05-02-2011 15:20

Chris is me

Re: pic: top view

I really don't like to do this most of the time but now I feel compelled to.

Your drivetrain is geared FAR too fast. You will not have much control of your robot nor will you be able to easily strafe on the field. This kind of set up is ASKING for big on field problems.

You're currently geared for a free speed of 38 feet per second with 8" mecanums. There is NO WAY you will be able to even get your robot that fast not to mention reliably control it. You'll have tons of problems with drawing too much battery current and overheating motors.

It's the end of week 4 but PLEASE consider doing a large redesign of your drivetrian.

05-02-2011 16:07

Tristan Lall

Re: pic: top view

Do what Chris said, immediately.

I just ran some calculations, and you're going to be drawing more than 40 A on each drive motor for about 3 s every time you accelerate from rest (and averaging roughly 75 A per motor during that period). Although there's some margin involved with the Snap-Action circuit breakers (i.e. momentary surges won't trip them right at 40 A), this sort of usage pattern is almost guaranteed to fail. Not to mention that this puts you at significant risk of throwing the Bussmann 120 A main breaker as well. (Datasheets: Snap-Action MX5, Bussmann 185120F.)

Furthermore, even if you accelerated diagonally from rest across an empty (i.e. no field elements) field, you'd only hit about 30 ft/s by the time you came to the opposite corner, even though the robot is geared for almost 39 ft/s. That's pretty much the definition of pointlessly fast.

If you can cut your speed in half, you ought to be on the limit of controllability and circuit breaker reliability. While you could do that simply by cutting your motor voltage in half (i.e. using the speed controllers to give them 6 V), you're still going to have dreadful acceleration and handling. The only way to actually have a chance to hit top speed on a regulation field involves changing the gear ratios.

It looks like your ground clearance is too small to allow you to switch to 4 in (non-mecanum) wheels without some reconfiguration. (That would have been another way to get a usable gear ratio out of the robot with relatively little modification.)

Bottom line: plan to either switch to some variety of AndyMark Toughboxes (the kit gearboxes from last year, or the Mini or Nano versions with similar gearing in a smaller package), or design and install a chain-drive reduction.

By the way, leaving the wheels cantilevered off of the gearboxes like that is not necessarily a good idea. Although the shafts and the gearbox casings ought to be able to withstand the bending load, the mounting flanges and screws for the gearboxes probably won't. They could easily become detached from your base plate, or at least misaligned. Your best bet is to support the wheels with a bearing on either side (but no more than two bearings on any shaft, unless you've carefully designed for that case).

05-02-2011 16:23

Jonathan Norris

Re: pic: top view

Please listen to Chris and Tristan, they are totally right on this.

The mechanical engineer in me is very worried about those gearboxes, they are already bending with almost no load on them. Its very possible that they are damaged already. If you have any questions on how to fix some of the potential problems, don't be afraid to ask.

05-02-2011 16:32

thefro526

Re: pic: top view

Chris, Tristan and Jonathan are all dead on.

If you happen to swap to another transmission but still keep the wheels cantilevered like they are, you really should have some sort of brace on the backside of the transmission. A few aluminum angles going down from the top of the transmissions to the wooded belly pan would do it, or you could run something from one transmission to the other.

05-02-2011 17:23

Chris is me

Re: pic: top view

If you want a very quick fix for this, here's a solution that will work. I don't like just giving people "the answer" to problems like this but it doesn't seem like you guys really know how to do motor calculations and having a useless robot all season is a pretty harsh price to pay for that.

If you replace the CIMple Boxes with stock 12.75:1 Andymark Toughboxes or Toughbox Nanos - your drive ratio should be a lot better. You will still be right on the edge of controllability and current draw (12.5 FPS) and it might not be pretty but it will at least be able to move around.

You will also need to support the gearboxes with something more substantial than bolting them to the piece of wood. A good trick is to take those big pieces of hexagonal tube you got in the Kit of Parts and connect each of your gearboxes to the one opposite it. That way they will both hold each other rigid.

Keep in mind this is by no means an ideal solution - this is just the quickest way to do it with AndyMark parts you could order or have now.

05-02-2011 23:34

papii911

Re: pic: top view

yea we have Aluminum bars to hold the Motors so that they don't bent
we will be gearing down the 4 motors

06-02-2011 00:20

Chris is me

Re: pic: top view

How much more than shown?