What is a capacitive sensor?
The capacitive sensor is an electronic switch that works without contact. These sensors take advantage of the effect of materials such as paper, glass, plastic, oil, water, as well as metals, to increase the capacity of the sensor when they are within the generated electric field.
They consist of a capacitor that generates an electric field. This capacitor is part of a resonator circuit so that when an object approaches this field, the capacity increases and the circuit begins to resonate.
Difference between capacitive and inductive
A cylindrical capacitive sensor is hardly distinguishable from the inductive sensor, as manufacturers typically employ identical housings. What the object does, being within the electric field, is to increase the capacity of that area, and therefore change its capacitance; This causes the sensor’s internal circuit to resonate.
The main difference between the sensors is that capacitive sensors produce an electrostatic field instead of an electromagnetic field
How does it work?
The sensing surface of the capacitive sensor is made up of two concentric metal electrodes of a capacitor. When an object approaches the sensing surface and it enters the electrostatic field of the electrodes, the capacitance in an oscillator circuit changes. This causes the oscillator to start oscillating. The trigger circuit reads the amplitude of the oscillator and when it reaches a specific level the output stage of the sensor changes. As the object moves away from the sensor the oscillator amplitude decreases, switching the sensor to its original state.
The capacitive sensor works inversely compared to an inductive sensor.
Advantages and disadvantages
It is important to note that the advantages of these sensors have to do with the fact that they detect all kinds of metallic elements, in addition to the fact that they can “see” through some materials and have many installation configurations in addition to having a fairly long service life.
However, it is also important to note that capacitive sensors have a short detection distance that varies depending on the material to be detected, and at the same time they are extremely sensitive to environmental factors.
Types of capacitive sensors
There are DC and AC capacitive sensors.
There are 2, 3 and 4-wire CDs, with sensing distances from 5mm to 20mm
Liquid level detector
Fluid detection in containers such as milk in cardboard cans
Barrier detection (for example water has a much higher dielectric constant than plastic this gives the sensor the ability to see through plastic and detect water