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My Thoughts on Digital Hydraulics

I remember that when I was studying for a master's degree in 1979, there were already analog computers in the school, and digital computers were added, all of which were the size of several large wardrobes. At that time, there was a debate about the advantages of analog machines and digital machines. Within a few years, the simulators became scarce. Beginning with the digital computer, digital technology has been marching triumphantly for the past fifty years. Music: Within a few years of the appearance of digital discs, analog records and tapes were pushed into the corner. The DDD on the disc means that the recording, processing, and storage processes are all digital. In the past, TV images often had "snowflakes fluttering", which means that the TV signal of analog technology has been interfered. After adopting digital technology, the image is much clearer and more stable. The same wire can transmit more than ten times the number of digital TV channels than analog TV.


To record video, (digital) discs replaced magnetic tapes using analog technology. In more than two decades, video formats have evolved from low-definition VCD (352×288 pixels) to DVD, Blu-ray, 4K (3840×2160 pixels), 8K ( 7680×4320 pixels), it is better than sitting in the front row to watch a movie. Digital cameras have replaced film cameras with analog technology, and the images are so easy to post-process that even professional photographers love them. Digital cinema is also making film films obsolete. It seems that digital technology is invincible wherever it goes, and it has replaced the long-term use of analog technology in the past in just a few years. So, will "digital hydraulics" also replace traditional hydraulics, and why? This is the question that this article tries to answer.


Simulation technology is based on analog quantities. Digital technology is developed on the basis of digital quantities. Some related concepts are defined differently in different categories. In the field of process control, the author understands this way. The analog quantity refers to the quantity that is continuous in time and space, gradually transitions, and may take any value within a certain range (definition domain). For example, the oil temperature and liquid level in the oil tank, the stroke of the piston in the hydraulic cylinder, the pressure in the load chamber, and the displacement of the proportional valve spool. Discrete quantity refers to the quantity that is discontinuous in time or space, and only takes several discrete values, and there is no intermediate value. For example, the position of the switch valve spool, under normal working conditions, ignores its opening and closing process, and only considers the two states of fully open and closed, and its position can be considered as a discrete quantity. For another example, several quantitative pumps are connected in parallel, and the flow that can be output is generally considered to be a discrete quantity—different combinations of the output flow of each pump. Discrete quantities are unordered.


There is no absolute boundary between discrete and analog quantities.


Analog and discrete quantities exist objectively.


Digital quantity is an artificially agreed, abstract and simplified description of the objective world. Numbers are made to convey information, not motivation. Using numbers can exert psychological pressure on people, but it cannot convey physical pressure.


Numeric quantities are ordered.


Has a sequence. For example, every time you add 1, the result is always 1, 2, 3, not 1, 3, 2. In binary, the numbers are only 0 and 1, but there is also a sequence: 0, 1, 10, 11, .... Numerals have different meanings in different locations. For example, 11, the value of the 1 on the left is 10 times the value of the 1 on the right in decimal. Thirty-five thousand, everyone understands. Thirty-five million won't work.


Unordered quantities cannot be counted as numbers.


Reasons and scope of application of digital technology

The popularity of digital technology is due to at least the following characteristics.


  • High anti-interference


The real world is basically a simulation. In the process of storage, transmission, and re-reception, the physical quantity actually used to store and transmit information will always be subject to unpredictable interference and undetermined losses. For example, the magnetism on the magnetic tape will weaken; electromagnetic waves in the air, wires, and light waves in optical fibers, after long-distance transmission, the intensity will attenuate; the reflection bumps on the optical disc will also have uneven heights due to oxidation after decades loss. In this way, the actual received signal strength will be different from the original one. For example, the signal strength at the transmitting end is 1.000±0.001V, and the receiving end may be 0.990±0.005V. If you use analog technology and process it according to the strength of the received signal, there will be distortion. The more retweets, the greater the distortion. The basic principle of digital technology is roughly as follows: the sending and receiving parties agree that the sending party samples the analog quantity to be transmitted to obtain discrete quantities, and then converts the discrete quantities into binary digital quantities composed of digital 0 and 1 one by one, and uses pulses in groups to obtain discrete quantities. sent to the recipient. The receiver regards the received pulses with an amplitude less than a certain value, for example, 0.3 pulses as 0, and the amplitudes within a certain range, for example, 0.7∼1.3 pulses as 1, and all pulses outside this range as 1. invalid. Then, convert the grouped binary numbers into octal, hexadecimal, or decimal numbers used in daily life where necessary, and then restore them to analog quantities. This has a very high anti-interference, and has achieved strong reproducibility, so that it can be forwarded many times. Principle of binary coded signal transmission A—received signal B—binary digital signal interpreted according to convention Anti-interference, binary is better than all other bases, for example, ternary. Therefore, the digital technology that is popular in the whole society is actually binary digital technology. The frequency of coded signal transmission is generally fixed, but instead of using Hz as the unit, it uses b/(bit/s bit rate, the number of digits that can be transmitted per second), from the early thousands of b/s to the current hundreds Thousands of b/s (Mb/s) or even billions (Gb/s).


  • Verifiability


It is easy to check after the analog quantity is converted into digital quantity. For example, send a check code at the end of sending a batch of data. It is agreed that the last two digits of the sum of this batch of data should be equal to the check code. Otherwise, this batch of data is all invalid and requests to resend. The reliability of the information is further improved by being verifiable.


  • Comprehensive expression ability


Using a series of digital combinations, digital technology can now express almost all the texts in use in the world at the same time, and express various sounds, colors, images, music, movies, etc., and its distortion has reached the level that ordinary people cannot perceive.


  • Programmability


Computer programs developed on the basis of digital technology can be executed by computers on the one hand, and can express extremely complex logic on the other hand, so that computers can realize complex process control and have a certain self-learning ability. Smart advance.


  • High-speed computing capability


The central processing unit (CPU) in mobile phones has been able to perform hundreds of millions of arithmetic and logic operations per second. Some mobile phones are equipped with 4 CPUs (4 cores) at the same time, and its calculation-the ability to execute complex programs is hard to imagine.


  • High speed transfer capability


Digital technology, on the one hand, has a speed of 300,000 km/s due to the use of electromagnetic waves and light wave propagation. On the other hand, transmission at extremely high frequencies (3G, 4G, and 5G of mobile phones essentially apply higher frequencies) can carry a lot of information. Now, the transmission rate of household optical fiber can reach 100Mb/s, and it is already possible to transmit high-resolution video synchronously. In less than a second, the four classic classics can be transmitted.


  • High level of integration


Now, the portable hard disk is the size of a palm and has a storage capacity of more than 2T, which is equivalent to 1 trillion Chinese characters and about 800,000 copies of A Dream of Red Mansions. However, the storage capacity of large-scale cloud servers is difficult to describe in everyday language. The binary digital signal is represented by two stable states of the circuit, therefore, the components in the circuit do not require high precision and can be compressed to a very small size. At present, the size of components in large-scale integrated circuits is only a few hundred or one thousandth of a hair, and an integrated circuit the size of a sesame seed can contain millions of electronic components.


  • Low price


Silicon, the main raw material for manufacturing integrated circuits, is the second most abundant element in the earth's crust, so it is inexhaustible. Over the past fifty years, as manufacturing technologies have matured and integrated circuits have shrunk in size, the price of integrated circuits with the same function has halved every 18 months.


  • The Internet


The powerful Internet developed on the basis of the above characteristics has entered every household and even everyone's pocket. Digital technology is based on the digitization of information, but the reason why it can replace analog technology is inseparable from the support of the above-mentioned characteristics in many aspects, all of which are indispensable. Imagine if the mobile phone is still as big as the big mobile phone 20 years ago, or the screen is only black and white, or the memory is full after taking 3 photos, or cannot connect to the Internet, or post a photo in Moments It takes an hour, or it costs 100 yuan to send a copy. Will digital technology still be so popular?


Even so, digital technology has its limits. It is only used to transmit information, the voltage is only a few V, the current is only a few µA, and the power transmitted is insignificant to the engineering technology of daily life.


There is still a lot of information, but it is a pity that it is still necessary to use language to make some inconspicuous and extremely subjective descriptions. There have been no public reports on the successful application of digitization, such as information about fragrance and food taste.


Can digital hydraulics be directly connected to a computer? This is a rumor!

The operating current between electronic components in a computer is less than a few µA. To control any hydraulic components, the required signal current is at least tens of mA, which is more than 10,000 times the internal current of the computer. Therefore, the computer needs power amplification to drive any hydraulic components, and it is impossible to connect directly. The electronic components in the computer transmit parallel signals through the internal bus, from the early 8-bit to the now common 64-bit. Any output signal requires special electronic components to take out the conversion and amplification from this internal bus, whether it is an analog signal to control a servo valve, a PWM signal to control a proportional valve, or a PNM signal to drive a stepping motor; even the simplest switch signal, also requires a dedicated electronic component to fetch a certain bit from the parallel bus. The output interface of a personal computer used to have a 25-pin parallel port for connecting to a printer and a 9-pin serial port for connecting to a mouse, but now there are only earphone jacks, HDMI and USB interfaces. None of these interfaces can be directly connected to hydraulic components. Some industrial control computers or PLCs have special interfaces, which can output analog signals, PWM signals, PNM signals and switch signals for controlling hydraulic components, but they are all specially set according to needs. Different electronic components and amplifiers have some price differences, but in today's highly developed electronic industry, these price differences are negligible compared to the cost of the entire hydraulic system. Therefore, it is true that "the computer controls the discrete hydraulic components without the need for D/A conversion components", but the so-called "digital hydraulics can be directly connected to the computer" is a rumor and a bluff!


About the definition of digital hydraulic pressure

In Europe, research on digital hydraulics is mainly concentrated in several research institutions such as Tampere University of Technology in Finland and Johannes Kepler University Linz in Austria. Mr. Linjama from Tampere University of Technology once proposed the following definition: Digital Fluid Power meanshydraulic and pneumatic systems having discrete valued component(s) actively controlling system output. Note: This definition only refers to digital systems and discrete components, not to digital components.


About technological innovation and replacement

There is no doubt that we need to constantly innovate and replace old products with new and better ones. And put customer needs first. Those who engage in technology and start a business should put the needs of customers first at all times, and what benefits their own technology can bring to customers. For example, hydraulic cylinders are used by customers to drive loads. Therefore, the first priority is the itinerary. Then, what is the rated working pressure, how high the pressure test has been done, and hundreds of thousands of durability tests have been done, how is the wear resistance and corrosion resistance of the piston rod? Crawling, whether it meets the minimum requirements of industry standards, and so on. At present, the vast majority of hydraulic cylinders, especially those used in construction machinery, are manually controlled, that is, manually closed-loop controlled, and do not require automatic positioning. If it is an automatic positioning hydraulic cylinder, it is necessary to talk about the positioning repeatability and accuracy when the load changes and the pressure of the oil source changes, and the dynamic response characteristics. Let's talk about the measured data. Domestically and internationally, there are at least a thousand companies that can manufacture hydraulic cylinders. It is relatively easy to say that it has reached the domestic advanced level. But to reach the international advanced level, it should at least have studied and inspected the level of eight or ten cutting-edge manufacturing plants in the world.


Key points that must be paid attention to in technological innovation and substitution

  • The functions and tasks must be basically the same. It is meaningless to compare components with different functions and tasks and talk about replacement.


  • The price-performance ratio is better: or the performance is better, but the price does not increase much; Or the performance is not bad, but the price is obviously low. The performance here should be generalized, including durability, environmental protection and so on. The price here should take into account not only the R&D, design and manufacturing costs, but also the replacement and substitution costs. Therefore, to say that one component is more advanced and can replace another component, it is necessary to comprehensively compare the performance data obtained through testing in a realistic way and compare various costs. It is not enough to compare only one point. For example, although the servo valve with feedback is obviously superior to the common open-loop electric proportional valve in repeatability, linearity, hysteresis, etc., its application quantity and scope are far inferior to that of the electric proportional valve, not only because of the complicated structure and the inability to reduce the manufacturing cost, but also because of the pollution sensitivity and other problems. In addition, different applications often have different evaluations on whether the performance is excellent or not. It is an excellent performance in one application, and it may not be taken seriously in another application. For example, the durability of work, ocean-going ships hope to last for 20 years, while missiles only need a few minutes.


  • Demons hide in details. Many ideas will not be exposed to the details until they are actually manufactured and used, so as to know whether they are suitable for popularization. The maglev train is an example. Theoretically, it is extremely beautiful: the train is suspended in the air by magnetic force, and there is no friction with the ground. However, when it was actually manufactured, it was felt that it was expensive and difficult to promote.


  • New technology may not be able to replace old technology. For example, the train traction locomotive, the earliest appeared steam locomotive, about 1825; Then came the electric locomotive, about 1840; Then came the diesel locomotive, about 1920. At present, steam locomotives are basically out of use, although diesel locomotives are still in large use, but now the 14 high-speed trains are all driven by electricity. For another example, the gasoline engine was invented in 1883. The diesel engine was invented nine years later, and it has been favored for many years because of its great output. It is almost the only power source in ships, trucks and mobile construction machinery, and it is equally divided with gasoline engines in cars. However, according to the recent report of the German Federal Ministry of Environmental Protection, diesel engines are the main emission sources of fine particles PM10 and NO 2 in Germany, so they have been criticized. In Germany, diesel cars meeting E5 emission standards were allowed to be put on the market in 2015, and they were banned from entering the urban area of Stuttgart in 2017. Munich, Cologne and other cities are also preparing to take similar measures [7]. The automobile industry is lamenting that the life of diesel engines is in jeopardy. Therefore, it is naive to claim that an artifact has been invented, which can replace all existing hydraulic components, and it is ignorant of the profoundness of hydraulic technology and its extensive and complex application.

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