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Five Defect Classes, One Robot-Held Camera
Overview
A large painted part presents more surface than a fixed camera arrangement can cover economically. A bumper, a door, a liftgate or a fender each have several faces with different curvature, and covering all of them from fixed positions needs a camera per face and a light per camera.
Mounting the camera on a robot changes that arithmetic. One sensor and one light travel to each face in turn, and the robot provides the coverage that would otherwise require an array. The cost is paid in motion: every view becomes a path, and every path has to be accurate enough that the lighting geometry is the same on the thousandth part as on the first.
This article describes what a robot-held station can inspect, how the lighting travels with the camera, and where the accuracy requirement actually lands. Big Bird Industrial (formerly Shimada Big Bird Industrial) has been building cleaning and deburring systems for automotive powertrain components since 2001, and the inspection approach here follows production stations for painted exterior parts.
1. Why Put The Camera On A Robot
The argument is coverage per sensor, and it becomes decisive as the part grows.
Many Faces, One Sensor
A painted closure has a top face, a side, an edge radius, an inner flange and often a compound curve between them. Each presents the surface to the camera at a different angle, and each needs the light at a specific angle relative to that surface. A robot can hold both camera and light in a fixed relationship and present that relationship to each face.
Variants Without New Optics
A station built for one panel is a station that becomes idle when the model changes. A robot-held head handles a new variant by teaching a new path, which is a programming task rather than a fabrication task. That property is why this arrangement appears on lines running several panel variants at modest volume each.
2. The Five Classes
A painted exterior carries defect classes that differ in size, contrast and how they respond to light, and a single station is expected to find all five.
Particles And Shallow Dents
A particle is a local bump that distorts the specular highlight, and a shallow dent is a local change in slope over a larger area. Both are revealed by a directional reflection: the highlight bends where the surface bends. Neither is visible under diffuse light, which is why a station that inspects only under diffuse illumination misses both.
Pinholes, Scratches And Runs
A pinhole is a dark point with a sharp edge, a scratch is a line that scatters light, and a run is a change in film thickness that alters the local slope over a long narrow region. The three respond to different light directions: pinhole to coaxial, scratch to low angle, run to a grazing source across its length.
| Class | Scale | Light that reveals it | Missed when |
|---|---|---|---|
| Particle | Sub-millimetre | Directional, highlight distortion | Light is too diffuse |
| Shallow dent | Centimetres | Grazing, slope change | View is normal to the surface |
| Pinhole | Tenths of a millimetre | Coaxial, dark against bright | Surface is inclined to the axis |
| Scratch | Millimetres long, microns deep | Low angle, dark field | Light arrives along the scratch |
| Run or sag | Long and narrow | Grazing across its length | Light runs parallel to the run |
3. Lighting That Travels
When the camera moves, the light has to move with it, and that changes the design of both.
Fixed Relationship
The practical arrangement is a head that carries camera and light in a fixed mechanical relationship, so that the angle of incidence at the surface depends only on the path and not on where the robot is within its envelope. That is what makes a taught path reproducible: the geometry is a property of the head rather than of the position.
Weight And Acceleration
A head carrying a light source is heavier than a bare camera, and the robot has to accelerate it between views without vibration that blurs the exposure. That constraint sets the head design as much as the optics does, and it is why robot-held heads use shaped light guides rather than large lamp housings.
4. Where The Accuracy Requirement Lands
Path repeatability is not measured in millimetres of position. It is measured in the stability of the lighting geometry at the surface.
Angle Matters More Than Position
A view that is a millimetre off target still sees the same surface, but a view whose light angle is a few degrees different produces a different reflection and therefore a different image. The requirement that matters is angular repeatability of the head relative to the surface, which on a curved panel varies along the path.
Stand-Off And Focus
Working distance has to be held closely enough that focus and magnification stay constant, because a changing magnification moves the defect scale relative to the pixel grid and upsets any size threshold. On a large panel the path therefore includes a normal-direction correction, not just a position.
5. The Cycle Budget
A robot-held station spends its cycle moving rather than exposing, which is the opposite of a fixed-camera station.
Views Set The Time
Each face needs at least one view per illumination mode, and a panel with five faces and two modes needs ten exposures plus the travel between them. On a bumper or a set of closures a cycle of fifteen seconds or less is achievable, but it is achieved by planning the path so that consecutive views are close together rather than by exposing faster.
Where Time Is Lost
Time is lost at direction changes and at the return to a safe position. A path that visits faces in a sequence with large jumps spends more time travelling than imaging, which is why the view order is an optimisation rather than an arbitrary list.
6. When Fixed Cameras Win
A robot-held station is not universally better. Where the part is small and presented in one orientation, a fixed multi-camera station has no motion time and no path to maintain, and it will beat the robot on both cycle and reliability.
The choice follows from the number of faces and the variant count. Many faces and many variants favour the robot; one face and one variant favour fixed cameras. A station built for a single high-volume panel with one critical face does not need a robot, and adding one adds maintenance without adding coverage.
7. Verifying A Robot-Held Station
Verification has to test the path as well as the optics, because the path is where the variability lives.
A reference panel with defects at known positions is presented several times, and the station must find them at the same rate each time and at the same measured size. A station that finds a particle on the first pass and misses it on the third has a path or a lighting-stability problem, and that is a different fix from a detection-threshold problem.
8. Planning The Path For Coverage
The path is the process on a robot-held station, and it is designed rather than taught by trial.
Grouping Views By Orientation
Faces that need the same light angle are visited together, even if they are not adjacent, because the alternative is switching modes between every view. On a closure that means grouping the top surfaces, then the sides, then the edge radii, with the mode changed at each group boundary rather than at each face.
Approach Direction
Every view needs an approach that keeps the head clear of the part on the way in and out, and on a curved panel the approach is not the same as the viewing direction. A path that ignores this will collide on a variant with a slightly different radius, which is why approach vectors are part of the program rather than a property of the robot.
A path designed this way runs in a consistent cycle with no surprises, and the same structure makes it teachable for the next variant without starting from an empty program.
Frequently Asked Questions
Can One Head Cover All Five Classes?
It can if it carries more than one illumination mode. A single mode covers some classes well and misses others, which is why a production head switches between at least two.
How Often Does The Path Need Re-Teaching?
After any mechanical change to the head, the fixture or the robot, and after a collision. Otherwise it is checked with the reference panel rather than re-taught.
Does Robot Speed Affect Detection?
Only if the exposure happens while the robot is moving. Indexed positioning before each exposure removes the effect, at the cost of cycle time.
What Is The Main Maintenance Item?
The head itself: lamp output, lens cleanliness and the mechanical alignment between camera and light. All three drift, and all three change the image without producing a fault message.
Conclusion
A robot-held camera buys coverage per sensor, which is what makes it economical on large parts with several faces and several variants. The price is paid in motion, and the motion has to hold the lighting geometry rather than merely the position.
The five defect classes need more than one illumination mode, and the acceptance test has to repeat the same reference panel several times. A station that passes once has demonstrated an image, not a process.
