Position Measurement with instruNet Data Acquisition Hardware & Software (i.e. a/d, digitize, potentiometer)

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 accelerometer measurement. For specifications, click here.

A potentiometer (pot) is an external device that measures linear distance or rotation angle. The i100xx conn-ects directly to pots by providing excitation at 4V and returns data in variable 0.0 to 1.0 units (e.g. 0.1 would represent 27 degrees on a 270 degree rotation pot). One can scale this 0 to 1 data to other units using the Channel Mapping dialog. We recommend 5K to 25K ohm pots since lower values may overload the Vout driver and higher values may incur excess noise. Figure 3.19 shows the Vin- terminal being wired to the sensor, yet a jumper wire between Vin- and Gnd will work as well.

Fig 3.19 - Potentiometer Measurement

When one selects "Potentiometer" in the "Sensor" field, an interview leads the user through the setting up of the device. One enters parameters such as pot maximum resistance (ohms). The excitation voltage for the pot is supplied by either the instruNet Vout terminal or by an external voltage source. instruNet calculates the pot position using the equation:

Potposition (0 to 1) = ((Vin+ - Vin-) - Vinit) / Vout

Vinit (Voltage measured in CCW position, used for calibration) 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. To do a Potentiometer measurement, you must:

1. Set the Sensor field in the Hardware settings area to Potentiometer.

2. Set the Range field in the Hardware settings area to +/- 5V.

3. Set the Ro field in the Constants settings area to the value of the potentiometer maximum resistance, in ohms units. 5K to 25K are recommended.

4. Set the Vout field in the Constants settings area to specify the voltage that is to be applied to the potentiometer (4V is typical). If you are applying an external excitation voltage, enter -Ro value in the Ro edit field (e.g. -10000 instead of 10000 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). 2 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. 11

5. Set the Vinit field in the Constants settings area to the voltage measured when the pot is in the CCW position, in Volts units. 8

6. Set the Wiring field in the Hardware settings area to Bridge.

7. Wire your potentiometer per figure 3.19, click here if you need more guidance setting up the software, and click here if the measured value is not correct. 5, 10