Table of contents

The design of sensing systems for the measurement of multiple physical quantities related to a dynamical system is considered. A multivariate sensor comprises several simple transducers, each measuring a scalar quantity that comes from the combination of the components of the quantity to be measured. From the collection of measurements of single transducers at different times, the desired information is extracted by analogue or digital processing. Besides the choice of technological characteristics of the transducers to be employed, the designer of multivariate sensors is usually allowed some freedom in choosing the number of transducers, their arrangement in the system, and the time scheduling of their measurements. These choices are the subject of optimal policies in the design phase, whose goal is to maximize some performance (or minimize some cost) criterion. We survey some of the existing approaches to optimal design of multivariate sensors, according to the different types of systems they are applied to. Two examples of optimal sensor design are discussed as an illustration of the methods.

Common mode voltages are particularly important in impedance spectroscopy because they will always have frequency components in common with the desired signal. A differential or instrumentation amplifier will imperfectly reject this common-mode voltage and consequently its output will be in error. This can produce unexpectedly large errors in the measured impedance. An analysis of various two- and four-terminal configurations is presented that shows how the errors in measurement of an impedance are related to the common-mode rejection ratios of the amplifiers. It also indicates how a suitable choice of experimental configuration can substantially reduce these errors.

We outline the design of a fully automated Millikan droplet apparatus that could detect a single free fractional charge in several hundred grams of matter even without the use of a prefilter. This would constitute an improvement over current limits by about three orders of magnitude. The experiment achieves high material throughput and high background rejection through on-line processing which allows for a feedback system that can concentrate the measurement effort on anomalous droplets. The task is simplified by generating a monodisperse stream of droplets which will be preprocessed to let only a very narrow range of charges enter the Millikan chamber. Because the droplets can act as carriers of finely dispersed materials it is also possible to search for fractional charge in matter that has not undergone extensive refinement that may have excluded fractionally charged atoms from the sample. In a large refinery style operation many such Millikan chambers could run concurrently to achieve extremely large material throughput.

A fully general control system capable of driving almost any kind of imaging array (including conventional CCDs of two, three and four phases, charge sweep devices, charge injection devices, and FET-switched arrays) has been developed. This paper describes the design and operation of both the system electronics and software. The control system is based around an IBM compatible computer, yet offers an unusual degree of versatility.

In this paper we describe the theory and application of a newly developed synthesizer that is especially suitable for cavities suffering instabilities and drift effects. The synthesizer presented here uses square-wave frequency modulation as its principle of operation. The synthesizer consists of two main phase-locked loops that are interconnected so that their outputs are alternately available. Spurious signals in the output spectrum have been measured to be more than 77 dB below the carrier at 405.782 kHz.

This paper describes an experiment where the flux-gate sensor broad band output signal is digitized with 8-bit resolution at a high rate, and subsequently numerically analysed in order to extract information on the external magnetic field. The results show that it is feasible to obtain a noise level of 1 nT for a data output rate of 100 Hz. The method will allow construction of a magnetometer based entirely on digital techniques, with only a minimum of analogue circuits.

In the development of the technique of reaction yield detected magnetic resonance (RYDMR), a novel simple accessory for optical detection of ESR (OD ESR), based on the S-band continuous wave magnetron generator is presented. Basic technical requirements for the RYDMR accessories are discussed. The successful use of an S-band magnetron as a microwave source for RYDMR experiments is reported for the first time. Along with the use of a simple loop-gap resonator it permits the studies of both polar and non-polar samples with good sensitivity and field resolution. With the help of this accessory, OD ESR spectra of some new acceptors, acridin and fluorescein, were detected and some new solvents, poly(ethylene glycol) and paraffin-based high-voltage insulator oil, were successfully utilized.

The development of a novel sensor for the measurement of bead size in tungsten inert gas (TIG) welds is described. A passive vision-based sensor combines infrared imaging of the weld pool and selective optical filtering, both to reduce excessive arc glare and improve contrast between weld pool and heat affected zone (HAZ). The arc interruption and external illumination techniques, used by others, are not required. Image quality is shown to be comparable to that obtained with external illumination and superior to that of arc interruption methods. The technique has been successfully applied to both pulsed and continuous TIG welding of stainless steel in order to maintain the bead size during a welding run.

An additional feature incorporated in a coaxial double-cylindrical tissue-equivalent proportional counter is the presence of a common tissue-equivalent dividing wall between the inner and outer counters of thickness equivalent to the corresponding maximum range of protons at the energy of interest. By appropriate use of an anti-coincidence arrangement with the outer counter, the inner counter could be used to discriminate microdosimetric spectra of neutrons at the desired low energy range from those of the faster neutrons. The construction of an A-150 self-supporting tissue-equivalent dividing wall and an anti-coincidence unit are described. Some operational characteristic tests have been performed to determine the operation of the new microdosimeter.

A versatile, low-cost, easy-to-implement, microprocessor-based scanning spectrophotometer suitable for time-resolved emission spectroscopy and time-correlated measurements is reported. Such a system, with high temporal and spectral resolution, incorporating pulsed lasers to excite the sample and with single-photon counting detection, has been used to investigate the photophysical properties of weakly emitting samples. This system is capable of measuring sub-nanosecond and nanosecond fluorescence lifetimes. Typical emission spectra are presented.

A new one-dimensional ion detector array on a silicon chip has been developed for use in mass spectrometry. It is much smaller and simpler than electro-optical arrays currently in use and in addition has a higher resolution and a zero noise level. The array consists of a one-dimensional array of metal strips (electrodes) with a pitch of 25 mu m on the top surface of a silicon chip, each electrode having its own charge pulse sensor, 8-bit counter and control/interface circuitry. The chip is mounted on a ceramic substrate and is preceded by a micro-channel plate electron multiplier. Chips are butted to give a longer array. Test results show a stable operating region. A digital-signal-processor-based interface is described, which controls the mode of operation and reads the accumulated array data at the maximum rate to avoid counter overflow.

Special stages have been used over many years for the study of magnetic domains and walls in ferromagnetics. The new stage described here allows direct observations of the interaction between structural defects and magnetic domain walls. With this new version, built in CEMES LOE in Toulouse, accurate measurements of microscopic hysteresis loops can be made, according to the very low beam deflection when applying variable external magnetic fields. Another advantage of the stage is the feasibility of very-low-temperature measurements (at 4 K). Experimental results are given on interaction between structural defects (grain boundaries) and magnetic domain walls at room temperature in a crystalline Fe-Si alloy.

Assessment of the quality of a digitized image is examined using an analytic model of the modulation transfer function of the image processing system. It is demonstrated that there is a significant and quantifiable effect on texture energy measures, as defined by the particular cases of the Laws convolution masks and a new adaptive approach to spatial filtering. In particular, the radius of the best-fit Gaussian to the point-spread function correlates inversely with the power of local operators to discriminate visually similar textures. Results indicate that the use of adaptive convolution masks improves discrimination significantly, and is more tolerant of instrumental variations.

A review of the operation of a streak camera is presented. This is followed by a discussion of measurement techniques with time resolutions on the order of 1 ps using the streak camera. A sweep circuit design is presented using power metal oxide semiconductor field effect transistors (MOSFETs). The sweep circuit generates two ramps of +or-2 kV in amplitude. With these ramps sweep times of less than 5 ns (125 ps mm^-1) to greater than 200 ns are achieved. The time-base variation is adjustable with a DC voltage provided by a front-panel potentiometer (knob) similar to an oscilloscope.

Interferometric fibre optic sensors are increasingly finding applications 'in the field'. Examples are provided by the use of fibre-based laser Doppler velocimetry (LDV) systems for wind tunnels and by laser vibrometry systems used for example in the monitoring of machine tool wear. Both the wind tunnel, and the factory floor are harsh environments for interferometric sensors. In particular the pick-up of acoustic noise from the air or from vibrating surfaces is a potentially serious problem, broadening and distorting the signal spectrum. A new broadening mechanism, called environmental noise broadening, is discussed and the magnitude of this effect is estimated for typical LDV systems. For flow velocity measurements in wind tunnels, this broadening leads to an overestimate of the turbulence intensity which is more serious for low-speed flows. The statistical formalism introduced here is applicable to any type of fibre-linked extrinsic interferometric sensor.

A method is described for discrete sampling type instruments to analyse the periodically time-dependent velocity fields found in turbomachines with elements rotating at different speeds. Velocity samples that are measured in the stationary frame are translated into the relative rotating frame and then organized by the relative angular position between all the independent elements of the turbomachine. At the instant of data acquisition the angular position of all independently rotating elements must be measured. Since the flow field is periodic, large velocity sample sizes can be obtained for every angular position combination of the elements of the turbomachine. By correct grouping and averaging the recorded samples, the unsteady flow field interaction of two or more components can be visualized. The technique was demonstrated for the turbine inlet flow field of an automotive torque converter using laser velocimetry. A torque converter has three components that are relevant to the flow field and which rotate at different angular speeds: pump, turbine and stator. The torque converter was tested for a turbine/pump speed ratio of 0.800. Throughflow velocity plots of the unsteady flow field are presented to demonstrate the method. A jet/wake region migrating through the turbine inlet plane at the pump-turbine blade interaction frequency is identified.

Pulsed field gradient nuclear magnetic resonance was performed on air-water mixtures flowing through a circular pipe. Analysis of the spin echo signals established the validity of the technique to measure the mean velocity and the velocity probability distribution in laminar and turbulent flows of liquid water (one phase) in a long circular pipe. The purpose of this study was to extend the method to the case of two-phase air-water flows. A full procedure is proposed and demonstrated for deriving different basic characteristics of these flows: void fraction, mean liquid velocity, liquid volumetric flux and mean velocity probability distribution. The experimental results, reported for various void fractions (0-0.5) and liquid volumetric fluxes (up to 25 cm s^-1), extensively illustrate the capabilities of this method.

A technique for making simultaneous temperature and velocity measurements, required for measuring temperature-velocity correlations used to determine turbulent heat fluxes, and temperature-velocity correlation coefficients, is described. This technique uses a two-component laser Doppler velocimeter (LDV) which gives a direct measure of velocity fluctuations and a 0.64 mu m cold-wire which gives a direct measure of the temperature fluctuations. This technique has the advantage over multi-element hot- and cold-wire anemometry systems in that there is no sensor interference between the velocity and temperature sensors, and there are no corrections which need to be applied to the velocity measurements because of temperature influences of the surrounding fluid.

By using the Peltier effect it is possible to measure the transport properties of thermoelectric materials. This so-called Harman method exhibits the advantage of measuring the properties of a sample without any change in preparation. Until now this method has only been used up to 100 degrees C. An apparatus and a preparation procedure is described to use this method in the temperature range 50-800 degrees C. Measurements of FeSi₂ and higher manganese silicide (MnSi_x) are given.

An improved version of a Marx bank circuit has been developed using 2N5551/2N5550 transistors as switching elements in avalanche mode which provides a negative pulse of about 4 kV amplitude and <or=ns rise time. The circuit has about 14 ns delay and <or=ns jitter.

An infrared radiometric temperature sensor based on multimode silver halide optical fibres was used for non-contact temperature monitoring during drilling and lathing. This sensor provides non-contact temperature measurements with an accuracy of +or-0.2 degrees C and can be useful for monitoring of machining procedures.