Alison Cleary

Golay code modulation in low-power laser-ultrasound ☆

The current work presents a correlation-based detection technique with application in modulated laser-ultrasonics. In standard use of coded sequences the impulse response of a system is recovered in the time domain with improved signal to noise ratio (SNR). The presented method is an extension of this technique, where the response to a chirped waveform is restored with improved SNR; hence, the response is in a well-defined frequency range. To achieve this goal the chirped waveforms are modulated by Golay codes. It will be shown that the response to this bandlimited carrier waveform can be recovered in the time domain with improved signal to noise ratio using a cross-correlation technique. Improvement in the SNR is discussed analytically and it is shown that this improvement is proportional to the square root of the length of the applied sequences. Experimental applications in laser-ultrasound are shown using modulated laser diodes as excitation sources with an output power of ∼1W. In the experiments a plate with a thickness of 50μm is investigated using Lamb waves in the MHz range to confirm the predicted improvement in the SNR. Golay codes with three different lengths were used with 7, 9 and 11 bits resulting in 2(7)=128, 2(9)=512, and 2(11)=2048 repetitions in an individual signal, respectively. The predicted improvements of 2 in the SNR between the 7 and 9 bits, and between the 9 and 11 bits waveforms, respectively, were well approximated by the experimentally obtained values of 1.83 and 2.17. As Lamb wave dispersion curves can be used for the characterization of plates or layered samples by inverse problems, it is also shown that by using multiple measurement points the recovered waveforms can be utilized in the evaluation of the dispersion relation.

pH tracking of silica hydrogel nanoparticle growth

The authors show that pH increase, due to removal by condensation of silicic acid, correlates with nanoparticle growth during the initial stages of silica hydrogel formation and becomes constant at a time tpH, the point when other particle growth mechanisms dominate. Absorption of common phthalein indicators is shown to allow effectively instantaneous tracking of the pH and nanoparticle size in alkaline and acidic hydrogels. Particle sizes are calibrated using the hydrodynamic radius determined from the fluorescence anisotropy decay. Tracking pH complements fluorescence anisotropy nanometrology by offering a lower cost, speedier, and simpler method of studying particle growth during silica hydrogel fabrication.

Development of a Novel Wear Detection System for Wind Turbine Gearboxes

This paper presents a low-cost, inline, gearbox lubrication monitoring sensor. The purpose of the research was to develop a sensor that can analyze wear particles suspended in gearbox lubricant systems. Current inline sensor systems rely on methods that prevent significant morphological classification. The size and shape of the particles are often indicative of the type of wear that is occurring and is therefore significant in assessing the gearbox state. A demonstration sensor consisting of a webcam that uses an active pixel sensor combined with a rectangular cross section optically transparent acrylic pipe was developed. A rig that simulates a gearbox lubrication system was used to test the sensor. Images of wear particles suspended in the lubricant were captured in real time. Image analysis was then performed to distinguish particles from the lubricant medium. Object characteristics, such as area and major axis length, were used to determine shape parameters. It was found that the sensor could detect particles down to a major axis length of 125 μm. Classification was also demonstrated for four basic shapes: square, circular, rectangular and ellipsoidal. ellipsoidal, was also demonstrated.

Low power laser generated ultrasound: Signal processing for time domain data acquisition

The use of low power modulated laser diode systems has previously been established as a suitable method for non-destructive laser generation of ultrasound. Using a quasi-continuous optical excitation amplified by an erbium-doped fibre amplifier (EDFA) allows flexible generation of ultrasonic waves, offering control of further parameters such as the frequency content or signal shape. In addition, pseudo-random binary sequences (PRBS) can be used to improve the detected impulse response. Here we compare two sequences, the m-sequence and the Golay code, and discuss the advantages and practical limits of their application with laser diode based optical excitation of ultrasound.

Low peak-power laser ultrasonics

Techniques for the successful excitation of guided ultrasonic waves using a low peak-power laser ultrasonic source are discussed and compared with more conventional Q-switched laser sources. The paper considers acoustic propagation in thin plates, in which the frequencies used, typically only the fundamental guided wave modes, are considered. Aspects of excitation and detection geometry are considered along with the physical mechanisms of photo-acoustic generation and the practical issues surrounding available source wavelengths and power outputs. Understanding of the effects of these constraints is critical for the successful application of the technique. Continuous wave excitation and fully arbitrary modulation schemes are compared, and a technique to control the bandwidth of Golay code modulation is introduced. It is shown that earlier work by the authors was capable of guided wave detection at peak-power densities of 104 W cm− 2. Later work has focussed on the use of erbium-doped fibre amplifiers combined with Golay code modulation to improve the recovered signal-to-noise ratio. Two key applications of the techniques are considered: material properties measurements (using inversion of dispersion curve data) and acoustic emission system calibration.

Crystal shape modification via cycles of growth and dissolution in a tubular crystallizer

Besides size and polymorphic form, crystal shape takes a central role in engineering advanced solid materials for the pharmaceutical and chemical industries. This work demonstrates how multiple cycles of growth and dissolution can manipulate the habit of an acetylsalicylic acid crystal population. Considerable changes of the crystal habit could be achieved within minutes due to rapid cycling, i.e., up to 25 cycles within <10 min. The required fast heating and cooling rates were facilitated using a tubular reactor design allowing for superior temperature control. The face-specific interactions between solvent and the crystals’ surface result in face-specific growth and dissolution rates and hence alterations of the final shape of the crystals in solution. Accurate quantification of the crystal shapes was essential for this work, but is everything except simple. A commercial size and shape analyzer had to be adapted to achieve the required accuracy. Online size, and most important shape, analysis was achieved using an automated microscope equipped with a flow-through cell, in combination with a dedicated image analysis routine for particle tracking and shape analysis. Due to the implementation of this analyzer, capable of obtaining statistics on the crystals’ shape while still in solution (no sampling and manipulation required), the dynamic behavior of the size shape distribution could be studied. This enabled a detailed analysis of the solvent’s effect on the change in crystal habit.

CW laser generated ultrasound techniques for microstructure material properties evaluation

Mechanical properties of materials may be obtained from the inversion of ultrasonic Lamb wave dispersion curves. In order to do this broadband excitation and detection of ultrasound is required. As sample size and, in particular, thickness, are reduced to those of microstructures, ultrasound frequencies in the range of the gigahertz region will be required. We look at two possible cw laser excitation techniques which, having far lower peak powers than the more frequently used Q-switched lasers, therefore give a negligible risk of damaging the sample through ablation. In the first method the modulation frequency of a sinusoidally modulated laser is swept over the required range. In the second, the laser is modulated with a series of square pulses whose timing is given by a PRBS (pseudo random binary sequence) in the form of a modified m-sequence.