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A capacitive proximity sensor is one of a class of sensors, called proximity sensors, which detects objects without actually touching them. Photoelectric sensors and inductive proximity sensors are also found in this category. The capacitive proximity sensor detects objects based on their dielectric strength, and has many uses which capitalize on this feature.
Capacitor
The heart of a capacitive proximity sensor is one capacitor plate (half a capacitor). A capacitor is one of the three basic passive elements in electric circuitry, along with inductors and resistors. While resistors resist electric current and inductors store electric current, capacitors store electric charge. A capacitor takes the form of two conductive plates separated by a dielectric material. A voltage difference applied to these plates creates an electric field across the dielectric between them. This electric field is what stores the electric charge, and if the source of electricity is cut off the electric field will collapse and release its energy as a voltage falling asymptotically toward zero from its initial level. Storing and releasing charge has the effect of dampening out changes in voltage in the circuit. This makes capacitors very useful for decoupling low voltage circuits.
Capacitance
The capacity for a capacitor to store charge as an electric field is called capacitance, and is measured as a unit called the Farad, after English physicist Michael Faraday (1791–1867), who conducted groundbreaking research which, among other things, lead to the development of Maxwell's Equations. Capacitance is influenced by the surface area of the conductive plates, the distance between them, and the dielectric strength of the material between the plates. Water has a very high dielectric constant, with a value of about 80, while air has a low constant of about 1. Most other materials fall somewhere between these values.
Capacitive sensor
The capacitive sensor is one half of a capacitor, that is, one capacitive plate mounted on the front of the sensor. When an object passes in front of the sensor, it acts as the second plate in the capacitor as well as the dielectric material, and the sensor measures the capacitance produced by this arrangement. So long as the object has a dielectric constant dissimilar to air, it will be detected at a short range. When the dielectric changes enough, it triggers the sensor's output, which sends a signal to some other device (usually a relay or a PLC) to do whatever it is it is supposed to do when an object is near the sensor.
Due to the capacitive sensor's ability to, basically, detect dielectric strength, it can be calibrated to ignore low dielectric values and detect high dielectrics. This finds application in certain manufacturing processes in which it is useful to see through things. If the contents of a box have a significantly higher dielectric than the box material, the capacitive sensor can be used to see through the box and detect the material inside, confirming the box is not empty or that it has been filled to some level. They are also used on sight glass tubes for level detection on bulk liquid storage tanks. The sensor will ignore the sight glass itself and detect the liquid inside of it, providing feedback for the liquid level in the bulk tank.
More often, however, the capacitive sensor is used as a replacement sensor for applications ill-suited to photoeye detection. Photoelectric sensors have difficulty with materials that are dark, highly reflective, or clear, but a capacitive sensor ignores all these properties. Unlike the inductive proximity sensor, which only detects metals, capacitive sensors function as efficient, all-purpose sensors to detect nearly any material.
Disadvantages
Capacitive proximity sensors do have a few drawbacks though. They typically have a very short range, because the dielectric influence of objects nearby drops off sharply with distance. Unlike the inductive proximity sensor, the capacitive sensor can fail if it gets too dirty because it will detect the dirt. Likewise it will have trouble detecting any material which has a dielectric constant similar to air, as these substances will be nearly invisible to it. Although they can be adjusted to be more sensitive, making them too sensitive can result in nuisance trips and false signals. Although the dielectric constant of air is measured as 1.0005, this is under ideal conditions. Very sensitive capacitive sensors can sometimes be triggered by a large increase in relative humidity. | 
