Knock sensor

[Steve=Ravet%Prj=Eng%PCPD=Hou at bangate.compaq.com]

sdbartho at cca.rockwell.com Wrote:
| Anyone know what voltage levels I could expect out of a knock 
| sensor
| on a GM V-6? Are we talking millivolts, microvolts, what range of 
| values?
errr, not sure.
| 
| I want to build a simple knock indcator light that lets the driver
| know when knock is past a reasonable level. The race PROM 
| disables the
| factory boost/timing retard vs. knock mechanism. 
| 
| The plan right now is to use a low input current op-amp fed into a
| FET to drive a lamp. Am I missing something obvious? How can I 
| "excite" a knock sensor? (Ball-peen hammer ain't gonna cut it) 
| I was thinking along the lines of a 5 khz signal generator tied 
| to a speaker, or is there a better way?
Actually, a ball-peen hammer would work.  An article in Performance
Engineering describes using a knock sensor to detect the opening and
closing of a fuel injector.  They are pretty sensitive.  You will have
to filter the output to a specific frequency range to see "just" the
knock noise.  Don't know what range, but I've been told 5-8 Khz.

Included is an early post to this list that discussed knock sensors.

--snip--
Knock sensors are basically just cheap accelerometers. They respond to 
vibration
by generating a voltage. There are three basic types - resonant , flat 
response,
and broadband resonant - which differ in their frequency response 
characteristics. Resonant sensors have a sharp response at a given frequency -
if the engine does not knock at that frequency then you'll never know it. 
Flat\
response knock sensors are just that - they have low output (typically a few
mV/g accel.) over a broad frequency range. They usually require amplification
of some sort and more extensive filtering. Broadband resonant sensors are an
attempt to take the best of the flat response and resonant sensors and combine 
them into one package. Broader response than resonant (peak response is spread
over 2-3 kHz) with higher output that flat response sensors. I believe 
AC-Delco
makes these type of sensors.

As far as a knock control system goes, knock occurs somewhere between 5 kHz 
and
8 kHz (from my experience) and the most important item to locate the sensor in 
a
place that won't pick up stray vibration. A crude knock control algorithm 
listens for output from the sensor of sufficient amplitude and then alters the 
timing.

There's not really any black magic in knock detection and control - just some
signal processing and good engineering in locating the sensor.

Jeff Armfield

--snip--

I think you will need a low noise op-amp since the signal is low level.
hang on, I've got that article here somewhere.  The article shows a scope
hooked to a knock sensor, the display indicates +-5mv straight from the
sensor.  sounds like a good idea, let us know if it works out....

--steve