Types of proximity sensors

A proximity sensor is a type of sensor that replaces contact detection methods such as limit switches. Proximity sensors aim to detect an object without actually contacting it. It is able to detect the movement information and presence information of the object via electrical signals. It uses the displacement sensor’s sensitivity to an approaching object to recognize the proximity of the object and subsequently output the corresponding switch signal. Because of this, proximity sensors are commonly referred to as proximity switches. This blog will discuss proximity sensors in detail, as well as their application in aircraft landing systems.

The working principle of proximity sensors is to send light from a light source into the modulator through proximity. The measured parameter then interacts with the light entering the modulation area, resulting in a change to the optical properties of the light (these include intensity, wavelength, frequency, phase, polarization, and more). This altered light is known as modulated signal light. In any kind of switch, there is a part or component that has the ability to ‘perceive’ any incoming objects approaching it. Proximity sensors, on the other hand, use outgoing signals to approach objects. Once this is done, it can then carry out the task of turning the switch on or off.

When an object comes within a certain distance of the proximity sensor, the displacement sensor will sense it and cause the switch to act. This distance is usually called the "detection distance" and varies from sensor to sensor. Different proximity sensors feature different response capabilities. This response characteristic is known as response frequency.

There are three main types of proximity sensors: capacitive, inductive, and photoelectric. Capacitive proximity sensors are electric capacitance proximity switches that use electrodes for detection. These consist of a high-frequency oscillation circuit, a detection circuit, an amplifier circuit, a shaping circuit, and an output circuit. An inductive proximity sensor is composed of a high-frequency oscillation circuit, a detection circuit, an amplifying circuit, a shaping circuit, and an output circuit. The detection element in these is the detection coil, an integral part of the oscillation circuit. Lastly, in a photoelectric proximity sensor, the beam axis of the light-emitting diode and the axis of the phototransistor are on a plane where the two form an angle. The two axes intersect at a given point ahead of the sensor. When an object is detected at the intersection point, the light reflected by the light-emitting diode is received by the phototransistor, generating an electrical signal.

An important application of proximity sensors is in aircraft landing gear systems. Here, they are used to detect the positioning of the landing gear. There are two main sensors for each set of landing gear: the lock-up sensor and the lock-down sensor. These are used to switch the transmission position signal when they are in place. Proximity sensors in landing gear are known as magnetoresistive proximity sensors, and comprise two parts: a sensory body and sensor excitation sheet.

The sensor body converts electrical energy into a magnetic field while the excitation sheet has the job of increasing the magnetic permeability. When the landing gear gets close to the excitation plate, it causes the magnetic permeability to increase. The sensor then sends a signal to alert the component to indicate the position of the landing gear. The distance the signal must travel directly affects the accuracy of the instructions, and the general adjustment requirements also vary. Using an inductive proximity sensor to detect the position of the landing gear will not only improve the life of the sensor, but the information transmission and information sharing with the avionics system will also be conveniently found in the control computer.

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