Limiting Drivetrain Motors

[NotInControl]

Originally Posted by Tom Line

I don't think that anyone can deny that FRC in general is in a bit of drive train war right now. The latitude we've been given in the last few years in motor choice and availability have made 6 cim drive trains increasingly common, with some few teams even moving to an 8 motor drive train.

That isn't a bad thing from an engineering standpoint.

Drive trains area already limited by the 120 amp breaker. Assuming the new CrossTheRoad PDB and current monitoring does what we hope it will, teams should be able to ride 'upper limit' of power while not popping their breaker.

I have to wonder, though, if we would be better served in FRC by limiting the total power output of the drivetrain. There is an argument to be made about the increasing price of multi-cim multi-speed gearboxes. A reasonable argument can also be made that the kitbot drive train has been rendered obsolete. This year, we saw an extremely brutal game. This made partially possible by the wide open field, but the high acceleration and high top-end speed that our drivetrains delivered were also responsible. A lot of robots left the field in pieces: even those of multiple-time world champions.

I think we've reached the point where it's time for FRC to consider reigning in the drive train power. I wouldn't be adverse to a max 6 cim, or even 4 cim and 2 mini-cim 'power' limit. I don't have my motor sheet in front of me to lay down the power numbers of those motors right now. What do other folks think?

I personally would like to see the rules more relaxed. I would rather the GDC do more of "I wonder what teams will come up with if we allow more of this..." rather than preventing what teams can create.

But playing devils advocate: Assuming the rule you described was implemented, how would one begin to regulate this rule? Drivetrain output power is determined by gear ratio, motor configuration, motor load, etc. All of these variables are engineering parameters that teams can choose based on their desires. To ensure robots are under the maximum power usage, the inspectors would need to measure or calculate power draw while the robot was under some worst case driving load to show it never pulls x much power. Is this even possible in the pits? Wouldn't this also mean that you would need a fully functioning/driving robot before you were able to pass inspection? Furthermore, unless the inspectors had their own current or power meters that they could connect to your robot while you were doing this demo, you would also be forcing every team to use the CAN interface. The inability to regulate this rule, my just be a reason why it can not be a rule.

In addition, the PDP we are currently using for Alpha Testing provides current monitoring for each channel on the PDP (1-16), not for the entire system. I suppose one could measure all channels, and then sum them up to provide an estimated system current draw.

However, I think I understand why you are thinking of this approach - to be a uniform solution to prevent main breaker trips, correct?
The 120Amp main breaker is not limiting at all. It takes a long time to trip and can handle 500+% of 120 amps for seconds before tripping under the right condition. The main breaker is a thermal device, not a current device. It trips after passing a certain temperature. Large current surges is just one way to raise the temperature of the device, however it is completely true that if you took a heat gun to the main breaker it would trip even under no current load. The proper way to prevent the main breaker from tripping is to design motors with the proper gearing ratios to prevent such cases.

However, If I were to design an automated control system to help prevent tripping the main breaker in 2015, this is how I would do it:
Since I know the main breaker trips based on temperature, that is the variable I want to measure. In control systems, when you have a system that is controlled by one variable (heat in this case), but you measure something else (current in this case) and use that to estimate the other, that is called an indirect measurement. This can leads to all sorts of trouble like making the wrong decision. When I am designing any new control systems, I want to directly measure all of the control variables, only if it is impossible for some reason (i.e. against the rules) then will I venture down the indirect measurement path.

I would use a thermocouple mounted to the power terminal of the main breaker to monitor the terminals temperature. When the temperature of the terminal goes above some threshold (based on spec sheet, or team experimental data) I would then warn the driver (via driverstation/lights on robot etc.) that the main breaker is approaching the tripping point. I could then automatically enter some "reduced power mode" of the robot where I would start scaling back the max power to the most power hungry systems determined by the max current draws measured on the PDP. For example, I could reduce the max output of the drivetrain to 75% while in reduced power mode, and stop the motor all together when I detect a stall condition (large current surge for some duration of time on some channel). I would keep doing this, until the temperature drops down below another threshold where I could then re-enter "normal power mode" and let the robot rip.

For added effect, I would mount the main breaker on top of a fan, and kick the fan on during reduced power mode to help it cool down faster. Or leave it on at all times to help prevent going into reduced power mode in the first place.

The system would also have the ability to be disabled by any of the drivers by pressing a button in real time.

Why would I do this over just measuring current? Well the main breaker is a thermal device. If I were measuring current, all I could determine is that the the current spiked over some threshold. This is not an indication that the breaker is approaching its trip point. The breaker can handle large loads for many seconds and even minutes depending on the conditions. The problem is that the time to trip is not deterministic. It can be milliseconds, seconds, or even minutes. In a match, I want to stay competitive as long as possible, for example staying in a pushing match for as long as possible. Prematurely reducing/disabling power to robot systems because I detected a current surge is very overly cautions and can reduce my competitiveness. The last thing I want to do is lose an elimination match because I disabled/reduced power to certain robot components because I thought the main breaker might blow, especially when it was no where near the tripping point. Measuring temp gives me the best information to make the best decision at that moment in time, I can let the robot rip it up for as long as I can until the temp of the main breaker reaches a worrisome point. I don't have to worry about prematurely reducing my systems. I have more useful information measuring temp and so I can make smarter decisions. I can also allow the robot recover back to full potential and rip it up again, once the temp drops down keeping my competitiveness, and the full advantage of my robot.

Remember these systems don't guarantee that the breaker will trip, they just inform you that the probability of the breaker tripping is higher. In a regional/district winning match, I would probably let the robot approach the trip rather than prematurely disabling/reducing power to critical systems. This is just me, I am more of a gambler and I trust the systems we design as a team. This is especially true in the last 30 seconds of a match, where I probably need my robot to push its limits to the end. I'd rather take the chance and hope it doesn't trip during a critical match, rather than reduce the capability of my robot where I can no longer be as effective/competitive. Its a trade off, its a gamble.

One could argue "couldn't you just use the PDP and average the current surge over some amount of time to help increase the probability that the main breaker is approaching the trip point.?" Yes you can, however this is still an indirect measurement. If measuring the temp of the main breaker is not feasible for some reason in 2015, this is the next best approach, and the one I would employ. It still has the problem that I do not know if the breaker is warm or not when the current surge is detected, but it is way better than nothing.

All of this doesn't replace good drive train design. These are just warning systems and can provide the drivers useful information in a match. The best defense against main breaker trips is to design appropriate drives in the first place. 2 speed gearboxes, ramping up your drive commands, and knowing when to switch to low gear can get you the 99% solution.

Main Breaker manufacture Spec PDF is attached

Hope this helps,
Kevin

[FrankJ]

Or you could just mount the main switch on a good heat sink & put a fan on it?

[Jon Stratis]

Originally Posted by NotInControl

I personally would like to see the rules more relaxed. I would rather the GDC do more of "I wonder what teams will come up with if we allow more of this..." rather than preventing what teams can create.

But playing devils advocate: Assuming the rule you described was implemented, how would one begin to regulate this rule? Drivetrain output power is determined by gear ratio, motor configuration, motor load, etc. All of these variables are engineering parameters that teams can choose based on their desires. To ensure robots are under the maximum power usage, the inspectors would need to measure or calculate power draw while the robot was under some worst case driving load to show it never pulls x much power. Is this even possible in the pits? Wouldn't this also mean that you would need a fully functioning/driving robot before you were able to pass inspection? Furthermore, unless the inspectors had their own current or power meters that they could connect to your robot while you were doing this demo, you would also be forcing every team to use the CAN interface. The inability to regulate this rule, my just be a reason why it can not be a rule.

Without getting into the big long analysis of how to (prevent) trip the main breaker, I wanted to address this from an inspection perspective. For at least the past few years, FIRST has provided a Motor Performance Data Sheet which contains information on most of the motors we're allowed to use, and probably all of the motors we're allowed that you would want providing locomotive power in your drive train.

Why should we measure output power when we can measure max motor power instead? Sum up the max power for each motor on the drive train and you're done. That leaves us with a practical upper limit on speed and power output in a drive train without getting into all of the specifics you mention. Further, with appropriate limits we can pass most teams without question quickly and easily. "4 CIMs on the drive train (like 75%+ of all teams)? You're good to go!" "4 CIMs, 2 mini-CIMs, and 2 775s? We'll have to do the math on that one..." And the best part... it doesn't require any special skills or knowledge from the inspectors in determining if it's legal - just some basic addition.

[Andrew Schreiber]

Originally Posted by Jon Stratis

Without getting into the big long analysis of how to (prevent) trip the main breaker, I wanted to address this from an inspection perspective. For at least the past few years, FIRST has provided a Motor Performance Data Sheet which contains information on most of the motors we're allowed to use, and probably all of the motors we're allowed that you would want providing locomotive power in your drive train.

Why should we measure output power when we can measure max motor power instead? Sum up the max power for each motor on the drive train and you're done. That leaves us with a practical upper limit on speed and power output in a drive train without getting into all of the specifics you mention. Further, with appropriate limits we can pass most teams without question quickly and easily. "4 CIMs on the drive train (like 75%+ of all teams)? You're good to go!" "4 CIMs, 2 mini-CIMs, and 2 775s? We'll have to do the math on that one..." And the best part... it doesn't require any special skills or knowledge from the inspectors in determining if it's legal - just some basic addition.

But what motors count? If I do a steered wheel system do the steering wheels count for this determination?

Truth, adding arbitrary design constraints like that just seems the wrong approach. I assert that with good game/field design it should be possible to minimize the impact of the increasing motor power. Make it so it's more worth my time to play the game than to ram others. Don't let me accelerate the length of the field into another bot by dividing the field up.

2014 was rough for reasons unrelated to the motor power. Most folks who were doing the worst of the ramming likely only had 4 motors in their DT anyway. It was the ability to accelerate cross field and overworked refs that caused the plethora of ramming issues.

[Chris is me]

I think the assumption being made by a lot of posters in this thread is that 6 CIM drives are a large competitive advantage. People have stressed this to the point of saying the Kit of Parts is "obsolete" because it has a 4 CIM drive.

While a 6 CIM drive definitely has some benefits, they are not magic. 6 CIMs do not increase your pushing force when you are traction limited. While most single speeds aren't geared to run indefinitely under traction-limited load (pushing against a wall), they are usually geared to be traction limited at SOME point before stall. Thus the pushing force "cap" for robots isn't determined by the number of motors.

Acceleration is trickier, as it's very hard to model several dynamic factors in play to make a general case situation for all robots.

Honestly, I think tons of non-top teams faced extreme defense for the first time this year. Their robots got damaged, they want something easy to blame, so the first guess was more CIMs in opponent drives. The game design sucked and we're scapegoating the motors.

[AllenGregoryIV]

Originally Posted by Chris is me

Honestly, I think tons of non-top teams faced extreme defense for the first time this year. Their robots got damaged, they want something easy to blame, so the first guess was more CIMs in opponent drives. The game design sucked and we're scapegoating the motors.

I agree, motor limit or not, high speed ramming would be in issue in this game. We have bumpers for a reason, hard clean hits between two robots that can take it is very entertaining and have been a part of most FRC competitions over the past two decades. Finding game designs that limit destructive actions is hard but should be part of every game design.

[Tungrus]

"More is better"? A good engineering is doing more with less (no, not Target's catch phrase). It would be awesome if some team can design an efficient and powerful drive train without adding more hardware.

[asid61]

Originally Posted by Jon Stratis

Without getting into the big long analysis of how to (prevent) trip the main breaker, I wanted to address this from an inspection perspective. For at least the past few years, FIRST has provided a Motor Performance Data Sheet which contains information on most of the motors we're allowed to use, and probably all of the motors we're allowed that you would want providing locomotive power in your drive train.

Why should we measure output power when we can measure max motor power instead? Sum up the max power for each motor on the drive train and you're done. That leaves us with a practical upper limit on speed and power output in a drive train without getting into all of the specifics you mention. Further, with appropriate limits we can pass most teams without question quickly and easily. "4 CIMs on the drive train (like 75%+ of all teams)? You're good to go!" "4 CIMs, 2 mini-CIMs, and 2 775s? We'll have to do the math on that one..." And the best part... it doesn't require any special skills or knowledge from the inspectors in determining if it's legal - just some basic addition.

I agree. If motors were ever limited, I would want to do it by maximum power.

[Richard Wallace]

I like motors.

If it ever becomes necessary to limit robot mobility, FIRST should prohibit wheels that contact the playing field surface.

----

Oh, and @the earlier comment comparing limited mobility games to tea parties / soccer: there's a guy named Dempsey who probably would not agree that soccer is anything like a tea party.

[yarden.saa]

tradeoffs are part of the engineering proccess. In previous years we had to calculate what motor we need for any mechanism (not only the drive train).
In this year most of the teams said, lets put cims/mini cims without even try another motors. The game this year was really easy. the task of throwing the ball isnt easy but when you have 6 cims and 4 minicims, you don't need tradeoffs in the drivetrain motors in order to get a good shooter/launcher. Most of the teams had 2/3/4 cims/mini cims in their shooter, but we could get the same result with less powerful motors with smarter mechanisms(you can find very few examples who did this, look at 254 shooter with only 2 small motors).
Some will say that more power is better but it is better only when it fits the game. Aerial assist is one of the easiest games because you only need to throw a ball, but has the highest power available for teams ever.
It doesn't make sense

[JamesCH95]

Lil'Lavery brings up a good point - it is inspirational to 'do more with less' and a lot of anti-power-limiting sentiment is based on flawed logic. However, I think that the 'enough rope to hang yourself with' philosophy is a better approximation of real-life engineering than 'try to do more with less.' I think that it's far easier to terminally screw up a project by getting too ambitious and overspending/over-promising than it is to exceed expectations.

ChrisIsMe and Andrew Schriber both hit the nail on the head - poor game design and (related) overworked refs lead to the serious amount of high-speed ramming we saw this year. Heck, our 4-cim robot managed to shear its deck during a bumper-to-bumper high-speed hit. Dropping the motor count down to 4 cims again wouldn't have prevented this from occurring.

I'll draw a parallel: F1 racing. Every few years the allowed engine displacement is reduced, or the amount of time an engine must be in service is increased. These rules are designed to limit the power available to F1 teams, but guess what? Lap times generally just keep dropping, and dropping.... So I don't think that dialing back on available motors will really solve the problem of 'robots that are too fast' unless our motor selection is reduced to the days of yore when 3/8 drill motors ruled the land. In 2013 no one complained about having too many motors, nor was there carnage on the field from high-speed hits despite having essentially the same motor selection available. The differences? Protected scoring and loading positions, non-open fields, and a scoring system that didn't rely on multiple refs poking through multiple menus to enter in scoring actions on their input consoles (taking time away from actually reffing the match).

Reducing motor count, barring severe restrictions, won't reduce high-speed impact damage. 4-cim robots are more than capable of causing severe damage with open-field bumper-to-bumper hits. Good game design will reduce high speed impact damage. See 2013, where defense was more cat-and-mouse than WWF smackdown, despite having essentially the same available motors.

Originally Posted by colin340

Fair point, but this year we got called for hitting the defense bots back too hard. Teams that are blockading / pinning a partner should design for 13 fps hits or get out the way.

i'm sick of playing tea party games / soccer.

Clearly you've never played soccer at a serious competitive level.

Originally Posted by Richard Wallace

I like motors.

If it ever becomes necessary to limit robot mobility, FIRST should prohibit wheels that contact the playing field surface.

----

Oh, and @the earlier comment comparing limited mobility games to tea parties / soccer: there's a guy named Dempsey who probably would not agree that soccer is anything like a tea party.

None of these guys would agree either.

/threadjack

[qnetjoe]

I think it is important to take a step back and look at basis/perspective on this, most of us come from team that have a good amount of engineering resources/experience.

Opening up the motor rules really helped middle to lower level teams reuse older parts and required less engineering expertise to field better bots. I know a lot of those teams that refuse to use to window motors (and some even the BB motors) because it is "too hard to" compared to me who has a broach for the window motor spline in my garage.

It is important that all of us realize that FIRST is more about helping the lesser teams do more with less support/resources, hence the push in 3D printing and lessening of the rules. Just like in business, regulations/rule changes do not affect the upper echelon teams as much since they have resources to work around them, it is the the small teams that take the blunt of it.

Just to kill some parallels, in F1 the top speed of the cars is what they seek to limit by the rules and that has stayed rather close in the last 15 years or so. Lap times have been going down due to advancements in suspension and handling. The reason they claim to need to limit top speed is due to safety.

If FIRST was going to do anything it would be to lower the rating on the main circuit breaker. That would limit the max power that could be delivered without some crazy rule change.

Just my two cents

[jman4747]

I do think robot damage is more of a game design problem the field was so open. When's the last time any robot could go anywhere at any time (excluding auto)?

I am however indifferent to a limit on drive train power or motor usage. The worst thing to do by far would be to reduce the limit on total allowed motors on the robot. Most of the innovations in robot design come from game specific systems, tank drive trains sporting 6cim inputs usually have few functional differences from 4cim drives.

[JamesCH95]

Originally Posted by qnetjoe

Just to kill some parallels, in F1 the top speed of the cars is what they seek to limit by the rules and that has stayed rather close in the last 15 years or so. Lap times have been going down due to advancements in suspension and handling. The reason they claim to need to limit top speed is due to safety.

I won't disagree that F1 lap times have been improved primarily by suspension and handling, but it doesn't negate the parallel. If motor availability (engine displacement) is reduced then teams will find other ways to make their robots (cars) just as fast as they were, like multi-speed automatic transmissions (improvements in suspension, brakes, and tires). This fits in nicely with your point - the restriction won't slow down the higher-end teams who can devote resources to making more advanced transmissions (develop suspension, tires, brakes) but it will hurt middle and bottom tier teams who do not have the resources to devote to these efforts.

[pfreivald]

Originally Posted by Richard Wallace

I like motors.

If it ever becomes necessary to limit robot mobility, FIRST should prohibit wheels that contact the playing field surface.

Walker-bots FTW! I would love to see this one year, just to watch peoples' brains explode at kickoff.

-------

On-topic, the destructive nature of this year's game had every bit as much to do with a completely open field as it did with drive trains. Those of us from the pre-bumpers era, I think, widened our eyes and thought, "oh, it's back to that, is it?" when we saw the game. Imagine how differently this would have played out were their barriers (Rebound Rumble), horizontal poles, humps (Breakaway), or the like dividing up the field....