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I have built a simple robot, my first project. When the wheels are off the ground, they spin, when they are on the floor they don't. At first I thought it was weight but it seems not, I tried removing all the components and holding them in my hand(with the cables still attached) so it was just a bare frame. I have 2 DC motors and 7 AA batteries. The Pi is powered separately. I used this tutorial

http://computers.tutsplus.com/tutorials/controlling-dc-motors-using-python-with-a-raspberry-pi--cms-20051

Below are some pictures. I realise you can't see the wiring too well, but i did it exactly as in the tutorial except the extra batteries

enter image description here enter image description here

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    This isn't my area of expertise, but it sounds like the motors aren't getting enough power. Why are you using 7 batteries? The tutorial calls for 4. Please post some pictures of how you have everything wired.
    – Jacobm001
    Jan 16, 2016 at 17:48
  • @Jacobm001 because 4 wasn't enough I thought I'd try 7. They do spin very fast so i don't think thats the issue. I'll add some pictures now
    – Hamzah Malik
    Jan 16, 2016 at 17:49
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    Spinning fast does not mean they have the power to push under load.
    – Jacobm001
    Jan 16, 2016 at 17:50
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    Your motors don't have enough torque.
    – Steve Robillard
    Jan 16, 2016 at 18:08
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    I couldn't say without much more detail, but you may want to consider something like this (or at least the geared motors) sparkfun.com/products/13166 Also you may want to check the specs on the motors you have and the amperage/voltage you are sippplying them.
    – Steve Robillard
    Jan 16, 2016 at 18:12

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Those appear to be ungeared motors. You need geared motors.

Can you stop the motor(s) turning by lightly holding the wheel with a finger?

When power is applied they will spin at something like 10 thousand RPM. You need motors which are geared to spin at something like 50 to 150 RPM.

As mentioned in the comments there is not enough torque. In your case torque is the ability to move the motor against a load.

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  • Ah thanks. So is lower RPM better then?
    – Hamzah Malik
    Jan 16, 2016 at 19:47
  • @HamzahMalik Essential. It will let your robot work. 100 RPM will work and move your robot at a foot a second, 10,000 RPM won't work as it tries to move your robot at 100 foot a second. I'm using approximate figures, the actual depend on your wheel's size.
    – joan
    Jan 16, 2016 at 20:01
  • Ok thanks ill look into it. So geared motors at low speed of about 100rpm?
    – Hamzah Malik
    Jan 16, 2016 at 20:02
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    @HamzahMalik Yes. There is no universal best RPM. It depends on what your robot is doing. Anything between 50-200 RPM is a good start.
    – joan
    Jan 16, 2016 at 20:06
  • You may want to read around robotic stack about a formula on how to calculate the correct gearbox setup. Motor to gear ratio and wheel radius. You may also want to use NiCD batteries or LiPO as they will last longer and allow for more power (amps) to be fed into the motors, also increasing output torque .
    – Piotr Kula
    Jan 17, 2016 at 22:50

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