paper: Drivetrain Acceleration Model

[Ether]

Thread created automatically to discuss a document in CD-Media.

Drivetrain Acceleration Model by Ether

[Andrew Schreiber]

Very cool. Now if only we could convince you to use variable names longer than 2 characters

Am I correct in assuming that Tm = Ts*(1-V/Vfree); on line 55 could also be used to figure out approximate current draw? Or am I over simplifying this?

[Ether]

Originally Posted by Andrew Schreiber

Am I correct in assuming that Tm = Ts*(1-V/Vfree); on line 55 could also be used to figure out approximate current draw?

Yes. Kt, the motor torque constant, is Newton_meters per amp. So you can calculate the motor amps A=Tm/Kt*. For a CIM, Kt≈0.018 Nm/A. You could modify the C code to output amps so it could be plotted.

However, the model does not account for voltage drops due to heavy currents through the wires, the wire connection points, the battery' s internal resistance, changes in motor resistance due to temperature, etc so the accuracy of Tm (and the current) will be affected. Also, Kt is not strictly constant - it varies somewhat with motor temperature.

Very cool. Now if only we could convince you to use variable names longer than 2 characters

I've been programming since 1969 and I've never liked long variable names. To me it makes the code much less readable. I like the code to look like the way I would write the equations. I know opinions differ on this matter.


* when not slipping. when slipping, you'd have to use the kinetic friction force and back-calculate the motor torque.

[Andrew Schreiber]

Originally Posted by Ether

Yes. Kt, the motor torque constant, is Newton_meters per amp. So you can calculate the motor amps A=Tm/Kt. For a CIM, Kt≈0.018 Nm/A. You could easily modify the C code to output amps so it could be plotted.

However, the model does not account for voltage drops due to heavy currents through the wires, the wire connection points, the battery' s internal resistance, changes in motor resistance due to temperature, etc so the accuracy of Tm (and the current) will be affected. Also, Kt is not strictly constant - it varies somewhat with motor temperature.

So, close enough for determining if it's bad or not but not close enough to be an "accurate model".

Originally Posted by Ether

I've been programming since 1969 and I've never liked long variable names. To me it makes the code much less readable. I like the code to look like the way I would write the equations. I know opinions differ on this matter.

Twas just a joke, I know the equations look like this but I've always found annotating them with english names helped me understand what was going on. Course, that was the second thing I did with this (first was read what heun integration was). Thanks for sharing this.

[Nemo]

This is cool. I wouldn't have looked up Heun's method if I hadn't seen this thing. I also wouldn't have looked up rolling resistance. This white paper has given me some homework. Hopefully I can include those two things into the calculator I've been playing with. Thanks very much for posting goodies like this.

[Ether]

Originally Posted by Ether

the model does not account for voltage drops due to heavy currents through the wires, the wire connection points, the battery' s internal resistance...

9/24/2013 added updated C code (build 2013-09-24_2231) which includes motor voltage drops due to circuit resistance.


9/24/2013 added an explanation how the voltage drop model was derived.

[Richard100]

Ether - Very nice, thanks for developing this. Do you think it could be extended to include wheelbase parameters and predict motor current draw when turning (say differential steering)? We've had trouble in this area during drive system design.

[Ether]

Originally Posted by Richard100

Ether - Very nice, thanks for developing this. Do you think it could be extended to include wheelbase parameters and predict motor current draw when turning (say differential steering)? We've had trouble in this area during drive system design.

Possibly. You can find several links here for static analysis of turning force (which relates to motor current) for skid-steer vehicles.

[cadandcookies]

The executable works very well functionally-- I was able to run it with no problems and create some nice graphs based on it.

The only problem I've noticed is that I really have no idea what any of the variables are until I look back into the original program for comments.

I don't think I've quite expressed my appreciation for this program enough-- it is really nice to have such a good model for generating a good estimate of how fast a given drive train setup will move.

[Ether]

Originally Posted by Ether

You can find several links here for static analysis of turning force (which relates to motor current) for skid-steer vehicles.

Due to changes made by the web host, many of those links are now broken.

So I collected everything together in one place and posted it here.

[lemiant]

Thank you so much for making this. You're my go-to resource for FRC physics and the community wouldn't be the same without you.
On line 84 voltage drop is calculated, but you don't ever seem to account for it when calculating the torque of the motors, is that correct?

[Ether]

Originally Posted by lemiant

On line 84 voltage drop is calculated, but you don't ever seem to account for it when calculating the torque of the motors, is that correct?

No. It is accounted for in the motor torque calculation. See the last paragraph in the voltage drop model rev E.pdf document:

In the C code, there is an accel(V) function which computes the vehicle acceleration given its speed V. The vehicle speed V is directly related to the motor speed Wm through the wheels and the transmission, assuming no wheel slip. This motor speed Wm is then used in the formula Tm=Toffset-Tslope*Wm derived above to obtain the motor torque (see line 77 in the code). By using this formula for motor torque, the voltage drops due to current are automatically included in the calculation.

If you change the values of Rone and/or Rcom, you will definitely see the effect of changed voltage drop in the model's output.

Thanks for the words of encouragement.

[Ether]

I posted an update/correction to the C code:

- fixed an error in the English-to-SI conversion constants

- modified the accel function to better reflect the relationship between friction and rolling resistance

- added total drive motor amps to output

[Nathan Streeter]

Can't look at this in-depth right now... but thank you very much for another great resource! This looks quite useful... particularly if k1, k2, and k3 are given real, semi-accurate values (not just assumptions)!

[Andrew Schreiber]

Originally Posted by Ether

I posted an update/correction to the C code:

- fixed an error in the English-to-SI conversion constants

- modified the accel function to better reflect the relationship between friction and rolling resistance

- added total drive motor amps to output

Dangit man, I just finished converting to coffeescript and started putting a front end on it! Mind posting your diffs?

http://bl.ocks.org/schreiaj/raw/6604...a78a8c98eda1b/ It's still missing the axis so it's kinda useless to get numbers out of but I'll post the updated version tonight once I get the axis un-inverted.

Current coffeescript port available at https://gist.github.com/schreiaj/660...a78a8c98eda1b/