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'[EE]: Knock sensor interface...'
2001\06\12@115005 by Stephen B Webb

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I thought I had seen a thread about automotive knock sensors on the list,
but I searched the archives and didn't find much.

I am tinkering with an ignition system for a car, and want to include a
knock sensor.  I imagine that the knock sensor is putting out some sort of
signal all the time, and you have to isolate the knock from other engine
noise, etc.  Does anyone know what an "knock" or "ping" looks like, and
the type of circuit that might be used to [robustly] detect it?

Thanks

-Steve

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2001\06\12@120441 by Spehro Pefhany

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At 11:16 AM 6/12/01 -0400, you wrote:

>I am tinkering with an ignition system for a car, and want to include a
>knock sensor.  I imagine that the knock sensor is putting out some sort of
>signal all the time, and you have to isolate the knock from other engine
>noise, etc.  Does anyone know what an "knock" or "ping" looks like, and
>the type of circuit that might be used to [robustly] detect it?

I don't think you'll have signal analysis problems, modern knock sensors
are just piezo "microphones" that pick up the rather severe mechanical
"detonation" in the kHz range. Expect output signal in the 1-volt range,
give or take. You can simulate knocking by tapping the engine block with
a hammer.

You can model it as a ~1nF capacitor in series with a voltage source to
the terminals, with a few megohms in parallel.

Best regards,

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Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2001\06\12@120450 by Spehro Pefhany

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At 11:16 AM 6/12/01 -0400, you wrote:

>I am tinkering with an ignition system for a car, and want to include a
>knock sensor.  I imagine that the knock sensor is putting out some sort of
>signal all the time, and you have to isolate the knock from other engine
>noise, etc.  Does anyone know what an "knock" or "ping" looks like, and
>the type of circuit that might be used to [robustly] detect it?

I don't think you'll have signal analysis problems, modern knock sensors
are just piezo "microphones" that pick up the rather severe mechanical
"detonation" in the kHz range. Expect output signal in the 1-volt range,
give or take. You can simulate knocking by tapping the engine block with
a hammer.

You can model it as a ~1nF capacitor in series with a voltage source to
the terminals, with a few megohms in parallel.

You'll need shielded cable to it, of course.

Best regards,

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

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2001\06\12@124049 by Dan Michaels

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>At 11:16 AM 6/12/01 -0400, you wrote:
>
>>I am tinkering with an ignition system for a car, and want to include a
>>knock sensor.  I imagine that the knock sensor is putting out some sort of
>>signal all the time, and you have to isolate the knock from other engine
>>noise, etc.  Does anyone know what an "knock" or "ping" looks like, and
>>the type of circuit that might be used to [robustly] detect it?
>

Some time ago, I had a friend who was interested in this, and had
some papers from the SAE [society automotive engineers]. Maybe you
can search their site on the web - I assume they have one.

From listening to my own engine, I would suspect that pinging is
rather higher in frequency than the normal engine noises [at least
if you have a 4MPG muscle car like mine], and comes in short,
intense bursts while the normal noises are more regular.

Set your PIC datalogger to hi-rate sampling, 20-40khz, put some
low-octane fuel in your car and drive up a hill in 4th gear on
a hot day, and capture lots of waveforms to your external RAM
buffer. You should hear plenty of pinging. There is probably an
optimal place to locate the piezo sensor - near the cylinder
heads.

I believe the normal means the in-car computers use to deal
with pinging is to retard the spark temporarily.

best regards,
- dan michaels
http://www.oricomtech.com
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2001\06\12@153125 by goflo

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> From listening to my own engine, I would suspect that pinging is
> rather higher in frequency than the normal engine noises [at least
> if you have a 4MPG muscle car like mine], and comes in short,
> intense bursts while the normal noises are more regular.

Try a patent search. Also, this was discussed to death on the
DIY-EFI list a few years ago. Don't know if it's still kicking,
but there was an archive.

4 mpg - Ouch! What kind of a BBC you got?

regards, Jack

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2001\06\12@163330 by Dan Michaels

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>
>4 mpg - Ouch! What kind of a BBC you got?
>

Actually, I have an old cherokee with EFI, and it gets 24 MPG
on the hi-way, recent trip thru the mountains to Grand Junction.
I was just trying to create the image of a throaty engine sound,
as compared to a higher pitched rattly pinging.

Roman will know about this pingy stuff, once he wakes up.

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2001\06\13@151859 by Peter L. Peres

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The knock sensor is a piezo microphone buried in the engine near the
cylinder heads. Its output is noise and becomes large (big pulses) when
knocking occurs. The ratio of knocking to not is at least 6 db afair. It
depends which motor, which car and which sensors you use. The microphone
needs to be tuned to the motor afaik.

Peter

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