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NI 9760 User Manual
Figure 6.
VR Input Pulse and Resulting Digital Output from VR circuit
The absolute maximum VR pulse amplitude allowed by the circuit is ±60 V. If the input signal
exceeds this voltage, damage may occur to the circuit. The amplitude should not exceed ±60 V
at maximum engine speed. The minimum VR pulse amplitude that will generate a digital output
by the VR circuit is ±30 mV.
The VR circuit implements adaptive noise rejection features during continuous incoming VR
pulses while in adaptive threshold mode. In general, an adaptive arming threshold voltage is
generated with each VR pulse at 33% of the previous pulse amplitude. The next pulse must have
an amplitude that exceeds the arming threshold in order for a digital output to be generated at
the rapid zero-crossing. If no arming event occurs for 85ms, the arming threshold is reset to
30 mV.
Alternately, a manually set arming threshold can be defined.
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, then the amplitude at 2000 RPM will be ±20 V. By using an oscilloscope
to measure the VR amplitude at a low speed, this relationship can be used to determine what the
maximum amplitude will be at the maximum speed. If the maximum amplitude of ±60 V will be
exceeded at maximum speed, then the sensor gap must be increased.
Input Channels Used for Hall-Effect Sensor
Inputs
The VR Hall module provides up to six identical hall-effect sensor input circuits. When using
hall sensors, the ground of the sensor should be connected to the IN - side of an input channel
and one of the GND pins. If the hall sensor has an opencollector output, requiring a pull-up
resistor at the collector, a pull-up resistor can be engaged through software. You must use an
external power supply to use the pull-up resistor.
The circuit's output to the RIO FPGA goes high when the input voltage is greater than the
manually set threshold voltage. The output goes low when the input is less than the manually set
threshold voltage.
Custom Configuration
Visit
ni.com/info
and enter Info Code
ADCOMBO
for more information about custom
configuration.
