Capacitive Sensors

Distance measurement is one of the main applications of sensors. Sometimes it is tough to measure the displacement of a moving object. Capacitance can be used for this purpose. But it is not only used for displacement measurement. It can also be used in sensing a lot of physical parameters. The following article explains the sensors that uses capacitance in detail.

Contents:

  1. Working Principle of Capacitive Sensors
  2. Widely used Capacitive Sensors
  3. Capacitive Proximity Sensors
  4. Capacitive Displacement Sensors
  5. Capacitive Keyboards
  6. Rain Sensing Capacitive Sensors
  7. Capacitive Humidity Sensors
  8. Capacitive Touch Sensors

Working Principle of Capacitive Sensors

Sensors that use capacitance for measurement are known as capacitive sensors. A capacitive sensor is similar to a capacitor. A capacitor is used to store energy in the form of charges and this principle is modified a bit for the working of capacitive sensors. The construction and working principle of a generalized capacitive sensor are as follows:

  • Generally, a capacitor consists of two metal plates that are placed in parallel to each other.
  • When voltage or current is applied across the capacitor, the capacitance across the plates will be developed.
  • The capacitance is inversely proportional to the space between the plates i.e. when the plates are close to each other, the capacitance produced is greater than the amount of capacitance produced when the plates are placed apart.
  • The same principle is used in the capacitive sensors. The only and main change is that instead of the second plate in a capacitor, the target to be sensed is used.
  • By measuring the amount of capacitance produced, the target to be measured can be calculated.

Widely used Capacitive Sensors

Capacitive sensors are one of the most commonly used sensors. They are used in various fields of technology. As the base principle all sensors are more or less same, it is a versatile type of sensor. By making some slight changes to the basic capacitive sensors, it can be used to measure an entirely different parameter. Some of the widely used capacitive sensors are listed below.

Sensor Input Output
Proximity sensors An object to be detected or the range of object to be measured Capacitance output which can be converted to a binary output
Displacement sensors The target that is moving The change in the position of the target
Capacitive keyboards The user presses on of the keys The value corresponding to the key pressed
Rain sensors Rainfall Amount of rainfall
Humidity sensors Humidity The moisture content present in the soil or in the atmosphere
Touch sensors Touch Change in initial capacitance

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Capacitive Proximity Sensors

Capacitive proximity sensors are non-contact type proximity measurement devices that are used in two different ways. One is to detect whether any object is present within the given range or not and the second type measures the distance between the object and the sensor.

The working of a simple proximity sensor that finds whether an object is present or not is as follows.

  • Just like a capacitive sensor, a proximity sensor consists of a metal plate which acts as a first plate in a capacitor.
  • If the target to be found is kept within the range of the sensor, then the target acts as the second plate in the capacitor.
  • When two plates are found, a capacitance is induced within the range.
  • If any capacitance is produced, it means that there is an object within the range and the proximity sensor provides the output as 1.
  • If no capacitance is produced, it means that there is no object within the range and the output of the proximity sensor will be 0.
  • One of the main advantages of this type of proximity sensors is that it can detect any object irrespective of the material of the object.

Capacitive Displacement Sensors

Displacement sensors are devices that are used to measure the amount of distance travelled by an object. In other words, displacement sensors can be used to detect the motion of the target. Capacitive displacement sensors are the same sensors which uses capacitance for the above-mentioned purpose.

The construction and working of a capacitive displacement sensor is explained below.

  • Capacitive displacement sensors use the same principle as the general capacitive sensor for measurement.
  • The initial capacitance when the plate in the sensor and the target object acts as two plates of the capacitor is measured.
  • When the target object is moved, the amount of capacitance induced between the plates is reduced and the change is inversely proportional to the displacement.
  • By measuring the capacitance change, the displacement of the object can be calculated.
  • These sensors are widely used in industrial based applications where the displacement between any two objects has to be constantly monitored.

Capacitive Keyboards

Capacitive keyboards are devices that are similar to the conventional keyboards, except for the fact that it uses capacitance to detect the key pressed. These keyboards have a relatively quicker response than the traditional keyboards and can detect the key pressed even when the press was light.

The working of a capacitive keyboard is as follows.

  • In the capacitive keyboard, there will be a capacitor like structure under every key.
  • The initial capacitance of all the keys is same when no keys are pressed.
  • When one of the keys is pressed, the distance between the plates of that particular key is reduced.
  • As a result, the amount of capacitance in that particular key is increased and the key pressed can be detected.
  • The capacitive sensors are better than the conventional keyboard in the other aspect as well i.e., the number of parts in a capacitive keyboard is very much lesser than the number of parts in a conventional keyboard.

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Rain Sensing Capacitive Sensors

Capacitive rain sensors are modules that are used to check whether it is raining or not. These sensors are widely used in automobiles. In an automobile, the automatic wiper system is always powered by the capacitive rain sensor.

The construction and the working of a rain sensing capacitive sensor is explained below.

  • A capacitive rain sensor consists of three main components – A capacitor, a heater and a temperature sensor.
  • The capacitance has a tendency to change when the temperature of the surrounding is changed.
  • If it rains, the initial capacitance is increased as the moisture level is increased.
  • The temperature sensor measures the environmental temperature.
  • By analyzing the capacitance change and the environmental temperature, the amount of rainfall can be calculated.

Capacitive Humidity Sensors

Humidity sensors are devices that is used to measure the amount of moisture content present in an object or in the soil or in the environment. The generalized working of capacitive humidity sensor is as follows:

  • Ordinary humidity sensors use relative humidity for measurement while the capacitive humidity sensor directly measures the humidity.
  • The capacitive humidity sensor consists of dielectric material which is placed within the plates of the capacitor.
  • Due to the presence of dielectric, the initial capacitance between the plates depends on the plates only.
  • When the temperature of the atmosphere is changed or the moisture content of the atmosphere is increased, the capacitance is increased.
  • By measuring the capacitance change, the humidity of the atmosphere can be calculated.

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Capacitive Touch Sensors

A touch sensor is used to detect a touch or a pressure applied. It acts as a switch in a circuit i.e., when a touch is detected, the circuit is closed and when there is no pressure applied, the circuit remains open.

The construction and working of a capacitive touch sensor are explained below.

  • The sensor consists of a glass panel which is to be touched.
  • When touch is detected, the printed panel which is placed under the panel is disturbed.
  • As a result, the initial capacitance of prints is changed.
  • If there is a change in capacitance, it means that there is a touch and vice versa.
  • These sensors are used in touch screens and also in some industrial applications.

Key Points to Remember

Here are the key points we need to remember about “Capacitive Sensors”

  • A capacitive sensor is similar to a capacitor and is used to measure various physical parameters.
  • A capacitive proximity sensor is used to detect the presence of an object within the given range.
  • Capacitive displacement sensors measure the amount of distance travelled by the target object using capacitance.
  • Capacitive keyboards are keyboards that uses capacitance for key detection and is better than a traditional keyboard.
  • Rain sensing capacitive sensors are devices that are used in automatic wiper systems in automobile.
  • A capacitive humidity sensor is similar to an ordinary humidity sensor except for the fact is that it does not use relative humidity for measurement.
  • Capacitive touch sensors are used in touch screens and touch sensitive devices.

If you find any mistake above, kindly email to [email protected]

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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