Only one motor moving

[pmaddocks]

My son had an NXT 2.0 for Christmas and there is an irritating issue. Whenever he creates a program (or downloads one that has already been created) only one motor will move when two are programmed, so the robot will only go around in circles. This happens if we change the program to try and make B and C move or A and B move. Both motors will move independently so there is no hardware fault. Any ideas or suggestions would be gratefully received.

[mattallen37]

Can you post your program?

[pmaddocks]

Hi.
It's the attached Bumper car bot from nxtprograms.com, but we have the same issue if we try to create a simple move program of our own. The motors will move individually, but the minute we try to get them to move simultaneously, only one will run. The other sort of twitches as if it's received an instruction, then stops. It happens if we use the mindstorms program running on XP 32bit or Win7 64 bit, so I can't see that it's a driver issue. lego refuse to accept that here may be a problem wih the brick and say that it must be the program, but I can't see how.

Thanks for taking an interest.

[mcsummation]

I downloaded that program into my bot and it worked properly.

Are you sure you have good batteries? What does the NXT-G system think the battery voltage is, both when the motors are idle and when the bot is trying to run?

[h-g-t]

Have you tried switching the motors over to see if the problem changes sides?

Have you tried replacing the motor which does not work?

Try switching the leads over or using different leads, the contact area inside the leads is very small and if there is a poor connection it can limit the power delivery. Lego motors are notorious for being inconsistent and if you have a sluggish motor connected with a defective cable it might cause that sort of problem, especially if the battery level is low.

You can put a block into your program which shows the battery level even when not connected to the computer, see the 7th post in this thread - https://sourceforge.net/apps/phpbb/mind ... tery+level

[hassenplug]

I'm pretty sure I can tell you where to look to solve this problem.

First, here's what's happening: when using the "move" block, the NXT tries to keep both motors moving at the same speed. If it detects that one motor is NOT moving, it will stop the OTHER motor, and simply try to drive the motor that it thinks is "stuck".

So, the motor that keeps spinning is not sending pulses back to the NXT, and the stopped motor is working as expected.

There are three possible problems:
1) Bad Connection
2) Bad Wire
3) Bad Motor

The first thing to do is make sure all wires are "clicked" into place. They may look like they are, but push them in again, anyway. Then, try the program again. This will solve the problem 99% of the time.

The second thing is to disconnect the wires where they plug into the two motors, swap them, and run the program again.
- If the other motor spins forever, the problem is in the wire. (check the connection at the NXT, yet again, then swap the wire with another)
- If the same motor spins forever, the problem is in the motor. (make sure you plugged it in well)
- If the motors work correctly, it was in fact a problem with the connection.

I have a couple hundred NXT motors, and I think I've only had one bad motor encoder. So, it can happen, but not very often. If a motor or wire is bad, LEGO will be very quick to replace it.

Hope that helps
Steve

[pmaddocks]

Thank you soooo much for all of those contributions people, especially Hassenplug, as it turned out to be a faulty motor, but it was the faulty one that always moved when the two were supposed to run together. The good one was the one that stopped. Would never have guessed that in a million years. Will get on to Lego now and get a replacement then we can build some more.

Thanks again, from one happy son and one very happy dad.

[mcsummation]

Incidentally, Mindsensors sells a cable tester - http://www.mindsensors.com/index.php?mo ... tion=50:50.

[hassenplug]

The good one was the one that stopped. Would never have guessed that in a million years.

When you understand what's happening, it all makes sense. The "twitching" from the good motor is what happens when that motor successfully moves a few degrees, then the NXT will shut it down and wait for the bad motor to indicate that it has moved the same distance, which it never does.

Glad we could help.

Steve