Servo - 201306

when the end of the stroke was reached. The large pulley on the left side of the photo was driven by the small orange and silver motor below it. This powered the flywheel while the robot jockeyed for position during a fight.

The rear bars of the flipping arm had bearings inside them so the shaft could turn the flywheel while they stayed still.

For this build, I allotted extra time for testing before the competition because I ran into so many problems on the previous version.

Figure 3 shows what happened on the first full power test flip. The Kevlar thread failed when the hard stop was hit at the end of the stroke.

Unfortunately, the Kevlar thread I used was the largest my supplier offered. I needed to find a suitable replacement or a new supplier in time to order the new thread and install it into the robot.

Mike Jefferies — a frequent contributor in the SERVO Combat Zone — suggested that I try out a kind of rope called Dyneema. This rope was available in many sizes, ranging from smaller than the Kevlar thread I was using to large enough to tow a car out of the mud. I decided to order a few different sizes and rebuild the robot to fit.

The Kevlar thread I was using was only .040” in diameter. I ordered .075”, .095”, and .125” thick Dyneema ropes from a spearfishing supplier and set about trying to fit them into the design while I waited for them to arrive.

The trouble with using the bigger rope was that the width between the rear flipper arms could not grow because I did not have time to move the main weapon rails. That meant I had to make space for them by making the flywheel and coil drum narrower.

FIGURE 4. New coil plate.

jammed against the insides of the weapon rails. The clutch engaged and disengaged properly but the system did not reset because of the rope jam. Once I realized what was happening, I decided to switch down to the thinnest rope that I had purchased to see if that would alleviate the pressure. One more test showed that it was strong enough to handle the load while leaving plenty of extra space on the coil drum shaft. This final version of the weapon assembly proved to be reliable enough to meet my goal of three unattended flips. In the end, the weight from the wider coil plates and the necessity of making the flywheel narrower forced the flywheel mass to be lower than the one in Reclipso.

While this meant that the flipper did not throw other robots as well as I wanted, the clutch mechanism performed flawlessly and that was a huge step in my quest.

I decided to call the new robot Threecoil, and its first competition was the 2012 Bot Blast in Bloomsburg, PA.

You can see the completed machine in Figure 5. The robot performed well beyond my expectations at the competition.

The clutch mechanism never jammed and the weapon fired multiple times in each match. I would have been completely satisfied if those were the only results but the robot also managed to win the award for Coolest Robot and the first place trophy.

These results were so exciting that I decided to see if I could scale the design up to a 30 pound sportsman class machine for Motorama 2013. You will have to wait for another issue to see how that went. SV

FIGURE 5. Ready for competition.

Once I did that, I had to replace the rear coil plates with wider pulleys that included a place to attach the new Dyneema rope. You can see one of the new coil plates in Figure 4.

The new coil plates sported a much wider and deeper channel for rope up to .125” in diameter, and an integrated pin for locking the rope to the coil plate instead of having it stick out the side like I did with the Kevlar thread.

Once I installed all the new parts, it was time for another test. I decided to start with the thickest rope to see if any of the sizes I bought could survive the forces in the flipper.