Inertia Measurement + Image Processing with MATLAB

A project log for Model rocket with active stabilization

By means of motor vectoring

Domus AmorisDomus Amoris 08/16/2019 at 19:280 Comments

At the moment, all the hardware is ready, only some software adjustments are necessary. One important remaining task is tuning the PID. For this purpose, I have to update the simulation parameters of the system with the latest values, and rerun the calibration process.

Today I set up a test bench in order to measure the rocket inertia, using the bifilar method, which basically consists on hanging the rocket with two chords, both at the same known distance of the center of gravity, and measure the period of the rotatory oscillation. You can find details at :

You can see the set up below. In order to measure the period of the oscillation, after considering different alternatives, I choose to record the tip of the rocket with my mobile phone. I draw a black dot in a piece of masking tape and the video was processed afterwards with MATLAB in order to obtain the change of position over time. I can provide further details if anyone is interested. You can see the animation below also.

I tested to configurations, with a length of 1.15m and 0.80 m, both gave the exact same inertia with a negligible error. However, the dimensions for the computation and the position of the CoG have a certain degree of uncertainty due to the poor tools I used to measure them. Nevertheless, I consider the results to be accurate enough for my purpose. There is also a low frequency mode which I do not clearly identify but should not affect the measurements.

This method was proved to be very suitable, cheap, easy to set up, there are not any sensor cables affecting the measures (compared for example to using an accelerometer), and does not rely on human ability (compared to using a manual timer).