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Load Cell |
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Home-made Analog to Digital converter. It was not made specifically for this purpose and it has more functionality than necessary for this application. Here is a schematic (gif) |
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MassLoad load cell |
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The electronics setup |
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The Encased ADC |
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Portable +/- 5V Power Supply for electronics |
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Load Cell installed in test stand |
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We use this load cell setup to test the thrust put out by our rocket motors. The stand is made out of mild steel and the loadcell can measure thrust up to 1000 lbs. The electronic components allow for easy upload and organization of thrust data on computers, which can then be changed into readable graphs. |
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Test Stand Component-Used to hold motors in place during firing. |
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Technical Info LOADCELL Model #: ML-0100 BC Output: 2.997 @ 1K output at 1000 lbs: 2.997mV/Volt @ 1K Our power supply is 5V, so there is ~ 14.985mV at 1000lbs thrust on the loadcell output. We are amplifying this to 2^12 (4096 -> 4.096 volts) so gain is 4096/14.985 or less = 273X gain LOADCELL CIRCUIT red--+5V black-GND green-1 white-2 shield-no connection instrumentation amplifier: the maxim (www.maxim-ic.com) part # MAX 4194 adjustable gain, single supply instrumentation amplifier Is used to get this gain: gain formula for this 8 pin chip: G = 1 + 50Kohms/RG G = 273X so 273 = 1 + 50000/RG RG = ~ 183 ohms We used a common 180 ohm resistor which gives a bit more gain (277X vs. 273X) Maxim offers free samples of these instrumentation amplifier chips from their website at www.maxim-ic.com MAX4194 AMPLIFIER CIRCUIT PIN1 RG1 (180 ohm 1% gain resistor - gives ~277X gain) PIN2 inverting input (white-2) PIN3 non-inverting input (green-1) PIN4 GND PIN5 GND (reference voltage - offsets output voltage) PIN6 ampOut (to ADC) PIN7 +5V PIN8 RG2 (180 ohm 1% gain resistor) analog to digital converter: MAX186 12 bit 8 channel analog to digital converter Purchased from Digikey electronics for a different project. It is hooked up to a PC's parallel port in SPP mode, also can be hooked up to a microcontroller. This chip samples the analog output of the instrumentation amp and this is fed into the C or Visual Basic analysis software on the computer. Power Supply: The power supply powers the loadcell and the instrumentation amp and the analog to digital converter. It consists of one 9V battery and a 5V regulator (7805 IC from RadioShack) Computer software: A custom visual basic application reads the analog to digital converter (ADC) and automatically senses when the rocket thrust starts and once finished, It outputs the thrust data into an excel graph. Here is the sourcecode we wrote for this project released free and opensource! NTPORT is required by our load cell software to communicate over the parallel port, And needs to be installed before the loadcell software will work correctly. |
