Can the butterfly effect of Beijing subway vibration affect precision instruments?

The Beijing Metro Line 4 train roared past 13.5 meters deep underground. 100 meters away, in the building of the School of Information Science and Technology of Peking University, "it seemed like a hurricane was blowing" inside an electron microscope.

To the naked eye, this white metal lens tube over 1 meter high stands firmly on the table. When you adjust it to the highest precision, you will find that the black and white image on the display has "burrs", and the original delicate atomic pattern becomes blurred due to vibration.

On the Peking University campus, the electron microscope worth millions of dollars is not the only precision instrument that has been affected by the subway operation. When Line 4 opened, Peking University had 1.1 billion yuan worth of precision instruments, of which 400 million yuan worth of instruments were affected.

In order to reduce the interference of subway vibrations on these instruments, Beijing and Peking University have made great efforts. In the Peking University East Gate section of Line 4, the subway company has laid the most advanced vibration-absorbing track. Peking University built a new comprehensive scientific research building farther away and moved some precision instruments, but the impact of subway vibrations is still difficult to eliminate. Some scholars can only conduct experiments in the middle of the night after the subway is shut down.

In 2019, the second phase of Metro Line 16, which is 600 meters away from the comprehensive scientific research building, will be opened. Precision instruments in Peking University will face the dilemma of being attacked from two sides. Zhang Zhiqiang, director of the Environmental Protection Office of the Department of Laboratory and Equipment Management of Peking University, believes that if more vibration reduction measures are not taken, the situation will not be optimistic.

Peking University is not the only scientific research institution facing subway vibration interference. The reporter learned that Tsinghua University, Chinese Academy of Sciences, Fudan University, Nanjing University, Capital Medical University, and Zhengzhou University School of Medicine have also encountered similar difficulties. Subways will be built around the University of Science and Technology of China, Zhejiang University and Nantong University.

The increasingly dense subway network in cities and the increasingly sensitive precision instruments in scientific research institutions are all signs of China's rapid economic and social development. But when high-precision equipment meets subway lines, who should give way becomes an irreconcilable conflict.

The planned 2020 Beijing subway network.

The butterfly effect of subway vibration

Subway tracks are growing rapidly in Beijing. By 2020, their total mileage will be nearly 1,000 kilometers. During peak hours, nearly a thousand trains will be speeding on the track at the same time.

While carrying passengers, these trains weighing more than 100 tons have also become huge sources of vibration. Vibrations travel through steel wheels, rails, tunnels and soil, spreading like ripples to the surface and into buildings.

Few people notice the impact this vibration has on cities. The Track Vibration Reduction and Control Laboratory of Beijing Jiaotong University is an early research team in China. The data they tested showed that in more than 10 years, the micro-vibration of the ground within 100 meters of the subway in Beijing has increased nearly 10 times.

Although the intensity of micro-vibrations caused by traffic is not large, it lasts for a long time and the impact is hidden and difficult to detect. It once caused cracks and then collapsed in an ancient church in the Czech Republic, affected the performance of the Bastille Opera House for a long time, and also interfered with Intel's engraving of nanoscale circuits on integrated boards.

Among the vibrations caused by the subway, the most serious interference to precision instruments is low-frequency vibration. This kind of vibration has a very long wavelength and is not easily attenuated in the soil layer. Lei Jun, director of the Environmental Vibration Monitoring and Assessment Laboratory of Peking University, once carried seismometers with his students to measure many subway lines in Beijing. They found that in the low-frequency range where precision instruments are more sensitive, the surface vibration intensity within 100 meters from the subway is higher than that without The train was 30 to 100 times taller when it passed.

For the precision instruments of Peking University and Tsinghua University, the subway almost means a "catastrophic blow."

Before the opening of the subway, at these two most famous universities in China, environmental vibrations caused by buses and railways had approached or even exceeded the safety values ??specified by some instruments. However, because these instruments leave a margin when formulating the vibration requirements for normal use environments, most of them can still work normally. Once the nearby subway line is opened, the vibration-sensitive precision instruments in the two universities are likely to be unable to work properly with the highest accuracy.

Some scholars believe that this results in huge waste. “The instrument bought for US$1 million can only be used for US$100,000.”

Many users of instruments do not know that subway vibrations can affect their instruments. A colleague once came to Lei Jun and complained that a precision instrument used to measure the age of rocks in the laboratory suddenly became abnormal.

The teacher called the manufacturer and adjusted it left and right, but he couldn't fix it, and the manufacturer couldn't figure it out.

Lei Jun asked: "When did it start to be abnormal?" The other party said: "Since 2009." In fact, it was not that the instrument was broken, but that after the opening of Metro Line 4, the vibration interfered with the instrument .

“There are less than a hundred domestic experts who study subway vibration problems, including equipment manufacturers.” Ma Meng, associate professor at Beijing Jiaotong University, lamented that this is a very small academic circle, and most of them The experts are still in the same WeChat group.

For more than 10 years, Lei Jun has been calling attention to the problem of subway vibration on various occasions. As a member of the Jiusan Society, he wrote proposals many times in the hope of reporting this issue to the National People's Congress. Whenever he gets the chance, he educates scholars and students who don’t know about the impact of subway vibrations.

For a long time, he was originally engaged in seismology, but he devoted himself to this unpopular academic field. His family often advised him not to "not do his job properly".

In Lei Jun’s view, this field is quite important. He knocked on the table and asked: "China is undergoing industrial transformation, but why are our scientific and technological achievements in these years so big? Why are some core electronic components, including the processing of chips, photolithography machines, grating thin materials and other parts in many fields? Even if we buy back a complete set of production lines from abroad, we still can't make the same thing? One big reason is that the environmental vibration exceeds the standard. Today we can produce rough industrial products. Our shortcomings are mainly in precision. No. ”

He has done environmental vibration assessments for two units. One is the China Institute of Metrology, which is the country's highest metrological scientific research center. The environmental vibration at the original site seriously exceeded the standard. It was later moved to Changping, but an evaluation found that the new site still had some problems. The other is a national defense measurement station, where the environmental vibration exceeds the standard by more than 100 times.

For experts who specialize in environmental vibrations, the micro-vibrations caused by the subway look like butterflies flapping their wings, but in high-precision fields that are sensitive to vibrations, they are enough to cause a catastrophic storm, thus restricting The development of a country: photolithography machines need to draw thousands of lines within 1 mm, and the external environment needs to be extremely stable; the high-speed rotating gyroscope in the missile system must ensure that the center of mass and the geometric center completely coincide during processing, otherwise it will point Fight here and there.

The Peking University campus adjacent to the subway line on the map.

A compromise that hurts both sides

Like many outside scholars, Lei Jun originally did not know that subway vibrations would affect precision instruments. In China, the fierce struggle between Peking University and the subway brought this issue to the surface for the first time.

In 2003, the plan for Beijing Metro Line 4 was announced, which would run all the way north from the east gate of Peking University. Several science and engineering colleges and many important laboratories of Peking University are closely distributed on both sides of the subway line. A considerable number of Peking University's precision instruments are concentrated in these scientific research buildings. Some scholars reminded Peking University to study whether the subway affects precision instruments.

Lei Jun’s previous studies on the earthquake resistance of buildings were all about larger-level vibrations, and he did not pay much attention to the impact of micro-vibrations. After starting to collect vibration data from other subway lines in Beijing, he discovered that "this problem is very complex and much more serious than imagined."

Because of the report he and his colleagues made, Peking University opposed the passage of Line 4. At that time, Peking University and the Metro Company repeatedly argued over two plans: either Peking University would be moved entirely, or Metro Line 4 would be rerouted.

Until the last seminar, the two sides were still at a stalemate. The meeting was chaired by a deputy mayor of Beijing and invited an academician and several experts from outside Peking University.

The academician said at the meeting that the track vibration isolation solution is feasible. He compared a project he had done, "When you touch it with your hand, you can no longer feel the vibration."

A representative from Peking University asked on the spot: "How sensitive is a sensor like the human hand?" The most sensitive electron microscope at Peking University is hundreds of times more sensitive than the human body.

The meeting finally reached a resolution to adopt a compromise plan - the 789-meter track section of Line 4 passing through Peking University will use the world's most advanced track vibration reduction technology, that is, laying it under the rails Steel spring floating plate. This floating plate was invented by a German company. It has a reinforced concrete plate about 50 centimeters thick on the top and supported steel springs on the bottom, which can isolate the vibration of the train from the track bed.

“For the train, this is equivalent to a very soft cushion, and the springs isolate the vibration.

"Associate professor Ma Meng of Beijing Jiaotong University told China Youth Daily? China Youth Online reporter that this track vibration reduction technology has reached its limit to a certain extent. If it is softer, the safety of train operation may not be guaranteed.

This kind of floating plate can generally isolate vibration very well, but it also has a big disadvantage: due to the principle of vibration isolation, it is of no use for vibrations below the natural frequency, and may even amplify them.

In 2009, after the Peking University East Gate Section of Line 4 was opened, Ma Meng and his colleagues conducted another test to verify this theory. In Ma Meng’s opinion, the vibration reduction measures in this section of track are still effective. , ensuring that many instruments with less demanding requirements can be used normally, but for some extremely sensitive equipment, it will increase the interference.

Peking University is not satisfied with this result. The vibration intensity at the old campus hospital is slightly lower. Peking University decided to build a comprehensive scientific research building there and move some of the affected equipment there. However, due to space and funding constraints, only about one-third of the equipment can be moved there.

In 2011, when the foundation of the building was laid and the lower floors were under construction, another news came: Metro Line 16 would bypass the west gate of Peking University, only 200 meters away from the comprehensive scientific research building.

Because. The precision instruments on campus had nowhere to move, and Peking University strongly protested. Lei Jun analyzed that the reason for this embarrassing situation was that the subway company thought that the vibration reduction was successful and did not know that Peking University was planning to move the instruments. The planning plan was notified to Peking University in advance.

The Beijing Municipal Government has allocated tens of millions of yuan in special funds to allow the Municipal Administration, Beijing Jiaotong University, China Electronics Engineering Design Institute, China Academy of Railway Sciences and Peking University to jointly form a research project. The group came up with a comprehensive solution, which in addition to subway track vibration reduction, also included redesigning the comprehensive scientific research building, considering installing a vibration reduction platform on the lower floor and using springs to levitate the entire building above.

Lei Jun remembers that in those months, there were meetings and discussions two or three days a week, and the parties often argued over specific plans. An expert from the Electronic Design Institute told reporters that Peking University’s requirements were too idealistic, and both parties were not familiar with the data. Different collection and analysis methods lead to several times the difference.

Some experts have heard a joke: If this matter is not handled well, it will affect Peking University's "Nobel Prize".

Just when all parties were arguing, the project came to an abrupt end. It is said that the leaders of Peking University and a city leader met at a meeting, and the two parties shook hands and took a step back, heading west for more than 300 meters. , got rid of the two stations, and Peking University no longer made any demands. Yang Yiqian, a researcher at the China Academy of Railway Sciences, is one of the experts on the project team. In his opinion, Peking University seems to have won in this game. But it is not. This is not a perfect solution, it is just a "lose-lose compromise"

The missing environmental standards

Yang Yiqian believes that taking a step back can reduce the burden on the subway. However, this distance is often not enough to eliminate the interference from Peking University’s precision instruments. On the other hand, after the subway was rerouted, it lost its role in attracting passenger flow.

He suggested at the time that Peking University move its precision instrument building to the suburbs to completely eliminate interference. But for many Peking University teachers, this suggestion is difficult to accept. Yang Yiqian can also understand. After all, Peking University was built first and the subway was later. No one would be happy to move.

Both he and Lei Jun agreed that to avoid such conflicts, planning should be on a first-come, first-served basis. Newly planned subway lines should avoid vibration-sensitive high-tech areas as much as possible, and newly built high-tech zones should be located in suburbs without subways as much as possible.

The crux of the current problem is that precision instruments for scientific research units are often purchased first, and the relevant impacts are not considered when the subway planning plan is formed.

Yang Yiqian is very familiar with relevant foreign laws, regulations and standards. Japan has a special "Vibration Law". The environmental impact assessment standards for rail transit in the United States involve vibration-sensitive equipment.

These two countries have also learned lessons. The University of Tokyo once suspended an entire building on springs, but the impact of vibrations was still unable to be eliminated. Because the light rail runs through the campus, the University of Washington adopted track vibration reduction measures and reduced vehicle speeds, but vibrations in 5 of the 15 sensitive buildings still exceeded the standard.

“Vibration reduction is a world problem, and the best way at present is to avoid it.” Lei Jun often cited the example of Tsukuba Science City in Japan. This city, which gathers Japanese scientific research talents, was founded in 1963. It was not until more than 40 years ago that the subway was connected, and the same city is 2.5 kilometers apart.

China does not yet have an environmental vibration pollution prevention and control law. Although the environmental protection standards include provisions on the impact of vibration on people in residential buildings, office buildings, hospitals, and schools, they do not cover interference with precision instruments. This results in the environmental protection department rarely considering this aspect when the subway planning scheme enters the environmental impact assessment stage.

Recently, the Ministry of Ecology and Environment issued the "Technical Guidelines for Environmental Impact Assessment for Urban Rail Transit (Draft for Comments)", but the impact of vibration on vibration-sensitive instruments is still not mentioned.

Yang Yiqian also found that even environmental protection practitioners have different attitudes towards this issue. Some people believe that this issue naturally belongs to the environmental protection department, while others firmly believe that it does not.

The lack of relevant evaluation standards has led to ill-considered subway plans that pass through scientific research institutions and industrial parks. When a provincial capital city was planning a subway, in order to facilitate patients' travel, they deliberately set up a subway station inside a university-affiliated hospital. Unexpectedly, some medical examination equipment could not be used normally.

By the time potential problems are discovered, it is often too late. Once a specific subway plan passes all levels of approval, "it is almost impossible to move it 100 meters out."

This often leads to confrontation between universities and subways. Line 15 was originally planned to pass through Tsinghua University, but Tsinghua University strongly opposed it. In the end, Line 15 only entered the Tsinghua campus for 120 meters and was not connected to Line 4, forming a transfer station.

As early as 1955, Tsinghua University had rerouted the railway line. The Beijing-Zhangjiakou Railway is located on the same side of the Tsinghua campus. Vibrations have seriously interfered with scientific research. With Tsinghua's efforts, the railway line was moved 800 meters eastward.

Not all universities have strong negotiating power. There are 985 colleges and universities that directly stamped the consent document without much consideration. Some colleges and universities have suffered losses and are unwilling to make them public.

When the subway plan becomes a reality, other vibration reduction measures can only be adopted. China Electronics Engineering Design Institute Co., Ltd. has provided vibration reduction solutions to Fudan University, Nanjing University and many other universities affected by subways.

Vibration Technology Research Center engineer Zuo Hanwen told reporters that the best solution currently is comprehensive vibration reduction. In addition to laying steel spring floating plates under the track, at the same time, installing springs at the beginning of the construction of the instrument building Supported vibration isolation bracket. If the building is already completed, a vibration damping table can only be installed under each instrument, which will greatly increase the cost.

After Line 16 is opened, Peking University can only adopt the second option. Zhang Zhiqiang, director of the Environmental Protection Office of the Department of Laboratory and Equipment Management of Peking University, estimates that a state-of-the-art air spring vibration damping table costs about one to two million yuan. The instruments that Peking University needs to dampen are "on the order of dozens or hundreds." .

Having witnessed the high-end German floating panels, tedious building construction and relocation, and expensive subway rerouting, Peking University’s most sophisticated electron microscope will also be equipped with a complex vibration-absorbing table in the future. But no one can guarantee whether it can escape the interference of subway vibration.

The above content comes from: Shangguan