Re: Standard Motor Driver Schematic for FRC
 

When I was a 9th grader most electronics in the home (radio,television,HiFi) were tube based. Even the sound system in our neighbor's car had vacuum tubes (you had to wait several seconds after turning the car radio on for the tubes to warm up before you could hear any sound).

When your TV or radio got flaky, you'd pull all the tubes out and take them to the corner drug store and test them in the tube-tester kiosk.

For my 13th birthday my parents bought me the RCA Tube Manual. I memorized large portions of it. I designed and built my own audio power amp using push-pull 6L6GC tubes. Almost electrocuted myself when I went to re-heat a bad solder joint and forgot about those 400V filter capacitors in the 5Y3 power supply.

It was a different world back then...

Re: Standard Motor Driver Schematic for FRC
 

What sort of continuous output current was your motor controller capable of? How efficient is it? How durable was it, especially when abused? How long has it lasted? How does it behave during fault conditions (does it fault in a safe and benign way)?

A schematic is only a starting point for a successful piece of equipment. A schematic for a good motor controller will look a lot like the schematic for a bad motor controller. You still need an enclosure to keep the circuit board from shorting to adjacent pieces of metal and to keep the metal shavings out. You will need to have a suitable heatsink. Do you know how to select a heatsink that performs adequately yet is not too big and expensive? The higher the current in the circuit and the higher the edge rate of the switching, the more critical the track layout on the PCB is. The layout of the circuit board suitable for FRC motor controllers will not be trivial. A poorly done layout will have high inductance paths, leading to high surge voltages and possibly destruction of the switching devices. A common cure for high surge voltages is to slow the edge rate of the switching but this causes higher losses and heating and lower efficiency and possibly destruction of the switching devices.

There is already a student designed motor controller out on the market. Test results published on this forum show that it is less efficient than other motor controllers. I have been told that it uses twice as many MOSFETs as other controllers indicating that the devices are not being operated in an efficient manner. It is quite likely that the board layout was not done in an optimum way (see above).


What is your team's time worth? Are you going to propose building your own controller too (because the processors are under $10) instead of purchasing a RoboRio ($435). This is a robot competition, not a motor controller competition.

It is likely that providing just a schematic will lead many teams to disaster. Not all of them will be able to obtain the proper parts and substitute a part that "looks close enough" (yes there is a difference between two parts with the same nominal rating). Most of the parts that will be suitable are "surface mount" types. Most people I have met (including engineers working in the field) do not have the skills needed to successfully hand solder surface mount components correctly.

There is at least one other individual who has posted in this thread who has indicated that they have to professional experience in this field to comment in an authoritative and credible way and they are all saying that there is a lot more to have a successful power electronic circuit than just having a schematic. My background is doing the design and development for motor controllers ranging from 5hp to 1200hp over the last 20+ years. I use many of the same design techniques necessary to make a successful FRC motor controller and it took me (and the others posting here) much longer to learn these techniques than a high school student would have. A few years ago, I visited a startup company that had a microprocessor guru/consultant who sold them on the idea that he could design a complete motor controller for them because he got hold of a generic schematic. I saw their prototypes (as well as the scorch marks on their equipment) and it was pretty easy to see what they were doing wrong. That start up closed down pretty soon afterward.

Re: Standard Motor Driver Schematic for FRC
 

There is a time and a place for saving money. DIY is sometimes the way to do that, but sometimes it is not.

2706 is a new team. We started from scratch last fall so we have had to buy everything new including all our tools, all the stuff on our robot and all the extra stuff we needed for prototyping and testing. Believe me we understand wanting to save money. There were many things we did ourselves in the interest of saving team money. Among them was judicious shopping for parts we could have bought from one popular supplier but found cheaper elsewhere. Motor controllers fall in this category, I think, especially with new controller options on the market now. You can even buy them from Amazon!

Re: Standard Motor Driver Schematic for FRC
 

It takes more time to make custom motor controllers then finding a sponsor or two that will give you enough money to buy them. Its a cool learning experience but a waste of time in a 6 week season.

Re: Standard Motor Driver Schematic for FRC
 

This is a very good point. In one Saturday of concerted effort our team can execute a community bottle drive that raises about $800-1000. You might be able to get even more by pounding the pavement and talking to shops and businesses in the area.

Re: Standard Motor Driver Schematic for FRC
 

I applaud my students who want to learn how to make custom circuits or custom anything. While our team doesn't do it much yet, my understanding is the RoboRio allows for this a lot more than past controllers (I may be wrong, programming/controls is my weak spot)
That being said, these are the things that control a 120 pound robot going fast enough to hurt someone. They also control a huge amount of amperage. I trust my students a lot, but not when it could be something where a mistake could greatly injure someone else.
Further, out of all of our costs, motor controllers are pretty small. Yes, we did dump about $2k into the Talon SRX's when they came out, and I do understand thats a lot of money, but you can easily get away with a couple hundred and still have the Talon SRX and not a cheaper option like the REV one if the options the SRX delivers are important to you
One big thing you're missing too is these are reusable. That was largely a one time cost compared to motors/gearboxes/framing/etc which far exceeds 2k a year.

It would be great to save money in FIRST, but I dont think this is where to look. If money is an issue with a team, look at the structural components as a way to save money. Reusing simple small things like hex clamps, sprockets, or bearings adds up to far more than a few speed controllers.

Re: Standard Motor Driver Schematic for FRC
 

Thank you for bringing this point into the discussion. FRC approved motor controllers are durable.

This year we are running several Talon SRs that we've used in two previous seasons. We also have the same number of new SPARKs, which are on our practice robot. After those have proved reliability we intend to re-use them on future robots. To date we have had no failures of Talon SRs (which are obsolete) or of SPARKs.

Re: Standard Motor Driver Schematic for FRC
 

I think that most people in this thread have summarized the reasons why letting teams make their own motor controllers is not a practical idea, as there are so many things that can happen when hand assembling something to handle this amount of power. Even if 99% of the motor controllers were done properly the 1% of them that fail can cause fire, robots not responding in a controlled fashion, and a bunch of other issues.


Some people might not have realized this, but we have published the schematic for the SPARK at the back of our user guide. We believe that people should be able to learn from the product itself and not just the use of the product. Here is the direct link (PDF) http://www.revrobotics.com/wp-conten...1200-UM-00.pdf

If any student wants to create their own "homebrew" version of the SPARK we encourage you to try, because reproducing is a great way of learning. For a low cost way to make your own PCB's, my friends own Dirtypcbs.com (and regardless of their name make good quality cheap boards)

Re: Standard Motor Driver Schematic for FRC
 

Greg, I didn't know this and I'm actually really happy. It'll at least help me figure out whether it's worth the effort to put together a custom ESC for a project that falls nicely in that gap between what a VexController 29 can drive (which is actually quite a bit more than you'd think) and where I'd use a Spark (I'm cheap, like REALLY cheap) There exist a handful of brushed ESCs from China in that range but, frankly, their english documentation and my knowledge of the Chinese language seem to share the property of being bad/nonexistent.

Re: Standard Motor Driver Schematic for FRC
 

I'm curios but those diacs on the motor out put, are those to protect the motor controller from ESD or from power backfeeding from the motor? Or am I way wrong?

Thanks

Re: Standard Motor Driver Schematic for FRC
 

I highly commend you for this.

Re: Standard Motor Driver Schematic for FRC
 

I'm sure that most of these teams when given the schematic will just arbitrarily place components onto the board layout and hit autoroute. :rolleyes:

There's a reason for everything!

Re: Standard Motor Driver Schematic for FRC
 

Also, the Victor SP was part of that voucher and part of FIRST choice, both of which make it exempt from the CAW.

Re: Standard Motor Driver Schematic for FRC
 

I'm somehow having difficulty picturing a young Ether. The only image that ever comes to mind is the one in the profile.

Re: Standard Motor Driver Schematic for FRC
 

How many of those issues are particular to high current designs? Does something sitting in the 15A constant 30A burst range have to worry quite as much about these issues?