Inductive Proximity Switches

Inductive type proximity switches incorporate a damped L-C oscillator driven coil generates a high frequency magnetic field that surrounds the active surface of the device (Fig. 1)

Introduction of metal (ferrous or non-ferrous) into the field causes damping of the L-C oscillator. The resulting change in operating current of the oscillator is sensed to provide an output.

Capacitive Proximity Switches

Capacitive type proximity switches incorporate a damped R-C oscillator. When a material ie. wood, plastic, water, glass, ferrous or non-ferrous metal, is placed within operating range of the switch, the resulting change in capacitance causes the oscillator to begin operating. The operating of the oscillator is sensed to provide an output (Fig. 2)

Magnetic-Inductive Proximity Switches

Magnetic inductive switches incorporate a damped L-C oscillator. Introduction of a permanent magnet into the proximity of the switch causes undamping of the oscillator with the resulting change in operating current being sensed to provide an output.

Light from the emitter is reflected to the detector by the object to be sensed. When the light reflected from the object reaches the detector's threshold an output is generated. Sensitivity is dependent upon is generated. Sensitivity is dependent upon the reflective index of the object. (Fig. 3 and 4)

For this reason it is necessary to take into account the difference in reflectivity between the object and the background.

 

Light from the emitter passes directly to the detector where it is converted to an electrical signal. When the object to be sensed interrupts the light beam, a change in detector current occurs. This change in detector current occurs. This change in current is sensed to provide an output. In this type of switch the emitter and detector are located in separate housings (Fig. 5).

Light from the emitter passes to the detector via an opposing reflector where it is converted to an electrical signal. When the object is detected in the same manner as in the interruptive type switch. The main difference being, that the emitter detector pair is located together in the same housing (Fig. 6).

 

Sensor using fiber optics for reflective detection operate on the same principle as previously described. The light passes to the object and back to the detector via a duplex fiber optic cable. The working end of the duplex cable is terminated in a single ferrule (Fig. 7)

When using fiber optic sensors in the light barrier mode, separate optical conductors are used for the emitter and detector. Light from emitter conductor is directed across the sensing gap to the detector conductor. Interruption of the light beam between the gap results in an output from the detector (Fig. 8).