Calibration of Delta 3D Printers using a calibration plate

One issue with the calibration of deltabots is that while it is relatively easy to probe bed level, it is difficult to determine an accurate XY position. If a linear-delta printer is built perfectly, with absolutely parallel towers, absolutely perpendicular to the bed, with perfect arms and end effector linkages, this is not a problem. However, in practice, there are always construction imperfections. These imperfections can cause distortions in the XY plane. It can sometimes be better to have calibrated parameters which sacrifice bed flatness for low distortion (higher accuracy) in the XY plane. The residual bed-flatness error can be removed by an automatic bed-leveling procedure (a.k.a. bed tram compensation).

See my earlier musings on calibration for a more in-depth discussion.

I had earlier proposed a technique which uses numerous measurements on a calibration print in the calibration procedure to reduce XY distortions. While this technique does work, it is limited by errors in measurement, printing, and part warpage when the plastic cools. It also assumes that the print spread is anisoplanatic, where on most printers it is not. I know that my blower is not completely uniform, and spread around the nozzle is not uniform.

What if we fitted the printer with a CNC probe needle sensor, and searched probes of a precision machined plate mounted on top of the print bed like this one. We could perform interactive bed probes, searching for the bottom of the calibration holes. This would give us printer settings at known, complete 3D XYZ positions to feed into a calibration parameter optimization.


Another way to accomplish this would be to create a flat calibration target with little fiducials, most likely + symbols, which can be attached to the printbed. A small camera, with macro focus, could be mounted to the end-effector, and used to track the fiducials for known XY positions. A traditional bed probe, also attached to the end-effector, could be used to estimate bed levels, albeit at an offset from the camera points.