Proximity Sensor

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Brief overview on how a Capacitive Proximity Sensor operates
Capacitive proximity sensors are capable of detecting both metallic and non-metallic targets such as powders, granulates, liquids and solids. This, along with their ability to sense through non-ferrous materials, makes them an ideal choice for level sensing applications; such as sight glass monitoring and high/low fill level detection of liquid or powder in tanks or hoppers.

Capacitive sensors mostly commonly function as an open capacitor. A capacitor can be best described as two conductors at different potentials, separated by an insulating material. In the capacitive sensor, these two plates, housed in the sensing head, are placed in a position such that they operate like an open capacitor. They use the air as an insulator: at rest there is little capacitance between the two plates. Like inductive sensors, these plates are linked to an oscillator, a Schmitt Trigger and an output amplifier (diagram). As a target enters the sensing range, the capacitance of the two plates increases, resulting in a change in the amplitude of the oscillator, which in turn changes the Schmitt Trigger state, thus creating an output signal. Note the difference between the inductive and capacitive sensors: the inductive sensor oscillates until the target is present while the capacitive does not start oscillating until the target is present. This operating principle does make the capacitive sensor a bit slower than an inductive, ranging from 10 to 50 Hz.

Many capacitive sensors are tubular in design with common sizes ranging from 12 to 30 mm in diameter with other housing styles available. Housing materials are usually metal and/or plastic (PBT). These are also available in shielded and unshielded mounting versions as well as normally open and normally closed. If the sensor is capable of operating as both normally open and normally closed it is said to have a “Complimentary Output.”

Like inductive sensors, capacitive sensors employ a limited sensing range, in most cases 3 to 60 mm. Their traditionally rugged design allows them to be mounted very close to the monitored process. Due to their ability to detect most types of materials mounting considerations such as proximity to detectable materials other than the intended target, must be considered in order to avoid false triggering. For this reason, if the intended target contains a ferrous material, an inductive sensor is a more reliable option.