FRC Blog - 2016 Motor Controllers

[cgmv123]

Originally Posted by Thad House

There were no rules saying you couldn't use those parts. Just that they couldn't directly control the robot with them. If you wanted to put a cRIO on the bot and use it for camera processing then communicate to the RoboRIO you legally could.

You could probably have gotten away with a cRIO, but I don't think you could have gotten away with an earlier PDB.

[Michael Corsetto]

Cross The Road Electronics has posted a detailed report of some Motor Controller Output Power Testing they preformed with the four main FRC speed controllers on the market:

• Victor SP
• Talon SRX
• SPARK
• SD540

Link to .pdf document here

Please take note of the test results on page 7. I'm very concerned about the SD540's performance, particularly that the SD540 appears to brown out at 9.5V.

2016 will be the year of the brown out...

-Mike

[Thad House]

Originally Posted by Michael Corsetto

2016 will be the year of the brown out...
-Mike

Programmers better bring their A game this year if teams want any sort of speed with 3 CIMs. Some very intelligent ramping and shifting code is going to be required for all the top teams.

Maybe someone who knows a lot about working with motor controls would be willing to do a white paper or something how how to limit current. Trying to find documentation online on how to do that has been impossible for me.

[Ryan Dognaux]

Originally Posted by Thad House

Programmers better bring their A game this year if teams want any sort of speed with 3 CIMs. Some very intelligent ramping and shifting code is going to be required for all the top teams.

With everything I've been reading about brown outs, I think we're just going to go back to the tried and true '4 CIMs on your drive' depending on the game. It just isn't worth the risk of being dead on the field in a match.

[Jared Russell]

Originally Posted by Thad House

Programmers better bring their A game this year if teams want any sort of speed with 3 CIMs. Some very intelligent ramping and shifting code is going to be required for all the top teams.

Maybe someone who knows a lot about working with motor controls would be willing to do a white paper or something how how to limit current. Trying to find documentation online on how to do that has been impossible for me.

It would be awesome if the Talon SRX gets a firmware update that lets it do this (current limiting). It can already do ramping.

[AdamHeard]

Originally Posted by Jared Russell

It would be awesome if the Talon SRX gets a firmware update that lets it do this (current limiting). It can already do ramping.

Yup Yup.

I'd love this for FRC, and a few non-FRC applications I use Talons for.

[notmattlythgoe]

Originally Posted by AdamHeard

Yup Yup.

I'd love this for FRC, and a few non-FRC applications I use Talons for.

A continuous sensor option would be nice too. Similar to the continuous sensor option in the WPILib PID Controllers.

[Thad House]

Originally Posted by Jared Russell

It would be awesome if the Talon SRX gets a firmware update that lets it do this (current limiting). It can already do ramping.

Even if its not built in, since we can access all the current and speed data, wouldn't we be able to do this on the RoboRIO? It wouldn't be anywhere close to as fast of a loop, but it should be good enough, right?

[notmattlythgoe]

Originally Posted by Thad House

Even if its not built in, since we can access all the current and speed data, wouldn't we be able to do this on the RoboRIO? It wouldn't be anywhere close to as fast of a loop, but it should be good enough, right?

I haven't spent nearly enough time trying to track down the correct way to do this, but I would love to know what it is.

[AdamHeard]

Originally Posted by notmattlythgoe

I haven't spent nearly enough time trying to track down the correct way to do this, but I would love to know what it is.

Hopefully someone knows the answer to this.

If you're just running 4 Talon SRXs on the CANbus, nothing else and you're in a separate thread running your drive, what's the fastest you can run that loop and still get a fresh current measurement over CAN for each one?

Same question, but w/ PWM speed controllers and reading current over PDP?

[Thad House]

Originally Posted by AdamHeard

Hopefully someone knows the answer to this.

If you're just running 4 Talon SRXs on the CANbus, nothing else and you're in a separate thread running your drive, what's the fastest you can run that loop and still get a fresh current measurement over CAN for each one?

Same question, but w/ PWM speed controllers and reading current over PDP?

I know that the PDP only reads new currents every 25ms, or at least it was last season. Don't know if that changed. In addition, PWM updates at max every 5ms, so you couldn't go any faster then that even if you had a faster sensor.

The default status update period for Talons is 10ms. However I know that you can set it even faster. I bet you could do 5ms easily, and could probably push about every 2ms if you are careful with your CAN usage.

I would bet that is plenty fast enough, as your mechanical system probably won't react much quicker then that.

[Jared Russell]

Originally Posted by Michael Corsetto

Cross The Road Electronics has posted a detailed report of some Motor Controller Output Power Testing they preformed with the four main FRC speed controllers on the market:

• Victor SP
• Talon SRX
• SPARK
• SD540

Link to .pdf document here

Please take note of the test results on page 7. I'm very concerned about the SD540's performance, particularly that the SD540 appears to brown out at 9.5V.

2016 will be the year of the brown out...

-Mike

This is a very interesting study, and props to CTRE in making it easy to reproduce the test.

Assuming the motor controllers are all linear, we can calculate series resistances from the test data. Averaging all non-brownout trials gives me:

R(Victor SP) = 0.0042 ohms
R(Talon SRX) = 0.0050 ohms
R(SPARK) = 0.0075 ohms
R(SD540) = 0.0175 ohms

We can roughly model a stalled CIM as a resistor R(CIM) = 12V/131A = 0.092 ohms.

Now put our stalled CIM in line with each speed controller (so the total resistance is R(CIM) + R(motor controller), the resulting current is calculated by I=V/R, and the resulting stall torque is the ratio of this current to the CIM's nominal stall current of 131A using motors.vex.com data):

Stall torque (Victor SP) = 2.30 N*m. This is 95.6% of the motor spec.
Stall torque (Talon SRX) = 2.29 N*m. This is 94.9% of the motor spec.
Stall torque (SPARK) = 2.23 N*m. This is 92.5% of the motor spec.
Stall torque (SD540) = 2.02 N*m. This is 84.0% of the motor spec.

These are large enough differences from the motor spec (even in the Victor/Talon case) that designers will want to keep these numbers close by when choosing gear ratios.

And of course, keep these numbers in mind when choosing speed controllers this season. In some applications, it won't really matter which one you choose...in others, it most certainly will.

[James Kuszmaul]

Originally Posted by Michael Corsetto

Cross The Road Electronics has posted a detailed report of some Motor Controller Output Power Testing they preformed with the four main FRC speed controllers on the market:

• Victor SP
• Talon SRX
• SPARK
• SD540

Link to .pdf document here

Please take note of the test results on page 7. I'm very concerned about the SD540's performance, particularly that the SD540 appears to brown out at 9.5V.

2016 will be the year of the brown out...

-Mike

Very interesting, although it would be nice if someone could do these tests with multiple of each controller (in case of either defective units or just generally varying units). In particular, I wouldn't want to judge the SD540 too much without confirming the results on multiple units.

[pnitin]

Originally Posted by Michael Corsetto

Cross The Road Electronics has posted a detailed report of some Motor Controller Output Power Testing they preformed with the four main FRC speed controllers on the market:

• Victor SP
• Talon SRX
• SPARK
• SD540

Link to .pdf document here

Please take note of the test results on page 7. I'm very concerned about the SD540's performance, particularly that the SD540 appears to brown out at 9.5V.

2016 will be the year of the brown out...

-Mike

Mike,

It is not brownout it is Seafty Feature, documents above says lockout due to Seafty feature,
I agree with this if you are going down to 8.2 V ( or even 10V ) you are seriously killing your battery.
Lead Acid battery do not like deep discharge, no matter what manufacturers say(deep discharge or what not), but they will not be same again if you discharge them down to 10.8V (1.8V per cell)

[Jon Stratis]

Originally Posted by cgmv123

You could probably have gotten away with a cRIO, but I don't think you could have gotten away with an earlier PDB.

Well, you could have used that old PDB, powered from a 40A breaker on the new PDP, to power your custom electronics (but not speed controllers, etc). It wouldn't really be worth doing, as there are smaller and cheaper ways to get power to a collection of custom electronics, but it would be legal.