The oil inlet and oil outlet of the traditional directional valve are controlled by a valve core, and the corresponding relationship between the two oil ports and the opening has been determined as early as the design and processing of the valve core, which cannot be modified in the use process, so that the flow or pressure passing through the two oil ports cannot be independently controlled and do not affect each other.
With the cost reduction of microprocessor controller and sensor components and the continuous improvement of control technology, double spool control technology has been applied in the field of construction machinery. Utronics Company of the United Kingdom has developed a double-spool multi-way reversing valve with its own technical and patent advantages, which has been widely used in excavators, quick connector manufacturers, loaders and backhoe loaders of JCB, Deere, DAWOO, CASE and other companies. So as to meet that functional requirement of China's construction machinery products on the hydraulic system. With the continuous improvement of stability and automation control, Youtuo's products entered the China market in time, and the prototype debugging of Xiamen Industrial (5t) loader and Zhanyang (8t) excavator has been initially completed and entered the experimental stage.
1. Defects of traditional single spool reversing valve
The traditional hydraulic system composed of single spool reversing valve is difficult to reasonably solve the contradiction between the following functions and control:
(1) In the design of hydraulic system, in order to improve the stability of the system, reduce the influence of load change on speed, or sacrifice some functions we want to achieve, or add additional hydraulic components, such as speed control valve and pressure control valve, to improve the stability of the system by increasing damping and improving the speed stiffness of the system. However, the increase of such components will reduce efficiency and waste energy; It will also reduce the reliability of the whole system and increase the cost.
(2) Due to the particularity of the reversing structure, users must purchase corresponding hydraulic components when realizing a certain function, and construction machinery manufacturers will design corresponding functions according to different end-user requirements, which causes manufacturers to purchase similar and multi-specification hydraulic control components to meet different functional requirements, which is not conducive to product generalization and product management, and will greatly increase product costs.
(3) Because the hydraulic oil in and out of the actuator is controlled by a spool, it is impossible to independently control the pressure on both sides of the actuator. Therefore, the back pressure on the oil outlet side acts in the opposite direction of the actuator movement. As the back pressure on the oil outlet side increases, the pressure on the oil inlet side must be increased to ensure the movement of the actuator. This will increase the functions consumed by the hydraulic system, reduce the efficiency and increase the heat.
For the hydraulic system with dual spool technology, because the position and control mode of the actuator in and out of the oil side valve port are different, the quick connector manufacturers are independent of each other. Therefore, through the different combinations of the two spool control modes, the problems that cannot be solved by the traditional single valve system can be well solved by software programming, and the functions that are difficult to realize by the traditional hydraulic system can also be easily realized.
2. Two basic control strategies of double spool reversing valve Because of the flexibility of double spool reversing and two oil ports control, the two oil ports can adopt flow control, pressure control or flow pressure control respectively. Actively introduce two simple control strategies.
(1) The load direction remains unchanged during the whole working process.
We know that for truck cranes, excavators, loaders, etc. The load direction of its hydraulic cylinder remains unchanged during the whole working process. Taking the luffing hydraulic cylinder of crane as an example, the control strategy of double spool is discussed.
During the working process of the luffing cylinder of crane, its stress and load direction always remain unchanged, so the control strategy of hydraulic cylinder with rod control and without rod cavity flow control can be adopted.
The flow control of rodless cavity is to detect the pressure difference between the front and back sides of the valve connected to the rodless cavity side, and then calculate the opening of the valve core according to the required inflow or outflow flow; Pressure control is adopted at the side of the cavity with rod, which keeps the side at low pressure and is more energy-saving and efficient.
Because we adopted the flow control in the rodless cavity, the balance valve used in the original control system can be replaced by the hydraulic control one-way valve by the quick connector manufacturer. This can eliminate the system instability caused by the balance valve, thus improving the system stability.
(2) The load direction changes during the working process.
In this case, "oil inlet pressure control, oil outlet flow control" is adopted, and the hydraulic cylinder has rod cavity side pressure control and no rod cavity side flow control.
If the load direction is unchanged, because the oil outlet side adopts flow control, we can use hydraulic control one-way valve instead of two-way balance valve, thus improving the stability of the system. On the oil inlet side, a pressure controller is used to maintain a low reference pressure, which on the one hand improves the efficiency of the system, and on the other hand prevents cavitation in the system.