Design: Hanging In There

Robots waiting for the match to start be like…

As we discussed in our Game Analysis post, a robot is that is capable of lowering from the Lander in autonomous and reattaching during the end game is capable of scoring a lot of points in not a lot of time. If done by both robots on an alliance, these actions alone can secure an alliance an additional 160 points. (Or the equivalent of 80 Minerals in the Depot, or 32 in the Cargo Hold)

So, what’s the best way for our MCC robot to negate at least some of this potential point swing in the opposing alliance’s favor? It’s simple, we should be able to perform both of these actions also!

Looking back at our robot feature list, the only requirements for this mechanism are:

  • Can start on Lander
  • Can lower robot from Lander
  • Can attach and lift robot off ground in less than 30 seconds

And remember, we want to be able to accomplish both of these actions with the same mechanism to optimize the robot, and save weight. We know we’re going to have to lift the robot off the ground. However, there are a few approaches we can take to get ourselves off the lander at the start of the match:

1.) Use a dedicated mechanism, such as a claw or latch that opens and drops us off the lander. We would not recommend this with traditional wheels, but the suspension we gain by using Flex Wheels on our drivetrain allows us to get away with this. Regardless this is going to be pretty violent on the robot, so it’s not exactly ideal.

2.) Reuse the mechanism that lifts the robot off the ground. Re-using the same mechanism to achieve multiple tasks is actually a common trick in competitive robotics, and has a lot of benefits. For one, it’ll be much less violent to lower ourselves off the lander instead of dropping entirely. Secondly, we’re reusing a mechanism we know we already have to have. This means we don’t have to add additional infrastructure (motors, structure, etc) to add a dedicated mechanism to get off the climber. Less additional infrastructure also means less weight.  One popular example of this in FRC is teams using their drivetrains, arms and elevators to power their climbing devices.

As far as climbing back up on the Lander goes, there are a number of different ways to do so. We didn’t feel we could easily achieve a climber powered by our drivetrain, so we didn’t go down that path. Some other examples are latching on and pulling the robot straight up, or you can always go for style points by flipping upside down or “taco-ing” *. No matter which route you decide to take, the only thing that matters is that the robot is off the ground at the end of the match. There’s not even a required height!

(* Side note, many of the engineers taking part in the FTC Build Blitz are mentors on FRC Team 148. We’ve got a lot of experience when it comes to “not simple” hangers, so it was very tempting for us to go that route again. Luckily, common sense prevailed.)

Like all the other sub-systems we’ve discussed thus far, we began our work on a Climber / Hanger (Clanger?) by building with spare VEX EDR parts we had laying around our lab. We determined early on that we would build it to be durable enough to lift more than than allotted weight limit of a robot. That way, we could guarantee that it would be compatible with lifting not only our robot, but theoretically, any Robot in all of FTC this season. (Not that it’s required, but still…)

First we wanted to establish what motors we wanted to use. We initially leaned toward a motor like the AndyMark NeveRest motor. However, we realized that that would require us to add a second Expansion Hub. If you read our robot requirements post from yesterday, you’ll know that we wanted to try to make this robot with a single Expansion Hub. We decided to first try VEX EDR 393 Motors. If we could get away with this, great. If we absolutely couldn’t, we could always fall back and add a second Expansion Hub.

Remember, in our robot requirements post we said that we can use 8x 393 motors and it would still be cheaper than adding a second Expansion Hub. Since we’re already using 4x 393 Motors on our intake and scoring mechanism, that left us with 4x more 393 motors to use. Since this would be our last mechanism, we decided that we would put all 4 on our climber.

To equate this kind of power, each 393 motor is about 4 watts of mechanical power. So 4x would be a little less than a single NeveRest. So if you’re reading this and know you’re going to have 2x Expansion Hubs, just use a NeveRest. If not, or you’ve already allocated your 8x motors elsewhere, then you can definitely make a climber with 4x 393 Motors.

The first few versions of this hanger were pretty rough. Our testing benchmark was that we wanted to have the arm lift a 25 lb weight. This would make sure that not only the motors could handle the weight, but the structure could support it as well. The first few attempts admittedly left behind a lot of bent metal. However, the great part about VEX EDR is that you can quickly play with different ways to strengthen and reinforce frames.

Finally, we got something that worked.

We used 4x VEX EDR 393 motors on this 49:1 reduction to lift 25 lbs. The hook is a standoff covered by surgical tubing. If you look closely at the picture you’ll see a small polycarbonate flap. We discovered that this is actually useless, and is no longer needed.

The idea for the hanging sequence is to deploy the arm out, turn into the Lander support and then curl up (taco) to get our wheels off the ground. More on this in our next post when we get an entire robot driving around.