PSC enquiry if is can perform the task

Wayne.W.K

Hi all,

I'm building this robot vehicle with 4 servo for steering 4 wheels and 4 steppers for driving each of the 4 wheels.
Initially want to control all with a BS2SX but realised to hold the servo for steering while actuating the stepper is almost impossible.

Then I found out that PSC might help....
Wanted to ask if PSC can control·the 4 servos such the servos will hold their position until a new position is send to the PSC. Cause after the direction·for the servo is set, the stamp will control the stepper to move and i need the servos to hold their positions when the vehicle is·moving...

I have not started on any program and just wanted to make sure it will work.

And·I wanted to connect the PSC directly to one of the output of the Stamp. I supposed·I should get the serial version right?

Thanks!


Wayne

Bruce Bates

Wayne -

Have you really thought out the potential problems you're may to have to face using four servos to steer four wheels using R/C servos? It seems to me you'd have a MUCH easier time of it if you used two R/C servos to control each of two PAIRS of wheels (front and back).

I, for one, try to keep the KISS principle in mind when it comes to mechanical linkages, steering geometries, and the like. As something of an aside to this, here is a nice presentation on the Ackerman Steering Principle at the R/C model level, which is really quite good: http://www.rctek.com/handling/ackerman_steering_principle.html

Here's the way I see it, if you haven't had a chance to think it through thoroughly. First, I'd guess you will need to test somewhere in the neighborhood of 12 to 24 R/C servos, all of the same brand and style, to find 4 that have about the same response time, overall throw, end points and rotational speed. You will probably need some rudimentry test jigs to test all these parameters as well. IF someone offers MATCHED servo PAIRS, you may be able to cut that number significantly.

Once you have that all sorted out, and you have two matched R/C servo pairs, or 4 well matched R/C servos, you will probably have to find a specific midpoint number for each of the servos, and use that as part of the error corrrection routines if/when one or more of the servos begins to wander. If this doesn't happen immediately (and it may well not), I can almost guarantee it will happen over time, as the tires, pivot bearings, servo bearings and other steering linkage parts begin to wear some over time.

In order to ensure that each of the four R/C servos is being positioned exactly where you want it to be, after being commanded to do so, I'd guess you'd need a rotational encoder (NOT like a wheel encoder, one in a perpendicular plane to that) in order to generate feedback to guarantee rotational steering compliance. If this isn't done, the vehicle may well tend to "crab" rather than roll. In the worst case, not only will the overall speed be diminished by excess toe-in or toe-out or wheel skew, but it's even possble that the undue strain on the wheels may cause one (or more) to break or fall off. In any case, such toe-in or toe-out is going to raise hell with the wheel bearings, and the tires, or if there are no wheel bearings (pivot bearings), then the bearing in the servos will take the brunt of the strain.

If you were to use two pairs of mechanically connected R/C servos using tie rods, and clevis ends (see next link) and ball linkage such as this (look above and below the item in the center of the screen):
http://www.aaaim.com/u/web/aaaimc/cgi-local/shop991/shop.pl/SID=401583025/page=MISC.htm#RC101

you would then afford yourself some adjustability, and be able to equalize or eliminate some of the potential sources for problems noted above. Atleast I hope that's the case.

Something even smaller than that shown above may be required, as I didn't check each of those clevis ends to note the various sizes offered. If that's the case, here is another source of even smaller tie rod-like units ("turnbuckles") and ball ends with which you can make similar adjustments (picture value ONLY, NOT the actual parts shown):
http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXJTS4&P=ML

If, OTOH, you have a method of avoiding all of these possible problems, I'd sure love to hear/see it, as I spent a good deal of time researching this very concept a few years back. Mars rover-like vehicles with their extremely accurately machined parts, with high precision motors on each wheel, and watch-like precision rotational components (in the steering and elsewhere) can easily do what you intend to do, and on more wheels than just four. We ALL WISH we had those kinds of components available to us at Erector Set or Lego prices!

I'd love to hear how you make out with this, if you still decide to go with the 4-wheel R/C servo steering. PLEASE DON'T let me DISSUADE you! I claim NO specific mechanical enginering expertise, and thus may have overlooked some very simple solutions to these potential problems.

Regards,

Bruce Bates

Wayne.W.K

I had already fabricated the body and the wheels and there's no turning back. I'm aware of the possible misaglinment problem and the turning problem.
Thanks for the reminder.

So can the card be used for my application?

Thanks

Wayne

Chris Savage

Wayne,

·· Yes, the PSC will hold the servos in the last position you commanded until you send a new position command.

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Chris Savage
Parallax Tech Support
csavage@parallax.com