# Eddy Current Sensor

#### bioamp

Joined Feb 7, 2008
4
I am trying to help my boy with an eddy current sensor project. He wants to build an eddy current sensor and display the distance results using a volt meter. Does someone know of resources on the internet that I can direct him to for further info?

#### bioamp

Joined Feb 7, 2008
4
He is setting the sensor up on a micrometer stage so that he can characterize the sensor by voltage output (change in impedance of sensor ) and distance

#### scubasteve_911

Joined Dec 27, 2007
1,203
This is a challenging project, what kind experience do you two have?

Eddy current sensors work by a few mechanisms all at once. The main measureable effect is change of inductance via the air gap's distance. There are other things that happen also, phase shift and the eddy current itself having an effect on the output.

I would aim to simply measure the inductance of the coil. You should draw a magnetic circuit of your sensor, then solve for total reluctance R with gap distance as a variable. Then, I believe L = N^2/R. Clearly, it can be seen that our inductance is non-linear.. Typically pot-cores are used for this.

So, you need a method to measure the inductance of the coil. There are a number of methods available, from very complex to pretty simple. You then need to get rid of your non-linearities and scale your output voltage to distance.

I had spent quite some time researching these with little answers. I found a couple of sites, but I could not get anything to work with. If you need some more specific help, then focus in on a particular question and ask it. I may be able to help out besides a general overview of the problem.

Steve

#### bioamp

Joined Feb 7, 2008
4
Thanks for the response.

I'm a Mech E with a lot of PLC controls back ground. I've used eddy current sensors for measuring different things over the years. I showed one to my boy and thats how he got interested in it. ( Who knows an analog Engineer in a Mech family.......... )

Since I've posted, I have found two sites of interest

http://archives.sensorsmag.com/articles/0998/edd0998/index.htm

Fig 8 shows a square wave coming off of the circuit... I prefer voltage so he doesn't have to run an O-scope

http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Instrumentation/bridges.htm

Which shows a Maxwell-Wien bridge. My short coming here is I don't know how to build the circuit across the bridge to change to voltage.

We've wound coils on other projects that we've done, so I'm thinkin we'll make a small coil for this project using info from these sites.

We just need a little guidance in getting the voltage out.

Thanks much

#### scubasteve_911

Joined Dec 27, 2007
1,203
No problem for the response My friend is an electrical and his father is a mech, he wishes he went into mechanical instead, maybe something in the blood?

Figure 8 is definitely an easy to implement schematic, I am surprised of it's inherent simplicity. You can easily create a frequency to voltage converter, it is a standard circuit that can be tweaked for this operation. You can even use a microcontroller to do the work for you, which is the best approach for the best possible dynamic range.

Steve

#### bioamp

Joined Feb 7, 2008
4