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#184 - More Axion G6

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#1 to #140 (Updates)

 

FLIGHT LOG

Each flight log entry usually represents a launch or test day, and describes the events that took place.
Click on an image to view a larger image, and click the browser's BACK button to return back to the page.

Day 67 - Test Stand
First version of the load cell amplifier and logger were both contained in the same box.
The load cell was at the end of a long cable away from the load cell amplifier.
On advice we moved the amplifier a lot closer to the load cell and placed it in a metal box.
This reduced the noise of the system considerably.
Here the test stand is on display in the kitchen.
The amplifier is mounted on top ...
... with the load cell underneath. The bottle is connected to the load cell here.
The rocket is supported by three wheels that barely touch it. This allows the rocket to move with very little friction.
The wheels can be adjusted out or ...
... in depending on the size of the rocket.
First tests set up outside.
First prototype of the nozzle release head. Pulling the string causes the release head to drop away.
   

Date:  21st September 2008
Location:
Workshop
Conditions:
Pleasant
Team Members at Event:
GK and PK

Test Stand Work

Over the past few weeks we have been building a sensitive test stand on which we will be able to test design ideas and see how they affect performance. The last test stand we built was not sensitive enough to measure the differences we are interested in. The new stand is based on a single point load-cell connected to a load cell amplifier and the amplifier in turn is connected to a data logger.

The guys on the Forum for Australian Rocketry were a great help in recommending which equipment to use, things to look out for, and generally how to set it up. Big thanks goes to Tarp, PK, PatchleAD, astroboy and others who helped us get the right parts and steer us in the right direction.

Load Cell

Load cells 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. We ended up paying US$24 for it + $9 delivery. Delivery was around 8 days.

We chose the 70Kg load cell as it was a good compromise between the upper and lower ranges of thrust we want to explore.

Data Logger

The data logger we chose was from DATAQ. We went with the DI-148U as it already had a USB connection. My laptop doesn't have a serial port so I would have needed a serial-to-USB converter anyway. If it did have a serial port we probably would have gone with the cheaper DI-194RS.

We ordered the logger from a local distributor called Total Turnkey Solutions (www.turnkey-solutions.com.au). All up delivered the DI-148U Starter Kit was AU$118.

I was very impressed with the delivery time. I placed my order just before lunch on Thursday and the package from Melbourne was waiting for me on Friday when I got home from work.

Load Cell Amplifier

The data logger and load cell aren't enough though, and we also need a load cell amplifier. The data logger reads in the -10V to 10V range with a 10-bit accuracy. The load cell only produces millivolts over the entire deflection range which is too low for the data logger to get enough resolution. The amplifier basically converts these small voltage changes to a voltage over the full dynamic range of the logger. It also provides the necessary excitation voltage for the load cell.

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. The amplifier is based on an instrument amplifier IC INA125P. The circuit is relatively straight forward and these IC's are designed for exactly this task.

Because it is not easily sourced within Australia, we ordered them from Texas Instruments in the US.

Circuit Diagram

You can adjust the amplifier's gain using the multi-turn trim pot in order to get the required thrust range.

Because we use two 9V batteries as the power source we don't quite get the full +/-10V range, but it is close enough. The excitation voltage used for the load cell is 5V.

Parts List

Since this was going to be a one-off we didn't bother with a PCB and just built the circuit on a strip board.

Part Designation Quantity Source
INA 125P Instrument Amp U1 1 Texas Instruments
16 Pin IC socket - 1 Jaycar
50 ohm Coax Connector JP2 1 Jaycar
5 DIN socket JP1 1 Jaycar
DPDT switch S1 1 Jaycar
Power LED LED1 1 Jaycar
500 Ohm multi turn pot R1 1 Jaycar
1.5KOhm R2 1 Jaycar
0.1uF Capacitor C1, C2 2 Jaycar
9V battery B1,B2 2 Jaycar
9V battery clip - 2 Jaycar
Metal enclosure - 1 Jaycar

Software

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 (14,400 samples/sec) 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, and looked at the data in a hex editor. The data was not encrypted, so it was relatively easy to reverse-engineer its format and write a small program that now allows me to convert their format into a text .csv file format that is directly readable by Excel. (saved $99 there :) )

Preliminary Testing

After initial testing it was discovered that there was a little bit of noise on the signal which allowed measurements down to about 20 grams. One of the guys (PK) on the Forum for Australian Rocketry made some good suggestions about how to reduce the signal noise. I ended up placing the load cell amplifier close to the load cell and enclosed the whole thing in a metal box. I also used a coax cable to run the long lead to the data logger. The noise level dropped off dramatically. It is now down to around 0.02V over the -9V to 9V range.

On the 70Kg load cell that allowed me to resolve down to ~5 grams = a sheet of A4 paper. (Yes I actually put a sheet of paper on it). This means we can get thrust measurement accuracy down to about 0.05 Newtons. This hopefully should be good enough to observe the subtle changes to components under test.

The expected thrust range of most rockets under test will be about 30N to 150N. That range applies for our typical rockets with pressures of around 120psi with nozzles under 10mm. The same setup will also be used for full bore tests of up to around 600N.

Test Stand

At the top of the test stand the load cell is bolted to a heavy steel plate which in turn is bolted to the stand. Above that is the load cell amplifier. The entire rocket is just suspended from the load cell. 

The arrangement at the bottom of the test stand stops the rocket moving side to side when thrusting. It consists of 3 adjustable wheels on ball bearings that allow them to be brought closer together for narrower rockets and further apart for wider rockets. The rocket only lightly touches these (a few mm clearance) so there is essentially no friction between them and the rocket during the test.

The rocket under test can be quickly disconnected from the load cell with the single pin that goes through the bracket connected to the load cell. This lets us take the rocket off, fill it with water and re-attach it back to the load cell.

The release mechanism consists of a brass Gardena mechanism with the spring removed and rubber bands are used to provide a retracting force. We added a non-return valve inside the release head as well as a hose quick connect adaptor to the bottom. We use a thick piece of plastic with a string attached wedged under the collar. To release the nozzle we simply pull on the string and the whole mechanism falls away.

First Tests

We ran 4 thrust tests yesterday with different nozzles on the new test stand. The tests were un-calibrated as we wanted to see what the overall performance was. We also set the amplifier gain for the range of thrusts we were interested in.

Here is an example of the raw data captured for both 7mm and 9mm nozzles at 100psi. There were 2 liters of water in the 7.2L rocket. Sample rate was 240 Hz.

7mm nozzle thrust curve

9mm nozzle thrust curve

After bringing the data into Excel, I corrected the offset and ran a 6 sample averaging window on the data to reduce the ringing seen initially. The data will be cleaned up further as we incorporate the linearity curve of the load cell, and average the data over multiple runs.

9mm nozzle thrust curve filtered in Excel

The initial ringing is caused by the initial launch pulse from the rocket hanging to pushing upwards. It is not too much of a problem as it can easily be filtered out.

The pre launch zero reading represents the weight of the entire rocket including water. At the end of the thrust curve the zero level is higher because the rocket is lighter. To get the useful thrust we essentially draw a horizontal line from the end level to the start of the thrust curve and only count what is above that since that is the actual thrust component that contributes to the upward acceleration of the rocket. The thrust value under that line is the thrust that just lifts the rocket off the ground balancing gravity. The change in weight of the rocket is non-linear due to the shape of the bottle and the decreasing pressure.

We will leave data analysis for another update once everything is calibrated. In the diagrams above the thrust can be approximated from the values as the difference between the initial zero level and final zero level is the weight of the water = 2Kg. = 19.6N.

Overall everything went really well and I am looking forward to doing the actual tests.

The total cost of the test stand not including the laptop has been about AUD$230.

References

Other sources of load cells, amplifiers and loggers we looked at:

 

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