Physics & Physical Science Demos, Labs, & Projects for High School Teachers

LEGO NXT Mindstorm Frustrations

Posted on: October 5, 2011

My two robotics classes have been working on an obstacle course.  I made it challenging, but it seems to be too challenging – not because the kids aren’t trying but because the robots do something a little different every time.  I’m confused.  I called LEGO Education support and they told me the motors are accurate to one degree per rotation.

I’m seeing errors much more than ten times that number.  I can’t seem to find a way to solve the problem.  We are using yellow RESET block, light green MOVE blocks, and orange rotational WAIT blocks.  We have a spot carefully marked on the floor for the start and way points.  Starting at the same spot may have the robot as much as an inch off within about 10 rotations.  The total motor error should be about 0.17 inches.  I can’t determine if it’s a problem in NXT-G or the motors.

I’ve searched the NXT books, blogs, web sites, and books, and I can’t find any references to the inaccuracy issues.  Has anyone else seen these problems?  How do I fix this?


6 Responses to "LEGO NXT Mindstorm Frustrations"

Is it possible to use a switch of some sort to close each rotation, thus using an external calibration check against the (what I assume is a) built-in rotation sensor? I don’t know how programmable the NXT system is, but if it can be programmed to re-calibrate the motor rotation every time it has a switch-closed condition that might help.

They have a counter reset that we use, but you can’t do any kind of software or hardware calibration. At least none that I’ve been able to discover.

I used the LEGO NXT Mindstorms with a group of middle schoolers. We found that the amount of battery life remaining had a strong effect on how well/consistently the motors performed. We had to keep the batteries fresh in order to ensure consistency in output.

My experience with the batteries has been decent. We try to charge them each day, but I’ve seen many of them go almost a week between charges. I should add here that I’m using the rechargeable packs that come with the education kits. If you are using alkaline AA’s, you are probably having a different experience. If you are running the motors, they don’t last real long. The rechargeable packs have a failure rate that I find unacceptable. Two out of thirteen died just after the first year, when they were out of warranty. I have enough kits across two classes that we just pull the batteries each class and the next class uses them. I like the rechargeable packs, but I would replace the dead ones with NiMH AA’s. For the price of one LEGO rechargeable pack ($50), I could buy a charger and a pile of NiMH AA’s.

I determined that the motors were very accurate for low accelerations but I encountered greater inaccuracies at greater accelerations. I am fairly certain the greater accelerations resulted in greater inaccuracies because the tires spin a small amount before gaining traction on the floor. When a bot starts from rest at a high power, the slippage is significant.

I just finished teaching a week of robotics summer camp. I had some time to experiment and played with the “motor control” button on the motor menu. I found that the control tries to compensate for anything bogging down the motor, such as friction. I was able to make a slightly more repeatable program compared to not using motor control. I agree that the wheels jump when starting from rest. I often tell the students that the programs are more repeatable at slower speeds for just this reason.

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