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# how do you measure speed of a vehicle using hall effect sensor? Answered

i mean where do you fix the sensor and how is it done!

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+1 to Jack and +1 to Steve...
With the correct viewer this pic shows interesting movement.
The sensor pickup is where you place a Hall device.  .  .  .   A

A hall effect sensor measures magnetic field strength, and the usual trick for using a HE sensor to measure speed (e.g. the angular speed of a turning shaft) is to attach a magnet to the shaft, and then place the sensor near this moving magnet.  The sensor produces a pulse, each time the magnet comes near it, once every revolution of the shaft.

Then you feed the pulses to some sort of counter, and count the number of pulses you get in a specific time interval.  E.g. 50 counted pulses in a one second time interval means the shaft is turning at:
50 rev/second = 3000 rev/minute

I was trying to find a picture of such a sensor, via Google images, and sort of the best I could find was this one:
http://www.tigersunited.com/techtips/PaulickCruise/Sensor425.jpg
http://www.tigersunited.com/techtips/PaulickCruise/rt-PaulickCruise2.asp

Although, the darned thing is, I don't the sensor in that picture is a Hall Effect sensor.  I think it is just a pick-up coil.   Which brings me to my next point, which is that a pick-up coil works too, and is often used as a speed sensor.

You might be wondering what is the difference between a pickup coil and a Hall Effect sensor.  Technically the HE sensor responds to magnetic field B, while the pickup coil responds to changing B, that is dB/dt.   What that means is if a magnet moves slowly past a pick-up coil, there is no measurable response, and thus there is some minimum speed needed to get measurable pulses out of a coil.

In contrast a HE sensor could in theory be used to measure arbitrarily slow speeds, like a shaft that turned only once an hour, or once a day.
But for measuring the speed of a drive shaft, speeds that low are just silly.

As a result, both pick-up coils, and HE sensors get used as speed measuring sensors for cars, and practically there is little difference between the two. Except that HE sensors, being semiconductors, can be made very tiny, and fit into very small places.

Which brings me to the next thing I wanted to mention:  There are other geometries that can be used to produce the varying magnetic field.  The example I gave is a simple one:  the magnetic field pulses just once, for each revolution of a shaft.

But there are other tricks too, like putting a sensor up close to a moving gear, so that there is a small change in magnetic field as each tooth of the gear goes by, and each revolution of the gear gives you as many pulses as teeth on the gear.

You put the sensor near a piece of wheel that's spinning, with a lug that spins closer too then further away from the sensor. Then you time the pulses.