Re: SD540 Motor Controller
 

Except in the case of CTRE's testing they were using power resistors instead of a motor (so no motor inductance), and commanding each motor controller to 100%. In that case, power output should equal power input minus losses in the controller.

Re: SD540 Motor Controller
 

Are you by any chance affiliated with MindSensors or FRC 540? If so, it would definitely be worth mentioning-- then your comments aren't being taken as being from some random person on the internet, but instead as someone who actually has knowledge of how this controller was designed, which is something many of the people in this thread are interested in, and lends your statements more weight.

If you are just some random person on the internet, ignore me, this back and forth is vaguely interesting (as someone who has very little knowledge of electrical engineering).

Re: SD540 Motor Controller
 

To calculate power loss in any circuit, you have to look at power in minus power out this is correct, but power is V*I so you have to measure I_in and I_out not just I some random current.

Re: SD540 Motor Controller
 

Do you know this to be true for the SD540? Please answer 'yes' or 'no'.

If the answer is 'yes' I think the SD540 will made a lot more sales. If the answer is 'no' then it is still up in the air because momentary drops below 9.5V are going to kill robots on the field.

Re: SD540 Motor Controller
 

Why take the risk at this point?

There are so many proven controllers on the market, let other teams be the guinea pig this season.

Speed controllers especially are very high on the list of FRC components where no level of failure or odd behavior is tolerable to any team.

Re: SD540 Motor Controller
 

It's a little different than a cell phone charger taking 110V AC input and 5V DC out...

For the conventional motor controller, you can make the assumption that Iin = Iout. Here's Why:

If you look at conventional motor controller designs, the current that drives the motor flows from the input of the motor controller, through a large output device (Power MOSFET or BJT), through the load (motor, or bank of resitors), and then back into the motor controller, through another large output device, and finally, back out the negative battery input on the motor controller.

You can think of the output devices in this case like a switch. When the output devices are "on" (transistors are in the saturation region), they have a small resistance (this is what causes the voltage drop between the input and output of the motor controller). This resistance here is in series with the load. Kirchhoff tells us that current through all components in the loop is the same. Operating on the assumption that the SD540 is in fact built like most conventional motor controllers, the input current will be the same as the output current (assume extra current consumed for control circuits etc in the motor controller is negligible). You know the resistance of the resistor bank, and you know the total power output, so you can easily calculate current through the motor controller. Now, knowing Iin and the delta V across the motor controller, you can calculate the amount of power consumed in the motor controller. This power is dissipated as heat.

As you can see, from the 50A load test on the MindSensors site, this motor controller does, in fact, get HOT (125C after 5 minutes at 50A), and still climbing... Seems to me like a lot of energy lost to heat in the SD540, not to mention a potential safety hazard...
http://www.mindsensors.com/content/7...haracteristics

Re: SD540 Motor Controller
 

Stupid questions but the casings for the SD540 controllers seem to be 3D printed. What is the plastic they are printed with? Is it ABS? Doesn't ABS begin to melt around 110C or so?

Re: SD540 Motor Controller
 

80-125 degrees C is the glass transition temperature for most variants of ABS. So it will start to get soft there but that doesn't mean the entire case will melt at all. If it was enough to liquefy the case then we wouldn't 3D print ABS at much higher temperature.

At worst it would likely melt the FDM layers together making it more a solid than it started.

I often print ABS over 225 degrees C and vacuum form over 150 degrees C.

To put that temperature in perspective your heated 3D print bed to keep that ABS from warping can be 110 degrees C.
So if the heated print bed isn't turning it into a puddle or ruining it...

Re: SD540 Motor Controller
 

Like I said, stupid questions... Thanks for explaining though.

Re: SD540 Motor Controller
 

Not a stupid question at all :). I had noticed this as well and wondered too.

Re: SD540 Motor Controller
 

Wow! Go on vacation and lots happens.

No! A typical FRC motor controller will not have any significant energy storage elements in them so the input current is going to be pretty close to the output current. The difference between the input current and the output current is the current consumed by controller circuitry and should be insignificant at high motor currents. The motor's inductance does not have the effect you think it has.


Yes! Stop talking about cell phone chargers. The circuit topology is totally different and the energy transfer mechanisms are also totally different and are not comparable with what goes on in the motor controllers used in FRC.


Thanks CTRE for posting test results that are well documented and give the test conditions. I used the values to estimate the losses in each of the motor controller using the data from the 11.05 V input case.

Iout = Vout / 0.2 Ohm
Pcont = dV * Iout

Victor SP - 12.4 W
Talon SRX - 15.1 W
Spark - 21.8 W
SD540 - 44.3 W


The high voltage drop and high watt loss in the SD540 is consistent with the data that Mindsensors has published showing excessively high temperatures on their heat sink (125 degree C, and climbing).

Does anyone have an SD540 where they have opened it up, or are willing do so, and report what sort of MOSFET's are used in it?

Re: SD540 Motor Controller
 

Thank you all for your feedback!

The SD540 is equipped with a battery safety feature that would disable operation when battery voltage drops below 9.5 Volts.

Based on your concerns we have realized that such a safety feature is not desired by the FRC community, so we have released a model that does not include this feature. The new SD540B will work down to 6 Volts without disabling any operations! Here is our voltage comparison chart:
http://www.mindsensors.com/content/7...540-and-sd540b

Since our design and production facility is local and our SD540s are made here in USA, we have the advantage to implement changes quickly. The new model SD540B is available now and ordering information is here: http://www.mindsensors.com/frc/159-sd540-model-b

Re: SD540 Motor Controller
 

Can you comment on the appreciable efficiency difference? Will you be able to fix that as well before kickoff?

Re: SD540 Motor Controller
 

regardless of anything else, making changes that quickly is pretty darn cool.

Re: SD540 Motor Controller
 

Which motor control microcontroller is buried in there?