Using inductive proximity sensors to measure displacement.

I don't know too much about this.

What is your shaft made of?  Is it smooth?  Is it shiny?  Is it magnetic?  How much and how so?  How long is it, how thick is it?  How do these modes happen in space?  Don't we need to sense in two directions, perpendicular both to each other and the shaft rotation?

Do the modes have individual acoustic signatures?

Can we use a hi-res camera and video analysis software to glean the information we need?

The shaft is smooth and shiny and is made of Stainless Steel. It is 0.8 m long and I'll be using three different shafts of different diameters, 5mm, 8mm, 10mm. The maximum deflection that each shaft will experience is around 8mm from the rotational axis. Yes we do need sense in two directions both perpendicular and to the shaft rotation as well. I highly doubt we can use a hi-res camera in this case since the deflection being measured is quite small.

These are the modal shapes that are formed at different frequencies. Although the image is 2D but the whole shaft flexes like that in 3D. I need to verify this deflection by it's measurement. My design includes a pair of sensors, one measuring deflection in the vertical axis and the other in the horizontal axis. These sensors will be attached to an arm which will have the capability to slide in a slot along the length of the shaft, enabling me to measure the deflections at different positions along the shaft. But since I was mistaken about the sensors, I want to find another alternative to it. The major problem is such a small displacement to be measured.

The shaft is smooth and shiny and is made of Stainless Steel. It is 0.8 m long and I'll be using three different shafts of different diameters, 5mm, 8mm, 10mm. The maximum deflection that each shaft will experience is around 8mm from the rotational axis. Yes we do need sense in two directions both perpendicular and to the shaft rotation as well. I highly doubt we can use a hi-res camera in this case since the deflection being measured is quite small.

These are the modal shapes that are formed at different frequencies. Although the image is 2D but the whole shaft flexes like that in 3D. I need to verify this deflection by it's measurement. My design includes a pair of sensors, one measuring deflection in the vertical axis and the other in the horizontal axis. These sensors will be attached to an arm which will have the capability to slide in a slot along the length of the shaft, enabling me to measure the deflections at different positions along the shaft. But since I was mistaken about the sensors, I want to find another alternative to it. The major problem is such a small displacement to be measured.

Your problem is just that you have the wrong kind of sensor:

look at this link:

http://www.micro-epsilon.co.uk/displacement-position-sensors/eddy-current-sensor/index.html

and Google for "eddy current sensors"

MK

A simple way to convert distance to voltage in this geometry is to use a capacitance-to-voltage circuit.

Form your variable capacitance by connecting one electrode to the metal bar and the other to a metal probe positioned 1 cm from the bar. Build a capacitance to voltage circuit that uses a fairly high frequency - well above the highest vibration frequency of the bar, and view the output voltage on an oscilloscope. As the bar vibrates the capacitance will oscillate and the scope will show both the frequency of vibration and the amplitude at the monitoring point. For best results ensure the probe wire is kept far from the bar except for the probe tip. You can look up capacitance to voltage circuits or capacitance meter circuits on the internet but be sure the one you choose has enough bandwidth to track the maximum vibration frequency.

Basically yes. Thanks alot I'll have a look at these. These seem quite useful

hi

I have a similar issue. I want to measure distance in micro meter range and then calibrate the movement to identify force applied to the system.

which sensor i should use ?

You need to specify or describe your system in  much more detail for anyone to make  a decent sensor recommendation:

The first question is " why not measure the force directly" - unless you describe the system we don't know.

Then you need to give other information like bandwidth (frequency range) resolution, accuracy, environment, size constraints, acceptable cost range etc etc.

MK

Well this is what the system looks like. As a replacement for now I am planning to use Linear Ratiometric Hall Effect Sensors to measure the displacement when the shaft vibrates. These are cheaper and for that I'll have to attach magnetic collars at different positions along the shaft.

please check details in this post

http://www.element14.com/community/message/142871/l/how-to-find-resolution-of-capacitative-sensors#142871

i Think you will be really good person to discuss this matter as you have way more experience in it.

If the frequency is high enough, you could try using guitar pickup sensors - or make your own.

For best performance, check out how to design a humbucker pickup and the various variants.

Doug