Inductive & Capacitive Proximity Sensor : Object Detector Circuit

Inductive & Capacitive Proximity Sensor (Last Updated On: October 2, 2018)

Proximity sensors:

A proximity sensor detects an object when the object approaches within the detection boundary of the sensor. Proximity sensors are used in various facets of manufacturing for detecting the approach of metal objects. In this post we will discuss about Inductive & Capacitive Proximity Sensor as an Object Detector.




Various types of proximity sensors are used for detecting the presence or absence of an object. The design of a proximity sensor can be based on a number of principles of operation, some examples include: variable reluctance, eddy current loss, saturated core, and Hall Effect. Depending on the principle of operation, each type of sensor will have different performance levels for sensing different types of objects.

Inductive & Capacitive Proximity Sensor

Common types of non-contact proximity sensors include inductive proximity sensors, capacitive proximity sensors, ultrasonic proximity sensors, and photoelectric sensors. Hall-effect sensors detect a change in a polarity of a magnetic field.

Variable reluctance sensors typically include a U-type core and coils wound around the core legs. Inductive proximity sensors have a lossy resonant circuit (oscillator) at the input side whose loss resistance can be changed by the proximity of an electrically conductive medium.

An electrical capacitance proximity sensor converts a variation in electrostatic capacitance between a detecting electrode and a ground electrode caused by approaching the nearby object into a variation in an oscillation frequency, transforms or linearizes the oscillation frequency into a direct current voltage, and compares the direct current voltage with a predetermined threshold value to detect the nearby object.

Ultrasonic sensing systems provide a much more efficient and effective method of longer range detection. These sensors require the use of a transducer to produce ultrasonic signals.

Eddy-current proximity sensors are well known and operate on the principle that the impedance of an ac-excitecl electrical coil is subject to change as the coil is brought in close proximity to a metallic object.




Proximity sensors often are employed in manufacturing industries in which the sensors are exposed to harsh environmental conditions. Inductive proximity sensors are used in automation engineering to define operating states in automating plants, production systems and process engineering plants.

Magnetic proximity detectors are commonly used on ski lifts and tramways for detecting a derope condition of the steel cable used as a haul line or haul rope.

Proximity sensors are widely used in the automotive industry to automate the control of power accessories. For instance, proximity sensors are often used in power window controllers to detect the presence of obstructions in the window frame when the window pane is being directed to the closed position.


Inductive & Capacitive Proximity Sensor:

Inductive Proximity Sensor:

Inductive sensor is an electronic proximity sensor which detects metallic objects without touching them. When the detecting distance is 5mm or less 1 inch and your application calls for metal sensing, the inductive proximity (IP) sensor provides the needed solution.

Inductive & Capacitive Proximity Sensor

Working Principal of Inductive Proximity sensor:

Inductive proximity sensors operate under the electrical principle of inductance. Inductance is the phenomenon where a fluctuating current, which by definition has a magnetic component, induces an electromotive force (emf) in a target object. In circuit design, one measures this inductance in H (henrys). To amplify a device’s inductance effect, the sensor twists wire into a tight coil and runs a
current through it.

Inductive & Capacitive Proximity Sensor


Component and working of Inductive Proximity Sensor: An inductive proximity sensor has four elements: coil, oscillator, trigger circuit, and an output. The oscillator is an inductive capacitive tuned circuit that creates a radio frequency. The electromagnetic field produced by the oscillator is emitted from the coil away from the face of the sensor. The circuit has just enough feedback from the field to keep the oscillator going. When a metal target enters the field, eddy currents circulate within the target. This causes a load on the sensor, decreasing the amplitude of the electromagnetic field. As the target approaches the sensor, the eddy currents increases, increasing the load on the oscillator and further decreasing the amplitude of the field. The trigger circuit monitors the oscillator’s amplitude and at a predetermined level switches the output state of the sensor from its normal condition (on or off). As the target moves away from the sensor, the oscillators amplitude increases. At a predetermined level the trigger switches the output state of the sensor back to its normal condition (on or off).

Advantages:

01. They are very accurate to other technologies.
02. Have high switching rate.
03. Can work in harsh environment condition.

Disadvantages:

01. It can detect only metallic target.
02. Operating range may be limited





Capacitive Proximity sensors:

Capacitive proximity sensors can be used to detect metallic and also non metallic targets like paper, wood, plastic, glass, wood, powder, and liquid.etc without physical contact. Capacitive proximity sensors sense “target” objects due to the target’s ability to be electrically charged. Since even non-conductors can hold charges, this means that just about any object can be detected with this type of sensor.

Inductive & Capacitive Proximity Sensor

Working Principal of Capacitive Proximity Sensor:

The capacitive proximity sensor works on the capacitor principle. Capacitive Proximity Sensors detect changes in the capacitance between the sensing object and the Sensor. The amount of capacitance varies depending on the size and distance of the sensing object.

Component and working of Capacitive Proximity Sensor: The main components of the capacitive proximity sensor are plate, oscillator, threshold detector and the output circuit.

Inductive & Capacitive Proximity Sensor

The plate inside the sensor acts as one plate of the capacitor and the target acts as another plate and the air acts as the dielectric between the plates. As the object comes close to the plate of the capacitor the capacitance increases and as the object moves away the capacitance decreases. The detector circuit checks the amplitude output from the oscillator and based on that the output switches. The capacitive sensor can detect any targets whose dielectric constant is more than air. The changes in the capacity generated between these two poles are detected.

Advantages:

01. It can detect both metallic and non metallic targets.
02. Good stability
03. High Speed
04. Good Resolution
05. Capacitive sensors are good in terms of power usage
06. Low cost

Disadvantages:

01. They are affected by temperature and humidity
02. Could be triggered by dust, moisture, etc.
03. Sensitive to noise
04. Difficulties in designing
05. Linearity is not good
06. Capacitive proximity sensors are not as accurate compare to inductive





Video Tutorial and Explanation:

Apart from the above explantion here is a video tutorial that explanins Inductive & Capacitive Proximity Sensor as an Object Detector.

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