Friday, 03 January 2020 04:01

how to improve the anti-interference performance of sensors in automatic doors?

Automatic doors first appeared in the 1930s. After decades of development and innovation, they have brought countless conveniences to our lives. The core of the automatic door system is the sensor. The sensor provides the signal for opening and closing the door. The accuracy of the signal provided by the sensor determines the performance of the automatic door. We will mainly discuss how to improve the anti-interference performance of sensors in automatic doors so that the sensors can provide more accurate control signals and prevent automatic doors from being wrongly switched.

Automatic doors are wildly used in public places in our lives. It is a product that people invent and innovate according to their own needs. Someone once said that the world's scientific and technological progress and inventions are driven by lazy people. It is precisely because people want to liberate their hands and save energy through their own ingenuity, so that a variety of inventions can appear. In public places, the ordinary door can no longer meet people's requirements. People hope that the door is intelligent, can sense the presence of pedestrians, and can be automatically opened and closed. The access control automatic control system is the core. The action of a person approaching the door or obtaining some kind of entry authorization can be regarded as a door opening signal, the door is opened by a driving system, and the door is automatically closed after the person leaves. Automatic doors bring great convenience to people's lives, and are now widely used in supermarkets, banks, hospitals, airports, shopping malls and other public places.

The automatic doors we see now come in many styles, such as: beautiful round doors, sensitive opening doors, remote-controlled garage doors, and so on. One of the core components of these automatic door access control systems is the sensor. Sensors are like human sense organs, eyes, ears, and nose, and are the medium through which automatic doors communicate with the outside world. The sensor is the core, and the peripheral circuit is equipped with safety devices, signal processing devices, motors, auxiliary control devices to form a complete automatic door control system.

The sensitivity, anti-interference, and blind spot size of automatic door sensors are all decisive factors for automatic door performance. If the sensitivity is not high, it will not bring convenience to people, and even may also cause injury. If the blind zone of the sensor is too large, it will reduce the working efficiency of the automatic door. People can only control the door opening by testing the position. Anti-interference is especially important in several performances. In the automatic door control system, the source of the control signal is a sensor. If the sensor sends an error signal, the signal processed by the entire control system is also wrong. For example, when the temperature of the sensor is high, it will generate a sensing signal by itself, which is not an external change caused by human movement. At this time, the control system processes this signal, and it is meaningless to control the automatic door opening and closing, which will only waste energy. We will discuss its anti-interference performance on the working principle of the commonly used automatic door sensor.

Automatic door sensor circuit introduction

At present, the commonly used automatic door sensors are mainly based on two principles: one is based on the principle of microwave induction. It is also called microwave sensor. It only senses the movement of objects, so it has a fast response speed and is suitable for being able to use normal steps. The main disadvantage of the sensor is that once the object is stationary, it will no longer respond and the automatic door will close. In this case, it is easy for people to be caught by the door. The other is based on the principle of infrared. It responds to the presence of objects, regardless of whether the person is moving. As long as the person is in the door of the visible area of ​​the sensor, the sensor can be sensed. The disadvantage is that the reaction speed is slow. These two sensors can be used together. For example, according to the characteristics of personnel flow, we can use microwave sensors on the lobby doors of office buildings and infrared sensors on office doors.

When the automatic door sensor detects someone entering, it sends a pulse signal to the main controller. The main controller judges the signal and then informs the motor to run. At the same time, it monitors the number of rotations of the motor so as to notify the motor to increase power and enter slow speed. run. After the motor receives a certain running current, it will run in the forward direction. The power will be transmitted to the synchronous belt, and then the power will be transmitted to the spreader system by the synchronous belt to make the door open. The controller will make a judgment after the door is opened. Drive the door in the opposite direction and close the door leaf.

The internal circuit of the automatic door sensor can be divided into signal detection circuit, signal amplification circuit, trigger blocking circuit, output delay circuit and relay control circuit.

Improving anti-interference through the unique design of the sensor

In nature, all objects above the absolute temperature can produce infrared spectrum. With the different surface area of ​​the object, the wavelength of the spectrum it produces is also different. The infrared wavelength and the energy of infrared radiation are related to the temperature. As we all know, human body temperature is stable at about 37 degrees. The wavelength of the infrared spectrum radiated by this constant temperature is also the same. The principle of a pyroelectric human body infrared sensor is actually easy to summarize: the pyroelectric effect is used to detect the infrared spectrum emitted by a person, and the detection is converted into an electrical signal output. Pyroelectric effect refers to that when some crystals are heated, opposite signs but equal amounts of charges are generated at the two ends of the crystal. This kind of thermoelectric electricity is called the pyroelectric effect, and this effect is only generated in specific crystal. This crystal is used to make a temperature-sensitive sensor.

The internal structural formula of the sensor is made of two pairs of very thin sheets of piezoelectric ceramic (PZT) material. Each pair of sheets leads to one electrode at each end, and the two pairs of sheets form two equivalent small capacitors C1 and C2. The two pairs of wafers are made on the same silicon wafer when they are manufactured, so that the equivalent capacitance formed by them can be polarized by itself, that is, the pyroelectric effect. The result of this effect is the formation of positive and negative charges with the same polarity and opposite polarity across the equivalent capacitor. At the time of production, the polarities of these two equivalent small capacitors are reversed in series, and the anti-interference performance of the automatic door sensor based on the pyroelectric sensor is formed by the principle of cancellation of the current generated by the two small capacitors.

When the sensor does not detect the infrared signal emitted by the human body, due to the thermoelectric effect of C1 and C2, they will have opposite polarities and equal charges at both ends of the capacitor. Positive and negative charges, but the polarities of these two capacitors are opposite in series, so the positive and negative charges cancel each other out. There is no current in the loop, and the sensor has no output. When the human body is still in the detection area of ​​the sensor, the energy of the detected infrared light energy is equal for C1 and C2, and the polarities are opposite. Photocurrents with the same energy and opposite polarities cancel each other out in the loop, the sensor still has no signal output; when the ambient temperature changes, the sensor itself is thermally released. In the case of electrical effects, because we made C1 and C2 on the same silicon wafer, the photocurrents they generate with opposite polarities and equal energy still cancel each other out in the loop, and the sensor has no output. The response of the sensor to a specific wavelength of infrared determines that it is only sensitive to changes in infrared radiation generated by the human body, and is not sensitive to human bodies that are stationary or moving slowly. Based on the direction serial connection on the same silicon wafer, the pyroelectric human body infrared sensor can withstand the interference of visible light and other infrared rays.

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