Safety distance between marine instrument EMC and compass
It was learned from the National Ship Industry Economic Research Center that last year China ’s total shipbuilding capacity was about 9 × 106t. The new ship Tingdan exceeded 17 × 106t, accounting for 17% of the global market. In addition, domestic shipyards already hold orders for ships of 37 × 106t, which is enough to build in the next three years. In the past two years, marine instrumentation has been growing at a rapid rate of 20% to 30% every year. Individual emerging areas such as shipborne navigation data recorders (ships above 500t are required to be installed), security alarm devices (all ships traveling on the high seas after "9.11" To be installed) is more rapid development.
Electromagnetic compatibility (EMC) means that a device or system can work normally in its electromagnetic environment and does not constitute unbearable electromagnetic interference to anything in the environment. It contains the duality of interference and anti-interference.
Due to special use occasions, the marine instrument has a narrow space (limited cabin position), long continuous working time (a voyage has several months or even a year), high safety and reliability requirements, and many types of instruments (including operation, Control, image, sound, alarm, storage, reception, playback, etc.), the signal type is complex (there are digital signal, analog signal, interference signal), and the frequency is from high frequency to low frequency (50Hz ~ 2GHz). Therefore, preventing the interference between the various systems, preventing the interference between the instruments and equipment, and preventing the interference between the wires, etc. The above-mentioned anti-interference forms the EMC of the marine instrument, which becomes an urgent problem to be solved.
Marine instrumentation EMC and compass safe distance Marine instrumentation has become one of the earliest and internationally integrated industries due to its particularity and international practice (all systems, instruments, equipment and pipelines used on ships must be approved by the classification societies of all countries of the ship) . The electromagnetic compatibility of marine instruments has become a mandatory test. The main test items are: energy fluctuation; conducted disturbance; radiation disturbance of the shell port; electrostatic discharge immunity; radio frequency electromagnetic field radiation immunity; electrical fast transient pulse group immunity; Surge immunity; low frequency conduction immunity; RF field induced conduction immunity: Luoj Jing safety distance, etc.
The compass, divided into standard compass and rudder compass, is installed in the cockpit. As the most important key part of the ship's navigation, it determines the direction, speed and time of the ship's navigation, which is directly related to the safety of the ship. Therefore, to ensure the safety of the compass, eliminating the interference of all systems, instruments, and pipelines has become the key technology of marine instruments. Now, more and more instruments in the cockpit are required to provide the minimum safety distance from the compass to determine the installation location of the product in the cockpit. Related international and domestic standards are also being formulated and enforced continuously (such as IEC60945).
The safety distance between the sample and the compass is defined as the distance between the closest point of the unit under test and the center of the compass or the magnetometer. This distance is to ensure that the standard compass does not produce a deviation of more than 5.4 ° / m. (H is the magnetic flux density at the test position in the horizontal direction) For the rudder compass, spare rudder compass, and emergency compass, the required deviation is 18 ° / H.
The test method for the minimum safe distance of the compass is to use the relationship between the earth's magnetic field strength and the angle to convert the measurement of the angle into the measurement of the magnetic field strength. The test methods in Shanghai are shown in Table 1. 
(1) Test to obtain the horizontal component of the local magnetic field strength in Shanghai;
(2) In an open place where there is no other magnetic interference except the geomagnetic field, place the magnetometer horizontally at 80 cm from the ground and adjust the direction of the magnetometer so that the reading is zero;
(3) Place the components of the device under test at the same height, move the components of the device under test, and find the longest distance where the magnetometer reads the last two items in Table 1;
(4) Place each part of the device under test in a DC field (103 / 4Ï€A / m) and an AC field (18x103 / 4Ï€A / m, 50Hz,) to perform magnetization, and repeat the above test.
Repeat the above test when the system is powered on or off, magnetized or not, to find the maximum value, which is the minimum safe distance from the compass. The smaller the compass safe distance reflects, the less the sample interferes with the compass, and the better the anti-interference ability of the sample.
The minimum safety distance between the test and the compass is a comprehensive project for a specific instrument, and it is completed and passed through energy fluctuations; conducted disturbance (CE); radiation disturbance (RE) at the shell port; electrostatic discharge immunity (ESD) ); RF electromagnetic field radiated immunity (RS); electrical fast transient burst immunity (EFT); surge immunity (Surge); low frequency conducted immunity; RF field induced conducted immunity (CS) ) The last test carried out on the basis of other tests. The minimum safety distance of the compass is a response to the comprehensive EMC capability of a product.
The EMC problems of common marine instruments compare the EMC problems of domestic and international marine instruments in the past two years, and find a very interesting phenomenon, that is, the anti-interference ability of domestic samples is very strong, such as energy fluctuation, low-frequency conduction immunity is almost unqualified ; Conducted disturbances, radiated disturbances on the shell port, electrical fast transient pulse group immunity, electrostatic discharge immunity, etc. often have problems. However, foreign samples have good interference capabilities, such as conducted disturbances and radiation disturbances on the shell port. Generally, they have poor anti-interference capabilities, such as energy fluctuations, electrical fast transient pulse group immunity, and RF field-induced conducted immunity. It is often more difficult to pass.
The following 48 samples were selected. The production sites are Zhongtong (35), France (1), Germany (3), the United States (3), Singapore (2), Japan (1), Sweden ( 3), the samples include shipboard navigation data recorder, data protection container, security alarm device, cabin liquid level control system, liquid level alarm device, fire alarm device, marine radar system, portable computer, etc. The unqualified statistics of the first test are listed in Table 2.
A few suggestions for EMC of marine instrumentation is a system project, which must be considered from the design. Generally, it can be selected from power supply design, printed circuit board design, chassis shielding, connector selection, power line / control signal line filtering, and grounding line. And cable selection.
a Power supply
(1) Separate the first and second circuits as much as possible (to reduce the impact of interference on the later signal);
(2) The rated voltage is preferentially 220V (in the EFT and Surge tests, the impact voltage of the power supply is the same, but the ratio of 24V and 220V relative to the impact voltage is different);
(3) It is better to use a power cord alone when the working current exceeds 5A.
b Printed circuit board
(1) Separate digital circuits and analog circuits (reducing interference);
(2) Try to increase the area of ​​the linear circuit ground (the interference can quickly go from the ground);
(3) The ground wire forms a closed loop (reducing the potential difference);
(4) The key parts are selected to pass the EMC test.
C chassis
(1) The chassis considers magnetic shielding and electrical shielding (radiation emits two types of magnetic field and electric field);
(2) Sealing gaskets and conductive rubber (to prevent electromagnetic leakage at the gap) should be used at the connection of the chassis, and the insulating paint at the opening must be cleaned (to prevent interference from accumulating on the chassis);
(3) Keep the wiring as far away from the interference source as possible (to prevent circuit interference and wire coupling interference, the signal line should be far away from the power line).
d Ground wire
(1) The cabinet body should be grounded as far as possible (grounding impedance should be ≤4Ω);
(2) The grounding wire should be thickened as much as possible (preferably ≥ 3mm, because the grounding potential changes with the current).
e Power line / control signal line filtering
(1) In the case of sufficient space, select a filter with a high rated voltage (can withstand a sufficiently high impulse voltage);
(2) Selection of ferrite magnetic ring:
â‘ Select the same frequency band magnetic ring as the interference frequency band to be suppressed;
â‘¡If the original common-mode common-path impedance is high and the added impedance of the choke is small, the effect of the magnetic ring is not obvious. Consider the bypass capacitor at the power supply end or replace the appropriate filter;
â‘¢ Eliminate other interference sources in the system.
f The cable is both an interference generator and an interference receiver. It is directly related to the reliability of data transmission, bit error rate, image distortion, control signals, etc. It is very important to choose a suitable cable. (Can be considered from the shielding, material, structure, etc. of the cable)
The conclusion is learned from the national shipbuilding industry economic research center. Last year, China's total shipbuilding capacity was about 9 × 106t. The new ship Tingdan exceeded 17 × 106t, accounting for 17% of the global market. In addition, domestic shipyards already hold orders for ships of 37 × 106t, which is enough to build in the next three years.
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