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ACCELEROMETER MEASUREMENT (g) The following instruNet hardware supports measurements from Accelerometer sensors that output voltage. This hardware provides high impedance inputs that do not pump current when the system switches channels, and is therefore suited for accelerometer measurement. If one is working with accelerometers that output charge (Q) porportional to acceleration (g), then an external charge-to-voltage converter, available from 3rd parties, is required.
An Accelerometer is an external device that measures acceleration in "g" units. They contain an internal constant mA excitation current and provide a voltage output to the instruNet (i.e. instruNet does not provide excitation). instruNet connects directly to accelerometer sensors that output voltage and returns g units to the end user. For example, if a sensor experiences 19.6 meters-per-second-per-second acceleration (9.8m/s/s * 2.0 = 19.6), then the instruNet software will return "2.0" g. When one selects Accelerometer in the Sensor field, an interview leads the user through the setting up of the device. One enters parameters such as maximum and minimum acceleration (g units) and mV per g sensitivity as specified on the physical package label (e.g. one enters "0.002" if the package label specifies 2 mV/g). Accelerometers are wired either single-ended (SE) or differential (DI). If one uses SE, then the sensor must be wired as shown in the above SE diagram, and the Wiring software field must be set to Vin - GND. If one uses DI, then the sensor must be wired per the above DI diagram, and the Wiring field must be set to Vin+ - Vin-. instruNet calculates acceleration using the equations:
Vmeasure (V) = (Vin+ - Vin-) or (Vin - GND) GF (Vmeasure per g) is a fixed value that is specified by the user in the Constants Settings area (indirectly if set via the interview), whereas Vmeasure is measured in realtime by instruNet. To do an Accelerometer measurement, you must wire your sensor per the above diagram and then set up your software via the Interview process (started after selecting sensor type in Channel Setup dialog) or by manually running through the below steps: 1. Set the Sensor field field in the Hardware settings area to Accelerometer. 2. Set the Measurement Range in the Hardware settings area. For details, click here. 3. Set the GF field in the Constants settings area to the sensors sensitivity in Vmeasure-per-g units. 4. Set the Vinit field in the Constants settings area to the voltage measured when the acceleration is 0g. This is used to calibrate the sensor when 0g is applied. 5. Set the Wiring field in the Hardware settings area to either Vin+ - Vin- (differential, figure a) or Vin - Gnd (single-ended, figure b). Click here if you need more guidance setting up the software, and click here if the measured value is not correct. 5, 10
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