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Inductive vs. capacitive sensors: differences and how it works

In this article we will discuss the differences between the inductive and capacitive proximity switches, and explain how they work with a number of applications as an example.
Proximity switches: Inductive sensors from Sensor Partners

What are proximity switches?

Before we delve deeper into the difference between inductive and capacitive sensors, it is first important to know exactly what a proximity switch is. A proximity switch, also called a proximity sensor, is a sensor that can detect the presence of nearby objects without having to make physical contact. This sensor works on the following principle: electromagnetic signal is emitted and it looks for changes in the return signal. When the sensor detects an object, the return signal will be shorter than when it detects no object.

The main advantage of a contactless inductive/capacitive sensor, compared to a contact-based sensor, is that it has a longer functional life. This is because no damage and wear can occur, due to the lack of contact between the sensor and the object. Difference in the material of the object to be detected determines the choice for the type of proximity sensor. For example, a capacitive proximity sensor may be suitable for a plastic object; an inductive proximity sensor always requires an object made of ferrous metal.

In addition to the material of the object, the questions below are crucial foer choosing the correct sensor:

  • Within which measuring range does the detection have to be done?;
  • Which protection class is necessary? (IP67, IP68 or IP69K?);
  • Which switching output must be chosen? (PNP or NPN)
  • How does a switching output work? (NO/NC)
  • How can the sensor be mounted in the application? (flush or non-flush);
  • Which type of connection is needed? (Connector or fixed cable)

What is an inductive sensor?

An inductive proximity switch is a sensor that is used in order to detect metal objects. The sensor does this in a way thast is insusceptible to shape or color of the object. An important property of inductive sensors is that these are cheap and reliable .

How does an inductive sensor work?

Inductive sensors work on the principle of change of inductance. What exactly is this? The sensor generates an electromagnetic field with the help of the electromagnetic coil located in the head of the sensor. In operation, this means that when a metal object comes close to an inductive sensor, the so-called impedance in the coil changes. The change of this impedance depends on the distance between the metal object and the sensor.

An important advantage of inductive sensors is that they are insensitive to non-conductive materials such as plastic, rubber and stone. In addition, the sensors are therefore not susceptible to external environmental factors such as accumulations of dust and dirt that can cover the sensor head or the exposure to (sun) light that can cause false detection with other sensors.

The most commonly used inductive sensor has a round construction, M18 housing with M12 connector and a switching distance of 8 mm. Examples of this are:

The reduction factor in inductive sensors

Warning! Inductive sensors have a so-called reduction factor. To understand what this means, it is necessary to come back to the way inductive sensors work; the difference in impedance in the coil allows the sensor to detect the metal object. However, not every type of metal conducts the energy of the electromagnetic field generated by the sensor equally well! That is why we work with a so-called reduction factor that differs per metal and sensor.

The reduction factor associated with a metal determines the sensor's performance and affects its effective measuring range. Ask one of our experts! Below is a table to indicate the reduction factor per metal type:

Numbers are indicative and can vary per model and material. Ask about it with our experts! (Source: MD - complete catalogue.)

What is a capacitive sensor?

Capacitive sensors can detect both metal and non-conductive materials. Think of viscose, but also aqueous liquids such as (petroleum) oil and water or all kinds of other solids, such as: plastics, stainless steel, brass, cardboard and animal materials such as leather. These are just a few of the many possible substances.

Detecting liquids or products can be used for the presence detection of grain in a feed silo, for example as a level measurement or full and empty detections. The shape and color of the object have no influence on the result. Examples of capacitive proximity switches are:

How does a capacitive sensor work?

The way capacitive sensors work is almost the same as that of inductive sensors. The sensor "looks" by means of an active capacitive field (also called dielectric). When this field changes, the sensor will detect. The air is in many cases the constant, when an object comes close to the sensor, the capacitive field changes. The object that passes the sensor has a higher density than air, so the sensor switches.

Mounting of inductive and capacitive sensors

Both inductive and capacitive sensors have two types of mounting: flush and non-flush. The mounting method may differ depending on the sensor model. The way in which the sensors are mounted in the application has an influence on its detection range and the moment when an object is detected.

In a flush mounting, the sensor is mounted so that the sensor head runs flush with the mounting surface. In a non-flush mounting, the sensor head protrudes above the mounting surface. This has direct consequences in the detection: a sensor with a flush mounting will only detect objects that appear directly in front of the sensor head. The sensor with non-flush mounting is also interrupted by objects that appear on the sides of the sensor head. This type of sensor can achieve a larger detection range.

Areas of application of proximity switches

To provide more insight into the applicability of a sensor. And to help you making a choice, industrial applications and examples are given below.

Applications inductive sensors

To further explain the difference between inductive and capacitive sensors, here is more information about inductive sensors. As mentioned earlier, an inductive sensor, also known as a proximity switch, is a simple, reliable and inexpensive solution to determine a position or end position. Therefore there are countless application examples for this, such as:

  • An elevator that arrives on a story floor;
  • A conveyor belt that passes at a specific spot;
  • Car wash installation, to determine the end and starting point;
  • Overhead cranes, to determine the start or end point;
  • As an encoder to detect metal strips, in order to determine position;

A short M18 sensor is often used for this, with a switching distance of 8 mm. Such as: AK1/AP-3H.

Below are some more specific applications of inductive sensors.

Position control of valves in the food processing industry

In applications with a high (er) hygiene standards, inductive proximity switches ensure the transition between production and cleaning processes. For example, the sensor switches when supplying cleaning agents or ingredients. 

By detecting the distinction between these two materials, the ingredients or cleaning agents are not supplied by the machine. The sensor ensures that the valve remains open or closed at the right time.

A sensor that comes in handy here is the PFM1/BP-3H from the PFM series of Micro Detectors. With a sealing degree of IP69K and a ECOLAB-certification this is a sensor specially designed for the food & beverage industry.

Position detection of mechanically moving objects

Placing objects or packaging on a conveyor belt is a simple process. However, great precision is necessary. Reliable detection and consistent alignment can be achieved through inductive sensors. The result is less waste and system interruptions.

Inductive sensor to detect moving objects

A suitable inductive sensor is the AE1/AP-3A from Micro Detectors. With a detection range of 2 mm and a fixed cable connection, it is a reliable solution for detections in pick-and-place applications.

Motion monitoring by gear detection

Different parts form the drive of a transport system. This includes gears. Due to the consistent and synchronized detection of the teeth, the drive of the gears can blend more easily.

For this application the AK1/AP-1H of Micro Detectors is ideal. The gears are detected flawlessly by this inductive sensor, so that its rotation can be monitored.

Full body INOX inductive

Challenging applications can be a burden for sensors. Inductive sensors are often made entirely of metal in these types of applications. This is also called “Full Body INOX”. These are corrosion-resistant and insusceptible to many aggressive industrial chemicals.

A good example of this type of sensor is the M18 sensor made entirely of stainless steel 316 and with IP67, IP68 and IP69K protection class: FMK6/BP-3H.

Applications capacitive sensors

Below are some more specific applications of capacative sensors.

Level monitoring of liquids

Capacitive proximity switches are used to determine and monitor the level of a liquid in a tank or basin. The sensor can detect from the outside whether a tank is sufficiently or insufficiently filled and thus provides the necessary information to start or stop the supply of liquid to the tank /silo /bassin.

A capacitive proximity switch that can be used here is the  C18P/BP-1E  from Micro Detectors, which can detect other materials in addition to liquids. Within a switching range of 8 mm.

Filling notification of bottles

Another application is to report whether a bottle on a conveyor belt is filled or not with, for example, soft drinks. By means of this determination, partially filled bottles can be separated from the properly filled bottles. The goal is to increase the quality of the end product.

A suitable sensor for this application is the C18P/BP-2E. This is a capacitive sensor with a detection range of 12 mm.

Non-metallic object detection on conveyor belt

Capacitive proximity sensors can be used to detect and count non-metallic objects such as glass, paper, cardboard, plastic or wood. This is possible because the sensor can detect more than one type of material unlike the inductive proximity switch. Think of the detection of plastic casings over a conveyor belt.

A suitable sensor is the  C30P/BP-2E of Micro Detectors in an M30 housing and a range of 25 mm. As a result, both smaller and larger packages are detected.

The difference between inductive and capacitive sensors

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