Vision inspection, laser, photoelectric and load-cell sensors: who does what

Vision inspection and sensors are not competing options. The practical rule on most lines is this: if the answer can be expressed as a number or an on/off state, use a sensor; give vision only the questions no single physical quantity can express — an action, a sequence, a shape, a surface. A station that carries a proximity switch, a load cell and an industrial camera at the same time is entirely normal, because each is answering a different question.

Rule one: if a sensor can decide it, do not use an image

A sensor returns a switch state or a scalar: present or absent, seated or not seated, a weight, a torque, a beam broken. The threshold is clear, the response is measured in milliseconds, the reading is immune to lighting and surface colour, the cost per point is low and there is almost nothing to maintain. Wherever the answer fits inside a numeric band or an on/off state, a sensor will almost always beat vision on cost and reliability. Two sensors can also produce a sequence — fire in order and you have a crude timing check — but once the sequence spans several stations and part numbers, or once you must tell “done wrong” apart from “not done at all”, the logic grows complicated fast, and wiring, programming and every later change get more expensive. The moment the logic stops being simple is the moment a Visual AI SOP System earns its place.

Three questions only vision can answer

• An action sequence. Scan, then label, then press. Do them out of order and the unit still passes. A sensor only knows the scanner fired; it cannot say whether that happened before or after loading.
• Shape and orientation. A reversed connector, a skewed seal, a label on the wrong face — all shape problems, with no threshold to set.
• Anything tied to the part number. One station may run a dozen part numbers in a shift, and the correct action differs for each. Sensor logic is fixed and hard to change; vision can switch its rules with the recipe.

What these three have in common is that none of them is asking for a quantity — they are asking for a state or a relationship. A sensor compresses the physical world into a number, and it does that very well. But once a judgement needs to understand “did this action happen before that one” or “is this part facing the right way relative to its neighbour”, a number is no longer enough and you have to go back to the image itself.

The boundary: when vision is the wrong tool

Vision is not a universal tool. Treat the following cases with caution, and in some of them drop the vision option altogether. Where judgement depends on feel, torque or internal structure the camera cannot see — weld penetration depth, thread engagement length. Where the defect criterion is itself unclear and nobody agrees on pass or fail, which means solving the standard first, as in five ways to prevent a missing operation. Where the cycle is very fast and the measurement is micron-level, which belongs to laser profilers and vision measuring machines rather than a general-purpose camera. Where lighting, dust, vibration or oil mist cannot be brought under control, because unstable imaging defeats any algorithm. And where volume at the station is very low and the consequence of an error is mild, because the payback will not close and manual confirmation is simply cheaper.

What to decide first