EM compatibility of library automatic equipments
[Abstract] We have briefly introduced the basic concepts of EM compatibility and we have analysed many EM compatibility problems in library automatic equipments. We have discussed some techniques and managing ways about EM interference.
[Key word] library, equipment, EM compatibility, EM interference
1 Issues raised
With the fast development of technology, many powerful and complicated auto equipments are used in scientific research, industry and agriculture fields widely. At the same time, they started entering our work or our daily life. On the one hand, they bring more convenience, but on the other hand, they bring serious EM interference. EM interference always makes equipments failure in libraries, and the expanding use increases the EM interference issue. We have to use auto equipments under the complicated environment. Maybe some people think it is not dangerous and the EM interference just causes little worse TV showing pictures or some cacophony in sound equipments.
In fact, this is not the case. It has been proved that, heart pacemaker for heart disease works abnormally with EM interference which endangers human life; and electronic aircraft navigation system stops working with EM interference which causes aviating accident; computer server cannot work with strong outer interference or the leaking EM information could be intercepted and resumed by modern techniques deliberately.
Past “social effects of pollution” mean water pollution or air pollution or noise pollution that threaten human. And now, the EM interference by so many kinds of auto equipments becomes another problem for us. The danger by EM interference is around us. How to improve the adapting capability of auto equipments in complicated EM environment efficiently? Library technicians need to know the EM compatibility of the auto equipments.
2 What is the EM compatibility (EMC)?
EMC is a new interdisciplinary comprehensive application science. As an edge technology, it is based on electric and radio technology and it refers to many new technical fields such as micro-electronics, computer technology, microwave technology, communication technology, network technology and new material application. EMC has a wide range which almost covers all the application fields of auto equipments such as electric power, communication, radio, spaceflight, war industry, computer, medical treatment.
There are detailed explanations of the related concepts or definition or terms of EMC in 1995 GB/T 4365 “EMC terms”. We will introduce some major concepts here.
2.1 EME (Electromagnetic Environment)
It means the summation of all the EM phenomena in given places. Given places, viz. spaces, mean all the EM phenomena including all time and all spectrums.
2.2 EMC (Electromagnetic Compatibility)
EMC is a capability that the equipment or system could normally work in EM environments without interfering any other equipment into serious EM interference. There are two meanings, viz. EM transmitting of EMC equipments or systems never has serious influence on other equipments or systems; EMC equipments or systems have a strong anti-interference capability. There are other two concepts for this -- Electromagnetic Interference (EMI) and Electmmagnetic Suseeptibilkr (EMS).
EMI may reduce the capability of equipment or transmitting channel or system.
EMI means all the EM phenomena that cause capability reducing of equipments or systems or that may do harm to life. EMI is composed by interference source, coupling channel and receiver. According to transmission of interference, there are two EM interferences and they are radiated interference (RI) and conducted interference (CI). RI is transmitted by space following the features and rules of EM wave but not all equipments could radiate EM wave; CI is transmitted by conductors, so there must be a complete circuit connection between interference source and receiver for CI.
Generally, higher sensitivity means lower anti-interference capability. EMS reflects anti-interference capability from different points of view. It is expressed by electric potential. Lower sensitivity electric potential (at the beginning of capability reducing) means higher sensitivity and lower anti-interference capability; and higher anti-interference electric potential means higher anti-interference capability and lower sensitivity.
There are two kinds of EMS sensitivity and they are radiated sensitivity and conducted sensitivity. The hotspot in present EMC are: feature and transmission of EMI source, harm effect of EMI, EMI suppression technology, use and management of EM spectrum, EMC standards, EMC test and measurement, EM leaking and static electricity distribution.
3 EMC issues of library equipments
Increasing number of all kinds of special equipments and large use of wireless equipments/electromotion equipments/high frequency equipments result in increasing EM interference, which results in EMC issue. The equipments, that would interfere with others or that are easy influenced, are mainly book detector and terminal and computer by observation and test. The interfering phenomena and reasons are discussed later.
3.1 Interference on detector
Book detector is special equipment for libraries and it is EMSS (Electronical Magnetic Surveillance System) equipment. It is based on EM induction principle.
Set detecting shelves immovably, and connect transmitting loops in one side of the shelf with sinusoidal oscillation signal to establish certain frequency EM field; and detect certain frequency EM signal by receiving loops in the other side of the shelf. When the books with detecting medium (EM strips) are inside detecting channels, the detectors will alarm with sound and light automatically. Its power frequency is normally 2KH~3KH sinusoidal fundamental frequency, when the f0 equals 2KH and n is on 4 level, the detecting frequency is f40=16KH. The outer environment has influence on it easily because it is working in thousands KH sinusoidal oscillation critical condition and its circuits design emphasized detecting sensitivity.
(1) Variable frequency & speed governing elevator interferes with detectors
The detector alarms when elevator starts up or stops, and the interference can not be avoided. The commuting bridge of transducer is non-linear load for power grid, and its harmonic bring harmonic interference to other electronic or electric equipments in the same grid. The inverter circuit of transducer is based on PWM technology and IGBT high-power tube is necessary. When controlling circuits bring corresponding on-off pulse in certain frequency and amplitude and IGBT high-power tube is in work, the output voltage and current wave will take corresponding high-order harmonic group. It is known that frequency of high carrier wave and high-speed switch dv/dt of field-controlling switching devices could be more than 1kv/Ls, so the caused radiating interference is obviously. And except transmitting interference by radiation, the variable frequency & speed governing circuit could bring interference into supply power circuits by impedance coupling or earthing loop coupling.
(2) Discontinuous interference on detector by copier.
Copier interference is from control circuit and static electricity eliminating circuit of DD motor as conducting interference through power supply, so if the copier and detector are not in the same phase power supply, then copier would not interfere with detector.
(3) Other interference.
When 150M interphones for guards is using in the area of 20 m2, it will interfere with detector by EM radiation in space into abnormal alarm. When the TV sets in reading room are too close to detectors, because their close frequency (fTV=15.625KH), the TV sets will interfere with detector easily by radiation. The transmitting antennae of personal handy-phone system on the roof of libraries, large used electronic daylight lamp, large power interphones, base station of cordless phone, and even the ring shape metal things with readers, and all kinds of peak pulse or high frequent harmonic wave from power will interfere with detector.
3.2 Interference on terminal and computer.
(1) The interference on terminal and computer from book detector.
Connect sinusoidal oscillation signal to loop inside transmitting shelf of detector, and a alternating magnetic field with 2KH~3KH excitation frequency will be established and the strength of strip signal field is determined by this field. Because the detector established alternating magnetic field that the frequency close to audio around transmitting shelf, so the detector will interfere with other closer equipments.
If the area around detector from terminal or computer is less than 3 m2, the CRT will be influenced with bending scanning beams in display screen with water wave image. If the parallel distance between computer communicating circuits (such as the fifth kind twisted-pair) and detector transmitting or receiving circuits is morn than 5m, the interference will be worse and percussing keyboard may cause alarm. Under 2KH~3KH detector working that is closer to audio frequency, there could be a harsh scream. This can be a noise pollution which influences reading and makes bad feel.
(2) Static electricity could form interference. Static electricity could be produced by rotational equipments or moving objects or friction in dry inner environment. Especially in winter of northwest china, static electricity can be easily produced because of dry air. Lots of static electricity charge accumulating on disk will cause read-write error. Static electricity accumulating on objects or human bodies will result in wrong order on computer or mainboard with touch (for instance, a man taking static electricity when percussing keyboard cause server being closed accidently). Static electricity may destroy CMOS connecting, too. Static electricity makes objects adsorb lots of dust and influence cooling and insulation of equipments.
(3) Doppler wireless anti-thief equipments interfere with computers.
60 channels Doppler wireless anti-thief alarm system in present library is based on separate Doppler probe to detect moving objects. The probe is designed by special RD627 integrate circuit, and its inner circuit is composed by oscillator, detector, Doppler signal amplifier, limitr transmitting circuit and UPS. Oscillator will produce microwave signal to form a solid microwave protecting space in a fan area. When a man or an object is moving in the area, reflected signal changes frequency with original signal, the weal frequency-changed signal will be processed by detector, and amplify obtained Doppler signal to produce 45~60M wireless signal by wireless transmitting circuit, and the host deals with it to carry out wireless centralization and auto alarm. Microwave signal and 45~60 wireless signal of wireless transmitting circuit from probe have a strong radiation EMI, and they cause serious cacophony which cannot be solved.
It is not very difficult to find that there still are many problems in the EM compatibility of book detector and variable frequency & speed governing elevator/wireless anti-thief equipment at present. The elevator has strong interference on detector, and the wireless anti-thief equipment are easy be influenced meanwhile it interfere with others. There are many other equipments has such problems with detector actually only we cannot find them obviously, but the potential treat has had influence on library auto equipments. Hidden safety trouble of EM leaking is contained in EM compatibility, too. EM leaking refers to leaking useful signal, though it is weak, it can be captured/amplified/decoded easily by modern ways for baleful people. The harm of EM leaking is as serious as interference, so to prevent such hidden safety trouble will be a main task for us.
4. Control the EMI of library equipment
The terminal aim is to suppress EMI rationally and effectively. In terms of the kinds and features of EMI, there are many ways.
4.1 Space separation.
Chose proper place and location. Separation by buildings. Control setting angle and vector direction of electricity field/magnetic field. In fact, we could avoid or lead it by nature separation of buildings or proper setting location/direction to control it by our best. For instance, it is necessary to consider transmitting/receiving direction of detector and to avoid elevator/TV/computer.
Avoid using the interfering equipments at the same time. Such as limit elevator when detector is in use.
4.3 Frequency management.
Frequency control/frequency modulation/digital transmission/photoelectricity conversion. Frequency control refers that avoid using equipments with same frequency together and attend multiple frequency among them. Frequency modulation means that to modulate frequency to avoid interference. All the interference could be controlled best if we converse the analog signal into digital signal. If condition permits, photoelectricity conversion and photoelectricity transmission will be good choices. Photoelectricity signal has a high signal-to –noise and anti-interference capability. Communicating by infrared or laser in visual range could be the developing trend in the future.
4.4 Suppress transmitting channel interference.
Material ways are earthing and wave filter and shield.
(1) Earthing is the basic requirement for circuits or equipments and systems. It is a basic method preventing interference. Because the current go through earth wire to form loop, so correctly earthing forms access for static electricity and interference signal and suppresses influence of interference signal on other equipments.
(2) Wave filter technology is an effect way in filter power interference. Generally, power pollution interference happen the most. Except the interference caused by EM radiation directly, most of interference is caused by entering into alternating power to interfere with its wave shape or to increase harmonic wave load or to pile up peak pulse. We usually use power filter and special separating transformer to filter interference out. According to frequency/type/method, we could also amplifier capacitance and inductance to absorb and bypass filter wave loop.
(3) Shield has two aims, for one thing, to limit radiating EM energy inside equipments being leaked to outside, for the other thing, to avoid outer radiation interference. There are three kinds of shield and they are electric shield and magnetic shield and EM shield. EM shield is always used to prevent interference of alternating EM field.
(4) Earthing, wave filter and shield are basic ways to enhance equipment EMC. Each of them can be used alone or can be used with each other. For instance, well earthing prevents static interference, and meanwhile reduces requirement on shield; well EM shield effectively prevents EM radiation interference and meanwhile reduces requirement on filter wave circuit. From the total point of view, well earth can reduce energy of interference frequency; shield can insulate coupling approach of EM radiation and reduce radiation energy; and filter wave can attenuate interference on power conduction.
It is hard to carry out completely EMC for each of equipments would conduct or radiate EMI. So the EMC problem can happen everywhere and the reasons and approaches are complicated. Neither we can not foresee or find a perfect solution. So libraries should solve EMC problems comprehensively by many technologies and management.
5. Brief summary.
EMC technology is very important for library auto equipments. Considering about EMC can be main task of library automatization engineering design. Eliminate EMI and carry EMC out to enhance equipment working stability/reliability and to make sure all kinds of equipments are under EMC. Offering the best service to readers is our terminal aim