Capacitive Sensors

The concept of Capacitive sensing relies on the phenomenon of capacitive coupling. In this technique, human body capacitance is the input to the system. In this article, you will read about capacitive sensors, their working principle and applications.

What is a Capacitive Sensor?

The non-contact device that can detect or sense the presence or absence of any object virtually irrespective of its material is known as a capacitive proximity sensor. They use the alteration of capacitance based on the change in the electrical field and the electrical property of capacitance and everywhere around the active face of the sensor.

Capacitive sensors can sense anything. The object should have a dielectric different from that of air, or it must be conductive; the only conditions for the detection by capacitive sensing.

This technique is used by many types of sensors. The main applications consist of the sensors used to detect position or displacement, proximity, fluid level, humidity and acceleration. Capacitive sensors can also replace mechanical buttons. Capacitive sensing touch screens as input devices are used by some modern technical devices such as digital audio players, mobile phones & tablet computers.

Working Principle of a Capacitive Sensor

Along with a sensing surface formed by two metal electrodes, the capacitive proximity sensor involves a high-frequency oscillator. When an object comes near the sensing surface, it changes the capacitance of the oscillator by entering into the electrostatic field of the electrodes. Due to this, the oscillator circuit begins to oscillate and variates the output state of the sensor when it reaches a certain amplitude. The oscillator’s amplitude decreases as the object moves away from the sensor, switching the sensor back to its initial state.

If the value of the dielectric constant of a target is larger, then it becomes easier for the capacitive proximity sensor to detect it. Regardless of the type of material being sensed, they naturally have a short sensing range of about 1 inch.

With an increase in the sensing surface size of the sensor, the surface area of the target and the dielectric constant of the target, the sensing distance increases while dealing with non-conductive targets.

In an environment with relatively constant temperature and humidity, capacitive sensors should be used for the best operational results.

Major Characteristics of Capacitive Proximity Sensor

The most important characteristics of capacitive sensors are:

• Non-metallic targets can be detected using capacitive proximity sensors.
• Mechanical limit switches that are unable to detect lightweight or small objects can be detected using capacitive proximity sensors.
• In object counting applications, the capacitive proximity sensors provide a high switching rate for rapid response.
• Capacitive proximity sensors can help to detect liquid targets through non-metallic barriers.
• With a virtually limitless number of operating cycles, capacitive proximity sensors have a long operational life.

Advantages of Capacitive Sensing

1. Capacitive sensing can sense diverse kinds of materials like metal, liquid, skin and plastic.

2. Capacitive sensing is wear-free and contactless.

3. With small sensor sizes, capacitive sensing has the capability to sense up to a large distance.

4. Capacitive sensing is a low power solution.

Points to Keep in Mind while Designing a Capacity Sensor

1. Maximise capacitance with large-area, close-spaced plates. Design electrode plates to measure the desired variable.

2. To handle lost capacitance from other circuits, surround the sensor with suitable guard or shield electrodes.

3. Calculate stray capacitance, output signal swing and sensor capacitance.

4. For low noise, choose a high enough excitation frequency. External and circuit-generated noise decreases when the excitation frequency increases.

5. Design circuit to provide immunity to environmental challenges and meet the accurateness of the specifications.

Charge Distribution between Capacitors:

Some Applications of Capacitive Sensing

1. Flow: Flow is converted into pressure or displacement, using many types of flow metres and orifices for volume flow, where displacement can be measured using a capacitive sensor.

2. Pressure: With the help of a spacing-sensitive detector and a diaphragm with steady properties of deflection, pressure can be measured.

3. Liquid Level: By the measurement of changes in capacitance values between conducting plates that are immersed in the liquid, capacitive liquid level detectors applied to the outside of a non-conducting tank sense the liquid level in a reservoir.

4. Spacing: The mutual capacitance is very sensitive in measuring spacing if a metal object is near a capacitor electrode.

5. Multi-plate scanned sensor: By using several plates, each addressed separately, the single-plate spacing measurement can be extended to contour measurement. Both dielectric conductive surfaces can be measured.

6. Measurement of thickness: If its dielectric constant is known, you can measure the insulator thickness of two plates that are in contact with an insulator. Or, if the thickness is known, the dielectric constant can be measured.

7. Ice detector: Using insulated metal strips on wing leading edges, the icing on the aeroplane wing can be detected.

8. Linear position or the angle of shaft: With a multi-plate arrangement giving high precision and digital output, capacitive sensors can measure angle or position.

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