Standard Motor Driver Schematic for FRC

[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.

[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.

[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!

[Ether]

Originally Posted by GreyingJay

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.)

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...

[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.

[Oblarg]

Originally Posted by Alan Anderson

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,

Should anyone doubt this, I'd suggest they direct their attention to the (now thankfully defunct) Jaguar motor controllers, which were made by a professional company who (ostensibly) knew what they were doing and still suffered from all sorts of reliability issues.

[ProfessorAlekM]

I was looking up the encoders on AndyMark. The encoders they sell are only $20 each when you buy them in bulk (50), and they sell them for less if buy more (it says I have to ask for a quote if more then 50), but they sell them for $40 each.

And I'm sure shipping +$5-10 in profit is not $20.

They also sold every team a base kit that can't even clear any of the obstacles, so that you would then have to buy another base kit or upgrades (Making them more money).

On top of that they knew what this years competition was, and they couldn't even keep Rhino Treads in stock during build. Some supply and demand could be going on.

I know that I'm getting off-track on my own thread, but I'm trying to conserve money for our school.

[Whippet]

Originally Posted by ProfessorAlekM

They also sold every team a base kit that can't even clear any of the obstacles, so that you would then have to buy another base kit or upgrades (Making them more money).

On top of that they knew what this years competition was, and they couldn't even keep Rhino Treads in stock during build. Some supply and demand could be going on.

I'm going to correct you on just this one point, since there will be plenty of people better able to address your concerns about motor controllers specifically. This year was the first time that FIRST revealed elements of the game to certain suppliers prior to kickoff. The only thing that Andymark or VEX knew prior to kickoff was that there would be obstacles for the robots to traverse and (I think) that it was a ball game. Nothing was disclosed of the size, type, or nature of the obstacles or balls. Accordingly, Andymark adjusted the Kit of Parts chassis to include 6 inch wheels rather than the 4 inch versions that have been provided for the previous two years. They did not know enough to guarantee that they would be able to move a sizable stock of Rhino modules, pneumatic wheels, etc. The same thing applies to VEX and West Coast Products.

The FRC suppliers have done an excellent job of handling supply shortages for the past few years. We recently purchased a set of hex shaft collars from Andymark and received a set of VEX collars - still in vex packaging - in the Andymark shipment. These companies do everything they can to help make sure your robot gets completed in time and I have nothing but good things to say about them.

[Kevin Sevcik]

Originally Posted by ProfessorAlekM

I was looking up the encoders on AndyMark. The encoders they sell are only $20 each when you buy them in bulk (50), and they sell them for less if buy more (it says I have to ask for a quote if more then 50), but they sell them for $40 each.

And I'm sure shipping +$5-10 in profit is not $20.

They also sold every team a base kit that can't even clear any of the obstacles, so that you would then have to buy another base kit or upgrades (Making them more money).

On top of that they knew what this years competition was, and they couldn't even keep Rhino Treads in stock during build. Some supply and demand could be going on.

I know that I'm getting off-track on my own thread, but I'm trying to conserve money for our school.

Andymark doesn't get specific game information ahead of time any more than any team does. I believe Andy said they were told there would be obstacles in this game or something like that, which inspired the rhino tracks. Makes it hard to design for specific obstacles. It also makes it hard to pre-buy enough stock to satisfy the demand of 3000 teams. It's obvious after kickoff that there will be a run on pneumatic tires, or banebot wheels, or whatever the hot item is. It's never obvious before kickoff.

On the cable, I suspect you didn't actually specify the encoder correctly on US-Digital. When you specify one spacer and centering tool per encoder, the price per 50 is $28. And then you add a $5 cable on top. Andymark and Vex don't have a huge markup. They can't because if they did, they couldn't compete with other suppliers like Robot Marketplace, Robotshop, Mouser, Digikey, various industrial supply houses, or any random Joe that sets up an EBay or Amazon shop. It's called capitalism.


Honestly, the best way to save money for your school is to not break things so you can save what's salvageable from year-to-year. After about 3-4 years of not exploding motor controllers or melting motors, you'll have plenty for whatever robot you're going to build. A few more years and you'll have plenty for two robots. The best way to save money in a particular season is to CAD and actually design your robot so you can minimize buying the wrong thing or express shipping something you need in 2 days before competition or bag.

[philso]

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.

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).

Originally Posted by ProfessorAlekM

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.

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.

[GreyingJay]

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!

[Sperkowsky]

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.

[GreyingJay]

Originally Posted by Sperkowsky

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.

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.