I. Performance characteristics
Among all kinds of switches, there is an element-displacement sensor, which has the ability to "sense" the approach of objects. Proximity switch uses the sensitive characteristics of displacement sensor to close the object to control the switch on and off.
When something moves towards the proximity switch and approaches a certain distance, the displacement sensor will "sense" it and the switch will act. This distance is usually called "detection distance". Different proximity switches have different detection distances.
Sometimes the detected objects move to the proximity switch one by one and leave one by one at a certain time interval, and so on. Different proximity switches have different response abilities to detected objects. This response characteristic is called "response frequency".
Second, the type
Because displacement sensors can be made according to different principles and methods, and different displacement sensors have different "sensing" methods for objects, the common proximity switches are as follows:
1. eddy current proximity switch
This kind of switch is sometimes called inductive proximity switch. It uses conductive objects to approach this electromagnetic field.
When approaching the switch, eddy current will be generated inside the object. This eddy current is reflected on the proximity switch, which changes the internal circuit parameters of the switch, thus identifying whether there is a conductive object approaching, and then controlling the on-off of the switch. The object that the proximity switch can detect must be a conductor.
2. Capacitive proximity switch
The measurement of this kind of switch is usually one plate of the capacitor and the other plate is the shell of the switch. During the measurement, the housing is usually grounded or connected to the equipment housing. When something moves close to the switch, whether it is a conductor or not, the dielectric constant of the capacitor will always change due to its proximity, so that the capacitance will change and the state of the circuit connected to the probe will also change, so that the switch can be controlled to be turned on or off. The detection object of the proximity switch is not limited to conductor, insulating liquid or powder, etc.
3. Hall proximity switch
Hall element is a magnetic sensitive element. Switches made of Hall elements are called Hall switches. When a magnetic object moves close to the Hall switch, the Hall element on the detection surface of the switch changes the internal circuit state of the switch due to the Hall effect, so that the presence of a nearby magnetic object can be recognized, and then the switch can be controlled to be turned on or off. The detection object of the proximity switch must be a magnetic object.
4. Photoelectric proximity switch
Switches made by photoelectric effect are called photoelectric switches. The light-emitting device and photoelectric device are installed in the same probe according to a certain direction. When a reflecting surface (the detected object) approaches, the photoelectric device will output a signal after receiving the reflected light, thus "sensing" that an object is approaching.
5. Thermoelectric proximity switch
Switches made of elements that can sense temperature changes are called thermoelectric proximity switches. In this switch, a thermoelectric device is installed on the detection surface of the switch. When an object with different ambient temperatures approaches, the output of the thermoelectric device changes, so that the approach of the object can be detected.
6. Other types of proximity switches
When the distance between the observer or the system and the wave source changes, the frequency of the approaching wave will shift, which is called Doppler effect. Sonar and radar are made by the principle of this effect. Ultrasonic proximity switch and microwave proximity switch can be made by using Doppler effect. When an object approaches, the reflected signal received by the proximity switch will produce Doppler frequency shift, so that whether the object approaches or not can be identified.
Third, the main use
Proximity switches are widely used in aviation, aerospace, aerospace technology and industrial production. In daily life, such as hotels, restaurants, garages, automatic doors, automatic air heaters have applications. In terms of security and anti-theft, such as information
Archives, accounting, finance, museums, treasury and other important places are usually equipped with anti-theft devices composed of various proximity switches. In measurement technology, such as length and position measurement; In control technology, such as the measurement and control of displacement, velocity and acceleration, a large number of proximity switches are also used.
Fourth, choose the matters needing attention
In general industrial production sites, eddy current proximity switches and capacitive proximity switches are usually selected. Because these two proximity switches have low environmental requirements. When the measured object is a conductive object or an object that can be fixed on a metal object, eddy current proximity switch is generally selected because of its high response frequency, good anti-environmental interference performance, wide application range and low price. If the measured object is nonmetal (or metal), liquid level height, powder height, plastic, tobacco, etc. Capacitive proximity switches should be selected. This switch has low response frequency, but good stability. Environmental factors should be considered during installation. If the object is made of magnetic conductive material or magnetic steel is buried in the object to be measured in order to distinguish the object moving with it, the Hall proximity switch should be selected, which has the lowest price.
When the environmental conditions are good and there is no dust pollution, the photoelectric proximity switch can be used. The photoelectric proximity switch has little effect on the measured object when it works. Therefore, it is widely used in demanding fax machines and tobacco machinery.
In the anti-theft system, automatic doors usually use pyroelectric proximity switches, ultrasonic proximity switches and microwave proximity switches. Sometimes, in order to improve the reliability of identification, the above proximity switches are often used in combination.
No matter which proximity switch you choose, you should pay attention to the requirements of working voltage, load current, response frequency, detection distance and other indicators.
1 overview
A new generation of photoelectric switch devices manufactured by integrated circuit technology and SMT surface mounting technology have intelligent functions such as delay, broadening, external synchronization, anti-interference, high reliability, stable working area and self-diagnosis. This new photoelectric switch is an active photoelectric detection system electronic switch with pulse modulation. It uses cold light sources, such as infrared light, red light, green light and blue light. , can control the state and action of solid, liquid, transparent body, blackbody, software, smoke and other substances quickly and lossless.
The contact travel switch has some shortcomings, such as slow response speed, poor accuracy, easy damage to the detected object and short service life, while the transistor proximity switch has a short working distance and cannot directly detect non-metallic materials. The new photoelectric switch overcomes these shortcomings, and has the advantages of small size, multi-function, long life, high precision, fast response, long detection distance, strong anti-optical, electrical and magnetic interference.
At present, this new photoelectric switch has been used in many fields, such as crop position detection, liquid level control, product counting, width discrimination, speed detection, fixed-length shearing, hole identification, signal delay, automatic door induction, color code detection, punching machine and shearing machine, safety protection and so on. In addition, it can also be used as an anti-theft alarm in banks, warehouses, shops, offices and other necessary occasions by taking advantage of the concealment of infrared rays.
2 Classification of photoelectric switches
2. 1 minute press detection method.
Taking the products of Shanghai zhonghu electronic instrument factory as an example, this paper introduces the classification methods of photoelectric switches: reflective, opposite and mirror according to the detection methods. The opposite detection distance is far, and the density (transmittance) of translucent objects can be detected. The reflection working distance is limited near the intersection of beams to avoid background influence. The specular reflection type has a long reflection distance, which is suitable for long-distance detection and can also detect transparent or translucent objects.
Table 1 gives the detection and classification methods and characteristics of photoelectric switches.
2.2 Classification by structure
According to the structure, photoelectric switch can be divided into three types: amplifier separation type, amplifier built-in type and power supply built-in type.
Amplifier separation is to separate the amplifier from the sensor, which is made of special integrated circuit and mixed installation process. Because the sensor has the characteristics of subminiature and diversity, the amplifier has more functions. Therefore, this model adopts terminal station connection mode and can be used for AC and DC power supply. It has the function of on-off delay, can set the switch between light and sound, can control six output States, and has two output modes: contact and level.
The built-in amplifier integrates the amplifier with the sensor, is made of special integrated circuit and surface mount technology, and works with DC power supply. Its response speed (0. 1ms and 1ms) can detect narrow and high-speed moving objects. Changing the polarity of power supply can switch between bright and dark motion, and self-diagnosis and stable working area indicator light can be set. It has two output modes of voltage and current, which can prevent mutual interference and is very convenient for system installation.
Built-in power supply is integrated by amplifier, sensor and power supply device, using application specific integrated circuit and surface mount technology. Generally, AC power supply is used, which is suitable for replacing contact travel switch in production site and can be directly used in high voltage control circuit. You can also set a self-diagnosis stable work area indicator light, and the output is equipped with SSR solid-state relay or relay normally open and normally closed contacts to prevent mutual interference, which can be closely installed in the system.
3 photoelectric switch working principle
Figure 1 is the working principle block diagram of reflective photoelectric switch. In the figure, the modulation pulse generated by the oscillation circuit passes through the reflection circuit and is then radiated by the light emitting tube GL. When the measured object enters the action range of the optical receiver, the reflected light pulse enters the phototransistor DU. In the receiving circuit, the optical pulse is demodulated into an electrical pulse signal, amplified by an amplifier, synchronously gated and shaped, and then the interference is eliminated by digital integration or RC integration. Finally, the driver is triggered to output the photoelectric switch control signal with delay (or without delay).
Generally, photoelectric switches have good return error characteristics, so even if the detected object shakes in a small range, it will not affect the output state of the driver, thus keeping it in a stable working area. At the same time, the self-diagnosis system can also display the light receiving state and stable working area, so as to monitor the working situation of photoelectric switch at any time.
4 characteristics of photoelectric switch
MGK series photoelectric switch is the product of the development of modern microelectronics technology and the upgraded product of HGK series infrared photoelectric switch. Compared with the previous photoelectric switch, it has its own remarkable characteristics:
● It has the functions of self-diagnosis and indication of stable working area, and can inform whether the working state is reliable in time;
● Opposing, reflecting and mirror photoelectric switches have the function of preventing mutual interference and are convenient to install;
● Setting the ES external synchronization (external diagnosis) control terminal can check whether the photoelectric switch works normally before operation. It can accept the interrupt or detection instruction of computer or programmable controller at any time, and the appropriate combination of external diagnosis and self-diagnosis can make the photoelectric switch intelligent;
The response speed is low, and the response speed of the high-speed photoelectric switch can reach 0. 1ms, and 300,000 detection operations can be performed per minute, which can detect tiny objects moving at high speed;
● Adopt special integrated circuit and advanced SMT surface mounting technology, with high reliability;
● Small size (minimum is only 20×3 1× 12mm), light weight, simple installation and debugging, and short circuit protection function.