Here's what the inside of a knocklink looks like.
Sorry it's a bit fuzzy.There's not much to them, a bandpass filter, op amp and LED driver chip.
This one has been in the car over 10 years, been calibrated against my datalogger, so I know what sort of knock counts bring up the big LED.
I've used a M5269L chip out of a dead ECU to make 2 driver circuits. I can use it to drive 2 relays and also 2 LEDs so I know when it stops working properly. I've added capacitors as a timer to extend the on period once triggered.
That can give me one relay output for medium knock and another relay output for high knock.
The intention is to have the BOVs open with medium knock, if it spikes too quickly then the 2nd output can kill the ignition.
Here's a quick video.
http://www.youtube.com/watch?v=FXng5_84 ... e=youtu.beIf you listen carefully you can hear the relay release. The LED is supposed to come on too but had a dodgy connection. My camera batteries are dead again.
It won't trigger with my signal generator, that only outputs maybe 5V P to P as a square or sinewave at max and will only light the lower 2 LEDs corresponding to less knock input.
I did a frequency sweep on the input. The bandwidth is maybe 2.5KHz to 7.5 or more KHz,Most sensitivity is about 4 to 5KHz . The knock sensor puts out about 10V P to P in the vid and so turns on all the LEDs. That's hitting it with something heavy. As you can see , the sensor has it's own resonant frequency but that doesn't mean it can't pick up noises outside that frequency.
As far as knock detection goes, I've read pages and pages of it on the net. I'd guess most if not all of it is a load of bollocks (to be expected, same for anything to do with ,cars, engines, computers and electronics).
Think of knock as being like an exposion, which it is. It's one initial wave front with many reflections of diminishing strenth.
The cylinder will resonate at many different frequencies, that's dependant on it's dimensions, most of the energy is above audible hearing range due to the head to piston distance.That's why it's so hard to hear with your ears, particularly with engines that have small cylinders.
In fact every object has it's own resonant frequency or several frequencies. That's what hammers are for, for testing.
What you'll read on the net is that the lowest frequency ties in with bore size. True, but there's not much bore when the piston is close to the top, the combustion chamber's surface is mainly top and bottom.
When knock happens, that initial shockwave travels through the block and gives the knock sensor a shake.....the knock sensor resonates at it's own frequency, that's what the knocklink picks up.
Or to put in another slightly different way, the knock sensor waveform on the scope.....it's known as "ringing". It's about 11.6KHz, tested on the scope. As a double check you can feed a signal into it and measure the current, where it changes, it's resonant. Doing it that way you still get 11.6KHz.
Ringing originates from bells that ring when struck with a hammer.
So , detecting knock, you're not trying to listen to the "bell" inside the combustion chamber, you're trying to detect the hammer.
The knock sensor is a "bell", as is the combustion chamber.
So, whatever the frequencies are inside the combustion chamber? ......it doesn't matter in the least.
That's put another spanner in the works...lol.
OK different manufacturers use different methods?
Afterall, what's a frequency? it's waves with a time between them.Because they're reflected waves, the time depends on the distance they travel before reflection.If you add 2 waves of different frequency, you still have the frequencies of both waves but you also have the sum and difference of those frequencies. OK that's why a CD recording has a 22KHz cut off frequency but ears can only hear up to 15KHz. So, sound, that's just what your ears pick up.
Regardless, these things DO work and work very well.They just need modifying.
BTW the jaycar kitset is very similar too and very cheap.
Got to fit it back in the box now and wire it back up in the car.
Then the next little project on it.
Steve
Login
Register
FAQ
Search
