What are Proximity Sensors?

Catalog

I Introduction

The proximity sensor is a general term for a sensor that replaces contact detection methods such as limit switches. It aims to detect without contacting the detection object. It can detect the movement information and presence information of the object into electrical signals. A proximity sensor is a device with the ability to sense the proximity of an object. It uses the displacement sensor's sensitivity to the approaching object to recognize the proximity of the object and outputs the corresponding switch signal. Therefore, the proximity sensor is usually called a proximity switch. 

II How do proximity sensors work?

The working principle of the proximity sensor is to send the light from the light source into the modulator through proximity. The measured parameter interacts with the light entering the modulation area, resulting in the changes in the optical properties of the light (such as light intensity, wavelength, frequency, phase, the polarization state, etc.), which called the modulated signal light. Functional type proximity sensors are made with a sensitive characteristic or function close to itself. Light transmission type proximity sensors are only playing the role of transmitting light. It installs other sensitive elements near the end or in the middle to feel the measured change.

proximity sensor

Among all kinds of switches, there is a component that has the ability to "perceive" the objects approaching it-displacement sensors. The proximity switch uses the sensitive characteristics of the displacement sensor to approach objects. Then it can achieve the purpose of controlling the switch on or off.

When an object moves to the proximity switch and approaches a certain distance, the displacement sensor will perceive and the switch will act. This distance is usually called the "detection distance". But the detection distance of different proximity switches is also different.

Sometimes the detected object is moved to the proximity switch one by one at a certain time interval, and then left one by one. It repeats continuously. Different proximity switches have different response capabilities to the detected object. This response characteristic is called "response frequency".

III Classification of proximity sensors

1. Capacitive proximity sensors

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

Capacitive proximity sensors

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 approaches the detection electrode, the detection electrode is subjected to electrostatic induction. Then a polarization phenomenon occurs due to the voltage applied to the detection electrode. The closer the detected object is to the detection electrode, the more the induced charge on the detection electrode. The electrostatic capacitance on the detection electrode is  . Therefore, as the amount of charge increases, the detection electrode capacitance C increases accordingly. The oscillation frequency of the oscillation circuit is  It is inversely proportional to the capacitance. So when the capacitance C increases, the oscillation of the oscillating circuit weakens or even stops oscillating. The two states of oscillation and oscillation stop of the oscillation circuit are converted into switching signals by the detection circuit and then output to the outside.

2. Inductive proximity sensors

The 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 sensitive element for detection is the detection coil, which is an integral part of the oscillation circuit, and the oscillation frequency of the oscillation circuit is 

When alternating current is applied to the detection coil, an alternating magnetic field is generated around the detection coil. When the metal object approaches the detection coil, the metal object will generate eddy currents to absorb the magnetic field energy and change the inductance L of the detection coil. As a result, 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 switch signal output by the monitoring circuit.

Inductive proximity sensors

It should be noted that the objects detected by inductive proximity sensors are also metal conductors, and non-metallic conductors cannot be measured by this method. The amplitude change varies with the type of target metal, so the detection distance also varies with the type of target metal.

3. Photoelectric proximity sensors

In the photoelectric proximity sensor, the beam axis of the light-emitting diode and the axis of the phototransistor is on a plane and form a certain angle. The two axes intersect at a point in front of the sensor. When the surface of the detected object is close to the intersection point, the light reflected by the light-emitting diode is received by the phototransistor, generating an electrical signal. When the object is far from the intersection point, the reflection area is not in the viewing angle of the phototransistor, and the detection circuit has no output. In general, the drive current sent to the light-emitting diode is not a direct current, but an alternating current of a certain frequency. In this way, the receiving circuit also obtains an alternating signal of the same frequency. If the received signal is filtered, only the signal of the same frequency is allowed to pass, which can effectively prevent the interference of other stray light and improve the luminous intensity of the light-emitting diode.

Photoelectric proximity sensors

IV Applications of proximity sensors

1. Application of human body proximity sensors in ATM cash machine monitoring

The human body proximity sensor is a control device for detecting human body proximity, which can accurately detect the proximity of nearby people. It is currently the best choice as alarm and status detection. Its sensing part has a high detection sensitivity to the movement of nearby people, and it has a strong anti-interference ability to suppress the sound signal of the surrounding environment. The internal use of micro-circuit chips for program control processing has high detection sensitivity and trigger reliability. The two parts of detection and control are combined into one, with low waiting power consumption.

Since the sensitivity to the human body is continuously adjustable, this makes the human body proximity sensor suitable for many different occasions. In terms of security and anti-theft, such as data files, accounting, finance, museums, vaults, etc., usually have anti-theft devices composed of various proximity sensors.

2. Application of proximity sensors in aircraft landing gear system

The aviation power plant, landing gear system, and navigation system are the top three systems in the aircraft service difficulty report data reported by airlines. Among them, the failure of the landing gear system is likely to cause abnormal events such as aircraft return and alternate landing, which will bring economic losses to the company and hidden dangers to aviation safety.

aircraft landing gear

In the general Airbus landing gear control system, the digital telex control system is usually adopted. The basic principle is to send the sensor signal to the control box, and then send instructions to the actuator after comprehensive calculation and comparison. The control link is redundancy control.

Modern civil aircraft landing gear retractable includes two systems of normal retractable and emergency release. The landing gear control system uses fly-by-wire manipulation in the operation mode and realizes cross-linking with other systems. Inductive proximity sensors are widely used to detect the position of the landing gear.

There are two main sensors for each landing gear, namely the lock-up sensor and the lock-down sensor, which are used to switch the transmission position signal when the lock-up and lock-down sensors are in place. This type of sensor is a magnetoresistive proximity sensor, which is mainly composed of two parts: the sensor body and the sensor excitation sheet.

The main body of the sensor converts electrical energy into a magnetic field. The excitation sheet mainly plays a role of increasing the magnetic permeability. When the landing gear is close to the excitation plate to a certain distance, the magnetic permeability increases. The sensor sends a signal to warn the component to indicate the position of the landing gear. The distance between them directly affects the accuracy of the instructions, and the general adjustment requirements are also strict. In addition, different models are slightly different, and you need to refer to the AMM manual of the model when adjusting.

Using an inductive proximity sensor to detect the position of the landing gear will improve the life of the sensor. In addition, the information transmission and information sharing with the avionics system is conveniently realized through the control computer.

3. Application of proximity sensors in the monitoring of railway track

In all railway accidents, train collisions account for a large part. The use of proximity sensors to monitor passing trains at intersections has become a very important part in improving railway safety.

The non-contact position measurement of the proximity sensor can be used to install them symmetrically at both ends of the rail at the intersection. When a train passes by, the proximity sensors at both ends of the rail can detect the changes caused by the wheels at each end. The signal of each sensor is analyzed by the crossing monitoring micro-processing system to determine the direction of the vehicle and the state of the vehicle when passing (passing or not, whether to stay). Eventually, the information is sent to the traffic control center by means of a cable or wireless communication, so as to dispatch the train.

4. Application of proximity sensors in automatic packaging machinery

Mechanized manufacturing has spawned the need for automatic packaging technology, and the manual packaging method is far from enough for mass production operations. Under the guidance of the control system, the automatic packaging machine can complete the packaging process of a series of items, which improves the packaging efficiency of the product and reduces the packaging cost. However, it will still inevitably cause leakage. For this reason, automatic packaging inspection has become an important link to ensure the quality of packaging. Among them, in the case of ferromagnetic substances in the packaging process, it is a common method to use proximity sensors for non-contact detection.

The coil near the sensor that can generate an alternating magnetic field will form an eddy current inside due to the principle of electromagnetic induction when the detected ferromagnetic object is in this environment. When the magnetic field generated by the eddy current is large enough, it will, in turn, change the original circuit parameters of the proximity sensor, thereby generating a signal output. Therefore, the proximity sensor can identify whether there is a magnetic or easily magnetized substance in a certain range nearby. In some automatic packaging processes, such as chocolate metal foil packaging, the proximity sensor can detect the presence of magnetic substances to determine whether there are packaging errors or unqualified products that are missing from the process.

5. Application of proximity sensors in robot hand gripper

The robot hand gripper is a mechanical structural part with multiple degrees of angles that can dexterously grasp objects. It can be used in various industrial automated production, assembly, and operation. It can perform tasks such as information detection, item collection, and detonation in a high-risk environment. Robot hand grippers generally use a pincer structure to grip objects in an open and closed manner. Therefore, the precise measurement and control of the opening and closing of the "jaw" is a key factor that directly affects the success of the gripping process.

robot hand gripper

Because the proximity sensor can sense changes in distance and position, it is also a common sensor device for measuring opening and closing conditions in robot hand grippers. It uses the relative positional relationship between the change of the magnetic field and the metal part to be measured. Generally, the sensor is installed on one of the clamps of the gripper. When gripping an object, the proximity sensor can judge the distance between the two (clamps) by sensing the change in the size of the magnetic field. Therefore, it can be compared with the set value, adjust the size of the opening of the hand gripper.

6. Application of proximity sensors in automotive electronics

The demand for proximity detection sensors in the field of automotive electronics has been steadily increasing. The possible applications of proximity detection in the automotive electronics industry are unlimited. For example, Car access control: detect the hand that is close to the door handle, and then start the unlocking process. When the palm is close to the screen surface, it can illuminate and wake up the touch screen. When the palm is close to the sensor, the interior lights can be turned on / off.

Door opening system with detection function

In response to the needs of various automotive electronics applications, there are a variety of proximity detection methods, such as capacitive sensing, infrared, ultrasonic, and optical. For proximity detection ranging from 5mm to 300mm, capacitive sensing technology has many advantages over other technologies: excellent reliability, simple mechanical design, low power consumption, and low cost.

An example of capacitive proximity detection is the application in the car access control system (see figure below). The proximity sensor that detects the approach of a human hand is located in the door handle (1). Once it is detected that an object is approaching, the main control unit (2) sends a wake-up signal through the low-frequency antenna (3); this signal activates the car key transmitter (4). The car key transmitter then exchanges information with the RFID receiver (5); if the encoded information matches the main control unit (2), the car door lock is opened. The entire process of proximity detection and ID recognition is about a fraction of a second.

Compared with touch detection, the advantage of using proximity detection in the car access control system is that it can preempt the time of identifying the owner. The result is that the door lock is already open before the door is pulled.

7. Application of proximity sensors in the screen door

For safety, subway screen doors are increasingly used by subway platforms. Proximity switch sensors will be heavily invested in the technical application of screen doors.

subway screen doors

At present, the common scheme used by the screen door system to detect the opening and closing of the door is generally through two proximity switches. Since the proximity sensor can detect in a non-contact manner, the object to be detected is not worn or damaged. This is also the main reason why it is suitable for installation on the subway. As the performance of the proximity sensor is further improved, it will be used on more occasions. In the future, it will be widely used not only for subway doors but also for other shielded doors such as buses.

8. Application of proximity sensors in touch screen mobile phone

The use of MEMS technology in proximity sensors has gained popularity in smartphones. At the beginning of the popularity of touch screen mobile phones, users discovered a defect of the touch screen: when we pick up the phone in the most common gestures, the face often touches the touch screen, and the on-hook key or the hands-free key is accidentally clicked. As a result, mobile phone manufacturers used MEMS technology to design MEMS proximity sensors into touch-screen phones. MEMS technology will automatically lock the screen when we answering the phone. In addition, the backlight can be turned off while the screen is locked, which can effectively save energy and extend the standby time.

touch screen mobile phone

Smartphones use MEMS ambient light sensors and proximity sensors, that is, ambient light detection and proximity detection. Ambient light detection is to judge the brightness of the surroundings based on the amount of light detected by the light-receiving unit according to the illuminance of the sensor. Proximity detection is that the light emitted from the light source with the sensor on the measurement object, and the distance from the measurement object is determined based on the amount of light reflected on the proximity light receiving unit of the sensor. The ambient light sensor can optimize the adjustment of the lighting of the LED backlight. Whether it is in a dim movie theater or outdoors with sufficient light, our mobile phone can adjust itself to a suitable brightness in any environment. The proximity sensor can turn off the touch screen when answering the phone so that we will not touch the buttons on the screen.

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