NI 9752 Automotive AD Combo Module User Manual

NI 9752 Automotive AD Combo Module User Manual

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

NI 9752 User Manual | © National Instruments | 7

Figure 7.

Negative Tooth Trigger Wheel

Figure 8.

Correct Signal Polarity for VR Input Circuit

Figure 9.

Incorrect Signal Polarity for VR Input Circuit

Each VR sensor input requires a positive sensor connection and a negative sensor connection.

Connect a positive sensor input to one of the NI 9752 pins labeled VR1 and VR2. Connect the negative sensor inputs to GND pins on the NI 9752. The polarity of the sensor connection to the

NI 9752 is critical. Connect the leads of the sensor so that the positive input of the VR circuit

receives the waveform shown in Figure 8. The waveform shown Figure 9 is incorrect, and the

VR circuit does not properly respond to this waveform. The rapid zero crossing of the VR signal must be in the negative direction.

The physical center of each tooth or gap corresponds to a known angular position of the trigger wheel. This physical center of the tooth or gap always corresponds to the rapid zero-crossing of the generated voltage pulse.

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| NI 9752 User Manual

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