Capacitive proximity sensors break all the rules

Citation

(2002), "Capacitive proximity sensors break all the rules", Sensor Review, Vol. 22 No. 3. https://doi.org/10.1108/sr.2002.08722cad.002

Capacitive proximity sensors break all the rules

Capacitive proximity sensors break all the rulesKeywords: Capacitance, Sensors

The new extreme range of sensors from Rechner sensors are not called extreme without good reason. Achievable sensing distances are up to ten times what could be expected for a standard sensor of the same size. For example, an M5 sensor could achieve up to 5mm sensing range and an M32 an amazing 120mm. The operating temperature range is from –200°C to +250°C. The sensors will sense the smallest changes in capacity and are independent of temperature (Plate 2).

The standard capacitive switch has electrodes arranged in the sensor head that form a measurement capacity. The field mainly of spherical or semi-spherical nature is influenced by the proximity of material with a di-electric constant of >1. This alteration of capacity is converted to a signal that can be evaluated, by a special oscillator circuit.

Plate 2

The new KXS range of capacitive proximity sensors patented by Rechner, are based on a principle called the "Three electrode principle". The key difference is that one electrode of the measuring system is now outside the sensor body. The machine or system potential now becomes one of the measurement electrodes of the capacitive switch and as such must be connected to the body of the sensor. Whereas a standard sensor reacts to a percentage change of the measurement capacity, the KXS evaluates an alteration in the absolute capacity, to a greater extent independent of the static, basic capacity. To clarify this point, an alteration of 0.1pF can be detected regardless of whether the basic capacity is 1pF, 10pF or 100pF. This also results in very small hystereses being achievable along with large distances that are independent of temperature right across the range of –200°C to +250°C. The large sensing distance can also be considered as an increase in the safety factor for previously borderline applications. A 30mm housing might well have been a requirement, using a standard capacitive sensor, to achieve a safe distance of 2-3mm on a single sheet of paper. With the KXS range this can be achieved with an 8mm sensor. The sensors can be mounted in any position, flush or non-flush not really applying to the KXS sensor. The sensors also have the advantage of being very small, the M5 for example being only 20mm long.

The sensors all work in conjunction with remote amplifiers with multi-turn adjustment. The housings are stainless steel/PTFE, allowing for use in many application areas including the food processing and chemicals industries. Housings are available in M5, M8, M12, M18, M30 and M32, all sealed to IP67. Amplifiers are available for PNP or NPN outputs and all are four-wire antivalent, thus normally open or closed by selection.

The characteristics of the KXS sensors certainly make them extreme in terms of performance and definitely unique in the market place. If the above is not extreme enough, Rechner have one special version in a stainless/ceramic housing which will work in a temperature range up to an unbelievable 800°C. Extreme indeed!

All enquiries to Ian Frais, Rechner, UK. Tel: +44 (0)118 976 6450; Fax: +44 (0)118 976 6451; E-mail: info@rechner-sensors.co.uk