My first tasks are to get the long lead activities under way. This includes designing a Nucleo Expansion PCB for the LCD and ordering parts. I actually started ordering parts as soon as I was announced as a participant, so I was able to unbox the I-NUCLEO-LRWAN1 module at the same time as the sponsored design kit.

To design the Nucleo expansion card I need to map out which pins on the MCU card will be used by each peripheral card. All of the expansion cards supplied with the kit only use arduino connectors, which doesn't seem optimal to me. I would have thought Nucleo expansion cards should use the Morpho connectors so they have control over the number of pin conflicts and leaving the arduino connectors available for arduino shields. As it is, the 3 expansion cards supplied with the kit take up the vast majority of arduino pins, making it pretty unlikely that another arduino shield can be used. Even amongst the 3 Nucleo expansion cards there are about 10 pins that overlap. It may be possible to resolve these conflicts as some pin assignments look like they can be altered with jumpers, but I have to fit a fourth Nucleo expansion (I-NUCLEO-LRWAN1) card onto the stack and it also uses only arduino pins. The Wi-Fi Expansion card alone connects to 23 of the 32 arduino pins.

Right now I have a major problem with the I-NUCLEO-LRWAN1 card because I cannot find out which pins it uses - there just doesn't seem to be any documentation on this. So this PCB design effort is semi-stalled until I can dig up more complete documentation. It is going to be painful and risky if I have to reverse engineer the LoRa card to discover its pinout.

Of course there are plenty of other things to do. One of them is to research whether it is feasible to measure fuel levels from outside the gas tank without tapping into the fuel plumbing or opening the tank. I will try to do this by measuring the resonant frequency of the air in the tank. The following video is a first attempt using a 2 liter bottle with water instead of gasoline.

I had to split the video to get it to upload...

The Rohde & Schwarz RTB2004 oscilloscope did a spectacular job of supplying both the excitation and the measurements for my tests. I literally did not have to build anything.

It clearly shows some well defined resonances that change pitch as the water level changes. However, my first look at the data yields very confusing relationships. The only thing that is clear at this point is that it is going to take lots of work to get the results I want.

Here is a scatter plot of resonant frequencies on the X axis versus air volume on the Y axis:

There doesn't seem to be much vertical alignment (indicating resonances that do not change with volume)

Below I assume the frequency increases linearly as the air volume decreases and draw some straight lines through possibly related resonances:

It doesn't make sense to me that all the frequencies would increase with decreasing volume, but maybe it is possible.

Next I tried to add water slowly and track the resonant frequency of a single initial resonance to see if these straight lines represent what is really going on.

This was a bit harder than I anticipated. It seems like a resonance dies off as volume changes by about 30 ml and a new resonance mode starts up at a different frequency. This may mean most of these resonances are not the air cavity, but the varying mass relationships can allow whatever is vibrating to resonate or it can damp them out. So I will need to investigate every resonance to see if it actually tracks the air volume ..... oh joy. At least this cool oscilloscope allows me to investigate without rebuilding a bunch of circuitry. I was hoping that the lowest resonance would be the Helmholtz resonance, but so far the data doesn't support this theory. (The Helmholtz resonance is the lowest resonance of the air in a cavity) Some people do a whole 2 year thesis on stuff like this - I need results faster than that.

One thing I didn't plot above is the amplitude of the resonant frequencies, but it seems to have some correlation with volume - the more water the higher the signal amplitude.

The investigation continues....


Project Links:

IoT On Wheels Design Challenge page



Links to other blogs on this project are included in the first blog: