Frequency waveform for 8-wire digital MAF

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Frequency waveform for 8-wire digital MAF

Postby AdvTechTS » Thu Dec 21, 2017 6:18 am

Hi all, I'm looking for some advice on the waveform I've take from a 2015 Holden Commodore 3.6L V6 SIDI which employs an Hitachi 8-wire MAFM (GM actually call it a multifunction intake air sensor). These new MAFM's output Air Mass, Humidity, Baro and 2 air temp values. With all of that said my capture is simply the MAF Value. I used the digital MAF settings that come preset in the software but had to adjust the channel scale and offset in order to see the signal.

I am used to seeing the interference of the ignition system in the MAF waveform but I have never seen anything like this. I don't believe there is an issue with this signal but I was hoping to get an explanation for the waveform.

***I am referring to the spikes on the frequency trace that are triggering the over-range warning***

Thanks in advance :D
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AdvTechTS
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Re: Frequency waveform for 8-wire digital MAF

Postby Steve Smith » Wed Jan 03, 2018 7:19 pm

Hello and thank you for the post, sorry for such a late reply.
Unfortunately the spikes we see here (Channel A) are a response to the inherent noise present on the
DC signal (Channel B)

IMAGE 1 NOISE PRESENT ON DC.jpg
Noise detection via Frequency Coupling


If you are using a 4425 or 4225 PicoScope, using the Bandwidth limit feature (Hardware Filter) will remove this noise entirely. Below is a BMW 3 Series N47 without the Bandwidth limiter active:

IMAGE 2 WITHOUT BANDWIDTH LIMIT.jpg
Without Bandwidth Limit


Below we have the Bandwidth limit feature active

IMAGE 3 WITH BANDWIDTH LIMITER.jpg
With Bandwidth limit


Activation of the Bandwidth limit feature can be found here:
https://www.picoauto.com/library/training/filtering

If you are using the 4423 or 4223 PicoScope the Bandwidth limit feature is not available, however all is not lost.

One option that will reduce the captured noise is to amend the Frequency Counting settings found under the Channel Options button.

IMAGE 4 AMEND FREQUENCY COUNTING.jpg
Amend Frequency Counting settings


Whilst the noise/spikes are reduced they remain during the WOT test.

Amending the Frequency Counting settings will allow for finer control of the crossing point value (both rising and falling edges) used by the software to calculate the frequency.

This is typically sent to + - 50 mV AC coupled which is fine but if “noise” is present within this “counting” region of the signal, the hardware is will detect and display the noise at the relevant frequency.

Amending the Frequency Counting input coupling setting to DC and shifting the crossing point to +3 V rising edge and 1 V falling edge (which may be more defined than + -50 mV AC) eradicates the noise at idle speed and reduces the noise at WOT

Alternately you can leave the Frequency Counting input coupling to the default AC but amend the crossing point values to a point within the signal where the noise level is reduced.

Below I have opted for + 1 V – 500 mV AC coupled

IMAGE 5 AMEND FREQUENCY COUNTING.jpg
Amend Frequency Counting AC Coupled


Experiment with these input values live whilst capturing the signal will help to fine tune these settings for optimal results.

Alternatively using a Math Channel will allow for filtering of the noise using the low pass filtering feature for the DC coupled signal on Channel B

IMAGE 6 FREQ MATHS.jpg
Freq Math Channel- Low Pass Filtering Active (20 kHz)


I hope this helps, take care…….Steve
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Re: Frequency waveform for 8-wire digital MAF

Postby AdvTechTS » Wed Jan 03, 2018 11:50 pm

Thank you Steve,
I do have the 4423 but it is good to know that all is normal. The interference doesn't bother me now that I know that it is normal. I will try your tweaks next time. Happy New Year :D
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