pic: Off Season CAD Practice

I taught myself how to use Inventor over the summer. This week, I decided to try my CAD skills, and this was the result. I tried to imagine what I would design if Logomotion, my “official” rookie game, were to have been played with the modern 2013+ rules. This design was meant to be built by a team such as my current one, with only basic tools and COTS parts.

Looks very lightweight. I do have to say though, I would expect that claw to wobble significantly due to the lack of stiffness in the structure.
Have you thought about adding intake rollers?

Note how the claw will be on the ground outside the bumper perimeter, and is liable to be ran into by other robots. The moment on the joint from such a collision is massive and could bring the whole tower down

I definitely think you need a powered roller intake.
If you touch a game piece, you need to own it.

Another thing to think about is how is the tube going to react when you pick it up?
The geometry of the current design is set so that once you pick it up, the tube is always going to have a certain angle relative to the arm. This means the robot can only score when the arm is at a certain angle (which makes it difficult/impossible to place tubes once that row is filled).
I would suggest something like what 148 did, where a piston changed the angle of the tube to be parallel to the scoring rack, making it easier to place on a peg. To do this, you will have to change the arm into a 4-bar linkage, or have some other method of keeping the grabbers at a same angle relative to the chassis as the robot lifts its arm.

148 2011 Reveal: Here

Nice work for your first large scale project in Inventor!

Learning that will be very helpful come January.

Also, some teams with two independently powered rollers were able to spin the tube in the claw, by rotating the rollers in the same direction.

Perhaps that could create a large enough moment to cause a tower failure, but I suspect there would be other failure modes that would occur well before the tower failed.

There are other ways to handle this than roller intakes, for example 217 had a single clamping grabber that was very wide and they seemed to do ok. Wonder what benefits it had over a roller intake (since 217 clearly knows how to build roller intakes, they did one in 2007)…

Claw intakes that were built and designed optimally work fine for Logomotion. It’s what we used for arguably our highest performing robot and what 217 used a similar looking claw. In fact I remember a few times where us and another roller claw came in contact with a tube and we just pulled it away from them because we had a tighter grip. That being said, our claw and arm had a bunch in built in features and control programming to make it efficient at grabbing tubes and scoring them.

Interesting critiques, but seems like most missed the key point.

“I taught myself how to use Inventor over the summer.”

Nice job Whippet.

How did you approach designing for proper reach to hang tubes on the top peg? Did you work out the basic geometry in 2D sketches or iterate your design through 3D modeling?


I was wondering a bit that you were a rookie and 2011, but are still a student. I guess by that you do not mean an **FRC **rookie, or are you on a team where you can play as a student six years?

I won’t add any more to the “roller claw” discussion, as you claimed “basic tools and COTS parts” in the OP, and most if not all of what I would have said has been covered, anyway.

Most importantly, as far as “modern 2013+ rules” ar e concerned: 2015 had no lateral “sprawl” rules beyond fitting on your alliance’s half of the field, but In 2012-2014, the robot was required to start the game completely within the “frame perimeter”, with a maximum height restriction. This arm does not seem to meet this requirement; you can’t get it to fit both requirements at the same time. The obvious answer is to have another articulated joint or two. I think there is an alternative, though I haven’t run the numbers: make the mobile segment of arm a bit shorter, and mount the claw on the opposite side of that arm. Give it a mobility of about 350 degrees rather than 170 degrees (doesn’t seem to be a problem from what I can see). Then, you could start with the arm stowed on what is currently the “front” side, pick up logo pieces off the floor from what is currently the “back” side, and score them high on the “back” side with less rotation issue than you would have with what you designed. If I recall the 2011 game correctly (I did not play it, but skimmed the rules a few months ago), this would probably also reduce the number of times you would have to turn the robot around.

Finally, I find that motor sticking out the side of the “wrist” to be cringe-inducing. This would be a great place to have the “thumb” actuator respond to a “muscle” pull along the arm. This could be done via pneumatics, a lead screw (or even threaded rod and a coupling nut), or a light chain or belt. Especially if you use pneumatics, consider actuating both jaws of the claw; this would allow you to “rotate” pieces for more solid placement.

Thank you all for the comments! I will upload some side view screenshots soon that show the full range of motion that this arm currently has.

My main issue with a roller claw would be counterbalancing the extra weight at the end of the arm. My team that year had a similar arm with a heavier claw, and the motor we used for the arm was constantly stalled out due to the forces involved. It seems like there is a lot of support for a roller claw in this thread, though, so I’ll see if I can incorporate one into this design.

The viewing angle in the picture is a bit deceiving, but the claw actually doesn’t contact the ground, and is fully contained within the frame perimeter during starting configuration. It sacrificed a floor pickup for it, but I decided that this design is pretty close to what my team can realistically manufacture in six weeks and would allow us to have some practice time before ship/bag.

I was envisioning the human player feeding the tube so that it would extend straight out from the arm, theoretically allowing the robot to score on all three levels without too much trouble. I do see how the tube could rotate because of a loose grip, so perhaps a 4 bar could be useful.

Thank you! I hope that having a detailed CAD model beforehand will enable my team to build more complex robots than our past years!

I have seen that, and will definitely try to incorporate such a controlled roller claw in the next iteration.

Could you point out elements of the design that would be likely to fail in this way?

Thank you! I built the model to be parametric, so I was able to easily change the lengths of the key different frame members to optimize the arm through the 3D model, and then built the support structure around it.

My old team, 3043, operates out of a combined middle-high school, and allows middle school students to be on the team under certain circumstances. I started hanging around the team in the 2010 season while I was in 6th grade, and was officially recognized as a member beginning in the 2011 season, when I was assigned to help build our (admittedly terrible) minibot. I also worked on the school’s FLL and FTC teams at the time, but FRC was my main thing.

By “modern 2013+ rules,” I was specifically targeting the new and smaller frame perimeter and the recent motor additions. The game-specific ones, such as the unlimited height during match play, the maximum size cylinder, and the starting configuration rules, were all left intact. Due to being a single-jointed arm and having to start within the frame perimeter, this arm did not include a floor pickup.

I hadn’t previously considered increasing the arm’s range of motion like that. It’s currently not possible due to the gas shock counterbalancing the arm, but if I moved the claw motor towards the shoulder as you suggested, and added a physical weight behind the shoulder as a counterweight, then the torque needed to rotate the arm might be small enough for the motor to handle on its own.

Sorry for the double post, but here are the screenshots:

Starting config:

Fully extended:

… and some closeups of the claw and shoulder mechanisms:

There are legitimate arguments against a roller claw in 2011, particularly for long-arm robots. A proper pinch claw can easily weigh just 2ish pounds, when a roller claw needs at minimum 1 gearbox + motor (usually 2), four shafts / pulleys, etc. Exaggerating the effect of this weight by putting it on a long arm and there’s a lot more torque that the lifting motor needs to deal with. Particularly for human load robots (as this one appears to be since it doesn’t seem to be able to extend past the starting config), the advantages of a roller claw aren’t definitive. It’s still likely the “better” choice, but many reasonable robots, great robots that made Einstein (217, 1503) were pinch claw.

None of them however were motor driven pinch claws, which are generally really bad. Motors alone can’t apply force to a tube without stalling, and even when they stall the force is usually minimal. Pneumatics are really the only good way to go for a pinch claw. I would also significantly change the gripper geometry to incorporate a traction material and a more rigid structure. Your tube is already compliant - no need to make your gripper compliant as well other than to open and close it.

As for your arm design - as others have said 1x1 is a bit flimsy for this application. I would at least go with a 2x2 tube if you’re set on a single piece of material. The more rigid the arm the better. Also, the live axle is definitely a mistake. You have a perfectly good sprocket with bolt holes right next to the arm, and instead of bolting the sprocket to the arm and transferring torque through multiple bolts on a 1.875" bolt circle, you’re concentrating the entire lifting force of the arm on a 1/2" diameter hex shaft. Torsional failure is a VERY real possibility here and there’s just no benefit to the way you’re doing it now. Easy fix though - bolt your Nano to the other tower using the other face of the gearbox, leaving your output shaft in the middle instead of off to the side of the tower. You also get the advantage of being able to relieve the cantilever of the gearbox here.

I would also run the CIM through at least one other reduction, possibly two, before going to the arm. 12:1 gearing even with a big sprocket reduction is ambitiously low. I would consider something more along the lines of a final reduction of 300 or more to 1. You could do this by feeding the CIM through a 10:1 VP before going through the Nano for example.

Finally, it looks like your sprocket is extending out past the bumper perimeter of the robot. Be careful about this. You can’t flush mount your tower supports to the back of your frame for this reason.

I wouldn’t worry too much about the angle of the tube - people in this thread are seriously overstating the difficulty of scoring a tube at anything but the perfect angle. Teams like 1503 had no mechanism to reorient the tube and did fine, provided you design the robot so that the angle you grab from the feeder is a good angle to score on the top row you’re fine.

An alternative to the roller claw for “rotating the game piece” purpose is to actuate the two halves of the claw independently around the same axis. Then, by rotating them in the same direction, the orientation of the game piece moves without the claw having to slip against the game piece. As I mentioned before, these can be driven from a motor mounted near the shoulder with a belt or linkage rather than out at the end of the arm. I made a smaller version of this a while back using servos whose axles faced into each other. I haven’t run the numbers, but I believe that you could get enough torque to hold up an inner tube with the larger FRC-legal servos, but it wouldn’t be very robust against defense.