3D Printing

Force Sensing Resistors Make for Auto Bed Levelling for 3D Printers

Johann Rocholl, inventor of the deltabot-style RepRap, is continually seeking to upgrade the designs for his 3D printers, when he’s not busy working at Google that is. After successfully building the Kossel 3D printer and the Mini Kossel, as well as seeing the Kossel Pro launched on Kickstarter with his friends at OpenBeam, Johann has made some progress in the realm of bed levelling for his Kossel series.

Bed levelling is a process that can make or break a print. If the bed’s not level or if the printer doesn’t account for the angle of a print bed, the extruded filament will not stick to the bed at all points of a print job. Previously, the Kossel relied on a retractable Z probe to calibrate the extruder to the printbed, using an allen wrench attached to the extruder and an Omron SS-5 micro switch. After gauging the angle of the printbed, the printer’s software can account for minute changes in the bed’s horizontality and print as though it were perfectly level. You can watch the Kossel’s old Z probe calibration tool below:

Because the retractable probe limited the reliability of the printer’s calibration, Johann has instead come up with an ingenious idea to place Force Sensing Resistors (FSR) in between the glass and the build plate.  Now, the extruder itself acts as the probe, pressing against the sensors and sending data back to the firmware to calibrate the bed. Johann explains in a Google group post, “When the hotend pushes down on the print surface, their resistance changes quickly from 500 kΩ to 50 Ω, depending on the force. The raw ADC value on the Arduino changes from 1023 to under 200, and the bed temperature display in Repetier-Host goes up from 0.0 to 240 Celsius. This allows calibrating the print surface with 0.03 mm repeatability, which is good enough for printing PLA on cold naked glass.” You can watch it in action in the following video:

Johann points out, however, that these resistors will only work without a heated bed. Members of the Google group have already begun searching for more powerful sensors that can withstand higher temperatures in order to perform prints with a heated bed. Like all of Johann’s designs, this one is open source, so there’s no doubt that some manufacturer will incorporate it into their own machines and launch on Kickstarter.

One tidbit you might know about Johann is that, besides building deltabots in his spare time and working at Google in his not-so-spare time, he toys with animating legos. At least, that’s what I gather from his YouTube channel:

Source: RepRap