Table of contents

The conventional split Hopkinson pressure bar system for materials testing at high strain rates produces loading pulses that have a high level of oscillation superimposed on their approximately trapezoidal shape. These oscillations, which are a consequence of the short risetime of the loading pulse produced by the impact of a projectile, cause difficulty in the interpretation of the stress-strain results obtained by such a system. A modification to the conventional system is described, which virtually eliminates these oscillations, resulting in a smooth loading pulse and a significant improvement in the interpretation of the stress-strain behaviour. The modification involves placing a pre-loading bar before the two main bars of the conventional system. By suitable choice of pre-loading bar material with lower strength than the main bars, the high-frequency oscillations resulting from the projectile impact are attenuated as the pulse travels along the pre-loading bar; the loading pulse entering the main loading bar is then smooth with virtually no oscillations. Results for copper and steel test specimens are presented, which illustrate the significant advantages that the three-bar technique has over the conventional method in producing unambiguous stress-strain data.

A device has been developed to determine the size of many soft biological material samples, which cannot be measured successfully with conventional micrometers because of distortion due to the force applied to the sample, It consists of a motor-driven, digital micrometer and control circuit, which is governed by the electrical conductivity of the material to be measured. The micrometer may be driven in an opening or closing mode by operating a three-position switch, but, when electrical contact is established with the sample surface, further closing is precluded.

A novel technique incorporating a laser scanner and diffraction pattern detection is developed for tracing a moving wire and gauging its diameter. The angular position of the wire under inspection is determined by the laser scanner. The gauged diameter of the wire is independent of the distance between the system and the target. The technique is particularly adequate for dynamic inspection of wires drifting within a wide spatial range.

Amplitude and velocity of vibration and their spatial distribution are very important parameters for ultrasonic motors. A complete automatic measurement unit built around a laser vibrometer has been developed in our laboratory. The main object of this non-contact measurement technique is to obtain experimental values of the particular displacement on the stator surface. Theoretical values can be obtained by modelling but the nonlinear effects are difficult to take into account in these complex structures. In this paper, we report on the measurement system and we show some results obtained with two types of motor.

An analysis of the dimension influence on the measurement accuracy of thermal conductivities by transient hot wire/probe method is made. The radial and axial effects are discussed separately under the supposition of constant boundary temperatures. The results show that the radial and axial lengths of the measured sample, the radius ratio and heat capacity ratio of sample to hot wire/probe, as well as the length to diameter ratio of the hot wire/probe are important parameters in error analysis. Former analysis underestimated the boundary effects. New criteria for error analysis are established.

A novel vertical Bridgman-Stockbarger furnace with the capability for real-time melt/crystal interface visualization is demonstrated. Radioscopic visualization of the cylindrical ampoule charge is considered in the furnace design. Sharp contrast of the 10.3% density difference for liquid and solid indium antimonide is obtained.

The feasibility and first results of a near sensor digitalization of fluxgate signals (digital fluxgate magnetometer) are presented. Applying the usual magnetometer electronics we have substituted the analogue section by a digital processing unit (DPU). The 2f₀ signal is digitized at its second harmonic and mean values are online phase-sensitively calculated. Based on this development we present a completely redesigned magnetometer experiment for applications on planetary surfaces. Sensor and electronics including serial interface have to be in one housing, its weight is less than 150 g and the number of electrical connections is limited to four (power lines+serial link). Applications for the digital magnetometer on Earth are also discussed.

Deconvolution techniques currently available for application in spectroscopy must presuppose a known system response function. An inexactly presupposed response function can degrade the obtainable resolution of instruments. In this paper we propose a practical method of blind deconvolution that can eliminate such an irrational presupposition. Simulations show that an improved resolution of instruments can be achieved by this new method. An advantage of the method is that it is easily implemented in an on-line way or by using modern digital signal processors.

A novel correction technique combined with a lock-in peak detection scheme for quantitative voltage contrast measurements using low extraction fields in the scanning electron microscope has been developed. This technique was found to be more accurate than the conventional feedback technique for quantitative voltage contrast measurements. Local field effects (of both types I and II) can be accounted for by this correction scheme without the use of very high extraction fields of 500-1000 V mm^-1. The use of a low extraction field of 50 V mm^-1 is especially advantageous for specimens that are sensitive to high fields and for measurements on specimens surrounded by regions of insulating material, which may charge up under high extraction fields. This scheme also allows the possibility of correction for minor charging due to a contaminant layer forming as a result of sample exposure to air or under electron beam bombardment.

Electrostatic condensing lenses of very short focal length were investigated via trajectory calculations with the goal of determining the minimum spot diameter that could be formed by focusing positrons of negative work function emitted from a 100 mu m diameter portion of a Ni(100) foil re-moderator. The specific applications for which the calculations were carried out were brightness enhancement and transmission-mode positron re-emission microscopy, in which positrons are accelerated to 5 keV onto the target, which also serves to terminate the electric field of the condensing lens. The minimum spot size that could be formed was 3 mu m using a 1 mm diameter two-cylinder lens with a tapered second cylindrical element.

A method for the collection of quantitative data from photographic prints is described. The technique uses a scanner for digitizing gray-scale photographs. The equipment is inexpensive and higher resolutions compared with a digital camera are attainable. Digital images are stored in non-proprietary TIFF format which allows the researcher direct access to each measured pixel. To complete the image analysis process for an application, computer software which corrects for parallax, geometric distortions and inconsistencies in lighting source is described. This quantitative, accurate but inexpensive, data collection technique is a convenient and effective method to analyse gray level photographic prints. Projects that require some image capture and processing, but cannot justify a dedicated capture system, may find this system more than adequate.

The standard cross correlation technique frequently used in particle image velocimetry to extract velocity vectors necessitates the assumption that the velocity gradients inside the interrogation area are negligible. However, the procedure is generally video-based, so such an assumption may no longer be valid. This is particularly so in re-circulation zones, in which the distortion between images can be dramatic. A new iterative procedure for re-building the second image, based on velocity gradients of particles due to displacement, rotation and shear, has been proposed. This improved cross correlation algorithm has been shown to be considerably more accurate for simulated uniform, re-circulating and bi-axial shearing flows, and has been applied to the case of natural convection due to a heated horizontal cylinder.

Theoretical and experimental studies of cross correlation techniques applied to non-restrictive velocity measurement of pneumatically conveyed solids using ring-shaped electrodynamic flow sensors are presented. In-depth studies of the electrodynamic sensing mechanism, and also of the spatial sensitivity and spatial filtering properties of the sensor are included, together with their relationships to measurement accuracy and the effects of solids' velocity profiles. The experimental evaluation of a 53 mm bore sensing head is described, including trials using a calibrated pneumatic conveyor circulating pulverized fuel and cement. Comparisons of test results with the mathematical models of the sensor are used to identify important aspects of the instrument design. Off-line test results obtained using gravity-fed solids flow show that the system repeatability is within +or-0.5% over the velocity range of 2-4 m s^-1 for volumetric concentrations of solids no greater than 0.2%. Results obtained in the pilot-plant trials demonstrate that the system is capable of achieving repeatability better than +or-2% and linearity within +or-2% over the velocity range 20-40 m s^-1 for volumetric concentrations of solids in the range 0.01-0.44%.

The spatial resolution of two X-probes is studied analytically and experimentally to evaluate the effect of separation between the two X-probes and the separation between the wires in each X-probe on velocity derivative statistics. The wire length effect is also considered. Correction ratios for the variances and spectra of the measured derivatives are calculated for isotropic turbulence. The calculations show that the separations, wire lengths and wire effective angles have significant effects on the variance and spectra of the velocity derivatives. Measurements at the centre-line of a cylinder wake provide reasonable support for the analysis.

A new pulsed power supply has been developed. The current model allows voltage pulses with amplitudes of about 100 kV. The pulse width is a few microseconds and repetition rates are up to some tens of kilohertz. The pulse energy is in the 1 J range. Pulses can be created independently of the preceding one in terms of their polarity and magnitude. The costs of production of this compact supply, which can be short-circuited, are lower than those of conventional supplies, which are even more difficult to tune. Fields of application include gas lasers and devices used in plasma technology.

A personal-computer-based low-temperature thermoluminescence system has been developed for recording thermoluminescence from photosynthetic materials, in the temperature range from liquid nitrogen temperature (-196 degrees C) to +60 degrees C. Programmable and linear temperature-time profiles are provided by a dedicated microprocessor-based PID temperature controller. The PC/AT with an add-on card acquires the thermoluminescence data. This paper describes the thermoluminescence set-up, the interface to the personal computer and the software developed for acquisition, processing and analysis of the thermoluminescence data.

The module is a low-inductance coaxial test facility incorporating integral voltage and current transducers. It allows measurements up to 50 kV at 1 kV ns^-1 and 5 kA at 10 A ns^-1. Tests with 11 kV distribution surge arresters enable their dynamic voltage-current responses to be examined. No voltage overshoot is observed.

A fast, high-sensitivity amplifier has been developed and successfully used to record transient photoconductivity current of high-impedance, hydrogenated amorphous silicon thin film samples mounted inside a cryostat. The key feature of this amplifier is adjustable, positive feedback to the guard conductor of the input coaxial line. The amplifier circuit is located entirely outside the cryostat. It is characterized by a useful bandwidth from DC to 5 MHz, current to voltage gain in excess of 8*10⁶ V A^-1, good linearity and low noise. Averaging over several hundred waveforms with the help of digital storage oscilloscope improves signal-to-noise ratios dramatically and allows transient photocurrents of the order of 1 nA to be accurately followed in a microsecond time scale. The amplifier was also successfully used to record the ion current of a Faraday cup in time-of-flight mass spectroscopy.

A scheme for the measurement of component values of inductors and capacitors is described. In this scheme the inductor or capacitor is connected in series with a known standard resistance and this series circuit is excited by an AC voltage source with appropriate amplitude and frequency. The phase differences between the supply voltage, voltage across the inductor/capacitor and the voltage across standard resistance are measured using a microcontroller. The governing equations giving the unknown inductance or capacitance are evaluated by the microcontroller. The technique is independent of the voltage across or current through the unknown inductor or capacitor. Because the excitation frequency is measured by the microcontroller, tests can be carried out at a desired frequency. The uncertainties in the measurement increase with the applied frequency for a given clock frequency of the microcontroller. The proposed scheme was implemented based on an INTEL 8751 microcontroller. For the prototype built, with a one decade span in the measurement range, an accuracy of +or-0.4% has been achieved.

In this design note we describe a low-cost system for scanning the translation stage and automatically maintaining the correct focal position of a microscope. The image signal is derived from a video camera and the focal position determined by measuring the energy in the edge-detected image. The microscope is a conventional laboratory microscope with stepping motors attached to actuate the stage and control is effected through a laboratory personal computer.

A PC-based thermocycler for polymerase chain reactions is described. It uses electric heaters and 4 degrees C water for heating and cooling respectively. The temperature control is built around the easily accessible parallel printer adaptor of an IBM-compatible PC. An IC temperature transducer LM335 is used for sensing the temperature. Pulse width modulation of the AC mains is used to control the energy output to the heaters. The thermocycler was tested in amplifying DNA in actual laboratory situations.

This design note presents magnetic field uniformity design data for several alternative current loop systems. Universal field symmetry properties of the class of current loop systems that is being considered are elucidated. A common property of the five loop systems that are investigated in detail is that they are all in a sense optimal. This 'Nth-order' optimality criterion is defined and discussed. Parameters of selected Nth-order current loop systems are quoted. Computations of the field uniformity of these loop systems are presented in graphical form, as 'isogauss' contours, and in tabular form, as the 'normalized volumes' enclosed by the isogauss contours. Information is provided about a current loop system that was actually constructed on the basis of the design data presented here.