Team 610 pre-season drivetrain

[cadandcookies]

Perhaps this discussion of acceleration versus top speed should be split off...

Regarding top speed versus acceleration, I think it's important to remember what (I believe it was BJC) said in a whitepaper: the goal of a drive train is to get you where you need to be, when you need to get there. To that end, I'd focus on time to go a given distance: a very nice calculator for this can be found here, which was written up by Andrew Schreiber (based on the drive train acceleration model, which Ether provides a link to in the next post). It's a little bit out of date but it's good for giving reasonably accurate numbers.

Other than that, it's all about team preference. Obviously there's some math behind it (if it's going to take you longer than 25 feet or so to reach your top speed, it probably isn't worth it), but nothing that hasn't already been mentioned elsewhere.

[Ether]

Originally Posted by cadandcookies

I'd focus on time to go a given distance: a very nice calculator for this can be found here.

That calculator was written by Andrew Schreiber¹, and was based on an early version (now superseded) of the Drivetrain Acceleration Model².

The model was updated a few days later to correct a small error and include the effects of reduced motor voltage caused by circuit resistance³. Andrew's calculator has not yet been updated to reflect the updated model.

Full C source code² is provided for the Model. All you need is a C compiler and you can change the parameters, compile, and run it. Compiling and running takes less than 2 seconds. The output is a CSV file which will open directly in Excel if you set file associations. Columns in the CSV file include time, distance, speed, acceleration, motor current, and motor voltage.

¹ See post#13 in this thread http://www.chiefdelphi.com/forums/sh...d.php?t=119442

² http://www.chiefdelphi.com/media/papers/2868

³ See post#23 in this thread: http://www.chiefdelphi.com/forums/sh...d.php?t=119442

[cadandcookies]

Originally Posted by Ether

That calculator was written by Andrew Schreiber¹, and was based on an early version (now superseded) of the Drivetrain Acceleration Model².

The model was updated a few days later to correct a small error and include the effects of reduced motor voltage caused by circuit resistance³. Andrew's calculator has not yet been updated to reflect the updated model.

Full C source code² is provided for the Model. All you need is a C compiler and you can change the parameters, compile, and run it. Compiling and running takes less than 2 seconds. The output is a CSV file which will open directly in Excel if you set file associations. Columns in the CSV file include time, distance, speed, acceleration, motor current, and motor voltage.

¹ See post#13 in this thread http://www.chiefdelphi.com/forums/sh...d.php?t=119442

² http://www.chiefdelphi.com/media/papers/2868

³ See post#23 in this thread: http://www.chiefdelphi.com/forums/sh...d.php?t=119442

Sorry, I didn't mean to post that without credit. I've edited my post to reflect the information you've presented.

At this point that model is something that I use quite often for getting a reasonable estimate of time to a given distance, and it just sort of permanently hangs out in my "Useful FRC tabs" bookmarks group. In the future I'll be using the up to date source code for critical applications.

[Ether]

Originally Posted by cadandcookies

At this point that model is something that I use quite often for getting a reasonable estimate of time to a given distance, and it just sort of permanently hangs out in my "Useful FRC tabs" bookmarks group. In the future I'll be using the up to date source code for critical applications.

Just remember the old adage "Garbage in, garbage out". The model is only as good as the parameters you use: the more difficult ones being the constant, speed-dependent, and torque-dependent drivetrain losses (Kro, Krv, Kf); and the circuit resistances (Rcom and Rone).

[cadandcookies]

Originally Posted by Ether

Just remember the old adage "Garbage in, garbage out". The model is only as good as the parameters you use: the more difficult ones being the constant, speed-dependent, and torque-dependent drivetrain losses (Kro, Krv, Kf); and the circuit resistances (Rcom and Rone).

Unfortunately I have yet to find an online C compiler that will give me the output csv file, so for back of the napkin calculations it will have to do.

[Ether]

I did not fully appreciate how much of an obstacle the need for a C compiler would be.

For those who do not have, or do not care to install, a C compiler, I will look into posting a compiled executable that grabs the parameter values from the environment.

[Ether]

Originally Posted by Ether

For those who do not have, or do not care to install, a C compiler, I will look into posting a compiled executable that grabs the parameter values from the environment.

Beta release posted. Constructive feedback would be appreciated.

[Mr. Lim]

http://youtu.be/lQLp_F88CEg

We had a chance to take the drivetrain for a spin last night. We rang out the encoders and gyro, and all seems to be in good shape.

Our driver for the upcoming 2013-2014 season had his first crack at driving it around, and the initial feedback is very positive.

[BrendanB]

Originally Posted by Mr. Lim

http://youtu.be/lQLp_F88CEg

We had a chance to take the drivetrain for a spin last night. We rang out the encoders and gyro, and all seems to be in good shape.

Our driver for the upcoming 2013-2014 season had his first crack at driving it around, and the initial feedback is very positive.

Wrong video? The cars are pretty sweet though!

[Mr. Lim]

Originally Posted by BrendanB

Wrong video? The cars are pretty sweet though!

Try clicking it again. There was some YouTube weirdness going on with the link.

Originally Posted by Mr. Lim

http://youtu.be/lQLp_F88CEg

We had a chance to take the drivetrain for a spin last night. We rang out the encoders and gyro, and all seems to be in good shape.

Our driver for the upcoming 2013-2014 season had his first crack at driving it around, and the initial feedback is very positive.

Looks solid. Something I noticed is your CG is between your back two wheels, so all of your turns spin around that back point, negating one of the useful effects of the 8wd and also preventing you from turning in place with almost no displacement of the robot.

[Akash Rastogi]

Originally Posted by Andrew Lawrence

Looks solid. Something I noticed is your CG is between your back two wheels, so all of your turns spin around that back point, negating one of the useful effects of the 8wd and also preventing you from turning in place with almost no displacement of the robot.

I'm quite certain 610 understands how weight distribution works Also, check again where the raised wheel is.

Edit: One can only assume this was quite intentional, for whatever reason.

Originally Posted by Akash Rastogi

I'm quite certain 610 understands how weight distribution works Also, check again where the raised wheel is.

Edit: One can only assume this was quite intentional, for whatever reason.

Ah, didn't see the raised wheel part. That makes much more sense. I was confused for a minute as to why 610 would have their CG in the back on a standard center dropped 8wd.

[BrendanB]

Originally Posted by Andrew Lawrence

Looks solid. Something I noticed is your CG is between your back two wheels, so all of your turns spin around that back point, negating one of the useful effects of the 8wd and also preventing you from turning in place with almost no displacement of the robot.

With the smaller robot size placing your gearboxes on one end allows you to keep a nice wide electronics area open in the robot without having to break it up. Having a small square robot you won't notice this off center turning as much and depending on what is build on top of it, the weight can distribute out. Our 2013 was weighted towards the back due to where our hanger was connected to our robot and we had both our gearboxes and battery as far back as we could. Even then when we hung on the tower we were the front end of our robot hung lower. Our driver had no issues or complaints and it was one of the smoothest 6wd robots I have driven. Also, when using gears to drive your robot, adding in wheels 7 & 8 is extremely easy so as an off-season robot it makes a nice test platform.