10-12-2012 01:00

Andrew Zeller

Re: pic: Sheet Metal with VexPro parts

EDIT: Nevermind I realized you are not direct driving. If you have the answers though feel free to reply.

Looks really slick. Simple and will get the job done. One question I have as I am also looking into using the vexpro components on our drivetrain:

For the wheel that is direct driven from the gearbox, did you place the inner of the two Hex VersaHub towards the center of the robot from the inward-most sprocket?

The reason I am asking is that I was thinking about the alignment of the sprockets since in a setup like this only directly driven wheel needs the two Hex VersaHubs while the other two need the bearings which do not add to the "stack" of the VersaHubs and Sprockets.

Maybe a render of the direct-driven wheel would be easiest to understand. Also are you planning on machining the output hex shaft to a certain length to fit your design?

10-12-2012 01:05

Madison

Re: pic: Sheet Metal with VexPro parts

I'm trying to find an angle that'll show things a bit better. This doesn't actually directly drive any of the wheels; it uses a two-stage ball shifter with two sprockets on the output shaft. The ratios available on the ball shifter don't really work to my liking for directly driving 4" wheels.

Were I to direct drive, I think I'd put the hex hub on the far side of the wheel and stack the sprockets and bearing hubs up on the other, otherwise the thickness of the hex hub's flange would throw off the stack up for the other wheels and your wheels would be slightly out of alignment.

10-12-2012 01:07

JVN

Re: pic: Sheet Metal with VexPro parts

I like this design a lot. We had a lot of discussions about how to mount the ball shifter into a drivetrain. This design is very similar to something I sketched on the whiteboard.

Face mount the 2-stage, then add another reduction via spur gear (my preference) or chain to one wheel.

-John

10-12-2012 01:12

Madison

Re: pic: Sheet Metal with VexPro parts

I started with spur gears, but I'm not confident enough with working in sheet metal on FRC robots to ensure the wheel/axle setup will remain rigid enough relative to the gearbox output to make it work. I switched to chain to give myself some room for error.

10-12-2012 01:17

ttldomination

Re: pic: Sheet Metal with VexPro parts

Super snazzy.

For someone of us that haven't had a chance to take these parts for a spin in Inventor, what's the output shaft length on those gearboxes? What's the distance between those railings?

And, just for general sheet metal design is "safe" (for lack of a better word) to use dead axle bolts directly like that on a sheet so thin?

- Sunny G.

10-12-2012 01:21

rcmolloy

Re: pic: Sheet Metal with VexPro parts

Madison,

I'm with everyone else as well on the design. It looks really awesome from an astethic perspective. Utilizing the extra sheet for mounting is what pops out for me. If you guys run an iteration of this guy in season you guys should definitely give us information on how it held up.

10-12-2012 01:25

Andrew Zeller

Re: pic: Sheet Metal with VexPro parts

When using JVN's design calculator, I found the low gear to be 9.3 ft/sec and high gear to be 21.2 ft/sec

What speed are you aiming for with the low gear? 6 or 7 ft/sec? Would 9.3 be considered an acceptable low gear speed?

10-12-2012 01:42

AllenGregoryIV

Re: pic: Sheet Metal with VexPro parts

Great concept, I've spent a little time with the VEXpro CAD files myself and one my favorite parts is how much ease of maintenance was included in the design. It looks like the rear wheel would block the awesome access holes that VEX designed into the gearboxes for removing the CIMs. That is one of the reasons their mounting bracket has the cutout in the middle.

Also what chain reduction did you use in the design? I've been playing around with a couple and haven't really made a decision yet.

10-12-2012 01:51

ttldomination

Re: pic: Sheet Metal with VexPro parts

21 fps...overkill's definition.

And low gear isn't necessarily about speed as it is about pushing power. Going to 3-4 FPS means nothing if you spin out your wheels in a pushing match.

- Sunny G.

10-12-2012 02:19

MichaelBick

Re: pic: Sheet Metal with VexPro parts

21 FPS isn't overkill if you're 254 with a 6 motor drivetrain

10-12-2012 03:51

Cory

Re: pic: Sheet Metal with VexPro parts

all speeds are not created equal. 21 FPS on 2 motors is probably slower than 13 or 14 FPS (You could do some quick math to figure this out).

Your acceleration would be SUPER sluggish.

10-12-2012 08:28

Andrew Remmers

Re: pic: Sheet Metal with VexPro parts

Its about time for me to crack out the old CAD machine again

Love the design.

- Andrew

10-12-2012 10:51

apalrd

Re: pic: Sheet Metal with VexPro parts

Quick spreadsheet:

2x CIM motors, 150lbs, 100% weight on driven wheels, 0.9 speed loss const
12.5v initial battery voltage, 0.03 ohm battery resistance

Gear ratio resulting in 21.33fps after speed loss:
2.07sec to 20ft
3.18sec to 40ft
1.27sec to 12fps
0.54sec to <160amp total current
3.23sec to 11v battery (battery remains under 11v for 3.23sec)

Gear ratio resulting in 13.29fps after speed loss:
2.00sec to 20ft
3.40sec to 40ft
1.17sec to 12fps
0.18sec to <160amp total current
1.20sec to 11v battery

Looking at the curves, most of the output distance curves for the two gears are relatively close, the velocity is worse than 13.29fps until ~1.5sec, and the higher gear will be under ~1.5x motor load and significantly lower battery voltage the entire time.

Edit:
When you choose a high gear ratio, you don't usually actually care about top speed. You really want to gear for either sprint-distance or time to speed, with the speed and distance adjusted based on game-specific strategy. Being tied to a specific ratio spread will also pull your high gear slightly based on where your low gear wants to be. You usually want low gear to be traction limited, at a current which is determined by your strategy (how much you want to push).

10-12-2012 14:06

Madison

Re: pic: Sheet Metal with VexPro parts

I have a section-view at home, but there's some build up around the axle slots to distribute the loading over a larger area and to deal with side-loading on the tensioning system. This is using a sheet-metal variant of the sliding bearing block tensioning method.

The output shaft of the 2-stage ball shifter is a bit more than 1.5" long measured from the face of its housing. The rails are 3" apart -- inside face to inside face.

The chain reduction is 1:2 right now; 16 tooth hex sprocket to 32 tooth sprocket bolted to a versahub. I just tossed those in there for the sake of figuring out the overall width of the assembly; I'd look more closely at the ratios if we were to build something like this. Generally, I'm less concerned with penciling out the math at this stage and more interested in packaging and assembly.

The ball shifter is face mounted to the inner rail and should be pretty easily removed. The slots in the belly pan around the CIM motor and the hole in the rail allow you to remove the entire transmission assembly as a whole. I've never had to replace a CIM motor on an FRC robot, so I'm not very worried about accessing their mounting holes without a bit more disassembly.

10-12-2012 16:18

ttldomination

Re: pic: Sheet Metal with VexPro parts

Interesting. I'd like to see an image if you have one handy.

- Sunny G.

10-12-2012 17:30

Andrew Zeller

Re: pic: Sheet Metal with VexPro parts

Thanks for the explanation. I was obviously oversimplifying the idea of robot speed and I missed the relationship between gear ratio and motor load on the CIM motors. Better to learn now. So from your numbers I can conclude that the gear ratio resulting in 21.3 fps has very similar acceleration to a gear ratio resulting in 13 fps while the higher top speed causes voltage to drop quicker.

Is there a specific equation you are using to relate output speed of CIMs to current draw or voltage drop?

Would you be able to PM me the spreadsheet you made?

Thanks for the help.

10-12-2012 17:46

billbo911

Re: pic: Sheet Metal with VexPro parts

Considering drive train design is by no means my area of expertise, I lean heavily on what I see other teams doing successfully and try to apply their knowledge in a way that supports our needs. I also make use of tools like the JVN Design Calculator for convenience.

What I'm looking for is some basic "Rule of Thumb" target numbers to use to help determine overall drive train gearing.

Let's assume we will be using 4" wheels.

Here are some first pass numbers I came up with using the JVN Calculator, do these look like a good starting point? Game requirements will need to be considered once we KNOW what the game is, but for now, are these numbers close? If no, why not?


VEXPro 2 speed Ball Shifter:
2 CIMs per tramsmission:
4" wheels:
Additional chain reduction of 28:16:

Low gear total reduction 14.6:1 Max speed = 5.33 ft/sec. Wheel stall 58 Nm
High Gear total reduction 6.4:1 Max speed = 12.12 ft/sec. Wheel stall 25.16 Nm

From your experience(s), are the numbers in the ballpark? Remember, I'm just looking for "Rule of Thumb" and why they are right or wrong.

10-12-2012 18:01

apalrd

Re: pic: Sheet Metal with VexPro parts

Basically, at 21fps, you're running with roughly the same final speed as 13fps (after ~1.5sec, before which the 21fps speed is worse), but running in the 'bad side' of the power curve, drawing significantly more current.

I use a spreadsheet originally created by John V-Neun to calculate most of this.
2008 version
2004 version

In general, everything can be modeled with basic physics equations, calculated iteratively. For example, you can use your current speed and voltage to calculate the motor output torque, which you can use to calculate your acceleration, which you can integrate to get velocity.

JVN's spreadsheet is very good. I usually use the 2008 version to model mechanisms (arms, elevators) and the 2004 version to model drivetrains. The 2004 version includes acceleration and sprint-distance graphs, which are very nice.

As to your original question, usually low gear is designed to be traction-limited at 40 amps per motor. Getting this right is usually slightly lower priority than getting high gear correct, usually low gear is what it is (especially since most gearboxes only come in one or two ratio spread options). For FRC robots with ~1.2cof and 4 CIMs, this is somewhere around 5.5fps.

17-12-2012 14:19

Madison

Re: pic: Sheet Metal with VexPro parts

Here's an okay look at what I roughed in for chain tensioning on the end wheel sets.



The white parts on the right side are part of the tensioning system. One is a U-shaped bracket that straddles the wheel and captures the axle in two places. The axle continues through that part and into the blue frame pieces where it can slide in a slot. The black pieces are acetal disks that act as spacers and as thrust washers and should help to spread side-loading out over a larger area than a typical washer or the bolt head.

The bracket also has a captive nut installed. Threading a bolt into that nut will pull on the wheel and tension the chain. The second, right-most white part is the fixed plate the bolt pulls against; it could just as easily use the blue, outer drive train cross member, but I wanted to avoid having the bolt head stick out into where the bumpers will sit and it also makes it possible to assemble each side of the drivetrain as a 'pod' without putting together the entire frame first.

17-12-2012 18:55

Andrew Zeller

Re: pic: Sheet Metal with VexPro parts

I like that tensioning design. May borrow it for our drivetrain this year.

What is the size of the bolt you are using to tension?

17-12-2012 18:59

Madison

Re: pic: Sheet Metal with VexPro parts

It's 1/4-20. Nothing fancy.

17-12-2012 19:43

Jay Trzaskos

Re: pic: Sheet Metal with VexPro parts

what clearance have you left so that the bolt will not interfere with the wheel as you continue to tension it? I'm guessing that the bolt is mounted below the axle plane, giving it increased clearance?

17-12-2012 19:52

Madison

Re: pic: Sheet Metal with VexPro parts

It's above the axle plane rather than below -- but yeah, it's closer than I'm happy with. That's something that I'd try to improve in a further iteration. Right now, there's probably only about 1/16" or so of clearance if it's fully tensioned.

18-12-2012 12:20

roystur44

Re: pic: Sheet Metal with VexPro parts

Madison,

Consider flipping the u bracket the other way and pushing the bracket to create tension. Brings your wheel out further on the frame and exposes more of the wheel for climbing ramps. Plus you won't have the screw sticking out into the thread.

18-12-2012 13:24

Alan Anderson

Re: pic: Sheet Metal with VexPro parts

I typically focus on less tangible matters like software and electricity, so I might not be as qualified as others to comment on this, but I'll do it anyway.

I have noticed that pulling tends to enforce appropriate alignment, while pushing has a habit of bending things out of the desired line. In the case of this chain tensioning system, the bracket holding the axle won't have a lot to keep it from rotating if you just push on it. If the spot you're pushing on isn't actually aligned with the axle, it will be actively encouraged to rotate out of the way.