Disadvantages of swerve/crab [Archive]

MichaelBick

29-12-2011, 20:18

Simply, there is none. An independent coaxial swerve, programmed perfectly, will be the best drive out there. The one performance disadvantage, is that if one module brakes, your whole drive system is down(this applies to all drive systems, except wcd and variations). However, the resources needed to get to this point are tremendous, and without planning and time, the benefits are nullified.

LeelandS

29-12-2011, 20:23

Competitively, Swerve is, perhaps, the best ALL AROUND drive train in FIRST. All around meaning it follows the old saying "Jack of all trades, master of none."

Swerve is so much desired because it offers the omni-directional movement of Mechanums, without relying on wheel slip and thus, is not as easily pushed. 4 Wheel Swerve will offer, more or less. similar power as a 4 wheel drive. Which is not particularly excessive, but well enough.

In addition, Swerve won't outrun a lot of drive systems. It's going to rely on that omni-directional movement to cut out the time used in turning to make up for the lack of excessive speed. Unless you're 973 using Emperor Swerve, you're not going to be moving at very fast speeds. That's going to be the result

Average speed + average power + omni-directional movement = one very effective drive system. But it will not handle drive systems that excel in power or speed very well. That is, of course, based on an average Swerve. Some teams who have become very well versed in Swerve drive can overcome these shortfalls and create a drive system with great power and speed. But your average Swerve will boast a decent level of speed and power, not a wealth of either.

Edit: And, yes, as many are mentioning, Swerve is very prone to failure. And recovering from a failure in a Swerve module is no easy task.

Edit-Edit: I apologize for my bad terming. I'm being quite unclear about a lot of things. Apologies, again.

Disclaimer: I'm no expert. Don't take my word as law.

connor.worley

29-12-2011, 20:28

Many many more possible points of failure.

Ninja_Bait

29-12-2011, 20:29

It's also likely to be devastated by a single part failure. If a module freezes up for any reason (even if it's perfect, there's a chance of damage) then the drive is almost useless.

Not that any other drivetrain is free of such problems.

sgreco

29-12-2011, 20:31

Mecanum wheels can drive in circles around swerve

MichaelBick

29-12-2011, 20:31

Competitively, Swerve is, perhaps, the best ALL AROUND drive train in FIRST. All around meaning it follows the old saying "Jack of all trades, master of none."

Swerve is so much desired because it offers the omni-directional movement of Mechanums, without relying on wheel slip and thus, is not as easily pushed. However, that in no way means the average Swerve can push. Your standard 4 wheel Swerve will lose a pushing match against most decently done 6 Wheel Drive Systems.

In addition, Swerve won't outrun a lot of drive systems. It's going to rely on that omni-directional movement to cut out the time used in turning to make up for the lack of excessive speed. Unless you're 973 using Emperor Swerve, you're not going to be moving at very fast speeds.

Average speed + average power + omni-directional movement = one very effective drive system. But it will not handle drive systems that excel in power or speed very well. That is, of course, based on an average Swerve. Some teams who have become very well versed in Swerve drive can overcome these shortfalls and create a drive system with great power and speed. But your average Swerve will boast a decent level of speed and power, not a wealth of either.

While you say that swerve does not have the same "power" as a 6 wheel, I'm not sure what you are referring to, as they will have the same power, assuming they have the same traction.

thefro526

29-12-2011, 20:32

Swerve is so much desired because it offers the omni-directional movement of Mechanums, without relying on wheel slip and thus, is not as easily pushed. However, that in no way means the average Swerve can push. Your standard 4 wheel Swerve will lose a pushing match against most decently done 6 Wheel Drive Systems.

In addition, Swerve won't outrun a lot of drive systems. It's going to rely on that omni-directional movement to cut out the time used in turning to make up for the lack of excessive speed. Unless you're 973 using Emperor Swerve, you're not going to be moving at very fast speeds.

These points aren't necessarily 100% accurate. Both of these will vary with the implementation of the swerve and the drive it's being compared to.

Pushing power is defined by the robots over all weight and wheel configuration, being a swerve doesn't really change much here - other than the fact that a 'unicorn swerve' (or any swerve with independently powered modules like 111's) has the issue of losing pushing power if one or more of the wheels lose the ground as the motor's power is wasted and the weight on the driven wheels will now be less - this is an issue that any drive with independent wheels can encounter.

For examples of fast swerves other than 973's Emperor Swerve, reference either 16 (In any year, but 2008 or 2011 would be best) or 148 (2008).

Tom Line

29-12-2011, 20:34

A crab drive will push with as much torque as you gear it to. Just like any other drivetrain. A crab drive will be just as fast as you gear it. Just like any other drivetrain.

Drawbacks to swerve include the interior space required, reliability, and serviceability. In addition, swerves can struggle to handle rough terrain as easily as 6 wheel drives.

Of course that can all be overcome by engineering. There were swerves that were quiet effective at going over the bump in '10, and swerves that have had lifting modules to go up a "stair".

The big disadvantage of swerve is the amount of time you are going to take away from working on game specific facets in any given year. Anything other swerve disadvantage (again ignoring the complexity and difficulty in making it) is really just an engineering challenge that can be overcome.

I don't think you can really ask what is better without taking complexity and manufacturing difficulty into account.

Akash Rastogi

29-12-2011, 20:36

These points aren't necessarily 100% accurate. Both of these will vary with the implementation of the swerve and the drive it's being compared to.

Pushing power is defined by the robots over all weight and wheel configuration, being a swerve doesn't really change much here - other than the fact that a 'unicorn swerve' has the issue of losing pushing power if one or more of the wheels lose the ground as the motor's power is wasted and the weight on the driven wheels will now be less - this is an issue that any drive with independent wheels can encounter.

For examples of fast swerves other than 973's Emperor Swerve, reference either 16 (In any year, but 2008 or 2011 would be best) or 148 (2008).

In addition to this- you (leland) keep saying "your standard swerve drive" but what are you defining that as?

The pushing thing can be whatever depending on the drive gearbox just as any other drivetrain can be. 2 speed solves all problems you mentioned.

Chris is me

29-12-2011, 20:39

How many of you guys have built and tested a swerve and compared it to a six wheel?

BJC

29-12-2011, 20:39

While you say that swerve does not have the same "power" as a 6 wheel, I'm not sure what you are referring to, as they will have the same power, assuming they have the same traction.

*Most swerves have one motor per wheel assembly. This means that when they are being pushed and are forced up on 2 wheels they have half the power to push back.

*118 had some wicked swerves specifically 2005 and 2007 are well known for the most powerful wicked fast drives ever. However, it could not rotate so to compensate they put their entire manipulator assembly on a turret.

LeelandS

29-12-2011, 20:46

In addition to this- you (leland) keep saying "your standard swerve drive" but what are you defining that as?

The pushing thing can be whatever depending on the drive gearbox just as any other drivetrain can be. 2 speed solves all problems you mentioned.

Apologies, I'm being quite unclear about these things right now.

By "Standard swerve", I'm not referring to anything particular about the Swerve itself. I'm referring more to a Swerve from a team who hasn't really mastered the Swerve yet (i.e. 16, 111, 118, etc.). Those teams have worked up good implementations to Swerve, and have made steps in overcoming the common short comings that go with it.

Again, I apologies. I'm being very unclear.

thefro526

29-12-2011, 20:50

sgreco

29-12-2011, 21:06

How many of you guys have built and tested a swerve and compared it to a six wheel?

2079 has kind of done this. We didn't really test any quantitative data, but we have run both in comparison during driver tryouts. The general sentiment from our drivers was that swerve is not as intuitive to drive (it does depend on the programming, but the programming is something that is really hard to get right).

And just from experience of doing both, 2079's 2nd gen swerve broke with much higher frequency than our 1st gen 6WD. Swerve requires a lot more fine tuning before you get it "right".

All 6 wheel drives have a zero turn radius (or at least most). Only well designed swerve drives can do this.

Both designs are good if implemented well, swerve just seems to require more effort to make it work well.

craigboez

29-12-2011, 21:54

Lil' Lavery

29-12-2011, 22:50

I'd say one big disadvantage is that it isn't very practical to implement a two speed drivetrain with swerve. You'd need to pick one multi purpose speed and gear for that, making your robot prone to being pushed around by a 2-speed 6WD bot.

Well, if we're ignoring the weight and manufacturing factors as instructed in the first post... :rolleyes:

Akash Rastogi

29-12-2011, 23:00

I'd say one big disadvantage is that it isn't very practical to implement a two speed drivetrain with swerve. You'd need to pick one multi purpose speed and gear for that, making your robot prone to being pushed around by a 2-speed 6WD bot.

What is impractical about making a multi-speed swerve?

AlexH

29-12-2011, 23:15

They eat a larger chunk of the weight budget than a normal tank drive.

Only effective if you have a good driver who can capitalize on the directional freedom that a swerve offers.

Mk.32

30-12-2011, 00:22

The extra driver practice it takes to be able to master a swerve and be able to use all its features, though personally I have never done it so I can't speak from experience.

Aren_Hill

30-12-2011, 00:35

The largest disadvantage I see to a swerve (once constructed and programmed) is it takes more driver practice to get to the same level as a 6wd, and then eventually surpass. Most silly humans get a little overwhelmed by the ability to go anywhere and spin any way.

sincerely,
someone with actual swerve credentials, who's spent the last few days driving around a unicorn swerve

AdamHeard

30-12-2011, 00:39

Complexity of the hardware.

Which is nothing compared to complexity of software/control philosophy.

Akash Rastogi

30-12-2011, 00:44

Apologies, I'm being quite unclear about these things right now.

By "Standard swerve", I'm not referring to anything particular about the Swerve itself. I'm referring more to a Swerve from a team who hasn't really mastered the Swerve yet (i.e. 16, 111, 118, etc.). Those teams have worked up good implementations to Swerve, and have made steps in overcoming the common short comings that go with it.

Again, I apologies. I'm being very unclear.

No need for apologies, I was just wondering what people consider a standard swerve to entail.

Also, it's worth mentioning that a team could build a 'fast' (Relative term), 'powerful' (Relative term), swerve using proven COTS components from AM and Team221 LLC - they've done the hard mechanical work for a team looking for swerve performance without swerve machining resources.

Just an anecdote of caution:
Using the 221 modules in 2010 was pretty cool; we learned a lot about the mechanics of a swerve. But with only a week to figure out how to program the swerve - our team just made an idiotic decision to go with this drive over our 8wd design, even after we were told we wouldn't get our modules until after week 3. This stupid decision was made because students and mentors got entranced by the "coolness factor" of swerve. It had no place in the 2010 game. Even with a full practice swerve to play with and program, we had only just started to figure out the best controls for the swerve WHILE in Atlanta that year. It is a HUGE learning curve for even the best programmers. (we had students and programming mentors working on it). All I'm trying to say is that even with the mechanics pretty much figured out for us through COTS parts, the programming and sensory involved took a long long time to figure out. It is hard to express my abhorrence for the decision making that went on that year.

Aren_Hill

30-12-2011, 00:48

Complexity of the hardware.

Ignoring the original posters statement?

Simply, there is none. An independent coaxial swerve, programmed perfectly, will be the best drive out there. The one performance disadvantage, is that if one module brakes, your whole drive system is down

Oh really? lost a drive chain on an 8motor swerve to one of the modules recently and kept on driving just fine, barely noticed...

I'd say one big disadvantage is that it isn't very practical to implement a two speed drivetrain with swerve. You'd need to pick one multi purpose speed and gear for that, making your robot prone to being pushed around by a 2-speed 6WD bot.

One of the largest Pro's to swerve is almost never having to get into pushing contest you don't want to, in many cases rendering 2 speeds not necessary. (keep in mind i had a 2 speed swerve in 08, using COTS AM gen2's....so the 2 speed part really didn't require much work on my end)

They eat a larger chunk of the weight budget than a normal tank drive.

Only effective if you have a good driver who can capitalize on the directional freedom that a swerve offers.

I've got a few concepts that could hit < 35lbs for complete chassis with completely independent module power and steering, so weights only really an issue if you aren't trying hard enough

Please only post things when you KNOW them, preferably from experience, or as some have done, put a proper disclaimer

craigboez

30-12-2011, 01:32

I'd say one big disadvantage is that it isn't very practical to implement a two speed drivetrain with swerve.

Should have clarified, it is impractical if you're using 4 wheel swerve with one CIM at each wheel. If you're using a central "power plant" then it becomes much more practical.

Chris is me

30-12-2011, 01:53

Should have clarified, it is impractical if you're using 4 wheel swerve with one CIM at each wheel. If you're using a central "power plant" then it becomes much more practical.

973 just posted a neat little CAD model of an 8 motor independent shifting swerve...

AdamHeard

30-12-2011, 01:54

973 just posted a neat little CAD model of an 8 motor independent shifting swerve...

Yup.

Any team with a manual mill and a waterjet/laser sponsor could make such a thing as well.

Most teams are capable of getting such sponsors and machines.

craigboez

30-12-2011, 02:00

973 just posted a neat little CAD model of an 8 motor independent shifting swerve...

Wow, very impressive. I didn't think that was practical, but they went and proved me wrong.

Garrett.d.w

30-12-2011, 02:07

As has been said earlier, driver practice is the main bump in the road (so to speak). If you plan on building a swerve drive, prototype it in the offseason. We didn't, so even though our swerve worked perfectly, we modified it to drive like a tank.

As with everything, practice makes perfect.

I wish you luck if you decide to go for it. It's one of the coolest drive systems out there.

davidthefat

30-12-2011, 02:48

Isn't it generally accepted that a more complex system fails even harder? More moving parts allow for more chances of something going wrong? I believe that is a serious flaw in performance.

evoluti1

30-12-2011, 03:09

Isn't it generally accepted that a more complex system fails even harder?

NASA even has a phrase for this: "The higher the tech, the bigger the wreck." It might be a little late in the game to be thinking about a crab drive at this point. My team wanted to do crab drive this summer, but earlier this month we realized we simply didn't have time to finish and refocused our efforts towards a WCD.

Your team may be different, of course. If you have the financial/technical/mentor resources to make a crab drive happen during season, go for it. But from what I've seen, only very elite teams have those kinds of resources, and even most of them prefer to stick with something simpler.

You have to remember, this is real life. Of course, a "perfect" crab drive beats any other drive on the field. But the level of quality/perfection that makes a crab drive competitive is very difficult to achieve in the real world.

Jared Russell

30-12-2011, 08:46

* Often less intuitive to drive than skid steer robots
* There is latency to all changes of direction because module steering is never instantaneous
* Geometry of swerve (assuming normal bumper zone rules) means that wheels cannot be put as far out to the edges of the chassis as in a skid steer case (may make being tipped easier)
* On uneven terrain, one or more motors may be unable to deliver power to the ground
* Unless you build an all-wheels-driven-and-steered-independently ("unicorn") swerve, there will always be certain combinations of manuevers that you cannot make.

Ether

30-12-2011, 09:55

Even with a full practice swerve to play with and program, we had only just started to figure out the best controls for the swerve WHILE in Atlanta that year.

In the above context, "the best controls" sounds like you are referring to the driver interface. What driver interface did you finally decide was the best for your team?

It is a HUGE learning curve for even the best programmers. (we had students and programming mentors working on it)... even with the mechanics pretty much figured out for us through COTS parts, the programming and sensory involved took a long long time to figure out.

Could you elaborate just a bit and list the top three (or whatever) things that made the learning curve huge and which took a long time to figure out? It might be helpful for other teams on the verge of making a similar decision.

sgreco

30-12-2011, 10:21

Even with a full practice swerve to play with and program, we had only just started to figure out the best controls for the swerve WHILE in Atlanta that year.

That's actually pretty quick as far as I'm concerned. 2079 built a prototype swerve before the 2008 season, we build a swerve drive for the 2008 season. We made our third iteration for the 2009 season. We just started to get it where we wanted it at the end our second regional in 2009. They have since ditched the idea of swerve do to complexity and lack of a fully intuitive algorithm for controls.

The real thing is that swerve is just harder to make well. It's not better or worse than any other drive train.

John

30-12-2011, 12:52

Theoretically, if you had unlimited resources, unlimited manufacturing time, unlimited programming time, and unlimited driver practice time, to build a "perfect" swerve drive, it would be clearly better than any other drive train. I would consider a "perfect swerve drive to be: all wheels independently powered, steered, shifting, never breaks :rolleyes:, low weight, very low delays in turning the wheels, driver can intuitively control all functions, no repercussions on other systems.

Its pushing strength would be as good as a low speed skid steer,
its speed would be as good as a high speed skid steer,
its acceleration would be as good as any robot of similar weight,
its omnidirectional ability would be as good as mecanum/omniwheel drive

However, any real team does not have unlimited resources and time. As a result, most swerve drives do not achieve that performance. Some teams have gotten close, and these teams have generally been very successful, (148 in 2008, 111 in 2009). The problems with a swerve have to do with the real challenges of making one work like it is supposed to without massive tradeoffs, and not with any inherent disadvantage to swerve.

Ether

30-12-2011, 13:19

Theoretically, if you had unlimited resources, unlimited manufacturing time, unlimited programming time, and unlimited driver practice time, to build a "perfect" swerve drive, it would be clearly better than any other drive train.

You didn't mention anything about waiving FRC rules, so I assume you are still limited to the legal type and number of motors.

its omnidirectional ability would be as good as mecanum/omniwheel drive

I think the jury is still out on whether it is possible to steer the wheels on a swerve fast enough to make swerve "omnidirectional ability" as good as a well-built omni or mec.

Siri

30-12-2011, 13:32

What Jared said.

Also, FIRST has literal strategies that jack-of-all-trades swerve just isn't the best for. Of course, if you want to master a single "trade", swerve isn't necessary. But even if you want more, swerve gives you two things--reasonable holonomic-ness and reasonable traction--all the time. If you want, say, better holonomic drive sometimes and unmovable traction other times, something like switchable wheels/Octocanum could be better. Etcetera.

What is impractical about making a multi-speed swerve?Depends. If you're doing a limited swerve/non-coaxial, adding a shifter isn't so hard. Maybe heavy, but at least you're not rotating it and probably don't need so many. In a co-axial, you're driving each wheel independently and rotating the drive motor/gearbox with the module. Adding a shifting mechanism gets really big and heavy (and expensive and complicated) really quickly. We do all our gear ratio adjustment via belt pulleys and planetary gearboxes. After two years of working on the drive, "impractical" still seems like a pretty good word for trying to mount an AM Supershifter on this (wiki.team1640.com/index.php?title=File:2011_pivot_annotated.jpg), but maybe there are more streamlined shifters?

That said, we tend to win our pushing contests.

Only effective if you have a good driver who can capitalize on the directional freedom that a swerve offers.Moreover, only if you have enough good drive time with your chassis to get this capitalization experience. We've done poorly in this area for the first two years, but now that we have essentially optimized hardware, getting-close programming and excited drivers, this is huge.

I've got a few concepts that could hit < 35lbs for complete chassis with completely independent module power and steering, so weights only really an issue if you aren't trying hard enoughWow, that's great! Our modules are at 9.1lb each with a 14.1lb chassis weldment. How'd you get it to 35lb total?

gblake

30-12-2011, 14:17

One obvious disadvantage is the endless debate they engender.

One clear advantage is the endless debate they engender.

;)

Blake

Akash Rastogi

30-12-2011, 14:52

In the above context, "the best controls" sounds like you are referring to the driver interface. What driver interface did you finally decide was the best for your team?

Sure thing! Best is the wrong word too, "better" is the correct choice. We started off with a single 3D Pro joystick to control the drive system, but did not get a chance to figure something else out until later in the season which was using a joystick for throttle and then used the 3d joystick for rotation. It isn't best, but it was as far as we got before we decided to put off swerve until future offseason projects. But even then, we ended up using the override button on the joysticks which turned it into a 4 wheel tank drive most of the time.

Could you elaborate just a bit and list the top three (or whatever) things that made the learning curve huge and which took a long time to figure out? It might be helpful for other teams on the verge of making a similar decision.

I'd say some of the biggest issues we had were 1. being dumb and not using a 1:1 reduction for the rotation, 2. getting the modules perfectly synched, and 3. realizing too late that we should have used higher quality sensors than we did.

Big problem was that our drivers expected a perfect response time from the modules everytime they wanted to move somewhere, but our gearing and sensors significantly delayed response time.

Andrew Lawrence

30-12-2011, 14:57

Big problem was that our drivers expected a perfect response time from the modules everytime they wanted to move somewhere, but our gearing and sensors significantly delayed response time.

How significant? Was it like major lag between controller input and movement, or more like a small yet noticeable change between input and movement?

Also, what gearing did you use, and which sensors? Which would you recommend of both?

Akash Rastogi

30-12-2011, 15:09

How significant? Was it like major lag between controller input and movement, or more like a small yet noticeable change between input and movement?

Also, what gearing did you use, and which sensors? Which would you recommend of both?

It did what seemed liked a small change between input and movement sometimes, but later on that was fixed. The delay later on was, to me, significant because I did not know what to expect. It was just not instant as we all wanted, and I think this also happens to a lot of people when they first try swerve. The team might be under the impression that they'll get prefect turning and response from the get-go. However, with optimized systems such as 973, 111, 16, I suspect they see the kinds of results we expected: instant response (or close to).

As for the gearing, I'd have to go back to the CAD as I don't recall. The CAD should be somewhere on the Autodesk website from 2010 if they still archive that stuff.

Aren_Hill

01-01-2012, 18:33

Wow, that's great! Our modules are at 9.1lb each with a 14.1lb chassis weldment. How'd you get it to 35lb total?

Well over thanksgiving I rebuilt 1625's 2010 practice chassis (was 6wd swerve) into an 8 motor independent swerve, that came in a 41lbs,

Mainly its just sitting there staring at a concept going "how do i get rid of that....and why is that even there?" when looking at parts :p

current one i have almost ready to go is 6lbs flat per module (including CIM and steering motor)