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csdude
01-05-2010, 09:58 AM
Hello all, (not been here for a good while)

After my experiences with the first rover I built, I am going to build a 2nd one. I learned some do's and don'ts from the first one :)
This one I am going to build from scratch, build a frame/platform myself and use RC axles/transmission/motor etc.

I am more of a software 'guy'. Right now I am looking for 2 small (sub micro?) servos for a range scanner. But it seems that all I can find are the ones that can rotate 60 degrees.
I need one that can rotate 90 degrees and one that can rotate 120 (or at least 2 that can rotate 90 degrees). Also is there a way to find out what the smallest "step size" of a servo is? (or does that depend on what kind of servo controller you use? (pretty much I want to divide 90 degrees up in as many positions as possible)

Ron

jes1510
01-05-2010, 10:06 AM
I can't answer the particular servo question but as to the smallest "step size", that will depend solely on the servo controller for analog servos.

csdude
01-05-2010, 10:27 AM
ok, cool. One of the controllers I used last time is the Robotics Connection serializer.

What does the "step size" depend on? On how accurate the length of the pulse is that you send?

jes1510
01-05-2010, 11:20 AM
It depends on the servo controller. If you have to send it a position in degrees then the resolution will probably be 1 degree. If you can send it pulse widths then it may be less than that. You need to look at the servo controllers first and then you can figure out what you can use.

Adrenalynn
01-05-2010, 12:44 PM
The step size on the Serializer is massively large and imprecise.

Just divide the range by the resolution.

ie. the Serializer is 0-255 = 256. A 180 servo wouldn't be able to get more than 180/256 =0.7

Toss it on a 10bit controller and you get 0-1023 = 0.16

But remember: It's impossible to build a rover without three dual-quadcore i7's and [email protected] of power. And the minimum size is about 4 feet long by 3feet wide. Anything less than that is known to be non-functional. ;)

csdude
01-05-2010, 01:13 PM
Hey Lynn,

Oh it is going to be functional, useful is another question though :)

Actually. The one I am going to build this time is going to have 2 pico boards, clustered running linux.
Also, it is going to have 2 axles that both can steer, that way it can negotiate turns better. (That 1:6 truck/rover from last year wasn't that good at making turns. Batteries, well.. this time I am probably going to use Lipo's

oh.. Happy New Year! btw

Ron

darkback2
01-05-2010, 01:21 PM
Hey,

Any reason why your not using tank stearing?

DB

csdude
01-05-2010, 01:25 PM
Uhm, actually.. no, no reason. (other than it is going to end up looking like a pickup truck)

jes1510
01-05-2010, 02:00 PM
Out of curiosity, why the clustered Pico boards?

Adrenalynn
01-05-2010, 02:14 PM
Happy NY to you too, Ron! Oh - I've got a new cell number, I'll pm it over to you later

zoomkat
01-05-2010, 06:06 PM
I can't answer the particular servo question but as to the smallest "step size", that will depend solely on the servo controller for analog servos.
From my tinkering the servo movement resolution available is also very dependent on the particular servo being used. I tested a standard ~$10 servo and estimated that no more than ~426 descrete positions were available in its full ~190 deg range of rotation. So using a mini-ssc II servo controller with ~255 possible positions, the controller would be the limiting factor. Using an ssc-32 with ~2000 positions, the servo would be the limiting factor. As for small servos, they probably more limited by the resolution of the smaller pot in them. I got several of the below small el cheapo servos to tinker with, and they seem to have less position resolution than a standard servo, but possibly sufficient for moving a sensor.

http://www.hobbypartz.com/topromisesg9.html

darkback2
01-05-2010, 07:40 PM
A couple of questions...

A) what resolution do you need from the servo...as in how many steps do you actually need, and

B) what is the resolution of your sensor.

I guess what I'm thinking is that perhaps accuracy is more important than resolution. (though they may go hand in hand) If you have a servo that can make 180 steps in 180 degrees, and they are each relatively close to 1 degree per step, then that would be a lot better than 2000 steps where they range by a grate deal in number of degrees.

Also, if your sensor has a resolution of 5 degrees then its all sort of pointless anyway.

If your using the servo to position a camera, then the resolution of the servo doesn't matter as much. It would more be about the accuracy, though again, I'm not sure if that ends up being similar because a servo that isn't accurate will have a low number of attainable positions.

Hope this is helpful.

Oh...and I looked up the HSP chassis...Talk about SWEET!

DB

csdude
01-06-2010, 01:38 AM
Out of curiosity, why the clustered Pico boards?

well, one is going to do all the network IO, pattern recognition etc etc. the other one is going to do all the driving related stuff. (I had some mishaps with the other rover because some things slowed things down considerably. Not a good thing when a rover is heading for a staircase *lol*)

csdude
01-06-2010, 01:45 AM
Happy NY to you too, Ron! Oh - I've got a new cell number, I'll pm it over to you later

ah alright, my number is still the same..

btw I finished another project, a media player (plays anything audio) My cd player broke and decided I could do better than what you can buy in a store.
Based on (of course) a pico ITX (1Gb ram, almpst 1TB drives. The HD audio is pretty damn good.
I bought a 3m touch screen monitor for it. It's a really cool setup.

scavenging that micro controller board from one of those cheap gane controllers worked pretty good for getting the front panel buttons to work. that with one of those usb/serial LCD screens makes it pretty slick.

csdude
01-06-2010, 01:56 AM
From my tinkering the servo movement resolution available is also very dependent on the particular servo being used. I tested a standard ~$10 servo and estimated that no more than ~426 descrete positions were available in its full ~190 deg range of rotation. So using a mini-ssc II servo controller with ~255 possible positions, the controller would be the limiting factor. Using an ssc-32 with ~2000 positions, the servo would be the limiting factor. As for small servos, they probably more limited by the resolution of the smaller pot in them. I got several of the below small el cheapo servos to tinker with, and they seem to have less position resolution than a standard servo, but possibly sufficient for moving a sensor.

http://www.hobbypartz.com/topromisesg9.html

That sounds reasonable. If you buy a cheap servo, well you probably get what you pay for.
I am looking at the "pololu serial 8-servo controller", that with two good servos should do the trick.
I am thinking about using sub micros for my scanner. I probably want something with good quality (I only need two) the beteer resolution the servos and controller have, the better the sanned images is going to be.

what was the highest resolution servo you have experience with?

csdude
01-06-2010, 02:12 AM
A couple of questions...

A) what resolution do you need from the servo...as in how many steps do you actually need, and

B) what is the resolution of your sensor.

I guess what I'm thinking is that perhaps accuracy is more important than resolution. (though they may go hand in hand) If you have a servo that can make 180 steps in 180 degrees, and they are each relatively close to 1 degree per step, then that would be a lot better than 2000 steps where they range by a grate deal in number of degrees.

Also, if your sensor has a resolution of 5 degrees then its all sort of pointless anyway.

If your using the servo to position a camera, then the resolution of the servo doesn't matter as much. It would more be about the accuracy, though again, I'm not sure if that ends up being similar because a servo that isn't accurate will have a low number of attainable positions.

Hope this is helpful.

Oh...and I looked up the HSP chassis...Talk about SWEET!

DB

well the higher the resolution of the servo/controller, the better. the sensor I choose depending on what I get with the servo (I am probably using the sensors from some cameras, 5,12,20 MP besides with a higher MP you can skip pixels and have the same effect as a lower resolution sensor.
(I don't know what I am going to use as a sensor yet, I also have access to a spectrometer that can be scavenged for parts). When it comes to accuracy, well it doesn't have to be terribly accurate. That pololu controller claims to have a 0.05 degree resolution, that would be plenty I think.
The servo is used to 'position' the lasers.

HSP chassis?

darkback2
01-06-2010, 08:12 AM
Sorry,

I'm so tired I'm starting to mix threads...don't worry next I'll be talking about servo bracket systems.

csdude
01-06-2010, 12:43 PM
Sorry,

I'm so tired I'm starting to mix threads...don't worry next I'll be talking about servo bracket systems.


:) that's funny

csdude
01-08-2010, 11:14 AM
Ok, back to servos and controllers. I think I'll get the "pololu serial 8-servo controller", that one seems to have a pretty high resolution.

Next question is, will "any" analog servo do? Meaning, if I hook it up to that servo controller, would I basically have a 180 degree (-90 0 90) degree throw?

I think I need a sub micro servo (just because of the size) however I would like it to be a better quality (metal gears, bearings?) for the scanner I am thinking of.

A guy at the RC shop mentioned a HS-65MG being a very accurate/dependable/durable one. any opinions?

thanks,

Ron

Adrenalynn
01-08-2010, 08:13 PM
The HS-65MG is a good choice if you can get away with such a tiny amount of torque (< 30oz-in)

The 5065 is the digital version, but needs to be modded for 180deg

csdude
01-09-2010, 12:45 AM
I almost don't need any torque. There's going to be a 1/2"x1/2" first surface mirror mounted on one of then, and that contraption mounted on the 2nd one.
It seems it is very accurate etc. that with that pololu board should probably do it.
(If not I can always mess with gears or something like that)

Adrenalynn
01-09-2010, 12:55 AM
Yeah, ~30oz-in should be able to handle that - assuming that isn't mounted on a few foot arm or anything. ;)

csdude
01-10-2010, 12:10 AM
it needs to go on front of a truck.. *S* so as compact as possible