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Robot Odometry

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New ( offline )
Posts: 8
Posted: 2007-06-22 00:23 
 Post subject: Robot Odometry
Hi,

I'm an EE student at Purdue and my senior project will be a room mapping robot. As you might guess, as an EE my mechanical skills are somewhat lacking.

I am wanting to use the 4WD-1 chassis with a slight modification. More specifically I'd like to remove the motors from the two rear wheels and instead have quaderature optical encoders.

Also I will have a digital compass mounted on the robot as I've read that odometry based systems alone tend to accumulate considerable orientation drift over time.

My reasoning for leaving the rear wheels unpowered is that if they are not powered I shouldn't have to worry about wheel slippage as much... If the encoder wheels are turning then the robot would have to be moving since the front wheels are doing the pulling.

Does anyone have any suggestions in implementing a system like this? Has anyone here mounted codewheels and optical encoders to this chassis before? Are there any recommendations for specific encoder modules/codewheels?

Any help that can be offered on this would be appreciated!

Thanks,

Scott


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Posted: 2007-06-22 10:54 
 Post subject: Re: Robot Odometry
scrook wrote:
I am wanting to use the 4WD-1 chassis with a slight modification. More specifically I'd like to remove the motors from the two rear wheels and instead have quaderature optical encoders.

You need to add quadrature encoders to the motors. :) From the assembly guide, it does not look like the motors will accept quadrature encoders. Only the GHM-04 motors have the extended shafts for the QME-01 encoders. If you have not yet purchased the 4WD-1 rover kit, you might be able to have the stock motors swapped out for the GHM-04 motors. The QME-01 quadrature encoders are not the same as optical encoders - they attach directly to the GHM-04 motors.

scrook wrote:
My reasoning for leaving the rear wheels unpowered is that if they are not powered I shouldn't have to worry about wheel slippage as much... If the encoder wheels are turning then the robot would have to be moving since the front wheels are doing the pulling.

For optical encoders, this might be an issue, but I don't think it will be for the motors LM sells. I have purchased GHM-04 motors and the QME-01 encoders and am looking forward to having my robot be able to navigate better. I will also be adding a Devantech CMPS-03 digital compass module, so will have a setup very much like you are talking about.

8-Dale

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Posts: 8
Posted: 2007-06-22 12:28 
 Post subject: Re: Robot Odometry
Quote:
You need to add quadrature encoders to the motors. Smile From the assembly guide, it does not look like the motors will accept quadrature encoders. Only the GHM-04 motors have the extended shafts for the QME-01 encoders. If you have not yet purchased the 4WD-1 rover kit, you might be able to have the stock motors swapped out for the GHM-04 motors. The QME-01 quadrature encoders are not the same as optical encoders - they attach directly to the GHM-04 motors.


So the 4WD 1 doesn't use the GHM-04 as stock? I'm confused... the on the 4WD-1 product page it says that it uses 7.2V 50:1 gearhead motors and the GHM-04's are the only motors on the motors page that fit that spec. Are the stock motors on the kit of a totally different make that LM doesn't sell separately then?

Also, from the looks of that encoder kit it would probably work way better for me as it's built to go with that motor and any non powered device I threw together would surely be of lesser quality so I will probably go with those and the GHM-04s.

I would still like to have the rear wheels be unpowered (free spinning) if possible. Really I want to have the stability of 4 wheels but I'd rather only power two motors to keep battery life up.

Do you think that 2 GHM-04's would be sufficient to move a 4WD1 with a few PCBs mounted and also a servo turret/ sharp IR sensors mounted on the top?


Expert ( offline )
Posts: 471
Posted: 2007-06-22 12:44 
 Post subject: Re: Robot Odometry
I think that with a 4 wheel bot without articulated wheels, the mechanism for turning is skidding. Even if they are unpowered, you'll still have sideways skidding throwing off your dead reckoning.

If you want to get tricky about it, use compass reading changes to try to adjust for slippage error in wheel odometry. Add an accelerometer and kalman across all three for an estimate.


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Posts: 471
Posted: 2007-06-22 12:45 
 Post subject: Re: Robot Odometry
Another option that's been used to solve this particular problem with tracked vehicles is to pull a 2 wheeled trailer which perform the odometry.


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Posted: 2007-06-22 15:46 
 Post subject: Re: Robot Odometry
scrook wrote:
So the 4WD 1 doesn't use the GHM-04 as stock? I'm confused... the on the 4WD-1 product page it says that it uses 7.2V 50:1 gearhead motors and the GHM-04's are the only motors on the motors page that fit that spec. Are the stock motors on the kit of a totally different make that LM doesn't sell separately then?

Maybe the photos on the assembly guild just don't show the extended rear shaft well then.

scrook wrote:
I would still like to have the rear wheels be unpowered (free spinning) if possible. Really I want to have the stability of 4 wheels but I'd rather only power two motors to keep battery life up.

Do you think that 2 GHM-04's would be sufficient to move a 4WD1 with a few PCBs mounted and also a servo turret/ sharp IR sensors mounted on the top?

I would say the 4WD-1 was designed with four motors for a reason, so that should probably not be messed with. However, since robotics is an experimental thing, nothing prevents you from doing just that - experimenting. You could try driving just two wheels with the GHM-04 motors and QME-01 encoders and let the other two wheels freewheel. :)

8-Dale

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Posted: 2007-06-22 16:54 
 Post subject: Re: Robot Odometry
linuxguy wrote:
scrook wrote:
So the 4WD 1 doesn't use the GHM-04 as stock? I'm confused... the on the 4WD-1 product page it says that it uses 7.2V 50:1 gearhead motors and the GHM-04's are the only motors on the motors page that fit that spec. Are the stock motors on the kit of a totally different make that LM doesn't sell separately then?

Maybe the photos on the assembly guild just don't show the extended rear shaft well then.

scrook wrote:
I would still like to have the rear wheels be unpowered (free spinning) if possible. Really I want to have the stability of 4 wheels but I'd rather only power two motors to keep battery life up.

Do you think that 2 GHM-04's would be sufficient to move a 4WD1 with a few PCBs mounted and also a servo turret/ sharp IR sensors mounted on the top?

I would say the 4WD-1 was designed with four motors for a reason, so that should probably not be messed with. However, since robotics is an experimental thing, nothing prevents you from doing just that - experimenting. You could try driving just two wheels with the GHM-04 motors and QME-01 encoders and let the other two wheels freewheel. :)

8-Dale


I think it would affect the skid steering though. Bassically becuase the Skid steering would be dragging the freewheels around instead of all four of them working in sync.

I'd also wait a while to purchase this kit, becuase Jim is getting new kinds of Hubs for the kits, and they are waaaaaay better :wink:

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Posted: 2007-06-22 18:54 
 Post subject: Re: Robot Odometry
I would make a custom robot with 2 drive wheels and a caster. I am using this on a firefighter robot and it is very accurate.

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Posted: 2007-06-22 19:59 
 Post subject: Re: Robot Odometry
spacejunk wrote:
I would make a custom robot with 2 drive wheels and a caster. I am using this on a firefighter robot and it is very accurate.

This is what I have with W.A.L.T.E.R. right now, but I am going to add two casters each, front and rear. I know this setup is rock solid stable, since it is the same setup my power wheelchair has, with just the two middle larger wheels being driven.

8-Dale

_________________
I can usually handle complexity. It's the [b][i]simple[/i][/b] things that always confound me..
Check out my [url=http://www.hybotics.me]blog[/url] now!
Open your mind, Read, Learn, Think, Apply..


Rookie ( offline )
Posts: 24
Posted: 2007-11-20 15:14 
Hello All,

I just read the posts regarding odometry with a four wheel drive rover. My design is quite the same. Although with a twist. I have all four motors equipped with the US Digital encoders. I plan on using two Acroname Moto 1.0 controllers with 4 H-Bridges to provide independent control to each motor. Skid steering projects I've done in the past with encoders on the two front wheels proved unreliable. I'm looking at being able to turn this rover on a dime, rather than it having go through several manuvers to accomplish this. Does anyone have any ideas or thoughts that might help me in this project? Or am I just expecting too much, and as usual going into overkill with this project. Thanks for your time!

Respectfully,
Michael G. Jessat


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Posts: 2251
Posted: 2007-11-20 19:33 
 Post subject: Re: Robot Odometry
If it has to turn on a dime, make a turning pad that is centered in the ROV such that it is lowered, lifting the rov wheels off the ground and rotating it as desired, then lowering the rov back to the ground.


Rookie ( offline )
Posts: 24
Posted: 2007-11-21 15:35 
Ok, I'm beginning to see some light on my problem while attempting to use the US Digital encoders on all four wheels of my 4WD1 rover. Correct me if I'm wrong. Being a non-holonomic rover the use of the four encoders on each motor would be of no great value. Am I correct in this assumption? If the design were holonomic I think the use of the four encoders would be of greater value. Since the rover lacks the ability of 'steering' the rover would still be reduced to a 'skid' steered robot. Can anyone correct me if I'm wrong.

Respectfully,
Michael G. Jessat


 

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