Hello,
Continuing in my many questions to explore drive trains.
What do you think about suspensions on robots, specifically mecanums (most common type with suspensions).
Is it worth the weight?
Is it helpful?
Thanks!
868 had an actuating Mecanum system, with pneumatics. This made somewhat of a suspension, and it weighed around 35 lbs. This took away from what we could do with the shooter, and everything else, so it really wasn’t worth it (except we got the xerox creativity award for it… which made it worth it)
Using this system, we found that at quick stops and changing direction (front to back) the robot would severly tip… but it really never fell over. We also found that one of our wheels lifts off the ground, and doesn’t touch, due to extremely slight differences in mounting, causing the robot to drive in biggish arcs, instead of straight lines.
Suspension isn’t really required for skid steering drive systems. It is a must for holonomic or mecanum drive systems if you want a good drive system. It shouldn’t be too heavy though if you just have one half of the chassis rotate or if you have the wheel mounted so that it has some travel up and down like 488’s mecanum drive system.
If you’re thinking about suspension because you’re worried of your electronics getting damaged if you go off a ramp or drop, then you could just mount the electronic boards on something to absorb the shock like rubber standoffs.
Team 357 Royal Assualt used a suspension on the back wheels of our jester drive
It helps alot. In order for this drive train to be able perform well all 4 wheels need to be in contact with the floor.
we have used this drian train for 2 years now. we used it in 2005, we did not use a suspension and in some cases it did not perform how we wanted it to.
In the 2006 game we put in a suspension and we saw a better result.
As a driver, i drove both robots in competition and it is much more controllable and more reliable with a suspension.
The kitbot frame is so flexy you might call it a suspension all of its own. I haven’t really decided if it is better to have a super rigid frame or not. For mecanum or omni, some sort of rubber pad may suffice. For any past game, most any real suspension would be too heavy for any benefit if you ask me. I wouldn’t mind seeing a game that really required some sort of suspension, although it will probably never happen.
Thanks for the responses!
How did you use suspension on only 2 wheels on Jester Drive? Why not 4? Also do you have any pictures? Thanks a lot!!
Probably because any three points of contact define a plane (e.g. the floor), and the fourth is redundant (in this case, not necessarily touching the ground). So, they leave their front wheels fixed, and let both of the rear wheels pivot so that they effectively have two planes of contact—either rear wheel plus both front wheels.
If you put it on all four, it works, but then the body of the robot becomes (potentially) unstable, because it’s now supported on four moving pivots. It’s like body roll in a car, only probably quite a bit more pronounced.
as soon as i get a picture i will give post it … but a team that i am pretty sure used independant suspension for each wheel is trinity team 40. if u want video on how it did … you might not be able to tell the difference but we went to the florida regional, drexel and champ. which are both supported by soap. i will get pictures of our frame asap
http://www.soap.circuitrunners.com/2006/movies/florida/ florida
http://www.soap.circuitrunners.com/2006/movies/philly/ drexel
http://www.soap.circuitrunners.com/2006/movies/archimedes/ champs
Personally i think, that if your are going for a high maneuverable drive system that does not really have that much grip to it, it would help to have an adjustable suspension because then you can shift the center of gravity and put more weight on a wheel or a set of wheel, which would increase traction. However if the weight of the suspension takes away from you a key scoring ability then it is not worth the time, effort, or weight
remember when you weld a frame, it warps because of thermal expansion. In 2005 my team had a frame with a lot of welds. Did one side first, flipped over and did the other side. When it cooled down, it was slightly cuved upward. Didn’t matter much, but imagine if it curved the corners up? A mecanum bot with a frame like that would not drive very straight.
I believe that a mecanum system actually needs every wheel on the ground because the direction is controlled by a different vector from each wheel. If a wheel isn’t touching the ground, the robot would drive at the wrong angle because force in one direction is missing, while the bot is programmed to use all four vectors. (Unless the robot was programmed to change its math based on which wheels are touching the ground…)
While I don’t have experience with the Mecanum drive, our team has done a harmonic drive for a couple of years which is pretty much the same thing. When we first put together our drive the chassis would steer off in random directions because the floor (and no floor will be ,ever) wasn’t perfectly flat (we had shimmed the wheels to be flat). This is why you use a suspension so that all wheel can stay in contact with the floor and cause good control of the vectors. Our first suspension was with each wheel mounted on a hinge and a spring, this would tilt whenever the robot would accelerate in any direction. So this year we used a “tractor” type of suspension. This didn’t add any weight really but kept all wheels in contact always.
http://www.chiefdelphi.com/media/photos/22700
We also added a rotational accelerometer to keep track of any variation in the bots direction of travel and last year when I broke out our original chassis for the first time in months so the programmers could work with it one of the motors wasn’t plugged and we didn’t notice. Despite the lack of an entire vector the accelerometer allowed the robot to drive straight (if a little drunkenly).
Hope this helps
Alex
I think you mean holonomic.