Aquarius Posted February 5, 2007 Posted February 5, 2007 I would like to build a static test stand for my homemade BP and KNSU motors. Reading has been done on Richard Nakka and James Yawns sites, but I would like a bit of "know-how" and see som pictures from homemade loadcell-equipment. My biggest "problem" is the small current from the loadcell itself, approx 0,1mV. I need to amplify this signal before it can be connected to a A/D-converter with a computer. Any suggestions? Comments on software and A/D-converter also appreciated.
Draco_Aster Posted February 6, 2007 Posted February 6, 2007 I was gonna to make a test stand for small rockets using a spring on a pivot controlling a variable resistor which is read by my dads chro. Simple and very accurate aslong as the springs elesticity doesen't degrade, in which case all you would need to do is re-position the resistor and make new benchmarks. I'm also interested to know how many people have these and what variations they use. I know theres a heap that just use a kitchen scale but I'm more interested in the actual trust curve than potential.
Boomer Posted February 6, 2007 Posted February 6, 2007 As kids, we made a spring-loaded lever of wood n' nails, with a pencil strapped to it, and some lego gear box + motor that ran a paper strip past the pen so it would draw a curve of the thrust over time. This was done after a motor exploded while we were watching it upside-down on a scale with a stop watch in hand.
Aquarius Posted February 6, 2007 Author Posted February 6, 2007 Thank you for your replies:I am familiar wirh both methods, the "Thrust-o-meter" as well as the upsidedown on grandmas old kitchenscale... What I am trying to do, is to use a loadcell, like Aerocons and amplify the signal. Then use a A/D-converter for computr hookup and more accurate measuring and testeing. The values will then be imported to Excel and written as a graph, thurst over time. I know the basics, but would like some comments and experiences with these kind of testrig. I have alot of new motors ready for testing, and it would be fun to se the differece in thrust, total impulse etc.
Joral Posted February 6, 2007 Posted February 6, 2007 Aquarius, Do you have an A/D converter picked out? How about data acquisition software? Load cell? The most direct way, depending on your level of electronics experience would be to build a small circuit around a precision op amp or instrumentation amp, keep your load cell signal wires short, and amplify it to 1V (Admittedly, a 10,000x amplification may very well be prone to noise.) Have you thought about doing something similar to the hydraulic cell on Nakka's website? Pressure sensors are pretty easy to get that will give a higher signal than 0.1mV.
Aquarius Posted February 7, 2007 Author Posted February 7, 2007 I am concidering the loadcell from Aerocon and a generic/homemade amplifier. The output from this cell is 2 mV, so it needs a good amplifier. I migyt buy one of those DIY-kits, a standars monoamp. And for D/A-converter, a single channel RS 232-hookup, comes with software (PicoLog).
Boomer Posted February 7, 2007 Posted February 7, 2007 It is 2mV *per Volt supply* at full load, i.e. 30mV for 15V supply (max. allowed). This sounds more reasonable, but remember it is for 44 pounds thrust. If your motor has 5 pounds, it puts out 3.4mV. Standard opamps have offsets in that range (typ. +/-2, max +/-7 mV for things like ua741, LM324/258...). Good opamps reach 0.1mV, and you get ones with 0.01mV which equals 0.015 pounds or 6 'grams force'. These should do without calibration. Paying 10 bucks for the IC should not be a problem here I guess! Plus, the offset voltage can be corrected for, as it simply adds to the motor weight. More important is drift usually, but within the few seconds it measures, forget that. All you must care about is linearity, so I would go with at least a 2-stage amp to get the 200-500 times amplification. P.S. Some sound cards can be abused as A/D inputs. These also come with microphone inputs including the millivolt-amp. You will need to convert to DC though. Google should find something in some electronics or computer forum, that's where I remember reading this.
Yafmot Posted December 18, 2009 Posted December 18, 2009 Most load cells use a strain gauge on a small metal member of known flexural rigidity. For this small a load, a button-type cell would probably work better, if for no other reason than an "S"-beam is a little bulkier and kind of clumsy. Omega Engineering has a whole, multivolume catalog of products, one volume of which is devoted to pressure and strain. They have a world of cells, amps, AD boards, and other signal processing gear. Keithley instruments has a lot of signal conditioners and other gear as well. There are many others, of course, but these are the ones that come to mind. Might I suggest subscribing to Test & Measurement World magazine. It's free, and full of DAQ stuff. But it sounds like you're more interested in the actual stand, at this point. I've built several thrust stands for both rockets and jets. All were based on the "trapeze" or parallelogram style of aparratus. It's very simple: two vertical members supported by a horizontal one, and supporting another one on the lower ends. Fixturing is added to mount the test article on the bottom "cradle," with a button cell on a fixed piece of the support frame, impinged upon by a steel "pad" attatched to the swinging cradle which has a threaded, fine adjustment to just barely make contact with the cell at static condition. Sort of like those little "scooter" things you see on a kid's swing set, with a seat at each end, and supported by the two vertical members. No matter where the thing swings, the two seats are always level with one another. I've used these setups for everything from small rocket motors to turbojets of over 3,000 lbs. of thrust. Simple, easy and fairly cheap.
dagabu Posted December 18, 2009 Posted December 18, 2009 Most load cells use a strain gauge on a small metal member of known flexural rigidity. For this small a load, a button-type cell would probably work better, if for no other reason than an "S"-beam is a little bulkier and kind of clumsy. Omega Engineering has a whole, multivolume catalog of products, one volume of which is devoted to pressure and strain. They have a world of cells, amps, AD boards, and other signal processing gear. Keithley instruments has a lot of signal conditioners and other gear as well. There are many others, of course, but these are the ones that come to mind. Might I suggest subscribing to Test & Measurement World magazine. It's free, and full of DAQ stuff. But it sounds like you're more interested in the actual stand, at this point. I've built several thrust stands for both rockets and jets. All were based on the "trapeze" or parallelogram style of aparratus. It's very simple: two vertical members supported by a horizontal one, and supporting another one on the lower ends. Fixturing is added to mount the test article on the bottom "cradle," with a button cell on a fixed piece of the support frame, impinged upon by a steel "pad" attatched to the swinging cradle which has a threaded, fine adjustment to just barely make contact with the cell at static condition. Sort of like those little "scooter" things you see on a kid's swing set, with a seat at each end, and supported by the two vertical members. No matter where the thing swings, the two seats are always level with one another. I've used these setups for everything from small rocket motors to turbojets of over 3,000 lbs. of thrust. Simple, easy and fairly cheap. Could you do a drawing or a picture for us? I am afraid I don't see your word picture well. D
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