Sensors and Actuators最新文献

New methods have been developed to implement position sensors based on electron tunneling. The electron tunneling methods enable position to be detected with sub-Ångstrom resolution using a compact mechanical structure and simple electronic control elements. A prototype accelerometer is the first sensor based on these principles; it shows reliable operation with a 10 μg/√Hz noise-limited sensitivity and abandwidth of approximately 3 kHz. Based on these results, it is expected that tunnel sensors optimized for many applications will enable an entirely new class of sensors to be developed.

The fabrication and characteristics of a prototype CdTe (cadmium telluride) detector for a β-ray thickness gauge are described. The detector consists of a pn-junction diode, operating in photovoltaic mode with no applied voltage. It needs no cooling because its dark current is less than 7 × 10⁻¹⁴ A/mm²at 0.01 V and 30 °C. An output current of 5 × 10⁻⁸ A with a statistical noise of 1.5 × 10⁻¹² A/(Hz)^{$\text{1}\text{2}$} has been achieved with ⁸⁵Kr radiation on an active area 16 mm in diameter. The (111) CdTe wafer used in the detector is 22 mm × 22 mm × 1.5 mm.

Single crystals with no twins and a low dislocation density (less than 1 × 10⁶ cm⁻²) are used because it is found that the generation-recombination centres of carriers related to crystallographic defects (such as twins and dislocations) increase the dark current and decrease the output current.

As the temperature coefficient of the output current for an ⁸⁵Kr radiation source is approximately 0.23%/°C, a thermostatistically-controlled module is used to keep the detector's operating temperature constant to within 0.1 °C, thus solving the problem of drift of output current with temperature.

The domain of micromechanical systems is an extensive, useful and yet largely unexplored area. It is likely that, in many applications, small mechanisms will prove to be faster, more accurate, gentler and less expensive than the macro systems presently used. This paper explores the advantages of micromechanical systems and analyzes the scaling of forces in the micro domain.

This paper is a feasibility study for the design and construction of a magnetic field sensor using a tunneling tip as the detection system. In this experiment, a scanning tunneling microscope is used to measure dimensional changes of a short, thin amorphous FeBSi magnetostrictive ribbon exposed to d.c. and a.c. magnetic fields. A response of the system to a.c. magnetic fields was observed down to 0.020 G at a frequency of 1 Hz. The calculated sensitivity of the system can be near 10⁻⁶ G if the vibrational stability of the device can be improved and if the demagnetizing field associated with short ribbons can be reduced. A mode of sensor operation involving the modulation of the tip-to-specimen separation is reported. Methods to improve the stability of the device are also discussed.

Optically-thin nickel films evaporated on the end of optical fibers show a reflectivity change when exposed to oxygen. This effect has been measured as a function of metal thickness and wavelength. Growth of NiO on the nickel surface reduces the thickness of the metal and thus the phase shift of the light passing through it. This phase change causes a change in the reflectivity. A simple classical model using Fresnel reflection coefficients fits all the data. The 4.2 Å reduction in nickel thickness deduced from the optical measurements agrees well with measurements made using modern surface analytical techniques. This new optical method for monitoring chemical interactions between adsorbates and metals may prove useful for gas sensing and corrosion monitoring. In addition, it may provide information on how chemisorption modifies the local electron density of states in metals.

A calorimetric sensor consisting of two identical platinum heater/ resistance thermometer films fabricated by a thick-film metallization process is used to monitor the steady-state concentration of hydrogen, carbon monoxide and hydrocarbon gases in a flow reactor. Platinum black and palladium are used as catalysts and are deposited either chemically or electrolytically on one of the platinum films, while the other serves as a compensating element. Experimental studies show that the sensors fabricated by this process have good sensing characteristics for the combustible gases examined.

Many inherent features available in the CMOS process lend themselves to the fabrication of certain micromechanical structures for sensor applications. These micromechanical structures are fabricated by implementing unconventional layout designs in CMOS technology without altering the process sequence. A single post-processing etching step is introduced to form free-standing microstructures on a CMOS IC without affecting the circuitry formed on the chip, thus allowing micromechanical sensors to be produced with pertinent on-chip circuitry for signal conditioning. Polysilicon micro-bridges, sandwiched oxide microbridges and cantilevers are produced using this technique.

Several semiconductive oxides are tested as sensors for the detection of NO, NO₂, CO and CO₂. The conductivity of Al₂O₃V₂O₅ is sensitive to 1–1000 ppm of NO and NO₂, but is insensitive to CO or CO₂. The mixed oxide (Al₂O₃)_0.5(V₂O₅)_0.5 is excellent not only in its selectivity for detection of NO and NO₂ but also in its sensitivity. The amount of NO adsorbed is greatly enhanced, but that of CO is unaffected by mixing V₂O₅ with Al₂O₃. The high sensitivity of Al₂O₃V₂O₅ to NO and NO₂ appears to result from the increased amount of adsorbed nitrogen oxide.

A coated piezoelectric crystal (CPC) detector system with high stability designed for the detection of pollutants in ambient air is described. The system is based on the use of two crystals, which are exposed to the air to be analysed. The incoming air flow is divided into two paths; one is led directly to one of the CPCs and the other flow is connected to a molecular sieve filter that absorbs the pollutants before the flow is led to the other CPC. The flows are controlled by magnetic valves in such a way that one crystal is exposed to either clean air from the molecular sieve filter or to polluted air. The other crystal is exposed in the same way, but in counter phase.

By careful design and control of the air flows, a high stability is reached and virtually no thermal drift is present. A long-term stability of ±0.002 Hz is demonstrated. The noise sources are analysed and methods to reduce their impact are demonstrated.