Novel sensors

A wealth of new ideas and techniques was presented at the recent MTEC sensors conference at the NEC BY Alan Reeve

At the MTEC sensor and transducer conference, held at the NEC in January, there were several new industrial sensing technologies introduced.

The conference was put together by Bob Bogue, and provided a fascinating insight into some of the newer and most innovative sensing principles that have recently reached the marketplace.

First on the line was a presentation from Michael Read, from Daco Scientific in Aldermaston, who explained an eddy current, non-contact measuring device from Kaman Instrumentation. This advanced inductive technology can be used for measuring position, thickness and displacement.

The sensor consists of a coil of wire driven by a high frequency current, which generates a magnetic field around the coil. If a non-magnetic conductive material is introduced into this field, eddy currents are induced in the surface of the target material. These currents generate a secondary magnetic field in the target, including a secondary voltage in the sensor coil. Change in coil impedance over the calibrated range of target motion is therefore related to sensor output.

However, sources of error can include reflectivity, and the mounting and fixing of the device is very critical. Also temperature changes should be kept to a minimum to get the best performance level and cost benefits of using this technology.

John Francis, from Positek in Cheltenham, also discussed inductive position sensing.

Position sensing

Francis introduced Pips, a sensor based on inductive coils and featuring a special microelectronic interface for the sensor.

Standard sensors have linear travels of 100-1,500mm and rotary displacements of 160sr. Both rotary and linear versions of Pips are currently available.

Coil-based sensors provide a good combination of robustness, performance and cost. However, this type of sensor has traditionally been complex to install, with multiple wiring connections needed for the separate electronic interfaces.

Ammonia sensing was the subject of Doug Meredith’s paper. He discussed a development by M-Squared, where low limits of ammonia detection, demanded by environmental authorities, have been met by a new probe. The new optrode can measure ammonia in the low ppb levels.

The optical chemical sensor uses a dye which is impregnated into a PTFE membrane, and is suited to handle measurements of ammonia in treated wastewater outfall monitoring applications.

Next to be presented were details on Giant Magnetoresitance (GMR) and magnetic sensors, by Carl Smith from Nonvolatile Electronics in Eden Prairie in Minnesota (USA).Rhopoint in Oxted has similar sensors.

Smith said that the large changes in resistance caused by magnetic fields, exhibited by recently discovered thin-film multi-layer structures, offer fresh opportunities in the sensors field.

He said that the use of GMR materials, with semiconductor underlays for detecting magnetic fields and gradients, now allows for the production of inexpensive, ultra-small smart sensors, with enhanced magnetic sensitivity.

Microwaves for moisture

Next, measurement of moisture by microwave resonance was the subject of a presentation by Alan Coventry, from Filtrona Instruments in Milton Keynes. He said that the measurement of the effect of microwave energy with various materials can lead to an accurate measurement of the percentage of water in a sample.

As a result, an in-line instrument has been developed which can be used for measuring water in solids, liquids, emulsions, granulates, pastes and even sheet materials.

Microwave energy is absorbed by water between 2-3GHz, and the percentage water present in the microwave field is proportional to the combined effect of signal attenuation and frequency decrease, due to the dielectric effect.

There was also a session on technology transfer, where developments were explained for a Hydrogel/fibre optic alarm, an in-line vibration-based fluid density sensor and a high temperature thick and thin-film thermographic sensor. Also discussed was the evaluation of Kenyan tea using an electronic sensor array and a dual triangular bluff body vortex flowmeter.

Additional sessions were held on gas sensors, and pressure, noise and vibration sensors. Proceedings are available at £45 from Trident Exhibitions on 01822 614671.

Quartz viscometers

Another interesting development is the quartz viscometer, recently announced by Flucon in Germany (tel: 00 49 5323 2212).

This viscometer is suitable for the continuous determination of viscosity in resting and flowing liquids. The sensor is based on an oscillating quartz crystal that responds to the viscosity properties of the surrounding liquid. This sensor is combined with a Pt100 RTD so that the temperature of the liquid is also measured.

Advantages of this sensor are that there are no moving parts and the unit is resistant to contamination and sedimentation, due to the high frequency oscillation which self-cleans the sensor.

The system produces results which are repeatable to within 1%. Also, the sensors can be used in hazardous areas at temperatures up to 300srC.