with the development of wireless network technology and electronic technology, wireless equipment has been widely used in petroleum, coal, chemical and other industrial production sites with flammable and explosive substances, wireless communication devices and other radio frequency source equipment is one of the most basic functions of intelligent handling robots, and the large number of intelligent handling robots undoubtedly make it one of the important potential danger ignition sources [1]. in order to meet different requirements, more and more wireless technology protocols have been formulated, and in order to make them work in explosive atmospheres, different explosion-proof technologies are used to improve the safety performance of wireless communication devices and effectively reduce the risk of explosion [2].
In this paper, an explosion-proof navigation instrument for exibIIAT6Gb explosive atmosphere mapping provided by a company (hereinafter referred to as Sample A) is used as an example, analyzes its structure and working principle, and conducts in-depth research on its explosion-proof performance.
1 application of explosion-proof technology for wireless communication devices in explosive atmospheres
wireless communication devices are used in explosive environments, and it is necessary to conduct comprehensive analysis and evaluation of potential hazardous ignition sources in electronic circuits, including static electricity, electric sparks, hot surfaces and radio frequency sources, and use explosion-proof technologies such as explosion isolation, intrinsic safety, and safety enhancement to meet the requirements of relevant standards.
1.1 structure of wireless communication devices in explosive atmospheres
The navigation principle of the navigator is shown in Figure 1, first using the Bluetooth at the PDA end to connect with Sample A, the omnidirectional antenna in the box machine receives the global positioning system (GPS), the global navigation satellite system (GLONASS), and the Beidou satellite signal, and transmits the received satellite data to the motherboard; the PDA uses the GPRS network module or WiFi module to obtain the differential data issued by the CORS base station, and then uses Bluetooth to transmit the differential data to the Sample A motherboard The Sample A motherboard uses the acquired satellite data and differential data to solve the high-precision coordinate data, and transmits the high-precision coordinate data back to the PDA side using the built-in Bluetooth for the software of the PDA side to call, so that the software on the PDA side can obtain and use the high-precision coordinates [3].
From the working principle of Sample A, it can be seen that Bluetooth, WLAN, GPRS, GPS, GLONASS, Beidou, etc. involve wireless technology, due to the high requirements of the test range and accuracy required by the navigator, some RF sources may have high power or energy conditions.
1.2 explosion-proof technical analysis of wireless communication devices in explosive atmospheres
Wireless communication devices are usually composed of housings, cables and cable entry devices, printed circuit boards, batteries and battery packs, radio frequency circuits (including antennas) and other parts. The Sample A housing consists of a light metal and plastic cover, the battery and battery pack, and the printed circuit board are designed with intrinsically safe technology, and the antenna as the RF source part is placed at the bottom of the plastic cover
IN VIEW OF THE POTENTIAL HAZARDOUS IGNITION SOURCE OF WIRELESS COMMUNICATION DEVICES, GB3836 SERIES OF STANDARDS HAS FORMULATED CORRESPonDING TECHNICAL MEASURES TO REDUCE RISKS, INCLUDING EXPLOSION-PROOF TYPE, SAFETY-INCREASING TYPE, INTRINSIC SAFETY TYPE, POURING TYPE AND OTHER EXPLOSION-PROOF FORMS, SO THAT THE EQUIPMENT HAS EXPLOSION-PROOF PERFORMANCE SUITABLE FOR THE EXPLOSION ENVIRONMENT. FOR WIRELESS COMMUNICATION DEVICE COMPonENTS WITH POTENTIAL HAZARDS SUCH AS CIRCUIT BOARDS, NON-metaLLIC PARTS, BATTERIES AND BATTERY PACKS, AND RF CIRCUITS, MEASURES SHOULD BE TAKEN TO DESIGN THEM SEPARATELY.
2.1 board
the overload, short circuit and open circuit state of the circuit board during the working process are very easy to generate hot surfaces and electric sparks. in order to avoid the corresponding ignition hazard, one method is to put the circuit board into a flameproof housing or an increased safety housing to isolate the explosion hazard, and the other method is to use intrinsic safety technology to limit the current and voltage in the circuit below the ignition value of the explosive atmosphere. the above two methods can achieve the purpose of reducing the risk of electric sparks. the spacing of housing protection, creepage distance and electrical clearance is the premise of achieving the corresponding safety factor.
2.2 non-metallic enclosures and non-metallic parts of the enclosures
NON-metaLLIC SHELL AND SHELL NON-metaLLIC PARTS OF THE MATERIAL IS MAINLY PLASTIC MATERIALS AND ELASTIC MATERIALS, IN ADDITION TO THE CORRESPonDING THERMAL STABILITY REQUIREMENTS OF THE MATERIAL, SUCH AS HEAT RESISTANCE, ConTINUOUS OPERATING TEMPERATURE (COT), RELATIVE HEAT INDEX (RTI- MECHANICAL IMPACT), ETC., NON-metaLLIC SHELL SHOULD BE DESIGNED TO AVOID IGNITION HAZARD CAUSED BY ELECTROSTATIC CHARGE DURING NORMAL USE, MAINTENANCE AND CLEANING. TO AVOID THE ACCUMULATION OF ELECTROSTATIC CHARGE ON ELECTRICAL EQUIPMENT, THE GENERAL METHOD: 1) REASonABLE SELECtION OF MATERIALS, THE ADDITION OF ANTISTATIC AGENTS, SURFACE ANTISTATIC COATINGS, ETC., SO THAT THE SURFACE INSULATION RESISTANCE IS NOT MORE THAN 109Ω; 2) ACCORDING TO GB 3836.1-2010 TABLE 6 LIMIT THE SURFACE AREA OF NON-metaLLIC PARTS OF THE SHELL.
2.3 batteries and battery packs
due to the defects in the structure and principle of the battery, when the battery is short-circuited externally or internally short-circuited, it will generate an excessive current, and the release of a large amount of energy in an instant will produce extremely high temperatures and even explosions. therefore, the battery must be subject to flame isolation measures or intrinsic safety treatment so that it can be applied to explosive atmospheres. carry out type tests such as charge and discharge tests, short circuit tests, and spark tests.
2.4 shell and whole machine
THE EFFECTIVE IMPLEMENTATION OF THE ABOVE TECHNOLOGIES DEPENDS ON THE MECHANICAL PROPERTIES AND AGING OF THE HOUSING AND THE WHOLE MACHINE, AS WELL AS THE SETTING OF THE CORRESPonDING SAFETY FACTOR. THEREFORE, FOR FLAMEPROOF TECHNOLOGY, THE IP PROTECTION LEVEL MUST MEET THE REQUIREMENTS OF IP54 AND ABOVE; FOR INTRINSIC SAFETY EQUIPMENT, IT MUST MEET THE REQUIREMENTS OF IP20 AND ABOVE; FOR PORTABLE EQUIPMENT, IT MUST MEET THE REQUIREMENTS OF THE DROP TEST; AND FOR THE metaL PARTS AND TRANSPARENT PARTS IN THE EQUIPMENT, IT MUST MEET THE REQUIREMENTS OF THE IMPACT TEST; FOR THE PORTABLE EQUIPMENT, IT MUST MEET THE REQUIREMENTS OF THE DROP TEST; AND FOR THE metaL PARTS AND TRANSPARENT PARTS IN THE EQUIPMENT, IT MUST MEET THE REQUIREMENTS OF THE IMPACT TEST; ACCORDING TO DIFFERENT EXPLOSION-PROOF FORMS, THE CORRESPonDING MAXIMUM SURFACE TEMPERATURE REQUIREMENTS ARE MET.
3 testing requirements and items
According to the relevant explosion-proof standards, combined with the application analysis of the above-mentioned wireless communication devices and explosion-proof technology, the corresponding inspection and testing items and requirements are formulated for the potential hazardous ignition sources in different parts of Sample A, as shown in Table 1. The explosion-proof performance of the equipment is guaranteed by various indicators, and any one of them that does not meet the requirements will bring a corresponding explosion risk. Therefore, a one-vote veto system is implemented when judging the test results, and even if there is only one nonconformance, the inspection conclusion should be judged as unqualified. The above testing items and requirements mainly include the application of general explosion-proof technology and intrinsic safety technology, through the detection of products, the basic explosion-proof performance of wireless communication devices can be completed and evaluated. The national relevant explosion-proof standards put forward the power or energy requirements of the RF source, see Table 2, the safety performance of the RF source has not been given corresponding attention, the type test method is not clear, this paper in this regard for further in-depth research and analysis.
4 analysis and discussion
STUDIES HAVE SHOWN THAT ELECTROMAGNETIC FIELDS AND ELECTROMAGNETIC WAVES ARE ONE OF THE IGNITION SOURCES. IT IS ALMOST IMPOSSIBLE FOR ELECTROMAGNETIC FIELDS TO DIRECTLY IGNITE EXPLOSIVE ATMOSPHERES, AND IGNITING A RADIATION SOURCE UNDER LABORATORY ConDITIONS REQUIRES HUNDREDS OF WATTS OF POWER [7]. HOWEVER, THE BIGGEST DANGER COMES FROM THE CURRENT INDUCED BY THE ELECTROMAGNETIC FIELD ON THE metaL STRUCTURE OR ELECTRonIC CIRCUIT WITH FAILED EMI PROTECTION, AS SHOWN IN FIGURE 3. THE INDUCED CURRENT WILL PRODUCE EXTREMELY HIGH SURFACE TEMPERATURES AND SPARKS. THREE ConDITIONS MUST BE MET FOR SUCH IGNITION TO OCCUR: 1) THE PRESENCE OF AN EXPLOSIVE ATMOSPHERE, 2) THE PRESENCE OF SUFFICIENT ENERGY, AND 3) THE PRESENCE OF A RING-SHAPED ConDUCTOR STRUCTURE AND THE PRESENCE OF A DISConNECT POINT OR A SUDDEN DISConNECTION OF THE ConTACT POINT [6].
Therefore, in order to avoid the above situation, based on the consideration of risk control, one of the ways to achieve the requirements of safety performance is to reduce the emission function or energy of the RF source to reduce it below the limit of the ignition energy of the explosive atmosphere. After years of research, the limits of different frequencies and forms of RF sources in different explosive environments have been recognized by the industry and unified, and finally the PD CLC/TR 50427:2004 "Guidelines for the evaluation of Accidental Ignition of RADIO Frequency Radiation in Explosive Atmospheres" was formed, and was adopted by the IEC 60079 standard. GB 3836.1-2010, which is based on the revision of the IEC 60079.0 standard, further improves the technical requirements related to RF sources and specifies the corresponding energy limit requirements[6]. Table 2 lists the possible RF source energy or power limits for explosion-proof wireless communication devices with different classes in the 9 kHz to 60 GHz range, which do not allow programmable or software to change them. Among them, the threshold power part includes continuous emission and the emission of the pulse time exceeding the thermal ignition time; the threshold energy part is the radio frequency emission with a pulse time shorter than the thermal ignition time [8]. Table 2 also takes into account higher power emissions near the hazardous area.
in the setup of wireless communication devices in explosive atmospheres, the rf source is usually in a flame-proof structure or other type of explosion-proof form. most of the flameproof shell is a metal structure, which will affect the electromagnetic waves emitted from the antenna, the solution is to install a directional antenna behind the glass, but the signal loss is more serious and limits the flexibility of use. another approach is to use an ampere-boosted external antenna, but ensure that there is no excessive current or voltage when the power supply is short-circuited with the input and output of the rf device. antennas are not allowed to be used in explosive atmospheres without corresponding testing. with the development of intrinsic safety technology and wireless technology, wireless devices using intrinsic safety technology have obvious technical advantages. the circuits in the wireless device are intrinsically safely designed, and after the antenna gain and emission energy are safely evaluated, it is ensured that the rf source cannot ignite explosive substances. whether it is the use of explosion-proof explosion-proof technology, or the use of intrinsic safety technology, or other explosion-proof technology, these technical measures are only for potential hazardous ignition sources such as electric sparks, static electricity, hot surfaces, etc., and the dangers existing in rf sources are not technically treated. according to the above analysis, the rf source processing method can be achieved by limiting the transmitted power or energy, since it is not in the standard
there are restrictions, the design methods used in the actual product are unpredictable, and the corresponding detection method standards do not give specific guidance, so it is particularly important to accurately and effectively test and evaluate the explosion-proof performance of rf sources. the test system is shown in figure 4.
Therefore, according to the test requirements and items formulated in Table 1, this paper tests Sample A, and the results show that in addition to the safety performance of the RF source, other items have passed the type test consistently. For the RF source safety of Sample A, this article reviews the technical data of Sample A in conjunction with relevant communication specifications and explosion-proof standards. After testing, Sample A mainly applies Bluetooth, GPS, GLONASS, Beidou satellite, from the drawing review can know that GPS, GLONASS, Beidou satellite power is far less than the standard threshold of 10 mW. The Bluetooth transmission power can be set by software, up to 25 W, much higher than the standard threshold, the risk of explosion is high. Therefore, the explosion-proof performance of RF sources must be tested and evaluated, which can effectively reduce risks and improve safety in explosion-hazardous places.
5 conclusion
On the basis of comprehensive analysis of the structure and explosion-proof technology of wireless communication devices in explosive atmospheres, taking Sample A as an example, combined with explosion risk assessment, relevant testing requirements and testing items are proposed, and judgment principles are given according to the standard testing requirements, providing a basis for product testing and certification of wireless communication devices. It is proposed that the safety performance detection and evaluation of the RF source is particularly important and urgently needs to be solved, and the accurate and effective detection and evaluation of the RF source is the key to reducing the explosion risk of the wireless communication device, which will further improve the safety performance of the wireless communication device used in the explosive environment.
