大型液压挖掘机双通道势能回收及负载平衡方法研究

Dual-channel Potential Energy Recovery and Load Balancing Methods for Large Hydraulic Excavators

为了解决气液平衡势能回收系统中蓄能器体积过大及“过平衡”和“欠平衡”等主机操控性问题,本文以大型液压挖掘机为研究对象,提出了一种由三腔室平衡缸、双蓄能器及风扇马达为主要构成的直/间接双通道动臂势能回收系统。该系统将原先的单蓄能器回收通道(单通道系统)变为双蓄能器回收通道(双通道系统),即,三腔平衡缸和蓄能器1构成直接回收通道,风扇马达和蓄能器2构成间接回收通道。一方面,通过间接调节变量风扇马达的压力可以避免蓄能器液压力和负载力之间的不平衡问题从而改善操控性;另一方面,利用双通道系统特性可以使蓄能器的工作压力设计具有灵活性并可大幅减小蓄能器体积。本文通过数学建模明确了风扇马达的控制规律,从而使双蓄能器提供的驱动力和负载力基本平衡。此外,通过参数分析获得了蓄能器参数配置规律从而使双蓄能器体积小于单通道回收系统中单个蓄能器的体积。为了验证以上理论分析结果,以70 t大型液压挖掘机为对象搭建了包含主泵、能量回收模块及机械结构在内的仿真模型,并将仿真结果和单通道系统的试验结果进行对比。结果表明,所提出的双通道系统可实现液压力和负载力的匹配并将蓄能器总体积减小50%以上。因此,所提出的系统可以在保留单通道系统优点的同时改善其对操控性的影响并节省装机空间,具有良好的推广应用前景。

Objective When recovering potential energy based on load balancing,the usual expectation is that the load force and the hydraulic force provided by the accumulator are fully balanced so as to reduce the energy provided by the hydraulic system.However,because the accumulator pressure is not controllable and the load force is uncertain,the two forces usually cannot be matched accurately.When the load force changes gently,the theoretical volume of the accumulator is too large,and handling problems such as over-balance or under-balance may occur.The specific manifestation of these problems is that the boom stalls or almost stops descending.To solve these problems,the research object here is a large hydraulic excavator,for which is proposed a direct/indirect dual-channel boom potential energy recovery system comprising a three-chamber balance cylinder,two accumulators,and a fan motor.Methods In this study,the traditional single accumulator recovery channel(single-channel system)is changed to a dual accumulator recovery channel(dual-channel system),where the three-chamber balance cylinder and accumulator 1 form a direct recovery channel,while the fan motor and accumulator 2 form an indirect recovery channel.The principle of the single-channel system in existing relevant literature is as follows.In the boom lifting stage,the accumulator provides lifting force through the energy storage chamber,thereby reducing the input power of the original drive system.In the boom descending stage,the gravitational potential energy of the boom is converted into hydraulic energy through the energy storage chamber and stored in the accumulator for auxiliary lifting in the next working cycle.Ideally,the auxiliary driving force provided by the accumulator fully balances the load force.However,because the pressure of the accumulator cannot adapt dynamically to the load force,handling issues or excessive accumulator volume often occur.Compared with the single-channel system,the principle of the proposed dual-channel system is as follows.1)By controlling the displacement of the fan motor,the load force can be balanced to the maximum extent,thereby avoiding the impact of over-balance and under-balance issues.2)Although only some of the energy is stored in the high-pressure accumulator for auxiliary lifting,the excess energy is not lost on the main valve but rather is used to drive the fan motor for useful work.3)Because part of the load is balanced by the fan motor circuit,the maximum and minimum working pressure of the accumulator can be adjusted within a large range,making it easier to choose a smaller-volume accumulator.Note that the fan motor in the proposed dual-channel system is the original device of the hydraulic excavator,so no additional installation space is required.In addition,although the proposed dual-channel system has two accumulators,their overall volume can be much smaller than the volume of a single accumulator in the single-channel system by designing its working pressure reasonably.A mathematical model of the proposed system is established,and the control law of the fan motor is deduced.Moreover,the influence of key parameters on system characteristics is analyzed.To verify the above theoretical analysis results,a simulation model including the main pump,energy-recovery module,and mechanical structure was built for a 70-ton large hydraulic excavator,and the simulation results are compared with the experimental results for a single-channel system.Results and Discussions The results show that the proposed system reduces the accumulator volume significantly(by more than 50%),saves in-stallation space, eliminates additional energy loss, and solves handling issues caused by the mismatch between the hydraulic pressure and load force of the accumulator. The direct recovery channel-which comprises the energy storage chamber and the high-pressure accumulator-re-duces the system energy supply during the boom lifting stage, and the indirect recovery channel-which comprises the rodless chamber of the bal-ance cylinder, fan motor, and low-pressure accumulator-can be used to drive the fan motor. By controlling the displacement of the fan motor, the pressure of both accumulators is reduced from the highest working pressure to the lowest throughout the entire boom lifting stage, thereby ensur-ing the control needs for the next working circle. Compared with the prototype with no system for recovering potential energy, this system has en-ergy-saving and emission-reduction effects while ensuring its original controllability. Because the hydraulic pressure provided by the dual chan-nels is equal to the maximum load pressure, a small amount of driving energy still must be provided by the pump during the boom lowering stage. In future research, predictive control or some detection methods can be combined with the proposed method to obtain the magnitude or variation pattern of load force. On this basis, by controlling the displacement of the fan motor, the hydraulic pressure provided by the dual channels can track the load force, thereby minimizing the supply pressure and balancing the load force to the maximum extent. Conclusion Therefore, the proposed system retains the advantages of a single-channel system while improving the impact on handling perform-ance and saving installation space. Moreover, the proposed method can be used in other applications such as wheel loaders, cranes, and other situ-ations requiring the recovery of gravitational potential energy, thereby giving it good promotion and application prospects. In addition, on the basis of load-force feedback, some precise closed-loop control methods can be used to achieve accurate load matching.