Motor Controllers

[aldaeron]

Originally Posted by JPBlacksmiths

So this is a question that I have had for a few years now, and never really gotten a good answer to it:

Which motor controller is the best for which situation? Meaning is there specific applications were one controller is better then the other, or is there just one motor controller which is the best, period?

This question has resurfaced for me especially with the creation of the new motor controllers such as the spark.

Thanks,
JP
Co-Captain Team 806

Getting back on topic to answer OP's questions...

As with most things in engineering design, the answer here is "it depends". In my mind there are three key features to look at: cost, capability and reliability. I am only going to focus on the controllers you can buy going forward (Talon SRX, Victor SP, REV Spark, MS SD540). Since you can't get spares for other controllers easily I would stay away from or use them sparingly (and with LOTS of spares ready just in case).

Talon SRX at $90 is the most capable, but also the most expensive.

Capability: Talon SRX allow use of a Controller Area Network (CAN) bus. The other option is Pulse Width Modulation (PWM). With a bus you get two way communication between controllers and the RoboRio.

Also, the Talon SRX has an input data port that allows you to do some routine control calculations directly on the controller instead of using the RoboRio. You can hookup small breakout boards to the data port and do a number of control loops. Some examples are that you can hookup an encoder to a shaft and thru the data port and tell the motor controller to drive forward until the encoder has reached 1000 ticks and then stop. Another example is that you can tell the motor controller to drive forward until the upper limit switch on your elevator is tripped and then stop. With PWM the RoboRio has to check all these sensors and issue the commands to stop. Since the RoboRio is doing a lot of calculations it may be slower to respond (by some number of milliseconds) to when the switch is tripped on your elevator. The Talon SRX offloads these mundane control calculations from the RoboRio.

Cost: You pay for all these nice features (CAN bus chip, extra connector for data port inputs). Over a whole bot this extra cost can add up.

Reliability: Last season the Talon SRX performed well with few (if any) failures. Since there was no defense and therefore few long duration high current pushing matches, there could be an argument made that some more testing is needed under these extreme conditions. Most people would agree it is a solid and FRC tested design. It also has a very small foot print and a sealed aluminum enclosure. This prevents debris from getting in and shorting out electrical components (a problem with older controllers). Aluminum is a great heat conductor and allows the heat generated by the controller to be moved away from the electronic chips without the need for a separate fan.



Victor SP at $60 is a robust, but basic controller.

Capability: Talon SP is a simple Pulse Width Modulation (PWM) driven controller. No fancy data inputs, just the ability to set the speed between 0 and 100%.

Cost: You pay for a smaller form factor and an aluminum case. More expensive than other PWM controllers and does not have data inputs.

Reliability: Same as Talon SRX. Most would agree it is a proven, dependable controller for FRC.


REV Spark at $45 is a new low cost controller with some nice looking features.

Capability: REV Spark is a simple Pulse Width Modulation (PWM) driven controller. You can set the speed between 0 and 100%. It also has a basic input data port that will allow you to directly wire in a set of limit switches to stop it. These limit switches are great for an elevator or a rotary arm with a top/bottom limit switch. No fancy encoder or other control capabilities. Pretty impressive for such a low cost device.

Cost: Cheapest one on the market for FRC right now.

Reliability: This is brand new this year. Preliminary test data looks positive. It has a plastic case, which will not transfer heat as well as aluminum. It claims not to need fan cooling, which is being tested in lab conditions (I believe results release soon). Obviously the real test is to see it during the season. Be cautious about switching to a new controller with limited testing unless you can afford to replace the controllers. It is a bit larger than the Victor SP and Talon SRX. For the price it looks promising.


MS SD540 at $50 is a new low cost controller with some nice looking features.

The SD540 implements many of the same features that the REV Spark does, albeit in a different layout. There are some concerns about the design choices made for the SD540 that are detailed in another thread. Same overall story - low cost plastic body controller with PWM input and limit switch control build in. Untested.



Hope that helps. PM me for more detail (I glossed some of the nitty gritty details)

-matto-

[electroken]

Originally Posted by aldaeron

REV Spark at $45 is a new low cost controller with some nice looking features.

Reliability: This is brand new this year. Preliminary test data looks positive. It has a plastic case, which will not transfer heat as well as aluminum. It claims not to need fan cooling, which is being tested in lab conditions (I believe results release soon). Obviously the real test is to see it during the season. Be cautious about switching to a new controller with limited testing unless you can afford to replace the controllers. It is a bit larger than the Victor SP and Talon SRX. For the price it looks promising.

Part of my own evaluation of the SPARK was having a look inside. While much of the case is plastic, the SPARK label in the center is covering a heavy c-channel shaped slug of aluminum with heat transfer pads to encourage cooling of the 4 MOSFETs and 3 electrolytic capacitors under it. My gut feel (with nothing but experience to back it up) tells me this device will survive a 2-minute pushing match just fine.

The footprint is close, but not identical to that of the Talon SR.

[aldaeron]

Originally Posted by electroken

Part of my own evaluation of the SPARK was having a look inside. While much of the case is plastic, the SPARK label in the center is covering a heavy c-channel shaped slug of aluminum with heat transfer pads to encourage cooling of the 4 MOSFETs and 3 electrolytic capacitors under it. My gut feel (with nothing but experience to back it up) tells me this device will survive a 2-minute pushing match just fine.

The footprint is close, but not identical to that of the Talon SR.

I don't have one to look at, but what I said is still true based on what you are describing. The internal aluminum heatsink will draw heat away from the components, which is helpful, but you need to look at the path the heat takes after that. In the case of the REV spark, this means conduction through a plastic case of some thickness then out to whatever the controller is mounted to. Compared with the aluminum body of the Victor/Talon that does not have this additional layer of restriction. Keep in mind that plastics have Thermal Conductivity of <1 W/mK compared with Aluminum that has Thermal Conductivity of 200 W/mK. While it is true the footprint sizes are different, my guess is that this will not have a large effect, especially if there is no thermal paste/pad between the aluminum heatsink and the plastic case.

For effective thermal management I like mounting the controllers to a hefty aluminum frame member or sheet metal belly pan of decent thickness. Wood or plastic or heavily pocketed aluminum belly pans limit the ability to conduct heat away from the mounting location.

Perhaps the REV will be fine for most situations, but what I said is still true - the Victor/Talon will be better because there is no plastic in the thermal conduction path. I believe REV is planning to release test data soon. Hopefully they will instrument down to the package level or heatsink right next to the MOSFET case and will provide ThetaJC values for the devices. The junction temperature is what is key for long term performance and reliability.

It really depends on current draw of the motors and efficiency of the controllers during matches as to how much heat you really need to dump. But keep in mind that in a Best-of-3 Finals match that you do not have as much time to cool off as during a normal match. It can be a problem.

TL DR: REV may be fine, but Victor/Talon will be better for dumping heat.

-matto-

[electroken]

I'm not questioning any thing you've said, and I fully understand that the slug of aluminum only buys time, and in the end the vast majority of heat must be radiated to the environment through the 1.5-ish square inches of aluminum exposed to the air.

Having spent some time within the Talon SR and the Spark, it is my opinion that teams familiar with the Talon SR will see similar results from the Spark.