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Rpm sensor conditioning


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5 replies to this topic

#1 Darren

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Posted 01 April 2007 - 06:54 PM

Hello support (or anyone else) I have fitted a crank position sensor (hall effect I think, taken from an automobile) to a diesel engine. The sensor is picking up a passing metal pin, attached to the front of the crankshaft. The signal I'm getting is a sin wave that increases in both frequency and voltage. I've attached a data capture I got streaming at 5khz from AIN0 on a UE9 pro. I didn't check but I guess I was sampling with the channel set to 0-5V I could (and probably will) move the sensor closer to the pin, which I'm guessing will increase the peak voltages. Am I right in thinking that if I get the lower peaks above 2.3 volts I should be able to use the timer function to show a real time rpm. If I can't get the lower peaks above 2.3V or this would not work could you suggest a cheap solution for modifying the signal so I can get a rpm using a timer? Thanks in advance. Darren

#2 LabJack Support

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Posted 01 April 2007 - 08:28 PM

I don't see an attachment. You might have to zip it to make sure the forum accepts it. If you can decide some threshold that is always valid (e.g. 1.0 volts), then you can use a comparator to convert the signal into a nice square wave.

#3 Darren

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Posted 02 April 2007 - 06:15 PM

File attached.

Attached Files



#4 LabJack Support

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Posted 03 April 2007 - 08:48 AM

Looking at your data, it appears that your signal sits right at 0.0 volts, and then every once in a while you get a quick pulse. Initially the pulses are 2-3 volts, but then settle down to consistent ~0.5 volt pulses. If you could get those pulses to go over 2.3 volts, then you can just connect the signal to a timer or counter input. As it is, though, you should use a comparator to condition your signal. Check out the MAX921CPA from maxim-ic.com. You can power this from VS on the UE9. That means that if IN+ is greater than IN-, you will get 5 volts out. If IN+ is less than IN-, you get 0 volts out. The typical operating circuit on page 1 of the MAX921 datasheet seems to show the input signal put through a voltage divider. Your signal appears to be within the input range, though, so I think you can just connect it directly to IN+ without the divider. In the typical operating circuit, they have REF=1.182 connected to IN-, so that is the threshold value. You want more like 0.3 volts, so pass REF through a voltage divider (try 100k/33k) before connecting to IN-.

#5 Darren

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Posted 03 April 2007 - 02:48 PM

Just to clarify a bit The data file I attached was a sample from a longer stream. The peak voltage of the pulses increase with frequency. The sample shows the engine slowing to tick over (idle). I think that the signal will be a sin wave and have troughs that match the peaks. I'm guessing the reason that these are not shown is that the AIN channel (by default?) was operating in 0-5V range. I was using the sample 'simple stream' modified to stream one channel at a faster rate. Thanks for your assistance hopefully I should be able to get things working now.....

#6 LabJack Support

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Posted 03 April 2007 - 02:56 PM

If your signal actually goes below zero, and you use the comparator as described before, you will need to protect the comparator from that negative signal. A 100k series resistor might do the trick.


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