NI 9752 User Manual
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© National Instruments
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Figure 5.
Analog Input Configuration for Thermistors and Switches
VR Sensor Inputs
The NI 9752 provides two identical VR sensor inputs. A VR sensor input is a standard, low-cost,
electromagnetic sensing device that contains a winding of wire around a permanent magnetic
core. The VR sensor relies on the movement of ferrous material, or steel teeth, past the tip of the
sensor to change the magnetic flux of the sensor. This movement creates a voltage pulse across
the leads of the wire coil. Figures 8 and 9 show a typical VR signal with respect to toothed
wheels, as shown in Figures 6 and 7. The VR signal is positive when a tooth approaches the
sensor tip and then rapidly swings back through zero precisely at the center of the tooth. As the
tooth moves away from the sensor tip, the voltage continues in the negative direction and then
returns to zero.
The polarity of the physical tooth or gap on the trigger wheel contributes to the polarity of the
voltage pulse from the sensor. Figure 6 demonstrates a positive physical tooth polarity and
Figure 7 demonstrates a negative physical tooth polarity. Assuming the lead polarity of a sensor
remains the same, one of the configurations generates the waveform shown in Figure 8 and the
other configuration generates the waveform shown in Figure 9.
Figure 6.
Positive Tooth Trigger Wheel
