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Author Topic: how to tell variable reluctance sensor  (Read 214 times)
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Melbourne , Australia
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Hi everyone, I have an old Alfa Romeo with a speedometer that uses a cable drive from the gearbox.
For various reasons, I want to convert this from mechanical to and electronic Speedometer.
A couple of years after my car was made, Alfa started using electronic speedometers that I assume were managed by a ‘controller’ that received a signal from a sensor attached to the gearbox..
I got hold of one of these Speedometers and when I cracked it open I found that it’s just a DC motor used to turn the needle to indicate the speed the car is travelling at.
I also got hold of the sender unit that sits in the gearbox and detects the rotational speed of the drive gear.
My problem is that I can’t seem to find a control unit that brings it all together, hence ... i want to use an Ardunio to act as the controller.
So my questions are ...
1) I think the Gearbox Sensor is a VR type. how can I be sure ? Is there a test I can do to tell?
2) if it is a VR sensor, then I assume I can use a comparator (like a lm393?) to bring it to a digital signal which I can feed into the Arduino?
I found a diagram on the net ages ago but I can’t remember where it came from.  I scrawled it on a piece of paper way back then and I just re-did it in pc Paint so I can attached it (see attached) . I’m not 100% sure I have it right, your opinions would be appreciated
Thanks

Sam


* lm393.jpg (36.41 KB, 1091x533 - viewed 23 times.)
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Melbourne , Australia
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 since I posted the above message  a friend of mine did some digging and found the Post where I must have gotten the diagram from all time ago.

 I had the diagram wrong anyway so if your interested  you should look at this post http://forum.arduino.cc/index.php/topic,157697.0.html

I also attached a correct version of the diagram \
The thing I didn't know about VR sensors is that they are passive, i.e. they done take any voltage in - they  produce a small voltage , I think its a sine wave +/- and the LM393 is used to amplify and turn into 0 or 1's

However , the question still stands, is there an easy way of determining if the sensor is a VR one?
In the above Link , user 'DC42' seems to have know how to tell but never actually mentions how .


* lm393 comparator.jpg (35.97 KB, 1091x533 - viewed 13 times.)
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If the sensor is VR, the best way to determine the output is to connect an oscilloscope to the leads of the sensor and just measure the voltage and frequency while the vehicle is moving (or the wheels are turning while the vehicle is up on a stand).  Perhaps someone with a portable oscilloscope can help. It would help to know if you have measured the resistance of the sensor with a multimeter (both possible connections).
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 since I posted the above message  a friend of mine did some digging and found the Post where I must have gotten the diagram from all time ago.

 I had the diagram wrong anyway so if your interested  you should look at this post http://forum.arduino.cc/index.php/topic,157697.0.html

I also attached a correct version of the diagram \
The thing I didn't know about VR sensors is that they are passive, i.e. they done take any voltage in - they  produce a small voltage , I think its a sine wave +/- and the LM393 is used to amplify and turn into 0 or 1's

However , the question still stands, is there an easy way of determining if the sensor is a VR one?
In the above Link , user 'DC42' seems to have know how to tell but never actually mentions how .


 The VR probes I've used at the refinery I worked on were simple 2 wire devices that feed a comparator to give a digital square wave output at a frequency proportional to the speed and number of teeth that the probe was sensing. I believe the internal construction is a magnet with a coil around it, and as a gear tooth passes the sensor face it changes the field of coil causing a change in the sensor output voltage. The sensor output voltage looks like a quai-sinewave and it's voltage increases with increasing gear speed, however once the voltage is above a certain minimum value the comparator doesn't care as it's just switching at zero crossings of the probe's output voltage.

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