blog; Motor Controller Options for 2015

[apples000]

Originally Posted by magnets

Perhaps an EE could help me out, but why go to 15kHz for the switching frequency? Is the 1kHz used on the Victor 888 not smooth enough?

From my small knowledge of EMI, the effective interference from the motors is proportional to switching frequency and current, both of which are very high in this application. Putting so much circuitry right next to the controller (like the CAN bus) makes me a little nervous.

The motors also generate really strong fields and though they drop off quickly, they still have an effect on our control system. Now that our main controller is no longer in a big heavy metal grounded enclosure, I'm slightly concerned about this.

That said, I am not an EE, but I dislike anything that increases electrical interference on the robots.

I also am not an EE, but have a lot of experience with this sort of stuff. In short, you're right, the higher frequency is proportional to higher interference. That said, I checked the spec sheets, and the old Talons are operating at 15kHz, and people have been fine.

I don't understand the reason to go up to 15kHz for the controllers. Switching up to 100 amps at 15kHz is not something to be taken lightly, as the high current and frequency causes parasitic capacitance to ground. This results in an effect called common mode coupling, which ends up "contaminating" your ground plane. Anything else that uses this ground can be affected by it.

Switching to a higher frequency also makes the system less efficient. Although the more efficient sign-magnitude switching could make up for this loss, you'd see higher efficiency if it switched at 10kHz than 15kHz. The reason is that the faster you switch the FETs, the more time they'll end up spending in their switching state. The transition is not instantaneous, and as you increase frequency, they'll spend more time in between, which is inefficient.

Also, I can't really think of a great reason for them to go this high. The 1kHz frequency was audible, and so will the 15kHz frequency (close to the frequency your CRT television's transformer makes). The talon is 15kHz and the Victor 888 is 1kHz. You really can't tell the difference between the two frequencies, other than the noise they make.