Failure in Sack Attack

We have found that this game has a high failure rate for designs. We are on our third or fourth substantive rebuild depending on how narrowly you define substantive. We have found the weight of the sacks to be the biggest obstacle in designing a robot that will score and descore all goals–in particular, the sacks backdriving motors when attempting to lift if the arm is extended beyond the 17.5 inch length of a 35 hole piece, e.g. using a slide to extend the arm 4-6 inches. (We had been trying to keep the height at <15 to play the whole field which necessitated some mechanical extension to reach the 30).

We went to one scrimmage and found stability to also be a big factor. We tipped once when extended to reach the 30. We are currently working on a 16 inch robot which will reach the high goal with a 20 inch chain lift arm and 1:15 compound gearing (which has less backdriving) and will work from there to bring it under 15–maybe getting an inch from changing the ratio to 1:9 which may also speed up the lift (using 2- 393). The question is how to make it stable–its wheel base is +/- 12 inches to allow for a wider intake. Boogie wheels do not look like the best option because of the possibility/likelihood of getting hung up on a sack but may be the way to go as a good driver can avoid getting hung up but a turtled robot is end of game.

Our other failures include using a differential to share motors between drive and lift(too much loss of power as well as very difficult to repair); a multi-stage elevator lift (alignment effecting retraction with shifting weight when fully extended); a fully flush passive spatula pick up(not a failure but undesirable e.g. not particularly efficient without some mechanical assist in the pick up)

Questions for discussion are what have been your failures and do you have any suggestions for stability–we have begun by moving our tower forward to get it under all four wheels vs. over the rear two and centering all weight (e.g. batteries, pneumatic reservoirs) as well. We are also using steel on our base and tower and aluminum on our arm/manipulator.

By boogie wheels do you mean mecanums because with a little thought into a design the sacks can either be easily avoided or can be easy to drive over. We use larger drive pods to prevent sacks from getting caught. Also by lowering the wheels in the drive train you can get increased clearance to drive over sacks. Plus the manuverability of MECANUMS is so useful.

Boogie wheels are the small grey wheels in the tank tread kits designed as inner tensioners with treads running over the top of them but which can be deployed from the rear of a robot to enhance stability.

We are already using an elevated drive train (4 -393) which has the motors above the wheels. If we wanted to use mecanums, we would have a zero clearance drive train as the mecanums sink like the they are in quicksand when they meet a sack and as easy to get out.

I’m assuming you meant “raising” not “lowering”?

My advice is just keep rebuilding as you have been doing until you find something that works. You could also use someone else’s idea if that is something you want to do. I have a thread but it hasn’t been updated in a little while: 24C has a good one here and 323Z has one here [

If you really want to use your own designs, just keep rebuilding and trying things out. We are on our fourth complete rebuild also and our robot now works extremely well. Try making your robot design simpler until it works, and then start adding complexity if you still feel it is necessary.

To avoid tipping, try using small omni wheels on the side the robot would tip to. These do catch on sacks, but it is better than tipping and it may be possible to put a metal skirt around them.](

Just as a suggestion, you may want to post a few pictures of your current robot if you want legitimate feedback on how to fix some of your issues. It’s much easier to give help when the community can see what is going on with your robot.

Thanks. We will redesign and rebuild and optimize without being urged to do so. The thread is a search for what does not work. Failure is an integral component of success. We’d prefer to try to innovate but if you know that a certain design will not fly we’d appreciate knowing it so we don’t build it and have to call it a duck to declare it a success.

And we say that with the caveat that even if you failed, we may still want to try the idea as sometimes you think you are Mark Zuckerberg and think you can do better what others can’t.

If you are on your fourth rebuild, what have you tried and why did you discard it? Some things are bad ideas, some are good ideas that are not executable and for some things it might be economics. For us the biggest limitation may be that we are extremely reluctant to cut, especially aluminum.

Yes that’s what i mean’t. We tried this on a more complicated mecanum prototype drive train and it actually worked pretty well I want to incorporate it into our current, super simple mecanum drive.

Our club has found failure in the lift, I think it is because we underestimated the power needed this year to pick up a reasonable amount of sacks.

I think the most effective thing to make good designs is to use design software, like Inventor. I’d say for designing, one hour of CAD is the same efficiency as 15+ hours of straight building. If you already use CAD, I’d suggest try thinking more about how the robot will interact, and make sure all your criteria is met (is the center of balance ideal, is it under 18 inches, ect).

Also did you use my design I posted on YouTube for the differential between lift/drive? I’d feel bad if you did, because that design wasted sooooo much power when I built it myself xD

To answer the original question, one way to add stability would be to add triangles to the towers since triangles are the strongest shape in engineering. But to add stability to the drivetrain, you could try to implement a wheelie bar at the back to make it harder for the robot to flip over. Also, you could try to add the 2.75 inch omni wheels up front like 4886A did on their latest robot on their thread.

I too have been through numerous redesigns, I’m currently on my 5th (I think). I too have found Sack Attack to be, let’s say, a bit troublesome, but I think it is great as it adds an extra level of challenge to the design/build process, which is something I like.

As for the robot design mentioned/described in the original post, I think needing an extra slide to reach 30" could be simplified much more. My robot was able to reach the 30" and still be under 15" with only 35 hole arms, you just need to make sure you get the most out of the lift and then it is pretty simple. For example, make sure the arms are ALL the way down when the intake is at the lowest height, and when I say that I mean so that the actual arms of your robot are just a few millimeters off the ground. This way you should be able to get much more movement out of the lift and therefore reach higher. If that still doesn’t fix it, you could try some other ideas such as adding a tilt to your intake so that it tilts backwards slightly as it lifts (there are some good posts abouts on the forums on how to do this if you aren’t sure). Another idea would be to consider scoring in ways that are a bit out-of-the-box. I’ll leave you to think of some of those ideas :wink:

As for tipping problems, I see you are using steel on the base with aluminium on the upper parts, that is great. I’m sure people have managed to make a robot with a light base and not have tipping problems, but ultimately, as long as you have plenty of strength to be able to drive a heavier chassis around, steel is a good option. You could also add some stops on the chassis, so that when the robot is balanced they are off the ground, but as soon as the robot tips forwards or backwards, the stops hit the ground, preventing the robot from tipping even further. The only problem with stops is their likelihood to get caught on sacks, if you intend on driving over sacks. You could also try and extend the length of the chassis and come up with another way for your intake to work with it.

Just a few suggestions, I hope they are helpful. Try and search the forums for more, there are loads of great options that have been discussed previously. :slight_smile: