Petr Benes

Comparison of Methods for the Measurement of Piezoelectric Coefficients

The charge constant of piezoelectric material is one of the crucial constant values. At the present time, the measurement of this value is mostly realized by means of three techniques: the frequency method, the laser interferometer technique, and the quasi-static method. These techniques have been practically applied to piezoelectric ceramic samples. Our paper presents comparison of the individual methods with regard to their accuracy and the demands placed on the instrumentation and preparation of the piezoelectric material samples. A “soft” ceramic product marketed under the production code of NCE51 was used in the experiments. The methods are described in detail, including the process of sample preparation, description of the experiments, and procedure of calculating coefficients from the measured values.

New design of the two-phase flowmeters

This paper is concerned with a new mass method of measurement of small flow of two-phase media, where the solid particles are carried by a carrying gas (air). The method exploits the fact that the solid particles being carried by a flowing air impacting a properly formed obstacle generates an acoustic surface wave (acoustic emission). The acoustic signal is indicated by the aid of a high sensitive transducer, which converts the acoustic signal to an electric one. The magnitude of the electric signal is proportional to the (average) mass of particles having a constant (unified) velocity. In general, in this way we get the information about the magnitude of particles being measured and about their quantity. The electric output signal of the transducer is processed in the electric section of the meter, using up-to-date electrical devices that are able to carry out amplitude or frequency analyses of the signal and performs its another processing. We may use the same idea of the particle sensing, but with two obstacles in the flow channel, to create a cross-correlation velocity flowmeter.

Measurement of particle size distribution by the use of acoustic emission method

The measurement of solid particle flow while measuring the sizes of particles at the same time presents a difficult problem. One of the possible solutions in relation to measuring the solid particle flow is the use of acoustic emission signal (AE) generated on the impact of the solid particles against a suitably chosen obstacle. The paper discusses the possible use of the acoustic emission signal to also establish the size distribution of the impacting particles. A simple model has been designed based on the Hertz theory of impact, which in theory describes the effect of particle sizes and velocities on the AE signal parameters. Practical experiments have been completed using mixtures of particles of different sizes from 4 to 16 mm and single-size particles hitting against an obstacle at different velocities, with the aim of verifying validly of the designed model. The theoretical assumptions have been confirmed by the experiments and it can be concluded that the acoustic emission method is suitable for orientation measurement of the size distribution of solid particle flow.

The determination of uncertainty in the calibration of acoustic emission sensors

The paper reviews the background and methodology of the primary calibration of Acoustic Emission (AE) sensors. Two practically realised basic calibration methods are described and explained. The method of reciprocity calibration is based on the reciprocity theorem. The input current and reception signal voltage for tone bursts of varying frequency are measured for each pair together with known reciprocity parameters. The step function calibration method is based on the generation of well-characterised displacements of capillary brakes on the surface of the block. The response of the sensor under test is then compared to the response of the reference transducer. The comparison of the results of both methods on a steel block is presented. The shapes of measured calibration characteristics are approximately the same, but there is a difference in amplitude between characteristics. The uncertainty of measurement by reciprocity calibration is determined and presented in the paper. The uncertainty in the range from 60 kHz to 285 kHz was ±3 dB and from 285 kHz to 1 MHz, up to ± 8dB.

Experience with Allan variance method for MEMS gyroscope performance characterization

The noise performance of four low cost and one reference classic gyroscope is estimated using the Allan variance (AV). The purpose is to highlight problems connected with this process and to decide which parameters are the most important if we want to choose the best gyro for a particular application. From the AV theory, basic kinds of noise and their characteristic slopes of the AV curve are known. For example, angle random walk has the slope of -1/2 and is caused by the white noise on the gyro output. However, the output of low cost gyro is usually corrupted by the more or less correlated noise. The level of correlation affects the slope of AV curve, so the better sensor has the slope closer to the theoretical value, thus a simpler model can be used. When collecting data for analysis, it is the most important to suppress temperature variations and vibrations. Attention should be paid to choose proper sampling frequency. Also the resolution and the connection of an acquisition card should be considered.

Comparison of methods of piezoelectric coefficient measurement

The paper compares the main methods used for measuring of the piezoelectric material constants. It outlines the principle of three measuring methods most commonly used today, i.e. the frequency method, the laser interferometry method and the quasi-static method. These methods have been practically applied to piezoelectric ceramic samples. The paper describes the production of the piezoelectric ceramic samples of defined sizes in accordance with the current regulations. An NCE51 production code soft ceramic was used in the experiments. A piezoelectric charge coefficient was measured. The final values of the piezoelectric charge coefficient obtained through all the methods were compared to the catalogue values of the piezoelectric ceramic. All three methods can be described as appropriate; compared with the frequency method, the laser interferometry and quasi-static methods are rather time-consuming and more demanding with respect to preparation of the measurement experiment. The frequency method provides results within a smaller value range.

Start-up response improvement for a MEMS inclinometer

A MEMS inclinometer SCA830 manufactured by Murata was used in a battery-powered handheld instrument designed for measurement of parallelism during fabrication of coil springs. The inclinometer offers a power down mode to reduce power consumption by stopping the internal clocks and resetting the control registers of the device. The power reduction is very significant; however, a transient response was observed in output data. This article describes a compensation technique to suppress this effect, and therefore significantly reduce the startup time. In this case, the model allowed to reduce the start-up time ten times.

In process measurement of particle size distribution

A new principle of particle size distribution measurement is described. We analyze the acoustic emission signal generated by a flow of particles impacting on a fixed obstacle. Changes of the power spectral density of the signal are well correlated with the particle size distribution. Our method was successfully used for measurement of particle size distribution in the range from 1 mm to 10 mm of ferrous sulphate monohydrate in a real chemistry process. Particles were measured at the output of the dry kiln (temperature 150-200/spl deg/C, mass flow 4 tons/hour), where it is not possible to use any other known principles such as particle vision, laser diffraction analyzers, ultrasonic analyzers, focused-beam reflectance measurement etc.

Comparison of measures of time-frequency distribution optimization

This article describes the problems of time-frequency representation optimization. The goal is to achieve compromise between good resolution and reduced cross terms. The quantitative measures used for this optimization are described. For the case of a simulated signal a measure based on mean squared error is proposed. The proposed method compares an optimized result to the ideal representation obtained as sum of Wigner-Ville distributions of mono-component signals. Three simulated signals are used to compare optimization of Choi-Williams Distribution, Gabor spectrogram, Short-time Fourier transform and S-method. The optimization is realized on the basis of four quantitative measures: Rényi entropy, Jones-Parks measure, Stanković measure and proposed method. Results show problematic use of Jones-Parks measure for optimization and comparable optimization of all tested time-frequency distributions by means of three usable measures. For practical use with real signal, S-method with Rényi or Stanković measure is recommended.

Measurement of temperature dependence in material coefficients of PZT ceramics for acoustic emission sensors

The behaviour of material coefficients of PZT ceramics at temperatures approaching the Curie point is a problem hitherto not examined in detail. However, this field offers several interesting perspectives, such as the possibility of controlled depolarization of the piezoceramic material in the volume of a sample. The measurement of temperature dependences in piezoelectric materials used for sensors is very significant with respect to determining the degree of stability that characterizes the sensitivity and accuracy of a sensor; moreover, such measurement enables us to establish the maximum temperature ranges for regular operation. In the experiment described within the paper, we applied two temperature ranges according to the parameters of the calibration furnaces. While the first range spanned between -20°C and 140 °C, the second one was defined by the boundary temperatures of 30°C and 330°C, where the latter value (330°C) corresponds to the Curie point of the applied NCE 51 piezoelectric ceramics. The samples were cycled at gradually increasing maximum temperatures, and changes of the material constants were observed. We applied the frequency method to measure samples for the radial, thickness, transverse longitudinal, longitudinal, and thickness shear oscillation types. In samples of PZT ceramics, the measurement of temperature dependences up to the Curie temperature is very demanding in view of the characteristics of the applied measuring devices. Thus, the authors have included in this article a detailed discussion of suitable apparatuses and measurement procedures. The major result acquired from the performed research activities consists in both the measured temperature dependences and the sum of recommendations concerning the correct approach to be adopted in the measurement of PZT ceramics at a wide range of temperatures and with minimum error.