Servo - August 2009

sympathy (yeah, right!).

Here are the gritty details: Hagetaka was being pushed to the limits on payload given the wider hip stance. While it ultimately provides a steadier gait, having a wider distance between the legs adds to the leverage required of the hip and ankle roll servos. The upper torso weighed in at a whopping 2.1 kg, and as such, I was playing a dangerous game while creating the walking gaits of trying to get the hip-roll and ankle-roll servos with a tight enough compliance to lift the foot without dragging.

Removing the battery was an instant solution, proving I was just slightly over my payload capabilities. Obviously, the robot wouldn’t work too well sans battery, and any smaller battery wouldn’t provide enough run time for a full 15 minute match. With only slightly over two weeks left until competition time, I didn’t think I had enough time to rebuild the upper torso using a lighter material, and really none of the equipment housed could be tossed overboard, so I stubbornly pushed on.

During my rush to get everything finished, I ended up damaging four of my RX-64s due to overheating. Dynamixels have a thermal shutdown which will normally prevent damage, but I was keeping them just short of that threshold for extended periods of time, which can be damaging. Generally speaking, if you’re developing walking gaits, you want to set this thermal shutdown threshold to a lower, safer value in the 60oC range. To add insult to injury, at some point a short occurred which damaged my LiPo (read: went nuclear) and blew out the Gumstix controller. The project was more or less grounded; the controller and damaged electronics would have to be rebuilt, the four RX-64s would need to be upgraded to EX-106s to handle the payload with less heat, and ideally, the upper torso would need a rebuild out of lighter material.

So, I did what any self-respecting tinkerer would do with two weeks left until said event: I tore everything apart and started over. Rob Farrell was kind enough to lend a hand getting my Gumstix controller back online and I set to work in an attempt to salvage what I had into a competition-ready robot. About 20 hours of CNC work later, Giger was born. I fell back to the “tried and true” humanoid configuration, built a much smaller, more lightweight upper torso, and changed the battery configuration into two 1,800 mAh packs mounted in the feet. Figure 5 shows the fruit of my labor.

Ultimately, my efforts were in vain; I managed to finish the physical build of the robot the day before I had to leave for RoboGames and simply did not have enough time onsite to finalize the walking gaits. This would prove beneficial for the competition as a whole; leaving me with more time to focus on running the event itself and making sure everything went smoothly.

Figure 5

Conclusion

Year One of Mech Warfare went great and given that I am no longer under a time crunch to get Hagetaka finished, I’ll be able to make the necessary changes to continue the project without having to rush to the finish line. Giger turned out better than expected, so I’ll likely keep that project intact for Humanoid Kung Fu competitions and rebuild Hagetaka with a mix of EX-106s and RX-64s — and a lighter upper torso. As this project is ultimately funded out of my own pocket and I’ll be looking at a considerable cost on the rebuild, the next article will likely be many months out. I learned a lot through this project, and hope that what I did right as well as what went wrong is beneficial for all that read this. Keep an eye out for those updates in the future and until next time, keep building! SV