FRC Blog - 2016 Motor Controllers

[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.