Hydraulic transmission control system

Hydraulic transmission control is a control method often used in industry, which uses hydraulic pressure to complete the process of energy transfer. Because of the flexibility and convenience of hydraulic transmission control mode, hydraulic control has been widely valued in industry. Hydraulic transmission is a subject that studies the realization of various mechanical and automatic controls with pressurized fluid as energy medium. Hydraulic transmission is to use this component to form various control circuits, and then organically combine several circuits into a transmission system to complete certain control functions to complete energy transmission, conversion and control.

In principle, the most basic principle of hydraulic transmission is Pascal's principle, that is to say, the pressure of liquid is the same everywhere, so that in a balanced system, the pressure acting on the smaller piston is relatively small, while the pressure acting on the larger piston is relatively large, which can keep the liquid still. Therefore, through the transfer of liquid, different pressures at different ends can be obtained, thus achieving the purpose of conversion. The hydraulic jack we often see uses this principle to realize force transmission.

Basic principle of hydraulic transmission

The components needed in hydraulic transmission mainly include power components, executive components, control components and auxiliary components. Among them, hydraulic power components are components that generate power for the hydraulic system, mainly including various hydraulic pumps. The working principle of hydraulic pump is volume change, so it is also commonly known as volume hydraulic pump. Gear pump is the most common hydraulic pump, which makes the liquid move through the rotation of two meshing gears. Other hydraulic pumps include vane pumps and piston pumps. The main problems to be paid attention to in selecting hydraulic pump include energy consumption, efficiency and noise reduction.

Hydraulic actuator is a device that converts hydraulic energy provided by hydraulic pump into mechanical energy, which mainly includes hydraulic cylinder and hydraulic motor. Hydraulic motor is a device that does opposite work to hydraulic pump, that is, the energy conversion of hydraulic pressure is called mechanical energy, thus doing work to the outside.

Hydraulic control elements are used to control the liquid flow direction, pressure level and expected flow control to meet specific working requirements. It is precisely because of the flexibility of hydraulic control elements that the hydraulic control system can complete different activities. Hydraulic control components can be divided into pressure control valve, flow control valve and direction control valve according to their uses. According to the operation mode, it can be divided into manual control valve, mechanical control valve and electric control valve.

In addition to the above components, the hydraulic control system also needs hydraulic auxiliary components. These components include pipes and pipe joints, oil tanks, filters, accumulators and sealing devices. Through the above devices, we can establish a hydraulic circuit. The so-called hydraulic circuit is the corresponding control circuit composed of various hydraulic devices. According to different control objectives, different circuits can be designed, such as pressure control circuit, speed control circuit and multi-cylinder work control circuit.

According to the structure and characteristics of hydraulic transmission, in the design of hydraulic system, the system should be analyzed first, and then the schematic diagram of the system should be drawn, which is represented by hydraulic mechanical symbols. Then, the hydraulic components are selected by calculation to complete the design and debugging of the system. In this process, the drawing of schematic diagram is the most critical. It determines the quality of the design system.

The application of hydraulic transmission is very strong, such as the hydraulic system of loading and unloading stacker, which is used as storage machinery to realize the mechanized loading and unloading of textile bags, oil drums and wooden barrels in modern warehouses. It can also be used in the production practice of hydraulic system of universal cylindrical grinder. These systems are characterized by high power, high production efficiency and good stability.

As a widely used technology, hydraulic pressure has a broad development prospect. With the in-depth development of computer, hydraulic control system can be combined with intelligent control technology, computer control technology and other technologies, so that it can play a role in more occasions, and can also complete the expected control tasks more skillfully and flexibly.

1, overview

Walking drive system is an important part of construction machinery. Compared with the working system, the walking drive system not only needs to transmit more power, but also requires the equipment to have higher efficiency and longer life. I also hope to have good ability in speed regulation, differential speed, changing the rotation direction of the output shaft and transmitting power in the opposite direction. Therefore, what kind of transmission mode is adopted and how to better meet the needs of various construction machinery walking drives has always been a subject that the construction machinery industry has to face. Especially in recent years, with the rapid development of China's transportation, energy and other infrastructure construction, the scale of building construction and resource development has been expanding. While the market demand of construction machinery has greatly increased, it also faces the challenges brought by worse working environment and more complicated working conditions, which further promotes the in-depth study of its walking drive system.

This paper attempts to discuss the development and law of hydraulic transmission technology in the walking drive system of construction machinery from the perspective of technical composition and performance characteristics.

2. Transmission mode based on single technology

At first, the walking system of construction machinery mainly used mechanical transmission and hydraulic mechanical transmission (except full hydraulic excavator). Now there are also hydraulic and electric drive transmission modes in the walking drive device of construction machinery, which fully shows the great promotion of scientific and technological development in this field.

2. 1 mechanical transmission

The average load factor of pure mechanical transmission engine is low, which can only be changed gradually, and the layout mode is limited. However, due to its advantages in steady-state transmission efficiency and low manufacturing cost, it still occupies a dominant position in the fields of general passenger and freight vehicles with relatively small speed regulation range and agricultural tractors with strict economic requirements and constant working speed.

2.2 Hydraulic transmission

The clutch in mechanical transmission is replaced by the torque converter in hydraulic transmission, which has the ability of stepless speed regulation in sections. Its outstanding advantage is that it has a hyperbolic output torque-speed characteristic, and it can automatically match the load with the rear power shift mechanical transmission to prevent power transmission overload. The characteristics of hydraulic torque converter, such as high power density, low load stress and low mass production cost, make it widely used in large and medium-sized earth-moving machinery, lifting and transportation machinery and high-speed vehicles such as automobiles and tanks. However, its characteristic matching and layout are limited, the torque variation range is small, and the dynamic braking ability is poor, so it is not suitable for occasions requiring speed stability.

2.3 Hydraulic transmission

Compared with mechanical transmission. Hydraulic transmission is easier to control its motion parameters (flow) and dynamic parameters (pressure), and hydraulic transmission ratio hydraulic transmission has better low-speed load characteristics. Hydraulic transmission has been widely used in construction machinery because of its outstanding advantages, such as high transmission efficiency, constant power output control, full power utilization, simple system structure, stepless speed regulation of output speed, forward and reverse operation, high speed rigidity and easy realization of action. Almost all construction machinery and equipment can see the traces of hydraulic technology, and many of them have become the main transmission and control methods. The development of closed-loop limit load regulation and engine speed regulation combined with constant pressure and constant power regulation variable system provides a broad development prospect for the application of hydraulic transmission in construction machinery walking system.

Compared with pure mechanical and hydraulic transmission, the main advantages of hydraulic transmission are convenient adjustment and flexible arrangement. Components such as engine, driving wheel and working mechanism can be arranged in reasonable positions according to the needs of construction machinery. When the engine works at any scheduled speed, the transmission system can exert greater traction, and the transmission system can still maintain high efficiency in a wide range of output speeds, and can easily obtain various optimized power transmission characteristics to adapt to various working conditions. The closed hydraulic drive device used in high-speed mobile machinery can adjust the speed stepless, which makes the vehicle start softly, change the speed quickly and change the walking direction without impact. This performance is very valuable for vehicles that need frequent starting and shifting and frequent shuttle in operation. However, compared with open circuit, the design, installation, debugging and maintenance of closed circuit have higher difficulty and technical requirements.

The combination of electronic technology and hydraulic technology can conveniently realize various adjustments and controls of the hydraulic system. The introduction of computer control and the application of various sensing elements have greatly expanded the working range of hydraulic components. Various state parameters of construction vehicles are monitored by sensors, and control target instructions are output by computer operation, so that the automatic control of vehicles in the whole working range can be realized, and the fuel economy, power performance and operation productivity of the machine can reach the optimal value. Therefore, hydraulic transmission can make construction machinery easy to realize intelligence, energy saving and environmental protection, which has become the development trend of construction machinery at present and in the future.

2.4 electric drive

Electric drive is a generator driven by an internal combustion engine. The generator generates electric energy to make the motor drive the running part of the vehicle to move. The rotation speed and steering direction of the motor shaft are adjusted by an electronic adjustment system, which has the advantage of large reduction ratio range. The input element (generator), output element (motor) and control device can be installed separately. Electric drive was first used in diesel-electric boats and diesel locomotives, and later extended to large-tonnage mining trucks and some large construction machinery. In recent years, small and medium-sized lifting transport vehicles such as forklifts and tractors driven by diesel engines have appeared. However, due to some technical and economic reasons, the power components suitable for mobile machinery are far less popular than those used for fixed equipment, and electric drive is only a "future technology" for most mobile machinery.

3. Develop compound transmission technology.

From the previous analysis, it can be seen that the transmission mode based on single technology adopted in the walking drive system of construction machinery is simple in structure and reliable in transmission, which is suitable for some specific occasions and fields. In most practical applications, these transmission technologies often do not exist in isolation, and there is mutual penetration and combination between them. For example, hydraulic, hydraulic and electric transmission devices all contain mechanical transmission links more or less, and new mechanical and hydraulic transmission devices are also equipped with electric and hydraulic control systems. In other words, targeted composite integration can give full play to the respective advantages of various transmission modes, foster strengths and avoid weaknesses, so as to obtain the best comprehensive benefits. It is worth noting that the hydraulic transmission device with flexible adjustment and high power density has played an important role in it.

3. 1 combination of hydraulic, mechanical and hydraulic transmission

(1) series mode

The series mode is the simplest and most common compound mode, that is, a mechanical transmission is set between the output end of the hydraulic motor or hydraulic transmission device and the drive axle to expand the high efficiency area of speed regulation and realize step-by-step stepless speed change. It has been widely used in loaders, combine harvesters and some special vehicles. Its development is to install the power shift planetary transmission which can change the transmission ratio when driving directly in the driving wheel to realize the wheel-side hydraulic drive with large transmission ratio, thus eliminating the driving axle and facilitating the arrangement.

(2) Parallel connection mode

It is the so-called "hydraulic-mechanical power split transmission", which can be understood as a transmission system in which hydraulic and mechanical devices are connected in parallel to transmit power flows respectively, that is, the power output by the engine is divided into two "power flows" of hydraulic and mechanical by using a multi-degree-of-freedom planetary differential, and the total output speed can be adjusted infinitely when the two power flows are reunited by relying on the controllability of the hydraulic power flow. This method combines the advantages of good stepless speed regulation performance of hydraulic transmission and high steady-state efficiency of mechanical transmission, and obtains a transmission device with good stepless speed regulation performance, high efficiency and wide efficiency area.

According to its structure, this compound transmission device can be divided into two types: the first is the external shunt type with planetary gear differential, and the common shunt transmission mechanism can be divided into two basic forms: input shunt type and output shunt type; The second type is internal shunt type, which uses the differential between the rotor and housing of hydraulic pump or motor to shunt.

This compound hydraulic transmission device developed by Komatsu Company in Japan has been applied to loaders, bulldozers and other construction machinery. By 2003, the total sales volume of agricultural tractors equipped with Vario CVT produced by German Fendt tractors exceeded 30,000.

It can be seen that this new transmission device is increasingly becoming a strong competitor of medium and high power hydraulic transmission and power shift transmission.

(3) Time sharing mode

For special vehicles with different working speeds and idle speeds under non-working conditions, adopting traditional mechanical transmission for high-speed driving and adding hydraulic transmission for low-speed driving can well meet the contradictory requirements of these two working conditions. Mechanical-hydraulic time-sharing drive has been widely used in this kind of vehicles, and this technology has also been applied to aircraft deicing vehicles, field transplanters and other vehicles and machines that need "crawling speed".

(4) Positioning mode

The "wheel-side hydraulic drive device" with hydraulic motor directly installed in the wheel is an auxiliary hydraulic drive device, which can solve the problem that the traction performance of construction machinery needs to be improved, but the all-wheel drive mode cannot be adopted, and it is difficult to arrange the traditional mechanical transmission device. The stepless speed regulation performance of hydraulic transmission makes the driving wheels of different transmission modes coordinate and synchronize. In a sense, it can also be regarded as a kind of power split transmission: the power of the power machine is distributed to several groups of driving wheels, and after ground coupling, it produces traction to promote vehicle movement. At present, many construction machinery manufacturers apply this technology to construction machinery with partial self-propelled driving ability, such as self-propelled graders and scrapers.

3.2 Combination of hydraulic and electric drive

Due to the development of modern technology, electronic technology has great advantages in signal processing ability and speed, while hydraulic and electric transmission have their own advantages in the characteristics of their respective power components. Therefore, in addition to the ubiquitous mode of "electronic nerve+hydraulic muscle", there are also many successful examples in the composite transmission of power flow, such as variable flow hydraulic oil source composed of variable frequency or DC speed regulating motor and high efficiency and low pulsation quantitative hydraulic pump, electro-hydraulic execution unit composed of integrated electric pump-hydraulic cylinder or low speed and high torque hydraulic motor, and drive system of hybrid industrial vehicles.

3.3 Secondary regulation hydrostatic transmission system

The secondary regulation hydrostatic transmission technology realizes the mutual conversion between hydraulic energy and mechanical energy by regulating hydraulic components. Generally speaking, its realization is based on the pressure coupling system. Constant pressure coupling is adopted between the primary component (pump) and the secondary component (motor), and the load torque is balanced by adjusting the motor displacement in real time. At present, the starting point of studying the secondary regulation hydrostatic transmission technology is to recover and reuse the energy in the transmission process, rationally configure the overall structure of hydrostatic transmission from a macro perspective, and improve the control characteristics of its hydrostatic transmission system.

In order to make the hydraulic motor and reciprocating hydraulic cylinder without bidirectional stepless speed change also run in the constant pressure network of secondary regulation system, a kind of "hydraulic transformer" adopting secondary regulation technology appears, which is similar to a power transformer to match the different demands of users on system pressure and flow, thus realizing the power matching of hydraulic system.

Compared with the traditional hydrostatic transmission system, the secondary regulation hydrostatic transmission system has the advantages of more convenient control, four-quadrant operation, energy recovery and storage without changing the energy form, acceleration power can be greatly improved by using hydraulic accumulator, and the system has no pressure peak. Because the first-stage components and the second-stage components are installed separately, the oil source can be provided to multiple hydraulic power components through one pumping station, which reduces the cooling cost, equipment manufacturing cost and system efficiency.

Compared with electric drive, hydrostatic drive with secondary regulation has the advantages of fast dynamic response, high power density, light weight and small installation space.

The hydrostatic transmission system with secondary regulation has many advantages and has been widely used in many fields. It has been successfully applied in shipbuilding industry, steel industry, large test bed, vehicle transmission and other fields abroad. Mercedes-Benz has applied secondary regulation technology to the driving of driverless traffic system.

4. Concluding remarks

Since 1990s, construction machinery has entered a new development period. With the wide application of new technologies, new structures and new products are constantly emerging. With the penetration of microelectronics technology into construction machinery, construction machinery is increasingly developing in the direction of intelligence and electromechanical integration, and the requirements for walking drive devices of construction machinery are getting higher and higher. In recent years, with the rapid development of hydraulic technology and the improvement of hydraulic components, the application of hydraulic transmission in the transmission system of construction machinery has made rapid progress, and the advantages of hydraulic transmission have become increasingly prominent. It is believed that with the close combination of hydraulic technology, microelectronics technology, computer control technology and sensing technology, hydraulic transmission technology will play an increasingly important role in the development of construction machinery walking drive system.