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EMC in Medical Devices
Release time:2014-05-26   Click times:4559
Electromagnetic Immunity transmission generally divided into two types: the conduction coupling and radiation coupling. The occurrence of any electromagnetic immunity necessarily exist immunity energy transmission and transmission channels (or transmission channel). Electromagnetic immunity transport is generally considered in two ways: one is conducting transmission; the other is the radiation transmission. Therefore, immunity from the sensor, the immunity can be divided into conduction coupling and coupling radiation coupling two categories. The transfer must be conducted between the source and the immunity of a complete sensor circuit connection, immunity signaling along this circuit is connected to the sensor, the occurrence of the phenomenon of immunity. The transmission circuit may include a wire, a conductive member equipment, power supply, public impedance ground plane, resistance, inductance, capacitance and mutual inductance components.
Radiative transfer through the medium of propagation of electromagnetic waves in the form of energy to the surrounding space launch immunity according to the law of electromagnetic fields. Common radiation coupling consists of three:
1. A electromagnetic waves emitted by the antenna B antenna accidentally accepted, called antenna to antenna coupling;
2. Space electromagnetic induction coupled via the wire, called field coupling to the line;
3. The high-frequency signal induction between two parallel conductors, called inductive coupling line to line. In practical engineering, immunity occurs between two devices typically contain many kinds of ways of coupling. Because of the presence of coupling a variety of ways at the same time, repeated cross-coupled together to produce interference, it becomes difficult to control the electromagnetic interference.
Medical equipment important role in the diagnosis and treatment of play, making the impact of electromagnetic immunity of its direct relationship to the safety of the patient, the current small, high sensitivity and intelligence to achieve medical equipment, making them more susceptible to electromagnetic immunity the degree of influence, particularly those of poor immunity (ie, electromagnetic compatibility poor diagnostic instruments, provides physicians with a distorted data, waveform images, and other information, so that the doctor can not make a correct diagnosis, of course, will affect the effective treatment and even endanger human life, there are many international reports in this regard.
Identified by the US FDA medical device that is suspected due to accidents caused by electromagnetic immunity; patients with a pacemaker in an ambulance first aid process, due to the use of two-way wireless communications ambulance personnel equipment failure caused the pacemaker. Patient Monitor immunity to electromagnetic influence, causing the patient can not be detected due to arrhythmia and death. CAT on the display device over immunity, the medical staff is difficult to judge the heart rate, causing the patient can not recover.
Mobile phone for baby incubators, infusion pumps, artificial dialysis, cardiac pacemakers, cardiac defibrillation device produces immunity, so US hospitals have banned the use of such equipment in the ward phone neonatal respiratory monitors ( neonatal respiratory arrest specially designed alarm device) affected by FM radio FM modulated wave emitted immunity, disrupting the rhythm of breathing cause alarm failure.
The above examples are merely talking about the influence of external electromagnetic immunity of medical devices, we do not know that modern medicine is used in various high-frequency, high sensitivity RF transmitter electrical and electronic components and parts and uses radio frequency energy as a diagnostic or treatment device or system (MAI), may serve as a source of EMI immunity through its work useful for different coupling ways to spread around different frequency ranges and electromagnetic field strength or unwanted electromagnetic waves, radio communication services and work around other equipment and they are in a common electromagnetic environment, but also may be subject to immunity between power and electronic equipment, and medical equipment around. So many medical device is both a source of immunity is sensitive equipment that is to say it exists immunity and the immunity duality, since such a problem worthy of our consideration, in this complex electromagnetic environment, how to achieve both a medical equipment is not affected or minimize other kinds of electromagnetic immunity, but also other devices or minimize electromagnetic immunity of the human body, so as to achieve a balance, electromagnetic facultative is one such concept.
Electromagnetic compatibility (EMC) refers to the device or system in its electromagnetic environment does not meet the requirements to run the ability to produce intolerable electromagnetic immunity of any device in its environment. Therefore, EMC includes two aspects: on the one hand refers to the electromagnetic immunity of the equipment during normal operation of the host environment can not exceed certain limits; on the other hand refers to the apparatus in the presence of an electromagnetic environment where the anti interference immunity of a certain degree, that electromagnetic sensitivity.
To achieve medical devices or systems in the same electromagnetic environment in their own work under normal circumstances, can achieve not interfere with normal radio communications work, without immunity to work around the equipment, we shall establish a rule, both to make the system of immunity capabilities of the device or a provision that the immunity level of the equipment can not be too low, the emission level and the level of immunity and limit the emission limits specified within the limits prescribed immunity equipment reached electromagnetic compatibility purposes. Any active medical electronic equipment will radiate electromagnetic field radiation just powerful magnetic field is small, different frequency, the stronger the field strength stronger immunity to the outside world.
Emission limit values ​​and intervals greater immunity, the greater the degree of electromagnetic compatibility, electromagnetic compatibility higher. So limit foreign medical equipment emission levels, to improve their immunity to electromagnetic environment, both, in order to achieve coordinated equipment and the environment.
With the electromagnetic compatibility of medical equipment to highlight the growing, many countries adopt international regulatory measures from the electromagnetic compatibility of medical equipment products to control, the Chinese government also attaches great importance to this issue, it was April 1, 2005, by the State food and Drug Administration approved made: "Y Y05 05-2005 medical electrical equipment electromagnetic compatibility requirements and testing" industry standard, after two years of implementation of the transition period, was formally implemented from April 1, 2007, which we need to implement in medical practice, the industry standard, and strive to improve the electromagnetic compatibility of medical equipment, enhance immunity capabilities of the device, the potential to minimize the risk of electromagnetic immunity.
Theory and practice show that the system regardless of the complexity or simplicity means any electromagnetic immunity occurs must have three basic conditions: First, the source should have immunity; followed by a propagation channel and interference energy pathways; the third also immunity must be responsive to the object. In the theory of the electromagnetic compatibility immunity are collectively referred to as target sensitive device (or sensor). Thus source immunity, immunity transmission (or transmission channel) and sensitive equipment called electromagnetic immunity of the three elements.
For medical devices and systems, not only require it to have no affect radio, television, or radio communications and other services do not affect the basic performance of other equipment and systems, and it requires a certain degree of immunity electromagnetic immunity, it the basic performance is not affected by electromagnetic immunity, immunity refers to the so-called means a device or system faces electromagnetic disturbance capacity without reducing performance, which is that the device or system in the face of electromagnetic immunity without degrading performance of one capacity, higher immunity, the more that it can withstand the external electromagnetic immunity.
Electromagnetic Immunity Immunity source can be divided into natural and man-made source source immunity, natural immunity to electrical noise sources include noise-day, explosions and sunspot activity generated by lightning on Earth produced everywhere like immunity electrical source or other electrical device generates electromagnetic immunity herein, this involved mostly artificial harassment.
Sensitive equipment to improve immunity is an effective means to achieve electromagnetic compatibility, immunity medical devices are divided into seven categories:
(1) electrostatic discharge (2) radio frequency radiation, (3) EFT, (4) surge, (5) RF field induced conduction, (6) frequency magnetic field, (7) short voltage sags when interruptions and voltage variations immunity, improve the immunity of these seven areas is a good way to improve the electromagnetic compatibility EMC problems just solve the above three requirements from the start, controlling the source of electromagnetic radiation immunity, suppression transmission of electromagnetic immunity, increase immunity ability of sensitive equipment, as long as the three elements is missing a feature, electromagnetic immunity can not be achieved.
As a medical device users, we are more consider the electromagnetic compatibility problems between systems, technical compatibility between systems is through shielding, grounding and filtering technology, but implementation in different ways.
Shielding means shielding willing barrier means. In different places, they have different meanings. Shielding also refers to isolation. Such as: shielding clothing shielding film and the like. Its role is to prevent static electricity and other radiation.
Shielding system is between two spatial areas metal spacer to control the electric and magnetic fields and electromagnetic waves from one region to another region and radiation sensors, which aims to cut off the electromagnetic field r combined way. It has two aspects: First, sensitive equipment or system is surrounded by a shield, to prevent interference by external magnetic fields. On the other hand is the immunity shielded to prevent magnetic field immunity outward diffusion, affecting other wireless devices or human. Source of immunity and sensitive electrical shielding is the use of a shield to prevent the spread of the principles of high frequency electromagnetic field in space, and reduce the impact of electromagnetic induction between systems, improve electromagnetic compatibility.
Shield against electromagnetic field from the outside or inside with the absorbed energy (eddy current loss), the reflected energy (electromagnetic waves reflected on the shield) and the offset energy (electromagnetic induction electromagnetic field on the reverse shield, partially offset by electromagnetic immunity ) role, to weaken immunity function. At low frequency electromagnetic fields, absorption loss is small, the shielding effect to reflect the loss-based, high-permeability material for shielding layer, so that the magnetic field lines is limited to shield the body, to prevent the spread outward. When the high frequency electromagnetic field immunity, absorption losses increase with frequency increases, the reflection loss decreases with rising frequency should be good conductivity metal material for shielding layer, using high-frequency electromagnetic field immunity, generated in the shielded metal vortex formed offsetting foreign electromagnetic waves.
Shield thicker or relative permeability is large, the shielding effectiveness is strong, but it is impossible to shield unlimited thickened, in order to enhance the shielding effect can be double shield law. Effect of the shielding effect of the main factors for the gap ventilation holes, power lines, signal lines, etc., to achieve a good shielding effect, requires each gap should be sealed electromagnetic, in practice we have adopted to increase the depth of the gap, reducing the length of the slit, in slit mixed conductive gasket or coated with conductive coatings are very effective method. Ventilation holes in the shielding effect is good or bad key points to improve the shielding effectiveness of the ventilation holes, we have taken measures in the mechanical structure, such as circular holes, holes reduce the area covered by a metal mesh on the hole, shielded cable do signal lines and power lines, or increase the filter on the input and output ports, etc., to achieve the purpose of improving the shielding effect.
Originally introduced grounding techniques to prevent other electrical or electronic equipment lightning protection measures taken, the purpose is to produce lightning lightning current through the lightning rod is introduced into the earth, and thus play a role in protecting the building. At the same time, the ground is also an effective means to protect personal safety, when certain causes phase (such as poor wire insulation, aging lines, etc.) and touch device housing, the housing of the device will have a dangerous voltage is generated, thereby generating fault current will flow through the PE wire to the earth, which play a protective role.
With the development of digital electronic communications and other fields, in the grounding system and only consider mine safety has been far from sufficient. For example, in a communication system, a large number of interconnect signals between devices requires each device must have a benchmark '' '' '' '' to '' '' '' '' as a signal reference ground. And with the complexity of electronic devices, more and more high frequency signal, therefore, in the design of the ground, mutual interference of electromagnetic compatibility problems between the signals must be given special attention, otherwise, improper grounding will seriously affect system operation and reliable and stability. Recently, high-speed signal return signal technology also introduces a "ground" concept.
Grounding circuits and electrical equipment by function into the safety ground or signal ground in two ways. Safety grounding conductor with low impedance is the housing of electrical equipment connected to the earth, so that people will not use the operator due to equipment failure or leakage housing discharge shock hazard, another security ground for the lightning protection and grounding. Signal ground is the use of the system and equipment in low-impedance conductor or ground plane for the common reference potential signals of various circuits provide a return path, flowing through the ground signal current circuits independently of each other, the main purpose of signal ground in order to suppress electromagnetic immunity, electromagnetic compatibility is the goal of the ground, including: (1) in order to prevent the shield grounding circuit due to the presence of parasitic capacitances produce immunity, radiated electric field circuit or external electric field sensitive and must be necessary isolation and shielding, these metal shield must be grounded, (2) filter grounding, filter typically includes a signal line and the power cord grounding bypass capacitor when the filter is not grounded, the capacitor is in a suspended state, from the less than a bypass, (3) noise immunity suppression, the internal and external noise immunity of the control device or system requires a number of points connected to ground, thereby providing immunity to signal "minimum impedance" channel (4 ) potential reference, the signal to be transmitted correctly between the circuit must have a common potential reference point, the reference point is the common potential, and thus interconnected circuit must be grounded. Signal grounding four categories, they are all circuits are classified according to the signal characteristics of the ground, forming four separate grounding system, each "land" subsystem using different grounding.
The first category, sensitive signal and small signal ground systems, these low-level circuit, the amplitude of the signal is weak, vulnerable to failure or immunity downgraded its ground to avoid confounding from other circuits.
The second category, insensitive and large signal-ground system, these circuits working current, ground current system is also large, small signal circuit and the ground shall be separated, otherwise it will cooperate with the ground for small signal r circuit caused by immunity.
The third category, immunity stream devices to the system, these devices work sparks or shock currents, etc., often have serious immunity to electronic circuits, in addition to shielded isolation technology, the ground should be separate from the electronic circuit settings.
The fourth category, the metal member is to prevent personal electric shock, external electromagnetic field immunity and static friction, etc. shall chassis ground.
The same circuit, according to ground connection in different ways, divided into single-point ground for low frequency "1MHz" and the common ground plane size is small, can be limited to avoid the resistance between points immunity); multi-point grounding, for large high-frequency "lOMHz" and the common ground plane size, the single-point and multi-point mixing ground: for frequency 1MHz-lOMHzo suspension grounded immunity prevents direct current r chassis is bonded to a signal circuit, but prone to static buildup, when the charge reaches a certain level, will produce electrostatic discharge, transformers and optocouplers is a typical floating.
Filtering techniques:
Filtering the signal is filtered in a specific band frequency operation, and is an important measure to suppress immunity prevented. According to observations of a random process, the other relating to the estimated probability of a random process theory and method. The term originated in the filtering communication theory, it is a technology to extract the desired signal from a received signal containing the noise immunity. "Reception signal" corresponds to the observed random process, "useful signal" is equivalent to the estimated stochastic process. Such as radar tracking aircraft, aircraft data measured position, contain other random measurement errors and immunity, how to use these data to estimate as accurately as possible the position of the aircraft at each moment, velocity, acceleration, etc., and forecast future aircraft position, is a filtering and prediction problems. Such problems in the electronics, aerospace science, control engineering and other scientific and technical sectors are abound. Is considered the earliest history of the Wiener filter, Kalman and R.S. Bucy later R.E. in the 1960s proposed Kalman filter. Now the study of general nonlinear filtering problem is quite active.
In short, as the main consumer of medical hospital equipment, we should pay attention to health care, procurement, maintenance personnel to operate the necessary electromagnetic compatibility (EMC) study and training of knowledge, according to the use of the site to buy the electromagnetic environment EMC compliant products and press the device correctly using manual or technical manual installation, operation, for the general public's health and safety, should be allowed in accordance with the design, immunity is not running.
 

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