Inductive safety sensors

Inductive safety sensors: construction and operating principle

In terms of their construction and function, inductive safety sensors are similar to traditional inductive sensors. However, thanks to some special features, they can be used for safety-relevant applications.

The sensors have an integrated copper coil at the end, which generates a magnetic field. This end is also referred to as the active surface. If a metallic object or a "target" approaches this alternating field, this process is referred to as damping. The resulting changes in vibration amplitude trigger a sensor switch. In other word, like classic inductive sensors, inductive safety sensors detect metallic objects, without any contact, and monitor their position.

However, in contrast to inductive sensors, they are equipped with additional functions, which make them suitable for use in safety-relevant applications. For example, they have self-monitoring output switching signal device outputs (OSSD outputs) and are constructed so as internally redundant.

The sensors check the OSSD outputs for deviations from standard switching patterns, and as such can detect short circuits, cross-connections, and switching failures. Should any one of these occur, the systems are switched off. Internal redundancy describes the presence of multiple identical functions within a sensor housing. For instance, each inductive safety sensor has two or three microcontrollers. In this way, the sensor is secured against the failure of certain functions.

Inductive safety sensors also differ from inductive sensors for traditional applications with regard to the manner in which switching takes place. A differentiation is made between the two switching methods: "securely damped" (NC) and "securely undamped" (NO)

Securely damped sensors switch to closed as soon as the "target" is within range of the magnetic field. As for securely undamped sensors, sensor switching takes place as soon as there is no (longer a) metallic object in front of the sensor. Here it should be noted that the damping and undamping must take place within so-called enable zones. Outside these zones, the outputs remain switched on or off accordingly.

Switching to safety mode is always important as soon as you have to guarantee that a system will not pose any danger to users.

The distance between the sensor and the metallic object is referred to as the switching distance. Because the switching distance is dependent on the size of the sensor coil, the bigger the sensor - the bigger the distance.

In addition to the nominal switching distance, with inductive safety sensors, the safe operating distance OFF, Sar, and the safe operating distance ON, Sao, must be taken into consideration.

Sar states the distance up wo which the absence of the target can be !00% guaranteed, taking into accoun any external influences and manufacturing tolerances.

Sao, on the other hand, defines the distance from the active surface of the sensor within which, taking into account external influences and object variations, the correct detection of the presence of the target is 100% ensured.

For which applications are inductive safety sensors suitable?

Inductive safety sensors can be used in all safety-relevant applications concerning the position detection of metallic objects. Therefore, inductive safety sensors are used, for example, in the following areas of application:

• End position detection
• Monitoring of protective covers and protective flaps
• Position detection for rotary tables
• Area detection for robotic arms
• Operation of mobile vehicles such as cranes or bin lorries

In comparison to safety switches, inductive safety sensors have the advantage that they have no external actuator.

ifm inductive safety sensors are now in stock at Automation24

Automation24 offers ifm inductive safety sensors for use in a safety engineering context. As a technological leader in the field of safety sensors, ifm is a reliable source for the highest quality and efficiency.

You receive cylindrical M12, M18 and M30 designs, as well as cuboid versions with flush and non-flush threads. The sensors are available in NO and NC-switching versions, cor various enable zones. They feature OSSD outputs, which can be analysed using the OSSD-compatible safety relays, the samos® PRO COMPACT safety relay control or fail-safe SIMATIC S7-1200F CPUs, likewise included in our product range.

The majority of the safety sensors available comply with the PLd category. For the most demanding of safety requirements, there are a number of Category PLe sensors available with complete redundancy.

It is important to mention that the selection of a suitable Performance Level (PL) can only be made following a prior risk assessment or based on standard specifications.

All ifm inductive safety sensors are suitable for various industrial facilities in which people have to be protected from machinery. The versions with the high IP69K protection class rating are extremely impact-resistant and are therefore intended especially for mobile applications, and for use within the foods industry.