Capacitive sensors operate based on changes in electrical capacitance. Capacitive sensors are the most precise of all electrical sensors (including resistive and inductive sensors) and are known for their extremely high sensitivity, high resolution (e.g., 0.01 nm), broad bandwidth (e.g., 1~100 kHz), robustness, long-term stability and durability, drift-free character, simple structures, low cost, and noncontact detection features. Most capacitive sensors are immune to humidity, temperature, target material, and stray electric field variations. Some can be integrated into a printed circuit board (PCB) or embedded into a microchip or a nanodevice to provide excellent accuracy and nearly infinite resolution, higher reliability, less weight, and lower power consumption.
The earliest capacitive sensing can be traced back to 1600 when William Gilbert experimented with frictional electrical charges on objects. He found that electrical charges cause objects to attract or repel each other. This attracting or repelling force was greatly affected by the distance between the objects. In 1745, the first capacitor, the Leyden jar, was invented independently by Ewald Georg von Kleist and Pieter van Musschenbroek.
Capacitive sensors, also called detecting probes, are traditionally divided into passive or active types, depending on whether or not there are any electronic components in the probe. Passive sensors do not come with any electronics; thus the sensor sizes can be minimized. They are also more flexible in probe design, more stable, and less expensive. Their disadvantages include cable length restrictions (≤3 m), narrow bandwidth, and lower drive frequency. The active sensors, on the other hand, have electronics in the probe. The electronics can be as simple as a few diodes, or as complex as an integrated circuit board (typically enclosed within the guard shield). Active sensors are not restricted by the cable length. They operate at much higher frequencies with broader bandwidths, and are particularly suitable to applications that involve stray electrical noise on the target. The disadvantages of active sensors are their higher cost and lesser design flexibility.
Capacitive Sensor Clasifications
Capacitive sensors can also be classified based on their configurations (e.g., parallel, cylindrical, and spherical), dielectric materials (e.g., polymeric and fluidic), measurants (e.g., acceleration and CO), mechanisms that cause capacitance change (e.g., space variation and area variation), or manufacturing means (e.g., micromachined and macroscopic). Micromachined capacitive sensors are built directly on a silicon wafer, usually integrated with an application-specific integrated circuit (ASIC) for high reliability and low production cost. A macroscopic capacitive sensor is usually manufactured as an individual component or is packaged on a PCB.
Capacitive sensors find their applications in precision motion detection, coating thickness gauging, liquid level and flow rate monitoring, pressure or force measurement, diamond turning, occupancy identification, fingerprint acquisition, chemical element selection, biocell recognition, engine rotational alignment, as well as on keyswitches, touchpads, and touchscreens.
Capacitive Sensors Base
Capacitive sensors based on their configurations (flat/parallel, cylindrical/coaxial, spherical/concentric, and array) and their sensing mechanisms that cause capacitance changes through
- Spacing variation: varying the space or distance between plates
- Area variation: varying the overlap area between plates
- Electrode property change: changing conductivity, charges, mass, or other physical or chemical properties of the electrodes
- Dielectric material property change: changing properties of the dielectric media
In each configuration, the sensing principles will be discussed first, followed by their features, designs, and applications. To assist readers understand these principles better, an overview of capacitors, capacitance, and associated physical laws.
I hope this information about “Capacitive Sensors” is easy to understood.