Alexander N. Kalashnikov

Errors and uncertainties in the measurement of ultrasonic wave attenuation and phase velocity

This paper presents an analysis of the error generation mechanisms that affect the accuracy of measurements of ultrasonic wave attenuation coefficient and phase velocity as functions of frequency. In the first stage of the analysis we show that electronic system noise, expressed in the frequency domain, maps into errors in the attenuation arid the phase velocity spectra in a highly nonlinear way; the condition for minimum error is when the total measured attenuation is around 1 Neper. The maximum measurable total attenuation has a practical limit of around 6 Nepers and the minimum measurable value is around 0.1 Neper. In the second part of the paper we consider electronic noise as the primary source of measurement error; errors in attenuation result from additive noise whereas errors in phase velocity result from both additive noise and system timing jitter. Quantization noise can be neglected if the amplitude of the additive noise is comparable with the quantization step, and coherent averaging is employed. Experimental results are presented which confirm the relationship between electronic noise and measurement errors. The analytical technique is applicable to the design of ultrasonic spectrometers, formal assessment of the accuracy of ultrasonic measurements, and the optimization of signal processing procedures to achieve a specified accuracy.

Compensation for temperature variation in ultrasonic chemical process monitoring

Chemical processes often involve heat exchange that causes changes in the temperature of the reactor. These temperature changes could affect ultrasonic monitoring of the processes to the same extent as changes in chemical composition. Discrimination between these two factors requires separate monitoring of the temperature. An ultrasonic reflector for pulse-echo monitoring of aqueous solutions with integrated temperature sensing was developed, implemented and experimentally tested. It contains a water filled cavity isolated from the test medium that is used as a reference. A compact 3 mm wide cavity provides changes in propagation delay of about 1 sample per 0.05 o C at a sampling frequency of 2430 MHz. The possibility of achieving even finer resolution is demonstrated.

Effects of frame jitter in data acquisition systems

This paper focuses on the analysis of frame jitter and the impact of data acquisition architecture on the associated disturbances to the acquired record. Frame jitter leads to the same random shift of all samples in an acquired record. It results in errors in the estimates of time intervals, and makes consecutive records slightly incoherent - compromising data averaging procedures. Two complementary algorithms are developed for the quantification of frame jitter, and their performance has been simulated and verified by experiment. They allow the estimation of the standard deviation of the frame jitter using a low-resolution instrument even in noisy environments. An expression for the minimum value of the standard deviation of the frame jitter has been obtained and verified experimentally for typical architectures of data acquisition systems. It is shown that this value could be reduced by specific improvements in the design of data acquisition system architectures.

VLSI architecture for repetitive waveform measurement with zero overhead averaging

The architecture and performance of a digital waveform acquisition instrument with built in averaging is discussed. The substantial time required for averaging in software was eliminated in the developed architecture by using hardware averaging at the speed of waveform occurrence, due to a fully pipelined operation of constituent units. This architecture was implemented using an FPGA development board, and was tested at a number of averages above 10,000 that should theoretically provide a reduction of the additive noise by more than 100 times. The effect of the noise reduction was clear from experiment. However, lower than predicted improvements were achieved when the level of the input noise before averaging was low. This occurred due to the quantization of the input signal, and should not be attributed to the averaging. Therefore, averaging of digitized data can impose particular limits on the achievable noise reduction but this only occurs when the noise level is very low and does not require much reduction per se.

Ultrasonic sensing of temperature of liquids using inexpensive narrowband piezoelectric transducers

We investigated the possibility of substantially reducing the cost of minimally invasive ultrasonic non-destructive evaluation (NDE) of liquids, in particular, temperature sensing, using inexpensive narrowband transducers. Although designed for operation in air, ultrasonic transducers enclosed in an aluminum case could be submerged in water and were found to be suitable for this application; however, their responses changed substantially when submerged. The test cell developed was complemented by an amplifier to operate as an oscillator and some other support electronics to supervise the sensors operation. The sensor was tested in temperatures ranging from 26 to 32°C at a nominal central frequency of 330 kHz and showed a sensitivity of around 280 Hz/K.

Super-resolution in situ ultrasonic monitoring of chemical reactions

This paper describes experiments to compare the sensitivity and robustness of ultrasound measurements with pH measurements when monitoring chemical reactions under laboratory conditions, the aim being to determine the relative suitability of the two techniques for process monitoring and control. It is shown that ultrasonic time-of-flight measurements, based on the centre of the area of an ultrasonic pulse, provides for super-resolution with respect to the sampling frequency. In comparison to reaction monitoring based on pH measurements, ultrasound was found to be superior in terms of its response time and resolution.

A field programmable gate array-based ultrasonic spectrometer

This paper describes a novel architecture for a low-voltage ultrasonic data acquisition system intended for process monitoring of liquid chemicals in an industrial context. The system is appropriate for operations in hazardous environments where limits are imposed on the working voltage levels of equipment, due to potential fire or explosion hazards. High SNRs in the captured signal records were achieved using 1 V Golay complementary sequences combined with cross-correlation for signal detection, all of which were implemented on a single-field programmable gate array chip. The system has an operating bandwidth of 40 MHz and a dynamic range at the receiver of 2 Vp-p with a resolution of 14 bits. Theoretical and practical design criteria are examined to optimize received signal SNR and data acquisition rate whilst minimizing hardware complexity. The system was implemented using a reconfigurable hardware platform of relatively low cost, providing a basis for developing more powerful and robust systems in future. Experiments show that the performance of the system is equivalent to that of a conventional high-voltage pulse-transmission system.

Ultrasonic monitoring of foamed polymeric tissue scaffold fabrication

Polymeric tissue scaffolds are central to many regenerative medicine therapies offering a new approach to medicine. As the number of these regenerative therapies increases there is a pressing need for an improved understanding of the methods of scaffold fabrication. Of the many approaches to processing scaffolds, supercritical fluid fabrication methods have a distinct advantage over other techniques as they do not require the use of organic solvents, elevated processing temperatures or leaching processes. The work presented here is centred on the development of a new approach to monitoring supercritical scaffold fabrication based on determination of the scaffold acoustic impedance to inform protocols for scaffold fabrication. The approach taken uses an ultrasonic pulse-echo reflectometer enabling non-invasive monitoring of the supercritical environment on-line. The feasibility of this approach was investigated for two scaffolds of different molecular weight. Acoustic results demonstrate that differences in the physical properties of the two scaffolds could be resolved, particularly during the foaming process which correlated with findings from time-lapsed imaging and micro X-ray computed tomography (μ X-ray CT) images. Thus, this work demonstrates the feasibility of ultrasonic pulse-echo reflectometry to non-invasively study supercritical scaffold fabrication on-line providing a greater understanding of the scaffold fabrication process.

Embedded processing of acquired ultrasonic waveforms for online monitoring of fast chemical reactions in aqueous solutions

Online ultrasonic monitoring of aqueous solutions involves acquisition of an ultrasonic waveform either propagated through the solution or reflected from a reflector placed inside it. Most conventional ultrasonic instruments upload the waveform to a separate computer which calculates from this waveform, e.g., the propagation delay and the attenuation coefficient. This approach limits the achievable update rate and requires use of a computer in addition to the ultrasonic instrument itself. This development utilizes embedded processing of the acquired waveforms in the ultrasonic instrument using a softcore MicroBlaze processor as a part of the overall FPGA design. A dedicated averaging subsystem (to reduce noise) and interleaved sampling subsystem (to achieve the equivalent sampling frequency of 1 GHz using a 50 MHz-clocked ADC) operate alongside each other, and are controlled by the MicroBlaze. After the completion of the waveform acquisition the MicroBlaze implements zero crossing algorithm to extract the sought after propagation delay. The instrument was applied for monitoring of neutralization reactions with the update rate of 5 measurements (i.e. acquiring and processing a full interleaved waveform then reporting the ultrasound propagation time) per second.

Quantification of frame jitter in data acquisition systems

The paper develops an algorithm for the estimation of the standard deviation of the frame jitter associated with digital oscilloscopes and other data capture equipment. Frame jitter is considered as a special case of the more general timing jitter expressed in the frequency domain. An approximation is used that maps the standard deviation /spl sigma//sub f/ of the time domain jitter disturbance into the quadrature component of the recorded signal consisting of required information plus noise. /spl sigma//sub f/ is estimated from the statistical moments of the phase and quadrature components of the disturbed signal in a manner that minimizes the influence of additive noise on the estimation. The method has been demonstrated on an ultrasonic pulse-echo spectrometer that features relatively narrow bandwidth, low time domain resolution and significant additive noise. The obtained estimate of the standard deviation of the frame jitter was about four times lower than the sampling interval.