Output Logic of Switching Sensors

Switching sensors have a binary output logic. The output knows only the two states "active" and "inactive" or "on" and "off." The switching element function determines whether a proximity sensor closes or opens the output when the object is detected, or whether this can be freely selected depending on the intended purpose.

NO Contact (Normally-Open—State of Unactuated Switch)

Electronic switching element function of a proximity sensor: switching output logic. When the switching condition is fulfilled (e.g., proximity sensor detects object in the switching range) the output is closed, i.e., current flows. When idle, the output is open, i.e., no current flows.

NC Contact (Normally-Closed—State of Unactuated Switch)

Electronic switching element function of a proximity sensor: switching output logic. When the switching condition is fulfilled (e.g., proximity sensor detects object in the switching range) the output is open, i.e., no current flows. When idle, the output is closed i.e., current flows.

Complementary

Electronic switching element function of a proximity sensor. A complementary proximity sensor has two built-in switching output stages—one of which is designed as a "normally-open" output type and the other as a "normally-closed" output type.

Complementary sensors can be used to reduce the variety of sensor types in the plant, thereby reducing storage requirements. Mostly, the complementary design of the switching output stages is used for diagnostic purposes. A plausible output behavior for the sensor is only possible if the two switching outputs have opposing states. If they have the same state, this is a sign of an error, e.g., lead breakage or lead short circuit.

Change-Over Contact

Physical switching element function of a relay contact unit.

Example: The relay contact unit may be connected to the NC contact when switched off. By turning on the relay, the relay contact unit is connected to the NO contact. Relay contacts are normally volt-free contacts. Galvanic isolation therefore exists between the sensor circuit and the load current circuit.

For the change-over contact, the relay contacts can be designed as electromechanical contacts or as electronic switches. Electromechanical contacts offer a high current-carrying capacity, but due to the inertia of the moving masses the switching frequency is limited to a few hertz. Change-over contacts with electronic contacts have a similar current-carrying capacity to other electronic switching output stages with correspondingly high switching frequency. The most important feature—the volt-free operation—is retained.

According to EN 60947-5-2, the repeat accuracy is the deviation value of the effective operating distance (sr) under set conditions. The value defines the switch point accuracy of successive switch events over a period of eight hours at an ambient temperature of +23 °C ±5 °C and at a constant operating voltage.

According to EN 60947-5-2, the hysteresis (H) is the distance between the switch-on point when the damping element approaches the proximity sensor and the switch-off point when it moves away from the proximity sensor. The switching hysteresis H is specified relative to the effective operating distance sr, measured at an ambient temperature of +23 °C ±5 °C and the rated operating voltage.

H < 0.2 * s_r

Proximity sensors from Pepperl+Fuchs typically have a hysteresis of 5%.