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oldcorollas
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Kita-Ku, Sapporo, Japan
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January 2003

Re: Knock Sensor Circuit Design ...	Tue, 22 July 2003 05:58

there are a few designs around on the net (and simple jaycar type circuits), but they all have the same problem (as do OEM ones) in that the definitions of the 'knock signature' is often completely different in different motors, and depends on engine size design, and even the geometry of parts the signal reverberates thru to get to the sensor. this signal will also change in both frequency and pattern with rpm. the second problem (and the other main issue) is the filtering of the noise that 'is meant to be there' such as valve noise, piston slap etc. to mimimise general noise, some OEM circuits only sample the knock sensors input signal in a certain time window (when knock is most likely to be). after that, it is a matter of creating a bandpass filter that works for the knock signals you want to detect. if you want it to respond to the specific noises in your motor, then about the only way, short of trial and error, is to hook up an oscilloscope to a knock sensor and record both the normal engine noise at different RPM and different throttle, and then do the same again with a hint of knock, and then compare the two traces to see what the knock signal is comprised of. when you know what it's frequency is, then you can pretty closely design a bandpass filter for it, and then maybe you can just adjust the gain and detection limit for that freq range. it's very do-able, but you have to know what frequency and amplitude the knock signals are first. (yup i realise that this is pretty much just repeating that thread, but this is what i've come to the conclusion ar the probs) i have a few knock sensors (the type that you put a bolt thru) and will be doing some of this testing when i put my MS on (when i finish my thesis) fwiw, the soon to be released 'ultramegasquirt' will have knock sensing with software adjustability, and also 'ion sensing' stuff.... so finally it will be an overall engine package. i haven't been following it too close, but maybe do an archive search thru the yahoo Megasquirt list for knock sensing and ion sensing Cya, Stewart [Report message to a moderator]

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