One speed vs Two speed gearboxes
 

My team is trying to decide whether we want to go with a two speed gearbox or one speed gearbox. For the last two years we have been running 2 speed gearboxes, this year our mentors are talking about possibly shifting(no pun intended) to a one speed. The main argument for the two speed is the flexibility it offers. The main argument against it is that they are expensive and they may not be worth the price. We are discussing this now among the team but I want some other perspectives on the issue.

Re: One speed vs Two speed gearboxes
 

Other than the actual cost, shifting gearboxes require more space on the robot, more weight, and more air.

Team 135 used a two-speed shifter last year, and I think it paid off since our drivers knew when to shift for the optimal performance. If your drivers know when to go fast vs. slow, then two-speed should be used despite the additional costs. Otherwise, if you really want to save money and weight, then you can eliminate the shifter.

Re: One speed vs Two speed gearboxes
 

Other things you should consider:
- the game and strategy, what's it gonna be?
- field layout and obstacles?
- is your robot going to be primarily defensive or offensive?
- if your primary design is offensive, will you be able to score points yourself or expecting minimal help from alliance partners?
- if its defensive robot, will you be able to successfully defend, are your drivers ready to defend irrespective of match result? If your partners are not able to score would your be ok?
- do teams in your region generally have a well designed offensive robots?

We have been using shifters and its a challenge for our drive team to switch in to defense mode, its more of mental block and hesitation.

Re: One speed vs Two speed gearboxes
 

We too have gone between shifting and non-shifting gearboxes over the years. The major argument I'm enforcing this year for shifters is getting through defense. I've never played on such an open field before*, and man was it eye-opening.

In recent years it wasn't so much about pushing others around, but rather having a high "sprint" speed combined with a low, more controllable speed. 2013/2014 were more about beating defense to a point on the field rather than outright pushing them. To this end, we've considered gearboxes which have multiple gear ratio options by swapping out internal gearing.

*I did mentor software for a small team in 2005/2006, but didn't go to an actual competition until 2007.

Re: One speed vs Two speed gearboxes
 

I generally see shifters as a good thing. Autoshifting code meanns that you can go 6 cim drive and have peak speeds of 18+ fps. You can get around defense.
Of course, if there's tons of obstacles, you could easily go to one speed and save some time and money.

To save money, you might consider switching to an in-house shifter.

Re: One speed vs Two speed gearboxes
 

I believe you can be highly competitive going single speed. If you accelerate quickly and manuever well, you can slip by defenders. You can also easily get in the way of others with those same attributes. You can achieve high acceleration, good turning, and sufficient pushing power with a single speed drivetrain.

That isn't to say that shifting isn't useful, but I distinguish between useful and absolutely mission critical. If you would rather reallocate your limited resources to making the stuff on top of your drive base better (I believe this is probably more important), I don't believe that is a bad move. 846 did better single speed than any year we ran shifting.

Your mileage might vary.

Re: One speed vs Two speed gearboxes
 

Big fan of two speed gearboxes and prefer to use the maximum differential ratio between high and low gears.

I value the ability to get the robot where you want it to be; either in a hurry or under duress. That is to say, I value the ability to either out run or out push an opponent to achieve the desired field position.

I also value the ability to maintain a desired field position once achieved.

Re: One speed vs Two speed gearboxes
 

Are you saying that a two-speed robot would generally be better at this than a single-speed robot?

Re: One speed vs Two speed gearboxes
 

I think it really comes down to preference and what you design for. There were plenty of teams who used single speed gearboxes to outmaneuver their opponents as well as play effective defense successfully, 118, 610, and 1986 off the top of my head.

Re: One speed vs Two speed gearboxes
 

More pushing power, so you can force them away to stop them from slipping past.
You can choose speed or pushing power, not both. A single speed drivetrain can't have both.

Re: One speed vs Two speed gearboxes
 

First and foremost, ask your drivers. How often did they shift in prior seasons? How useful did they feel it was?

Based on my experience with shifting drivetrains, I'd argue that for most teams it's really not worth the cost and effort. For teams looking to get from the 80th percentile up to the 90th and who finish with plenty of time for driver training, they could be worth it. For teams who are building up to the last minute or are in the middle of the pack currently, it's probably worth focusing your resources elsewhere (such as finishing earlier or your manipulator).

Especially with COTS 6-CIM options and high traction wheels, single-speed drivetrains can execute most defensive and offensive strategies. Holding position is more a function of your mass, bumper design, and traction than the maximum torque you can transmit. Geared properly, single-speed drivetrains can often give all the accelration and velocity most drivers can handle accurately. A true "pushing match" is relatively rare. Simply getting in a position before the opposing team often suffices. If an offensive team has to bulldoze their way through your robot, you're playing great defense already. If you beat the defensive robot to your scoring position, it's better to invest the rest of your resources into a manipulator that can score quickly and reliably than building a drivetrain that can mask a poor manipulator.

Finish early. A practiced driver on a single-speed will almost always best an inexperienced driver with a dual-speed.

Re: One speed vs Two speed gearboxes
 

I appreciate all the input and different POV.

Thank you

Re: One speed vs Two speed gearboxes
 

There have been many posts on this topic and there is no overlying correct choice. It all depends on your team.

If it is a problem with cash then, imo, I would go with a single speed drivetrain. They are not necessarily a bad thing due to them offering a mix of push power and speed all in the same gear. Also a SS gearbox will make your robot lighter meaning you can put weight towards other things. I truly believe both are perfectly good options and if you are struggling with other things it is best to take the cheaper not to mention easier of the two options so that you can put your time elsewhere.

Re: One speed vs Two speed gearboxes
 

It really depends on your strategy. That said, a reliable shifting gearbox will never hurt.

If you look at some very successful teams, you'll see that some teams like 254 have run two speed transmissions for over 10 years. On the other hand, team 118 didn't have a two speed gearbox in 2011, 2012, 2013, or 2014, and they were very successful. 118 has also put brakes on their gearbox in the recent past too.


I've seen you say this a few times. How would you set up an auto-shift system? When do you shift up and shift down?

Re: One speed vs Two speed gearboxes
 

Autoshifting is done by either current sensing or heat sensing on the main breaker. When the current spikes (or the heat) for an extended period of time, you shift down. You can find a curve fit for the graph of the main breaker current limit versus time, or just have hard limits and times. This prevents breaker blows.
Generally you only need to utilize this in a pushing match, and the driver gets distracted. You could potentially also use collision detection, where the driver is pushing on the joystick but the robot is not moving. Then the program shifts down after a couple seconds. Shifting up is manual.
There is a number of ways to accomplish autoshifting.

Re: One speed vs Two speed gearboxes
 

Maybe you could use encoders to determine the speed the wheels are running, and go from there setting a threshold above which to use 2nd gear and then below that value it goes back down to first gear. We used auto-shifting this year, and it worked pretty well. We also had a manual override in case we were in a pushing match or other scenario where auto-shifting may not be entirely reliable. However, all in all, it definitely worked well for us. One less thing for the driver to worry about, although we are looking into single-speed gearboxes over the offseason.

We also had a timer that did a 1 second lockout to prevent rapid repetitive shifting between 1st and 2nd gear when reading the values.

Re: One speed vs Two speed gearboxes
 

I know no team is autoshifting off heat currently.

I'm unsure any teams are using current (as you technically could with the jaguars).

Teams that autoshift now are primarily shifting based on encoder feedback, once they hit a certain rpm in low gear they shift up (for a full voltage output). Once they hit below a certain rpm in high gear (for full voltage output) they shift down. There is sometimes a timer on it to prevent gear hunting (rapid back and forth).

Easy current monitoring will make the down/up shift easier next year.

By checking rpm and assuming full voltage output, the above method was essentially shifting based on current.

Re: One speed vs Two speed gearboxes
 

These seem like very different goals to me. Maintaining a desired field position seems to imply an offensive strategy. For example, this year it might have been beneficial for a robot to hold its shooting position for a few seconds while getting ready to shoot. Forcing away opponents to stop them from slipping past seems to imply a defensive strategy.

Re: One speed vs Two speed gearboxes
 

They are not necessarily different. We didn't see this last year, but in 2013, a robot that could maintain a desired field position could be VERY desirable for shutting down a FCS-type robot. I mean hanging out in their line of fire with a maximum-height blocker and STAYING there--but not necessarily keeping them from moving, just keeping them from moving YOU. In other games, a similar form of defense could have been played. I believe it's known as "plugging the gaps" in some sports.

Your reasons for holding a desired field position are your own. Your method had better have a lot of traction, though... Which is why you want a low gear. But getting to that position so that you can hold it may need a high gear. Depends on the game whether the low gear you need and the high gear you need are close enough to do it single-speed or far enough a part to use a shifter.

Re: One speed vs Two speed gearboxes
 

Using current alone doesn't work.
The initial current spike when beginning a pushing match (where you want to shift) looks the same as the initial current spike during acceleration.

Autoshifting is really, tricky.
There are so many instances where the robot can't possible know that shifting would be an issue at the current point in time. When you're turning, drifting to a stop, or grabbing a game piece, you don't want to shift.

Yes, it's possible, but I think it may be more efficient for the driver to just remember where the shift button on the joystick is, and the coach can instruct the driver on what gear to be in (if the driver is too busy to make the decision himself.)

Re: One speed vs Two speed gearboxes
 

How does having a low gear increase your traction?

Re: One speed vs Two speed gearboxes
 

It doesn't increase your traction, but it increases your use of traction (until your wheels slip, that is). If you're being pushed backwards, and your wheels aren't slipping, you aren't using enough.

Re: One speed vs Two speed gearboxes
 

I should've been more thorough; You don't just do a greater/less than check on current, you need to look at it over time (how it's changing), along with applied voltage to make intelligent decisions.

Re: One speed vs Two speed gearboxes
 

If your wheels are slipping, you're reducing your traction.

Re: One speed vs Two speed gearboxes
 

+1.
On heat checking: That's the best way to do it IMO, as I believe the breaker trips based on heat, but it also something that hasn't been done before. You would need to gen an IR gheat sensor and direct it at the breaker.

Re: One speed vs Two speed gearboxes
 

It doesn't cool down very rapidly at room temp though.

Re: One speed vs Two speed gearboxes
 

True. So only an emergency measure then.

Re: One speed vs Two speed gearboxes
 

I don't think auto-shifting makes a lot of sense in the world of FRC. The "correct gear" to be in at any time is a function of two things:

1) What am I doing now?
2) What am I going to be doing in the near future?

Only the driver knows #2.

Re: One speed vs Two speed gearboxes
 

I would beg to differ on all accounts. You can definitely make light and small 2 speed gearboxes. Additionally the amount of air required to shift is extremely small, for example the volume of air required for a single shift on 192's 2014 gearbox was (9/16)^2 * pi * 3/8 = 0.373 in^3 of air, which is nothing as many tanks hold around 35 in^3 of air.

Re: One speed vs Two speed gearboxes
 

You don't need full autoshifting. Just enough such that your driver doesn't blow the breaker.

+1, but some caveats. COTS shifters weigh a lot and take up a lot of space. A custom shifter is much easier to work in like 192's design this past year and hang over the wheels, taking up less "valuable space" than a single speed, depending on the chassis.

Re: One speed vs Two speed gearboxes
 

Or just design your robot not to trip and make sure your drivers know its limits.

Re: One speed vs Two speed gearboxes
 

Even if some teams can find ways to minimize the additional costs in space, weight, and air doesn't mean they don't exist. By default, adding the additional gearing and shifting mechanisms will take up more space and weight than not having them. By default, triggering a pneumatic cylinder to shift will consume more air than not triggering that cylinder.

Heck, some teams might not even install a pneumatic system if it weren't for two-speed drivetrains.

Re: One speed vs Two speed gearboxes
 

I agree with the posters that say you can't decide this until you've seen the game.

I've considered designing a mechanical 'flyball governor' to autoshift. Have any teams ever fielded one of these? Seems easy enough to conceive of. No air, no software and it would down shift when you got slower automatically.

Re: One speed vs Two speed gearboxes
 

I can see why you would do that, but that severely limits your speed. A 20fps+ drive can trip in seconds, and even an exeperienced driver can make mistakes (see: 1678). To prevent that you have to cap out your speed, thus limiting your competitiveness. One less thing for the driver to worry about IMO.

Re: One speed vs Two speed gearboxes
 

Team 71 won a world championship with a dominant robot in 2002 spending the majority of the match traveling only inches each second. If you think you need 20+ ft/s to be successful in FRC I suggest you take a step off the California powerhouse hype train and re-evaluate what a drive system is supposed to do. Better yet, attend my class on Strategic Design at the WRRF workshops on December 13th and I will personally explain to you why this isn't necessary.

I understand where you're coming from - I once had a similar mindset as well - luckily I had some good mentors knock some sense into me. We're fortunate to have some of the best mentors to learn from here in California - especially in the Silicon Valley.

Re: One speed vs Two speed gearboxes
 

In a time when moving was a major challenge and almost everything was custom and there was less power available in the KoP. Things were slower then.

Re: One speed vs Two speed gearboxes
 

If I understand correctly, asid61's stated hypothesis is this: limiting speed reduces competitiveness.

In the context of Aerial Assist, I agree with this. I believe our speed on the field was a component of our success this season.

All aboard the California powerhouse hype train. Next stop, Einstein 2015 ;)

-Mike

Re: One speed vs Two speed gearboxes
 

I agree, I think speed was one of the factors that separated a good team from a great team. 1678 is a perfect example of this.

Re: One speed vs Two speed gearboxes
 

For Aerial Assist, if your strategy played that way it is very true, however it's dangerous to think that an extremely high speed is required for all games, and that was the point I was trying to get across to asid61. Your strategy for this year was more successful with a higher speed and you pulled it off amazingly well. That being said, running your same speeds in a different year - 2012, for instance - may not play as well to your strategy than slightly lower, more controllable speeds. Of course I know you understand this, but I'm posting this to clarify for asid61.

Re: One speed vs Two speed gearboxes
 

I was watching one of the Behind The Lines videos which was on drive train design and they discussed modifying a 2-speed shifting gearbox to be one speed and when you shift, the motors run a different system on your robot. This is great for a game where you are either driving or doing another high load/speed task. For example, climbing in games like 2013 and 2010. Has anyone done this before? How effective was it?

Re: One speed vs Two speed gearboxes
 

Look for "PTO" gearbox in search. Power Take-Off rigs allow the full power of the drivetrain (or half of it) to power something else. They've been used as far back as 2004 (60 and 254 winched themselves up to a bar with a PTO from their drivetrain, as I recall). They can be very, very effective, but only if used properly. For example, you probably never want to be driving at the same time the PTO system is on; you could do some serious damage to your robot!

You could also modify a 2-speed gearbox to be a 3-speed, with the third speed being the PTO.

Re: One speed vs Two speed gearboxes
 

I understand many games require slowness. 2002 is not a good example, as IIRC there was only two cims in the KOP. Going 20fps was not an option. Now, 2010 was a year where speed was not very applicable, and where I think a single speed drive would make sense.
However, for games 2011-2014, fast robots are good. And to go fast and avoid trips (even accidental ones) you should really have a shifter.
Now 2012 I could see going under 20fps. However, six cim drives make going under 18fps almost pointless if you don't have to worry about breaker trips. So you slap on a shifter. The game is not just speed, it's distance/time. The greater that ratio is, the more maneuverable a robot is (depending on the driver, of course).
The primary downside of a shifter is space usage. Custom options are good for this. A team that needs to every inch of space on their robot for manipulators, etc. would need to go non-COTS.

Re: One speed vs Two speed gearboxes
 

Team 118 didn't shift in 2011, 2012, 2013, or 2014. They were still incredibly good in each of those years. They prove that you don't need 2 speeds to win the same way that Einstein proves, year after year, you don't need an omnidirectional drive to win.

Re: One speed vs Two speed gearboxes
 

Yes, they were good. But could they have been better? That's not said as a challenge, but I am legitimately interested in why they go with a single speed over a two speed.
As a counterexample, 254 has been shifting for about a decade now, and they have two championship wins.
I would like somebody from 118's opinon on this.

Re: One speed vs Two speed gearboxes
 

Can I visit the magical world where this is the case?

I feel like what you've been saying about extremely high gears is true if and only if power management is not a concern.

Re: One speed vs Two speed gearboxes
 

It's called a shifting gearbox.
My point is that with a shifter you don't need to worry about power managment, as long as you are either a) careful and shift or b) use encoders or other methods to automatically shift down in collisions. I prefer the latter method, as the first has much less use.

Re: One speed vs Two speed gearboxes
 

Chooo Chooo!

I think it's fair to argue for 95+% of teams the difference between a 16fps high gear and a 20+ fps high gear is not what will make them more competitive.

Gearing that high requires a higher attention to detail mechanically, as well as a good supply of batteries. Many teams don't realise their batteries are in bad shape because their robots simply don't use that much juice.

Re: One speed vs Two speed gearboxes
 

I don't know of any teams that can just "slap on a shifter" without any substantial time commitment from their build team. Additionally, shifting necessitates both more coding time and more practice time for the drivers. Shifting is a tradeoff, just like every other part of the robot. No robot will instantly just become better if a shifter is just slapped onto it.

Maybe I am reading too much into your words asid, but it seems to me that you believe that every robot would instantly just be better if a shifter were used instead of a single-speed. On a team with infinite resources, this might be the case, but on my team at least, we always have to make tradeoffs in our designs, and we will likely not be using a shifting gearbox next year so that we can focus on other aspects of our robot.

Re: One speed vs Two speed gearboxes
 

Arguably you can slap one of these on just as easily as slapping on a single speed.

http://www.vexrobotics.com/vexpro/ge...llshifter.html

Re: One speed vs Two speed gearboxes
 

As a matter of fact, it's can be kind of hard to do that on a ballshifter. We are planning out a test WCD (again) this year, and we found it hard to integrate a ballshifter without having to make our own bearing blocks. We could add our own bearing blocks, but we wanted to try as much COTS as possible, so we're actually going with a WCP shifter for the chassis. But depending on the chassis, WCP or Vex is easy.

Shifting gearboxes do cost a lot of cash, as far as resources go. More coding, okay, more practice, okay. However, both of those can be done pre-season as long as you release the code for the shifting as open-source. Other things might be a higher priority, but if you have the money and pre-season time it definitely worth it.

Re: One speed vs Two speed gearboxes
 

As a counterpoint, though, for most teams - who don't build a practice bot and therefore don't have the time to train a driver for super-high speed manuvering or develop robust autoshifting code - super high ( I view that to be above the 16-17 fps range) speeds can be absolutely terrible.

Case in point: Us last year. While our drive was a monster and for the most part dominated the field, our driver was, for our first competition, terrified to drive in high gear because of the high risk of blowing the breaker. It took him the full competition to really get the hang of obstacle avoidance and shifting for plowing.

Re: One speed vs Two speed gearboxes
 

The 3 cim ballshifter is designed to mount with half of a versablock set.

Re: One speed vs Two speed gearboxes
 

... <facepalm>

Well, we already ordered everything, and it's not like we're any less off for that. WCP is still fine.
We thought we were supposed to use the "gearbox bearing block" like on a WCP box.

Re: One speed vs Two speed gearboxes
 

You can use the versablock on both actually.The "Gearbox Bearing Block" is just for teams that can CNC a hole/want a smaller bearing block.

http://www.buildblitz.com/final-cad-files/

The Copioli robot should show the setup with Ball Shifters + VersaBlock.

Re: One speed vs Two speed gearboxes
 

Absolutely agree.

-Mike

Re: One speed vs Two speed gearboxes
 

I would challenge the notion that a smaller gear ratio will always result in a quicker robot on the field. We did some modeling a couple years ago and found that because of the tradeoff of top speed and acceleration, 16 to 16.5fps free speed with two CIMS and full weight robot is fastest for a half field distance. Our thinking is that for most games, it is rare that you can have an uninterrupted run across the length of the field, and that having top speed here is not worth trading off performance during shorter runs.

We ran this speed last year and were keeping up gamewise with those geared faster. This could be because 2014 game play ended up being much further from an open field than it looked like, but I think another reason to not lose sight of is that once your robot is fast enough, it is more important how it is driven than what the exact speed. Our driver gets a lot of practice, and a fundamental of what we practice is how to accomplish whatever needed in the most efficient means possible.

Re: One speed vs Two speed gearboxes
 

Re: One speed vs Two speed gearboxes
 

This year, we had 6 Cims on our drive. During competition season, we were geared for 10.5 fps. With the wheels we had, this was traction limited, and we never got even close to popping breakers. But once we got to the highest level of competition at worlds, we were too slow to be competetive. We switched to 16FPS at IRI. The speeds with this actually allowed us to be competitive with the rest of the field. But in eliminations when our driver was playing rougher defense, we did pop breakers.

I would argue that with 6 Cims, the optimal points are about 10FPS for the low speed, and 16+fps for the high speed. With a 6 Cim drive, 10FPS will allow you to get around your half of the field basically as fast as possible, yet still allow you to not get in trouble with defense. Then if you need to cross the field, or the defender is faster then you, you can shift up to 16+fps and get across field very quickly.

If the field is split like 2010 or 2012, I would argue that a single speed 6 Cim 10 fps robot is the best you can get, because there is not enough room to use any higher of speed. For years like 2011, 2013 and 2014, you want to be able to shift up for extra speed to cross the field quickly. Those are the types of years shifter are useful, because they allow cross field travel faster.

Re: One speed vs Two speed gearboxes
 

I don't understand this, there are numerous factors as to whether a team wins or not, and I doubt shifting is a major one.

I would hesitate to use wins/losses as evidence to back up certain points, mainly because correlation is not causation.

As a counterexample to your counterexample, 610 in 2013 was extremely competitive to the point that they took home the Championship. They also only used one speed - I recommend looking up their philosophy on that, I believe Mr. Lim and Mr. Rob Stehlik have some in-depth posts about it. Back to my point, personally while I thought they had a very good robot, I would not attribute the win to them having the single best robot on the field, but rather to 1241, 1477, and 610's capability to play as an alliance instead of playing as three teams.

I still believe one choice is not inherently better than another, both have benefits and drawbacks, and it just depends on the game or team preference.

Re: One speed vs Two speed gearboxes
 

I have to disagree with 6 cim single speed 10fps. You get a maximum of a few hundreths of a second less time to get to a point versus 4 cims, regardless of the distance. If you're gearing for 10fps, I would definitely just use a 4 cim drive.

My example was actually to show that having a shifter isn't uncompetitive, although it's rather rather unnecesary. I wonder how many shifters were on Einstein these past years? That would provide more usable data.

Re: One speed vs Two speed gearboxes
 

You are also likely to wear out your batteries much faster. You are consuming energy to accelerate your robot and move it across the field. Higher speeds means that you will have to pull more energy out of the battery in each match than if you used a lower top speed. Thus, it is harder to avoid deeply discharging your batteries and damaging them (see page 9 of the Enersys "Genesis NP Applications Manual").

Our experience at work with using the NP7-12's in our UPS' is that applications that cycles the batteries more frequently and discharge them more deeply will shorten the life of the batteries, dramatically. There is also a line that states "When considering discharge currents exceeding 6C, consult with an EnerSys Application Engineer." Notes from manufacturers like this usually indicate that their experience is that such operating conditions are likely to permanently damage or shorten the life of the component. The 6C discharge rate corresponds to 103A which is easily exceeded in many of the recent drive train designs.

www.enersys.com/WorkArea/DownloadAsset.aspx?id=489

Re: One speed vs Two speed gearboxes
 

Acceleration isn't the only benefit. 6 cim drives are more efficient as they operate higher on the motor efficiency curve.

Re: One speed vs Two speed gearboxes
 

How did you determine that?

Re: One speed vs Two speed gearboxes
 

With a simulation model. As you add more cims, the torque load (and current) is split to more motors. Map this new torque on the cim motor curve, and you will see the motors spin at a higher speed creating more mechanical power. This is most noticable during times of high load such as turning and pushing where you operate closer to peak power instead of peak efficiency.

Re: One speed vs Two speed gearboxes
 

All else being equal, the CIMs on a 4 CIM drive spin the same speed as the CIMs on a 6 CIM drive, for any given robot speed (assuming the wheels are not slipping).

Re: One speed vs Two speed gearboxes
 

I agree speeds are same with no slip.

I don't think they will consume the same eletrical power though.

Re: One speed vs Two speed gearboxes
 

All else being equal, which do you think will consumer more power, and under what operating conditions?

Re: One speed vs Two speed gearboxes
 

In a constant velocity situation, when I did the math, it looked like the 4 Cim system drew more power. This was because each motor required more current to run at the same speed vs a 6 cim drive. If you're drive system takes 100 amps to hold the speed it is currently at, the 4 cim drive will be pulling 25 amps per cim, vs the 6 cim drive at 16.66 amps per cim. This means you can apply less voltage per cim in order to keep the same speed. This is only in the physical motors though. The entire system together will draw the same amount of power. Its just that the cims can be ran in a more efficient part of the curve.

When accelerating, the 6 cim will draw more power, but it will reach the constant velocity situation faster, when it will start drawing less current.

Re: One speed vs Two speed gearboxes
 

Could you please clarify? Are you saying that according to your calculation the 4 CIM system drew more power than the 6 CIM system, or the same power, or less power ?

Re: One speed vs Two speed gearboxes
 

The 4 cim system takes the same amperage, but more voltage to reach that amperage. So the 4 cim system does use more power to hold an identical velocity.

Re: One speed vs Two speed gearboxes
 

That doesn't make sense. It should take more amps than a six cim system per motor. Adding all the motors, a four cim system should use the same number of amps as a six cim system, with a bit of wiggle due to the efficiency curve of the cim.

Re: One speed vs Two speed gearboxes
 

They do use the same amperage. But the 6 cim version uses less power to to hit the same amperage, because each motor can use less voltage to hit its share of the amperage. Ex. If each cim needs to draw 25 amps in a 4 cim system, it might require 12 volts to do that. That would equal 1200w total. For the 6 cim, each motor only has to draw 16.66 amps, and might only need 9 volts to do that.
Thats only 900w. So less power is used by the 6 cim system to hold the same speed.

Re: One speed vs Two speed gearboxes
 

I would argue that for a significant number of teams an increase in top speed results in a less competitive robot because their drivers won't be able to handle the "twitchier" controls. I think slowing down the kit gearbox is one of the easiest knobs FIRST has to increase match scores, because it should be easier for good robots to get around the D and teams with less good mechanisms will have an easier time scoring. If I had a nickel for every time I watched a robot spend 2 minutes to pick up a game piece...

Re: One speed vs Two speed gearboxes
 

It's straightforward to do the calculation. Let's pick an operating point, say 2000 rpm at 240 ozin total torque of CIMs.

Calculate the CIM operating condition at 2000 rpm and 60 ozin (for 4CIM drive), and 2000 rpm and 40 ozin (for 6CIM drive):

4 CIM mechanical watts out = 4*88.7 = 355 watts
6 CIM mechanical watts out = 6*59.2 = 355 watts

4 CIM amps total = 4*24.3 = 97.2 amps
6 CIM amps total = 6*16.5 = 99.0 amps

4 CIM electrical power in = 6.62*97.2 = 643 watts
6 CIM electrical power in = 5.92*99.0 = 586 watts


So under certain operating conditions (in this case, 2000 rpm @ 355 watts out), 6 CIM draws less total power.

Re: One speed vs Two speed gearboxes
 

So we've been thinking about auto-shifting some more, and how to increase our efficiency. I've also heard that 1678 achieved speeds of 22 ft/s on the field and wondered how they did that, especially if they started in first gear then shifted or if they were in 2nd the entire time (assuming they used shifters). We were happy with our auto-shifting system last year and would like to improve it further. This led to a few questions:

How do we figure out the correct gearing for the low gear supposing we know we want to have a high gear of 17 ft/s? This needs to maximize acceleration as well as battery life.

What exactly does it mean to be traction-limited and what are the physics equations and calculations surrounding that?

We are planning on using either 4 CIMs, 4 CIMs and 2 MiniCIMs, or straight up 6 CIMs, so any advice on those configurations would help as well. I didn't want to start a new thread since there has been so much discussion on the topic, but since using auto-shifting as a method of avoiding blowing the main breaker was discussed here, I figured it was apt. We didn't have to worry about that last year since we only had 4 CIMs on the main drivetrain.

Re: One speed vs Two speed gearboxes
 

We were GEARED for 22fps(no loss), we measured somewhere between 17 and 18 actual. About 12 miles per hour or if you want to give a good value to some pit scouts, that's 5.3% the speed of sound!

Re: One speed vs Two speed gearboxes
 

As a general rule, most speeds posted for gearboxes and drivetrains are free speeds.
Because everybody posts free speeds, 22fps is still extremely fast, even if the actual is 17 or 18fps. A robot geared for 10fps may only reach 8.

Re: One speed vs Two speed gearboxes
 

Honestly, most people just post what the JVN calculator spits out with the default constant values. Unless someone says "free speed", I don't generally assume they are posting free speed. Otherwise a "12FPS" drivetrain would be a lot less impressive. Certainly not "everybody".

Re: One speed vs Two speed gearboxes
 

Oh ok, I see. What was your lower gearing set to, and did you have to upshift/downshift in order to get to that 17 fps, or were you able to start in 2nd gear just fine?

Re: One speed vs Two speed gearboxes
 

If you want to make it sound less impressive, call it Mach 0.053

Re: One speed vs Two speed gearboxes
 

I don't know what our high/low free speeds were.

However, I do know we did not auto shift. We just stuck it in high gear until we ran into so traffic, or the drive train was stalling (to prevent tripping the breaker).

Remember kids, it's not about the speed of your drive train, it's how you use it ;)

Merry Christmas!

-Mike

Re: One speed vs Two speed gearboxes
 

Thanks for the advice, I appreciate it. Merry Christmas to you too!

Re: One speed vs Two speed gearboxes
 

Figuring out the ideal High / Low gear split requires a bit of thinking, a bit of math, and honestly becomes a bit of a preference issue in the end. If you're planning on auto-shifting, especially if you're trying to optimize acceleration, you want to look at the acceleration of both options, and pair them together in a way that is going to give you the most smooth accel. If your split between high and low gear is too extreme, you'll notice a "dip" in acceleration, between the top end of low gear, and it's shift into high, where the robot appears to struggle a bit. Anecdotally, anything beyond a 2.5 split will stair having a significant "dip" between high and low gears, but it's been a while since I've looked at the issue in detail.

With that being said, and as Michael mentioned above, you can get away without shifting from low gear to high gear even when gearing aggressively - if your driver / team knows what they're doing. If you get used to driving at speed, and or optimizing paths so that you're not changing directions a lot, you're minimizing the amount of time spent getting up to speed.

As far as being "Traction Limited" is concerned - it's a function of you're drivetrains traction, relative to the "thrust" of the drive. Essentially if your drivetrain can produce enough torque (thrust) to spin the wheels before your breaker would trip, that'd mean you're traction limited. To calculate this, you need to first find the force of friction (Ff = Fn x CoF - is a good start) and then find the effective "Trust" (wheel torque) of the Drivetrain. (Input Torque x Gear Ratio / Wheel Radius - as a starting point) From there, you look at the current draw for that torque, and then compare that to the performance of your breakers.

One of the biggest things to be aware of is the current per motor, and the number of motors in the system. In a 4 CIM drivetrain, designing to be traction limited usually means designing around the 40A breakers in the PDB, so that you're not drawing enough current to trip them - But in a 6 CIM drivetrain, it's better to design around the 120A main breaker, specifically not exceeding a total current draw of about 150% (180A) which works out to about 30A or so a motor - doing this ensures that you shouldn't ever (or at least rarely) trip the main breaker...

Re: One speed vs Two speed gearboxes
 

I know that for the last few years, we've had high/low splits of right around 3:1 and free speeds of ~5/15 FPS, and I can't imagine wanting them to be any closer together than that. Even with only 4 CIMs, starting in high gear is fine and low can be almost exclusively reserved for pushing and fine control. A simple software adjustment like squaring the drive joystick values makes control in high gear noticeably more manageable, and if we were to change anything, I would say we would try to push it faster before deciding to slow it down.

Re: One speed vs Two speed gearboxes
 

On our 2013 robot, which did not auto shift, we often had trouble starting in 2nd gear, and it seemed to take a toll on the batteries whenever we tried. The robot simply did not like to get going very quickly in 2nd gear, so we started in 1st and upshifted to 2nd once we had some motion.

Do you have any guesses as to why this may be?

Re: One speed vs Two speed gearboxes
 

Well, I took a glance at your season highlight video, and the first thing that struck me was your wheels. How much did they weigh? They look relatively heavy. If you were rolling on heavier wheels (especially if a lot of that weight is concentrated ~4" from the center in the thick rubber tread), the wheels' moments of inertia can come into play. It probably took a lot more torque to get those wheels rolling than lighter ones like the VexPRO traction wheels (6" in 2013, 4" in 2014) that we've used.

One of the first things that comes to mind when you wonder about the physics of accelerating a robot is inertia, but rotational inertia can become a big factor too in some designs. That's my best guess as to what it was, especially if you didn't have as much of a problem with it in other years.

Re: One speed vs Two speed gearboxes
 

Hmm, yes, they were 8 inches Colsons, so pretty heavy. We were planning to use 6 inch wheels but then some complications arose that caused us to use bigger wheels, We didn't think it would have that adverse of an effect on the drive performance.

Could electronics be an issue? Or is that a minor concern, efficient wiring?

Thanks for the help!

Re: One speed vs Two speed gearboxes
 

The rotational inertia of the wheels does little to hinder robot acceleration.

For anybody interested, I compared the kinetic energy of the robot moving at 10 feet per second to the rotational energy of the wheels on a robot moving at 10 feet per second. If we compare these energies, we'll see how much of our power goes to spinning the wheels, and how much goes toward moving the robot.

A fully loaded robot weighs 150 lbs, which is 68 kg. 10 feet per second is 3.05 meters per second.

KE = 1/2*mv^2 = 0.5(68)(3.05)^2 = 316.285 Joules
That's the amount of energy it takes to bring your robot up to speed.

For the rotational energy, we've got to find the moment of inertia for the wheels. The radius of the wheel is 4", which is equal to 0.1016 meters, and I'll guess that the mass of the wheel is 2 pounds (probably heavier than the actual wheel), which is equal to 0.91 kg. The wheels are disc shaped, so we can use I = 1/2 * m *r^2 = 0.5(0.91)(.1016)(.1016) = 0.0046968 kg * m^2

One rotation of the wheel causes the robot to travel 8*pi inches = 25.1327 inches = 0.63872 meters/revolution.

3.05 meters/second (divided by) 0.63872 meters/revolution = 4.77517 rev/second = 30.00 radians/second

For the rotational energy, E = 1/2 I*omega^2 = 0.5(0.0046968)(900) = 2.114 Joules per wheel.

For six wheels, that's 12.684 Joules for a robot.

tl;dr, it requires 328.969 Joules to bring your robot to 10 feet per second, and 12.684 of these Joules (3.8%) are used to get your wheels up to speed. This assumes that you have 6 colsons with a diameter of 8" and a mass of 2 pounds.

The fact that the wheels spin when the robot moves makes the robot feel 5.7 pounds heavier to the drive system.

Re: One speed vs Two speed gearboxes
 

Good to know. I was actually in the process of working out this math myself, but you beat me to it. I guess I'm not all that surprised that it works out to be so small, but it was the only reason I could come up with other than "it just is." I did expect it to be more like 8-9% of the robot's weight rather than <4%, but kudos for proving it.

And those wheels are just under 2 pounds each.

Re: One speed vs Two speed gearboxes
 

I expected it to be more, which is why I did the math. I wonder how much the gears/chain/shafts contribute to the rotational inertia.

Re: One speed vs Two speed gearboxes
 

Those should be very little. The squared term in the rotational inertia of axially symmetrical objects is the radius, and almost nothing else in the drive system comes close to the wheels in radius. The only examples I can come up with are the transmissions we made in 2012 and 2013. The first reduction was done with Gates belts instead of gears, and the large pulleys were about 4" in diameter. They were spinning faster than the wheels, so they needed more energy put in to get going than if they were further along in the reduction sequence, but they weren't that heavy. They were aluminum and pretty well lightened, so I don't think they mattered that much.

Re: One speed vs Two speed gearboxes
 

Have you guys purchased new batteries since 2013/used the same batteries this year? I've found that many teams that run 4cim drives have no idea how bad the condition of their batteries is. A battery that is in bad condition can lead to sluggish accelerations because it's ability to release, or store energy has decreased. This type of problem is more prevalent among 6CIM drivebases, because the acceleration is higher, thus energy demand is increased. However, i could definitely see this impacting a 4cim setup if the battery has degraded enough.

As a general rule, we only run competition batteries for 2 years before retiring them to a non-competition use, such as a practice bot.

Re: One speed vs Two speed gearboxes
 

Electrically speaking the age alone of a battery is irrelevant to it performance. What you should really be checking for is internal resistance. A lightly used, well cared for battery can last longer than just two years, while a heavily used battery that is poorly mantained may not even last that long. However you are right that the battery will increase in resistance with aged, but the lagest factor in a batteries life expectancy is not the age but the usage and care. You may have more batteries that are perfectly fine to use. While I like your rule I encourage you to also check you internal resistance.

Also if you think that inefficient wiring is the issue try either:

1) wire the PDB, motor controllers and the motors as close together as possible. Short wire means less resistance.

2) If your components have to be spread out but you can affored a little extra weight try using a wire size bigger than FIRST requires for the motor wire. Doing this will decrease the wire resistance allowing for better energy transfer.

3) Finally, one of the best ways to get the most out of your motors is to be using the top of the line motor controllers. The Talons will be more efficient than say a Victor 884.

While I wish I could offer more insight on to what happened to 624 in 2013 with their shifting, I believe that all that I could have said has already been spoken. So I leave you with these electrical tips that can help get the most out of your motors.

Re: One speed vs Two speed gearboxes
 

Would you please elaborate a bit what you mean by "get the most out of your motors" and "more efficient"?

Do you have some test data you could share?

Re: One speed vs Two speed gearboxes
 

I don't have easy access to the data right due to the fact that I am on vacation.

The place I reamber finding the data was here on CD as an attachment. Once I get more time I will look for the data again.

Re: One speed vs Two speed gearboxes
 

Not to interfere with your vacation, but in the meanwhile could you explain what you meant by "more efficient" ?

Re: One speed vs Two speed gearboxes
 

You're absolutely right, but I think the point of doing this is less that something magic happens after two years to make batteries terrible, and more that batteries don't' cost that much to replace and it's easier to cycle out batteries early than to discover too late that you have a sub optimal battery.

I mean, the way batteries are used in FRC is inherently suboptimal. We use the batteries hard for two minutes, then throw them on a charger well before we've mostly discharged them, and usually to charge at a higher current than they should. We're already abusing these batteries.

Re: One speed vs Two speed gearboxes
 

Nicely presented! You might also want to consider the moment of inertia of the CIM motor rotors.

For example (using the motor rotor inertia figure given in another recent thread) if the 8" wheel 6WD robot in your example is driven using CIM motors with 14:1 speed reduction ratio between motors and wheels, each motor rotor will have a "reflected inertia" of J_rotor * (ratio^2) = 0.015 kg-m^2, or about three times the figure you used for one wheel.

For a different example, consider a drivetrain with 4" wheels and half the speed reduction ratio. Will a robot with the same mass get up to speed quicker if it is on smaller wheels?

Re: One speed vs Two speed gearboxes
 

We take good care of our batteries and purchase new ones every year. I don't think it's a battery issue because our 2014 robot has had very few issues regarding the drivetrain, and we interchange 2013 and 2014 batteries with it. This makes me think the issue in 2013 was mechanical efficiency of some sort - we used omni wheels in 2014 which meant there was very little wheel scrub so it took very little power to turn rapidly. In both 2011 and 2014 we used 4 inch wheels and those were our best drivetrains in terms of acceleration as well as turning ability.

So we were thinking the cause was the heavy and big radius 8 inch wheels, but correlation does not necessarily mean causation and Jared's equations prove that there is little effect. So next we'll be looking at electrical possibilities like Munchskull brought up. We're not extremely concerned because our 2014 drivetrain worked out so well, but it's always nice to figure out what went wrong and learn from our mistakes.

Re: One speed vs Two speed gearboxes
 

Are you saying you think it's better for lead-acid batteries to be mostly discharged before being charged??