BEST 08 CPH: Second Day With LEGO Mindstorm

Today's lesson was about sensors for mobile robots. The sensors are needed for position measurements, detection of obstacles and measurements of internal states. We got a short introduction in odometry, encoders and kinematics of mobile robots. We also heard about reflectance sensors and obstacle sensors working with infrared and laser scanners. Professor Nils Andersen showed us videos of various robotors and GPS systems like the traktor HAKO and we got to know the cleaning robot "Kaercher RC 3000".

Here you can see the cleaning robot at work:

Today we also got two homework assignements.
The first one is about the ultrasound sensor:

Test the sensor by taking measurements from 5 to 30 cm with 1 cm resolution and measurements from 30 to 150 cm with 5 cm resolution using a cardboard box as target.
Take 5 measurements at each distance.
Make a plot of actual versus measured distance.

To prepare for the measurements we took a ruler and a card box. We used the biggest side of this box for our experiment.
The following table is showing the results of the first set of measurements (5 to 30 cm, 1 cm resolution).

Here you can see the measures in a graphic chart.

The absolut error is shown in this picture. It's the difference of the real distance and the average of the measured distance.

Here you can see the relativ error of our measurements.

To summerize our results, we can say that the biggest error occured in the closest range. The optimal result was achieved after 21 cm.

Here you can see the results of the second set of measurements (30 to 150 cm, 5 cm resolution).
This table shows the numerical results:

Here you can see the graphic referring to the table above:

The absolute error is shown in this figure:

In this picture the relative error of our second set of measurements is shown:

Comparing to the first set of measures, we can see that in the second case there is nearly no big error occuring. This lets us conclude that in farther distances the measurements are more precise.

Execise two is about odometry:

Calculate cm for your tribot
Measure the distance between the wheels b
Measure cm by making the robot go forward 2m
Try to check b by calculating n for a 90 degree turn and compare with what the robot actually does.
Try to calibrate the odometry by long straight line movements and multiple turns around itself.

For the first point we used the following formular:
c_m = Pi*D/C
Where D = 56mm is the diameter of the wheels and C = 360 degrees.
Hence c_m = 3.14159*56/360 = 0.4887 mm/degree.

Next we measured the distance between the wheels: b = 12 cm.

We programmed the TriBot to go for 11.368 turns of wheel which should be exactly 2m. Trying this out, we got 1.97m as a result. So there's a difference of 30mm.

We already got the task for the final competition. There are five competition assignments

1.) Find and Measure Distance to Box