Membrane Measuring: Light Pipe & Diode Tech

[Michael Zager]

Hi!
I found a nice thing via google for a measuring forces by using a light pipe. The light gets coupled into a light pipe and the amount of light is measured by a diode (http://www.dr-iguana.com/prj_LightPipe/). I think you can also use such a device for measuring the deflection of a membrane, made out of polycarbonate or silicone rubber or something which acts like a light pipe. This would be a nice thing to measure the defection of a audio membranes or of all kinds of other membranes which get deformed or stretched by a external force.

 

[Mech_Engineer]

It's an interesting design application, but it looks to me like its being developed for use as a touch sensor rather than a strain gauge. When bent around a robot's foot the deformation of the tubing causes a drop in transmitted light. It seems to me that this would work best when the light guide is bent such that the LED does not shine directly on the diode, but instead depends on internal reflection. It may be the light response may not change much in a straight configuration.

Perhaps the applications you're describing could be better achieved using a Fiber Bragg Grating (Application note: http://www.ni.com/white-paper/12338/en/) or optical fiber strain gauge?

 

[Michael Zager]

Hi! Thanks for your feedback, that also sounds interesting. But I think it will not work for my application since i want to measure a more or less flat surface at once and I cannot use a Grating because it would be to expensive.
I think I have to give it a try, by just testing the idea from the webpage. Since I just want to measure a the peeling behavior from a part sticking on a surface and detect if the surface is bending or not / or if its in its relaxed position. A simple setup with a photodiode and a light source must somehow work, if the membrane is guiding the light.
Otherwise I will just measure it with a DMS to detect the peeling of the surface, this has been done for that application before and it is working.

 

[tom bradley]

Dear Michael,

It's a nice and easy way to measure forces via a optical setup! I checked the webpage, very smart. What kind of application are you looking for?
If I get it you like to measure the adhesive force acting on the PDMS substrate.

 

[tom bradley]

By the way:
http://hackaday.com/2011/10/21/building-optical-flex-sensors/
:-)

 

[JBA]

Have you considered using photostress film for your application? It takes a bit of equipment, polarized light source, etc; but, you may be able to rent that equipment if this is a one time or short time requirement. You would not need to do a detailed analysis because the visible fringes would indicate the stressed areas vs the relaxed areas.

 

[Michael Zager]

Hi all!

Thanks for your input, @ Tom thanks for the link for the optical sensor :-)

@JBA can't use a photostress film since I want to have output of the sensor which I can directly feed into a controller unit and it should be cheap!
I think the LED and the PD is the easiest way, for the system I have in mind. My application is not directly just measuring a membrane.

Greetings

 

[superdadmom]

Hi

sounds really like a very interesting tool to use a LED and a Photo diode to measure forces or even stress?
I think Important is the detector to get a nice voltage vs. force behavior like shown in http://www.dr-iguana.com/prj_LightPipe/.

Just an idea i recently bought me a top down SLA-System, i could use this for finding the home position of the z axis or measuring other stuff.
For homeing it would be great also in FDM based systems, touching the plate with the nozzle.
I think you could also speed up the process….
But implementing would take some time….