AC Coupling

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AC Coupling

Postby Steve Smith » Tue Jan 09, 2018 12:48 pm

I want to share a recent diagnostic "journey" that had me chasing my tail until I took the time out to analyse the data. Whilst the vehicle in question has now been resolved I would value any opinions on the theory below:

To cut a long story short I attended a local garage requesting support with a Mini Cooper ABS fault.
No Speed signal from the Left Hand Front Wheel Speed Sensor

The sensor had been replaced with a reputable Aftermarket component along with a wheel bearing containing the magnetic pick up ring. PicoScope had been used to confirm a speed signal was present (using AC coupling) at both the sensor and ABS controller, however, there was still no road speed reported by the vehicle scan tool along with the relevant ABS sensor fault code

The waveform below was taken from the fore mentioned Mini Front Left Wheel Speed sensor. On the surface everything looks fine as we can clearly see the speed signal formed in the power supply (AC and DC coupled) along with the current flow through the sensor via a x10 current multiplier. It was only when comparing the current flow through the Front Right Wheel Speed sensor did alarm bells start to ring


The waveform below is taken from the Mini Front Right Wheel Speed sensor


Once again on the surface everything looks fine, but the keen eyed would have spotted the current flow measurement from each sensor differs. (I did not spot this initially)

Using the reference waveform and scaling feature of PicoScope we can overlay both the Front Left Wheel Speed sensor (Magenta) and Front Right Wheel Speed sensor (Black) and see the immediate difference


The horizontal blue line indicates the theoretical crossing point used by the ABS controller to calculate the frequency (Approx. 6 mA). Each time the current level rises above 6 mA and then falls below 6 mA the ABS controller measures the time taken between each of these events, so calculating the frequency and hence “wheel speed”.

Looking at the Magenta waveform, the signal never crosses the theoretical crossing point as the current never falls below 7.5 mA and so the ABS controller simply cannot calculate the frequency. (Hence no speed signal recorded via the scan tool for the Left Front Wheel and the generation of the applicable fault code)

My first thought here was to blame the measurement technique (which is good practice) as we must question our approach/connections etc. before confirming the anomaly is real. The current clamp zero point and number of windings of the break out lead (x10) through the clamp were confirmed and so the anomaly proved to be both actual and relevant.(The image below describes the connection/setup)

Current Multipler

The cure for the Mini was to replace the Aftermarket speed sensor for an Original speed sensor where the wheel speed values returned and fault code cleared. Unfortunately the vehicle was reassembled and handed over to the customer before I could return to the workshop for a final capture which is disappointing to say the least!

So what does AC coupling mean and how does effect the acquired signal. I think James Dillon has this summed up beautifully using the ocean as an analogy.

DC coupling of your input signal allows all the voltage present (including voltage fluctuations/ripple) to be captured and displayed on screen. This could be likened to measuring the depth of the ocean including the height of the waves.

AC coupling your input signal rejects all the DC voltage to display only the fluctuations/ripple (centred about zero volts) This could be likened to measuring only the height of the waves on-top of the ocean.

The message here from the experience above is to use AC Coupling with caution as the signal fluctuations/ripple are centered about zero volts regardless of the fundamental signal level. (Removing all bias voltages) This does not not mean that AC Coupling is not to be used, in this instance it serves as an initial rapid measurement to confirm activity from the ABS sensor only. Here we can confirm power, ground, signal and "pick up" to be functional.

However, to categorically confirm the ABS sensor and circuit, measuring the current flow whilst comparing the offending sensor signal level with a known good (LH Front with RH Front) has to be paramount.

Has anyone experienced something similar to the above as I would love to know as I am sure others would too?

I hope this helps, take care......Steve
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Re: AC Coupling

Postby Autonerdz » Tue Jan 09, 2018 8:18 pm

Has anyone experienced something similar to the above as I would love to know as I am sure others would too?

Indeed. There is no need to use AC coupling with a PicoScope unless you have some compelling reason to do so. DC should always be your default.

I learned this the hard way working with analog ABS more years ago than i care to admit...

I made some comments about this here:

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Re: AC Coupling

Postby mr.diag » Wed Jan 10, 2018 3:59 am

Sorry for my ignorance, but I'm not fully grasping the AC coupling part of your post. Also, you used the current flow to figure out the fault. How did using AC coupling throw you off? Thanks.
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Re: AC Coupling

Postby Steve Smith » Wed Jan 10, 2018 8:01 pm

Hello and thank you for the feedback.

AC Coupling threw me off the scent as at first glance we can clearly see there is a speed signal from the offending Front LH Wheel speed sensor.

You can also see this ripple in the DC coupled signal, but using AC coupling improves the vertical resolution.
With such results you would be forgiven for assuming the speed signal from the ABS sensor is correct but not so!
The ABS controller is not looking at the voltage ripple which we have amplified using AC Coupling

Remember that AC Coupling centres the signal "ripple" about zero volts, but the ABS Controller is monitoring current flow via the ABS sensor, not voltage pulsation/ripple.

The voltage ripple is a consequence of the “switching” current flow via the ABS sensor, in exactly the same fashion as loading a circuit will pull down voltage with an increase in current

The ABS controller is also expecting the current flow to switch from low (approx. 5 mA) to high (approx. 11mA) and will determine the frequency based on the rate at which the current rises and falls about the theoretical crossing point. See the image below.

Frequency calculation via switching current

I hope this helps with a description of why I tripped up with this fault, and having discussed this within the office we have now renamed AC Coupling “Apply Caution Coupling”

We live and learn together, take care……Steve
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Re: AC Coupling

Postby rossk » Mon Feb 12, 2018 9:53 pm

Nice Steve

One question for you, you got to the bottom of this issue only when you considered and or compared the crossing point! But how did you get to the theoretical crossing point value of 6mA ?

Was if from some tech data? Educated Guess? Or just surmised based on a percentage of the good signal from the O/S WSS signal?

Keep up the good work
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Re: AC Coupling

Postby Steve Smith » Tue Feb 13, 2018 6:04 pm

Hello Ross and thank you for the feedback.

You are correct, theoretical crossing point was exactly that (Theoretical)

The only basis I have for the 6 mA value was the fact that this value could not be reached by the LH Wheel Speed Sensor

I guess you could say Back to Back testing highlighted a difference between sensors and this is my theory.

The value could have been slightly higher or lower and I wish we could find out from the designers etc. as knowing the true value would be a piece of gold from a diagnostic point of view.

Take care......Steve
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Re: AC Coupling

Postby volrem » Tue Feb 13, 2018 6:52 pm

Different application have different current for sensors.

Most Volvo applications have current between 7mA and 14mA. No info in documentation about crossing point current though. May well be 10 mA.
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