With the further improvement of the modernization of medical instruments and equipment, the equipment is not working properly due to interference, and the phenomenon of damage to the system is becoming more and more serious. All kinds of operating electrical equipment are related to each other and interact with each other in three ways: electromagnetic conduction, electromagnetic induction and electromagnetic radiation. Under certain conditions, it will cause interference, influence and harm to the running equipment and personnel. Based on the analysis of the impact of interference on medical equipment, this paper will introduce the common methods and interference suppression techniques for medical equipment to suppress interference.
1 Ways, types and effects on medical equipment
1.1 Ways of interference
Interference is divided into differential mode interference, common mode interference and series mode interference. Differential mode interference is also called normal mode interference, transverse mode interference or symmetric interference. It refers to the interference superimposed on the line voltage sine wave and is the interference between the current carrying conductors. Such as the grid over- and under-voltage, transient mutations, spikes and so on. Common mode interference is also called longitudinal mode interference, asymmetric interference and grounding interference. It refers to the interference generated between the power grid and the neutral line. It is the interference between the current carrying conductor and the earth. It is coupled into the circuit by radiation or interference. Come. Such as spike interference, radio frequency interference, steady-state voltage between the neutral line and the ground line. Serial mode interference refers to interference caused by an external magnetic field electric field.
1.2 Type of interference
All electronic devices produced today contain electromagnetic interference filtering circuits. Similarly, all switched-mode power supplies have an internal EMI filter. However, in some environments, the EMI filters of these electronics require an auxiliary filter to meet more stringent electrical noise regulations or to protect the device from excessive external noise sources. Types of power supply interference include voltage drop, power loss, frequency offset, electrical noise, surge, harmonic distortion, and transients.
1.3 The impact of interference on medical equipment
Cardio-electron electrogram machine, monitor, ultrasonic diagnostic apparatus, acupuncture electrotherapy instrument or silver needle directly contact with human body equipment, especially instruments for detecting human bioelectrical signals, because the signal is very weak, if it is interfered, it will The detection results, such as waveforms, graphics, and images, are superimposed with a distortion similar to some lesions, which can cause misdiagnosis, and also cause micro electric shock. In serious cases, there is danger to life. If it is a medical instrument with a computer system, when the peak interference amplitude in the common mode interference reaches 2V ~ 50V, the time lasts for a few microseconds, can cause computer logic errors, loss and so on.
2 Common methods for suppressing interference
The common methods of suppressing interference are grounding and shielding. These two methods are described in detail below.
2.1 Grounding
Before declaring grounding, the basic concepts of grounding and change, protective grounding, and protection to zero must be clarified. That is to say: the ground wire refers to the metal connecting wire connecting the earth to the earth, and the neutral wire is the working circuit provided by the electric power department of China; the protective grounding is to connect the metal casing of the instrument and equipment to the ground wire, and when the outer casing is charged due to interference, The current flows into the earth along the ground to achieve the purpose of protecting the safety of people and equipment. The protection of zero is to connect the metal casing of the instrument and the neutral wire of the power supply. When short circuit, immediately burn the fuse to achieve the purpose of cutting off the power.
2.1.1 Signal grounding of equipment
1 floating ground to insulate the "zero" potential of the circuit or the "zero" potential of the device from the common grounding system, or the common conductor that may cause the circulating current, that is, not grounded, making this "zero" potential relative to the zero potential of the earth A floating "zero" potential. Common methods are transformer isolation and optocoupler isolation. The advantage of the floating ground is that it has strong anti-interference ability, and the disadvantage is the accumulation of static electricity. When the charge accumulates to a certain extent, the potential difference between the device ground and the common ground may cause severe electrostatic discharge, which becomes a highly destructive source of disturbance. The solution is to bridge the bleeder resistance and the magnitude of the resistance between the floating ground and the common ground so as not to affect the leakage current of the device.
2 A single-point grounding circuit and any point in the equipment that needs to be grounded is connected to a point where only one physical point is defined as the grounding reference point. For a system with a single point grounding, each device must have its own single point ground point, and then the ground of each device is then connected to the only specified reference ground point in the system. The disadvantage is that the system has a high reactance effect when the operating frequency is high, resulting in poor grounding effect.
3 Multi-point grounding Multi-point grounding refers to the point in the equipment that needs to be grounded, and is directly connected to the ground plane closest to it. The advantage is simple, the high frequency standing wave is small. The disadvantage is that the amount of maintenance is large.
4 mixed grounding set single point and multi-point grounding length, the point that needs to be grounded nearby, directly connected to the ground plane or the point where high-frequency grounding is required, connected to the ground plane through the bypass capacitor, the other points are adopted Single point grounding. When the wavelength of the circulating signal is lower than 0.05λ, single-point grounding is adopted, and when the length of the grounding wire is 0.05λ or more, multi-point grounding should be adopted.
2.1.2 Grounding of instruments and equipment
1 The grounding of the instrument and equipment in addition to the signal grounding inside the instrument equipment in practice, but also the signal ground, the casing and the earth ground of the instrument equipment, and the ground as the grounding reference point of the instrument equipment, thus ensuring the personal body Safety and stability of circuit operation.
2 Grounding method The size of the grounding resistance is an important indicator to measure the effectiveness of the grounding. It depends on how the grounding electrode is made and the nature of the earth itself. Usually, because the underground metal pipe has a large contact area with the earth, the grounding resistance is small, and people are accustomed to use it as a grounding electrode. It is worth noting that the fault current and stray current flowing into the pipeline can cause damage to the pipeline maintenance personnel. Some heating pipes are installed in the underground channel, and poor contact with the earth is not suitable for grounding. Gas pipelines, liquid fuel pipes, explosive gas pipes, and neutral lines of power lines are absolutely prohibited from being grounded to avoid danger.
The correct method of grounding is to bury the grounding electrode yourself. First, in the damp place on the ground, dig a pit with a depth of more than two meters, and put a copper rod with a wire diameter of 1cm~2cm and a length of 2m~4m, then bury the wet soil and put the guide line out of the ground. If the soil is dry, fill the copper rod with an appropriate amount of salt and water to reduce the grounding resistance. The grounding resistance of the device is generally less than 4 ohms.
The grounding of medical instruments and equipment must be treated separately according to the specific equipment. For example, ECG machines, EEG machines, electrogastrogram machines, B-mode ultrasound, etc. must be connected to the earth separately. Do not connect to the same place, especially not with The X-ray machine, CT/MRI and other grounding wires are connected at the same point, otherwise it will cause strong interference through the grounding wire, resulting in failure to work properly.
2.2 shielding
In order to effectively suppress electromagnetic interference that radiates electromagnetic energy inside and outside the device through space, the usual measure is shielding. Specifically, there are three types of electric field, magnetic field and electromagnetic field shielding. Practice proves that the instrumentation with computer system has a good inhibitory effect on the interference caused by electromagnetic interference and static electricity. Different shielding methods and materials have different effects.
2.2.1 Electric field shielding
The mutual induction between different potentials in the instrument can be seen as the voltage distribution between the distributed capacitors. In order to reduce the interference of the interference source to the inductive object, the usual measures are: increasing the distance between the interference source and the object to be sensed, and reducing the distributed capacitance; as much as possible, the object to be sensed is close to the ground plate, and the capacitance to the ground is increased. ; Add a metal shield between the two. The shield must be a conductive conductor with sufficient strength and grounding. For example, the electrocardiograph, monitor, acupuncture electrotherapy or silver needle should be in direct contact with the human body. The equipment should be kept away from the ultrashort wave therapy machine, high frequency electrosurgical unit, X-ray machine, CT, MRI and all medical equipment capable of radiating electromagnetic waves. In the area, the importance of the high voltage cable shielding layer of the X-ray machine.
2.2.2 Magnetic field shielding
Magnetic field shielding refers to the shielding of DC or low frequency magnetic fields. The shielding principle is to utilize the magnetic permeability of the magnetic flux by the high magnetic permeability and low reluctance characteristic of the shielding body, thereby weakening the magnetic field inside the shielding body. In order to reduce the magnetic reluctance of the shield, the material used must be a material with a high magnetic permeability and a certain thickness. The shield should be placed in the center of the shield as much as possible, paying attention to the gap. Ventilation holes and the like should be distributed along the direction of the magnetic field. Electromagnetic shielding is one of the main measures of electromagnetic compatibility technology. That is, a metal shielding material is used to enclose the electromagnetic interference source so that the external electromagnetic field strength is lower than the allowable value; or the metal shielding material is used to close the electromagnetic sensitive circuit to make the internal electromagnetic field strength lower than the allowable value. Measures.
2.2.3 Electromagnetic field shielding
The circuit includes an inverting device coupled to a turn of the secondary winding for generating a phase opposite to a phase of a voltage signal induced from the secondary winding to the anode; an oscillating device for oscillating from the a voltage signal outputted from an output node of the inverting device and matching the oscillating signal to the high voltage level; and an electromagnetic field generating device for applying a voltage signal outputted from an output node of the oscillating device Generating an electromagnetic field in response to the voltage signal substantially surrounding the periphery of the front portion of the kinescope and eliminating and shielding the electromagnetic field generated from the anode. As a result, the circuit can be applied to cathode ray tubes of various sizes at a lower cost, thereby contributing to production efficiency.
3 Techniques for suppressing interference
3.1 dedicated line
In order to suppress mutual interference between instruments and devices, the simplest method is to use a phase separation power supply system. That is, in the three-wire power supply line, one phase is identified as the power supply for the sensitive device; one phase is used as the power supply for the external device; and the other phase is used as the power supply for the common test instrument or other auxiliary equipment. This measure is often used in large medical equipment power supply systems.
It is worth noting that in the modern medical electronic equipment system, due to the use of non-linear loads in the distribution line, the harmonic currents in the line are present, and the zero-sequence component harmonics cannot cancel each other in the neutral line, but are superimposed. A line that is too fine will cause an increase in line impedance and an increase in interference.
3.2 transient interference suppressor
Gas discharge tube: commonly known as lightning protection tube. The advantages are high insulation resistance, small parasitic capacitance, and strong surge absorption capability. The disadvantage is that the response to the surge voltage is low.
Metal oxide varistor: The main parameters of varistor are nominal voltage and flow capacity. When in use, the voltage selection of the varistor should take into account the fluctuating voltage that may be present on the protected line, generally taking 1.2 to 1.4 times. If it is an AC circuit, pay attention to the relationship between the effective value of the voltage and the peak value. For example, the nominal voltage of the varistor at 220V should be 220 × 1.4 × 1.4 = 430V. The former is due to the high-speed performance level of the varistor for transient interference absorption. The longer the lead wire is, the larger the induced voltage is, and the latter is due to the inherent capacitance of the varistor.
Silicon Transient Voltage Absorption Diode: TVS tube is also called transient voltage suppression circuit. When the transient voltage protection diode is subjected to reverse transient high energy shock, the high impedance between the two poles becomes a low impedance at a speed of 1×10-12 s, and the surge power of up to several kilowatts is absorbed, so that the two poles The voltage clamp is at a predetermined value, effectively protecting the sensitive components of the electronic circuit. Specifically divided into one-way and two-way. The main parameters are breakdown voltage, leakage current and capacitance. It features fast response time, high surge absorption capability, large transient power, low leakage current, easy control of clamping voltage, no damage limit and small size. It is widely used in the static electricity of medical equipment, the transient voltage generated when the inductive load is switched, and the overvoltage protection generated by lightning strikes.
Solid discharge tube: The solid discharge tube is characterized by fast response speed, large absorption current, stable operating voltage and long service life. The working principle is: when the external interference is lower than the trigger voltage, the discharge tube is in an off state; when the interference voltage exceeds the trigger voltage, the discharge tube operates in a negative resistance region. At this time, the current is extremely large, causing the interference energy to shift. As the interference decreases, the current through the discharge tube falls back. When the interference current is lower than the holding current, the discharge tube returns from the low resistance region to the high resistance region, and the discharge process is completed.
3.3 power line filter
The power line filter is installed between the power supply and the electronic device, and mainly functions to suppress parasitic electromagnetic interference in the power transmission and improve the reliability of the device. A commonly used single-stage line consisting of passive concentrated parameters. As shown in Figure 1. In the figure, Cx is a differential mode capacitor, which acts to attenuate differential mode interference. It is tens to hundreds of nF in 220V AC power supply and has a withstand voltage of 250VAC. Cy is a common mode capacitor that acts to attenuate common mode interference. Generally take 1nf~4.7nf, withstand voltage 3~6KVDC. L1 and L2 are common mode inductors, and the inductance is related to the magnitude of the passing current, which has a good filtering effect on the common mode current. Multiple inductors are connected in series: for a more demanding filter, a large inductor can be decomposed into a larger inductor and a small inductor with different inductances. By connecting these inductors in series, the bandwidth of the inductor can be expanded. But the price paid for this is volume and cost. Also note the same problem as paralleling the capacitor, which introduces an additional series resonance point. The impedance of the inductor at the resonance point is small.
Measures to improve filter performance: First, use a filter with a grounding inductance. This can suppress interference on the ground. The second is to use a multi-stage filter. The third is to use a filter in combination with an absorbing device. The fourth is the use of new soft magnetic materials. The fifth is to add a consumable component.
3.4 isolation transformer
The principle of an isolating transformer is the same as that of a normal transformer. They all use the principle of electromagnetic induction. Isolation transformers generally refer to 1:1 transformers. Because the secondary is not connected to the ground. There is no potential difference between any of the secondary lines and ground. Safe to use. Often used as a maintenance power source.
Ordinary isolation transformer: The common isolation transformer does not have a shielding layer between the primary and secondary. It is electrically isolated from the input and output to solve the problem of public ground. The advantage is that it has a certain inhibitory effect on common mode interference, and its size can be estimated by the ratio of the distributed capacitance between the primary and secondary and the distributed capacitance of the device to the ground. Generally, the distributed capacitance between the primary and secondary is several hundred Pf, and the distributed capacitance of the device to the ground is several to several tens of nF, so the attenuation of the common mode interference is about 10 to 20 times. The disadvantage is that the suppression of common mode interference is reduced by the distributed capacitance between the windings as the frequency increases.
Isolation transformer with shielding layer: Adding a shielding layer between the primary and secondary of the transformer, and grounding the shielding layer reliably, can obtain better suppression of common mode interference, and can also use the shielding layer to suppress differential mode interference. This is done by connecting the shield of the transformer to the centerline of the primary. For example, for the 50Hz power frequency, since the primary and shielding layers have a high capacitive reactance, they can still be transmitted to the secondary through the transformer effect without being attenuated. For the common mode interference with higher frequency, the capacitive reactance between the primary and the shielding layer becomes smaller, so that this part of the interference directly returns to the grid through the distribution capacitor and the connection between the shielding layer and the primary neutral line, and enters the secondary loop.
Super isolation transformer: The isolation transformer is a safe power source, which is generally used for maintenance, lightning protection and filtering of machine maintenance. The isolation transformer is a 1/1 transformer. Primary single phase 220V, secondary is also single phase 220V. Or the primary three-phase 380V, the secondary is also three-phase 380V. First of all, the AC power supply voltage we usually use is connected to the ground by a line, and the other line has a potential difference of 220V from the ground. Human contact can cause electric shock. The secondary of the isolation transformer is not connected to the earth, and there is no potential difference between any two lines and the earth. It is safer for people to touch any line without electric shock. Secondly, the output of the isolation transformer is completely "opened" from the input, which effectively acts as a good filter for the input of the transformer (the supply voltage supplied by the grid). Thereby providing a pure power supply voltage to the consumer
3.5 AC voltage regulator
Since the supply voltage of the mains is unstable due to various reasons, especially in some power supply places, the voltage fluctuation range is large, which affects the normal operation of the electrical equipment, and may also cause damage to the electrical equipment, and the AC voltage regulator is capable of A voltage stabilizing device that uses a working voltage of an electrical device to be substantially stable.
The role of the AC voltage regulator is to stabilize the output voltage within the allowable range as the input voltage and load current change. Commonly used are AC regulated power supplies such as ferromagnetic resonance, parameter adjustment type, servo type, step adjustment width, super isolation, switching type, uninterrupted and clean.
Ferromagnetic resonant AC regulated power supply: An electronic device that provides a stable AC power supply to the load. Also known as the AC voltage regulator. For the parameters and quality indicators of the AC regulated power supply, please refer to the DC regulated power supply. Various electronic devices require relatively stable AC power supply, especially when computer technology is applied to various fields, and the method of directly supplying power from the AC grid without taking any measures is no longer sufficient. The working principle is to adjust the saturation of the inductor and make the inductor and capacitor resonate to achieve the adjustment. When the input voltage is too high or too low due to some factor, its output voltage can be automatically adjusted according to the input voltage, so that the output voltage remains stable. The advantages are simple circuit, high output impedance, strong overload capability and high reliability. The disadvantages are low voltage regulation accuracy, large distortion of the output voltage waveform, phase shift and noise. It is not suitable to start a load with a large current.
Parameter-adjusted AC regulated power supply: Typically, the 614 series regulators of the early years. The power supply has been improved on the basis of 614. The working principle is to use the phase control of the thyristor to change the parameters of the inductor and adjust to make the output voltage stable. The advantage is that the voltage regulation accuracy is high, and the partial harmonics in the AC output voltage can be suppressed as compared with the first type. The disadvantage is that the current harmonics on the input side are large, the power factor is low, and there is a phase shift. In particular, there may be low frequency oscillations with a non-linear load.
Servo AC regulated power supply: This power supply is an early multi-tap auto-coupled voltage regulating transformer. The working principle is to monitor the output voltage of the transformer to drive the servo motor to change the position of the output nozzle of the transformer so that the output voltage is within the allowable voltage range of the load. The disadvantage is that the response speed is low and many spikes and ringing disturbances occur during adjustment.
The grading-adjusted wide-band AC regulated power supply: This power supply is similar to the servo-type AC regulated power supply. The difference is that the tap position of the multi-tap autotransformer is converted by the relay. Due to the low price of the power supply and the wide adaptability of the input voltage, it is applied to the AC voltage regulation of household appliances. The disadvantage is that the voltage regulation accuracy is not high, and the peak interference caused by the spark is easily generated during the relay conversion process.
Super isolation transformer: In order to solve the problem of miniaturization, digitization and low power consumption of modern electronic equipment, especially sensitive to transient interference of power grid, the control of multi-tap winding adopts non-contact two-way controllable Silicon, digital circuit or microcontroller. Sometimes referred to as a CNC-type purification power supply. The advantages are: the regulated voltage has a wide voltage adaptation range, fast response to grid or load changes, and strong transient interference suppression capability in the power grid.
Switching AC regulated power supply: Switching AC regulated power supply adopts advanced high frequency switching power supply technology. Advantages: small, lightweight, efficient, and fast response. Disadvantages: complex and expensive.
ups:
1 Motor One engine group is mainly composed of an inertial flywheel driven by a DC motor and an alternator. When the grid voltage is cut off, the inertial energy storage of the flywheel is used to make the generator continue to supply power in a short time; at the same time, the standby diesel generator set is started, and when the engine speed is the same as the generator set speed, the oil machine clutch and the generator Connected to complete the conversion from the mains to the oil machine. It is an uninterruptible power supply that was developed earlier. Advantages: stable and reliable. Disadvantages: large size and high noise.
2 When the static backup grid is normal, the static backup uninterruptible power supply is in the bypass state. When the utility power is cut off, the static transfer switch is switched to the inverter side. After 2~4ms, the inverter starts. The electrical energy stored in the battery is converted into alternating current and delivered to the load. Advantages: simple, compact and inexpensive. Disadvantages: The output voltage is directly affected by the fluctuation of the power grid, and the ability to resist sudden changes in the power grid is poor.
3 static online type The working process of the power supply is that the commercial power is first rectified to charge the battery, and then the power is supplied to the inverter by the power storage, and the AC power is supplied to the load after the inverter, the voltage regulation and the frequency stabilization. When the power is off, the battery is no longer charged, and the state of the inverter power supply is unchanged. When the inverter has output overvoltage, overcurrent or uninterruptible power supply failure, the inverter will automatically shut down and go to the side through the static transfer switch. Through the position, the mains supply power directly to the load. Advantages: Strong protection and expansion capabilities. The power supply has a large capacity, and the three-phase high-power common hospital electronic computer and monitoring system.
4 Application in medical equipment
The above various methods and anti-jamming technologies have been widely used in medical diagnostic and therapeutic instruments such as cardio-electroencephalographs, monitors, ultrasonic diagnostic instruments, electronic pulse therapy instruments, acupuncture electrotherapy devices or silver needles directly contacting the human body. The signal detected by the energy detector is relatively weak, and it has high requirements against power supply interference. If the signal is superimposed, the interference signal will be superimposed on the signal and cannot be correctly diagnosed. In addition to the grounding of the casing, the internal part of the machine is strictly used for the power supply. Shielding measures, but the requirements for the surrounding environment are also very high.
1 Ways, types and effects on medical equipment
1.1 Ways of interference
Interference is divided into differential mode interference, common mode interference and series mode interference. Differential mode interference is also called normal mode interference, transverse mode interference or symmetric interference. It refers to the interference superimposed on the line voltage sine wave and is the interference between the current carrying conductors. Such as the grid over- and under-voltage, transient mutations, spikes and so on. Common mode interference is also called longitudinal mode interference, asymmetric interference and grounding interference. It refers to the interference generated between the power grid and the neutral line. It is the interference between the current carrying conductor and the earth. It is coupled into the circuit by radiation or interference. Come. Such as spike interference, radio frequency interference, steady-state voltage between the neutral line and the ground line. Serial mode interference refers to interference caused by an external magnetic field electric field.
1.2 Type of interference
All electronic devices produced today contain electromagnetic interference filtering circuits. Similarly, all switched-mode power supplies have an internal EMI filter. However, in some environments, the EMI filters of these electronics require an auxiliary filter to meet more stringent electrical noise regulations or to protect the device from excessive external noise sources. Types of power supply interference include voltage drop, power loss, frequency offset, electrical noise, surge, harmonic distortion, and transients.
1.3 The impact of interference on medical equipment
Cardio-electron electrogram machine, monitor, ultrasonic diagnostic apparatus, acupuncture electrotherapy instrument or silver needle directly contact with human body equipment, especially instruments for detecting human bioelectrical signals, because the signal is very weak, if it is interfered, it will The detection results, such as waveforms, graphics, and images, are superimposed with a distortion similar to some lesions, which can cause misdiagnosis, and also cause micro electric shock. In serious cases, there is danger to life. If it is a medical instrument with a computer system, when the peak interference amplitude in the common mode interference reaches 2V ~ 50V, the time lasts for a few microseconds, can cause computer logic errors, loss and so on.
2 Common methods for suppressing interference
The common methods of suppressing interference are grounding and shielding. These two methods are described in detail below.
2.1 Grounding
Before declaring grounding, the basic concepts of grounding and change, protective grounding, and protection to zero must be clarified. That is to say: the ground wire refers to the metal connecting wire connecting the earth to the earth, and the neutral wire is the working circuit provided by the electric power department of China; the protective grounding is to connect the metal casing of the instrument and equipment to the ground wire, and when the outer casing is charged due to interference, The current flows into the earth along the ground to achieve the purpose of protecting the safety of people and equipment. The protection of zero is to connect the metal casing of the instrument and the neutral wire of the power supply. When short circuit, immediately burn the fuse to achieve the purpose of cutting off the power.
2.1.1 Signal grounding of equipment
1 floating ground to insulate the "zero" potential of the circuit or the "zero" potential of the device from the common grounding system, or the common conductor that may cause the circulating current, that is, not grounded, making this "zero" potential relative to the zero potential of the earth A floating "zero" potential. Common methods are transformer isolation and optocoupler isolation. The advantage of the floating ground is that it has strong anti-interference ability, and the disadvantage is the accumulation of static electricity. When the charge accumulates to a certain extent, the potential difference between the device ground and the common ground may cause severe electrostatic discharge, which becomes a highly destructive source of disturbance. The solution is to bridge the bleeder resistance and the magnitude of the resistance between the floating ground and the common ground so as not to affect the leakage current of the device.
2 A single-point grounding circuit and any point in the equipment that needs to be grounded is connected to a point where only one physical point is defined as the grounding reference point. For a system with a single point grounding, each device must have its own single point ground point, and then the ground of each device is then connected to the only specified reference ground point in the system. The disadvantage is that the system has a high reactance effect when the operating frequency is high, resulting in poor grounding effect.
3 Multi-point grounding Multi-point grounding refers to the point in the equipment that needs to be grounded, and is directly connected to the ground plane closest to it. The advantage is simple, the high frequency standing wave is small. The disadvantage is that the amount of maintenance is large.
4 mixed grounding set single point and multi-point grounding length, the point that needs to be grounded nearby, directly connected to the ground plane or the point where high-frequency grounding is required, connected to the ground plane through the bypass capacitor, the other points are adopted Single point grounding. When the wavelength of the circulating signal is lower than 0.05λ, single-point grounding is adopted, and when the length of the grounding wire is 0.05λ or more, multi-point grounding should be adopted.
2.1.2 Grounding of instruments and equipment
1 The grounding of the instrument and equipment in addition to the signal grounding inside the instrument equipment in practice, but also the signal ground, the casing and the earth ground of the instrument equipment, and the ground as the grounding reference point of the instrument equipment, thus ensuring the personal body Safety and stability of circuit operation.
2 Grounding method The size of the grounding resistance is an important indicator to measure the effectiveness of the grounding. It depends on how the grounding electrode is made and the nature of the earth itself. Usually, because the underground metal pipe has a large contact area with the earth, the grounding resistance is small, and people are accustomed to use it as a grounding electrode. It is worth noting that the fault current and stray current flowing into the pipeline can cause damage to the pipeline maintenance personnel. Some heating pipes are installed in the underground channel, and poor contact with the earth is not suitable for grounding. Gas pipelines, liquid fuel pipes, explosive gas pipes, and neutral lines of power lines are absolutely prohibited from being grounded to avoid danger.
The correct method of grounding is to bury the grounding electrode yourself. First, in the damp place on the ground, dig a pit with a depth of more than two meters, and put a copper rod with a wire diameter of 1cm~2cm and a length of 2m~4m, then bury the wet soil and put the guide line out of the ground. If the soil is dry, fill the copper rod with an appropriate amount of salt and water to reduce the grounding resistance. The grounding resistance of the device is generally less than 4 ohms.
The grounding of medical instruments and equipment must be treated separately according to the specific equipment. For example, ECG machines, EEG machines, electrogastrogram machines, B-mode ultrasound, etc. must be connected to the earth separately. Do not connect to the same place, especially not with The X-ray machine, CT/MRI and other grounding wires are connected at the same point, otherwise it will cause strong interference through the grounding wire, resulting in failure to work properly.
2.2 shielding
In order to effectively suppress electromagnetic interference that radiates electromagnetic energy inside and outside the device through space, the usual measure is shielding. Specifically, there are three types of electric field, magnetic field and electromagnetic field shielding. Practice proves that the instrumentation with computer system has a good inhibitory effect on the interference caused by electromagnetic interference and static electricity. Different shielding methods and materials have different effects.
2.2.1 Electric field shielding
The mutual induction between different potentials in the instrument can be seen as the voltage distribution between the distributed capacitors. In order to reduce the interference of the interference source to the inductive object, the usual measures are: increasing the distance between the interference source and the object to be sensed, and reducing the distributed capacitance; as much as possible, the object to be sensed is close to the ground plate, and the capacitance to the ground is increased. ; Add a metal shield between the two. The shield must be a conductive conductor with sufficient strength and grounding. For example, the electrocardiograph, monitor, acupuncture electrotherapy or silver needle should be in direct contact with the human body. The equipment should be kept away from the ultrashort wave therapy machine, high frequency electrosurgical unit, X-ray machine, CT, MRI and all medical equipment capable of radiating electromagnetic waves. In the area, the importance of the high voltage cable shielding layer of the X-ray machine.
2.2.2 Magnetic field shielding
Magnetic field shielding refers to the shielding of DC or low frequency magnetic fields. The shielding principle is to utilize the magnetic permeability of the magnetic flux by the high magnetic permeability and low reluctance characteristic of the shielding body, thereby weakening the magnetic field inside the shielding body. In order to reduce the magnetic reluctance of the shield, the material used must be a material with a high magnetic permeability and a certain thickness. The shield should be placed in the center of the shield as much as possible, paying attention to the gap. Ventilation holes and the like should be distributed along the direction of the magnetic field. Electromagnetic shielding is one of the main measures of electromagnetic compatibility technology. That is, a metal shielding material is used to enclose the electromagnetic interference source so that the external electromagnetic field strength is lower than the allowable value; or the metal shielding material is used to close the electromagnetic sensitive circuit to make the internal electromagnetic field strength lower than the allowable value. Measures.
2.2.3 Electromagnetic field shielding
The circuit includes an inverting device coupled to a turn of the secondary winding for generating a phase opposite to a phase of a voltage signal induced from the secondary winding to the anode; an oscillating device for oscillating from the a voltage signal outputted from an output node of the inverting device and matching the oscillating signal to the high voltage level; and an electromagnetic field generating device for applying a voltage signal outputted from an output node of the oscillating device Generating an electromagnetic field in response to the voltage signal substantially surrounding the periphery of the front portion of the kinescope and eliminating and shielding the electromagnetic field generated from the anode. As a result, the circuit can be applied to cathode ray tubes of various sizes at a lower cost, thereby contributing to production efficiency.
3 Techniques for suppressing interference
3.1 dedicated line
In order to suppress mutual interference between instruments and devices, the simplest method is to use a phase separation power supply system. That is, in the three-wire power supply line, one phase is identified as the power supply for the sensitive device; one phase is used as the power supply for the external device; and the other phase is used as the power supply for the common test instrument or other auxiliary equipment. This measure is often used in large medical equipment power supply systems.
It is worth noting that in the modern medical electronic equipment system, due to the use of non-linear loads in the distribution line, the harmonic currents in the line are present, and the zero-sequence component harmonics cannot cancel each other in the neutral line, but are superimposed. A line that is too fine will cause an increase in line impedance and an increase in interference.
3.2 transient interference suppressor
Gas discharge tube: commonly known as lightning protection tube. The advantages are high insulation resistance, small parasitic capacitance, and strong surge absorption capability. The disadvantage is that the response to the surge voltage is low.
Metal oxide varistor: The main parameters of varistor are nominal voltage and flow capacity. When in use, the voltage selection of the varistor should take into account the fluctuating voltage that may be present on the protected line, generally taking 1.2 to 1.4 times. If it is an AC circuit, pay attention to the relationship between the effective value of the voltage and the peak value. For example, the nominal voltage of the varistor at 220V should be 220 × 1.4 × 1.4 = 430V. The former is due to the high-speed performance level of the varistor for transient interference absorption. The longer the lead wire is, the larger the induced voltage is, and the latter is due to the inherent capacitance of the varistor.
Silicon Transient Voltage Absorption Diode: TVS tube is also called transient voltage suppression circuit. When the transient voltage protection diode is subjected to reverse transient high energy shock, the high impedance between the two poles becomes a low impedance at a speed of 1×10-12 s, and the surge power of up to several kilowatts is absorbed, so that the two poles The voltage clamp is at a predetermined value, effectively protecting the sensitive components of the electronic circuit. Specifically divided into one-way and two-way. The main parameters are breakdown voltage, leakage current and capacitance. It features fast response time, high surge absorption capability, large transient power, low leakage current, easy control of clamping voltage, no damage limit and small size. It is widely used in the static electricity of medical equipment, the transient voltage generated when the inductive load is switched, and the overvoltage protection generated by lightning strikes.
Solid discharge tube: The solid discharge tube is characterized by fast response speed, large absorption current, stable operating voltage and long service life. The working principle is: when the external interference is lower than the trigger voltage, the discharge tube is in an off state; when the interference voltage exceeds the trigger voltage, the discharge tube operates in a negative resistance region. At this time, the current is extremely large, causing the interference energy to shift. As the interference decreases, the current through the discharge tube falls back. When the interference current is lower than the holding current, the discharge tube returns from the low resistance region to the high resistance region, and the discharge process is completed.
3.3 power line filter
The power line filter is installed between the power supply and the electronic device, and mainly functions to suppress parasitic electromagnetic interference in the power transmission and improve the reliability of the device. A commonly used single-stage line consisting of passive concentrated parameters. As shown in Figure 1. In the figure, Cx is a differential mode capacitor, which acts to attenuate differential mode interference. It is tens to hundreds of nF in 220V AC power supply and has a withstand voltage of 250VAC. Cy is a common mode capacitor that acts to attenuate common mode interference. Generally take 1nf~4.7nf, withstand voltage 3~6KVDC. L1 and L2 are common mode inductors, and the inductance is related to the magnitude of the passing current, which has a good filtering effect on the common mode current. Multiple inductors are connected in series: for a more demanding filter, a large inductor can be decomposed into a larger inductor and a small inductor with different inductances. By connecting these inductors in series, the bandwidth of the inductor can be expanded. But the price paid for this is volume and cost. Also note the same problem as paralleling the capacitor, which introduces an additional series resonance point. The impedance of the inductor at the resonance point is small.
Measures to improve filter performance: First, use a filter with a grounding inductance. This can suppress interference on the ground. The second is to use a multi-stage filter. The third is to use a filter in combination with an absorbing device. The fourth is the use of new soft magnetic materials. The fifth is to add a consumable component.
3.4 isolation transformer
The principle of an isolating transformer is the same as that of a normal transformer. They all use the principle of electromagnetic induction. Isolation transformers generally refer to 1:1 transformers. Because the secondary is not connected to the ground. There is no potential difference between any of the secondary lines and ground. Safe to use. Often used as a maintenance power source.
Ordinary isolation transformer: The common isolation transformer does not have a shielding layer between the primary and secondary. It is electrically isolated from the input and output to solve the problem of public ground. The advantage is that it has a certain inhibitory effect on common mode interference, and its size can be estimated by the ratio of the distributed capacitance between the primary and secondary and the distributed capacitance of the device to the ground. Generally, the distributed capacitance between the primary and secondary is several hundred Pf, and the distributed capacitance of the device to the ground is several to several tens of nF, so the attenuation of the common mode interference is about 10 to 20 times. The disadvantage is that the suppression of common mode interference is reduced by the distributed capacitance between the windings as the frequency increases.
Isolation transformer with shielding layer: Adding a shielding layer between the primary and secondary of the transformer, and grounding the shielding layer reliably, can obtain better suppression of common mode interference, and can also use the shielding layer to suppress differential mode interference. This is done by connecting the shield of the transformer to the centerline of the primary. For example, for the 50Hz power frequency, since the primary and shielding layers have a high capacitive reactance, they can still be transmitted to the secondary through the transformer effect without being attenuated. For the common mode interference with higher frequency, the capacitive reactance between the primary and the shielding layer becomes smaller, so that this part of the interference directly returns to the grid through the distribution capacitor and the connection between the shielding layer and the primary neutral line, and enters the secondary loop.
Super isolation transformer: The isolation transformer is a safe power source, which is generally used for maintenance, lightning protection and filtering of machine maintenance. The isolation transformer is a 1/1 transformer. Primary single phase 220V, secondary is also single phase 220V. Or the primary three-phase 380V, the secondary is also three-phase 380V. First of all, the AC power supply voltage we usually use is connected to the ground by a line, and the other line has a potential difference of 220V from the ground. Human contact can cause electric shock. The secondary of the isolation transformer is not connected to the earth, and there is no potential difference between any two lines and the earth. It is safer for people to touch any line without electric shock. Secondly, the output of the isolation transformer is completely "opened" from the input, which effectively acts as a good filter for the input of the transformer (the supply voltage supplied by the grid). Thereby providing a pure power supply voltage to the consumer
3.5 AC voltage regulator
Since the supply voltage of the mains is unstable due to various reasons, especially in some power supply places, the voltage fluctuation range is large, which affects the normal operation of the electrical equipment, and may also cause damage to the electrical equipment, and the AC voltage regulator is capable of A voltage stabilizing device that uses a working voltage of an electrical device to be substantially stable.
The role of the AC voltage regulator is to stabilize the output voltage within the allowable range as the input voltage and load current change. Commonly used are AC regulated power supplies such as ferromagnetic resonance, parameter adjustment type, servo type, step adjustment width, super isolation, switching type, uninterrupted and clean.
Ferromagnetic resonant AC regulated power supply: An electronic device that provides a stable AC power supply to the load. Also known as the AC voltage regulator. For the parameters and quality indicators of the AC regulated power supply, please refer to the DC regulated power supply. Various electronic devices require relatively stable AC power supply, especially when computer technology is applied to various fields, and the method of directly supplying power from the AC grid without taking any measures is no longer sufficient. The working principle is to adjust the saturation of the inductor and make the inductor and capacitor resonate to achieve the adjustment. When the input voltage is too high or too low due to some factor, its output voltage can be automatically adjusted according to the input voltage, so that the output voltage remains stable. The advantages are simple circuit, high output impedance, strong overload capability and high reliability. The disadvantages are low voltage regulation accuracy, large distortion of the output voltage waveform, phase shift and noise. It is not suitable to start a load with a large current.
Parameter-adjusted AC regulated power supply: Typically, the 614 series regulators of the early years. The power supply has been improved on the basis of 614. The working principle is to use the phase control of the thyristor to change the parameters of the inductor and adjust to make the output voltage stable. The advantage is that the voltage regulation accuracy is high, and the partial harmonics in the AC output voltage can be suppressed as compared with the first type. The disadvantage is that the current harmonics on the input side are large, the power factor is low, and there is a phase shift. In particular, there may be low frequency oscillations with a non-linear load.
Servo AC regulated power supply: This power supply is an early multi-tap auto-coupled voltage regulating transformer. The working principle is to monitor the output voltage of the transformer to drive the servo motor to change the position of the output nozzle of the transformer so that the output voltage is within the allowable voltage range of the load. The disadvantage is that the response speed is low and many spikes and ringing disturbances occur during adjustment.
The grading-adjusted wide-band AC regulated power supply: This power supply is similar to the servo-type AC regulated power supply. The difference is that the tap position of the multi-tap autotransformer is converted by the relay. Due to the low price of the power supply and the wide adaptability of the input voltage, it is applied to the AC voltage regulation of household appliances. The disadvantage is that the voltage regulation accuracy is not high, and the peak interference caused by the spark is easily generated during the relay conversion process.
Super isolation transformer: In order to solve the problem of miniaturization, digitization and low power consumption of modern electronic equipment, especially sensitive to transient interference of power grid, the control of multi-tap winding adopts non-contact two-way controllable Silicon, digital circuit or microcontroller. Sometimes referred to as a CNC-type purification power supply. The advantages are: the regulated voltage has a wide voltage adaptation range, fast response to grid or load changes, and strong transient interference suppression capability in the power grid.
Switching AC regulated power supply: Switching AC regulated power supply adopts advanced high frequency switching power supply technology. Advantages: small, lightweight, efficient, and fast response. Disadvantages: complex and expensive.
ups:
1 Motor One engine group is mainly composed of an inertial flywheel driven by a DC motor and an alternator. When the grid voltage is cut off, the inertial energy storage of the flywheel is used to make the generator continue to supply power in a short time; at the same time, the standby diesel generator set is started, and when the engine speed is the same as the generator set speed, the oil machine clutch and the generator Connected to complete the conversion from the mains to the oil machine. It is an uninterruptible power supply that was developed earlier. Advantages: stable and reliable. Disadvantages: large size and high noise.
2 When the static backup grid is normal, the static backup uninterruptible power supply is in the bypass state. When the utility power is cut off, the static transfer switch is switched to the inverter side. After 2~4ms, the inverter starts. The electrical energy stored in the battery is converted into alternating current and delivered to the load. Advantages: simple, compact and inexpensive. Disadvantages: The output voltage is directly affected by the fluctuation of the power grid, and the ability to resist sudden changes in the power grid is poor.
3 static online type The working process of the power supply is that the commercial power is first rectified to charge the battery, and then the power is supplied to the inverter by the power storage, and the AC power is supplied to the load after the inverter, the voltage regulation and the frequency stabilization. When the power is off, the battery is no longer charged, and the state of the inverter power supply is unchanged. When the inverter has output overvoltage, overcurrent or uninterruptible power supply failure, the inverter will automatically shut down and go to the side through the static transfer switch. Through the position, the mains supply power directly to the load. Advantages: Strong protection and expansion capabilities. The power supply has a large capacity, and the three-phase high-power common hospital electronic computer and monitoring system.
4 Application in medical equipment
The above various methods and anti-jamming technologies have been widely used in medical diagnostic and therapeutic instruments such as cardio-electroencephalographs, monitors, ultrasonic diagnostic instruments, electronic pulse therapy instruments, acupuncture electrotherapy devices or silver needles directly contacting the human body. The signal detected by the energy detector is relatively weak, and it has high requirements against power supply interference. If the signal is superimposed, the interference signal will be superimposed on the signal and cannot be correctly diagnosed. In addition to the grounding of the casing, the internal part of the machine is strictly used for the power supply. Shielding measures, but the requirements for the surrounding environment are also very high.
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