FLL 2021: 'It works sometimes. Why?' - 10 out of 10 on how to accomplish the Train Tracks mission Pro Preview

"Sometimes it works, sometimes it does not work" - this is the most common case in FIRST LEGO League competitions. In this tutorial, we demonstrate and discuss such a case where the first part of accomplishing the mission always works but the second part has about 60-70% success rate. The robot is not very consistent. Let's take a moment to see it and explain why so that we can resolve the problem in the next lesson.

  • #2077
  • 21 Feb 2023
  • 5:51

Consistency and reliability

The consistency and reliability of the robot come from the use of the sensors. In this 10 out of 10 tutorial, we've decided to demonstrate the robot's behavior when we don't effectively use the Motion and Light/Color sensor of the robot. In the second part of the program, we don't use sensors, and we rely on aligning/squaring the robot to the field border. We also rely on moving a set of degrees forward. Thus we reach the predictable result that the robot's behavior can not be predicted (pun intended).

With this tutorial, we hope we can help you identify such behaviors in your robots and then arrive at a solution to improve them. In the next lesson, we are improving the behavior of the robot.


In this video tutorial we accomplish a complex mission, the train track. It requires us to close the train track and then to push or pull on the train. And that's a mission model that involves two actions. We are doing a ten out of ten tutorial to demonstrate the consistency and reliability of this LEGO Education SPIKE Prime robot and its attachments. And for this program, what we are doing is it's a program without the use of too many sensors, just one sensor to accomplish this mission. Let's see how consistent it is.

First from restart the robot, download the program and start.

We fix the train tracks, we go to the other end of the mission model, we square the robot to the border and we push on the train. And we almost got it. So the train is not completely locked. We couldn't accomplish the mission just for a centimeter. Let's do a second run as we discuss in the tutorial. In the first part of the program we use the motion sensor and you can see how precise the route is in the first part of the program. But then we don't use any sensors, we just rely on moving with the robot, rotating and it's possible. We got it. Second run. We are successful. We got it. We managed to accomplish the mission, but it's one to one. It's interesting how for the first part we use and you can see how the robot compensates. We use the motion sensor and the robot compensates for its errors. But then because we don't use the sensors and here it got stuck.

These were three runs. What will happen with the forth? So one successful, two unsuccessful. We try to go to the other end, we rotate with square with a mission and we push. So two successful, two unsuccessful. It's kind of getting us there now in some of the next tutorials and practically in the next lesson we'll improve this program to use more sensors. But we decided to split the tutorial in these two parts in order to demonstrate how you see how the robot got stuck here previously. In the first part we are using sensors, after that we are not using sensors and the robot is doing things that we can't even predict. Five runs. Try again.

We can't compensate for the errors here we kind of got lucky.

Then we rotate and we manage. And that's some of the most difficult things on the competition is that you somehow sometimes manage to accomplish the mission, but then it fails and it is always in home on the competition field that we are preparing. It always works, but then when we go to the competition, it never works.

And here again it was a good run, but we couldn't get the robot to push the train all the way to the other end of the train track. A few more runs just to make a point. And our point is simple. If you limit yourself with the use of the sensors, you get to a robot that kind of works, but it won't be very consistent and reliable. Here the program is complex. It has three parts close the track, go to the other end, push the train back, and in the three parts, we use the sensors only in the first part of the program, we don't use the sensors in the second and the third part. We use the square. The square, in case you can see. But we don't use sensors and we get it working in about 60 70% of the cases. A few more runs. In the next lesson from the course will improve this robot with additional programming to use additional sensors like the light sensors. I hope you like it, hope you find something useful in this tutorial. I hope it is useful to you and I'll see you in the next tutorial.

Courses and lessons with this Tutorial

This Tutorial is used in the following courses and lessons

Image for FIRST LEGO League 2021-2022 Cargo Connect solutions and review with LEGO Education SPIKE Prime
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