NI 9760 User Manual
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© National Instruments
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5
Figure 3.
Negative Tooth Trigger Wheel
Figure 4.
Correct Signal Polarity for VR Input Circuit
Figure 5.
Incorrect Signal Polarity for VR Input Circuit
Each VR sensor input requires two connections. The VR Hall module pins labeled IN 1+ through
IN 6+ are the positive sensor inputs. The negative sensor inputs must be connected module pins
labeled IN 1- through IN 6-. The polarity of the sensor connection to the module is critical. The
leads of the sensor should be connected such that the positive input of the VR circuit sees the
waveform shown in Figure 4. The waveform shown Figure 5 is incorrect, and the VR circuit will
not properly respond to this waveform. The rapid zero crossing of the VR signal must be in the
negative direction.
The polarity of the physical tooth or gap on the trigger wheel will contribute to the polarity of
the voltage pulse from the sensor. Figure 2 demonstrates a positive physical tooth polarity and
Figure 3 demonstrates a negative physical tooth polarity. Assuming the lead polarity of a sensor
remained the same, one of the configurations would generate the waveform shown in Figure 4,
while the other configuration would generate the waveform shown in Figure 5. Triggers wheels
are designed so that the physical center of each tooth or gap corresponds to a known angular
position of the wheel. This physical center of the tooth or gap always corresponds to the rapid
zero-crossing of the generated voltage pulse.
The VR circuit is designed so that the rapid negative zero-crossing of the raw sensor signal
corresponds to the rising edge of a digital pulse sent to the RIO FPGA. The VR output signal to
the FPGA will go TRUE at the rapid negative zero crossing of the external VR pulse and remain
TRUE until the external VR pulse rises above the arming threshold voltage. An example of this
is shown in Figure 6. Within LabVIEW FPGA the system designer can route this digital signal
to the EPT CrankSig or CamSig input. The signal can also be routed to any other speed
measurement sub-VI.
