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humble
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I supported Toymods

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May 2002

Re: Knock Sensor Circuit Design ...	Tue, 22 July 2003 17:22

Whoa! this is a bit of a long thread... Ed, If you want an ideal knock sensor solution, what you would want is a DSP (digital signal processor) hooked up to your knock sensor and an interesting algorithm (for example, a FFT [fast fourier transform] that would convert the discrete knock sensor input samples into it's frequency spectrum and look for strong signals out of the engines characteristic frequency band at any given RPM). Look at Motorola web site and some of their automotive industry application notes for DSP products. However, this is a costly and time consuming approach. Contrary to popular belief, most knock sensors are just simple piezo electric pickups. The high pass filters are implemented in the ECU. And most ECU designs tend to have a simple high pass (or low pass) filter followed by an integrator (AC to DC converter if you must) and then a comparator (voltage difference sensitive trigger of sorts). The simplest setup is to find a cutoff frequency for the filter (say the frequency, for example at which 95% [variable number] of the power concentration in the frequency spectrum is divided at a high [close to redline] RPM, yet lower than the knock frequencies at lower RPMs). Then the output of the filter (both the low and high component) are integrated seperately, and then run through a comparator. The comparator is triggered whenever the power concentration of the frequency spectrum is significantly different from 95:5 (as per our example), biased to the high frequency side (i.e. potential knock). Most simple circuits you find on the net are of this nature (with different cutoff frequencies). IMO, the engine RPM/AF ratio/Intake air temp and AFM (or MAP/TPS) signal presets can be used to create a "potential knock zone" trigger succesfully, by marking conditions that are most likely to cause knock. The knock sensor trigger can be a secondary "shit, it's all bad" trigger. Sorry if I repeated what's been said already... it's too early in the morning and the thread is just too long... Cheers [Report message to a moderator]

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