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Author Topic: DC motor drive?  (Read 950 times)

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Offline oioimacoyTopic starter

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DC motor drive?
« on: June 07, 2013, 02:57:26 PM »
This is my first post on this forum, plus I don't tend to post on forums so I don't really know the general protocol. I am fairly sure I am doing it right though.
I am also very new to electronics, let alone robotics. I'm a software engineering graduate so all the software stuff is a breeze. Right now I am actually struggling with getting my robot to move forward haha.

I basically have two 6v-16v dc gear-less motors. I am controlling them independently via two transistors in which I am supplying the base voltage via  setting a pin high on the Ardunio Uno, thus allowing 9v to pass through the transistor to ground completing the circuit making to motor turn. I'm sure you know all of this. It all works great until I put the robot on the floor. The two motors have no torque what so ever to even budge the robot a hairs width distance. Are gear-less dc motors pretty much useless then? I mean the robot is made from foamcore, with two center wooden wheels, an Arduino Uno, 2 9v batteries and a breadboard. It doesn't have a great deal of weight tbh.

But if I was to purchase geared motors instead, for the same amount of money I could purchase continuous servos which I believe already have a fair bit of torque.

This is essentially what I am making: http://blog.makezine.com/2013/06/04/projects-with-ryan-slaugh-hack-bot/
However I chose to buy the DC motors because the continuous servos where too expensive.

I guess I'm really asking: What the hell can you use a DC motor for? They're sh*t! And what is a good solution for the drive of a small robot like the one I am making?

Apologies If there is a topic on the forum like this already.
« Last Edit: June 07, 2013, 03:11:37 PM by oioimacoy »

Offline waltr

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Re: DC motor drive?
« Reply #1 on: June 08, 2013, 07:09:50 AM »
Yes, 'bare' motors have little torque but lots of speed (rpm). Check rpm & torque specs on almost any DC brushed motor to see.
A gear box (like in your car) is need to convert rpm to torque. Baring losses, if a gear box reduces rpm by 10 the torque increases by 10.

To chose a gear motor calculate the required maximum RPM output from the robots wheel diameter and the desired speed. This gets you the gear ratio needed.
The required Torque is a bit harder to calculate but can be done from Physics basics. Or use the motor calculator found by clicking on "Robot Tutorials" in the upper right corner of this screen.

Offline jwatte

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Re: DC motor drive?
« Reply #2 on: June 08, 2013, 09:46:09 PM »
Motors themselves are almost always geared. Your car has a gearbox. Servos have gear boxes. Belt drive motors typically use the belt itself as gearing, with a small pinion on the motor and a large receiving wheel on the other end.

Note that "gearhead" motors are just regular motors, with a gearbox added. The motor you have, plus appropriate gearing, would work fine. (Note that large wheels count as "inverse" gearing)

So, you can try very tiny wheels, and see if the robot moves. Or you can try using a rubber band and a small cylinder on the motor, driving a large wheel on the wheels. Or you can buy new motors with gearboxes. Pololu has some good mini- and micro gear motors for 4.5V or 6V:

http://www.pololu.com/catalog/product/1594 -- $6
http://www.pololu.com/catalog/product/1094 -- $16
http://www.pololu.com/catalog/product/1109 -- $20 (likely draws too much current for a 9V battery)
http://www.pololu.com/catalog/product/1595 -- $23 (likely draws too much current for a 9V battery)

You can either put 9V into them, which will make them work harder and wear out sooner, or use PWM on the Arduino pin to give it a 1:1 or 2:1 duty cycle that will be equivalent to feeding 4.5V or 6V to the motors.

Offline ROBOT420

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Re: DC motor drive?
« Reply #3 on: June 10, 2013, 09:25:20 PM »
That is probably not enough amps. I tried to use two 9 volts in series (18V) to rig up a car horn with remote key fob that I could hide random places and scare my friends. it would only make a slight click. The same setup on a car 12v worked fine with even less volts (but with MORE AMPS), I would check that first. Good luck, keep us posted! ;D
 
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