Capacitive Proximity Sensor Working Principle


A capacitive proximity sensor is proximity or touch sensor. Unlike inductive proximity sensors, it does not rely on coupling electrical fields to sense their environment. Instead, it feels the capacitance (i.e., capacity to store an electric charge) between one conductor and another.

The capacitance may be measured between two conductors (for example, the sensing component and ground) or between one conductor and an insulating material (like air). Like other types of sensors, they come in mechanical switch form or as part of an IC package mounted on a circuit board. A proximity sensor can detect nearby objects; for example, it can see a finger moving near a computer screen or the presence of an object in a bin.

The capacitive proximity sensor works by sensing the change in the capacitance of a conductor as the object is brought nearer to it. The human body can change the local dielectric constant, which measures how easily an electric field will propagate through a material. As objects come nearer to this conductor, they alter the ability of electric fields to bear through it and cause charge separation due to like charges repelling each other. This change in dielectric constant then alters the capacitance of the conductor, and a signal is produced that can be detected by an electronic circuit that can measure these changes.

The sensor employs what is called a pair of conductors, which form an alternating-capacitance circuit. One of the conductors is grounded, while the other connects to a sensing component mounted on a printed circuit board (PCB) or glued to the surface of the material being sensed. A tiny electrical current flow into this conductor as the sensor picks up charge separation between it and its ground terminal, causing the capacitance to change due to surface electric field effects. The change in capacitance between the sensing component and ground is proportional to the distance between these two points. This signal is amplified by a voltage amplifier, which changes the output signal into a digital code, then fed into an analog-to-digital converter (ADC) or digital-to-analog converter (DAC) stored as numerical values. The ADC can convert this signal into a numerical value between 0 and 65535 (16 bits), or perhaps even higher.

How does a capacitive proximity sensor work?

The sensing component is typically a chip with a capacitor glued to it, or the circuitry can be built on printed circuit boards. These components are connected to the PCB with wires (conductors).

As an object approaches the sensor, it will change the capacitance of one of the conductors between itself and the ground. The sensing component senses this change and produces a voltage. This voltage is then used as feedback for controlling other features in an electronic circuit (for example, an amplifier for increasing its gain).

While proximity sensors work by measuring capacitance, they should not be confused with capacitive touchscreens that require conductive objects to touch two contact areas arranged in parallel.

A proximity sensor that uses electrical fields to measure capacitance is sometimes called a capacitive sensing proximity sensor.

The capacitive proximity sensor is a device that enablsenes us to detect the distance of an object. It works by measuring capacitance. Capacitive sensors are based on the principle that two conductors separated by insulating material create a capacitor when an object is brought near the conductors, their mutual capacitance changes depending on their relative permittivity and the distance between them.

The Capacitive Proximity sensor differs from inductive proximity sensors, which rely on electromagnetic induction to measure distances from objects in close proximity through changes in inductance in a resonant circuit or due to coupling of electromagnetic fields with varying impedance.

The capacitive proximity sensor is a device that can detect a change of capacitance, the value of which depends on the distance from an object. If we imagine two perfectly conducting plates that are insulated from each other and placed close to each other, a capacitor will appear that stores energy between them.

When near the plates attached to one plate (for example, hand), some of this energy is spread into space through your body, and between the plates, there is also stored part of this energy. Since the hand has been in contact with one leaf, capacitance decreases. Thus, it affects changes in capacitance. It is why it can be used as a measuring device.

What is the principle of the capacitive sensor?

The principle of this sensor is a change of capacitance. It is based on the fact that when we bring an object near a conductor, there is a change in capacitance. This change depends on the permittivity of the conductor and its distance from the thing that makes it possible to determine its length.

Capacitive sensor type

There are many types of sensors, but we can divide them into three main categories: mechanical, optical, and electrical. The first two are parts of automatic machines and only detect changes in objects at certain distances. In contrast, electrical sensing changes in the space only between conductors and not within any other medium (for example, air).

The capacitive proximity sensors are typically based on a resistive photo sensor invented in 1959 by William E. Buhl. The resistive photo sensor uses a transparent conductive film coated with an opaque layer that changes the light incident onto it into an electrical signal proportional to the distance between the source and the detector.

The principle of this device is similar to that of carbon arc or infrared detectors, which use some material such as phosphor or a photosensitive surface to emit photons. Still, this time around, we detect electrons that have passed through our object instead of detecting infrared photons. In addition, we can control the amount of sensitivity and resolution by adjusting the resistance at the input (solder) and output terminals (connector).

The output voltage of a resistive photo sensor is directly proportional to the intensity of the incident light. This output voltage can then be transformed into an analog signal (usually with an op-amp) that can be read by a microcontroller and then converted into a digital code. This digital code can control other electronic systems such as motors, LEDs, or computers.

Photo resistive sensors are sensitive to visible light and infrared radiation, making them incredibly useful in detecting objects without touching them.