NI 9752 User Manual
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© National Instruments
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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 becomes TRUE at the
rapid negative zero crossing of the external VR pulse and remains TRUE until the external VR
pulse returns to 0 V shown in Figure 10. Within LabVIEW FPGA, the system designer can route
this digital signal to the EPT CrankSig input, EPT CamSig input, or any other speed
measurement subVI.
Figure 10.
VR Input Pulse and Resulting Digital Output from VR circuit
The absolute maximum VR pulse amplitude the circuit allows is 60 VDC. If the input signal
exceeds this voltage, damage might occur to the circuit. The amplitude must not exceed 60 VDC
at maximum engine speed. The minimum VR pulse amplitude that generates a digital output by
the VR circuit is ±200 mV.
The VR circuit implements adaptive noise rejection features during continuous incoming VR
pulses. In general, an adaptive arming threshold voltage is generated with each VR pulse and
bleeds down thereafter. The next pulse must have an amplitude that exceeds the arming threshold
for a digital output to be generated at the rapid zero-crossing. The initial arming threshold is set
to approximately 70% of the amplitude of each pulse.
Given a constant gap between the sensor and the trigger teeth, the amplitude of a VR pulse is
directly proportional to the speed of the trigger wheel. For example, if the VR amplitude at
1000 RPM is ±10 V, the amplitude at 2000 RPM is ±20 V. By using an oscilloscope to measure
the VR amplitude at a low speed, you can use this relationship to determine the maximum
amplitude at the maximum speed. If the maximum amplitude of 60 VDC will be exceeded at
maximum speed, you must increase the sensor gap or obtain a custom VR circuit configuration
from NI.
Hall-Effect Sensor Inputs
The NI 9752 includes two identical Hall-effect sensor input circuits. The Hall-effect inputs take
a digital input from a Hall-effect or proximity sensor. Typical sensors include an open-collector
output that requires a pull-up resistor at the collector. The Hall-effect inputs read active TTL
compatible signals. The standard configuration includes a 4.7 k pull-up to 5 V for use with
open-collector-type inputs. The input is protected against typical automotive battery voltages
and can be connected to actively-driven battery voltage signals. Channels with this configuration
are protected from voltage swings of ±30 V.
