I'm doing the same thing with Vex. I'm a robonewbie, so I haven't much
to offer. I puzzle over how to connect the front wheel to the rest of
the chasis, though. I envision something like a trike, with perhaps
two horizontally parallel bars extending from the rear chasis to put
bearings into that will strongly enough support the front. But exactly
how the servo will fit into this, I'm not clear about yet. Above?
Connected to what?
Sigh... These sorts of things seem to be my biggest challenge. I wish
there were some site which had construction tips for this (and lots of
other things I've puzzled out very slowly.)
Hope others will comment on your post since I'm quite interested in the
answer to your question.
Ya I was thinking off putting the servo above the wheel like the pic below,
so when the servo rotats the wheel turns,
If there is someone more expereanced here, Please tell me is this would
work. Thanks, Donncha.
| | --- Servo
/ | | \
/ |_| \
\ / ------ Wheeele
It might work, try to find wheels that have just the right amount of
traction because if you use too much the servos will burn out and it
wont turn properly, too little and it wont turn the robot, also dont
use a shoping cart desing where the wheel is offset from the center of
the rotational axis or when you back up the it wont back up straight.
It will have to go in a semi circle to get the wheel orientated
properly. Something that I think might work better for you is having
the wheels turn in oposite directions or different speeds to turn
I'm a noob to robotics and a thought this would be the easiest way to make
my first one, I was going to mount the servo directly over the wheel with it
aligned to the center, So all I have to have is one channel for speed and
the other for direction. Putting the batteries and stuff at the front to
make it front heavy so the front wheel will get a grip.
Perhaps f these sites will help. Nothing like looking at different designs to
give yourself new ideas: http://www.vexlabs.com /
This site also has a Vex specific forum to post to:
Good luck !
Here's the problem with this design: If you have two drive wheels driven
by a single motor, and one free-wheeling steering motor, the drive
wheels will encounter added friction in in turns because one must go
slower than the other to follow the turning arc. If you're lucky the
wheels will just harmlessly slip, but usually what happens is that the
wheels will "chatter" because of the friction between rubber and road.
In a rear-drive car the differences in rear wheel speed is handled by a
differential gear; you could make a differential gear for your bot, but
it's not all that easy to get something nice. (If you're interested you
can check out some of the designs the LEGO people have come up with.)
The Arrick robot gets around this by using one driven wheel, and a pair
of unpowered streering wheels coupled with a tie rod to the steering
servo. It's an excellent design you should study.
If you're planning on powering each drive wheel with a different motor
(you mention you have two 9 volt motors, and the steering motor is a
servo), then you don't need the steering servo. You steer the vehicle
using traditional differential steering, as found on most bots.
Differential steering works whether the wheels are in the center or to
one end of the robot.
I just re-read your post and finally noticed 2 things - the "What else will I
need" question being most important. For the design
you are talking about you would also need these things:
- batteries and a holder (of course)
- some kind of speed controller
- a servo controller (possibly)
- some kind of microcontroller ( like a Basic Stamp from Parallax.com )
You might be able to control a couple of servos using just the
The 2nd thing I noticed - only after seeing that Gordon McComb picked up on it -
is that you have 2 drive motors, and I would
suggest as did he that you use those for steering instead of a servo on a third
wheel. There are 2 big advantages to that approach:
1) You don't need the servo controller, necessarily. This also saves weight,
and power requirements.
2) By using the 2 drive wheels to steer, you get the robot to move in place
should you need to. It could rotate about it's
vertical axis in place, most likely. With the servo steering wheel approach, you
have to move either forward or backward in order to
Well, all of these are just suggestions. Once you do some actual building you
learn even more!
Have fun !
I have a bot that uses a variation on this. It uses two large wheels in
back, each driven off it's own motor and the front wheel is a simple
swiveling caster. It's easy to control by varying the relationship of
the drive to the right and left wheel motors, it is very maneuverable.
Ok I think I will go with the differential system, Yes I have two motors so
how would you make that, I will just use the front wheel as a castor for
balance, Why would I need a micro-controller?
I forgot to say why, sorry about that.
I mentioned a microcontroller because you need something to be used as the
brains of your robot. It doesn't *have* to be a
microcontroller, I just assumed that was the best solution because I assumed
your bot is small - say less than 1 square foot of
space to mount various components on.
One alternative - and I have done this for development purposes - is to tether
your robot with a cable to a PC and control it
directly from your PC instead of using a microcontroller onboard the bot
itself. But even so, many speed controllers are designed
to work with pulses and/or other signals that are easily downloaded to a
microcontroller from a PC.
It all depends on what you want your bot to do. A good start is to just have it
follow along a wall using whatever sensors you want
to start with. Some bots use "whiskers" to feel along the wall; some use
infrared sensors to measure the distance; others use sonar
to measure the distance. Once you get comfortable getting your robot to do
things like that, you can move on to more complicated
tasks such as finding a beacon, navigating with vision, etc.
If you can post some description of what you want your bot to do I am sure
people on this newsgroup will be glad to provide more
assistance and guidance.
You might still need one; or you might not. It all depends on exactly what you
want the robot to do. I'd say you need to outline
that first and then we all can help you more on this newsgroup.
Good luck !
You don't *need* a microcontroller if you're using regular DC motors
(and not R/C servo motors) for the drives. You can hook up wired
switches as a test. This lets you "drive" the robot around the room with
only you controlling it. You need double pole, double throw switches
with a center off position for this. Use Google to look up how to wire
up these kinds of switches to a motor, or look on page 256 of the Second
Edition of my book, Robot Builder's Bonanza for a picture.
(Go to Amazon.com here: (Amazon.com product link shortened) />/,
click the Search Inside This Book link, type in "dpdt" -- no quotes --
as the search string, then find the lnk to page 256.)
When you tired of manual control you can use whatever control circuit
you want, which may indeed include a microcontroller. You will, of
course, also need a motor driver circuit, which provides the current
needed to operate the motors. But worry about that after you get the
You don't need a micro-controller for the RC car that you mentioned in
another post. The micro-switch arrangement mentioned by Gordon will be
usable if your vehicle is not too fast. Another way to do this is to use
a pair of reversable speed controls for RC cars and an aircraft radio
with elevon mixing capability. Another possibility is a dual channel,
mixing speed control like the Roboteq one, but it will be more money.
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