If someone says proximity sensor, what is the first thing that comes to mind? My guess is inductive and justly so because they are the most used sensor in automation today. There are other sensing technologies that use the term proximity in describing the sensing mode. These include diffuse or proximity photo electric sensors that use the reflectivity of the object to change states and proximity mode of ultrasonic sensors that use high frequency sound waves to detect objects. All of these sensors detect objects that are in close proximity of the sensor without making physical contact.
One of the most overlooked or forgotten proximity sensors on the market today is the capacitive sensor. Why? Perhaps it is because they have a bad reputation from when they were released years ago, as they were more susceptible to noise than most sensors. I recently heard someone say that they don’t discuss capacitive sensors with their customers because they had a bad experience almost 10 years ago. With the advancements of technology this is no longer the case.
Recently, I was visiting a packaging customer that had an A-frame device that automatically dispensed various sized packages with various types of packaging. The packaging varied from dull or dark cardboard to glossy plastic. He was trying to come up with a sensor that would signal a low-level in a gravity fed bin and was having issues with photo electrics producing false triggers. I suggested a capacitive sensor and he looked at me and said “I never thought of using a capacitive sensor”.
Capacitive sensors are versatile in solving numerous applications. These sensors can be used to detect objects such as glass, wood, and paper, plastic, ceramic, the list goes on and on. The capacitive sensors used to detect objects are easily identified by the flush mounting or shielded face of the sensor. Shielding causes the electrostatic field to be short conical-shaped much like the shielded version of the inductive proximity sensor.
Just 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. Since capacitive sensors can detect virtually anything, they can detect levels of liquids including water, oil, glue and so forth and the can detect levels of solids like plastic granules, soap powder, sand and just about anything else. Levels can be detected either directly, the sensor touches the medium or indirectly where the sensor senses the medium through a non-metallic container wall.
The sensing distance of a capacitive sensor is determined by several factors including the sensing face area, the larger the better. The next factor is the material property of the object or dielectric strength, the higher the dielectric constant the greater the sensing distance. Lastly, the size of the target affects the sensing range. Just like an inductive sensor you want the target to be equal to or larger than the sensor.
Most capacitive sensors have a potentiometer to allow adjustment of the sensitivity of the sensor to reliably detect the target. The maximum sensing distance of a capacitive sensor is based on a metal target thus there is a reduction factor for non-metal targets.
Capacitive sensors are ideal for detecting non-metallic objects at close ranges, usually less than 30mm and for detecting hidden or inaccessible materials or features. Just remember, there is one more proximity sensor…the capacitive one!