Hey I’m new to the forums and my team and I are considering using either a two speed transmission that would shift using pneumatics, or using four direct driven mecanum wheels. Which would you guys suggest and why?
You should use mecanum wheels.
The most important goals in sack attack are the troughs and high goal. These goals are all on a line. Being able to strafe along this line is probably the most important quality a robot can have. Without this, you have to waste time EVERY TIME YOU NEED TO CHANGE A TROUGH. That is bad.
Strafing is the most critically important feature of any drive train in sack attack, other than perhaps the ability to not stall when a sack works its way under the drive.
tl;dr: Strafing is important. Go mecanum.
We are also planning to push around a lot of sacks using a sort of cow catcher, will the mecanum wheels with direct drive have enough torque and traction to push around sacks?
I would respectfully disagree that straffing is of the utmost importance in Sack Attack.
I am also not sure that a shifting transmission is needed…
However, if you want to do something cool and complicated like hold on the order of 40-60 sacks (hoarding) a fun idea is to use a shifter to “shift” power from the drive to the arm. This allows you to have 8 393 motors the drive AND in the arm. Basically you’re just more efficiently using motors to get the fastest/most powerful drive and the fastest/most powerful drive ~ just not at the same time. Ideal for a hoarding robot.
Of course, it’s up to you what if any you want to do. There are/will be many robots that are very successful without shifting or strafing.
I like the way you think sir!
But to the OP, It really depends on your strategy and what you want your robot to do.
Personally I like shifters they can be fun if done correctly, and I also like omni now as well. So the question should be, do you think that the strategy needs omni directional at all?
That sounds way too complicated for my team, but definitely awesome.
I think I’m siding more towards mecanum as great as a transmission would be. We’ll probably try out both, just in case though
We have an H-Holonomic drive. It works great. You get the same stuff out of each. We just hated how big the mecanums were. That is why we went with the omni wheels.
We also thought that strafing would be a big part of this years game.
While I do agree with you that strafing is an important quality, I don’t think that it is the most, if there is one, important quality.
Strafing is a great ability to have, and if you possess that ability you could have an advantage, but there are other things that are important in a drive train as well, such as: Strength(pushing power) and speed.
If you are looking to strafe, I would honestly go with a holonomic x-drive. Our team built this type of holonomic drive because it goes in all directions, at the same speed, unlike with mecanums whereas they strafe slower. However, the chassis for a x-drive is a bit harder to design around when compared to a mecanum chassis. Best of luck!
have you thought of using 4 omni wheels with the 2 speed and a strafe wheel? best of both worlds.
This is a good point. If you want the omni-directional motion (also called holonomic motion) you arent just limited to Mecanum Wheels. While they are the most common case for a holonomic drive, and one of the easiest to set up, the VEX wheels create relatively slow strafing speed as compared to the speed in a straight line.
Another option is the one provided above, and H-Drive, which would be able to produce a similar motion to mecanum, but requires some extra wheels and motors. The H-Drive also requires space in the center of the robot for the extra wheel(s).
A third option, and also what my team has chosen to use, is called an X-Drive. This uses 4 omni-wheels and is installed in a similar manner to mecanum wheels. The 4 wheels would be placed at angles (usually 45 degrees) in the corners of the robot and actually makes to robot faster in a straight line than a robot using the same gear ratio, but with a tank or H-Drive. This is the most complicated of the three that I have suggested. The also has the lowest tourqe of any of the designs when driving.
Our team likes X drive too; we’ve used it a lot. If you want to try an X-holonomic with a little (or a lot) more space in the front, you could try a similar chassis design to what we had last year:
WASABI QF 2-1
WASABI SF 2-3
Green Egg’s Clean Sweep robot is another good example of this.
This is kind of true. X-holonomics will always move at sqrt2 times the gear ratio of the wheel itself. So, if you use a motor on torque mode directly driving each wheel, it will have less power than a torque motor on direct drive on a tank chassis, but more power than a speed motor on a tank chassis.
Please edit that last post replacing the word power with either torque or speed.
I get what you are saying and the intended person may also but later down the line it might confuse a vex novice.
Feel free to delete this post after it has been corrected.
Dear Vex novice,
I apologize for my misuse of the word “torque.” The Official American Representative of New Zealand Robotics was correct in saying that I should not confuse power, which is a rate of energy, with torque, which is the tendency of a force to rotate an object about an axis.
Also Vex novice, I would recommend that you not try building a two-speed pneumatic transmission because it is very complex, though I commend your interest in this thread.
I deserved that.
Edit(Might have been just trying something to do to avoid doing math homework.)
The word should not be torque or speed or power but force:
But people in vex usually say either “pushing power” (which makes sense) or “torque” (which doesn’t) when they mean force in this context. Just a little nitpick. Most of the time we can afford to be sloppy because most people understand the physics, but careless talk costs lives :p.
Keep your drive simple. Mecanum wheels driven directly from 393 motors, can’t get much more simple than that.
You need to evaluate your resources as a team and decide where you are going to focus your time and energy. Odds are your mechanism will take the most time to perfect, so I wouldn’t spend any more time than you have to building your frame and drive.
Thanks for all the advice guys! We’re probably going to go with the mecanum drive since strafing seems pretty advantageous for our design.