LOAD CELL FORCE MEASUREMENT (psi, Kg, LBs)
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Load Cell Measurement with the i100
The i100xx box provides 8 differential analog input voltage channels (14bit) and 8 voltage output channels (i.e. for excitation) that can be used for load cell measurement. For specifications, click here.
A load cell is an external device similar to a strain gage that measures force in Kg or LBs units. The i100xx connects directly to load cells by providing an excitation voltage and returns Kg or LBs units to the end user. When one selects "Load Cell" in the "Sensor" field, an interview leads the user through the setting up of the device. One enters parameters such as device resistance (ohms), excitation voltage specified on package label (V), maximum force (Kg or LB) at specified excitation voltage, and mV/V sensitivity (e.g. one enters "2" if the package label specifies 2mV/V).
Fig 3.17 - Load Cell Measurement
Load cells are set up in a bridge circuit where a voltage is applied across the bridge, and the voltage across the two intermediate bridge nodes is measured via a pair of instruNet Vin+ and Vin- input terminals. The excitation voltage for the bridge is supplied by either the instruNet Vout terminal or by an external voltage source. instruNet calculates the force using the equations:
Vratio (V/V) = ((Vin+ - Vin-) - Vinit) / Vout
Force (Kg) = Vratio / Sensitivity
Sensitivity = Vmeas per Vexc per Kg, as specified on package label
Ro (bridge resistance in ohms), GF (Vmeasure per Vexcitation per Kg, as specified on the physical sensor label), Vinit (Voltage measured with 0 force, used to calibrate 0) and Vout are fixed values that are specified by the user in the Constants Settings area (indirectly if set via the interview), whereas (Vin+ - Vin-) are measured in realtime by instruNet. For more details, please see Maximizing Strain Gauge Accuracy and Voltage Ratio Measurements.
To do a Load Cell measurement, you must:
1. Set the Sensor field in the Hardware settings area to Load Cell.
2. Set the Range field in the Hardware settings area to +/- 10mV.
3. Set the Ro field in the Constants settings area to the value of one Ro bridge completion resistor, in ohms units.
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4. Set the GF field in the Constants settings area to the sensors sensitivity in Vmeasure per Vexcitation per Kg units.
5. Set the Vout field in the Constants settings area to specify the voltage that is to be applied to the bridge (1V is typical). If you are applying an external excitation voltage, enter -Ro value in the Ro edit field (e.g. -100 instead of 100 ohms) to tell the software that the excitation is external, and then enter the value of the external excitation voltage into the Vout field (e.g. 4V).
In high current cases (e.g. >2mA), it is often helpful to alternate the polarity of the excitation voltages to evenly burden the +/-12V supplies.
6. Set the Vinit field in the Constants settings area to the voltage measured when 0 force is applied, in Volts units.
7. Set the Wiring field in the Hardware settings area to Bridge.
8. Capacitors across the voltage input terminals are highly recommended for reducing errors caused by RFI. With 350ohm sensors, 0.1uF caps create a low pass filter at 4KHz [4K = 1 / (6.28 * 350 * .1e-6)], and are ideal at minimizing RFI effects.
9. Wire your voltage source per figure 3.17, click here if you need more guidance setting up the software, and click here if the measured value is not correct.
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