I want to measure deflection of cantilever beam in micrometers.
Dear Ahmed,
You can use some high frequency transducers f > 100 MHz and the accuracy can be estimated as one quater of wave length. Using high frequency You probably will have to face a problem of a high attenuation.
Best regards
Chris
The resolution of a measurement system based on a wave cannot be much better than the wavelenght, unless you use phase, which can be noisy... I don't think ultrasound can deliver what you want.
A differential capacitive sensor is perhaps the simplest solution. It depends of course on the physical dimensions and geometry of Your beam in relation to the sensor. Also consider signal bandwidth of the beam movement. This sets a lower limit to the carrier frequency used regardless of the measurement method. For very fast movements a charge amplification method (similar to an electret microphone) may work best, or some similar non modulated method.
For slower movements a laser interferometer or laser vibrometer is a very good lab method.
Link to one make: http://www.polytec.com/int/products/vibration-sensors/
I have used one of these occasionally and it can give You a very good picture of the movement of the entire beam.
Ultrasonic sensors may not solve the purpose which you are looking for. Position sensor based on inductance principle can be tried. We are successful in some of the applications using our in-house developed pulsating sensor based instrumentation using inductance probe.
How big is the cantilever? Is it located in air? How big is the deflection?
Well if it is big enough, it is in air and the deflection is not large, then you can put a 1 MHz air c-oupled transducer at 2 mm and operate it at pulse-echo mode and take the FFT of the received echo.A 2mm aIr cavity (between transducer and beam) will have a first resonance located at 85 kHz. As you are using a 1MHz transducer the resonance order that you can best measure will be the 11th (0.935 MHz) or the 12th (1.02 MHz).
If the beams deflects and the thickness of the air gap (cavity) changes, then you will have a shift in this resonances, for example, if it inrceases by 5 micron, 11th and 12th resonances will be now at: 0.9327 MHz and 1.0175 MHz, respectively, this is a small but measurable quatity.
In my opinion, the use of optical methods is better in this case , you can use the optical beam deflection method wich is very simple and usually used in Atomic force microscopy to measure the cantilever deflection.
It's hard to follow such little displacement with US, you can try using water as coupling media and a probe with the highest frequency as possible. In this way the echo pulse may became more visible. In ancient times i used US as a Non destructive technique to inspect carbon fiber composites, with almost 10-20 microns precision. Otherwise you can use Eddy Current, if the cantilever is metallic.
As Fernando already said, the resolution belongs to the wavelength "lambda".
Lambda = c_medium / f_transducer.
So, regarding of your medium the ultrasonic wave has to travel (air, steel...) you have to find the suitable transducer frequency for your purpose. BUT you have to fight with an increase of the damping of your waves amplitude the higher the frequency is chosen.
Nevertheless the resolution can be increased by using coded signals, (high) oversampling, correlation and interpolation techniques.
E.g. if you want to measure distances in steel (c ~ 5500m/s) you can apply a 25MHz transducer (--> lambda = 220µm) and with the techniques above you`ll reach a resolution in the area of a few µm.
You didn`t tell us something about your purpose and the medium for ultrasonic waves.
Have fun.
Can you post some more details of the cantilever? There are many techniques that could be used, but the most appropriate will depend on the physical size of the cantilever and the material it is made from.
Hello, I agree with some of the previous comments, if you add some more details it would be easier to give you an answer. For small deflections, say in the order of 100 microns it should be fairly easy to implement a self-mixing interferometer. If you used a NIR LD around 800 nm you would end up with a resolution of at least 400 nm.
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