Hydraulic technology is one of the key technologies to realize modern transmission and control, and its level directly affects the quality of construction machinery products. To this end, the development of hydraulic technology in the world has given great attention. At present, hydraulic technology is widely combined with high-tech achievements, such as automatic control technology, computer technology, microelectronics technology, friction and wear technology, reliability technology and new processes and new materials, so that traditional technology has a new development, but also make the quality of hydraulic systems and components, the level has a certain improvement. At present, as far as the construction machinery industry is concerned, hydrostatic technology has shown a more and more extensive application trend.
Hydrostatic transmission technology
The word "static" in front of the word "hydraulic" was in fact not intended to be unique in hydraulic technology at the beginning, but only to correspond to the more general fluid transmission field that was once translated as "hydrodynamictransmission". However, the latter term is now called "hydraulic transmission" in China. Without the contrast of "dynamic hydraulic", "hydrostatic transmission" is just the name of a special class of closed circuit systems in hydraulic technology. In fact, in terms of physical concepts, it seems more appropriate to use "hydrostatic" and "hydrodynamic" to characterize these two fluid power transmission modes, which are mainly potential energy and kinetic energy conversion, respectively.
Examples of closed loop systems
The hydrostatic drive device composed of the closed oil circuit system retains the advantages of flexible control mode, convenient layout and strong overload protection, etc., while it has the ability of stepless speed regulation and continuous operation in all four quadrants composed of the output speed vector and output torque vector of the hydraulic motor as the coordinate axis. In many ways, it is more suitable than pure mechanical transmission, hydraulic transmission and electric transmission for medium and low speed walking machinery with high speed regulation performance requirements and special transmission route layout. However, due to the low steady-state efficiency of the hydrostatic drive device relative to the pure mechanical transmission and the current high cost, the high-speed performance of the components used and the power level that can be achieved are not as good as the hydraulic and electric transmission, so that its application results in high-power high-speed special vehicles and walking machinery are not much. But that is changing. Through the combination of various transmission technologies, people are injecting new vitality into the hydrostatic drive device technology, and will promote its application to a greater depth and breadth of development.
Meet the new development opportunities of hydraulic technology
The development direction of hydraulic control system to digital electronic control transformation is undoubtedly correct. At present and in the future, the trend is to separate the control function of the hydraulic device from the electronic circuit, forming the pattern of electronic nerves and hydraulic muscles.
The complex hydraulic resistance network system in hydraulic control is gradually replaced by electronic control system. But as is the case in other fields, the electronic technology used has also undergone a process of development from analog computers to digital computers. Now, the "digitalization" of hydraulic devices mainly refers to the problem of the control interface of components. Most of the digital hydraulic components developed at home and abroad have no change in the basic functional principle of force transmission and power transmission, etc. What changes is that the input control instructions are converted into electrical signals by mechanical (displacement and force value, etc.) and hydraulic (pilot control pressure). Similarly, the amount of hydraulic components to be controlled is also developing from analog to digital, such as servo valves and proportional valves are converted into changes in electricity resistance to control the size of the flow. For example, the multi-cylinder synchronous system used to be controlled by a very complex hydraulic resistance network and an expensive displacement sensor, but now it only needs to input an equal amount of hydraulic micro-elements to each cylinder through the digital valve. In multi-user hydraulic systems such as 6-DOF emulators, quantitative control with hydraulic microelements can also effectively isolate the interference between hydraulic cylinders, which is a great progress.
The digital principle is integrated into the structure of the power transmission hydraulic components in addition to valve components. At present, there are radial piston variable pumps and variable motors with numerical control distribution devices developed by Artemis, a British company owned by Mitsubishi, Japan, which use pulse width control to change the effective stroke of the plunger. The hydrostatic transmission device composed of this component has been used in the world's current 7MW offshore wind turbine developed by Mitsubishi Heavy Industries, which was put into trial operation in early 2013.
When it comes to hydrostatic technology, there have been some debates in the industry.
In the 1970s and 1980s, after the digital stepper motor fully replaced the electro-hydraulic servo hydraulic motor in the machine tool industry, this trend spread to other areas that were originally the protagonist of hydraulic devices such as high-end plastic injection molding machines, and then people saw the strong development of electric energy and electric transmission vehicles represented by electric and petrol-electric hybrid vehicles. Heard in some areas for "all-electric" products of high profile publicity. Some people are beginning to regard hydrostatic drive technology as a "sunset industry" that will be fully replaced by electric transmission.
However, today we see that the position of walking hydraulic technology, especially hydrostatic drive technology, in the field of off-highway vehicles and walking machinery has not only not been weakened, but has been further strengthened.
In addition to the progress of materials and processes, the three major breakthroughs made in the field of hydraulic technology in recent years are important reasons for its strength.
First, the large-scale manufacturing of hydrostatic mechanical power shunt continuously variable transmission has made it occupy the commanding heights in the transmission technology of off-highway vehicles and walking machinery.
Second, the maturity of series hybrid technology has opened the door for it to enter the automobile industry with strict technical and economic barriers.
Third, the successful development of high-efficiency hydraulic pumps and hydraulic motors with electronic control distribution has pushed the technology of hydrostatic components to the forefront of electromechanical hydraulic integration and information equipment.
What these three breakthroughs have in common is that they all benefit from a combination of technologies. An important development issue in today's world is low-carbon and energy conservation, which requires both "open source" and "reduce expenditure". The hydraulic energy storage system based on the principle of gas compression energy absorption and hydraulic power transmission is one of the most practical ways of recovering and regenerating energy with good comprehensive efficiency. In the field of vehicles and walking machinery, the hydrostatic drive system has a very "hard" potential in both hands.
We are standing on the threshold of a period of new development opportunities for hydrostatic drive technology. Fluid transmission and control technology, including hydrostatic drive, will undoubtedly have a broad space for sustainable development in the future, and continue to move from the kingdom of necessity to the kingdom of freedom. We should continue to discover, invent, create, and move forward with the idea of inclusion.
