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Capacitive sensors

Capactive sensors for detection ofnon-metalic objects. High noise immunity leading to increased operational reliability. Adjustable sensing ranges up to 60mm. Housing styles from M12 cylindrical to 105 x 80mm block. Assortment of tank and sight glass mounting acccessories.

Introduction
Not only metal: Capa-citive sensors detect almost all materials, here for example a log in a saw mill. Capacitive sensors are used for the non-contact detection of any objects. In contrastto inductive sensors, which only detect metallic objects, capacitive sensors can alsodetect non-metallic materials.Typical applications are in the wood, paper, glass, plastic, food and chemical indu-stries. Capacitive sensors for example monitor that the contents of a cardboard box are complete or check the presence of the non-metallic caps. Operating principle The capacitance between the active electrode of the sensor and the electrical earthpotential is measured. An approaching object influences the electrical alternating field between these two ”capacitor plates”. This applies to metallic and non-metallic objects.In principle, capacitive sensors work with an RC oscillator. A very small change incapacitance is enough to influence the oscillation amplitude. The evaluation electro-nics converts this into a switched signal. The sensitivity can be set with a potentio- meter. When detecting objects very small changes in capacitance of 0.02 pF (with a basiccapacitance of the electrode of 0.2 pF!) must be reliably converted into useful swit-ched signals. This makes high requirements for the electronics as the circuit anddesign-related parasitic basic capacitances (e.g. conductive tracks, input capacitances of the components) can be much higher thus making a precise capacitance measure- ment much more difficult.ifm electronic therefore developed a future-oriented solution to this problem. A newsensor circuit effectively avoids the indicated problems of the RC oscillator with anacceptable level of input. The new circuit concept achieves much better values with respect to all relevant noise parameters.Special attention was given to very common noise sources in practice (frequencyinverters, switched-mode power supplies, stepper-motor controllers, etc.). Increased noise immunity Full product and technical information from www.ifm.co.uk

ifm English catalogue 2008 - 544 40