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Author Topic: New robot builder, H-bridge/Motor question  (Read 2858 times)

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Offline bshibleyTopic starter

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New robot builder, H-bridge/Motor question
« on: March 12, 2007, 01:55:56 PM »
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.

Offline Eco19R

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Re: New robot builder, H-bridge/Motor question
« Reply #1 on: March 12, 2007, 05:01:44 PM »
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.




« Last Edit: March 12, 2007, 05:02:53 PM by Eco19R »

Offline bshibleyTopic starter

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Re: New robot builder, H-bridge/Motor question
« Reply #2 on: March 12, 2007, 05:31:28 PM »
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?

Offline Eco19R

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Re: New robot builder, H-bridge/Motor question
« Reply #3 on: March 12, 2007, 06:09:38 PM »
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.


Offline sdk32285

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Re: New robot builder, H-bridge/Motor question
« Reply #4 on: March 12, 2007, 07:01:56 PM »
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.
Robots for Roboticists Blog - http://robotsforroboticists.com/

Offline dunk

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Re: New robot builder, H-bridge/Motor question
« Reply #5 on: March 14, 2007, 06:56:43 AM »
hmm,
i'd try and keep things as simple as possible on your first bot.
2 wheels and a caster is the simplest configuration.
if you really want 4 wheels, i'd stick with 4 motors rather than introducing chains and gears if you want to keep things mechanically simple.
you will be able to use lower power motors this way so there wouldn't be much difference in the power used.

my first bot was a failure because i went with motors that were far too big for the job, which in turn needed huge motor controllers, bigger batteries, etc.
if you want the best chance, start with a nice simple design. the fewer components there are the better. once you have this working you can move on to version2...

dunk.

 


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