Jesus D. R. Valera

Impact force measurement using an inertial mass and a digitizer

A novel method for accurately measuring impact forces with small peak value and steep slope is proposed. In the method, a mass, which is levitated with a pneumatic linear bearing and hence encounters negligible friction, is made to collide with an object under test. The Doppler frequency shift of a laser beam reflecting from the mass is calculated by using an optical interferometer whose output signal waveform is recorded with a digitizer (i.e. a high-speed analogue-to-digital converter). The velocity, position, acceleration and inertial force of the mass are calculated from the measured varying frequency shift. The performance of the proposed method is demonstrated by evaluating the viscoelasticity of a small rubber block under an impact load with small peak value and steep slope.

Electronic speckle pattern interferometry using diode lase stroboscopic illumination

An electronic speckle pattern interferometer (ESPI) using stroboscopic illumination from a diode laser, and its application in vibration measurement, are described. Diode lasers are readily frequency (FM) and power modulated (AM) via control of their injection current; they are inexpensive and compatible with the single-mode optical fibres used in the described system. However, their modest peak power output demands the use of high-duty-cycle pulses, so that it was necessary to synchronize the illumination with the turning points of the vibration. This was achieved by operating the ESPI system in conjunction with a laser vibrometer. Injection-current-induced FM of diode lasers is non-uniform for modulation frequencies less than about 10 MHz, and to achieve successful stroboscopic illumination an electronic equalization stage was included to shape the current pulse, and hence compensate for the non-uniform FM.

Lasers measure up for the car industry

This paper details the use of laser interferometry techniques in automotive engineering. Optical (laser) instruments can provide measurements over extended areas, yielding simultaneous sub-micron scale information about the shape, vibration and deformation of components, including complete car bodies. Laser measurement techniques can be non-contacting and non-destructive and data are captured as an image of the object. This article concentrates on electronic speckle pattern interferometry (ESPI) and related developments in structured-light techniques. The theory of interferometry and the techniques used are described. Some applications include stress analysis of engineered components, used in engine development work: the ESPI system produced for this purpose is detailed. Structured light techniques are used to investigate wind-induced distortion of a convertible soft-top sports car. Further uses include vibration analysis. Potential future developments of the laser measurement systems are considered.

Combined fibre optic laser velocimeter and electronic speckle pattern interferometer with a common reference beam

A novel optical fibre system, for out-of-plane vibration analysis, comprising the complementary techniques of laser velocimetry and electronic speckle pattern interferometry, is described. The laser velocimeter (LV) and the electronic speckle pattern interferometer (ESPI) share a common reference arm and phase modulator. The LV locks to the motion of a point on the vibrating surface and since the ESPI and LV share a common reference arm, automatic heterodyning of the time-averaged ESPI fringes is achieved. The experimental results are described.

Contrast enhanced and phase controlled stroboscopic additive fibre optic TV-holography for whole field out-of-plane vibration analysis

Abstract A new technique for real-time contrast enhancement and phase control of fringes in additive Stroboscopic TV-holography applied to out-of-plane vibration analysis and its implementation on a fibre-optic electronic speckle pattern interferometer (FOESPI) are presented. Synchronous Stroboscopic illumination, firing two pulses per object vibration period, is combined with simultaneous inter-pulse (high frequency) and inter-frame (low frequency) phase modulation in the reference arm of the ESPI yielding a sequence of frames (interferograms) that are grabbed and processed in real-time. With this technique both speckle and fringe phases are independently controlled by means of the parameters of modulation enabling speckle contrast inversion, as required to enhance the visibility of fringes by sequential subtraction, as well as dynamic fringe phase shifting to solve peakvalley ambiguities.

A Planar Waveguide Nd:YAG Laser Using Active Q-Switching of a Hybrid Unstable Resonator

A planar waveguide laser operating in a negative branch unstable resonator is Q-switched by an acoustooptic modulator in a new configuration, providing effective, high-speed switching. The laser using a 200-mum Nd:YAG core, face pumped by 10 laser diode bars, has produced 100-W output in a good beam quality at 100-kHz pulse rate, and 4.5 mJ at lower frequency with 15-ns pulse duration.

Strain and vibration analysis by fibre based speckle shearing interferometry

An optical fibre based, speckle shearing interferometer is described. The instrument uses a highly birefringent optical fibre to illuminate a test object with equal intensities of light guided by the orthogonal polarisation eigenstates of the fibre. A Wollaston prism is used to obtain two sheared images with adjustable shear. Optical phase changes between the sheared images are readily achieved, without mechanical movement of components, by straining the optical fibre. Object strain determination, by fringe analysis with phase stepping techniques, is readily achieved. Vibration analysis by heterodyning is also reported.

Automatic heterodyning in fiber optic speckle pattern interferometry using laser velocimetry

We combine the operational advantages ofthe complementary techniques of electronic speckle pattern interferometry (ESPI) and laser Doppler velocimetry (LDV) for out-of-plane vibrational analysis. A cw laser was used in a fiber optic system to generate time-averaged ESPI fringes. By mutual phase-locking of the ESPI and LDV signals, automatic heterodyning of the ESPI signal was achieved (i.e., without operator intervention) together with compensation for unwanted nondeterministic out-of-plane whole-body motion of the object under study. Automatic heterodyning extends the operating range of time-averaged ESPI systems to higher vibrational amplitudes and reveals phase relationships in the object vibration. A practical demonstration of the technique is described.

Bi-modal vibration analysis with stroboscopic heterodyned ESPI

An ESPI system, utilizing stroboscopic illumination, was developed for bi-modal vibration analysis, in which the timing of the illumination pulses is controlled to isolate the individual modes. An object vibrating simultaneously at two frequencies, that are related by an irrational number, was examined experimentally. It was observed that when the frequency of the illumination pulses corresponded to one of the vibration frequencies and the pulse train was phased with the vibration, the fringe pattern observed only showed information on the other component frequency. In this way, the fringe pattern associated with either of the component frequencies could be made to disappear, and each mode could be investigated independently. The vibration amplitude map of the remaining component was obtained using heterodyned fringe analysis techniques.

Absolute phase measurement in fringe projection using multiple perspectives.

A technique for absolute phase measurement in fringe projection for shape measurement is presented. A standard fringe projection system is used, comprising a camera and a projector fixed relative to each other. The test object is moved to different orientations relative to the fringe projection system. Using the system calibration parameters, the technique identifies homologous surface areas imaged from different perspectives and resolves the 2 π phase ambiguity between them simultaneously. The technique is also used to identify regions of the phase maps corresponding to discrete surfaces on the object. The methods described are suitable for automatic shape measurement with a lightweight fringe projection probe mounted to a coordinate measuring machine.