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What is Capacitive Sensor?
   It is a proximity sensor that detects nearby objects with the electric field it creates. Simple capacitive sensors have been commercially available for many years and are one of the solutions for non-metallic object detection for the machine industry. However, they have short working distances (max 4cm).

   The way a capacitive proximity sensor works is very similar (theoretically) to the working principle of a capacitor. If one of the conductive elements or plates is inside the sensor, the other is the object to be detected. The inner plate is connected to an oscillator circuit that generates an electric field. The air gap between the inner plate and the outer object acts as the insulator or dielectric material. When an object is found, it changes the capacitance value and saves it as the existence of the object.

   Attention is important; As we mentioned above, the object opposite represents the other pole of the capacitor. So, for example, a capacitive sensor may not respond to an object at a distance of 2mm, but to an object at a distance of 1 cm. Unfortunately, this fact is ignored, and from time to time, our customers make the mistake of thinking that the capacitive sensors they receive are defective.


   One of the most common types of sensors is the capacitive proximity sensor. As we mentioned in the description, capacitive proximity sensors work by indicating a change in the capacitance read by the sensor. A typical capacitor consists of two conductive elements (sometimes called plates) separated by some type of insulating material, which can be one of many different types, including ceramic, plastic, paper, or other materials.

   Capacitive sensors are relatively weak in their ability to detect conductive materials while seeing insulating materials such as wood or plastic. Therefore, they may not work as accurately as metal-to-metal inductive sensors. They have simpler structures, so they are potentially smaller, cheaper, and consume less power.



   They are not used for distance estimation or distance measurement. They only give on-off straight logic digital output. They do not give a direct indication of how far away the detected object is. A more distant and stronger target may give the same response as a weaker target nearby.

What Can It Detect?

   Due to its non-directional nature, the capacitive sensor measures some capacitance from objects in the medium that are always present and therefore no additional signal objects. Commercial capacitive sensors typically operate at intervals of 4 cm or less. In these ranges, the object capacitance approaches the background capacitance. However, the capacitance change at 1 meter is orders of magnitude smaller and much less than the background capacitance. It serves to detect objects that we cannot detect with inductive sensors such as plastic, wood, water, cement, grain.


   How do capacitive sensors work?
   Capacitive sensing is proximity sensing technology. Capacitive sensors work by detecting nearby objects by creating an electric field and detecting if this field is disturbed. Capacitive sensors can detect anything that has a significantly different permeability from conductor or air, such as the human body or hand.

   Capacitance (capacitor) and Distance

   Non-contact capacitive sensors work by measuring changes in an electrical property called capacitance. Capacitance describes how two conductive objects with a gap between them respond to the difference in voltage applied to them. When a voltage is applied to the conductors, an electric field is created between them which causes positive and negative charges to accumulate on each object. If the polarity of the voltage is reversed, the charges will also be reversed.

   Capacitive sensors use an alternating voltage that causes the charges to constantly reverse their position. The movement of charges creates an alternating electric current, which is sensed by the sensor. The amount of current flow is determined by the capacitance, and the capacitance is determined by the area and proximity of conductive objects. Larger and closer objects cause greater current than smaller and farther objects. Capacitance is also affected by the type of non-conductive material in the space between objects.

   Technically, capacitance is directly proportional to the surface area of ​​objects and the dielectric constant of the material between them, and inversely proportional to the distance between them.

   In typical capacitive sensing applications, the probe or sensor is one of the conductive objects. The target object is the other. (Using capacitive sensors to detect plastics and other insulators is covered in the section on non-conductive targets.) The dimensions of the sensor and target are assumed to be constant, as is the material between them. Many capacitive sensors have an integrated ambient reset trimpot. Therefore, any change in capacitance is the result of a change in the distance between the probe and the target. Electronic circuits are calibrated to produce specific voltage changes for corresponding changes in capacitance. These voltages are scaled to represent specific changes in distance. The amount of voltage change for a given distance change is called sensitivity. A common sensitivity setting is 1.0V / 100µm. This means that for every 100µm change in distance, the output voltage changes exactly 1.0V. With this calibration, a +2V change in output means the target is 200µm closer to the probe.



What is capacitance (capacitor)?

Capacitance describes how the spacing between two conductors affects the electric field between them. If two metal plates are placed with a gap between them and voltage is applied to one of the plates, an electric field will exist between the plates. This electric field is the result of the difference between the electric charges stored on the surfaces of the plates. Capacitance refers to the "capacity" of the two plates to hold this charge. A large capacitance has more charge holding capacity than a small capacitance. The amount of charge available determines how much current must be used to change the voltage across the plate. It's like trying to change the water level by 1mm in a bucket compared to a teacup. It takes a lot of water to move the level one mm in the chamber, but a teacup takes less than a bucket.

When using a capacitive sensor, the probe's sensing surface is the electrical plate, and what you're measuring (the target) is the other plate (we explained above about measuring non-conductive targets) the driver electronics constantly changes the voltage across the sensing surface. This is called excitation voltage. The amount of current required to change the voltage is measured by the circuit and indicates the amount of capacitance between the probe and the target. Conversely, a constant amount of current is pumped in and out of the probe and the resulting voltage change is measured. This change is sent to the machine center (maybe PLC or other receivers) as a digital signal.

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