wifi robot for the lads

[lightening_fingers]

ok, so I'm somewhat of a long time lurker and heres my first real pi project.

I've got a 7 year old and a 5 year old lad both itching to start a robotics project.
we're going to go for a simple small mechano based wireless robot, controlled from an android tablet and hopefully eventually with a camera and some headlights. (future mods would include collision detection, range finding self-drive).
I've written android and iPhone/ipad apps in the past so I'm happy about the coding side of things.
I work in IC design so I "should" be ok on the hardware side of things, I say "should"

I've got my pi, its now wireless and hopefully should be soon battery powered off of a 7.2V RC car power pack (suitably stepped down via a linear regulator to 5v).
My question is, which stepper motors (2 centre wheels with swivelling dollies at either end)
I've been looking at loads but I'm stumped on the whole supply V vs torque vs cost consideration.
what sort of size/power will i need for a small mechano based robot?
heres one I'm considering:
ROB-09238
http://www.hobbytronics.co.uk/robotics/stepper-motor
any good? rated at 12V but CAN i drive it with 7V from the rc power pack?

Then theres the drivers for the steppers, 2 I've been looking at are:
http://www.ebay.co.uk/itm/Dual-H-Bridge ... 0743762410

and
http://www.schmalzhaus.com/EasyDriver/

I seem to remember the GPIO outputs of the pi are open drain (can only pull down) so I presume they need a pull up turned on or an external pull up. (?)

Considering the easydriver, it needs 2 gpio outputs, one for pulse and one for direction.
I notice the pulse is active on the rising edge of the input (GPIO output), wont this be slow because of the "weak" pull-up ?
it says
"EasyDriver requires a 7V to 30V supply to power the motor and can power any voltage of stepper motor"
does this mean that with a 7V supply, you need a stepper motor that can handle 7V?

alot of silly beginner question here, I'd really appreciate any help anyone can offer

S

[joan]

I'd get a pair of geared DC motors. They are cheaper and much easier to drive. I'm sure stepper motors have their uses - just not for a general purpose robot.

[lightening_fingers]

Thanks joan, do you mean something like:
http://www.hobbytronics.co.uk/robotics/mm-gear-motor-10

I presume I can use the H bridge driver from ebay that I linked to.
Also it looks like these motors play nice at the 7.2V I'll have available.

am I right in thinking that the H bridge board needs a pwm input per motor to be controlled?
does the pi have 2 pwm outputs?

S

[joan]

Unless you are after a speed record I'd look for something around the 100 rpm mark. That will move along the floor quite smartly and give you a chance to stop it before it smashes the brains out of the guinea pig!

e.g. http://www.ebay.co.uk/itm/6V-DC-100RPM- ... 0809454899

Any old H-bridge should do.

e.g. http://www.ebay.co.uk/itm/L298N-DC-Step ... 0973372764

I have three of those and they work fine for me from the Raspberry Pi. Others say they don't work for them.

You don't need PWM to switch the motor on or change direction. PWM is used to change the speed (by rapidly switching the motor on/off). The Pi has one (perhaps two I'd need to double check) gpios which can be directed to be in PWM mode. There are other solutions (one is a library I have which provides PWM on all the gpio pins).

[joan]

It shouldn't need saying, but I'll say it. For every two forum members there'll be at least three different opinions!

[southpaw]

what about either of these, cheap but do take a couple of weeks to arrive on the free shipping method

http://dx.com/p/2wd-robot-raider-car-ki ... ino-153021

http://dx.com/p/batman-robot-raider-car ... ino-153016

[lightening_fingers]

at these sorts of prices I wnet for:

http://www.hobbytronics.co.uk/robotics/ ... ot-chassis

now to order up an H bridge.

S

[joan]

Good choice. If you have some spare motors you can make two bots.

img_2309a.jpg (39.43 KiB) Viewed 2281 times

[lightening_fingers]

Joan,

The h bridge I linked has arrived and works a treat, can I use it straight off of the raspberry pi GPIO output, or do I need to level shift the 3.3V GPIO up to 5V for the h bridge module?

ta

LF

[joan]

The three I have work quite happily direct from the 3.3V gpios. A case of try and see.