Electronics > Electronics

New robot builder, H-bridge/Motor question

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bshibley:
Hello,
I'm new to building robots, but I'm a junior computational physics major, so I should be capable.  I started simple with a LEGO Mindstorm and have been reading the tutorials and such for a while to build a robot from scratch.  I have a Digilent Cerebot board (http://www.digilentinc.com/Products/Detail.cfm?Prod=CEREBOT&Nav1=Products&Nav2=Embedded) which I've been using for my microprocessors course.  This is what I plan to use for the logic.  It supports up to 8 servos and 3 Digilent H-bridge modules (http://www.digilentinc.com/Products/Detail.cfm?Prod=PMOD-HB5&Nav1=Products&Nav2=Peripheral or http://www.digilentinc.com/Products/Detail.cfm?Prod=PMOD-HB3&Nav1=Products&Nav2=Peripheral).
For this project, I would like to build a 4wd rover which I can continuously build upon while learning robotics.  The first step is to have it drive with just a simple program controlling the motors.  Since the board only supports 3 of the Digilent H-bridge modules, is it possible to run two DC motors in parallel (ie. the right side motors) off of the same H-bridge by doubling the current through the H-bridge?  If so, is this wise?  I figured I could make my own H-bridges and work around the limitations of the board, but I was curious about how an experience builder would approach this problem.
Thank you for your time.

Eco19R:
Why do you need 4 motors? just use two motors with gears or chains or w/e -  drive the two right side wheels with one motor and the two left side wheels with one motor.




bshibley:
Thats a good idea.  I planned to use 4 motors since thats what the other 4wd robots/kits I had seen used.  How much efficiency is likely to be lost by using gears/chains?

Eco19R:
I really don't know but your going to be saving allot of battery power by not using 4 motors.

Your going to need to geardown the motors anyway so I doubt you will be having a lack of power.

Unless you want an offroad robot or a sumo bot or your gonna have a really big base 2 motors will be fine.

Kohanbash:
Hi
If this is your first robot you might want to start with only 2 wheels and leave the front wheels to swivel on their own. After playing with your code/design/aspect ratio you can add actuate the front 2 wheels.

Another reason that it might be good to only actuate the back wheels is for ease of turning. with the 4 wheel gearing option you can use differential drive (slip steering) to turn (similar to a tank) where the wheels on each side are turning with a different velocity. if all 4 are actuated separately than you can use an addition motor / linkage to turn with the front wheels.

With only two wheels actuated the turning becomes easier as the front wheels swivel. The Down side is you can't manipulate rough terrain as well.

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