Spray paths for coating robots, planned from a scan

A robot needs a program for every part

Powder coating robots are good at doing the same part all day. Most coating shops do something else: a set of rims this morning, a bike frame after lunch, a batch of brackets tomorrow. Every new part needs a program, and someone has to write it, test it and fix the thin spots it leaves. For a one-off set of four rims nobody does that, so the parts go to a person with a hand gun.

Raincoat is our attempt to make the program write itself. It is an Iron Rainbow product and it is still in development. It has not driven a real robot yet. Everything below runs in simulation on real part models.

Scan first, ask nothing

The part arrives on the hook and nobody types in what it is. Four depth cameras measure it while the hook turns. From one position they see 38 % of a rim to within 1.5 mm; after six stops, 96.8 %. The screen shows the operator the same map, blue for measured, amber for seen but rougher, red for still unseen. The scan is then matched against the Iron Rainbow part library, or used as it is.

The path draws itself

Raincoat turns the rim into a model of about a square meter of surface and samples it every 6 mm. Each of those 34,633 points needs the right film. It works out the 6,665 places where the gun could stand and still reach the surface at the right distance, then plans the path the way a good coater sprays: inside out, inner barrel first, then the back of the spokes, bead seats and outer barrel, then the rim turns round for the front face and the spoke roots.

The hanger turns the rim while the gun sprays, so on this rim the gun is spraying for 98.6 % of the cycle. The whole program is 44.7 m of gun path in 1 minute 55 seconds, and the gun body, nozzle and hose were checked against the rim and hanger on every move. Planning it took 23 seconds.

Faraday pockets on screen, before the oven

Corners and deep pockets push charged powder away, and the thin spots usually show up after the oven. Raincoat simulates the charge instead. On this rim it marks 7.6 % of the surface as Faraday pockets. Pointed into one of them, the gun hits its 20 µA current limit and its voltage drops to 36 kV, against 49 kV on an open face. In the pocket 52 % of the powder lands on the part, on the open face 74 %. Spray too long and the layer pushes the next powder back, and Raincoat hatches that back-ionisation in red where it starts.

Working with the line a shop already has

Most shops will not replace their reciprocator line. So Raincoat can take the line as it is (guns per side, spacing, stroke, conveyor speed), predict the film after the automatic guns, and plan a robot path for the touch-up areas only. On a steel rim on a two-by-four gun line at 2 m/min, the touch-up needed 78 seconds while the part was in reach for 21.5. At about 0.6 m/min it fits. A shop finds that out before it buys an arm.

The model is tuned on seventeen test pieces with drawings: U-channels, cups, a slotted box, a wheel spoke window. The shop sprays them in its own booth, reads them with its film gauge and enters the numbers, and Raincoat fits its powder model to that booth.

Where it stands

The planner, the scan cabin software, the electrostatics and the touch-up check all run, on real part models and simulated scans. Robots and guns come from a library where every figure links to the maker’s own sheet: twelve arms, five guns, three gun controllers and a reciprocator so far. The next step is the one that counts, a first run on a real arm in a real booth.