10-07-2015 03:04

asid61

Re: pic: WCD Summer Project

Is this 34lbs as pictured?
I woud change the rectangular lightening pattern in the front into a triangular one, to preserve more strength, but that's just me.
Is that 15.35fps free speed or realistic?
I think you can get your gearbox lighter if you wanted to by adding a lightening pattern or just redesigning into a flipped-cim verison like 192 in 2014.
Ugh bumpers. I hate mounting bumpers to a WCD. It always looks so awkward (or weighs a lot) when I try. I look forward to seeing those when you add them in.

10-07-2015 06:57

GeeTwo

Re: pic: WCD Summer Project

It looks like this was based on a frame efficiency of 81%; I get 18.95fps free speed.

10-07-2015 07:57

Kevin Leonard

Re: pic: WCD Summer Project

This thing looks to have little to no torsional stiffness right now. A structural bellypan would help immensely, or some sort of cross-bracing between your drive gearboxes. 20 generally runs standoffs between their drive gearboxes in our WCD's to help provide that torsional stiffness you're currently lacking.

10-07-2015 12:14

pmangels17

Re: pic: WCD Summer Project

It is 34 pounds as pictured, hopefully going to stay there. The top speed is calculated with efficiency allowed for (A La the JVN Design Calc). I hadn't thought about the flipped CIM thing, but now that you mention it, it probably would save space. I'll have to toy around with it. Also, I still have a lot of lightening to do, the rectangular thing was just to get a rough estimate of how much weight I could expect to save by lightening.

I definitely will look into cross bracing. It'll probably be in the form of a cross brace between the gearboxes, because I have other plans for the bellypan, most likely a corrugated plastic thing.

Thanks all for the help thus far, it is much appreciated. If anyone could tell me how to attach a file to a post, I can get a STEP file up if y'all want to take a look at it.

10-07-2015 21:39

Travis Schuh

Re: pic: WCD Summer Project

I would drop 2 of the CIMS and add that weight back in to the front/back rails and adding a bellypan. 6 CIMS will be a liability next year with the brownouts.

I get worried about taking so much weight out of frame rails like that, the weight savings are usually not great compared with the greatly increased risk of bending a frame rail.

Also, any reason you don't have your gearbox in 2 stages given that you are running 4in wheels?

10-07-2015 23:08

GeeTwo

Re: pic: WCD Summer Project

Do you have some intel on next year's game? It certainly would have been overkill for Recycle Rush, but next year's game may call for fast climbing or other need for high traction while at speed, for which 3 CIMS would do better than two, even after taking brownouts into account.

As the high gear is only 4.89:1, a single stage is adequate (11 tooth pinion, 56 tooth bull). Adding a second stage would cause more frictional losses than a single stage.

10-07-2015 23:20

Knufire

Re: pic: WCD Summer Project

Are you sure? From the data someone (234?) posted before last season, it seems like the voltage drop from simply accelerating from a standstill with six CIMs on a fully charged battery would result in a brownout condition. Of course this can be mitigated by setting a max dV/dt or doing more elaborate forms of current control, but you quickly run into a scenario where producing an effective six CIM system is substantially harder than buying a different COTS gearbox.

10-07-2015 23:21

pmangels17

Re: pic: WCD Summer Project

I figured I'd design for more CIM's rather than less. Plus they can always be replaced with MiniCIMs or removed later, it's a lot harder to add a motor that wasn't designed in than to remove one that was.

As far as the gearbox, there are currently three reductions going on:
The 12t CIM Gear to the 50t Cluster Gear
The 50t Cluster Gear to 40t dog gear or 30t Cluster Gear to 60t dog gear
And the final stage 30t spur gear to 44t output gear.

I couldn't package the speed I wanted in only two stages, so I added the third stage to get the speed I wanted without going to something like a three inch wheel.

There is still lots of lightening to go, and the bellypan will be non-structural, only to support electronics, so I'll have to find some other way to stiffen the chassis. As far as removing weight, I'm designing this assuming there will be bumpers next year, in which case some added rigidity can also be included in the bumper construction, most likely.

10-07-2015 23:27

pmangels17

Re: pic: WCD Summer Project

As a mechanical guy I usually just plow ahead and let the programmers figure that out

But on a more serious note at stall in low gear these motors will each draw 27-ish amps, so though that is enough to throw the breaker, I don't plan on stalling them, I plan on pushing people out of the way or breaking traction. In previous years we ran similarly geared 6CIM drives without issue unless we stalled in high gear. And I feel that the extra pushing power is always nice to have, motors can always be removed later.

11-07-2015 09:27

GeeTwo

Re: pic: WCD Summer Project

From a standstill, full-forward or full-reverse situation, hitting six CIMs hard would drop the voltage enough to cause motor brownouts. However, if you were only pushing on one side, or if you needed to still have some torque going after you were at some speed, having that third CIM on each side could come in handy. It's not likely, but it's possible if the game (and your strategy) requires this sort of activity.

11-07-2015 10:52

Jared

Re: pic: WCD Summer Project

At stall, a CIM motor will draw 130 amps at 12 volts, regardless of gear ratio.

11-07-2015 11:06

pmangels17

Re: pic: WCD Summer Project

Whoops, that was not what I meant to say. The 27 amps is the "Pushing Match Motor Current" value from the JVN calculator. *walks away embarrassed*

15-07-2015 18:48

Chris is me

Re: pic: WCD Summer Project

I believe the reason Travis is specifically mentioning next year is that it will be the first game with the new control system where anyone would have any reason to build a 6 CIM drivetrain. Some have suggested that the system is more vulnerable to brownouts than in the past. (I'm not particularly informed on this, I don't know if people have tested this etc)

However, at least in the context of the previous control system, gearing your low gear so that your motors draw <30 amps each when traction limited is the right move. It's a conservative number that should help prevent pushing matches from tripping your main breaker. Be careful to manage current draw throughout the rest of the robot as well.

As for the design, it's not too bad. I would remove the lightening pattern from the front and back rails. The square lightening versus a triangle or X pattern is substantially weaker, and most any lightening pattern in 1/8" wall tube is going to result in a weaker and less rigid tube than a 1/16" wall unlightened tube. And it won't even be lighter! If you want to save weight, you can usually get away with 1/16" wall tube for the crossmembers.

While I wouldn't say this chassis would have "little to no" torsional stiffness, a lot could be gained with a structural belly pan for relatively little cost of weight (since you already need something to mount electronics to regardless). The waterjet or lasered aluminum belly pan is a slick way to do it, but a composite like garolite left solid can also do the trick for a lot less machining time. You could also add some standoffs connecting your two gearboxes together at the mounting holes. Also do keep in mind that your manipulator design may also provide some cross bracing, and in that case your drivetrain doesn't necessarily need to be structurally perfect independent of other subsystems.

15-07-2015 22:37

GeeTwo

Re: pic: WCD Summer Project

While we haven't done any intentional testing of the brownout, we have definitely found a difference with the RoboRIO in practical (non-competition) situations. In previous years, we didn't always realize when it was time to change batteries, because there would be different symptoms. We would lose acceleration, or we might drag a bit as we tried to do a kick or other high-current function, but the degradation was usually gradual, until it caused one of the digital components (most commonly our raspberry pi vision processor, but occasionally the D-Link radio, and in one case apparently the cRIO) to reboot. In each case, the failure took several to many seconds for recovery. The roboRIO has a defined brownout sequence, which cuts power to the motors first, then proceeding to other specific busses until the cRIO itself is eventually shut down. In practice, this means that our robot runs just fine at extended practices and demos until the drive system enters a "jitter" phase in which it is energized, draws the voltage down, shuts off, allows the voltage back up, is re-energized, and shuts off again, creating a cycle that executes about 7-10 times per second (based on my non-calibrated biological clock, but with a lot of acoustics experience). In Recycle Rush, this appears to have rarely occurred during a match unless a team was working unusually quickly, or using particularly old batteries. I suspect that in the near future, especially if we get more defensive games, teams will have to monitor battery voltage and total current draw, and implement software limits that prevent or at least limit the impact of these brownouts. The good part of this change is that at least we are much less likely to spend most of a match waiting on a reboot!