Our micro controller teacher assigned robot projects for the class, using the pic 16f877a, my friend and i got the line follower robot and we finished assembling it yesterday and everything was going fine, later she tells me that one of the motors stopped working (and the one parallel to it followed as well) she tried to test the motors separately and they worked fine, but once assembled, it won't start, we checked the circuit and everything was fine, and we detected voltage on the motor pins but it just won't start.

Does anyone have any idea what the issue could be? Any help would be much appreciated.
We are using a DFrobot pirate-4WD mobile platform
So I hear you saying the following... When you test the motor(a) standalone they appear to work, but when implement back into the vehicle assebly they do not? But though they do not rotate, you measure the correct drive voltage across the motor terminals?

Assuming this is indeed the case, a couple possibilities are:

1. Check battery. Make sure it is fully charged. There is also a proportional relationship between available current and torque to rotate a load.

2. Usually such DC motors have step down gearing integrated,else it would rotate too fast for driving line following robots. If the motors are (too) cheap, they simply may start to fail. They may rotate under no-load, but not very well under load. Can easily verify this in standalone test. See if motor still exhibits torque to turn though under load.

3. You may have implemented motors in the assembly in such a way that any attached axles, etc. are binding, hence it locks up and prevents rotation. Make sure there is no wobble, etc. in assembly. You should be able to rotate the wheels by hand, and inspect if something is interfering with smooth rotation.

Hope this helps...

By Grey.CC
Hello Douaa,

Yeah, WA is right, there is a lot of factors,
What is your Micro controller? And how do you connect?
Do you have a  SMPS? If you have, you you could check the signal motor working current.
If you den't, you also could use a multimeter to check its working current and voltage. It is the best way to check whether the motor have been broken.