Capacitive Sensors: Versatile enough for most (but not all) detection applications

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Capacitive sensors are versatile for use in numerous applications. They can be used to detect objects such as glass, wood, paper, plastic, ceramic, and more. Capacitive sensors used to detect objects are easily identified by the flush mounting or shielded face of the sensor. This shielding causes the electrostatic field to be short and conical shaped, much like the shielded version of an inductive proximity sensor.

capacitive 2Just as there are non-flush or unshielded inductive sensors, there are non-flush capacitive sensors, and the mounting and housing look the same. The non-flush capacitive sensors have a large spherical field which allows them to be used in level detection, including detection of liquids and granular solids. Levels can be detected either directly with the sensor making contact with the medium, or indirectly with the sensor sensing the medium through a non-metallic container wall.

Capacitive sensors are discrete devices so once you adjust the sensitivity to detect the target while ignoring the container, the sensor is either on or off. Also remember that the sensor is looking for the dielectric constant in the case of a standard capacitive sensor or the conductivity of a water based liquid in the case of the hybrid technology.

Recent technology advances with remote amplifiers have allowed capacitive sensors to provide an analog output or a digital value over IO-Link. As previously mentioned, these sensors are based off of a dielectric constant so the analog value being created is dependent on the media being sensed.

While capacitive sensors are versatile to work in many applications, they are not the right choice for all applications.

Recently a customer inquired if a capacitive sensor could detect the density of an substance and unfortunately the short answer is no, though in some applications the analog sensors can detect different levels of media if it can be separated in a centrifuge. Also, capacitive sensors may not detect small amounts of media as the dielectric constant of the media must be higher than the container that holds the media.

There are three important steps in applying a capacitive sensor — test it, test it and test it one more time. During your testing procedures be sure to test it under the best and worse conditions. Also like any other electronic device temperature can have an affect although it may be negligible there will be some affect.

For more information on capacitive sensors visit www.balluff.com.

Sensing Types of Capacitive Sensors

Similar to inductive sensors, capacitive sensors are available in two basic versions.  The first type is the flush or shielded or embeddable version however with capacitive sensors they are sometimes referred to as object detection sensors.  The second type is the non-flush or non-shielded or non-embeddable version however again with capacitive sensors they are sometimes referred to as level detection sensors.

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The flush or object detection capacitive sensors are shielded and employ a straight line electrostatic field.  This focused field is emitted only from the front face of the sensor allowing the sensor to be mounted in material so that only the face of the sensor is visible.

The highly focused electrostatic field is perfect for detecting small amounts of material or material with low dielectric constant.  The typical range of a flush 18mm capacitive sensor is approximately 2 to 8mm depending on the objects dielectric constant.  As with any capacitive sensor the sensor should be adjusted after installation.

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If the sensors are mounted adjacent to each other the minimum gap should be equal to the diameter or the adjusted sensing distance whichever is less.  These sensors can also be mounted opposing each other however the distance should four times the diameter of the adjusted sensing distance whichever is less.

CapacitiveTypes3Shielded or flush capacitive sensors are perfect for detecting solids or liquids through non-metallic container walls up to 4mm thick.  If you are detecting liquid levels through a sight glass with the sight glass mounting bracket then the flush mounted sensor is the preferred choice.

CapacitiveTypes4The non-flush or level detection capacitive sensors are not shielded and employ a spherical electrostatic field.  This field is emitted from the front face of the sensor and wraps around to the sides of the sensor head.  Unlike the flush sensor this version cannot be mounted in material where only the face of the sensor is visible.  Non-flush sensors have better characteristics and better performance in applications with adhering media.

CapacitiveTypes5The spherical electrostatic field provides a larger active surface and is perfect for detecting bulk material and liquid either directly or indirectly.  The typical range of a flush 18mm capacitive sensor is approximately 2 to 15mm depending on the objects dielectric constant.  As with any capacitive sensor the sensor should be adjusted after installation.

CapacitiveTypes6If the sensors are mounted adjacent to each other the minimum gap should be equal to three times the diameter or the adjusted sensing distance whichever is less.  These sensors can also be mounted opposing each other however the distance should four times the diameter of the adjusted sensing distance whichever is less.

Shielded or flush capacitive sensors are perfect for detecting solids or liquids through non-metallic container walls up to 4mm thick.  If you are detecting liquid levels through a sight glass with the sight glass mounting bracket then the flush mounted sensor is the preferred choice.

Capacitive sensors are perfect for short range detection of virtually any object regardless of color, texture, and material.

To learn more about capacitive sensors visit www.balluff.com.