What are capacitive sensors and what are they used for?

Professional capacitive sensors measure a variety of industrial variables, e.g. the quantity of granulate, the fill level of liquids or the filling of tanks. Another task of capacitive sensors is the detection of metallic and non-metallic materials in a stack, such as paper elements or liquid in a conductive element. The versatility is the great advantage of these sensors.

The operating principle of non-contact capacitive sensors is akin to that of a capacitor, relying on the measurement of changes in capacitance. This parameter reflects the electric field’s characteristics and describes how two conductive bodies respond to a voltage difference applied between them. In the upcoming sections, we will provide a more detailed explanation of the sensors’ measuring functions and operating principle, and compare their components with those of inductive sensors.

Capacitive sensor - what is it?

Industrial capacitive sensors are electronic devices that produce an output signal when they detect the presence of an object within their magnetic field. The signal is processed by an electronic circuit and microprocessor, which enables tasks such as material detection and distinguishing between plastic, metal, and wood.

These sensors are often referred to as capacitive proximity sensors because they detect changes in the electromagnetic field caused by the approach of an object. The primary distinction between capacitive and inductive proximity sensors is in their design.

Inductive sensors rely on an embedded coil for detection, whereas capacitive sensors consist of a cover that functions as a capacitor and the surface of the object being detected. Unlike inductive sensors, capacitive sensors are more resilient to failure and mechanical damage

What materials does a capacitive sensor detect?

A primary application of capacitive sensors is the non-contact detection of both electrically conductive and non-conductive materials. These sensors are typically used to detect solids such as plastics, metals, and wood, as well as various liquids. Their ability to detect liquids makes them ideal for measuring the fill level and detecting the presence of liquid in bottles.

How does a capacitive sensor work?

We have briefly discussed the functional principle and applications of capacitive proximity sensors. These sensors operate similarly to capacitors. An alternating voltage is applied to the sensor’s electrodes, causing the positive and negative charges to shift positions continuously. This movement generates an electric field, and the changes in this field form the basis of the sensor’s measurement.

The proximity sensor acts as one cover of a capacitor, while the object being measured serves as the other cover. The capacitance in the measurement system is directly proportional to the surface area of the two conductors and inversely proportional to the distance between them. Additionally, the relationship between capacitance and the dielectric constant of the material helps identify the nature of non-metallic objects.

A key factor influencing the measurement is the switching frequency, which is typically in the range of a few tens of Hertz. This makes capacitive sensors less suitable for measuring systems with rapid dynamic changes. The standard sensitivity of these sensors is 1 V/0.1 mm, meaning that for every 0.1 mm change in the sensor-object distance, there is a corresponding 1 V change in the output voltage.

When selecting a sensor, it is important to consider factors that may negatively affect the measurement. One of the key elements influencing measurement accuracy is electromagnetic interference and contamination between the sensor and the object. These factors can impact the change in the dielectric constant. Additionally, it is crucial to consider the temperature limits specified by the manufacturer and the sensor’s susceptibility to corrosion.

Application of capacitive sensors in various industries

The versatility of capacitive sensors allows them to be utilized across numerous industries, including food, automotive, and logistics, with industry being the primary beneficiary of their advantages.

In particular, precise capacitive proximity sensors are used for non-contact detection of packaging contents, checking the fill levels of liquids in bottles, and monitoring conditions in various processes. Additionally, they play a crucial role in detecting the orientation of products on conveyor belts, ensuring efficient and accurate handling of materials.

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