Lithion-Ion Batteries

[ToddF]

Originally Posted by JVN

This is the first time I've seen brushless motors suggested as a cost saving measure. Do you have a rough BOM of what you'd put in the kit spec'd out with pricing?

-John

I'll take our shooter from last year as an example. We used a pair of RS-550 motors ($7.25 ea) driven by a pair of Jaguar drives ($119 ea), for a total of $252.50 system price.

That motor is 36mm in diameter, has a Kv of 1608 rpm/V, max current draw of 85A and max power of 254 Watts.

For an apples to apples comparison, Hobby King has a 36mm diameter brushless inrunner (S3674-1660) for $39.20 each. This motor has a Kv of 1660 rpm/V, a max current draw of 60A, and max power of 1600 Watts.

To drive it, we would need a 60A ESC. The HobbyKing Red Brick 60A ESC is $13.20 each. This ESC is a fraction of the size and weight of a jaguar, though it is controllable via PWM only.

So, using the motors and power drivers mandated by last years rules, we had a total of 508 Watts of power available at our shooter wheel for $252.50 total cost. If we were allowed to use brushless motor technology, we could have had 3200 Watts of power at our wheel for $104.80 total.

More power, less weight, less than half the cost. Note that the real driver of the cost difference is the drastically lower cost of the drive electronics. They can get that cost so low by removing all the smarts from the controller and making a bazillion of them. I wouldn't want to eliminate Jags and CIMS altogether, just allow these better technologies as an option for use where appropriate.

[Jon Stratis]

Lets take a look at that S3674-1660 motor a bit more...

For an apples to apples comparison, Hobby King has a 36mm diameter brushless inrunner (S3674-1660) for $39.20 each. This motor has a Kv of 1660 rpm/V, a max current draw of 60A, and max power of 1600 Watts.

power = current x voltage. Running at 12V and 60A, the maximum power input into the motor is 720W. Where's the other 880W coming from? It's a perpetual motion machine!

Realistically, that 1600W comes from running it at higher current/voltages. That motor is supposedly rated to 22.2V, according to the website. But even at that voltage, 60A only provides 1332W of input power!

So, all of these numbers need to be taken with a huge grain of salt. Without actually testing the motor with our system (in other words, take your proposed set up and hook it up to an FRC robot) and measuring the output, the comparison can't really be Apples to Apples.

Your numbers significantly overestimate the power this motor/controller combination would provide you.

[AdamHeard]

Originally Posted by ToddF

I'll take our shooter from last year as an example. We used a pair of RS-550 motors ($7.25 ea) driven by a pair of Jaguar drives ($119 ea), for a total of $252.50 system price.

That motor is 36mm in diameter, has a Kv of 1608 rpm/V, max current draw of 85A and max power of 254 Watts.

For an apples to apples comparison, Hobby King has a 36mm diameter brushless inrunner (S3674-1660) for $39.20 each. This motor has a Kv of 1660 rpm/V, a max current draw of 60A, and max power of 1600 Watts.

To drive it, we would need a 60A ESC. The HobbyKing Red Brick 60A ESC is $13.20 each. This ESC is a fraction of the size and weight of a jaguar, though it is controllable via PWM only.

So, using the motors and power drivers mandated by last years rules, we had a total of 508 Watts of power available at our shooter wheel for $252.50 total cost. If we were allowed to use brushless motor technology, we could have had 3200 Watts of power at our wheel for $104.80 total.

More power, less weight, less than half the cost. Note that the real driver of the cost difference is the drastically lower cost of the drive electronics. They can get that cost so low by removing all the smarts from the controller and making a bazillion of them. I wouldn't want to eliminate Jags and CIMS altogether, just allow these better technologies as an option for use where appropriate.

Your comparison is completely unfair.

You are comparing US made versus Chinese made, you are also comparing an esc optimally sized for operation versus one far oversized for operation.

I love RC cars, and spend way too much money on hobbyking and can say with certainty I have used more motors and ESCs from them than anyone on chief. I love them for personal use, and a lot have surprised me with their performance and durability, but I would not put them on an FRC robot.

[ToddF]

Originally Posted by Jon Stratis

Realistically, that 1600W comes from running it at higher current/voltages. That motor is supposedly rated to 22.2V, according to the website. But even at that voltage, 60A only provides 1332W of input power!

...

Your numbers significantly overestimate the power this motor/controller combination would provide you.

Very true, and good catch. I just took their numbers without checking their math first. My bad.

Let's take a look another similar motor listed: XK3674-1650KV Brushless Inrunner.

Specs.
# of poles: 2
Max Amps: 66A
Max Volt: 14.8V
Max Watts: 975W
Rpm/V: 1650kv

Running the math, the quoted max power actually equals the max current x max voltage. Derating to the 12 V on a FIRST robot (nominally) gives 792 Watts.

This one costs $31.86, so we get 1,584 Watts of power for $90.12 total.

[Andy A.]

Originally Posted by JVN

This is the first time I've seen brushless motors suggested as a cost saving measure. Do you have a rough BOM of what you'd put in the kit spec'd out with pricing?

-John

I couldn't even begin to price that kind of thing, but it occurred to me I couldn't really price out the motors currently in the kit either. I know many moons ago nearly everything in the kit, motors included, were donated.

These days the number of kits has obviously grown and motor selection is a little more elaborate. Does anyone know how much of the in-kit motors are donated, purchased or ___? What about motors made available for purchase by teams? Are those discounted somehow?

Now that I think about it, I miss some of the old motors. The Globe motor was awesome.

[Jon Stratis]

Originally Posted by ToddF

Very true, and good catch. I just took their numbers without checking their math first. My bad.

Let's take a look another similar motor listed: XK3674-1650KV Brushless Inrunner.

Specs.
# of poles: 2
Max Amps: 66A
Max Volt: 14.8V
Max Watts: 975W
Rpm/V: 1650kv

Running the math, the quoted max power actually equals the max current x max voltage. Derating to the 12 V on a FIRST robot (nominally) gives 792 Watts.

This one costs $31.86, so we get 1,584 Watts of power for $90.12 total.

Also reduce it to the maximum currents allowed on an FRC robot (it has to sit behind a 20, 30 or 40A breaker, remember!), and you're down to 480W.

I find it impossible to believe that this motor operates at 100% efficiency. There's no power loss at all due to friction in the bearings, or in generated heat in the motor?

The motors we deal with in FRC have been tested to death by teams, and the posted numbers are typically peer reviewed and confirmed to represent the output power available from the motor. The math we've done here can easily be applied to any of our motors to find the input power available to the motor, with identical results.

The key here is to look at the efficiency of the motor - that's where you'll be able to see actual gains in output power to your mechanisms. A supplier like BaneBots provides their motor efficiency right on the website. I ran a quick search, and I can't find any posted efficiency numbers for either of the motors you mentioned.

Granted, brushless motors are typically more efficient than brushed, but unless we know exactly how much more efficient, we can't really do a comparison. Is going from 70% efficiency to 80% efficiency (for example) worth ditching our current suppliers and sponsors? Is it worth ditching the known performance and longevity of our motors for new motors we have no experience with? Remember the debacle with the RS-775 case short a couple of years ago? I wouldn't want to see us make a drastic switch in our motors only to encounter something like that again... or worse!

[AdamHeard]

Keep in mind you can't run at max power, nor even close.

These are non-sensored brushless motors, they aren't meant for torque or control loops, they will stutter (known as cogging) and fail to start.

They might work for conveyors, some might work for flywheels if they are way overpowered, but in most applications these low cost non-sensored brushless motors are not suitable for FRC.

[IndySam]

Originally Posted by Jon Stratis

Also reduce it to the maximum currents allowed on an FRC robot (it has to sit behind a 20, 30 or 40A breaker, remember!), and you're down to 480W.

I find it impossible to believe that this motor operates at 100% efficiency. There's no power loss at all due to friction in the bearings, or in generated heat in the motor?

The motors we deal with in FRC have been tested to death by teams, and the posted numbers are typically peer reviewed and confirmed to represent the output power available from the motor. The math we've done here can easily be applied to any of our motors to find the input power available to the motor, with identical results.

The key here is to look at the efficiency of the motor - that's where you'll be able to see actual gains in output power to your mechanisms. A supplier like BaneBots provides their motor efficiency right on the website. I ran a quick search, and I can't find any posted efficiency numbers for either of the motors you mentioned.

Granted, brushless motors are typically more efficient than brushed, but unless we know exactly how much more efficient, we can't really do a comparison. Is going from 70% efficiency to 80% efficiency (for example) worth ditching our current suppliers and sponsors? Is it worth ditching the known performance and longevity of our motors for new motors we have no experience with? Remember the debacle with the RS-775 case short a couple of years ago? I wouldn't want to see us make a drastic switch in our motors only to encounter something like that again... or worse!

I get what you're saying, but we can't be afraid to try new things just because they are new and untested in a FRC robot.

We have all been bitten by that bug in the past but we can't be afraid to move on. Who can't forget the debacle of the BaneBot gearboxes in 2007? Those were fun times.

[Stephen Kowski]

Originally Posted by artdutra04

Lithium Iron Phosphate chemistry shows potential, but is still too new and costly.

I find the cost concern here difficult to reconcile. Every year there are many teams with a double digit number of batteries housed in large rolling cabinets. They have them labeled as practice, competition, good, ok, bad etc.

It seems to me that teams have a large volume of batteries to counteract a few apparent problems:
1) the batteries maintain inconsistent charge levels
2) these batteries degrade quickly over time needing consistent replacement and
3) the batteries cannot cope with a large C ratings while charging or discharging which causes the electrode/cathode degradation and results in 1) and 2).

Personally, I would rather have 1 or 2 batteries that work reliably even if they cost more rather than 5-10 batteries I have to babysit. At the very minimum it would be nice to have an option, any option, because right now there is no alternative.

[Ether]

Originally Posted by ToddF

Running the math, the quoted max power actually equals the max current x max voltage. Derating to the 12 V on a FIRST robot (nominally) gives 792 Watts.

792 is the electrical power input to the motor, not the mechanical power output.

For reference, the brushed motors used in FRC all have less than 50% efficiency at max power. Brushless motors share many of the same losses that brushed motors have, like windage, bearing friction, eddy currents, winding resistance, etc etc. I doubt that brushless motors are anywhere near 100%.

Sensorless brushless motors cannot operate at high torque at low speeds because of commutation issues.

[Jon Stratis]

Originally Posted by IndySam

I get what you're saying, but we can't be afraid to try new things just because they are new and untested in a FRC robot.

We have all been bitten by that bug in the past but we can't be afraid to move on. Who can't forget the debacle of the BaneBot gearboxes in 2007? Those were fun times.

I have no problem trying new things, I didn't mean for my posts to come off that way. I'm really trying to say here that we just need more information and testing done before these motors should be seriously considered for use by all teams. We'd need to understand their properties and be able to compare their specs directly to the "tried and true" motors we're familiar with, something no one has been able to provide yet.

I'd love to see a team adopt a couple of these motors and run them through their paces through a couple of off-season competitions. Find out how well they hold up, figure out how efficient the motors are. We're supposed to be engineers here... do we field prototypes as production equipment? Should we include a new motor we know little about and have no experience with in a Kit of Parts for thousands of teams, when we aren't even sure what the benefit (or drawback) of such a motor is?

I'm not saying we shouldn't look at these... I'm saying we need to make sure we understand the true benefits and costs before we start arguing for adopting them.

[ToddF]

I'm a fairly new mentor, so some of my thoughts may not be as seasoned as the veteran folks out there. I'm seeing some assumptions in some of the responses that I don't understand.

There seems to be an assumption that for something to be legal for use on a FIRST robot, 1) its performance needs to be completely characterized, 2) it needs to be supplied to all teams in the kit of parts, and 3) it replaces whatever components were previously legal.

Where I can see how this MIGHT apply to control system components running software, I don't see why it should apply to things like actuators. I don't see a downside in allowing brushless motors and drivers to be included in the list of options we consider. Maybe they aren't right for certain applications, but for others, they might be. Why not allow teams who want to use them to save money to do so?

AdamHeard rightly pointed out that the Jaguars (and Victors) are far oversized for the shooter wheels. So why do the rules limit us to using those and only those motor drivers? Why not allow teams to use other options?

Reading between the lines, it seems that at some point in the past, cheap motor drivers were not available, so in order to ensure a supply of reasonably priced drives, FIRST worked with suppliers to come up with custom motor controllers. To ensure that the market was large enough to keep prices down, they made rules that limited teams from using anything else. I would argue that the commercial marketplace for inexpensive motors, drives, and controllers has caught up and bypassed the needs of FIRST. Inexpensive, rugged motors and drives, both brushed and brushless, are commonly available in the RC marketplace. For the $285 cost of the cRIO FRC II Controller, you can buy EIGHT Raspberry Pi computers (I have an order of three arriving today.)

During build season, I'm all about doing the best you can with the limited set of components allowed. But, with cheaper, possibly better, components available in the commercial marketplace, why not expand the rules to allow us to use them?

Again, I apologize for hijacking the OP's thread. If anyone knows how to split this topic off into a new thread, please do so...

[EricH]

Todd, I think I can address some of those. We'll let the third assumption slide as that's not necessarily true--the ARA donation and the BaneBots motors are good examples.

The second assumption is VITAL. If you're going to make a new technology legal, ALL TEAMS must be able to obtain it, preferably with minimal research. The easiest way to do that--and cheapest--is for FIRST to buy about 2000 sets of the item and distribute it via the KOP. Otherwise, you get the vouchers which may or may not be of any practical use. If it isn't in the KOP, it should be available from AndyMark/IFI/BaneBots/local retailers.

As far as the first assumption, YES! You do have to have characterizations! (We'll assume fairly complete, but not completely complete.) Why, you ask? How else are you going to size motors properly for the applications? Not all teams have the equipment to test the motors, or the budget to replace ones that fry during testing. Not having even an incomplete characterization can result in teams frying motors left and right--and even with an incomplete one... See Tetrix motors on minibots in 2011.


The paragraph on the reason for the rules, however, missed even the hay bales behind the target. That's not the reasoning at all. The original reason for the restrictions on the control system were so that FIRST technical staff (who are still comparatively few in number) didn't have to troubleshoot 40 different systems at a single event--and that is still a huge reason. Once IFI came along, it made life easier, as it was fairly easy to get teams a new control system every year--and teams with experience still managed to make dumb mistakes every year. The cRIO system was more powerful--and had a lot more issues the first couple years. See "limited technical staff not having to troubleshoot 40 different systems at each event"--and read the Einstein report again for how some teams, who've been using the system for years, made mistakes.

Having the control system the same is important, not because cheaper systems aren't powerful enough or tough enough, but because if you open the rules to more than one system, you introduce nightmares. FIRST recently put out documents indicating that they are looking for a new system--look at those, and see if you can come up with anything within their guidelines.


The motor system is the same way, except that it's limiting the maximum available power from motors instead of making the technical group's life easier. Same available motor pool = same available maximum power = no advantage unless you can put more motors from the pool on the robot and control them. It's been opening up a bit lately, but I think the main reason for brushless motors not being allowed is that the legal speed controllers can't handle them. But when you open up the pool of legal speed controllers, you have to make adjustments to the control system, somehow, or force the controllers to work with the system.

That's why, until the new control system comes out, I don't see brushless motors being legal, unless it's on a very limited basis (as in, the 2 motors must be Brand X, Motor Y, and each running on one ESC with BEC Model Z).

I'm not opposed to brushless, under the following conditions (legality is assumed):
--They have to work with the existing control system.
--There has to be a specific type or limited pool to draw from. (This would be opened up as teams gained more experience.)
--They have to come in either in the KOP or with an easy-to-use system to obtain them for a steep discount. Teams don't necessarily have the budget to go all-in quickly.

[apalrd]

The current system has four types of regulated outputs:
-Servo PWM
-two digital lines (relays)
-CAN
-moderate current high-side drivers (solenoids/lights)

Almost any motor controller that exists in the hobby world will accept Servo PWM. The current code even allows tuning the pwm max pulse width - A Victor is narrower than a Jaguar, which is narrower than a servo.

There's nothing the control system really cares about, as long as a removal of the PWM signal guarantees the motor shuts off.

Also, as teams are regulated by the PD board (limited 40a breaker spots, and 40a max breaker size), the 120a main breaker, and the 50a battery connector, teams are already power limited electrically, without limiting motors.

As long as a motor is a COTS part that can be bought by anyone, I see no reason to make it illegal. You are limited by the main breaker and PD board, which effectively limit the power of individual motors and all motors on the robot as a whole.

[Ian Curtis]

Originally Posted by AdamHeard

You are comparing US made versus Chinese made, you are also comparing an esc optimally sized for operation versus one far oversized for operation.

I love RC cars, and spend way too much money on hobbyking and can say with certainty I have used more motors and ESCs from them than anyone on chief. I love them for personal use, and a lot have surprised me with their performance and durability, but I would not put them on an FRC robot.

Can you expound on this Adam? I don't know much of anything about their ground vehicle stuff, but I have spent in the 4 digit range on HorizonHobby (USA) and Hobbyking (Chinese) products. For starters, I think it's pretty unfair to say US made vs. Chinese made, as to my knowledge essentially all of their airplane products are manufactured in Asia. Are cars different?

Multiple times I have received defective components from Horizon, but I have yet to receive a defective product from HK. In my experience there is something like a 20% derate from advertised Chinese specs, but I've never had so much as a servo fail without reason. (And these are servos that you can buy for $4, so they cost significantly less to make) I realize I have a small sample size, but the fact that I've had 3 Horizon orders where components have failed within an hour of first flight just doesn't sit well with me.