Half a part in frame: trigger and encoder faults that look like the model

If the pictures are wrong, nothing downstream can be right, and a surprising number of inspection projects spend weeks on a training set to fix a photoelectric sensor mounted 30 millimetres too far along the belt. Streaks across a surface, a part cut off at the frame edge, the same piece imaged twice, a piece never imaged at all — all four are acquisition faults, all four have physical causes, and three of them are settled by arithmetic rather than by opinion.

Start with a count, because it costs ten minutes and it partitions the problem before anybody opens a configuration screen.

Ten minutes: count triggers against parts

Run 100 parts and record three numbers: triggers fired, images acquired, and parts actually produced as counted at the machine. Three numbers, one shift-supervisor-sized task, and the pattern between them tells you which half of the system to open. Note the third row in particular — triggers matching parts while images fall short is a throughput ceiling rather than a triggering fault, and it belongs to it keeps up on the bench and falls behind at line speed.

The blur arithmetic, done once, ends the exposure argument

That last sentence is the trap. Cutting exposure from 1 millisecond to 200 microseconds removes four fifths of the light reaching the sensor, so either the illumination gets five times brighter or the images get five times darker, and darker images lose exactly the low-contrast defects the shorter exposure was meant to preserve. This is why a strobe exists: it delivers a large pulse over tens of microseconds and freezes motion without asking a continuous lamp to be five times more powerful. It also introduces a new way to fail, because a strobe that fires outside the exposure window produces a dark frame with no obvious cause.

• Check the strobe fires inside the exposure window, not merely at the same time as the trigger. A scope on both signals settles in minutes what a week of image review will not.
• Check the strobe is not being asked to fire faster than its duty cycle allows. At high part rates some units skip pulses rather than overheat, which produces intermittently dark frames.
• Check whether the exposure was shortened by someone chasing blur after a line speed increase. This is the most common undocumented change on an inspection cell, and it degrades every low-contrast class at once — the same contrast loss described in the model that finds the scratch on day shift.
• Check that the smallest defect you must find still spans several pixels after any resolution change. If a class was marginal at feasibility it will be the first casualty, which is what the pre-spend gate in fifteen checks that tell you whether a camera can judge this part exists to catch.

Clipped parts are geometry: offset, latency and jitter

A part cut off at the edge of frame is almost never a cropping bug. Between the sensor detecting the part and the sensor array being exposed there is a chain of delays — sensor response, controller scan, cable, camera trigger handling — and during that time the part keeps moving. At 500 mm/s a total latency of 20 milliseconds carries the part 10 millimetres past where the trigger thought it was. That offset is constant and can be compensated with a trigger delay.

What cannot be compensated is jitter. If the latency varies by 5 milliseconds — a controller scanning on its own cycle rather than responding to an interrupt is the usual reason — the part lands 2.5 millimetres either side of nominal on every image. Set the field of view so that the part fits with a margin bigger than the jitter and you have a robust cell; set it tight to maximise resolution and you have a cell that clips one part in fifty and nobody can say why.

1. Measure the offset. Image 20 parts, mark the leading edge position in pixels, and take the mean. That is your fixed offset, convertible to a trigger delay.
2. Measure the jitter. Take the spread of those same 20 positions. That is what your margin has to absorb.
3. Compare the jitter against the free space at the frame edge. If the spread is larger, the cell will clip parts regardless of the delay you set.
4. Fix the source before the symptom. A trigger routed through a controller's scan cycle inherits that cycle's variability; a hardware trigger wired to the camera does not.
5. Re-measure after any change to belt speed, part length or the sensor's mounting. All three move the numbers.

Encoder faults have a signature nothing else has

On a line-scan station the image is built one line at a time, clocked by an encoder on the drive. Every geometric property of the image along the travel direction therefore depends on the encoder being right, and encoder faults are unique in leaving distortion along travel only, never across it. A round hole photographs as an ellipse; a square part comes out as a rectangle; the cross-web dimension stays correct throughout. That asymmetry is the diagnosis.

There are two versions. The first is a wrong constant: an encoder giving 1000 pulses per revolution on a 100 mm diameter roller produces one pulse every 0.31 mm of belt travel, so a station configured for 0.1 mm per line is stretching every part by roughly three times unless the divider is set correctly. This is consistent and repeatable, and it shows up the moment you image a calibration artefact of known length. The second is slip: the encoder wheel loses grip on a dusty or loaded belt, so the stretch varies part to part and within a single part. Consistent distortion is arithmetic and is fixed in configuration; varying distortion is mechanical and is fixed with a hand on the encoder mount.

Distortion along the direction of travel and nowhere else is an encoder telling you it is not measuring the belt. No amount of training data will teach a model to un-stretch a part by an amount that changes every piece.

Duplicated and missing frames: debounce, pitch and what the sensor can see

A part with a slot, a hole or a stepped profile presents more than one edge to a photoelectric sensor, and each edge is a trigger. The result is two or three images of the same piece, which then arrive downstream as separate parts with separate verdicts, and a reject on any of them looks like a reject on the part. The fix is a one-shot lockout sized from the minimum part pitch: parts 300 mm apart at 500 mm/s arrive every 600 milliseconds, so a lockout around 400 milliseconds suppresses the extra edges while never suppressing a real part.

• Size the lockout from measured pitch, not from a default. Too long and it deletes genuine parts during a burst; too short and it does nothing.
• Match the sensor to the material. A diffuse-reflective sensor may not see a matt black or a translucent part at all, and swapping to through-beam or background suppression solves a triggering problem that looks like software.
• Watch changeovers. A shorter part at the same belt speed halves the pitch and can walk straight into a lockout tuned for the previous product.
• Keep the acquisition parameters in the recipe, not in an engineer's memory. Exposure, delay, lockout and encoder divider are per part number and belong with the part's other master data — the argument in master data before a plant goes live.
• Exclude bad frames from the training set explicitly. A labelling standard needs an 'unusable image' verdict, or blurred and clipped frames get labelled as parts and taught to the model, which is one of the adjudication rules in labelling defect images so two inspectors agree.

The decision tree: steel, wiring or code

1. Counts unequal? Fix triggering first — sensor type, mounting, lockout. Nothing else can be assessed until one part produces one image.
2. Smear across the whole image, in the direction of travel? Exposure against speed. Compute the blur, then decide between a shorter exposure with more light or a strobe.
3. Smear on some frames only? Strobe synchronisation or duty cycle. Put a scope on the trigger and the strobe.
4. Distortion along travel only? Encoder. Consistent means a divider or resolution setting; varying means slip, and that is a mechanical job.
5. Parts consistently off-centre? Trigger offset, correctable with a delay. Parts randomly off-centre? Jitter, correctable only with margin or a hardware trigger.
6. Everything measured and correct, images still poor? Now it is optics or presentation, and the model still has not been touched.

The organising principle is that acquisition is cheap to measure and expensive to assume. A morning with a tape measure, a scope and a calibration artefact answers questions that months of model work cannot, and it produces numbers a supplier and a plant team can argue about on the same terms. When we scope an inspection build, these measurements sit at the front of the plan rather than in a commissioning week at the end — the shape of work described under MVP and product builds. The rest of the cluster, from feasibility to acceptance, sits in visual inspection and defect detection, part of our manufacturing and industrial vision work.

• machine vision
• triggering
• encoders
• image acquisition