In this article, I am going to discuss the capacitive proximity sensor working principle, and my commentary will increase your knowledge on this topic.
A capacitive proximity sensor senses the presence of an object (usually called the target) without physical contact.
They can detect both metallic and nonmetallic targets. They are ideally suited for liquid level control and for sensing powdered or granulated materials.
Capacitive Proximity Sensor Working Principle
The capacitive proximity sensor consists of a high-frequency oscillator along with a sensing surface formed by two metal electrodes. When an object comes near the sensing surface, it enters the electrostatic field of the electrodes and changes the capacitance of the oscillator.
As a result, the oscillator circuit starts oscillating and changes the output state of the sensor when it reaches up to a certain amplitude. As the object moves away from the sensor, the oscillator’s amplitude decreases, switching the sensor back to its initial state.
The larger the dielectric constant of a target, the easier it is for the capacitive proximity sensor to detect. This constant makes possible the detection of materials inside nonmetallic containers because the liquid has a much higher dielectric constant than the vessel, which gives the sensor the ability to see through the vessel and detect the fluid.
They typically have a short sensing range of about 1 inch, regardless of the type of material being sensed. While dealing with non-conductive targets, sensing distance increases with an increase in
- the sensing surface size of the sensor.
- the surface area of the target.
- the dielectric constant of the target.
For best operation, we should use them in an environment with relatively constant temperature and humidity.
The point at which the proximity sensor recognizes an incoming target is known as the operating point. The point at which an outgoing target causes the device to switch back to its normal state is known as the release point. The area between operating and release points is called the hysteresis zone.
The sensitivity adjustment can be made by adjusting a potentiometer provided on the sensor. If the sensor does not have an adjustment potentiometer, then the sensor must physically be moved to get the optimum installation position.
Optimum sensitivity provides a longer operating distance. However, the operation of the oversensitive sensor is very much affected by temperature, humidity, and dirt, etc. and may cause false triggering of the sensor.
Most proximity sensors are equipped with an LED status indicator to verify the output switching action. Capacitive proximity sensors are available in various sizes and configurations to meet different application requirements.
One of the most common shapes is the barrel type, which houses the sensor in a metal or polymer barrel with threads on the outside of the housing. Due to the threaded housing, we can easily adjust the sensor on a mounting frame.
Characteristics of Capacitive Proximity Sensor
The major characteristics of capacitive proximity sensors are as under:
- They can detect nonmetallic targets.
- They can detect lightweight or small objects that cannot be detected by mechanical limit switches.
- They provide a high switching rate for rapid response in object counting applications.
- They can detect liquid targets through nonmetallic barriers (glass, plastic, etc.).
- They have a long operational life with a virtually unlimited number of operating cycles.
- The solid-state output provides a bounce-free contact signal.
Typical Applications of Capacitive Proximity Sensors
- High/low liquid level.
- Dry tank.
- Material present/absent.
- Product present.
- Product count.
Inductive Proximity Sensor
Inductive proximity sensors are used to detect both ferrous and nonferrous metals (such as copper, aluminum, and brass). An inductive proximity sensor operates on the eddy current principle.
When a metal object moves into the electromagnetic field of the sensing head, eddy currents are induced in the object. This current causes a change in the loading of the oscillator, which then operates the output device. We can summarize the operation of an inductive proximity sensor as under:
- The oscillator circuit generates a high-frequency electromagnetic field that radiates from the tip of the sensor.
- When a metal target enters the field, eddy currents are induced in the target.
- Eddy currents in the target absorb the radiated energy of the sensor, which results in a loss of energy and change in the field strength of the oscillator.
- The sensor’s detection circuit monitors the oscillator’s strength and triggers a solid-state output device at a specific level.
- Once the metal object leaves the sensing area, the oscillator returns to its initial value.
The type of metal and size of the target are essential factors that determine the effective sensing range of the sensor. Ferrous metals may be detected up to 2 inches away, while most nonferrous metals require a shorter distance, usually within an inch of the device.
Typical Applications of Inductive Proximity Sensors
- Detection of the rotating motion.
- Zero-speed indication.
- Speed regulation.
- Shaft travel limiting.
- Movement indication.
- Valve open/closed.
Thanks for reading about the “capacitive proximity sensor working principle”. To learn more about sensors visit balluff.com or watch the video.
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