However, the weightlessness of the aircraft in orbit is not the result of the disappearance or substantial reduction of gravity (in fact, at the height of 100KM, the gravity of the earth is only about 3% lower than the surface of the earth). Weightlessness mainly occurs in orbit or space or other abnormal situations (far away from planets or heavy objects) in a zero-gravity environment at the point of gravity balance, which should be called complete weightlessness. The phenomenon that the pressure of the object on the support is less than the gravity of the object is called weightlessness (if the gravity of the earth disappears, then people will fly in one direction forever with a slight jump).
In the movie Crouching Tiger, Hidden Dragon, the stunts of the heroes "walking on clouds and flying over the walls" are a piece of cake in space flight. As long as you press your feet gently, people will fly freely in the air, and their skills are beyond people's imagination. These phenomena are often mistaken for weightlessness and should be called microgravity-like phenomena.
The centripetal acceleration in orbit is provided by the acceleration of gravity, so the resultant force on the circular spacecraft provides the centripetal force of the circular spacecraft. And the instantaneous acceleration at each time point will point to the center of the earth. In outer space, astronauts and everything in spacecraft move around the earth at the same speed because of gravity, so there will be weightlessness in space and all substances can't be measured.
Weightlessness characteristics
One of the most important signs to judge whether an object is completely weightless is that there is no interaction between the internal parts of the object and the particles, that is, there is no stress such as tension, compression and shear.
Human weightlessness
Balance is the state of motion of our most common objects. However, the balance of force and weightlessness are completely different. For example, people standing on the ground, people sitting in chairs, people lying in bed, planes flying at the same speed, etc. , in a state of force balance, but not weightlessness. Because in these cases, there is interaction between various parts of the human body. The real weightlessness simulation should be to make all parts of the human body, especially the internal organs and internal organs disappear. In this case, due to weightlessness, otoliths in human vestibular organs no longer contact with surrounding nerve cells and transmit signals to the central nervous system, thus losing their directional function. Vestibular organs are closely related to autonomic nervous system, which is responsible for breathing, digestion, circulation, excretion and sweating. Therefore, once the vestibular organs don't work, the normal interaction between internal organs of the body disappears, which will cause dizziness, nausea, vomiting and other symptoms for space pilots.
In the past 30 years of space flight, Russian and American scientists have collected some preliminary data. These data show that weightlessness has a certain influence on endocrine, the production of red blood cells and white blood cells, and the osteoporosis of balanced organs and bones in the inner ear, but the most obvious physiological weightlessness is the water loss in space and some symptoms caused by it, such as space anemia, endocrine decline, muscle atrophy in both legs and so on. Weightlessness can also lead to decalcification of bones, which is very similar to osteoporosis in the elderly.
Great progress has also been made in microgravity physics and chemical science. A large number of data of combustion, materials and fluids under weightlessness were collected. More breakthroughs have been made in the growth of protein crystals. Every space flight brings hundreds of protein crystallization experiments, which has made great contributions to human medicine. Complete weightlessness is an ideal state. In actual space flight, spacecraft will be subjected to some non-gravity external forces from time to time besides gravity. For example, there is the resistance of the residual atmosphere near the earth, the pressure of sunlight, and the atmospheric force on the planet when it enters the atmosphere. According to Newton's second law, the result of a force acting on an object is to accelerate it. When the spacecraft flies in the gravitational field, the non-gravitational force is generally very small and the acceleration is also very small. This non-gravity acceleration is usually only one tenth or even less than the ground gravity acceleration. In order to compare with normal gravity, this micro-acceleration phenomenon is called "microgravity". In fact, even if the spacecraft is only subjected to gravity, there is actually microgravity inside it, because the spacecraft is not a particle, but an object with a certain size. People often use 10-6- 10-4g to represent the microgravity level of spacecraft. The smaller the microgravity, the more complete the weightlessness. In short, complete weightlessness is only an ideal state, and microgravity is the actual situation.
Quantitative analysis of complete weightlessness:
When a=g, the supporting force is n, according to Newton's second law:
Mg-N = mA = mg
So N=0
According to Newton's third law, the pressure of the object on the support is 0, which is obtained by Newton's second law: N+ma=mg, so n = M (G-A).
According to Newton's third law, the pressure of an object on a support
Quantitative analysis of complete weightlessness;
When a=g, the supporting force is n, according to Newton's second law:
Mg-n = ma = mg, so N=0.
According to Newton's third law, the pressure of an object on a bracket is zero.
It is concluded that downward acceleration and upward deceleration: downward acceleration leads to weightlessness.
So as long as the acceleration direction is downward, it is weightless, regardless of the speed direction. 1. Traditional continuous measurement method
There are many methods to measure or control the composition of building materials, grain and oil, mines and other bulk materials online. Typical ones are: belt scale, perforated flowmeter, nucleon scale and disc feeding scale. These measurement forms have their own characteristics, but they are very limited, greatly influenced by mechanical changes of equipment, with low accuracy, complicated installation and adjustment, and large maintenance.
Process introduction and flow of belt weigher;
The belt weigher integrates the load signal received on the unit area (weighing section) with the change speed (belt speed) signal to obtain the flow value as the controllable object.
Note: By controlling the speed of the traction belt and changing the amount of the material to be towed, the thickness of the material will be stable and uniform after it is shaped by the outlet of the feeding chute, and the load on the belt will be constant regardless of the speed of the belt conveyor. Compared with other feeding methods, this method has better measurement and control accuracy.
Note: Feeding and weighing functions are realized on two belts respectively.
Application status of continuous metering method in continuous mixing equipment
Continuous mixing equipment includes stabilized soil plant mixing equipment, cement continuous mixing equipment and asphalt continuous mixing equipment. As far as metering accuracy is concerned, these devices cannot be compared with intermittent devices. Therefore, the continuous mixing mode is not favored by the majority of users, which is also one of the reasons. Scientific analysis shows that the mixing process determined by these two metering methods has its applicable occasions, and the application of continuous mixing cannot be affected by temporary technical restrictions.
In China, continuous mixing equipment is measured by volumetric method or belt scale/screw scale. Since the introduction and development of continuous mixing technology from Europe in the 1970s, there has been no breakthrough. In fact, these two measurement methods can achieve high accuracy when used in Europe. For example, the belt batching scale in Schenk, Germany, has a dynamic batching accuracy of 2%. But not in China, because of the constraints of China's basic industries such as machinery manufacturing and materials. At present, the measurement accuracy of belt weighers used in highway industry in China can only reach about 5%, which is almost the same as volume measurement and has poor long-term stability. The Revolution of Continuous Weighing —— Differential Weighing Scale
Since 1990s, the weightlessness method has been applied to the continuous measurement of industrial process weighing. Weightlessness scales gradually replace belt scales, spiral scales and even stacking scales. As a brand-new measurement method, it is gradually applied to more and more material handling.
1. Basic principles:
Taking the weighing hopper and the feeding mechanism as a whole weighing body, the weighing signal of the weighing body is continuously sampled by the instrument or the upper computer, and the change rate of weight per unit time is calculated as the instantaneous flow, and then the "actual flow" which can be used as the control object is obtained through various filtering technologies of software and hardware.
It is very important to obtain this kind of flow, which is the basis of accurate measurement of weightlessness scale. A classic method is introduced in the figure: then FC approaches the target flow through PID feedback algorithm, and outputs adjustment signals to control feeder controllers such as frequency converters.
2. The application of differential decrement scale (weightless scale) in practice:
From the principle, it can be seen that it is not affected by the mechanical changes of the scale body and the feeding mechanism, and only calculates the weight difference (differential weight). Compared with the traditional dynamic measurement method, its advantages are self-evident.
When the controlled object is flow (t/h, kg/min) and the material has good transportability and high measurement accuracy, the weight loss method can be used as an optimal scheme.
3. Matters needing attention in the design of weightless scale and factors affecting accuracy:
The weightless scale has the characteristics of both static scale and dynamic scale. Therefore, when designing the system, it is required to:
The correct delivery range is generally 60%~70% of the rated delivery. If AC speed regulation is adopted, the strain frequency is 35-40Hz. This ensures a wide adjustment range. But also because the system stability is poor when the transmission rate is too low. Select the appropriate sensor range according to the formula.
In other words, the sensor also uses 60%~70% of its range, and the signal changes widely, which is very beneficial to improve the accuracy.
Mechanical structure design should ensure good fluidity of materials, and at the same time ensure short replenishment time and not too frequent replenishment. Generally, it is required to supplement the materials every 5- 10 minutes.
The matching transmission system should ensure smooth operation and good linearity.
4. Application prospect:
With the rapid development of electronic control technology, the measurement accuracy of weightlessness scale is improved from 0.3% to 0.5% by adopting new technology. The application of digital weighing sensor is the core of this new technology, which is increased to 0. 1%~0.2%, even exceeding the static scale. Application of weighing sensor
In order to meet the needs of dynamic measurement, the sensor is very important as the input of the weighing system. Especially in the need of intelligence, the direct or indirect data security of sensors is very important. At this time, measurement uncertainty and measurement speed are often a pair of contradictions, so it is difficult to have both, and a compromise choice needs to be made according to the actual situation. In the field of weighing, a large number of traditional analog sensors are produced and applied in China, and the output of analog signals is very few. Take the weighing sensor with the largest production capacity and the principle of resistance strain as an example, the maximum output is generally 30-40mV. Therefore, its signal is easily disturbed by radio frequency, and the cable transmission distance is short, generally within 10m m. In the container weighing system (hopper scale batching scale), platform weighing system or scale bridge (truck scale or track scale) with multiple sensors connected in parallel, "self-calibration" can be realized by using digital system. This is because there is no impedance matching problem in multi-channel digital sensor system. Users only need to input the address, weight and sensitivity of each sensor, and they can automatically balance the "four corners" or "four corners" of the scale without repeatedly adjusting the letters. However, after connecting multiple sensors in the analog system, the characteristics of each sensor are no longer distinguishable. In the calibration process, it is necessary to apply a weight to each sensor and use the voltage divider in the junction box for adjustment. Because there is interaction during the adjustment, it is repeated many times. In a digital system, it is allowed to inspect each sensor individually as a unit. Therefore, the time spent calibrating the digital sensor system is only 1/4 of that of the analog system.
Digital system can realize "self-diagnosis", that is, the diagnostic program constantly checks whether the signals of each sensor are interrupted and whether the output is obviously out of range. If there is a problem, it will automatically display or give an alarm on the instrument or controller panel. Users can use the buttons on the panel to find each sensor, independently determine the cause of the problem and troubleshoot it. This intuitive ability to diagnose and troubleshoot is obviously an important advantage for users, but it is unforgettable to realize it at low cost in analog sensor systems.
In the field of weighing, the resolution of analog-to-digital converter of typical analog sensor system is 16 bits, that is, there are 50000 available counts; In the digital system, the resolution of each sensor is 20 bits, that is, there are 1000 000 available counts. Therefore, a system equipped with four digital sensors can provide a resolution of 4,000,000 counts. This advantage of high resolution is especially suitable for occasions where the scale frame is heavy and the weighed object is light. For example, in the batching weighing system, sometimes a certain material only accounts for a small proportion in the formula, but the accuracy requirement is still very high. This is also difficult for the traditional simulation system to achieve.
1. Application status at home and abroad (cement plant, metallurgy, plastics, chemical fiber and other industries)
Many industries have rich experience in applying weightlessness scales. Such as: cement plant ingredients. It has been widely used in engineering plastics, chemical fiber, optical fiber and many other industries. In some industries, continuous weightlessness measurement can ensure the proportional mixing of blanking, weaken the necessity of stirring and simplify the process. This product is very mature in developed countries abroad, such as Schenk company in Germany, brabender company in Switzerland and ktron company, and its technology is in the leading position in the world. Thanks to the adoption of digital sensor technology, the dynamic accuracy of the Capital Company can reach 0.25%. In industrial process weighing, the accuracy of static scale has been reached. Application and prospect influence in continuous mixing machinery: Because the domestic continuous mixing equipment measurement stays in the traditional method, the application prospect of popularizing weightlessness scale will be very broad, which will revolutionize the processes of stabilized soil factory mixing, cement continuous mixing and asphalt continuous mixing, and the accurate control of flow will produce very qualified and ideal mixture. Due to the simple structure and low maintenance cost of continuous mixing process, once the product level is properly matched, the current situation of low market share of continuous mixing will be completely changed. Especially the high-yield equipment needed by highway and hydropower industries, is of great significance.
Seymour Weight Loss Scale uses static scale to weigh the whole feeding system (silo, feeder and bulk materials), and controls the discharge flow of bulk materials through variable speed motor or electric vibrator. When the material is discharged from the system (through the screw, vibrating tube or trough), the measured weight loss (dv/dt) per unit time will be compared with the required feed rate (preset value), and the difference between the actual (measured) flow rate and the expected (preset) flow rate will be corrected by the feed controller (MT2 104), which can automatically adjust the feed rate. When the measured weight in the silo reaches the low level (reloading) of the silo, the controller controls the feeding system according to the volume, and then the silo is quickly reloaded (manually or automatically), and the weightless controller acts again. In the batch weighing weightlessness system, the design is similar to the continuous weightlessness system, however, the accuracy of the final weight of the feeding (batch) cycle is higher than the actual feeding control. 6 104 controller completes fast feeding by providing high feeding signal to variable speed drive, and then switches to low feeding control signal for precise control at the end of batch. After satellites, spaceships and space shuttles enter orbit, people and things inside will be in weightlessness. After satellites, spaceships and space shuttles enter orbit, they can be regarded as objects that move in a circle around the earth. The direction of speed is always changing, so there is acceleration, which is equal to the gravity acceleration of the satellite height. This is similar to the situation in the gravity acceleration elevator, where people and things in the spaceship are inside.
Can you imagine what will happen in the case of complete weightlessness? Imagine what will happen on the earth and in the spaceship once gravity disappears. Objects will float in air, droplets will be absolutely spherical, and bubbles will not float in liquid. Astronauts are as comfortable to sleep standing as lying down, so they must be careful when walking. If they are not careful, they will not be able to reach the ground. Food should be made into paste like an image block or toothpaste to prevent food residues from "floating" into the air.
You can think again, what can human beings do in weightlessness? The following example will help you think. Although the examples mentioned here have not been fully realized, scientists are working hard to explore and may realize them in the near future.
Under the condition of weightlessness, the droplet shape of molten metal is absolutely spherical, and it can become an ideal ball after cooling. On the ground, the ball manufactured by modern technology is not absolutely spherical, which is one of the important reasons for bearing wear.
Glass fiber (very fine glass fiber with a diameter of tens of microns) is the main component of modern optical fiber communication. It is impossible to make a long glass fiber on the ground, because it will be pulled into small pieces by gravity before solidification. In space orbit, it will be possible to make glass fibers hundreds of meters long.
In the orbit of space, a new kind of foam material-jujube foam metal can be made. Under the condition of weightlessness, when gas is introduced into liquid metal, bubbles will not "float" or "sink", but will be evenly distributed in liquid metal. After solidification, it will become foam metal, so that it can be made into foam steel as light as a cork, light and strong enough to be used as a wing.
By the same token, in weightlessness, the mixture can be mixed evenly, thus making a special alloy that cannot be obtained on the ground.
There is an increasing demand for high-purity materials in electronic industry, chemical industry, nuclear industry and other departments. The purity requirements are "six 9s" to "eight 9s", that is, 99.9999% ~ 99.999999%. On the ground, metal smelting needs to be carried out in a container, and there are always some trace elements mixed with the smelted metal in the container. In space, "suspension smelting".
When the crystal used in electronic technology is produced on the ground, the size of the crystal is limited due to the influence of gravity, and it is polluted by containers. Under the condition of weightlessness, the crystals produced are more uniform and much larger. In the near future, if a factory can be established in space to produce pure crystals of gallium arsenide, it will be much better than the existing silicon crystals. It will lead to a major breakthrough in electronic technology. Wingless fish and ants can float leisurely in the air. This is not a scene of magic show, nor a simulated weightless environment in space, but a real scene in the laboratory of Northwestern Polytechnical University. Xie Wenjun, who is in charge of this experiment, is a material physicist at Northwestern Polytechnical University. Of course, scientists are not joking with these small animals on purpose, but are conducting a study on acoustic levitation. Ordinary objects and animals, because of their own gravity, can't overcome gravity and float freely in the air without the help of external forces. Of course there are exceptions. Astronauts will also experience the feeling of negligence in space and can be suspended in the air. This is because the astronaut's spacecraft trajectory is at the gravity balance point of two celestial bodies, for example, the gravity of the earth and the moon cancel each other out, and then the spacecraft is in a weightless environment, and the gravity is zero, so it can naturally float. Are these fish and ants floating in the air because scientists have created a weightless environment for them through special means? "The floating of fish and ants is not a weightless phenomenon." The guess of weightlessness was immediately denied by Wen Jun, and it seems that the answer is not so simple. If fish and ants are still not out of their own gravity, then from the perspective of force balance, there must be external forces to help them overcome gravity and eventually float. Where does this invisible power come from? Xie Wenjun told us that they actually just skillfully used sound waves. In the experiment, the acoustic emission end at the top emits sound waves, which reach the acoustic reflection end at the bottom and then reflect back. The reflected sound wave overlaps with the sound wave that continues to propagate to the reflecting end, thus forming a standing wave. Standing waves don't move forward like sound waves, but vibrate up and down in the same place. The place with the largest amplitude is called antinode, and the place with the smallest amplitude is called node. As long as small animals such as fish and ants are placed at the nodes, they will be still. When doing experiments, as long as the distance between the reflector and the emitter is adjusted first, the position of the node is fixed. At this time, it is enough to clamp small animals such as ants, ladybugs and small fish in this position with tweezers. When floating in the air, these animals seem to be very nervous. Ants danced and tried to swim around, while ladybugs flapped their wings as if to fly away. However, their bodies were not harmed, but the vitality of small fish was obviously affected. Because he left the water environment, Xie Wenjun kept "bathing" the small fish when they were floating in the air. In fact, as early as 2002, Xie Wenjun and his colleagues used sound waves to suspend solid iridium and liquid mercury. Since 2003, they began to pay attention to the acoustic levitation of living things. So, if the sound wave reaches a certain intensity, is it possible to suspend people? Xie Wenjun said that experiments have proved that acoustic suspension can suspend any certain volume of solids and liquids in principle. In their experiments, suspended animals include crawling on the ground, swimming in the water and flying in the air, but the size of small animals does not exceed 1cm. This is because the principle of acoustic levitation determines that the size of suspended objects must be less than half a wavelength. For ultrasonic slicing, the size of the object that can be hung should not exceed 1 cm. There is no experiment to prove that the acoustic levitation device can levitate an object as big as a human being to levitate a living animal, and some scientists abroad have tried it. 1997, physicist Andre of Nijmegen University, Netherlands. Jim and Sir Michael Bailey of Bristol University in England used magnets to float frogs. They used superconducting magnets to float a live frog in mid-air. Frog itself is not a magnet, but it becomes magnetic by the magnetic field of electromagnet. In addition, superconductors will float automatically because of the repulsive force to the magnetic field. This principle has been verified in Japan. 1996, a Japanese sumo wrestler weighing 142kg was suspended by a metal plate in the magnetic field suspension experiment. The same principle is also used to develop maglev trains. Although most of the maglev trains used are realized by electromagnetic fields, the principle is the same.