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Measuring Thrust — Parallax Forums

Measuring Thrust

everesteverest Posts: 141
edited 2009-12-19 19:16 in BASIC Stamp
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

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-12-12 20:33
    I assume you're talking about some kind of rocket engine? If so, you could mount it sideways on the arm of a heavily-weighted pendulum and measure the pendulum's displacement during the burn. Then compute the thrust from the displacement, the weight of the pendulum, and the positions of the weight and engine on the pendulum arm.

    -Phil
  • everesteverest Posts: 141
    edited 2009-12-12 20:41
    Yes, I've already built something like that actually. . .and I use a video camera to take my samples, but it's not terribly accurate. I'm looking for something more precise, and one that let's me get the data right into my PC.

    -Jeff
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-12-12 21:09
    You could use a TSL1401-DB to get the pendulum position directly into your PC, as shown here: http://forums.parallax.com/showthread.php?p=814265. Using it in conjunction with a MoBoStamp-pe will provide the most precision, since the AVR firmware does pixel interpolation. I'm not sure whether such a setup would meet your standards for accuracy, though.

    -Phil
  • dredre Posts: 106
    edited 2009-12-12 21:20
    You can find low-force load cells for much less than $100 on e-bay. Most however put out a low voltage signal [noparse][[/noparse]say, 30 millivolts full scale] but instrumentation amplifiers are also inexpensive. I like the INA 125 as it also has a bridge supply output.

    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
  • SRLMSRLM Posts: 5,045
    edited 2009-12-12 22:27
    You could also build/buy a cart with low rolling friction, put a bunch of weight on it and the motor, then measure the acceleration of the cart on a flat surface. If you tilt the motor then you can reduce the weight on the cart by some amount. In short, there are a bunch of ways that you could measure force. Look in any physics book that discusses kinematics and you can replace the "F" (force) by your motor. There are usually lots of helpful diagrams on the homework problems that might give you an idea.

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-12-13 00:10
    Here's another alternative: Go to www.dealextreme.com, and do a search on scales. There, you'll find a mother lode of digital weighing scales, most for under $20, including shipping. Any of these could be fitted with various lever arms and hacked to provide force data to an external controller.

    -Phil
  • LeonLeon Posts: 7,620
    edited 2009-12-13 21:46
    An old-fashioned spring balance would work, as well.

    Leon

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  • James LongJames Long Posts: 1,181
    edited 2009-12-14 00:57
    There are a number of methods that will work, but the old trusty load cell and capture is the best.

    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!
  • everesteverest Posts: 141
    edited 2009-12-14 16:01
    Yes, I'm coming to the conclusion that a load cell is the only way to go. I've already tried a spring scale, etc. but the accuracy just isn't all that great, and the dial tends to bounce around a lot. A load cell will be much cleaner. I'll have to keep digging for schematics, I've found a few, but none that are truly "newbie" types. If anyone knows of one, let me know. I'm particularly interested in a good simple amplifier circuit. I think I can handle everything else.

    -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
  • sylvie369sylvie369 Posts: 1,622
    edited 2009-12-14 17:07
    If you haven't looked at these guys yet, you should:

    http://www.aeroconsystems.com/
  • everesteverest Posts: 141
    edited 2009-12-15 01:09
    I have. . .in fact I have one of their motor casings. [noparse]:)[/noparse]

    -Jeff
  • everesteverest Posts: 141
    edited 2009-12-15 01:12
    Thanks for all the help so far. . .and now for an actual question specific to Stamp2. . .once I manage to get my Stamp2 to actually start taking readings from my load cell (I'll get there eventually!), I can foresee a challenge. . .how do I calculate the precise amount of time that passes between the execution of lines of code in my Stamp2? For example, I intend to have the unit either dump the data to EEPROM, or just spit it out to my PC, where I can capture it as text and drop it into an Excel spreadsheet. . .BUT, how do I know the precise amount of time that's passed between each thrust report?? I have to know that to generate a thrust curve. . .I think?

    -Jeff
  • $WMc%$WMc% Posts: 1,884
    edited 2009-12-15 02:09
    everest

    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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  • James LongJames Long Posts: 1,181
    edited 2009-12-15 02:19
    everest said...
    Thanks for all the help so far. . .and now for an actual question specific to Stamp2. . .once I manage to get my Stamp2 to actually start taking readings from my load cell (I'll get there eventually!), I can foresee a challenge. . .how do I calculate the precise amount of time that passes between the execution of lines of code in my Stamp2? For example, I intend to have the unit either dump the data to EEPROM, or just spit it out to my PC, where I can capture it as text and drop it into an Excel spreadsheet. . .BUT, how do I know the precise amount of time that's passed between each thrust report?? I have to know that to generate a thrust curve. . .I think?

    -Jeff

    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

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    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!
  • SRLMSRLM Posts: 5,045
    edited 2009-12-15 04:33
    To figure out how long a loop takes, a) make it deterministic (takes the same amount of time each loop, guaranteed) then b) time a large number of loops, c)divide the count by the time. This will give you a fairly precise amount of time between cycles.

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  • James LongJames Long Posts: 1,181
    edited 2009-12-15 04:58
    SRLM said...
    To figure out how long a loop takes, a) make it deterministic (takes the same amount of time each loop, guaranteed) then b) time a large number of loops, c)divide the count by the time. This will give you a fairly precise amount of time between cycles.

    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

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    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!
  • sylvie369sylvie369 Posts: 1,622
    edited 2009-12-15 09:21
    You might consider sending your measurements out through the serial port into an Excel spreadsheet using PLX-DAQ, and including a TIME directive there. You could have a thrust curve on the screen while the motor is still smoking. I'm not sure about the precision you get, but in light of the variation between purportedly identical motors, I'm not sure you need that kind of precision.
  • dredre Posts: 106
    edited 2009-12-15 23:50
    Jeff,

    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 Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-12-16 00:09
    Doesn't (or didn't) Parallax offer an analog sensor conditioner (i.e. Wheatstone bridge / instrumentation amp module) for apps like this? I seem to recall it from a couple years ago, but I don't remember what the product was called.

    -Phil
  • dredre Posts: 106
    edited 2009-12-16 00:47
    Jeff,

    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
  • everesteverest Posts: 141
    edited 2009-12-16 02:00
    Thanks everyone, I've ordered a few INA-125 unites. . .I've found a few schematics on the web that seem to show how to wire it up. . .but there are still lots of questions I have. I'll post them when the parts show up. Thanks again to everyone. These forums are just the BEST! So much knowledge!

    -Jeff
  • Mohamed RefkyMohamed Refky Posts: 47
    edited 2009-12-18 11:38
    The AD7730 is a complete bridge transducer ADC connected directly to the load cell and outputs a serial word and easy interface to BS2.

    http://forums.parallax.com/showthread.php?p=718875
  • everesteverest Posts: 141
    edited 2009-12-19 17:09
    For anyone else looking to do this. . .here is a web site that provides a wonderful schematic and instructions for wiring up an INA125 to perform this function:

    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
  • dredre Posts: 106
    edited 2009-12-19 18:43
    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
  • dredre Posts: 106
    edited 2009-12-19 19:16
    Jeff,

    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
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