Why is swerve so slow?
 

Just wondering why swerve drives all seem to be geared for very low top speeds on the order of 10-15fps. Even 1717 with their shifting swerve has a top speed of ~16fps. Why is this? Is it due to the 4 cim drives?

Re: Why is swerve so slow?
 

16fps is not a low speed. The (few) fastest FRC robots are geared for a little more than 20fps and the kitbot is geared for about 10fps. Normal low gears for FRC robots are about 4-7fps.

Most drivetrains, including swerve drives, have a < ~20fps top speed because a drivetrain geared too high will accelerate slowly, turn badly and possibly trip breakers.

Re: Why is swerve so slow?
 

Not sure about turning, but yeah, acceleration would be a problem. Thank you!

Re: Why is swerve so slow?
 

Speed is a interesting concept and a top speed is a really weird way to state it.
We ran swerve this year using something almost identical to what 1640 uses. We ran something at like 12.5-13.5 fps single speed. Acceleration matters much more when you are supper maneuverable in a tight space then when you just want to out run your competition. We wanted something just above the median speed range but not so fast that we would never reach the speed. So a acceleration that we can reach top speed in about 3ish robot lengths. The key is not to be to fast to out run your opponent nor to slow only be where you need to be in such a way that it would appear effortless.
https://www.youtube.com/watch?v=Lpb2...DIBcQ&index=37
Us team 2517 were always where we needed to be to play defense in just the right time in such a way that it would be hard to see where we were going.

Now there is some cool things you can do with swerve that make it appear that you are travailing much slower then you really are. By making small circles where a normal tank drive would drive back then ram you can be essential ramming at full speed even though it takes up the same amount of space as a tank drive would take to break and start moving again.

Re: Why is swerve so slow?
 

Exellent driving in that match! You definitely look faster than you are.

Thinking about acceleration, if you could take the battery load then you could "warm up" the motors by spinning in place until it's time to move.

One thing to note is that acceleration apparently barely changes from 10fps to 20fps. One parent on our team made me a spreadsheet (with graphs) that detailed acceleration given motor specs and robot weight, although it did not include friction in the calculations. It showed that for speeds up to ~30fps the distance vs. time was almost the same. By going at lower speeds you would get an advantage on the order of a few inches.

Increasing the number of cims did help acceleration a lot, but nothing else except robot weight helped that much (according to the spreadsheet).

Re: Why is swerve so slow?
 

Most teams don't "spin in place" because that is also known as "stalling your motors."

A look at Ether's Drivetrain Acceleration Model might be worthwhile.

Re: Why is swerve so slow?
 

There's something wrong with that spreadsheet...

Re: Why is swerve so slow?
 

What model for acceleration are you using? What factors is it taking into account? Are these results backed up by empirical data? There are models available for acceleration in FRC, but they are just that - models. There isn't a difference "on the order of a few inches" in acceleration between 10 and 20 FPS, unless you perhaps mean the time it takes to make a 40ft sprint (and if it's a difference of a few inches, the 20 FPS drive is extremely inefficient and performs worse at close range).

I've made the mistake before of using spreadsheets to design drivetrains without really understanding what the values on the screen meant. I'm not saying that's what you're doing here, but that model seems to conflict with my empirical data so I'm curious how it works.

Re: Why is swerve so slow?
 

Here, I'll put the spreadsheet into google drive. Like I said, it doesn't take into account friction so the results might be a little off. Please wait a moment.
See here: https://drive.google.com/file/d/0BzU...it?usp=sharing
You can download and open it in excel.

Note that everything is in meters/second, not feet/second. 10m = 32ft.

Re: Why is swerve so slow?
 

The only team to do swerve in NE was 2067. They were one of the fastest on the field (didnt see too much pushing robots out of the way from them due to speed. Instead they would go around.)

Re: Why is swerve so slow?
 

Are you reading the graphs correctly?

At 2 seconds, the 10fps bot is more than 9 feet ahead of the 20fps bot.

Re: Why is swerve so slow?
 

Ether, it's in meters/second. 3.04m and 6.08m is what should be compared for 10fps vs. 20fps.

Re: Why is swerve so slow?
 

I think what you meant was 3.04m/s and 6.08m/s.

If so, the difference in distance traveled at 2 seconds is almost 13 feet, according to the spreadsheet.

The maximum distance traveled at 2 seconds occurs with 30fps gearing, according to the spreadsheet. The lack of friction and electrical resistance in the model probably is responsible for this unrealistically high number.

Re: Why is swerve so slow?
 

If this discussion is about calculations, then please carry on -- theory is always amusing, and frequently provides insight.

However, if anyone seriously believes ALL swerve drives are slow, then I think there are several highly successful robot drivers who can offer very convincing, practical refutation. <insert well known team numbers here>

Just as one example, Team 16 has not built a robot that anyone could reasonable consider "slow" for a quite a long time. Circa 2004, I recall a very young John Taylor Novak (all 42 inches of him) seated on the St. Louis Regional inspection station table, explaining to a group of grey haired engineers why swerve drive is the best thing since sliced bread.

Re: Why is swerve so slow?
 

To be fair, while he's MUCH taller now he still does that...

Re: Why is swerve so slow?
 

Something else to note swerve drives are typically heaver then a skid/west coast ect decreasing acceleration.

How does the mechanical efficiency of a swerve drive compare to that of other drives?
How much does Mechanical efficiency matter any way? Does removing a gear set or a chain really do that much?

Re: Why is swerve so slow?
 

And he doesn't actually sit on the inspectors' table now...the under 12 rule went into effect the year he turned 12. Could have been ugly.

Re: Why is swerve so slow?
 

So you're telling me the 120 lb robot with swerve will accelerate slower than the 120 lb robot with 6WD because it weighs more.

:rolleyes:

Swerve drives are not inherently slower than any other drive system.
/thread

Re: Why is swerve so slow?
 

Its a know fact that swerve is heavy.
Comparing a robot that is identical to another with a swerve drive. The one with the swerve drive will almost always be the heavy one.
This year was a big point in that when robots would typically only weigh 90ish pounds.
I'm not saying that swerves will always be slower I am just pointing out they are heavy in comparison.

Re: Why is swerve so slow?
 

Correct but note that in high gear drive trains are power limited so 6 cims in a tank drive will lead to faster acceleration.

Re: Why is swerve so slow?
 

What may give the impression of slow for swerves (4 wheel independent) is what happens when rotation and translation (spin and strafe) actions are combined. Here each motor is going at a different direction and speed - the fastest motor is at the limit - the other are less - net result is a slower translation motion. But, If you drive pure translation (all 4 motors at their limit speed) and no rotation you go faster.
(You can see this effect if you use Cory's 2067 swerve simulation shown on a different thread)

A second consideration is most swerves are not shifters - so you will typically see in the 10 - 14 fps range. We were around 12 fps which was actually faster than we have been before (with tank, and were at ~10 fps in our development swerve off-season robot). Tank shifters will often have high speeds at 14 - 18 fps (true speeds) (there are shifting swerves out there though).

Thirdly most are 4 CIMs - compared with the ease (now) of implementing 6 CIM shifting tank drives - will typically show less acceleration for a given gear ratio.

On the other hand a swerves ability to directly go in the direction desired and to spin around a defender can speed up getting from A to B.

I believe the open field this year favored faster. NE, I believe, has traditionally run slower with more emphasis on defense/pushing. I think this year CA taught us that fast and maneuverable was the way to go - though with 6 CIM shifters to maintain pushing capability.

As some have posted, in 2013 on a "shorter" field, 610 showed a conservative 10-11 fps cycler that was "Faster" than most. Going with single speed 6 CIM drive has advantages for simplicity; mechanical and driving, and low weight.

In the end you may be perceiving a swerve to go slower but if applied (correct gear ratio(s) used, and reliable) and driven well can be more efficient and "faster" in executing its goals.

Anyone seeing 1717 in 2012 would say they were clearly the fastest (IMO) in collecting balls and shooting reliably, though you watch there slalom video and would say they looked slow.

Re: Why is swerve so slow?
 

It seems the inability to carry momentum through a change in translation direction causes a swerve to be a bit slow.

For example, on a skid steer setup, you can be moving very fast and basically pin one side to the ground. This allows the robot to use its own momentum to swing around the pinned wheel side. You exit the turn with almost as much energy as you had before, just moving a different direction.

I think most swerve drive software implementations are pretty naive and will just turn the wheels 90 degrees when the driver wants to start moving to the side. Field centric control only makes this worse as there is another controller between your driver and the wheels.

If we ever built a field centric swerve, I would almost certainly allow a drive mode that allows quick momentum saving direction changes. (Maybe a single button click? Maybe a press and hold button that locks 'forward' to the direction of your last translate command then allows throttle/turn steering?)

Re: Why is swerve so slow?
 

16 does this and it's why everyone thinks they are so fast.

We had variations of it in 2012 but never quite practiced them enough to utilize it.

Re: Why is swerve so slow?
 

Just throwing this out there: I've got a drivetrain calculator spreadsheet on here.

http://www.chiefdelphi.com/media/papers/3038

It used Ether's algorithms, but it's in Excel format. There are some pretty graphs as well.

Re: Why is swerve so slow?
 

I was thinking about this too. It is very possible to do this using a swerve drive. However, not only can swerve turn like that, it can also do a "basketball roll" around a defensive robot by spinning while going around the opposing robot to keep its momentum.

I recall 254 made some swerve modules in 2008. Where did that go?

Re: Why is swerve so slow?
 

148 did swerve in 2008, and I doubt anyone can call that robot slow.

Re: Why is swerve so slow?
 

Those modules were for the 2008 world champions :rolleyes:

Re: Why is swerve so slow?
 

I always assumed that they were a prototype, or for the EWCP swerve project!

*Mind Blown...*

Re: Why is swerve so slow?
 

They were definitely 148's in 08. 254/968 just posted the picture, they didn't make them.

254 isn't even involved in EWCP either. Did they ever do a swerve, I don't recall that.

Re: Why is swerve so slow?
 

There are maneuvers that allow the momentum to transfer they just are not instantaneous.
The faster you want to translate the less momentum you can save. Mind you this is for pure translation only you can save a marginal amount of momentum by translating and rotating at the same time. http://youtu.be/1oVNrp2L1EQ?t=2m27s For instance this is the best example of us translating with the rotation.
Using banked turns on a swerve drive is best as it allows for the most momentum saved over instantaneous turns.

Re: Why is swerve so slow?
 

I recall a well respected 254 mentor going on record with a clear statement that sideways motion is a waste of time.

Re: Why is swerve so slow?
 

Ha! I should have appended "for our team" on the end of that comment.

We try to build robots that can complete the game challenges in the simplest, most efficient manner (in both design and implementation). We have not seen a game where our primary scoring objectives cannot be efficiently accomplished by a 6/8 wheel drive. There may be games in the future where this is no longer true, but (I'm practicing my Pat Fairbank-isms here) we'll burn that bridge when we get to it. We've talked about building one as an academic exercise but I think there is enough community experience on the subject that we could put something together, if we needed to, without a homegrown proof of concept.

Now for other teams, this could be different. Swerve drives are incredibly cool and building them definitely inspires a lot of students every year. It's a hard engineering challenge which some people may want to try out, and that is awesome. We just concentrate on other things.

Re: Why is swerve so slow?
 

Even this shows that a 11fps drive will go only ~6" farther than a 21fps drive maximum. after about 3/4 of a second, the 21fps drive will go farther. This is with 4 cims.

So 254 has never actually built a swerve drive then?

Re: Why is swerve so slow?
 

Could you or someone else elaborate? I'm not entirely sure I understand how this works

Re: Why is swerve so slow?
 

Essentially their primary drive mode is Monster Truck steering. It's a traditional omnidirectional swerve derivation (like the type Ether posted) but with strafe locked out.

It will only ever let you drive smooth splines essentially, so you're always carrying your speed with you. You can still do very tight radius (and at no speed zero radius) turns.

They then have multiple (maybe just one?) other modes they toggle into for strafing variants.

16 could explain better, but I think I got the jist of it. I feel video of their 2012 bot shows this pretty well.

Re: Why is swerve so slow?
 

We are in 4-wheel (Ackermann?) steering that is NOT field-centric by default. In that mode we maintain a lot of our momentum because the turns are very smooth. When the driver needs to make a quick maneuver, he pulls (and holds) the trigger on the joystick to enable crab that IS field-centric from the orientation of the robot when the trigger was pulled. The driver has the x and y motion on one joystick and a twist motion on the other. When the trigger is released, the driver is back in 4-wheel steering mode.

Re: Why is swerve so slow?
 

Well said.

We have toyed with other driving/aiming modes, but they are just variants of those two basic functions.

Re: Why is swerve so slow?
 

What do you mean by monster truck steering?

Re: Why is swerve so slow?
 

That is so weird. Man, that must take a lot of practice to get used to!

Re: Why is swerve so slow?
 

List of reasons swerve is "slower" than a traditional skid steer. Reasons may not be valid for all swerve variants.

-Likely has more stages of power transmission, and if using bevel gears has more inefficient power transmission. A good skid-steer is typically 2-3 stages of decent spur gears. A swerve could be 3-4 stages easy. Motor in module variants are an exception. This changes a drive efficiency from a possibly 90% to maybe around 70% (made up numbers!)

-For the same reasons as above, there is likely more rotating mass (inertia) in the drive to accelerate, this could easily be a few percentage points slowdown.

-Steering error. The odds of all your modules pointing perfect is low, there are likely appreciable losses due to friction here.

-Steering time. It takes time to steer the modules to where they want to go, and teams have different methods for dealing with this. Some assume it's instantaneous as they gear it fast, but this leads to a lot of the above error on the way there, which wastes battery and heats the motors more. Some teams turn power down until wheels are pointing where they want to go, and this also obviously adds a delay as the drive must wait before it starts really going.

-Weight Transfer. Depending on the exact gearing, CG, overall robot geometry, tread, etc... it's possible that during initial acceleration enough weight is transferred such that half your wheels are now breaking traction. Even if this happened for .1-.2 s it would appreciably slow you down. This is only valid for independently powered modules.

-Driving style/Control Scheme. Except for 16, most famous swerves have been driven by point to point drivers. As Tom pointed out top drivers on skid steer are forced to learn to carry their momentum to be quick. Many swerves don't do this. If we swerve again, we will spend time looking at this.

There are more reasons, and if I think of them I'll edit and add them.

None of the above are huge, but a few small things could easily add up to make something seem/feel a bit slower. Maybe if I get a good block of time I'll model the above and see if I can point out a difference.

Re: Why is swerve so slow?
 

JTN could comment better about the difficulty of learning it (I'm a pretty terrible driver), but it's probably not as difficult as you think. You just have to put yourself "inside" the robot. The tough part comes when you change orientation (twist) in crab. You have to keep up with where the "front" of the robot is/was.

Like most robots, it's not hard to learn the basics, but it takes hours and hours of practice to get good. There's no time to stop and think behind he glass.

Re: Why is swerve so slow?
 

You have gas (speed) and steering angle inputs. All four wheels turn to create the desired robot turn. Wheel stop turns will spin about the center of the robot with no translation.

Re: Why is swerve so slow?
 

Oh ok, so the rotation point is at the center of the robot.
I didn't know monster trucks drove like that :P

Re: Why is swerve so slow?
 

I'll say we assume they are correct and taking off. The modules are actually really fast and we always take the shortest route to the desired angle. This requires inverting the drive motor direction if necessary.
We noticed our drivetrain chewed up batteries much faster when we swapped the Denso window motors for the AM 9012.

Re: Why is swerve so slow?
 

You killed that chair...

Re: Why is swerve so slow?
 

But it was quite a spectacular killing!

If you're going to test controls changes, do it with confidence. Full throttle or go home!