NI 9752 Automotive AD Combo Module User Manual

NI 9752 Automotive AD Combo Module User Manual

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.

NI 9752 User Manual | © National Instruments | 9

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