And according to the datasheet, The PIC you used (2550/4550) support “full” speed.

How does this work?

> One thing that stuck out (IMO) was your use of a spreadsheet for the results, you may be interestind in looking into programs like octave (GPL) or matlab (expensive) for doing numerical analysis, as they’re a lot more optimised for doing things like this than a spreadsheet application.

I guess this is a case of using what I know. I haven’t used any of the programs that you describe. Excel does the trick for me. I admit though that the spreadsheet could use some work. The 32.000 data point limitation is a real show-stopper, which would become worse when the sample speed would be increased.

I would be gratefull if someone who is more proficient in Excel would take a look at improving it, or perhaps even writing a custom application to analyse the data.

> A little tip for text to Excel spreadsheets: insert a comma after each entry in a row, and use carriage returns for new rows, then save the file as a .csv.

This creates a Comma Separated Value sheet that Excel and other spreadsheet apps can read natively.

I use CSV files often, but decided against them in this case. One would have to open the CSV file in Excel and then copy-and-paste the data into the analysis spreadsheet anyway. I prefer to copy-and-paste from a plain text file.

> On increasing the sample rate: Since the numbers you are sending over the serial link are 10 bit, you could fit them into 2 bytes (16 bits) easily (just don’t convert them to ASCII in the PIC before transmission!). This way you wouldn’t have to bother with calculating deltas on the PIC.

That’s a really good idea, I hadn’t thought about that one myself. I guess the simplest solutions are most often overlooked. It shouldn’t be too difficult to convert the two bytes into an integer on the client side - I might adapt my firmware and Excel sheet to use this trick.

> Final thought - can you not just increase the baud rate of the serial connection? I have a vague recollection that Microchips CDC code can work at up to 576000 bit/s. Unfortunately I can’t find the documentation to prove that at the moment

The USB communication is running at 115kbps at the moment, which is the maximum Microchip’s CDC will support. It would be possible to increase the USB spreed, one would need to write a custom driver then. I chose the easy route. The 6000 Hz of sample speed is plenty for measuring rocket motor thust curves anyway.

> How about the rocket motors, do they meet spec? Are they Estes?

The rocket motors are home-made, as per http://jamesyawn.net/modelrocket/index.html or http://nakka-rocketry.net/pvcmot12.html . Sometimes they meet specs, sometimes they don’t and sometimes they explode, but they always offer opportunities to learn something new! Ofcourse this system could be used to test Estes motors just fine.

> Very Nice! I did something almost exactly the same- Except I made the loadcell. http://colinkarpfinger.com/projects/rocketry/rocketry.htm

Yes, I previously found and took a look at your nice website. Looks like some fine work you are doing! I was thinking of making my own load cells as per Nakka myself, but that’s when I found the digital scale on sale, so I ripped that apart to salvage the load cells. I expect the homemade version to be much more accurate, and will probably make a few myself in the future.

On increasing the sample rate: Since the numbers you are sending over the serial link are 10 bit, you could fit them into 2 bytes (16 bits) easily (just don’t convert them to ASCII in the PIC before transmission!). This way you wouldn’t have to bother with calculating deltas on the PIC.

I realise this adds complexity on the host PC end. I think you could solve that (for example) by using a very simple Python script and pyserial: get the python script to read data out of the serial port, convert to ASCII text and output to a CSV file for loading into Excel later.

Let me know if you want help with the Python script - I’ve got something from a similar (Arduino) project lying around.

Final thought - can you not just increase the baud rate of the serial connection? I have a vague recollection that Microchips CDC code can work at up to 576000 bit/s. Unfortunately I can’t find the documentation to prove that at the moment

http://en.wikipedia.org/wiki/Comma-separated_values