Coaxial Swerve Derivation with Paired Modules

[Clem1640]

Some benefits of pairing swerve modules would be:
* Ability to drive (2) swerves with (3) CIMS
* Easier to add gear shift

But, what kind of drive action/control are you seeking: Crab or Snake? Either could be accomplished through pairing, but not both together.

For example of crab & snake, see:
http://www.chiefdelphi.com/media/papers/2400
http://www.chiefdelphi.com/media/papers/2401

[Craig Roys]

Originally Posted by Clem1640

Some benefits of pairing swerve modules would be:
* Ability to drive (2) swerves with (3) CIMS
* Easier to add gear shift

But, what kind of drive action/control are you seeking: Crab or Snake? Either could be accomplished through pairing, but not both together.

For example of crab & snake, see:
http://www.chiefdelphi.com/media/papers/2400
http://www.chiefdelphi.com/media/papers/2401

This is one of the reasons we were considering looking at two pairs...we wanted to see about getting 6 CIMS to power the 4 wheels.

[Tyler2517]

Originally Posted by Craig Roys

This is one of the reasons we were considering looking at two pairs...we wanted to see about getting 6 CIMS to power the 4 wheels.

Why do you want 6 cims for a swerve drive? It seems a bit over kill when a swerve is best at out maneuvering not out pushing/running.
You could try a 3 wheel 6 cim having the best out of both worlds.

[Chris is me]

Originally Posted by Tyler2517

Why do you want 6 cims for a swerve drive? It seems a bit over kill when a swerve is best at out maneuvering not out pushing/running.
You could try a 3 wheel 6 cim having the best out of both worlds.

6 CIM drives really aren't about pushing harder; they don't, at least not with shifters. They're about accelerating better when geared for a high speed at full weight. A swerve that accelerates more slowly than the standard tank drive would be less agile - this would be bad for outmanuvering.

[asid61]

6 cims is not for pushing, because the 120a breaker basically limits your torque, even if it delays a few seconds. Your wheels will lift before that happens at lower speeds (~7fps IIRC). It does, however, give you much better acceleration.
The main problem with a 6 cim swerve is that you lose all your cims that could go to a manipulator. Plus, you either have to do a central gearbox or go with a 6-module swerve.
Using centralized gearboxes saves weight (chain weighs very little as a matter of fact) but at the cost of not being able to do complex maneuvers. For programming purposes, a simple crab would work better in my opinion, but mechanically it will be hell to line them up due to chain spacing.

The chain for crabs also has to be designed around for anything that goes above the chassis. The high number of chains also can lead to faster breaks, so using #35 chain could be an advantage maintanence wise.
Chain weight is 0.087lbs for #25, 1.8lbs for #35 per foot.

One way to line up swerve modules is to stretch a piece of surgical tubing between the end of one wheel's axle to the end of another and tie it off there. That keeps both wheel facing forwards. Then you calibrate at 0, and remove the tubing.

[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.

[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.