Coaxial Swerve Derivation with Paired Modules

[Ether]

Originally Posted by asid61

6 cims is not for pushing, because the 120a breaker basically limits your torque... It does, however, give you much better acceleration.

How do you get "much better acceleration" if you don't have more torque?

[pwnageNick]

Originally Posted by Ether

How do you get "much better acceleration" if you don't have more torque?

This. Without more torque, then there is no reason why your wheel spin-up would be decreased, thus helping your acceleration. What's your reasoning behnd saying you do not get more torque?

Originally Posted by asid61

The main problem with a 6 cim swerve is that you lose all your cims that could go to a manipulator.

I think some teams on Einstein this season would have disagreed with you.

-Nick

[asid61]

I think the reason a 6 cim drive gives you more acceleration is because it spikes the current initially. I'm not 100% sure why it is faster at accelerating (it definitely is though, one of parents did some calculations) but that would be my guess.
It won't help in a pushing match because the current spike will just kill you, because the current will not decrease over time like when you start driving. Insert blown breaker here.
The reason I say torque is limited is because torque is directly related to current on a motor. Because we have (usually) a maximum 120a-180a going to all cims, and the stall current on each cim is 133a, you are not getting more torque. The delayed blow on the breaker will allow the current to spike for all the cims, giving you a torque boost with more cims, but it can't hold those current levels for long. So the max torque cap is pretty much the same if you don't want to blow breakers.

Torque is also inversely proportional to speed in electric motors, so having 6 cims divvy up a given amount of current (torque) will increase their speed as well compared to 4 cims.

Let me rephrase what I said about manipulators:
6-cim drivetrains are fine. You are still left with many motors for doing all kinds of stuff on the robot. However, when you have a bunch of turning motors too (which you will want, crab or swerve) then you end up being left with some wimpier motors. On a single centralized turning gearbox, it might not be a problem, but if you want to turn the modules quickly then you would want more than 1 turning motor there.

Just my opinion of course. 6 cim swerves have been done in the past, and have done beautifully. However, in my opinion, it doesn't matter what you pick as long as it's excecuted well and you get driver practice. Focus more on the little picture.

[Ether]

Originally Posted by asid61

Torque is also inversely proportional to speed in electric motors, so having 6 cims divvy up a given amount of current (torque) will increase their speed as well compared to 4 cims.

Using the above logic, a 6 CIM drive should have a 50% faster top speed compared to a 4 CIM drive, all else being equal.

[asid61]

Originally Posted by Ether

Using the above logic, a 6 CIM drive should have a 50% faster top speed compared to a 4 CIM drive, all else being equal.

Hm, that's correct. How would you explain it then? 6 cims definitely increase acceleration.
However, I stand by by statement that torque is limited by the breaker.

[Knufire]

You can pull huge amounts of current for a short amount of time without tripping the breaker.

Source: http://www.cooperindustries.com/cont...UITBREAKER.pdf

[Ether]

Originally Posted by asid61

Hm, that's correct. How would you explain it then? 6 cims definitely increase acceleration.

At a given voltage and speed, 6 CIMs draw more current than 4. More current means more torque. More torque means more acceleration.

As the speed approaches motor free speed (for the given voltage), current draw approaches zero no matter how many motors you have (due to back emf).

However, I stand by by statement that torque is limited by the breaker

With 6 CIMs you also have 6 40-amp breakers, so the associated total current limit increases. And the main breaker is slow acting - it can sustain high overcurents for a significant duration.

[Ether]

Originally Posted by Ether

At a given voltage and speed, 6 CIMs draw more current than 4. More current means more torque. More torque means more acceleration.

Further detail for those interested:

Drivetrain Full-Throttle Acceleration Simulation Model with traction limiting and voltage drops:

http://www.chiefdelphi.com/media/papers/2868

see these attachments:

PDF Drivetrain Acceleration 2013-09-25 RevC

Derivation of Voltage Drop Model rev E

C source Drivetrain Acceleration 2013-09-24_2231

See attached chart of accel vs speed for one set of model parameters, using data generated with attachment ready-to-run model 2013-12-18

You can change the parameters to whatever you think is appropriate for your drivetrain and run the model to see how they affect the performance.

[Ether]

Originally Posted by Ether

See attached chart of accel vs speed...

Hmm. I thought I'd get some comments or questions about the shape of the 6CIM accel curve.

[Oblarg]

Originally Posted by Ether

Hmm. I thought I'd get some comments or questions about the shape of the 6CIM accel curve.

If I'm guessing correctly, the cusp is due to the transition between traction-limited and motor-limited. The 4CIM at that gearing is never traction-limited so there is no cusp. For a different gearing you might see an identical plateau at the beginning for the 4CIM, but it would drop off sooner.

I wonder why there is a slight decrease in acceleration while traction-limited as the speed increases, though?

[Ether]

Originally Posted by Oblarg

If I'm guessing correctly, the cusp is due to the transition between traction-limited and motor-limited.

Correct.

I wonder why there is a slight decrease in acceleration while traction-limited as the speed increases, though?

Yes, that's the one I was expecting to get questioned about.

See if you can figure it out with the following hint: Notice that Krv is set to a non-zero value.

[Oblarg]

Originally Posted by Ether

Correct.



Yes, that's the one I was expecting to get questioned about.

See if you can figure it out with the following hint: Notice that Krv is set to a non-zero value.

Ah. The rolling friction losses are being applied (incorrectly, I'd think) even when the wheels are slipping.

[Ether]

Originally Posted by Oblarg

Ah. The rolling friction losses are being applied (incorrectly, I'd think) even when the wheels are slipping.

It's not incorrect if you look closely at what Krv is supposed to be. It's supposed to be rolling resistance force proportional to vehicle speed (not wheel speed).

[Oblarg]

Originally Posted by Ether

It's not incorrect if you look closely at what Krv is supposed to be. It's supposed to be rolling resistance force proportional to vehicle speed (not wheel speed).

Yes; I meant incorrect compared to what is actually physically happening.

I imagine even if you corrected it to be wheel speed it would not be quite right, since it's supposed to be accounting for rolling friction losses, and clearly the physics are pretty different when you're slipping the wheels from when you're rolling.

[Gregor]

Originally Posted by asid61

Let me rephrase what I said about manipulators:
6-cim drivetrains are fine. You are still left with many motors for doing all kinds of stuff on the robot. However, when you have a bunch of turning motors too (which you will want, crab or swerve) then you end up being left with some wimpier motors. On a single centralized turning gearbox, it might not be a problem, but if you want to turn the modules quickly then you would want more than 1 turning motor there.

I still don't buy it. Even if you somehow manage to allocate 6 cims and 4 turning motors (say Banebots), you still have 4 Minicims/Bags, and 4 AM 9015's, in addition to many other motors of decreasing value.

Get back to me when you find a need for 18 motors.