- 1. displacements on the surface of a little plate which can be deformed by a PZT actuator,
- 2. amplitude of the vibrations when the PZT is excited by a sinusoidal signal.
Digital holography provide images of submicrometer displacements on an object surface. These techniques allows to analyse vibrating structures.
Classical holography can now be advantageously replaced by these compute-br ased techniques for industrial measures.
Our experimental bench allows the students to discover several measurement techniques based upon such digital holography devices.
We observe with a CCD camera the interference pattern between a laser beam reflected by the object to be analysed, and a reference channel.
The object is a little plate, a PZT actuator can apply a static deformation of its surface or excite vibration modes according to a DC ou sinus excitation.
Brief labworks description
As the object scatters light, we observe speckle patterns on interference images (Figure 2). Then the first object of study in this experiment is speckle.
The CCD camera is linked to a computer, some numerical processing allows the students to adjust the experimental parameters to optimize the visualization of interference patterns.
To measure displacements on the plate surface, two methods can be performed. The principle is always to compute the numerical difference between two images, pixel by pixel:
1. direct interference patterns, before and after deformation of the plate.
1. or both obtained by phase-shift method using 4 ou 5 different images, stored after little shift ofthe light phase. This phase shift is performed by a second PZT which move a mirror for a chosen distance less than the wavelength.
In this two cases, the result is an image which measure the variations of phase, on the laser beam reflected by the object, due to the deformation.
The refresh frequency of the camera is 50Hz, therefore, when the plate vibrates at frequency near 10kHz, our device integrate vibration signal. However, numerical processing allows to have a measure of amplitude of vibration along the surface of the object. Indeed, it can be shown that two methods derived from the previous ones can give a map of the vibration mode as schown in figure 3.
The vibration of the structure of an electrical motor causes accoustic noise. This experimental bench has been used to analyse the vibration modes of an electrical motor. Some students have performed theses measures in an experimental project.
All theses experiments give spectacular images and provide valuable teaching support.