What is a Proximity Sensor?

What is a Proximity Sensor

The proximity sensor is a general term for sensors that replace the contact detection methods such as limit switches and aim to detect without touching the detection object. It can detect the movement information and presence information of objects and convert them into electrical signals. Among the detection methods of electrical signals, there are methods that utilize eddy currents generated in a metal object to be detected by electromagnetic induction, methods that capture changes in the capacity of electrical signals due to the proximity of the object to be detected, and methods that use sharp stones and guide switches. The proximity sensor is a device with the ability to sense the proximity of objects. It uses the sensitive characteristics of displacement sensors to approach objects to identify the proximity of objects and output corresponding switch signals. Therefore, proximity sensors are usually called proximity switches.

In the JIS standard, the JIS standard (JIS C 8201-5-2 Low-voltage switchgear and control device, fifth control circuit device and switching element, second section proximity switch) was established based on IEC60947-5-2 non-contact position detection switch. In the definition of JIS, products that can also detect the proximity of an object in a non-contact manner and the presence or absence of a nearby detection object are collectively referred to as “proximity switches”. It is composed of induction type, electrostatic capacitance type, ultrasonic type, photoelectric type, magnetic type, and so on. In this technical guide, inductive proximity sensors that detect the presence of metals, capacitive proximity sensors that detect the presence of metallic and non-metallic objects, and switches that utilize a DC magnetic field generated by magnetic force are defined as “proximity sensors”.

Proximity Sensor Types and Working Principle

Capacitive Proximity Sensor

The capacitive proximity sensor is an electric capacitance proximity switch with electrodes as detection terminals. It consists of a high-frequency oscillation circuit, detection circuit, amplifier circuit, shaping circuit, and output circuit.

Usually, there is a certain capacitance between the detection electrode and the ground, which becomes an integral part of the oscillation circuit. When the detected object is close to the detection electrode, due to the voltage applied to the detection electrode is C=Q/V, the detection electrode will be subjected to electrostatic induction to generate polarization. The closer the detected object is to the detection electrode, the more induced charges on the detection electrode. Since the electrostatic capacitance exits on the detection electrode, the capacitance C of the detection electrode increases as the number of charges increases. Since the oscillation frequency of the oscillation circuit is inversely proportional to the capacitance, when the capacitance C increases, the oscillation of the oscillation circuit weakens or even stops the oscillation. The two states of oscillation and stop of the oscillation circuit are converted into switching signals by the detection circuit and then output to the outside.

Inductive Proximity Sensor

Inductive proximity sensor consists of a high-frequency oscillation circuit, a detection circuit, an amplifying circuit, a shaping circuit, and an output circuit. The detection sensitive element is the detection coil, which is an integral part of the oscillating circuit, and the oscillating frequency of the oscillating circuit is. When the detection coil is connected to alternating current, an alternating magnetic field is generated around the detection coil. When the metal object approaches the detection coil, the metal object will generate eddy current and absorb the magnetic field energy, so that the inductance L of the detection coil changes. Thereby, the oscillation frequency of the oscillation circuit is reduced, and the oscillation is stopped. The two states of oscillation and vibration stop are converted into switching signal output by the monitoring circuit.

It should be noted that: the same as the capacitive proximity sensor, the measured object detected by the inductive proximity sensor is also a metal conductor, and non-metallic conductors cannot be measured by this method. Amplitude changes vary depending on the type of target metal, so the detection distance also varies depending on the type of target metal.

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