Servo Magazine ( January 2015 )


By Matthew Spurk    View In Digital Edition  

I have been trying — rather unsuccessfully I might add — to build a 150 gram combat robot for several years. It seemed that I just couldn't get a functional robot in that weight class. When my son and I started talking about new designs for a combat robot, the conversation of what weight class came up, and when he said “skeeter weight,” I knew I was in trouble.

This robot began as they all do with a 3D model of the components. The first thing I needed to do was to determine if it was even possible. I weighed all the components I had on hand and started working on the 3D model of the assembly. I knew it was possible if I could keep the frame weight down.

When I design my frames, I begin by laying out all the components in their approximate locations and get the spacing and orientation worked out. Then, I go back and build the frame based on the structure I need to support all the parts. I follow that up by rebuilding the virtual assembly, except now everything is referenced off the frame instead of floating in space.

Slade was going to be a robot that incorporated a slanted blade. (SL-anted bl-ADE, SLADE, get it?) The blade began life as a 3-3/8" circular saw blade. Due to weight, I cut off every other tooth with a Dremel tool.

The saw blade was directly mounted to a 1,100 kV brushless motor. I 3D printed a hub that secured the blade. I made certain to print up three spares because I wasn't convinced the hub would survive an entire match and I may need to replace it.

I was very wrong. In one fight, the robot lost an entire tooth (sheared completely off), but through the entire event I didn't lose a single hub.

The frame was also 3D printed. In order to improve the print speed and quality, I decided to print the frame as two components and then glued and bolted them together. I began by printing the weapon motor mount. I printed this part by standing it up on its back to eliminate the need for support material.

Next, I printed the frame. The frame featured a large opening where the motor mount would be installed. Both components combined took about three hours to print with a 45% infill. I probably could have gone with a lesser infill, but again, strength was a concern.

I glued the two frame components together and added two mounting screws to ensure they stayed together. I made the top armor by cutting out a piece of plastic bottle and screwing it into four holes in the frame.

While the frame parts were printing, I soldered up the 7.4V 270 mAh battery pack, the hacked micro servo drivetrain, the servo speed controllers, and the FingerTech robotics on/off switch. By the time I finished all my soldering, the frame was nearly complete. I had a little extra time, so I started giving the saw blade a flame-ish look. I thought it turned out cool, albeit not what I was going for originally.

Overall, I was disappointed in this bot. It didn't fare well in competition, losing its first two matches. The weapon hit suitably hard for my liking, but the hacked servo drives let us down. The servos were older and I had a lot of trouble getting them to function reliably. I had smoked one of the speed controllers about a week before the event, which meant buy a new one. I even paid for expedited shipping. Then, the servo case cracked and that caused alignment problems.

While I was at the competition, I picked up two FingerTech robotics Tiny ESCs (electronic speed controllers) and two Pololu gearmotors. My intention is to upgrade the drivetrain and the speed controllers for a future version of this robot. Not only should this setup be more reliable, I should also have a slight weight and space savings over the hacked servos.

In speaking with other competitors at the event, the discussion was that gearmotors are somewhat weak — especially during large impacts — but when supported properly and not directly coupled to the wheel, they can work quite well.

My intention is to 3D print a flexible spider tooth coupler (a.k.a., a love-joy coupler) or use a small length of rubber tube glued to the motor and wheel shaft. We'll see what the future holds for this robot.

My son has already discussed a drum, a wedge, and a flipper, so stay tuned to future issues of SERVO to see what we end up with.  SV

Article Comments