Measuring Thrust
Hello Everyone,
I've been furiously researching ways for the Stamp2 to accurately measure thrust/force (up to around 100 pounds or so). There are a ton of load cells out there, and some that will even output 0-5v directly, which I could read with an ADC. . .but those are rather expensive, around $100. I was wondering if anyone has build their own system or has a creative way to solve this problem without spending a ton of $$ on load cells.
-Jeff
I've been furiously researching ways for the Stamp2 to accurately measure thrust/force (up to around 100 pounds or so). There are a ton of load cells out there, and some that will even output 0-5v directly, which I could read with an ADC. . .but those are rather expensive, around $100. I was wondering if anyone has build their own system or has a creative way to solve this problem without spending a ton of $$ on load cells.
-Jeff
Comments
-Phil
-Jeff
-Phil
You can also make your own load cell but the strain gages which make up the sensing element are somewhat expensive [noparse][[/noparse]but can also be found on e-bay]. There is an excellent article on building a load cell for measuring the thrust of a model rocket motor. As I am not at my home computer I cannot give you the link for a month or so. Let me know if you want more information or if you want the link in mid January.
cheers, David
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Powered by enthusiasm
-Phil
Leon
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Amateur radio callsign: G1HSM
Rocket guys (including myself) capture the data to analyze it further. Most of the people using load cells are building their own rocket motors, and need the data to reverse analyze it. If certain assumptions are made, chamber pressure and many other details can be obtained.
The best bet is to get on Ebay buy a load cell, build the capture system with the built in amplifier, and go from there.
The only other route is to read chamber pressure directly and go forth in that direction. Reading chamber pressure is a little more difficult, but can be successfully done.
I would say there are many methods, but the lead cell is the easiest direction to go.
James L
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James L
Partner/Designer
Lil Brother SMT Assembly Services
Are you addicted to technology or Micro-controllers..... then checkout the forums at Savage Circuits. Learn to build your own Gizmos!
-Jeff
UPDATE: I just picked up a 30lb load cell on Ebay for $16.
Post Edited (everest) : 12/14/2009 4:27:18 PM GMT
http://www.aeroconsystems.com/
-Jeff
-Jeff
Take a look at "strain gauges" This is what most load cells are.
The RCTIME command could be used here (the loadcell is the variable resistive part)
I have a home made motorcycle dyno and this is what I use.
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The Truth is out there············___$WMc%___···························· BoogerWoods, FL. USA
Jeff,
If you just put a regular counter in the loop, that should suffice to be pretty accurate. At least accurate enough for what you are doing.
James L
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James L
Partner/Designer
Lil Brother SMT Assembly Services
Are you addicted to technology or Micro-controllers..... then checkout the forums at Savage Circuits. Learn to build your own Gizmos!
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Powered by enthusiasm
Or even better,
If you capture the firing circuit, you could video tape it. and play it back in slow motion. That would give you a reference for time. This is based on the fact you have an easy way to do that.
Still, SRLM's suggestion is probably be the best.
James L
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James L
Partner/Designer
Lil Brother SMT Assembly Services
Are you addicted to technology or Micro-controllers..... then checkout the forums at Savage Circuits. Learn to build your own Gizmos!
Have your wired in your load cell yet? The reason I like to use an analog instrument amplifier is to see the output of the load cell prior to sending the amplified signal to the A-D converter; and to scale and zero out by viewing on a voltmeter.
Most load cells have strain gages wired in a wheatstone bridge arrangement that can be zeroed out using a 10 turn pot placed across adjacent bridge arms. The INA 125 instrumentation amplifier uses a single pot to adjust gain as well as providing for several levels of bridge supply voltage; which makes the analog setup easy and able to be calibrated.
cheers David
-Phil
I forgot to mention that an excellent source of knowledge regarding strain gages is the MicroMeasurements Division of Vishay. Their Tech Notes [noparse][[/noparse]TN] offer an indepth study of gages. [noparse][[/noparse]They make excellent gages too.]
Also, you can recheck the gain calibration by temporarily shunting one arm of the bridge with a high value resistor. Again I believe that there is a TN on shunting. This shunt procedure is an excellent check on all the bridge associated electronics.
cheers, David
-Jeff
http://forums.parallax.com/showthread.php?p=718875
http://www.jamesyawn.net/electronicstand/amp/board2.html
As soon as my INA125s show up, this is the approach I'm going to take. I plan to use an MCP3301 12bit ADC to get the amplified signal into my Stamp2, and then PLX-DAQ software to import the data into a spreadsheet. . .I should be all set!!!
-Jeff
The site you listed is, I believe,·part of the model engine thrust teststand description I refered to earlier: an excellent site.
The procedure of using a 24 bit A-D processor might be better [noparse][[/noparse]I am not·well versed in·electronics] but I like to see the load cell output on a voltmeter before the conversion. I have a very sensitive altimeter which uses the direct to 24 bit A-D [noparse][[/noparse]Ramsay kits] so I assume that that is the best way nowdays to get high sensitivity.
I can, however, hopefully help you with the strain gage portion of your project. Some 30 years ago I·was the assistant instrumentation engineer on an airplane structural test program·[noparse][[/noparse]some 2000 gages with maybe 500 read out per test condition; and some 100 loadcells] and become a believer. Hope you are having success.
cheers, David
I stand corrected. The site you refered to is not the site I had in mind. The site I remember also had a discussion on building a rather complex geometry load cell. Sorry.
A simple homebuilt load cell can be constructed by thinking of a minature diver's 'spring board', made by clamping a thin bar of, say,·aluminum to the edge of a table top. The maximum strain [noparse][[/noparse]and stress] occures·just outboard of the support end of the cantilever beam and the gages are mounted there. 2 gages are mounted on the top surface and are in tension [noparse][[/noparse]increasing in resistance] and 2 are mounted on the bottom, acting in compression. The downward force on this load cell is placed at a constant distance outboard.
The wheatstone bridge output comes from having the tension gages in opposite arms of the bridge and the compressive gages making up the other two opposite arms. So the voltage at the midpoint junction on one side of the bridge increases by a small amount from the initial 1/2 value of the bridge supply voltage; and the other side·midpoint voltage decreases. Also, any changes in gage resistance due to temperature is neatly canceled out.
The 'small' amount of voltage output comes from the fact that the strain in the metal is small and the gage resistance changes by some 2x this value [noparse][[/noparse]gage factor = 2].
cheers, David