Electromagnetic compatibility analysis of medical equipment

The propagation pathways of electromagnetic interference are generally divided into two types: conduction coupling and radiation coupling. The occurrence of any electromagnetic interference must involve the transmission of interference energy and the transmission pathway (or transmission channel). It is generally believed that there are two ways of electromagnetic interference transmission: one is conduction transmission; the other is radiation transmission. Therefore, from the perspective of the interfered sensor, interference coupling can be divided into two categories: conduction coupling and radiation coupling. Conduction transmission must have a complete circuit connection between the interference source and the sensor. The interference signal is transmitted to the sensor along this connection circuit, and interference occurs. This transmission circuit may include wires, conductive components of the equipment, power supply, common impedance, ground plane, resistors, inductors, capacitors, and mutual inductance elements.

Radiation transmission is the propagation of electromagnetic waves through the medium, and the interference energy is emitted to the surrounding space according to the laws of the electromagnetic field. There are three common radiation couplings:

1. The electromagnetic waves emitted by antenna A are accidentally received by antenna B, which is called antenna-to-antenna coupling;

2. The electromagnetic field in space is coupled through induction of the wire, which is called field-to-wire coupling;

3. The high-frequency signal induction between two parallel wires is called line-to-line inductive coupling. In actual engineering, interference between two devices usually involves coupling in many ways. It is precisely because of the simultaneous existence of multiple coupling paths, repeated cross-coupling, and the joint interference that electromagnetic interference becomes difficult to control.

Medical equipment plays an important role in diagnosis and treatment, so the impact of electromagnetic interference on it is directly related to the personal safety of patients. At present, the realization of small, highly sensitive and intelligent medical equipment makes them more susceptible to electromagnetic interference, especially those diagnostic instruments with poor anti-interference ability (i.e. poor electromagnetic compatibility), which provide doctors with distorted data, waveforms, images and other information, making it impossible for doctors to make correct diagnoses, which of course affects effective treatment and even endangers human lives. There are many reports on this internationally.

The US FDA has determined that the accident was suspected to be caused by electromagnetic interference of medical devices; the patient with a pacemaker was in an ambulance and the pacing failed because the rescuers used two-way wireless communication equipment. The patient monitor was affected by electromagnetic interference, causing the patient to die because of the failure to detect arrhythmia. The excessive interference on the CAT display of the equipment made it difficult for medical staff to judge the heart rate, resulting in the patient's inability to resuscitate.

Mobile phones interfere with infant incubators, infusion pumps, hemodialysis machines, pacemakers, and defibrillators. Therefore, hospitals in the United States have issued prohibitions on the use of mobile phones in wards with such equipment. Neonatal respiratory monitors (specially designed alarm devices for neonatal respiratory arrest) are affected by the interfering modulation waves emitted by FM radio stations, which disrupt the respiratory rhythm and cause the alarm to malfunction.

The above example only talks about the impact of external electromagnetic interference on medical equipment. However, modern medical treatment uses various high-frequency, radio frequency transmitters, highly sensitive electrical, electronic components and parts, and equipment or systems (MAI) that use radio frequency energy for diagnosis or treatment. When working, they may act as an EMI interference source and transmit useful or useless electromagnetic waves of different frequency ranges and electromagnetic field strengths to the surrounding through different coupling paths. The radio broadcasting communication business and the work of other surrounding equipment, and they are in a common electromagnetic environment, and may also be interfered by surrounding power, electronic equipment, and medical equipment. Therefore, many medical devices are both interference sources and sensitive devices, that is, they have the dual nature of interference and interference. Therefore, a question is worth thinking about. In this complex electromagnetic environment, how can medical equipment achieve a balance that is not affected by or minimizes the impact of various other electromagnetic interferences, and can minimize electromagnetic interference to other equipment or the human body, so as to achieve a balance. Electromagnetic compatibility is such a concept.

Electromagnetic compatibility (EMC) refers to the ability of a device or system to operate in accordance with requirements in its electromagnetic environment and not produce intolerable electromagnetic interference to any device in its environment. Therefore, EMC includes two requirements: on the one hand, it means that the electromagnetic interference generated by the equipment to the environment during normal operation cannot exceed a certain limit; on the other hand, it means that the equipment has a certain degree of immunity to the electromagnetic interference in the environment, that is, electromagnetic sensitivity.

In order to ensure that medical equipment or systems in the same electromagnetic environment can work normally without hindering normal radio communication and normal operation of surrounding equipment, a rule must be established to stipulate the anti-interference ability of the equipment or system, that is, the anti-interference level of the equipment cannot be too low, and the emission level and anti-interference level are limited to the specified emission limit and the specified anti-interference limit, so that the equipment can achieve the purpose of electromagnetic compatibility. Any active medical electronic equipment will radiate electromagnetic fields, but the strength and frequency of the radiated magnetic field are different. The stronger the field strength, the stronger the external interference.

The greater the gap between the emission value and the immunity limit, the greater the electromagnetic compatibility, and the higher the electromagnetic compatibility of the equipment. Therefore, it is necessary to limit the external emission level of medical equipment and improve its immunity to the electromagnetic environment. Only by taking both into account can the coordination between the equipment and the environment be achieved.

[page] As the electromagnetic compatibility problem of medical equipment becomes increasingly prominent, many countries in the world have taken measures from the perspective of laws and regulations to control the electromagnetic compatibility of medical equipment products. The Chinese government also attaches great importance to this issue. On April 1, 2005, the State Food and Drug Administration approved and issued the industry standard "Y Y05 05-2005 Medical Electrical Equipment Electromagnetic Compatibility Requirements and Tests". After a two-year transition period, it was officially implemented on April 1, 2007. This requires us to implement this industry standard in medical practice, strive to improve the electromagnetic compatibility of medical equipment, enhance the anti-interference ability of equipment, and minimize the potential risk of electromagnetic interference.

Theoretical and practical research shows that no matter whether it is a complex system or a simple device, the occurrence of any electromagnetic interference must meet three basic conditions: first, there should be an interference source; second, there should be a path and channel for propagating the interference energy; third, there must be a response from the interfered object. In the electromagnetic compatibility theory, the interfered objects are collectively referred to as sensitive devices (or sensors). Therefore, the interference source, the interference propagation path (or transmission channel) and the sensitive device are called the three elements of electromagnetic interference.

For medical equipment and systems, it is required not to affect the basic performance of radio broadcasting, television, radio communications and other services or other equipment and systems, but also to have a certain degree of immunity to electromagnetic interference, and its basic performance is not affected by electromagnetic interference. The so-called immunity refers to the ability of a device or system to face electromagnetic disturbance without reducing its operating performance. This indicates that the performance of a device or system does not decrease when facing electromagnetic interference. The higher the immunity, the more it can withstand external electromagnetic interference.

Electromagnetic interference sources can be divided into natural interference sources and man-made interference sources. Natural interference sources include atmospheric noise generated by lightning in various places on the earth, noise generated by sunspot explosions and activities, etc. Interference sources are electromagnetic interference generated by electrical appliances or other electrical devices. This article mostly involves man-made interference.

Improving the immunity of sensitive equipment is an effective means to achieve electromagnetic compatibility. The immunity of medical equipment is divided into 7 categories:

Improving the immunity to (1) electrostatic discharge, (2) radio frequency radiation, (3) fast-changing pulse groups, (4) surges, (5) conduction of radio frequency field induction, (6) power frequency magnetic fields, and (7) voltage sags, short-term interruptions, and voltage changes is a good way to improve electromagnetic compatibility. To solve electromagnetic compatibility problems, you only need to start from the above three requirements, control the electromagnetic radiation of the interference source, suppress the propagation path of electromagnetic interference, and increase the anti-interference ability of sensitive equipment. If one of the three elements is missing, electromagnetic interference cannot be achieved.

As a user of medical equipment, we are more concerned about the electromagnetic compatibility between systems. The compatibility technology between systems is also achieved through shielding, grounding and filtering technologies, but the implementation methods are different.

The meaning of shielding is to cover or block. It has different meanings in different places. Shielding also means isolation. For example: shielding clothing, shielding protective film, etc. Its function is to prevent static electricity and other radiation.

Shielding between systems is to isolate two spatial regions with metal to control the induction and radiation of electric fields, magnetic fields and electromagnetic waves from one region to another, with the purpose of cutting off the combined path of electromagnetic fields. It has two aspects: one is to surround sensitive equipment or systems with shielding bodies to prevent interference from external magnetic fields. The other is to shield the interference source to prevent the interference magnetic field from spreading outward and affecting other wireless devices or the human body. Shielding interference sources and sensitive electrical appliances is to use the principle of shielding bodies to prevent high-frequency electromagnetic fields from propagating in space, reduce the impact of electromagnetic induction between systems, and effectively improve electromagnetic compatibility.

The shielding body has the functions of absorbing energy (eddy current loss), reflecting energy (reflection of electromagnetic waves on the shielding body) and offsetting energy (electromagnetic induction generates a reverse electromagnetic field on the shielding layer, offsetting some interfering electromagnetic waves) from the outside or inside of the electromagnetic wave field, thereby achieving the function of reducing interference. When the frequency of the electromagnetic field is low, the absorption loss is small, and the shielding effect is mainly based on reflection loss. High magnetic permeability materials are used as shielding layers to limit the magnetic lines of force within the shielding body to prevent them from spreading outward. When the frequency of the interfering electromagnetic field is high, the absorption loss increases with the increase of frequency, and the reflection loss decreases with the increase of frequency. It is advisable to use metal materials with good conductivity as the shielding layer, and use the high-frequency interfering electromagnetic field to generate eddy currents in the shielding metal to form an offsetting effect on external electromagnetic waves.

The thicker the shield is or the larger the relative magnetic permeability is, the stronger the shielding effectiveness is. However, it is impossible to make the shield thicker indefinitely. In order to enhance the shielding effect, a double-layer shielding method can be used. The main factors affecting the shielding effect are gap ventilation holes, power lines, signal lines, etc. In order to achieve a good shielding effect, each gap should be electromagnetically sealed. In practice, we use methods such as increasing the gap depth, reducing the gap length, rolling conductive pads in the gap or applying conductive paint, which are very effective methods. Ventilation holes are also the key point of the shielding effect. In order to improve the shielding effectiveness of ventilation holes, we take measures in the mechanical structure, such as using circular holes, reducing the hole area, covering the holes with metal mesh, using shielded cables as signal lines and power lines, or adding filters to the input and output ports, etc., to achieve the purpose of improving the shielding effect.

Grounding technology was originally introduced as a protective measure to prevent electrical or electronic equipment from being struck by lightning. The purpose is to introduce the lightning current generated by lightning into the earth through the lightning rod, thereby protecting the building. At the same time, grounding is also an effective means to protect personal safety. When the phase line caused by some reason (such as poor insulation of the wire, aging of the line, etc.) touches the equipment shell, a dangerous voltage will be generated on the equipment shell, and the fault current generated will flow through the PE line to the earth, thereby playing a protective role.

[page]With the development of electronic communications and other digital fields, it is far from enough to only consider lightning protection and safety in the grounding system. For example, in the communication system, the interconnection of signals between a large number of devices requires that each device must have a reference ground as the reference ground of the signal. Moreover, with the complexity of electronic equipment, the signal frequency is getting higher and higher. Therefore, in the grounding design, electromagnetic compatibility issues such as mutual interference between signals must be given special attention. Otherwise, improper grounding will seriously affect the reliability and stability of the system operation. Recently, the concept of "ground" has also been introduced in the signal return technology of high-speed signals.

The grounding of circuits and electrical equipment is divided into two aspects according to their functions: safety grounding and signal grounding. Safety grounding is to use a low-impedance conductor to connect the outer shell of the electrical equipment to the ground, so that the operating personnel will not be in danger of electric shock due to leakage of the equipment shell or fault discharge. Another safety grounding is lightning protection grounding. Signal grounding is the use of low-impedance wires or ground planes in systems and equipment to provide various circuits with a signal return path with a common reference potential, so that the signal currents of each circuit flowing through the ground line do not affect each other. The main purpose of signal grounding is to suppress electromagnetic interference. It is a grounding method aimed at electromagnetic compatibility, including: (1) Shielded grounding. In order to prevent the circuit from generating interference due to the existence of parasitic capacitance, the circuit radiating electric fields or being sensitive to external electric fields, necessary isolation and shielding must be performed. The metal of these shields must be grounded. (2) Filter grounding. The filter generally contains bypass capacitors for grounding signal lines and power lines. When the filter is not grounded, these capacitors are in a suspended state and cannot play a bypass role. (3) Noise interference suppression. The control of internal noise and external interference requires that many points on the equipment or system be connected to the ground, thereby providing a "lowest impedance" channel for interference signals. (4) Potential reference ground. For signals to be transmitted correctly between circuits, there must be a common potential reference point. This common potential reference point is the ground, so the interconnected circuits must be grounded. There are four types of signal grounding methods. They classify all circuits according to signal characteristics and ground them separately to form four independent grounding systems. Each "ground" subsystem uses a different grounding method.

The first category is sensitive signal and small signal ground systems. These circuits have low operating levels and weak signal amplitudes and are easily affected by interference, failure or degradation. Their ground lines should be avoided from being mixed with other circuits.

The second category is insensitive signal and large signal ground systems. The working current in these circuits is large, and the ground system current is also large. They must be separated from the ground of small signal circuits, otherwise they will interfere with the small signal circuits through the r-cooperation of the ground.

The third category is the ground system of interference source equipment. This type of equipment generates sparks or impact currents when working, which often cause serious interference to electronic circuits. In addition to using shielding isolation technology, the ground wire must be separated from the electronic circuit.

The fourth category: To prevent personal electric shock accidents, interference from external electromagnetic fields and static electricity generated by friction, the casing of metal components must be grounded.

In the same type of circuit, according to the different grounding point connection methods, it is divided into single-point grounding, which is suitable for low frequency (1MHz or above) and the small size of the common grounding surface, and can avoid ground resistance interference between points to a limited extent; multi-point grounding, for high frequency {>lOMHz) and the large size of the common grounding surface, single-point and multi-point mixed grounding: suitable for frequencies between 1MHz-lOMHz. Floating grounding can prevent the interference current on the chassis from being directly coupled to the signal circuit, but it is easy to accumulate static electricity. When the charge reaches a certain level, static electricity discharge will occur. Transformers and photoelectric couplers are typical floating grounds.

Filtering technology:

Filtering is the operation of filtering out the frequency of a specific band in a signal. It is an important measure to suppress and prevent interference. It is a probability theory and method for estimating another random process related to it based on the result of observing a random process. The term filtering originated from communication theory. It is a technology for extracting useful signals from received signals containing interference. "Received signal" is equivalent to the observed random process, and "useful signal" is equivalent to the estimated random process. For example, when tracking an aircraft with radar, the data of the measured aircraft position contains measurement errors and other random interferences. How to use these data to estimate the position, speed, acceleration, etc. of the aircraft at each moment as accurately as possible, and predict the future position of the aircraft, is a filtering and prediction problem. Such problems exist in large numbers in electronic technology, aerospace science, control engineering and other scientific and technological departments. The earliest consideration in history was Wiener filtering, and later RE Kalman and RS Busch proposed Kalman filtering in the 1960s. Research on general nonlinear filtering problems is now quite active.

In short, as the main user of medical equipment, hospitals should attach importance to the necessary electromagnetic compatibility (EMC) knowledge training for medical staff, purchasing and maintenance personnel, select products that meet EMC requirements according to the electromagnetic environment of the use site, and install and operate the equipment correctly according to the equipment instruction manual or technical manual. For the health and safety of the general public, it should be allowed to operate normally according to the design without interference.