Hi there

I'm after making a simple rotational speed sensor for a little project at work and have been looking at magnetic pick ups designed specifically for this sort of application, i.e. variable reluctance sensors. The data sheets all seem to require ferrous targets (toothed wheel for example) which makes sense. However my rotating device has to be 316 stainless (a non magnetic grade).

What's bugging me is my EDIS toothed wheel for megasquirt is aluminium. Surely that shouldn't work as it wont vary the reluctance of the magnetic circuit around the VR sensor??

But obviously it does work! So is the ford VR sensor not actually a simple VR sensor? If it is and detects aluminium, will it detect stainless (which is mostly ferrous afterall even if non magnetic)?

I know you can get inductive proximity sensors that sense various non-ferrous metals, but these aren't specifically designed for speed sensing. They switch rather than provide an output waveform. Is the ford VR sensor perhaps one of these (though i know it gives an ac output)?

I guess one of these could provide an output waveform, only they are higher voltage and would dissipate appreciable power if continuously switching. A VR sensor could be better but will it pick up the stainless part? It might be possible to fit some ferrous material (e.g. a washer) to the rotating part, but not ideal.

Hmmmmm... well any advice appreciated. Cheers,

Liam

[Edited on 6/12/06 by Liam]

it's all to do with eddy currents innnit? you need metal to conduct them so it should work with non-ferrous & non-magnetic metals; the signal may be lower though.

Don't the old cable driven car speedo use eddy currents too?

see more about it here: http://www.infolytica.com/en/coolstuff/ex0128/

have a look inside a mouse. if youve got a toothed wheel, a light one side & reciever the other may work (speed dependent obviously)

quote:

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Originally posted by Liam
Hi there

I'm after making a simple rotational speed sensor for a little project at work and have been looking at magnetic pick ups designed specifically for this sort of application, i.e. variable reluctance sensors. The data sheets all seem to require ferrous targets (toothed wheel for example) which makes sense. However my rotating device has to be 316 stainless (a non magnetic grade).

What's bugging me is my EDIS toothed wheel for megasquirt is aluminium. Surely that shouldn't work as it wont vary the reluctance of the magnetic circuit around the VR sensor??

But obviously it does work! So is the ford VR sensor not actually a simple VR sensor? If it is and detects aluminium, will it detect stainless (which is mostly ferrous afterall even if non magnetic)?

I know you can get inductive proximity sensors that sense various non-ferrous metals, but these aren't specifically designed for speed sensing. They switch rather than provide an output waveform. Is the ford VR sensor perhaps one of these (though i know it gives an ac output)?

I guess one of these could provide an output waveform, only they are higher voltage and would dissipate appreciable power if continuously switching. A VR sensor could be better but will it pick up the stainless part? It might be possible to fit some ferrous material (e.g. a washer) to the rotating part, but not ideal.

Hmmmmm... well any advice appreciated. Cheers,

Liam

[Edited on 6/12/06 by Liam]

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As Mr 02GF74 rightly states, it's all to do with Eddy currents innit?

The tooth moving towards the magnetic field generated by the VR sensor will have a current induced into it. This current will create a magnetic field around the tooth that opposes the field from the sensor and creates a voltage.

This is not the way VR sensors are designed to operate however! The tooth is supposed to be ferrous and it used to lower the reluctance of the magnetic path within the sensor.

Did you make your own trigger wheel from alloy? I'm very surprised it works reliably to be honest. The output from the VR sensor will be far lower than it would be a ferrous trigger wheel.

Cheers guys!

I was playing around with an inductive prox. sensor today and it seems it'll do the trick. It's transistor output and has no probs switching at the speeds I need to measure, detects the stainless fine, and doesn't get warm or owt. Luckily the tacho box doesn't mind the non-TTL 10V pk-pk output from this sensor .

As for VR sensors, cheers for clearing that up, Mike. Interesting they still work with non ferrous but using a slightly different mechanism. The alloy wheel I have for my megasquirt is off an OE ford pulley so I assume it will work fine for my megasquirting.

Cheers

liam