Theory of Capacitive Proximity Sensors | How they Works?

Theory
of Capacitive Proximity Sensors | How

Capacitive Proximity Sensors Works?

This knowledge of Sensors will help you better understand customer
applications. In addition, you will be better able to describe products to
customers and determine important differences between products. You should know
Basics of Electricity and Basics of Control Components before attempting Sensors. 
Today
we learn the Theory of Capacitive Proximity Sensors and how they work? Let’s
Start.
Capacitive
proximity sensors
are similar to inductive proximity sensors. The main difference
between the two types is that capacitive proximity sensors produce an electrostatic field instead of an electromagnetic field. Capacitive
proximity switches will sense metal as
well as nonmetallic materials
such as paper, glass, liquids, and cloth.


The sensing surface of a capacitive sensor is formed by two concentrically
shaped metal electrodes of an unwound capacitor. When an object nears the
sensing surface it enters the electrostatic field of the electrodes and changes
the capacitance in an oscillator circuit. As a result, the oscillator begins
oscillating. The trigger circuit reads the oscillator’s amplitude and when it
reaches a specific level the output state of the sensor changes. As the target
moves away from the sensor the oscillator’s amplitude decreases, switching the sensor
output back to its original state.


Standard
Target and Dielectric Constant

Standard targets are specified for each capacitive sensor. The standard
target is usually defined as metal and/or water. Capacitive sensors depend on
the dielectric constant of the target. The larger the dielectric number of a
material the easier it is to detect. The following graph shows the relationship
of the dielectric constant of a target and the sensor’s ability to detect the
material based on the rated sensing distance (Sr).

The following table shows the dielectric constants of some materials.
If, for example, a capacitive sensor has a rated sensing distance of 10 mm and
the target is alcohol, the effective sensing distance (Sr) is approximately 85%
of the rated distance, or 8.5 mm.

 

Detection
through Barriers

One application for capacitive proximity sensors is level detection
through a barrier. For example, water has a much higher dielectric than
plastic. This gives the sensor the ability to “see through” the plastic and
detect the water.

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