Proximity Sensor and Switches

A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. A proximity sensor often emits an electromagneticor electrostatic field, or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. The object being sensed is often referred to as the proximity sensor's target. Different proximity sensor targets demand different sensors. For example, a capacitive or photoelectric sensormight be suitable for a plastic target; an inductive proximity sensor requires a metal target.

Proximity sensors can have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between sensor and the sensed object.

Inductance is the property in an electrical circuit where a change in the electric current through that circuit induces an electromotive force (EMF) that opposes the change in current (See Induced EMF).

In electrical circuits, any electric current, i, produces a magnetic field and hence generates a totalmagnetic flux, Φ, acting on the circuit. This magnetic flux, due to Lenz's law, tends to act to oppose changes in the flux by generating a voltage (a back EMF) in the circuit that counters or tends to reduce the rate of change in the current. The ratio of the magnetic flux to the current is called the self-inductance, which is usually simply referred to as the inductance of the circuit. To add inductance to a circuit,electronic components called inductors are used, which consist of coils of wire to concentrate the magnetic field.

A capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by adielectric (insulator). When a potential difference (voltage) exists across the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the conductors. The effect is greatest when there is a narrow separation between large areas of conductor, hence capacitor conductors are often called plates.

An ideal capacitor is characterized by a single constant value, capacitance, which is measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. In practice, the dielectric between the plates passes a small amount of leakage current. The conductors and leads introduce anequivalent series resistance and the dielectric has an electric field strength limit resulting in a breakdown voltage.

Inductive switches can be used for counting of metal objects and detection of metal objects with any contact.

Capacitive Switches can be used for level control of water, plastics, foam products or food products.

Both Inductive and Capacitive switches can be made with NAMUR outputs so that the switches can be put in hazardous areas. The connections do need to go through a intrinsic safety barrier.

Most proximity switches are barrel shaped in the 5mm, 12mm, 18, and 30mm diameters. Also very common is the limit style proximity switch. Barrel materials are crastin, plastic, stainless steel, and nickel plated brass.

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Sords Electric sells Carlo Gavzzi, ATC, Autonics, EMX and Pepperl and Fuchs proximity switches.