Proximity Sensor Working Principle | Inductive Proximity Sensor | Capacitive Proximity Sensor

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Proximity Sensor Working Principle

A proximity sensor is an electronic solid-state device used to indicate the presence of an object without making physical contact.  The proximity sensor is a very useful device in hazardous areas such as oil refineries and not so hazardous areas such as car door detection systems. Proximity sensors do not use any type of physical moving parts instead they allow signals to transmit through them when something that is being monitored comes in close proximity of the sensing area. Despite the process, they are still referred to as proximity switches.

Proximity Sensor Uses

Proximity sensors are to be used when the object that needs to be detected is too small, lightweight or too soft to operate a mechanical switch.  When there is a need for rapid response and high switching rates such as the counting objects, proximity sensors are ideal for the task. Proximity sensors should also be used when there’s a need to sense material through nonmetallic barriers such as glass, bottles, plastic, or paper cartons or when working in hostile environments that demand electrical isolation from the product being monitored. Proximity sensors are also needed when there is a need to have a device that provides long life and reliable service or when there are vast electronic control systems that need to be free of chattering contacts to produce an accurate analysis of what’s being monitored. There are two main types of proximity sensors, that achieve these need, these include the inductive and capacitive proximity sensors.

Inductive Proximity Sensor

The inductive proximity sensor as seen in Figure 1 (a) is used to detect both ferrous metals that contain iron and can be magnetized and nonferrous metals such as what we use to conduct electricity and copper. Inductive proximity sensors operate under the electrical principle of magnetism when a fluctuating current induces the voltage in a target object.

Inductive Proximity Switch

Fig. 1 (a): Inductive Proximity Switch

The inductive proximity sensor contains a certain type of solid-state control system. It contains an oscillator circuit that generates a high-frequency magnetic field. When the metal object enters the field, it disturbs the magnetic field, this disturbance results in a change of state in the high-frequency circuit. 

Inductive Proximity Switch Circuit Diagram

Fig. 1 (b): Inductive Proximity Switch Circuit Diagram

Inductive Proximity Sensor Types

Inductive proximity sensors are intended to be wired into motor control circuits or electronic control circuits with a voltage rating of 24 V DC. Most proximity sensors come in two basic configurations one is 3 wire configuration while the other one is a 2 wire configuration. The three wire proximity sensor is required to be powered up at all times so the sensor must have two wires connected to a constant voltage source to operate the electronic circuitry of the device. The third wire of the proximity inductive switch is the contact wire which can come normally open or normally closed. Some proximity switches can contain both so the total number of wires coming from the unit can total to be 4. The two wire proximity sensor is intended to be connected in series with the load is to control. In its natural state, there’s enough current that must flow through the circuit to keep the sensor active. This current is referred to as the leakage current and typically may range from 1 to 2 mA. 

Capacitive Proximity Sensor

The capacitive proximity sensor is similar to the inductive proximity sensor, as shown in figure 1 (b). The main difference between the two, capacitive proximity sensor produces an electrostatic field instead of a magnetic field and the sensing area of the capacitive proximity sensor can be actuated by both conductive and non-conductive materials.  A capacitive proximity sensor contains a high-frequency oscillating circuit along with a sensing surface formatted by two metal plates. When an object or some type of material gets in the sensing range it disturbs the electrostatic field of the metal plates, changing the capacitance of the proximity sensor, this change results in a change of state in the operation of the proximity sensor.

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