Standard Motor Driver Schematic for FRC

[ProfessorAlekM]

Hello!

Recently I've been calculating the cost of our robot, and it is waaay overpriced. Part of the problem is that the rules force you to buy motor drivers from specific companies like VEX, who charge way more that they should. I get that the company has to make a profit, as well as pay for the cost of design, but $60 for a victorSP?!?! It would only cost $5-$25 to make a motor controller with the same functionality.

I hear that the reason why they don't want people making their own motor drivers is because they don't want people making circuits that could break, or build a motor driver that may keep driving even after it stops receiving power from the roboRIO.

My proposed idea:
Create a set of standard and FRC approved schematics so low budget teams can make many more motor drivers for their bot with much less money.

[GreyingJay]

The Talon SRX is only slightly more expensive than the Victor SP and has many nice things under the hood including full software control over the CAN bus and the built-in ability to interface with encoders, limit switches, and do PID control.

From a value proposition - and taking into account the typical skill level of high school students - this is hard to beat.

While you are correct that, at heart, a basic FRC motor controller could just be a PWM-controlled H-Bridge, I think there's a lot to like about buying a COTS part versus making our own. The fact that it's COTS, already, gives a lot of advantages when playing by FRC rules in terms of being able to swap them out easily for repair, replacement, or upgrade. I would hate to be in a competition situation, with 15 minutes until our next match, combing over a half dozen custom-made motor control circuits trying to figure out why our robot doesn't move. Plus they're sealed, packaged robustly and compactly, made to interface with standard components and connectors, and offer quick easy tools for changing brake/coast modes, recalibrating, etc.

And to top it off, they can be free for FRC teams through the rookie kit and through yearly product donation vouchers such as FIRST Choice. My team ordered four Victor 888 controllers through FIRST Choice and had the choice of either 2 Talon SRX's or 3 Victor SP's from the PDV, all "free". In addition, we got another two Victor SP's in our rookie kit. That's plenty for a robot drivetrain and mechanisms.

FRC is an expensive competition, but motor controllers are not where I'd peg most of the cost.

[RyanN]

I'll just leave this here: http://www.amazon.com/REV-Robotics-S...tor+controller

Comes with spare screws and a PWM cable for $45 shipped.

[Knufire]

Yeah, the VictorSP is the "premium" product in FRC. You pay more for the compact size, conformal coated board, the aluminum heatsink body, etc. If you're looking for a better price/performace ratio, look at the REV controllers.

[AdamHeard]

All of the motor controllers from the various FRC approved vendors are priced very, very well for what you get.

[Jon Stratis]

Originally Posted by ProfessorAlekM

My proposed idea:
Create a set of standard and FRC approved schematics so low budget teams can make many more motor drivers for their bot with much less money.

How would you suggest making this inspectable? The inspectors are not all electrical engineers, and looking over each circuit to make sure it was both correct and safe, even from someone with applicable experience, would take some time, and I've heard complaints on CD before about inspections taking too long as it is. And when one doesn't work, most teams don't have the expertise to properly debug a circuit like that, and finding the expertise to help at an event may be difficult.

[Coach Seb]

Since the Talon SRX were part of the KOP as a voucher from Vex, you do not need to account on the CAW for your robot.

[Jared]

I don't think too many teams have the resources/skills to make motor drivers for less than the cost of something like the Spark given only schematics.

Even though you might be able to buy all the components for real cheap on Digikey, you don't have a circuit board. Some weird parts like gate drivers might only come in surface mount packages sometimes. Even if you did find all the parts in through hole packages, you need to worry about all sorts of electrical interference issues - the FRC controllers don't have isolated logic and power, so you'd need to be clever with how you laid out your board to prevent transients from messing with the microcontroller.

Then, you'd have to figure out how to program the microcontroller. I have yet to find a through hole microcontroller that supports a direct USB connection, so you'd need to buy some sort of programming board or get really good a surface mount soldering.

[teslalab2]

Originally Posted by ProfessorAlekM

I get that the company has to make a profit, as well as pay for the cost of design, but $60 for a victorSP?!?! It would only cost $5-$25 to make a motor controller with the same functionality.

Unless you started mass producing it would actually be very expensive to too make a motor controller even like the old talons or victors, there is a lot of engineering that goes into them first off. PCBs are very expensive in small quantities, and components are very expensive in small quantities. These PCB's are not necessarily something a home gamer could produce either, you need via's in the board to connect top and bottom traces, as well as plated through holes for that parts to handle the current and connect top and bottom traces. I know this because I work in a circuit board shop.

[GreyingJay]

Originally Posted by Jared

I don't think too many teams have the resources/skills to make motor drivers for less than the cost of something like the Spark given only schematics.

Even though you might be able to buy all the components for real cheap on Digikey, you don't have a circuit board. Some weird parts like gate drivers might only come in surface mount packages sometimes. Even if you did find all the parts in through hole packages, you need to worry about all sorts of electrical interference issues - the FRC controllers don't have isolated logic and power, so you'd need to be clever with how you laid out your board to prevent transients from messing with the microcontroller.

Then, you'd have to figure out how to program the microcontroller. I have yet to find a through hole microcontroller that supports a direct USB connection, so you'd need to buy some sort of programming board or get really good a surface mount soldering.

The expected audience for this particular scenario seems very small: a team with a low budget that does not want, or cannot afford, to buy motor controllers, but can buy the discrete components, with the students having access to PCB manufacturing equipment, and who are (will become) skilled in circuit design, component layout, soldering, testing, etc.

If this describes your students, then a home-made motor controller might be an excellent off-season project to develop an intimate familiarity with how they work. I daresay that after the experience they will also gain an appreciation of how much work goes into a motor controller and how much value you get for the $40-60.

When I learned to do woodworking one of my first projects was an Adirondack chair. I found plans online, purchased pine boards, and started marking, measuring, cutting, shaping, sanding, routing, drilling, assembling, gluing, clamping, more sanding, staining, more sanding, etc. I probably spent $100+ in raw materials, and dozens of hours of my time. Then one day I walked into Costco and they had Adirondack chairs for $49 each...

[ProfessorAlekM]

You would be surprised would students can do. It's really not hard to make motor controllers, I've made them myself before and I'm only in 9th grade.

Although it doesn't seem like it, the point I'm trying to argue is that FRC would be much cheaper if they would give students the ability to create and buy their own motor controllers. When you calculate it, our team has spent hundreds of the dollars on motor controllers.

Just because something is in the kit, doesn't mean it's free.

[efoote868]

Originally Posted by Jared

I don't think too many teams have the resources/skills to make motor drivers for less than the cost of something like the Spark given only schematics.

Even if someone were to do the board layout, and get the PCBs fabricated and sold at cost, I would not trust a high school student populating the boards.

Troubleshooting even some of the simplest circuits can take a very long time, and without the right equipment (something a low budget team likely does not have) nearly impossible.

Also, having a 17 amp hour battery connected to a bad solder job is a fire hazard...

[Richard Wallace]

Originally Posted by ProfessorAlekM

It's really not hard to make motor controllers, I've made them myself before and I'm only in 9th grade.

I applaud your motivation. However, as someone who started making motor controllers more than 30 years ago, I should advise you that making them inexpensive and reliable (at the same time) requires significant attention to details that you have not had time or experience to learn yet.

The lead designers at CTRE and REV watch CD and contribute regularly. They are very smart, experienced people. And they are competing with each other. Good, competitive designs are driving motor controller costs down, and performance up. Maybe you can help one of the competitors do that even better, very soon.

[GreyingJay]

Originally Posted by ProfessorAlekM

You would be surprised would students can do. It's really not hard to make motor controllers, I've made them myself before and I'm only in 9th grade.

I'm not saying I don't think students can do it. But I am saying students of your skill level, especially at your age, are not common. And as others have said, there's so much potential for error and frustration and even danger that I can see why they don't allow it.

When I was in grade 9 I took out a sheet of paper and sketched a schematic for a simple computer featuring a Z80 CPU (with EEPROM, RAM, basic I/O, etc.) Sadly I never got to build it. I applaud your ability and the opportunities you have already taken advantage of.

Originally Posted by ProfessorAlekM

When you calculate it, our team has spent hundreds of the dollars on motor controllers.

Just because something is in the kit, doesn't mean it's free.

Well, sure. But there's a lot of things that we spend money on. The entire RoboRio control system, especially with the PDP, is quite pricey. Gearboxes are expensive. Wheels, once you add the hubs, are pricey. And did you see how much the boulders cost this year?! Turns out the motors themselves are actually some of the cheapest components in the robot!

[Alan Anderson]

Originally Posted by ProfessorAlekM

You would be surprised would students can do. It's really not hard to make motor controllers, I've made them myself before and I'm only in 9th grade.

Ah, to be a teenager and know everything again.

It's not hard to make a motor controller.

It's hard to design one that interfaces with standard hobby servo signals, provides sustained performance at 40 amps and occasional transients approaching 200 amps, is proven to perform as advertised so the robot can be certified as following the rules, is rugged enough to withstand the abuse it will take on an FRC robot, doesn't pose shock or fire hazards to its users, etc. It's hard to build one to those specifications without having resources, skills, and experience.

...our team has spent hundreds of the dollars on motor controllers.

If you're expecting to be able to save "hundreds of the dollars" by building your own motor speed controllers, I suspect you're failing to consider a lot of details.