Table of contents

Many measurement tasks in industrial automation, e.g. non-contact distance measurement, room surveillance, object identification and gas flow measurement can be accomplished by sensors using airborne ultrasound. Their performance is determined by the properties of the ultrasonic transducers. The authors compare electrostatic and piezoelectric transducers. Piezoceramic transducers combine a high efficiency factor and ruggedness. Composite transducers consisting of piezoceramic and polymer materials show a wider relative bandwidth of about 30% at the expense of efficiency. Ultrasonic transducers based on piezopolymer foils offer a variety of acoustic properties at low expenditure. Their bandwidth corresponds to that of composite transducers. Composite, as well as piezopolymer, foil transducers are suited to the design of phased arrays. Foil transducers have the additional advantage of very small crosstalk between neighbouring transducer elements. By electronically controlled deflection of the sound beam, lateral details and consequently the spatial structure of objects can also be detected.

A front face area control system has been develop for DC-pulsed tungsten inert gas (TIG) welding. The system is based on a binary vision system built to interface to the BBC range of microcomputers. The system utilizes a fast charge coupled device (CCD) camera, to enable an image of the weld surface to be captured during a 3 ms interruption of the main arc current. The control system regulates the bead area by controlling the pulse duration of the TIG process. Comparisons are made between the required and the actual front face bead areas in real time, the arc current being returned to the background level when the required area is reached. The front face area control system has been tested extensively on type 304 stainless steel, using plate thicknesses from 1.65 mm to 2.7 mm, and was found to perform well. The image of the weldbead does not have to be in the centre of the screen, as long as the whole bead appears within the field of view of the camera. This leads to simple operation with a short set-up procedure.

To measure the bulk volume of solid materials accurately by mercury displacement, a pycnometer consisting of four parts was developed. The pycnometer was designed to have a repeatable inner volume when assembled. In addition, the weight of mercury, with which the pycnometer was filled, was corrected for the deviation of the free surface of the mercury from its inlet. The device and the correction allow determination of the bulk volume within an error of 0.1% even for 0.1 ml of sample volume.

The authors present the results of an investigation into clamping problems associated with the flexural testing of specimens which are clamped at both ends for dynamic mechanical thermal analysis (DMTA). Two major factors which must be considered when modelling the specimen's behaviour have been identified. It is shown that the use of simple beam theory to extract complex moduli is not justified, and more accurate models are required. An appropriate model is presented which shows that a knowledge of factors other than force, deflection and specimen geometry are necessary to obtain accurate and reproducible results. Alternatively, the clamping system may be changed, and this option was pursued further. A new type of clamp has been designed, built and tested, which allows the specimen's behaviour to be described by simple beam theory. Static and dynamic tests have been performed, and accurate results have been obtained.

A linear amplifier capable of providing a single-ended output in excess of 3 kV across a predominantly capacitive load requiring charge/discharge currents of order 10 mA is described. High-voltage capability, low-power consumption and excellent linearity and harmonic distortion performance are all obtained from a simple circuit, employing negative voltage feedback around a high-voltage low-power MOSFET ladder. Results are presented which highlight the performance of the amplifier in its intended role as a triangular-wave voltage driver for an electro-optic crystal.

The susceptibility of the sample is measured, using an AC bridge technique at 703 Hz. The typical minimum measurable susceptibility (dipole moment per unit field) is 2*10^-9 m³, equivalent to the susceptibility of 4 mg of magnetic recording material such as fine acicular gamma -Fe₂O₃. The remanence of the sample is measured using two aligned horizontal fluxgate probes, connected in series opposition to combat environmental noise. The minimum measurable sample moment (total dipole moment) is about 5*10^-5 A m₂, equivalent to the saturation remanence of about 1 mg of randomly orientated gamma -Fe₂O₃. The magnetic susceptibility, remanence, temperature and pressure signals are multiplexed into a microcomputer. The data is stored on disk, processed and displayed on a visual display unit or printer. Some preliminary results involving the pressure- and temperature-induced phase transition (inversion) of gamma -Fe₂O₃ are presented.

It is impossible to construct an ideal phase shifter, one whose phase shift and gain is independent of frequency, using a linear circuit. The reasons for this are discussed and the compromises necessary in some practical implementations are examined. A novel circuit using programmable delay lines is presented. Such circuits have proved particularly useful in NMR systems.

A pulse programmer designed for imaging experiments using pulsed EPR techniques is described. The programmer consists of two sections. The first, of traditional design, provides the time base for the pulse sequences; the second, based on two digitally programmable delay generators, model AD 9500 (Analog Devices) makes it possible to generate pulses ranging from 10 ns to a few milliseconds in steps of 0.4 ns. On a 50 Omega load the rise and fall time of the output pulses is 5 ns. The programmer can drive a two-channel radio frequency transmitter and permits the use of one, two or three pulse sequences through an external microcomputer.

An experimental method is proposed for the determination of temperature profiles during laser irradiation of supported thin films. The method is applied to the measurement of the evolution of the temperature profile during the oxidation of thin vanadium films on glass, induced by a CW argon ion laser beam. A temperature decrease in the centre of the laser beam due to negative feedback is detected. The advantages and limitations of the method are discussed.

An electron spectrometer employed to record gas phase photoelectron-photoelectron coincidence (PEPECO) spectra following valence shell single-photon double ionisation is described. The spectrometer is based on two opposed hemispherical electron energy analysers, and employs a helium discharge lamp as a source of ionizing radiation. To achieve a practical coincidence count rate each analyser is designed to gather electrons from a large solid angle. The successful operation of the experiment is illustrated by new PEPECO results recorded following double photoionization of xenon at 40.8 eV. These spectra record the excess energy distribution between the pair of ejected photoelectrons following population of the 5p^-2 (³P and ¹D) states of Xe²⁺, and confirm that the double photoionization mechanism is predominantly indirect.

A versatile electron-ion coincidence spectrometer for photoionization studies is described. The spectrometer incorporates a 127 degrees cylindrical electrostatic energy analyser and a time-of-flight energy analyser. The two analysers are mounted on a turntable that can be rotated with respect to the polarization axis of the photon beam. Both analysers have a penetrating field stage which provides very high detection efficiency over a large solid angle and excellent threshold energy resolution ( approximately 3 meV) for the detection of charged particles. The analysers can also detect energetic particles. The time-of-flight analyser is used specifically for the separation and detection of different charge states of molecular photodissociation products. The use of the spectrometer to observe photoelectron-photo-ion coincidences and photoelectron-photoelectron coincidences in a variety of experiments (TPEPICO, PEPICO, TPEPECO) is illustrated by a study of the photo-double ionization of argon and of the dissociation products from the v=0 level of the c state of oxygen at 24.56 eV. Additionally, a spectrum of coincidences between threshold photoelectrons (TPESCO) is presented in the region of the ³P double ionization potential of argon.

A fast acquisition method for extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements has been developed to observe in situ dynamical processes. Typically, a spectrum is obtained about ten times faster than by the classical method. This new method offers the possibility of following chemical reactions within a characteristic time of the order of one minute.

Determination of the time since deposition of a sediment sample from the optically stimulated luminescence of component quartz and feldspar grains using green light has previously employed an argon ion laser as light source. Test results are presented which show that green light emitting diodes provide a convenient and economical alternative for this dating technique, and their potential for routine use is assessed.

The design of a simple system for generating uniaxial stresses is described. The improvement on existing systems centres on the incorporation of a threaded bar, steel spring and load cell into the main push rod assembly. The use of a load cell with an electrical voltage output facilitates interfacing to computer-based data collection systems leading to automatic recording of the stress applied to the sample. The apparatus compares favourably with commonly used alternatives in terms of accuracy and sensitivity, with the advantage of being simple to construct and convenient to operate. Uniaxial stress measurements are reported to indicate the quality of spectra obtained using this system.

Using a limacon approximation to linearize the minimum zone circle (MZC) problem, and then by applying duality theory of linear programming, Chetwynd (1980, 1985) obtained a geometric algorithm exhibiting an 'alternation property.' It is demonstrated that the alternation property can be obtained using a classical result due to Bonnesen (1924). The equivalence is demonstrated using elementary arguments.