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Usable Caster Drive Design

Bryce2471

By: Bryce2471
New: 05-17-2018 05:09 PM
Updated: 05-17-2018 06:09 PM
Views: 1330 times


Usable Caster Drive Design

With this design, I wanted to create a caster drive that wasn't massive, or enormous, or impossible to manufacture, so that maybe something similar could eventually be used on a competition robot.

I have also wanted to try using Onshape for a while now, so I took the opportunity to kill two birds with one stone. My experience with Onshape was a positive one in general, but it had some blemishes. I might go into more detail in a later post.

Here is a link to the CAD model. Because it's in Onshape, you should be able to view it without downloading any software.

Discussion

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05-17-2018 06:25 PM

gorrilla


Unread Re: pic: Usable Caster Drive Design

Is the only difference between a traditional swerve and this being that the wheel is not inline with the axis of rotation?

Is this something anyone's done before? because I have a similar concept to this involving the motor in module cad I post a few days ago



05-17-2018 06:59 PM

Lil' Lavery


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by gorrilla View Post
Is the only difference between a traditional swerve and this being that the wheel is not inline with the axis of rotation?

Is this something anyone's done before? because I have a similar concept to this involving the motor in module cad I post a few days ago
https://en.wikipedia.org/wiki/Caster_angle



05-17-2018 07:00 PM

Bryce2471


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by gorrilla View Post
Is the only difference between a traditional swerve and this being that the wheel is not inline with the axis of rotation?
That's the principal difference.
Quote:
Is this something anyone's done before? because I have a similar concept to this involving the motor in module cad I post a few days ago
I have been told that this type of dive system has been used in industry in a few applications. I think there is a related patent out there somewhere. I've been thinking about making an FRC size caster drive for for several years, but as far as I know, no FRC team has built one.



05-17-2018 07:14 PM

SPang


Unread Re: pic: Usable Caster Drive Design

I love your packaging of this design, its so compact - it looks like you're using the underside faces of the bolt heads to retain the main bearing and gear for azimuth rotation?

How well would this module (when implemented in a set of 4) stand up to the load of a fully weighted FRC robot and the rigors of competition?



05-17-2018 07:15 PM

Jared Russell


Unread Re: pic: Usable Caster Drive Design

Seems pretty straightforward to give the caster a suspension if you wanted to.



05-17-2018 07:30 PM

toppestK3K


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Jared Russell View Post
Seems pretty straightforward to give the caster a suspension if you wanted to.
Well, there goes my afternoon.



05-17-2018 07:38 PM

gorrilla


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Lil' Lavery View Post
Does this not still rotate around the Y axis of the bevel gear? I don't see how increasing the distance in the x axis between the wheel shaft and bevel gear shaft does anything other than increasing the load on the rotation gear/motor since the wheel can't pivot around it's own forks the way a bicycle does because it is bolted to a flat plane in the frame.


Edit: because the steering shaft is still 90 to the floor.



05-17-2018 07:54 PM

AlexanderTheOK


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Lil' Lavery View Post
Actually, the pivot on this module is still perpendicular with the ground, but bypasses the center of the wheel, so the castor angle isn't quite relevant here.

The design and motivations are explained in this thread as well as this thread

TL;DR: a conventional swerve drive still needs to reorient modules for small movements, whereas a castor swerve can produce lateral movement perpendicular to the wheel through steering. This allows for very precise and instantaneous movement.



05-17-2018 08:15 PM

gorrilla


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by AlexanderTheOK View Post
Actually, the pivot on this module is still perpendicular with the ground, but bypasses the center of the wheel, so the castor angle isn't quite relevant here.

The design and motivations are explained in this thread as well as this thread

TL;DR: a conventional swerve drive still needs to reorient modules for small movements, whereas a castor swerve can produce lateral movement perpendicular to the wheel through steering. This allows for very precise and instantaneous movement.
After reading that thread I see what the caster affect could theoretically do, but if you laid out a grid on a flat floor and had the robot drive forward then translate left or right without stopping, would the caster drive pass through less grids or just cut the corner slightly compared to a traditional swerve.

How is this any different that powering your drive wheels while rotating the swerve module at the same time, would both robots not move through almost the exact same arc.



05-17-2018 08:22 PM

AlexanderTheOK


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by gorrilla View Post
but if you laid out a grid on a flat floor and had the robot drive forward then translate left or right without stopping
This isn't actually the intended use case. In fact, in such a use case, a conventional swerve module would likely be able to make a tighter turn, as the castor swerve also has to apply steering torque when accelerating perpendicularly with the robots motion.

The use case where this has an advantage is the one I mentioned. When the robot is stationary, a castor swerve can trivially move a few millimeters in any direction instantaneously, whereas a conventional swerve has to point the wheels in the correct direction first.

I'd bet that castor swerves would be at a terrible disadvantage in FRC, where precise movement has very little value, and where the "turn 90 degrees without stopping" use case is far more common. (when juking defense of course)



05-17-2018 08:26 PM

Bryce2471


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by SPang View Post
I love your packaging of this design, its so compact - it looks like you're using the underside faces of the bolt heads to retain the main bearing and gear for azimuth rotation?
Thank you. Yes the inner race of the turning bearing is pressed and bolted onto a lip that's machined into the top of the turning gear.
Quote:
Originally Posted by SPang View Post
How well would this module (when implemented in a set of 4) stand up to the load of a fully weighted FRC robot and the rigors of competition?
It's very hard to say for sure, considering that there are no examples that I can turn to for reference. But I suspect that it is over engineered if anything.
Quote:
Originally Posted by Jared Russell View Post
Seems pretty straightforward to give the caster a suspension if you wanted to.
That's true.

It would certainly be a cool feature to add, but I've already designed and posted a crazy caster drive, so for this one I wanted to go back to basics, and design something with a minimal amount of machined parts. I might come back and design a suspension version, but not until I've seen one built and deemed suspension worth while, or I've become very bored.

The design has 10 parts that require machining.

1 large turning gear needs milled out.
3 milled plates one on top and two on bottom
1 small turning gear just needs faced down on a lathe.
2 bevel gears need bored and broached.
2 hex shafts need their ends turned round.
1 VP output shaft needs shortened.

Besides the 3D printed encoder gear, the remaining parts are COTS.



05-17-2018 08:47 PM

Chak


Unread Re: pic: Usable Caster Drive Design

The advantage a true swerve has over crab is that you can do fancy movements by putting different angles on the wheels. Would caster swerve lose that functionality? Wouldn't pointing one wheel at the different angle than the others and then bringing back require dragging the wheel sideways on the ground? I'm not quite sure whether that's right (haven't done the math), but in the extreme case when the 4 wheels are oriented like an X (top view), putting the wheels back to "drive straight" configuration obviously requires overcoming scrub.

Maybe it would be forced to behave like crab. Or maybe it would be like WCD skid steer and work anyways. What do you think will happen?

Someone please build one!



05-17-2018 08:53 PM

s_forbes


Unread Re: pic: Usable Caster Drive Design

That's a very elegant and simple design, great work on all the packaging! I have drawn up several versions of swerve drive modules (just as concepts) and always put the goal to minimize the number of parts as the priority, but they still always have too many for my liking. Looks like you've done a much better job of it. The tucked away low profile of the whole module is a nice result.

Quote:
Originally Posted by Bryce2471 View Post
(...)
The design has 10 parts that require machining.

1 large turning gear needs milled out.
3 milled plates one on top and two on bottom
1 small turning gear just needs faced down on a lathe.
2 bevel gears need bored and broached.
2 hex shafts need their ends turned round.
1 VP output shaft needs shortened.

Besides the 3D printed encoder gear, the remaining parts are COTS.
Well, hop to it!



05-17-2018 08:57 PM

AlexanderTheOK


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Chak View Post
but in the extreme case when the 4 wheels are oriented like an X (top view), putting the wheels back to "drive straight" configuration obviously requires overcoming scrub.
Nope. Say you point all of the wheels "in", and want to go forward. The rear wheels begin to rotate "inward" and drive forward. The outer wheels rotate the same way, but initially, drive "backwards". Once they achieve 90 degrees, they begin driving "forwards" again as they continue to rotate into place. The wheels themselves are always travelling in the direction they're pointed. No scrub.



05-17-2018 09:00 PM

Jared Russell


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by gorrilla View Post
After reading that thread I see what the caster affect could theoretically do, but if you laid out a grid on a flat floor and had the robot drive forward then translate left or right without stopping, would the caster drive pass through less grids or just cut the corner slightly compared to a traditional swerve.

How is this any different that powering your drive wheels while rotating the swerve module at the same time, would both robots not move through almost the exact same arc.
If you want to do a sharp 90 degree direction change on a standard swerve, you need to translate, stop, rotate module, stop, translate.

If you want to do a sharp 90 degree direction change on a caster swerve, you need to translate, stop, then do a coordinated module rotation + translation that smoothly blends into pure translation.



05-17-2018 09:10 PM

Jared Russell


Unread Re: pic: Usable Caster Drive Design

How much caster (offset) do you have?

One potential drawback of any caster swerve is that your support polygon moves well inside your frame perimeter in your direction of motion (depending on how much offset you have). That's an argument for keeping the caster to a minimum.



05-17-2018 09:17 PM

gorrilla


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Jared Russell View Post
If you want to do a sharp 90 degree direction change on a standard swerve, you need to translate, stop, rotate module, stop, translate.

If you want to do a sharp 90 degree direction change on a caster swerve, you need to translate, stop, then do a coordinated module rotation + translation that smoothly blends into pure translation.
So in a race on an L shaped path, assuming the exact same wheel and modual rotation speed the caster swerve would theoretically win by a very small amount?



05-17-2018 09:19 PM

s_forbes


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by gorrilla View Post
So in a race on an L shaped path, assuming the exact same wheel and modual rotation speed the caster swerve would theoretically win by a very small amount?
If you're racing, you'd never take an L-shaped path to begin with.



Quote:
Originally Posted by Jared Russell View Post
How much caster (offset) do you have?
I have not used Onshape before seeing this model, but it appears to have a measuring tool that you can use. Looks like ~2.6"



05-17-2018 09:30 PM

AlexanderTheOK


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by s_forbes View Post
If you're racing, you'd never take an L-shaped path to begin with.
Yup. The race you're most likely to see in FRC is "I'm going forward and now I want to go left as fast as possible." For a conventional swerve, this time is only bounded by traction. For a castor swerve, this time depends on how much torque your motors can produce.



05-17-2018 09:39 PM

s_forbes


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by AlexanderTheOK View Post
Yup. The race you're most likely to see in FRC is "I'm going forward and now I want to go left as fast as possible." For a conventional swerve, this time is only bounded by traction. For a castor swerve, this time depends on how much torque your motors can produce.
I think in most cases you'd path a smooth arc for the path, to minimize how much you'd have to slow down while traveling along the path. The type of swerve mechanism doesn't play a large role as there aren't any abrupt changes in direction in that case. If you aren't planning a smooth arc for your path, then any type of drivetrain can go faster.



05-17-2018 09:39 PM

Bryce2471


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Jared Russell View Post
How much caster (offset) do you have?
It's designed with 2.7" of offset mostly because that was a dimension that minimized the height of the module while using a belt I can buy from VEX.
Quote:
Originally Posted by Jared Russell View Post
One potential drawback of any caster swerve is that your support polygon moves well inside your frame perimeter in your direction of motion (depending on how much offset you have). That's an argument for keeping the caster to a minimum.
Yes, and its a good argument at that. One argument for maximizing the caster distance is that it will give the system more time and distance to smoothly transition from rotating to driving the module. The primary benefit of caster drive in my mind is a more predictable and immediate response in the motion of the robot according to joystick inputs. This benefit likely scales with the caster of the module to some extent. However, I am once again trying to solve an optimization problem with very little information to go off of, so I won't know if this answer is wrong or right without a lot of testing.



05-17-2018 10:06 PM

AlexanderTheOK


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by s_forbes View Post
I think in most cases you'd path a smooth arc for the path, to minimize how much you'd have to slow down while traveling along the path. The type of swerve mechanism doesn't play a large role as there aren't any abrupt changes in direction in that case. If you aren't planning a smooth arc for your path, then any type of drivetrain can go faster.
Smooth arcs aren't really a test of a drives agility are they? Anyways, I'm not sure drivers want smooth arcs. They pull the joystick to the right and they want to see the robot go right, in which case a non castor swerve really does beat out the rest of the competition.

A tank drive needs to slow down its inside edge (and thus the whole bot) to turn.
A castor swerve can change direction without slowing down, but in doing so is limited by the torque the steering motors can create.
A non castor swerve can also change direction without slowing down, but the module can turn as fast as it wants, in practice limiting you to either the speed of the steering motor, or to the traction of your wheels.



05-17-2018 10:24 PM

Bryce2471


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by s_forbes View Post
That's a very elegant and simple design, great work on all the packaging! I have drawn up several versions of swerve drive modules (just as concepts) and always put the goal to minimize the number of parts as the priority, but they still always have too many for my liking. Looks like you've done a much better job of it. The tucked away low profile of the whole module is a nice result.
Thanks, it follows a similar basic packaging architecture to this swerve I posted a while back.
Quote:
Originally Posted by s_forbes View Post
Well, hop to it!
I would love to, but I probably won't be building it for the foreseeable future. Thankfully, however, I will be helping a design student build a T-shirt cannon robot that is planned to have a three wheel caster drive with 6" pneumatic wheels. So the general premise of caster drive for FRC will get some testing this summer!
Quote:
Originally Posted by s_forbes View Post
If you're racing, you'd never take an L-shaped path to begin with.
Yep, any path that involves a stop is usually a loss for an offensive robot.
Quote:
Originally Posted by s_forbes View Post
I have not used Onshape before seeing this model, but it appears to have a measuring tool that you can use. Looks like ~2.6"
I just changed it, so now it rounds to 2.7" because I realized the module could be shorter.
Quote:
Originally Posted by AlexanderTheOK View Post
Yup. The race you're most likely to see in FRC is "I'm going forward and now I want to go left as fast as possible." For a conventional swerve, this time is only bounded by traction. For a castor swerve, this time depends on how much torque your motors can produce.
I agree that maximum acceleration on a curved path is probably diminished in a caster drive, but I wonder if the improved response time couldn't help to juke defenders as well. A caster drive would retain its ability to roll around defenders as well, so a large opening from the initial juke should not be required to get around a defender.



05-17-2018 10:52 PM

Jared Russell


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by AlexanderTheOK View Post
Smooth arcs aren't really a test of a drives agility are they? Anyways, I'm not sure drivers want smooth arcs. They pull the joystick to the right and they want to see the robot go right, in which case a non castor swerve really does beat out the rest of the competition.

A tank drive needs to slow down its inside edge (and thus the whole bot) to turn.
A castor swerve can change direction without slowing down, but in doing so is limited by the torque the steering motors can create.
A non castor swerve can also change direction without slowing down, but the module can turn as fast as it wants, in practice limiting you to either the speed of the steering motor, or to the traction of your wheels.
I'm not sure why you think a powered caster drive has so much more difficulty changing direction? Yes, there is a steering moment, but it's not like a swerve magically has infinite steering torque or zero turning resistance. The total amount of work done by the system is nearly identical in both cases (if you assuming perfect rolling/non-slipping traction wheels), though it's true how much of the work is done by the steering axis vs. the traction axis is a function of how much caster you have.

In my experience, driving a standard swerve takes some practice because the robot lags whenever you change direction because the modules need to catch up. Powered caster drives do not have this issue and are more intuitive to drive IMO. (I have teleoperated a couple of powered caster robots, in non-FRC settings)

The biggest disadvantage of powered casters in my experience is doing rapid reversals. Your casters need to quickly swing (close to) 180 degrees, which exposes any flaws in the synchronization between steering and traction and forces you to devote a decent chunk of power to the steering motors to keep things snappy. The robot can wiggle left and right during the reversal in a situation where a standard swerve just needs to reverse the traction wheel.



05-17-2018 11:03 PM

gorrilla


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Bryce2471 View Post

I agree that maximum acceleration on a curved path is probably diminished in a caster drive, but I wonder if the improved response time couldn't help to juke defenders as well. A caster drive would retain its ability to roll around defenders as well, so a large opening from the initial juke should not be required to get around a defender.
Of all the normal swerve setups, wouldn't a crab drive provide the best Juke-abilility, with one stick for forward/reverse and another for rotation on the modules?(and another button for whole robot rotation) Granted this would require extreme driver effort to utilize effectively.



05-17-2018 11:48 PM

Bryce2471


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Jared Russell View Post
In my experience, driving a standard swerve takes some practice because the robot lags whenever you change direction because the modules need to catch up. Powered caster drives do not have this issue and are more intuitive to drive IMO. (I have teleoperated a couple of powered caster robots, in non-FRC settings)
This is highly valuable insight. Would you be willing to give more details on what you drove, and what its driving performance was like?
Quote:
Originally Posted by Jared Russell View Post
The biggest disadvantage of powered casters in my experience is doing rapid reversals. Your casters need to quickly swing (close to) 180 degrees, which exposes any flaws in the synchronization between steering and traction and forces you to devote a decent chunk of power to the steering motors to keep things snappy. The robot can wiggle left and right during the reversal in a situation where a standard swerve just needs to reverse the traction wheel.
This is an interesting problem, and I'm very curious how much it would affect FRC driving.
Knowing this could be a potential issue, the steering and drive setups for this design have the same power available, and the same relative gearing. Which should mitigate the problem, but making the transition smoothly would still be a major controls problem.
Quote:
Originally Posted by gorrilla View Post
Of all the normal swerve setups, wouldn't a crab drive provide the best Juke-abilility, with one stick for forward/reverse and another for rotation on the modules?(and another button for whole robot rotation) Granted this would require extreme driver effort to utilize effectively.
I'll say no, because a normal crab drive is not good at translating and rotating simultaneously, which is critical for getting around defense if you ask me.



05-17-2018 11:50 PM

Nick_Coussens


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Jared Russell View Post
...

In my experience, driving a standard swerve takes some practice because the robot lags whenever you change direction because the modules need to catch up. Powered caster drives do not have this issue and are more intuitive to drive IMO. (I have teleoperated a couple of powered caster robots, in non-FRC settings)
...
Can you expand on this point a bit? I'm not sure I follow how the caster would be better/quicker at changing direction, and the rapid reversal of direction seems like a huge downside.

Overall, I feel like I must be missing something as I don't see the huge benefit a caster swerve has over a regular swerve.



05-18-2018 06:15 AM

Gdeaver


Unread Re: pic: Usable Caster Drive Design

Visualize the module movements need for this years near side 2 or 3 cube auto.
Do you still think castor swerve is a good idea? Think about the far side 2 cube. do you still want to go down this path?



05-18-2018 07:18 AM

hrench


Unread Re: pic: Usable Caster Drive Design

I think caster (rake and trail) is designed to make wheels follow the particular direction of the rest of the body while they remain stable.

I think swerve drive is designed to be unstable, to turn whenever you want.

So I think these two ideas are against each other.

Also, seems many people here need to learn about rake and trail.
https://en.wikipedia.org/wiki/Bicycl...cycle_geometry



05-18-2018 10:26 AM

Jared Russell


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Nick_Coussens View Post
Can you expand on this point a bit? I'm not sure I follow how the caster would be better/quicker at changing direction, and the rapid reversal of direction seems like a huge downside.

Overall, I feel like I must be missing something as I don't see the huge benefit a caster swerve has over a regular swerve.
Powered casters can theoretically change direction more quickly because they can induce instantaneous lateral torque instead of having to rotate the module first. Whether or not this has a practical advantage is a function of the torques and velocities of your traction axis vs. your steering axis.

Having to rotate the caster 180 when doing reversals is a nuisance because of the requirements it imposes if you want forward or reverse motion from a stop to feel exactly the same (equivalent torques and speeds until the module reorients). It's not like the robot sits idly in place while the casters reorient. Standard swerves have to if the modules are aligned to 0 but the driver wants to start moving +/-90, unless you just send it and accept a little bit of forward/backward motion as you start moving.

This is what a direction reversal looks like on a powered caster robot: traction wheel reverses while the caster begins rotating, the traction wheel stops entirely as the module passes 90 degrees offset, then the traction wheel spins forward again as the caster homes in on its final position. The only downside here is having to coordinate steering and traction closely, which means you really want similar speed/torque characteristics on both axes. If you use a dinky motor for steering a powered caster, you will be sluggish when changing direction, and if you don't achieve good software coordination, the robot will jiggle left or right during this motion. For the former reason, the differential caster drive is not the craziest idea in the world: the feel of the drive (and the simplicity of autonomous path following) is best when both axes have similar characteristics. Moreover, the trail of the caster (there's no rake in this design) actually makes coordination of two motors in a differential mechanism easier than in a standard swerve - the caster naturally wants to trail the motion of the robot.

The value of the benefits of powered casters is arguable, and there's a ton of known and unknown risk in pursuing a powered caster drive in FRC. But it's Summer Chief Delphi (TM) so this is the sort of stuff we talk about



05-18-2018 11:04 AM

AriMB


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Jared Russell View Post
Powered casters can theoretically change direction more quickly because they can induce instantaneous lateral torque instead of having to rotate the module first. Whether or not this has a practical advantage is a function of the torques and velocities of your traction axis vs. your steering axis.

Having to rotate the caster 180 when doing reversals is a nuisance because of the requirements it imposes if you want forward or reverse motion from a stop to feel exactly the same (equivalent torques and speeds until the module reorients). It's not like the robot sits idly in place while the casters reorient. Standard swerves have to if the modules are aligned to 0 but the driver wants to start moving +/-90, unless you just send it and accept a little bit of forward/backward motion as you start moving.
Has anyone ever tried adjusting the power given to the drive motor by the cosine of the error in the module angle? That way when the wheel is pointed in the right direction (0 error) the drive motor gets full power, when it's perpendicular (90 error) the drive motor gets no power, and in between the drive motor ramps up. In theory, you'd get less wiggle in the "wrong" direction (compared to not waiting for reorientation) and some movement in the "right" direction (compared to waiting for full reorientation).

Quote:
Originally Posted by Jared Russell View Post
The value of the benefits of powered casters is arguable, and there's a ton of known and unknown risk in pursuing a powered caster drive in FRC. But it's Summer Chief Delphi (TM) so this is the sort of stuff we talk about
During the season, everyone loves WCDs. But offseason is swerve season



05-18-2018 11:28 AM

PatrickW


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Jared Russell View Post
In my experience, driving a standard swerve takes some practice because the robot lags whenever you change direction because the modules need to catch up. Powered caster drives do not have this issue and are more intuitive to drive IMO. (I have teleoperated a couple of powered caster robots, in non-FRC settings)
We haven't noticed any lag with our standard swerve from this past year. I don't think anyone on our team has driven another teams swerve drive though. If we are accepted and able to make it to Chezy Champs we would be happy to let you guys drive our robot.

Quote:
Originally Posted by Jared Russell View Post
It's not like the robot sits idly in place while the casters reorient. Standard swerves have to if the modules are aligned to 0 but the driver wants to start moving +/-90, unless you just send it and accept a little bit of forward/backward motion as you start moving.
This past year we totally just "sent it". Including at the start of auto when we didn't bother to make sure all the wheels were pointed forward when placing the robot on the field. Doing it this way didn't seem to cause any real world issues.



05-18-2018 12:36 PM

Jared Russell


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by PatrickW View Post
This past year we totally just "sent it". Including at the start of auto when we didn't bother to make sure all the wheels were pointed forward when placing the robot on the field. Doing it this way didn't seem to cause any real world issues.
Thanks for your input. Your swerve was awesome to watch this year!

That you are momentarily applying torque in an undesirable direction is definitely something people think about in academia and industry, but how much it matters in FRC is certainly up for debate. It could certainly be that the swerves I've driven are not as snappy as yours.



05-18-2018 01:07 PM

Gdeaver


Unread Re: pic: Usable Caster Drive Design

Autonomous was a real tough but very valuable task for 2018. Auton navigation is a real problem for swerve teams. I won't go into all the sources of error and problems. To do dead reckoning with swerve requires very tight precise mechanical and control. A few teams with swerve could do far multiple cube autons. For most this was off the table. Looking at the moves and speed we needed to accomplish the far 2 cube and the issues that will come up with castor swerve makes this a don't go there for me. I would ask what problem does castor swerve solve over conventional swerve?



05-18-2018 01:42 PM

balloman


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Gdeaver View Post
Autonomous was a real tough but very valuable task for 2018. Auton navigation is a real problem for swerve teams. I won't go into all the sources of error and problems. To do dead reckoning with swerve requires very tight precise mechanical and control. A few teams with swerve could do far multiple cube autons. For most this was off the table. Looking at the moves and speed we needed to accomplish the far 2 cube and the issues that will come up with castor swerve makes this a don't go there for me. I would ask what problem does castor swerve solve over conventional swerve?
I would argue that autonomous navigation is easier with swerve, as you can go in any direction without having to steer there first, and keeping track of the robot is easier



05-18-2018 04:37 PM

pkrishna3082


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by balloman View Post
I would argue that autonomous navigation is easier with swerve, as you can go in any direction without having to steer there first, and keeping track of the robot is easier
Try driving straight without a gyro. You can honestly get pretty close with just %Vbus on a tank/WCD. Have fun with swerve!

(Use a gyro please)



05-18-2018 06:20 PM

Bryce2471


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by PatrickW View Post
This past year we totally just "sent it". Including at the start of auto when we didn't bother to make sure all the wheels were pointed forward when placing the robot on the field. Doing it this way didn't seem to cause any real world issues.
The last swerve robot I drove had a small but noticeable delay before accelerating in the desired direction, and I think the worst part about that, is that the delay time varies depending on the current angle of the modules, and the desired direction of travel.

However, the last swerve robot I drove used a 9015 motor, and was controlled with a PID on the RoboRio. So if yours used a 775pro and an SRX PID, I would expect a much faster response time.
Quote:
Originally Posted by AriMB View Post
Has anyone ever tried adjusting the power given to the drive motor by the cosine of the error in the module angle? That way when the wheel is pointed in the right direction (0 error) the drive motor gets full power, when it's perpendicular (90 error) the drive motor gets no power, and in between the drive motor ramps up. In theory, you'd get less wiggle in the "wrong" direction (compared to not waiting for reorientation) and some movement in the "right" direction (compared to waiting for full reorientation).
This is exactly how a caster drive would work, and it would probably be a good idea for a regular swerve as well. Theoretically, the benefit of doing it with caster is that the steering of the module would also contribute to movement in the right direction, and would compensate for movement in the wrong direction from the drive wheel.
Quote:
Originally Posted by AriMB View Post
During the season, everyone loves WCDs. But offseason is swerve season
QFT.
Quote:
Originally Posted by pkrishna3082 View Post
Try driving straight without a gyro. You can honestly get pretty close with just %Vbus on a tank/WCD. Have fun with swerve!

(Use a gyro please)
The swerves I have driven were better with a gyro, (mostly because of field orientation) but they were perfectly drivable in teleop without one. I fact, I suspect that a caster drive would be very drivable in teleop with open loop control only.
Quote:
Originally Posted by Gdeaver View Post
Autonomous was a real tough but very valuable task for 2018. Auton navigation is a real problem for swerve teams. I won't go into all the sources of error and problems. To do dead reckoning with swerve requires very tight precise mechanical and control. A few teams with swerve could do far multiple cube autons. For most this was off the table. Looking at the moves and speed we needed to accomplish the far 2 cube and the issues that will come up with castor swerve makes this a don't go there for me.
I would argue that with field relative odometry, highly competitive autonomous routines would be very achievable with caster drive. (Or with swerve drives, but potentially to a lesser extent.)

Imagine that you take the normal casters on a pit cart, and put encoders on them, then push cart around the field, using the encoders and a gyro for feedback, it should be possible to calculate the absolute position of the cart on the field with fairly good accuracy over 15 sec. Using this calculated position to feed the uppermost loop of your control system should result in a robot that gets to where it is supposed to go. Then put a good feed forward system underneath, and you should be driving autonomously with the best of them.

Would it be hard to make a swerve bot move quickly and accurately in auto? Yes, but probably not that much harder than with a WCD. (maybe I'm wrong about this, but I would have to try again after working on a WCD motion profiling system to find out)

Would it be significantly harder to make a caster drive bot do the same? Maybe, but I doubt it.
Quote:
Originally Posted by Gdeaver View Post
I would ask what problem does castor swerve solve over conventional swerve?
Sense no one in FRC has tried it, it's hard to say. But it could potentially have a faster and more predictable response to joystick input, especially from a standstill.



05-18-2018 07:09 PM

CVR


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Jared Russell View Post
How much caster (offset) do you have?

One potential drawback of any caster swerve is that your support polygon moves well inside your frame perimeter in your direction of motion (depending on how much offset you have). That's an argument for keeping the caster to a minimum.

Couldn't this be a huge advantage for Power Up (and similar games?)

If you had a ~3" caster, you could rotate the modules to increase your wheelbase or track by 6", which would significantly increase stability. It would put your wheels just at the edge of your bumper (still fairly protected).

I see the major challenges to this being bumper height (two low and the casters can't rotate underneath, and being able to keep the wheels in an unstable orientation (that's not my problem, it's the programming team's problem...)



05-19-2018 06:38 AM

pkrishna3082


Unread Re: pic: Usable Caster Drive Design

Quote:
Originally Posted by Bryce2471 View Post
The swerves I have driven were better with a gyro, (mostly because of field orientation) but they were perfectly drivable in teleop without one. I fact, I suspect that a caster drive would be very drivable in teleop with open loop control only.
My apologies, I was referring specifically to autonomous mode, in response to the post that I had quoted. Looking back my post was pretty ambiguous, sorry about that.



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