Inductive sensors

Inductive sensors detect metal or electrically conductive objects, which move into their sensing range, without any contact. They provide the user with a binary signal that indicates whether an object has been detected or not. Inductive sensors are also called proximity switches or position sensors.

Design and operation

Inductive sensors consist of three functional units: an oscillator, an evaluation unit and an output stage. As part of the oscillator, the coil uses the oscillating circuit to generate an alternating magnetic field that then radiates from the active surface of the inductive sensor. When a metallic object enters the magnetic field, eddy currents develop that reduce the energy from the oscillator. The amplitude and frequency of the oscillating circuit change and the output stage is adjusted by a Schmitt trigger or the amplitude is converted into a distance. If several coils are used, the sensing accuracy of the inductive sensor is increased.

Advantages of inductive sensors

Thanks to their contactless operating mode, inductive sensors have numerous advantages:

  • Long service life thanks to hardly any mechanical wear
  • No downtime caused by dirty or damaged contacts
  • No contact bounce and thus no switching errors
  • High switching frequency
  • Vibration resistance
  • High degree of protection thanks to a fully encapsulated housing
  • Any mounting position possible

Inductive sensors from Festo

Festo inductive proximity sensors are virtually wear-free, easy to operate and of very high quality. We offer inductive sensors in numerous designs for a wide range of applications.

For example, the proximity switches SIED and SIEN with standard switching distance are suitable for use in applications with either DC or AC voltage. They are characterised by their round design and metric thread.

Our inductive proximity sensor SIEH -...-CR is particularly sturdy thanks to its fully enclosed stainless steel housing. Unlike conventional sensors, it offers high resistance to almost all gases and media. Other features include the increased switching distance, flush mounting, LED switching status display and metric thread.

The Festo proximity switch SIEF is extremely easy and reliable to use. It is welding field-resistant, has a weld spatter-resistant housing, LED status indicator and can be installed flush, partially flush or not flush.

Our inductive sensor SIES-8M is especially suitable for position detection on electric axes and grippers with T-slot. The SIES-8M has 2 LEDs for indicating the switching status and better visibility regardless of the direction from which it is approached. The mounting technology helps to position and fit the proximity switch quickly and easily.

The inductive proximity sensor SIES-Q is extremely space-saving thanks to its cuboid-shaped design. It can be flush mounted and also has an LED for indicating the switching status.

Inductive sensors

An inductive proximity sensor operates contactlessly, i.e. it does not have any direct contact when responding to an approaching metal or galvanic object.

Benefits:

  • No mechanical wear, which in turn means a longer service life
  • No downtimes due to dirty or welded contacts
  • No contact bounce and thus no switching errors
  • High switching frequencies
  • Vibration resistant
  • High degree of protection thanks to a fully encapsulated housing
  • Any mounting position

Types of sensors

They are indispensable in modern automation solutions, inductive sensors. They are sometimes also called inductive proximity sensors. Inductive displacement sensors are able to detect objects without having any contact with said object. In practice, this functionality can offer many advantages. Think for example of low wear.

Sensor inductive can only be used to detect metallic objects, because of the fact that the signal is only influenced by ferritic materials. In addition to an inductive sensor, we also have capacitive sensors. These sensors use a capacitive field, which allows a wider range of materials and substances to be detected. The differentiating characteristic of the inductive sensor is primarily the low cost price and the high level of reliability.

Inductive sensor operation

The operation inductive sensor is based on changes in an external magnetic field that has been created. In a normal situation, in which only air offers resistance, the sensor receives a stable return signal. When this return signal deviates from the norm, detection will take place, for example of metallic object.

The magnetic field is generated with the help of a spool, through which an electric current is transmitted. The spool is integrated in the top of the housing, which in many cases consists of a barrel with a metric male thread. The most commonly used thread sizes are M12, M18 and M30. When a metal object enters the field that has been created, the magnetic field is “disturbed”. This is also called impedance. The impact of the impedance is dependent on different factors, such as distance but also the metal that has been detected.

Because of the impact of different metals, these are quantified for each sensor using a reduction factor. This means that due to the fact that not every metal conducts well in the magnetic field. That is why a reduction factor is provided for each sensor, per type of metal. This factor has an effective impact on the measuring range of every sensor.

Because of the fact that inductive sensors can only detect metals, this offers opportunities for sorting, for example metal targets and other materials. In addition, inductive sensors are relatively insensitive to external factors, or non metallic objects; think of dust, dirt or deposits on the sensor. In many processes this can be a interfering factor. From an optical point of view, too, this offers advantages, as an inductive switch is insensitive to glare from (sun)light.

As part of the product segment from Festo we have designs that generate an output signal. These are available in PNP or NPN transistor versions. This can be very important for the inductive sensor’s connection diagram. The difference between both transistors is the switching principle, with PNP meaning positive-negative-positive and NPN negative-positive-negative. Before making a choice, it is advisable to determine the required switching principle and look up the specifications of the PLC used. In the EU, PNP is the most well-established choice, with the exception of the mobile automation industry (for example automotive) where NPN transistors are often still used. A combination of inductive sensor/arduino is possible, for example, as long as a PNP/NPN input has been prepared.

Proximity Sensors

When choosing inductive proximity sensors, a number of elements are of great importance.

Firstly, how does the signal need to be created? Normally open (NO) or normally closed (NC)? The normal status has, in effect, a great influence on the switching values and on the inductive sensor diagram and electrical diagram. Does it need to drop or increase. When thinking about this, safety often also plays a role, think for example of when the current drops.

Secondly, which output has to be chosen? An NPN or PNP transistor? The type can often be deduced from the symbol inductive sensor.

Thirdly, which connection is required? It is possible to choose a connector (M8 for example) or a fixed cable.

Fourthly, which protection class is required? IP67 or IP69K?

Fifthly, what is the measuring range needed. At which distance does the product still need to be detected? The fact is that magnetic induction has a limit, which translates into the maximum measuring range.

Once these questions have been answered, a correct choice can be made for the correct inductive proximity sensors.

If you have any questions about the different types of inductive proximity sensors, magnetic proximity sensors, electrical signal, stainless steel or connection type, our team will be happy to support you. Festo have a complete portfolio and we are used to work in wet or dirty conditions, with non metallic substances, ferrous metals (iron, aluminum...). We will work together to find the ideal solution.

Difference between inductive and capacitive sensors

Besides the much used inductive proximity sensor, the capacitive sensor is a second approach technology. The difference between inductive/capacitive sensor is primarily to be found in the way detection takes place. This is with a capacitive field, which in principle looks very much like induction, but is not limited to metals. This actually means that all solid materials and non-conductive materials can be detected. Think of a level measurement of fluid, oil, cardboard but also food for example. In order to move over to capacitive sensors, Festo refers to third-party supplies as Festo only offers inductive sensors.