This blog covers the day to day progress of water rocket development by the Air Command Water Rockets team. It is also a facility for people to provide feedback and ask questions.

Saturday, August 16, 2008

New test stand work

We have been working on a side project for the past couple of weeks. We are keen to build a new thrust measuring test stand for testing design changes, but doing it the old way of recording a kitchen scale on video and then manually transferring the data into a spreadsheet just wasn't going to cut it for accuracy, and especially for making multiple runs.

We are doing it properly this time with an electronic data logger and a load cell hooked up to the laptop. The main problem with trying to do it the proper way is the higher cost involved as the equipment typically isn't your regular mass produced consumer electronics. The guys on the Australian Rocketry forum were a great help in recommending which equipment to use, things to look out for, and generally how to set it up. The data-logger we chose was from DATAQ. They have a number of different loggers with different capabilities at a very reasonable prices. I was very impressed with the delivery time from the time that I ordered. I placed my order just before lunch time on Thursday with the Australian distributor, and the package from Melbourne was waiting for me on Friday when I got home from work.

The load cells also tend to be quite expensive - easily upwards of $150-200+. We had been researching a lot of different companies and finally found a new 70kg load cell from a Chinese manufacturer whose cost and delivery were far cheaper than local distributors. It is still likely to be about 10 days before it arrives. I don't know what the quality is like yet, but I believe it should be great for our purposes.

The data-logger and load cell aren't enough though, and we also need a load cell amplifier as the datalogger only reads in the -10V to 10V range. The load cell only produces miliVolts over the entire deflection range which is too low for the datalogger to get enough resolution. The amplifier basically converts these small voltage changes to a voltage range usable by the logger. It also provides the necessary excitation voltage for the load cell. Again you can buy these off-the-shelf but you will pay upwards of $100+. Being on a limited budget, we decided to build our own load cell amplifier instead, which is based on an instrument amplifier IC. The circuit is relatively straight forward and these IC's are designed for exactly this task. The IC has been ordered and is on its way from the US.

I'll publish the full details of the entire setup and suppliers when it's calibrated and working properly.

The data logger came with free "lite" software for capturing and viewing the data. However, being free there are some limitations in terms of exporting the data and maximum allowed sample rate. The max sample rate is 240Hz which is more than ample for us. The full software with higher allowed sample rates is another $200. You can also buy a $99 software add-on that lets you save the captured data into Excel friendly format. (doh)

The free software, however, does record the data into their own proprietary binary file format. I recorded a number of waveforms today, and looked at the data in a hex editor. I noticed that it was not encrypted, so it was relatively easy to reverse-engineer its format and write a small program that allows me convert their format into a text .csv file format that is directly readable by Excel. (saved $99 there :) )

The entire thrust measuring test stand + software should cost us under $300 when finished.


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