Improved piezo excitation

In spite of the clean amplitude and phase curves obtained by the magnetic and photothermal excitation methods, the piezo excitation method has been the most widely used, even in liquid. This is mainly because of its simple setup and wide applicability. The method does not require any modifications on the cantilever, which is very attractive for commercial instruments. Therefore, considerable efforts have been made for improving the performance of the piezo excitation method. 

Recently, another approach has been proposed by Asakawa et al Instead of using an acoustic wave, they used the deformation of a flexure hinge prepared in the cantilever holder to drive a cantilever. To illustrate the effect of the flexure drive mechanism, they made two cantilever holders with the same structure but made of different materials. One of them is made of SS316, and the other is made of polyetheretherketone (PEEK). The former is too hard to be deformed by the impulsive force generated by the vibration of the piezo actuator, while the latter is compliant enough to be deformed by that. This difference is illustrated in the simulation results obtained by the finite element method (Fig. 18.3a,b). 

The amplitude and phase curves obtained with the SS316 holder are distorted by the random peaks caused by the spurious resonances (Fig. 18.3c,e). On the contrary, such peaks are suppressed in the curves obtained with the PEEK holder (Fig. 18.3d,f). While the amplitude curve obtained by the PEEK holder shows almost an ideal response, the phase curve deviates from the ideal response. This is because of the linear phase delay caused by the finite delay time required for the deformation of the flexure hinge. However, 

The relationship between the Q factor and the phase curve is given by 

Owing to the linear phase delay, the apparent Q factor (Qa) measured from the experimentally observed phase curve is larger than the true Q factor of the cantilever resonance. As a result, the apparent frequency shift A fa observed in the experiment is smaller than the true A/ expected from the ideal phase curve. The true Af can be recovered with the following equation. 

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Figure 6 shows an outline of a PAS instrument designed for fast time-resolved measurements. The excitation light is a laser pulse of some 20 ns duration, at a wavelength which falls within the absorption spectrum of the sample (e.g. 337 nm with a nitrogen laser). Total absorption of this pulse then deposits an energy E in the sample and this will decay in the course of time into heat which will give rise to the pressure sensed by the detector usual microphones have slow response times, so that piezo-electric devices are used to improve the instrument s time resolution [43]. 

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