ifm capacitive sensors

Capacitive sensors are used for the non-contact detection of any objects. In contrast to inductive sensors, which only detect metallic objects, capacitive sensors can also detect non-metallic materials. Typical applications are in the wood, paper, glass, plastic, food and chemical industries.
Operating principle
The capacitance between the active electrode of the sensor and the electrical earth potential 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 in capacitance is enough to influence the oscillation amplitude. The evaluation electronics converts this into a switched signal. The sensitivity can be set with a potentiometer.
Increased noise immunity
When detecting objects very small changes in capacitance of 0.02 pF (with a basic capacitance of the electrode of 0.2 pF!) must be reliably converted into useful switched signals. This makes high requirements for the electronics as the circuit and design-related parasitic basic capacitances (e.g. conductive tracks, input capacitances of the components) can be much higher thus making a precise capacitance measurement much more difficult. ifm electronic therefore developed a future-oriented solution to this problem. A new sensor circuit effectively avoids the indicated problems of the RC oscillator with an acceptable 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 (frequency inverters, switched-mode power supplies, stepper-motor controllers, etc.).
More about capacitive sensors: www.ifm.com/de...