Let's Make Robots!

Recommendations for steering

I'm building a yard bot. The initial blog is posted for my Yard Gnome already.

This robot will steer like a car, with one drive motor pushing two rear wheels, and a two-wheeled steering mechanism up front.

I'm looking for recommendation for how to automate steering on this thing.

Is there an affordable servo that is strong enough?

Should I build my own servo?

How about linear actuators? I've heard they are kind of slow. This bot may go up to 5 miles per hour.

Maybe I'll need a powerful stepper?

Or perhaps a simple geared motor with limit and position switches?

WWLD (What Would LMR Do?)

 

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jollyrodger13's picture

I dabbled in "robotics" off and on over the years, when i had the time & life wasn't getting in the way. I recently started to be more serious about it, when i get the chance I'll post a couple of projects i'm working on myself. Good luck with yours.

jollyrodger13's picture

Hi, I looked up that article in servo magazine, it was April 2008, the article has a lot of information you could use in your project.

the author used a BS2 stamp as a controller, for steering he used 2-sm ball bearings/length threaded rod/2-switches/2-hall effect switches/3-magnets/2-relays/1-motor w/2 gears (from battery drill). I don't know if its legal to post the article on this site, maybe I could email it or something.

ignoblegnome's picture

Thanks. I was able to find a copy of the article online. The approach is very different than the one I am trying. I will keep this article handy for future reference. If my idea doesn't work out, it will be the very near future...

ignoblegnome's picture

One problem with a wiper motor is that just about all of them are built for chassis ground systems. I'm not sure I want that for Yard Gnome.

I found this nice 24VDC geared motor on eBay:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=300372429085&rvr_id=&crlp=1_263602_263622&UA=%3F*I8&GUID=dcc93eaa1280a0aad5b1e4a1ff81d84b&itemid=300372429085&ff4=263602_263622

Horse Power Cont.

1.5 W

Length of Motor Body(excluding spindle) (BL)

50.2mm

Gear ration

1:200

Full Length of Spindle (SL1)

14.0mm

Voltage & Current

24 V DC

Length of Spindle (SL2)
12.0mm

RPM

30

Length of Spindle Flat(SL3)
10.0mm

Reversibility

Reversible

Diameter of Spindle (SD)

4.0mm

Length of Motor (including spindle)

64.2 mm

Height of Spindle (no a flat ) (SH)

3.5mm

Diameter of Motor (D)

25.0 mm

Center Distance of Mount Hole(H)
16.8mm

Torque

41 LB-Inch

It'll only cost me $20 including shipping. Think that'll handle the job? I'll still need an h-bridge to drive it, and a servo controller (purchased or hacked from an old servo) to control it.

 

Regards,

 

Andrew

 

Rudolph's picture

It doesn't have to be chassis grounded, just run the negative wire to the motor's housing. You could use a shim and nylon (or other nonconductive) bolts to totally isolate it from the chassis if necessary.

Also see Frits' tutorial on making your own servo

ignoblegnome's picture

I thought about isolating the motor. However there's plenty of risk that it will be accidently grounded at some point during use. It may not cause an issue, but it turns out I can get a 24VDC gear motor with decent speed and torque (mentioned elsewhere in this post) for $20 delivered. Then I have no grounding to worry about.

I will definitely read Frits' post. How did I miss that!

ignoblegnome's picture

For the same price I can also get the same motor at a different gear ratio that delivers 60 rpm and about half the torque.

I did a quick check and one "high torque" hobby servo has a 6V speed of .18 seconds/ 60 degrees. This translates to 64.8 rpm, so I think a motor that turns about 30-60 rpm is in the right speed range for the steering servo. I just need to decide if I want more like 40 lb-in of torque at lower speed or if 20 lb-in is enough and get more speed.

Yard Gnome only goes about 5 mph, so maybe the lower speed steering will be OK and I won't have to worry about an underpowered steering under heavy load.

jollyrodger13's picture

I am new to LMR but I remember an article in servo magazine about one of those childs cars that they can drive in the yard, the author of the article modified it to be autonomous and steered using elec motor/threaded rod/limit switchs, i'm on vacation so I can't tell you which issue it was in 2009 though. I hope this helps you,  good luck!!

Chris the Carpenter's picture

I just remembered this one going by. You know, there really is everything you need here at LMR!

http://letsmakerobots.com/node/2653

Link is for the "Giant servo kit" (of which an even stronger version is available) that will allow you to turn any gear motor and any pot into a big-ass servo! I'm thinking power window motor or winshield wiper motor --I have seen a lot of these type of motors go by around here with good results.

ignoblegnome's picture

Unfortunately, this item is no longer available! Rats, that would have been perfect!