Drive Train Calcs - Not trusting my results

[Nemo]

Originally Posted by Gregor

At about what speeds are these according to your calculations?

It is highly dependent on several factors. Let's say I assume the following:
CoF = 1.2
Drive efficiency = 0.80
Carpet-wheel efficiency = 0.93
Enough resistance to cause voltage to drop to 7.8 V / 6.7 V for 4/6 CIM drive
Charged battery voltage = 12.8 V

4 CIM is traction limited at about 11 ft/s
6 CIM is traction limited at about 14 ft/s

Change any of the conditions I listed, and you get an appreciable change. I encourage you to download the spreadsheet and start messing with the numbers along the left side.

In practice, we had to gear our drive closer to 9 ft/sec to get the tires to spin from a stop using 4 CIM's, Versawheels, and 1 stage Vex gearboxes plus a chain stage (we have since gone to ballshifters). I can play with numbers to make the spreadsheet report that 9 ft/s result, but I don't know which of the factors listed above is off by the most. That's why I'd like to put together some testing in the fall.

[KrazyCarl92]

Originally Posted by Nemo

4 CIM is traction limited at about 11 ft/s
6 CIM is traction limited at about 14 ft/s

Using your spreadsheet with your assumptions I am getting:
4 CIM traction limit at 17.8 fps
6 CIM traction limit at 26.5 fps

Are you sure you're not looking at the average speed over distance fields from your spreadsheet? Those actually come close to matching your claims.

For reference, from my own spreadsheet I get:
4 CIM traction limit at 16.4 fps actual speed
6 CIM traction limit at 25 fps actual speed

[KrazyCarl92]

Comparing both of your spreadsheets, I see that your first one gives different results than the second. This appears to be due to the fact that you use voltage as a variable in determining the gearbox output torque.

However, I have observed robots which experience voltage drops when initially engaging in a pushing match then quickly recover. This would mean that the second spreadsheet would be misleading for determining traction limits because this only considers sprints, assuming that the traction limit occurs during the initial drop while disregarding the recovery of battery voltage during a pushing match.

For example, a robot may drop to 8V when initially engaging in a pushing match, then within half a second rise back up to 11V and remain there for the duration of the pushing match. Therefore, the traction limit should be evaluated at 11V and not 8V.

Seems like hard experimentation should be explored before blindly trusting any of these models.

[Nemo]

Originally Posted by KrazyCarl92

Comparing both of your spreadsheets, I see that your first one gives different results than the second. This appears to be due to the fact that you use voltage as a variable in determining the gearbox output torque.

However, I have observed robots which experience voltage drops when initially engaging in a pushing match then quickly recover. This would mean that the second spreadsheet would be misleading for determining traction limits because this only considers sprints, assuming that the traction limit occurs during the initial drop while disregarding the recovery of battery voltage during a pushing match.

For example, a robot may drop to 8V when initially engaging in a pushing match, then within half a second rise back up to 11V and remain there for the duration of the pushing match. Therefore, the traction limit should be evaluated at 11V and not 8V.

Seems like hard experimentation should be explored before blindly trusting any of these models.

I certainly wouldn't argue with your last statement.

The older first spreadsheet doesn't take voltage drop into account, so the second one should be a better predictive tool. IF there isn't anything drastically wrong with it.

For a robot's voltage to increase from 8 V to 11 V, the wheels have to start turning. That can happen if the tires spin or if the other robot decides to go backwards. In a situation in which both robots are stalled at a dead stop while trying to push each other, I care about being traction limited at 8 V. If it's an option to back up and ram the other robot to develop a bit of wheel speed, or if the other robot might let up for a second to let me gain an inch, then I agree that being traction limited at 11 V could be just fine in those cases.

[KrazyCarl92]

Did some testing last night. We have a drivetrain which is geared to 8.8 fps in low gear and 22.6 fps in high gear with 4 CIMs. We are using KOP wheels (Andymark says .95-1.0 CoF) and the robot weighs about 145 pounds with battery, bumpers and all. We are aware this gearing is not absolutely ideal, but using 6 in. wheels and looking at COTS options meant we were going for whatever retrofits best to 6 in. wheels out of designs made for 4 in. wheels.

We set the robot up against the wall and drove into it to test our traction limits. Our robot is traction limited in low gear. Didn't watch the voltage drop in this case since I fully expected it to be traction limited.

The high gear when starting statically was not traction limited. The voltage reading on the driver station dropped just below 8 Volts and then hovered between 7.5 and 8.5 Volts while the robot's wheels failed to turn.

We then tested dynamically going into the wall such that the wheels would be spinning when we hit the wall in high gear. The observed effect is that we were traction limited in this case. The wheels spun while the robot pushed against the wall not moving. The observed voltage readings were an initial drop below 8 Volts, then a quick recovery to between 9.5 and 11.5 Volts. If you have suggestions for other things to test with this set up I would be glad to give stuff a try.

One thing I have done is use Weighted Objective Tables to evaluate the trade-off between "Time to Distance" and motor/circuit breaker abuse. Basically it's an objective way of determining which gear ratios are reasonable for a given strategy. If your strategy requires driving long distances across the field often, go with time to travel 25 feet. If the game dictates that the longest distance a team will reasonably be able to travel often in a sprint is 10 feet, use that. Then consider current draw under certain conditions and how long it may take your breakers to trip in the worst case scenario. Weight them accordingly. It's a good way to objectify this inevitable trade-off in gearing decisions, especially since all of these things are quantifiable with decent models. This can also be used to objectify motor allocation and shifting in addition to gearing choices.

[Chris is me]

Originally Posted by Nemo

It is highly dependent on several factors. Let's say I assume the following:
CoF = 1.2
Drive efficiency = 0.80
Carpet-wheel efficiency = 0.93
Enough resistance to cause voltage to drop to 7.8 V / 6.7 V for 4/6 CIM drive
Charged battery voltage = 12.8 V

4 CIM is traction limited at about 11 ft/s
6 CIM is traction limited at about 14 ft/s

Change any of the conditions I listed, and you get an appreciable change. I encourage you to download the spreadsheet and start messing with the numbers along the left side.

It is really important to note that these calculations are for traction limited at *stall*, not at 40 amps or max power. If you're trying to gear for traction-limited pushing, you definitely want to gear slower than this.

For Carl's data above - note that 20 uses KoP wheels in their drive. Your mileage will vary with higher and lower traction wheels, and even different wheel diameters!

[Nemo]

Originally Posted by Chris is me

It is really important to note that these calculations are for traction limited at *stall*, not at 40 amps or max power.

Yes. I'm reading the first line of my spreadsheet, where speed = 0 and current is whatever maximum is achieved at the lowered voltage that occurs at startup.

Originally Posted by Chris is me

If you're trying to gear for traction-limited pushing, you definitely want to gear slower than this.

How do you know?

[Nemo]

Originally Posted by KrazyCarl92

Did some testing last night.
(snip)
We have a If you have suggestions for other things to test with this set up I would be glad to give stuff a try.

Thanks for sharing the testing you did - empirical data points are helpful. One thing you could do if you're curious is keep adding weights to your robot and run it in low gear, and then record the total weight at which the drive ceases to be traction limited from a stall. Knowing that, one can backtrack a bit and calculate the gear ratio at which the robot would be barely traction limited with the actual robot weight. We did something like that during the season to figure out where we'd be traction limited, which is how we arrived at the 8.7 ft/s we ran at the start of the season.

That's the type of thing I want to have our team do in the fall; test the CoF of the wheels using the angled ramp test or whatever; measure the voltage; drive the robot through some sprints and measure its actual speeds and rates of acceleration; use weights to figure out at what point it becomes traction limited from a stop. Once we have all of that stuff, we can adjust the unknown spreadsheet parameters accordingly and hopefully have a model that makes decent predictions for other sets of circumstances.

[Chris is me]

Originally Posted by Nemo

How do you know?

If you want to be able to push indefinitely, your motors can't draw more than 40 amps* each while pushing, or else you'll begin to trip the auto-resetting breakers on the PDB. The 160A total current draw across 4 CIMs is a lot, but not enough to immediately (or even quickly) trip a functional 120A breaker. The speed that roughly correlates with traction limited at 40 amps, for an at weight robot using high traction wheels, is ~5.5 feet per second.

You can definitely still push if your drive isn't geared that slowly for a variety of reasons. For one, circuit breakers don't trip immediately. There are also quite a lot of factors in play during each individual pushing match that affect how successful it will be - wheel traction, pre-existing heat in the motors, chassis rigidity, center of gravity, weight transfer between robots, etc. But if you design a two-speed transmission with a low gear that is traction limited near 40A, you can be confident that you'll always be able to push as well as you're able. This is one of the major selling points of two speed transmissions.

*In reality, the snap-action breakers have a huge safety margin, so there's some wiggle room here.

[Nemo]

Originally Posted by Chris is me

If you want to be able to push indefinitely, your motors can't draw more than 40 amps* each while pushing, or else you'll begin to trip the auto-resetting breakers on the PDB. The 160A total current draw across 4 CIMs is a lot, but not enough to immediately (or even quickly) trip a functional 120A breaker. The speed that roughly correlates with traction limited at 40 amps, for an at weight robot using high traction wheels, is ~5.5 feet per second.

You can definitely still push if your drive isn't geared that slowly for a variety of reasons. For one, circuit breakers don't trip immediately. There are also quite a lot of factors in play during each individual pushing match that affect how successful it will be - wheel traction, pre-existing heat in the motors, chassis rigidity, center of gravity, weight transfer between robots, etc. But if you design a two-speed transmission with a low gear that is traction limited near 40A, you can be confident that you'll always be able to push as well as you're able. This is one of the major selling points of two speed transmissions.

*In reality, the snap-action breakers have a huge safety margin, so there's some wiggle room here.

Ok, now I understand that you're talking about not tripping breakers. Certainly a slower gearing will draw fewer amps, and there is a cutoff somewhere at which a robot can no longer be driven a certain way for 2 minutes straight without tripping those breakers.

I'm thinking about that, and it seems more complicated than getting traction limited at 40 Amps (or 50 or 55 Amps or whatever won't trip the breakers in 2 minutes). For example, let's say I enter a CoF = 1.1 into the spreadsheet and gear it for 5.5 ft/s and otherwise use the same values as before. With those conditions, the spreadsheet reports that the robot can be traction limited when it's going about 3.5 ft/s while pulling around 40 amps. If that same robot pushes as hard as it can at 1.0 ft/s, it's pulling about 76 amps. So it depends on a few things, including what type of pushing the robot does and under what conditions the wheels actually start to spin out during a shoving match.

[KrazyCarl92]

For reference, Team 358's website has the links to electrical datasheets, like our circuit breakers:
http://www.team358.org/files/electri...ec%20Sheet.pdf

http://www.team358.org/files/electri...ainBreaker.pdf

These are quite useful when evaluating gearing options.