pic: Sheet Metal with VexPro parts

[ttldomination]

Originally Posted by Madison

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.

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

- Sunny G.

[Andrew Zeller]

Originally Posted by apalrd

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).

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.

[billbo911]

Originally Posted by Madison

The chain reduction is 1:2 right now; 16 tooth hex sprocket to 32 tooth sprocket bolted to a versahub..........

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.

[apalrd]

Originally Posted by boarder3512

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.

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.

[Madison]

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.

[Andrew Zeller]

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?

[Madison]

Originally Posted by boarder3512

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?

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

[Jay Trzaskos]

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?

[Madison]

Originally Posted by Jay Trzaskos

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?

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.

[roystur44]

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.

[Alan Anderson]

Originally Posted by roystur44

Consider flipping the u bracket the other way and pushing the bracket to create tension.

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.