ABOUT CAPACITIVE PROXIMITY SENSORS

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

Capacitance is a function of the surface area of either electrode, the distance between the electrodes, and the dielectric constant of the material between the electrodes.

When a conductive target enter the sensor's field, it forms a counter electrode to the active face of the sensor, thus decreasing the distance between electrodes and increasing the average surface area of the electrodes. The capacitance with a metal target present is always greater than the capacitance of the circuit in the abscence of the target. Reduction factors for different metals for different metals are not a consideration when using capacitive sensors.

When a non-conductive target enters the sensorenters the sensor's field, it acts as an electric insulator between electrodes. The dielectric constant of the material (e symbol) is a measure of its insulation properties. All liquids and solids have a greater dielectric constant than air (e symbol = 1). Therefore, the capacitance with a non-metallic target present is always greater than the capacitance of the circuit in the absence of a target. This change is nevertheless very small, so only a short sensing range can be achieved.

The nominal sensing ranges refer, as with inductive sensors, to a metal target (mild steel). On different materials, different sensing ranges apply.


LINKS

Capacitive Proximity Sensors Product Info (PDF)