Thursday, December 14, 2017

Dinner Time 2 / Margin of Safety 4: 3D printed modular combat robot

For this robot, the goals were to 1) mostly 3D print the chassis on my home printer, and 2) connectorize the weapon modules. The 3d printing requirement is driven out of laziness. The result of this endeavor was Dinner Time 2, which I reverted back to the Margin of Safety name to restore continuity of the record. I just like coming up with names.
I promise it's not made of wood.
3D printed parts very isotropic, that is, the part strength varies depending on the direction of force. Tensile forces that pull along the layers are well resisted, while tensile forces that pull layers apart (delamination) are weaker. To get the maximum strength while avoiding forces that pull layers apart, I designed the frame elements to have metal parts clamp around 3D printed components. Most of the screws do not thread directly into the plastic, but instead go into standoffs that go through the plastic part. This way, when assembled, the plastic is held in compression by the screws and standoffs, preventing delamination when the plastic is hit by an opponent.
Rear picture of the chassis without armor, showing standoffs.
The 3D printed parts serve as armor, absorbing impact energy from opponents and distributing the loads across the standoffs. The standoffs connect the 3d printed parts to the top and bottom plates and prevent delamination.
Side view, showing how the top and bottom plates clamp around the 3D printed sections. Front interface is at right.
Due to strength requirements and space constraints, the front interface is held on with two large turned standoffs. For quick manufacturing, the rear standoffs are off the shelf hex things. The frame is made of four 3D printed components: front interface, two motor supports, and rear armor. The three top and bottom plates hold the 3D printed parts together.
Bottom view showing rear armor at left, motor support pieces, and front section at left. Also seen are wheelguards, drive motors, and weapon connector.

I was not happy with the minutes required to switch weapon modules. If someone else had a modular robot, with easy to swap modules, they could out-switch me before a match. As a result I designed a connector embedded in the interface between the weapon module and frame.
The weapon module interface on the frame side.
I used a six position blade connector, using two blades per phase. Each blade is rated for seven amps, resulting in a total rated capacity of 14 amps. My controller is set to limit battery current to 10 amps; phase current can be much higher. However, given the short duty cycle of the system (three minutes on, non repeating), I could get away with overrating the connector.

Placing the connectors into the structure required a lot of work. It had to be firmly connected, in a compact space. It could not result in any weak spots in the connector area. It cannot be impossible to assemble.
Detail on connector interface.
Custom PCBs were made to mount the connectors and provide large solder pads for the wires. The connector assembly was put into fitted sockets in the frame and weapon module. The white bar in the image above is to provide access to the connector so I can solder the motor wires on. The overall dovetail design is retained, though a little narrower.

Both weapon modules were more or less copied from the previous robot, with the connectorized interface. On the horizontal weapon this worked out great. In the vertical weapon this resulted in loads pulling apart the 3D print layers. This was deemed unavoidable for maximizing part reuse so I printed it extra thick.
Connnectorized vertical attachment.
Showing the suboptimal screw placement into the 3D printed plastic.
The control board was remade removing the encoder and Maxon motor connectors to reduce the length slightly.
All the electronics, showing drive motors, control board, receiver, and weapon motor connector.
Finally, for fun, I covered the entire robot in two tones of wood grain shelf liner.
A little too much time went into this.
Totally worth it though.
Undercutter inverted, showing hub motor.
The stately appearance qualified it for the Dragoncon Teacup Racers, running without the blade, of course.
The tea set is a Dowton Abbey Christmas ornament.
This robot went 5-0 at Dragoncon 2017 for first place. It was a close match at the end due to a broken connector, with no thanks to a "totally not entanglement on purpose" rubber latex wheels.
Entangled latex wheel coating. The blade had to be removed to free the motor.
Getting lazy with hot glue.
Instead of gluing the connector housing to the PCB with epoxy like a regular person I used hot glue. This in combination with what was likely slightly too short wires resulted in the PCB getting pulled off the mounted connector. This could also be averted by mounting the connector assembly to the frame via the PCB, not the connector housing. Lessons learned.