Sensing a change in measurement
The variety of sensors available to perform different kinds of measurement can be bewildering and is ever increasing.
Although there are some new contact sensors on the market, there does seem to be a general trend toward the use of non contact sensors, even in applications that have been traditionally measured using direct contact methods. It seems there are no longer any simple, hard and fast rules to follow when it comes to knowing what is best for a particular application.
Take position measuring, for example. One can use a non contact solid state laser, capacitative sensing, linear variable differential transformers (LVDTs) or an old fashioned reel of wire used to rotate a potentiometer contact or an encoder.
But, even here, there is innovation. ASM Sensor has recently introduced the Positape series that use a reel of very thin, smooth stainless steel tape instead of wire. The tape is so thin that, even when repeatedly bent over a succession of pulleys, the life of the WB17KT series is, 'virtually unlimited'. Measurement ranges are 3.5, 6.5 and 10m.
Variohm, on the other hand, has recently announced the SR1V pull out cable sensor from Celesco that, it says, is rugged enough for use on off-road construction equipment and has a rated life of 250,000 cycles. Alternatively, Leuze Electronic has brought out protective housings that allow nearly 200 of its optical sensors to be used to measure the force of explosions.
Suited for even harsher environments still are pressure transducers from Impress Sensors and Systems. Designed to be used in environments such as the nuclear industry, the sensors use LVDTs to measure the movement of metallic pressure diaphragms.
For more mainstream pressure and temperature measurements, Kistler has designed its type 9210A, for monitoring indirect micro injection mould pressures up to 3,000bar. The sensor is only slightly larger than a single quartz grain and is part of its miniature accelerometer, force and pressure sensor range.
Also very small, is a sheathed thermocouple, made by the Okazaki Manufacturing Company which has an outside diameter of just 0.08mm, similar in scale to the human hair. Response time is less than 1ms and the thermocouple can be bent on installation without causing damage or loss of performance.
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