Sensor Selection Guidelines, Capacitive Proximity Switch

Capacitive Switches

The active element is formed by two metallic electrodes positioned much like an "opened" capacitor. The electrodes are placed in a feedback loop of a high frequency oscillator. When no target is present, the snesor's capacitance is low, therefore the oscillation amplitude is small. When a target approaches the face of the sensor, it increases the capacitance. This increase in capacitance results in an increased amplitude of oscillation. The amplitude of oscillation is measured by an evaluating circuit that generates a signal to turn on or off the output.

Now, in plain English:

From the explanation below (left column), small wonder then that capacitive prox switches are largely misunderstood devices. Capacitive switches work off of dialectric constant, often referred to as "mass". If a target has enough dialectric constant, the switch can sense it. If the target does not have enough dialetric constant, or "mass", the switch cannot sense it. In the TURCK catalog there is a listing of all common materials and their dialectric constant ratings.

Now, in plainer English:

In the example, above left, two targets are coming into the field of the capacitive switch. The first bottle is empty; the second bottle has product in it. In both cases, the dialectric constant of the plastic bottle is insufficient for the capacitive switch to sense it. So, when the empty bottle enters the field, the capacitive switch is sensing "nothing". There is not enough "mass" for the switch to sense the bottle, and there is nothing in the bottle. Therefore, the switch detects a "no-fill" condition. The second bottle has product inside. The capacitive switch, again, cannot sense the bottle, because it does not have enough "mass", but the switch does sense the product inside the bottle, because it does have a high dialectric constant rating, and therefore can be sensed by the capacitive switch. Hence the switch detects a "fill" condition. All is well. There is product, because there is "mass".