?

Choose by the available light path. Beam interruption avoids relying on light reflected from the pallet; one-sided target-return sensing is useful when a receiver or reflector cannot fit across the conveyor.

Two accessible sides: through-beam

A separate emitter and receiver face each other. An opaque pallet feature reduces the received light when it crosses the beam. This is a useful starting point for mixed surface colors, provided both devices can be mounted, wired and aligned. A clear opening in the pallet can still let the beam through.

One wired side, with reflector space: retro-reflective

The emitter and receiver share a housing, with a reflector opposite. Check the specified reflector and the required beam interruption. Shiny film can return unwanted light; a polarized or application-specific version may help, but the actual wrap and angle still need a trial.

No opposite hardware: diffuse or background suppression

A diffuse sensor receives light from the target itself. Background suppression (BGS) adds distance discrimination to help reject a surface behind it. This is useful at a defined block or deck edge, but not a promise that every dark, glossy or recessed face will be detected. Identify both the nearest and farthest target positions before setting the cutoff.

A long catalog range does not resolve a missing target feature. First establish the optical arrangement; then compare the exact model's range, spot footprint and setting limits.

Why do pallet openings cause flicker or extra counts?

A pallet is not a solid box. At one sensing height, blocks and openings can alternate as it moves past. A point sensor may therefore report several real target transitions during one pallet passage. Increasing sensitivity cannot make wood exist in a hole.

Map the sensing path over the full pallet length, including permitted damage, lateral offset and both transport orientations. If no suitable continuous feature exists, a light band or multiple sensing points may be more appropriate than a narrow beam.

Leuze's RK46C application guidance describes using a light band for pallets with openings. That is evidence for a particular sensing approach, not a guarantee that any wide beam bridges any gap. Field orientation, interruption threshold and the smallest feature still determine the result.

Also define damage limits. A broken pallet that must be rejected belongs in the rejection test set; it must not be treated as an acceptable pallet merely because the presence sensor still sees it.

When does ultrasonic sensing help with plastic or wrapped pallets?

Ultrasonic sensing is worth evaluating when optical return is inconsistent and the pallet presents a usable sound-reflecting surface. A broader acoustic field may span recesses, but it can also include a roller, bracket or neighboring pallet.

Pepperl+Fuchs documents two versions of its conveyor pallet detector: an application-specific optical version for standard pallets and an ultrasonic version for recessed plastic pallets. The useful lesson is to compare sensing fields against the actual construction—not to assume that all plastic requires ultrasound or that all wood requires optics.

Check what the sound field actually sees

For an echo-based ultrasonic sensor, keep the closest permitted pallet outside the specified blind zone. Review its detection-zone drawing at the installation distance, and test tilted faces, nearby structures and adjacent active sensors. Synchronization or multiplexing, where supported, can alter the available update rate.

With shrink wrap, first decide whether you need the pallet structure or the load envelope. Loose film may arrive before the deck edge or obscure a fork pocket. A successful film detection is not necessarily the event your conveyor or vehicle needs.

What changes at long range or in harsh environments?

More distance and harsher conditions increase the importance of the complete sensing path. Compare the nearest and farthest target positions, the clear-state background and the remaining signal margin after realistic contamination—not just the largest range number.

Separate range from usable detection margin

For optical sensing, excess gain describes received light relative to the switching threshold. A curve helps assess reserve against attenuation, but it does not prove that an unwanted reflection or pallet opening will be rejected. At the farthest position, verify the actual target or reflector, beam footprint and alignment tolerance.

For distance measurement, check what surface generates the value as well as accuracy, repeatability and update time. A precise reading of the rack through a pallet opening is still the wrong measurement. Stack-height sensing has the same issue: a spot that falls between deck boards may read a lower layer.

Turn “harsh environment” into specific exposure tests

• Dust and deposits: plan access to clean the lens, reflector or transducer. Check diagnostic options and the signal before the planned cleaning interval ends.
• Cold storage: verify operating temperature and behavior during entry and exit. An IP rating does not establish that frost or condensation will leave the sensing face clear.
• Washdown: check the housing, mating connector, cable and chemical exposure as an installed assembly. Water resistance does not establish detergent compatibility.
• Lighting and air movement: include sunlight, nearby optical devices and, for ultrasonic sensing, the actual airflow and temperature changes along the sound path.
• Impact and vibration: protect the sensor and cable without blocking its field. Keep a repeatable mounting datum so replacement does not require guessing the original position.

Define the required maintenance condition before the trial. A system that only works with freshly cleaned optics has not demonstrated operation through a dirty production shift.

Will the PLC capture the signal and stop the pallet in time?

Check event capture and stopping travel separately. The sensor output must remain in each required state long enough for the input and logic to recognize it. After recognition, the pallet still travels while the drive responds and the conveyor stops.

Trace the event beyond the indicator LED

Match supply voltage, output type, input common and the exact connector assignment. PNP/NPN describes the electrical interface; Light-ON/Dark-ON describes optical switching behavior. Neither label alone defines your application's “pallet present” state.

Record the physical target, raw sensor output, PLC input and program state during the same controlled pass. If the raw output breaks up, investigate geometry or sensing. If it stays correct but the PLC misses the event, check input thresholds, filtering and acquisition before changing the sensor.

Siemens' S7-1200 filter guidance illustrates why a short pulse can disappear before the program sees it. Use the installed input module's actual filtering and capture rules; do not transfer a different controller's default. Longer filtering can suppress useful transitions as well as noise.

Define where stopping-distance measurement begins

For constant speed before the stop command, a simple process-position estimate is:

Travel after detection = speed × pre-command delay + travel from stop command to rest

A switching sensor can trigger a controlled stop without measuring continuous distance. If the final position tolerance cannot be met consistently, consider position feedback or a different control method. This estimate is not a personnel-safety separation-distance calculation.

When does an AGV or AMR need 3D pallet perception?

Use spatial perception when autonomous pickup needs pocket locations and orientation that are not already constrained by the handling system. A single presence bit cannot tell the vehicle how far left, high or rotated the pallet is.

ifm's PDS Pick documentation describes processing a 3D point cloud into X, Y and Z coordinates, yaw, pitch and roll, plus left/right pocket locations. These outputs support an approach decision that a simple switching sensor cannot provide.

Check supported pallet families, working distance, field of view, camera-to-vehicle calibration and how invalid or obscured detections are reported. Test covered pockets, skew and permitted damage. A recognized pallet is not proof that its pockets are unobstructed or that pickup is acceptable.

Keep final load-presence or insertion-depth verification distinct from approach guidance. Neither the perception result nor an ordinary confirmation sensor replaces the vehicle's protective safety functions.

What must a pallet-detection trial prove before approval?

It should demonstrate the required machine event across the approved pallet population, including clear conveyor states and abnormal flow. Agree the acceptance criteria before testing; one clean sample at low speed is not a qualification.

For a useful supplier review, send a dimensioned mounting sketch, representative pallet photographs, the required event, speed and spacing limits, environment and PLC interface. Request the exact proposed model and suffix, sensing-field drawing, response in the selected mode, required accessories and configuration instructions.

Freeze the accepted model, firmware where relevant, teach settings, bracket position and controller settings with the trial record. Choose the simplest sensing arrangement that passes the defined task—not the one with the longest catalog range or the most impressive demonstration.