Study On Control And Energy Saving For Electric Driving Swing System Of Hybrid Excavator

Electric driving swing system is an effective way to reduce further the fuel consumption and emission of hybrid excavators. It is helpful to improve the low efficiency, high emission and heavy pollution of excavators. The presented design and control methods may also provide the similar problems of other construction machinery with references.Since the swing driving configuration and energy flow in a hybrid excavator are altered in comparison with a conventional machine, how to obtain a good operability and coordination in compound operations simultaneously becomes the key problem which restricts the practical application of electric driving swing system. Firstly, the architecture of the electric driving swing system is presented and an equivalent method of parameter design is proposed. To solve the problem of operability, this dissertation adopts a novel idea to simulate the working principle of central-normal-opened-valve-controlled hydraulic motor system in the electric driving swing system and presents a multi-mode control strategy to maintain the same operability and function. A smooth speed control strategy is also presented to guarantee the speed smooth transition of platform with high inertia under the mode switch. Under the condition of operation on slope, hostiting and swift joystick operation, a robust speed control strategy including a reference speed variation limitation unit, a PI controller and a disturbance observer, is presented to guarantee the smoothness and consistency of rotary speed. A limitation is imposed on the SOC control of ultra capacitor when the compound operation of the swing and other actuators is performed and operation point of engine is adjusted so that the SOC of capacitor is maintained on the medial level. Therefore the compound operation can be performed independently. Besides, the key factors to energy consumption of electric swing system are analyzed. The energy saving is evaluated compared to hydraulic motor swing system. The design and control methods of electric driving swing system proposed in the dissertation, which can realize the good swing operability, the noninterference of swing compound operation and obvious energy saving, strongly speed up the practical application of electric driving swing system in hybrid excavators.The dissertation is organized as follows:In Chapter 1, the significance of study on the swing system with high efficiency and energy saving for excavators is discussed under the background of energy crisis and environment pollution. The advantages and existing problems of present swing systems are analyzed in detail. The state-of-art is summarized of the swing smoothness control, operability control and speed control of swing system with high inertia and strong disturbance. The characteristics of electric driving swing system and the difference from the electric vehicles are discussed. The existing problems of electric driving swing system are also analyzed. Finally, the main research contents of this dissertation are proposed.In Chapter 2, the configuration and parameter design of the electric driving swing system are researched. Firstly, the working condition and performance requirement of swing opration are analyzed based on the valve-controlled hydraulic motor swing system. An equivalence design principle is presented to obtain the same function and operability. A configuration of electric driving swing system is given and key parameters are designed. Modeling and analysis are conducted for the design of control strategy.In Chapter 3, the research on the control of electric driving swing system is conducted. Base on the analysis of working condition, the operation of central-normal-opened-valve-controlled swing system is divided into four modes:acceleration mode, deceleration mode, netural positon control mode and idling mode. The mode identification is also presented. The working performance in each mode is analyzed and the operation law is abstracted. A multi-mode control strategy of electric driving swing system is presented to guarantee the function and the variable gain proportion controllers are designed to ensure the same operability as hydraulic driving system. In addition, a smooth speed control strategy is presented to guarantee the smooth speed transition under the mode switch. Simulation and experiment results show the presented control strategy is effective in function and operability.In Chapter 4, the speed control is researched on the swing operation on the slope and the hoisting condition. The variation in swing inertia, disturbance caused by platform gravity on the slope and rough operation of the joystick have great influence on the rotary speed, which is adverse to the swing operability on these conditions. The rough operation of the joystick easily causes the saturation of the controller, hence leading to the impact of the platform and swing of the object. Therefore, a referenc speed rate limitation is imposed to avoid the saturation of controller. Meanwhile, a PI controller plus disturbance observer is presented to suppress the influence of the variation of inertia and disturbance on the swing speed. The simulation and experiments are conducted under three control strategies:variable gain proportional controller, PI controller and reference speed rate limitation plus PI plus disturbance observer. Results show that reference rate limitation plus PI plus disturbance observer is the best choice for the working conditions.In Chapter 5, the power flow management under the swing compound operation is investigated. Firstly, the energy flow and dynamics of the hybrid power train is analyzed and it is concluded that too high or low SOC of capacitor easily causes the interference of the swing and other actuations in the compound operation. Therefore, a constant SOC strategy is presented. Modeling of a 20-ton hybrid excavator is presented and verified. The thermostat control strategy and SOC constant strategy are compared in the SOC control, fuel consumption and interference in swing compound operation. Simulation results show the contant SOC stategy has good SOC maintaining performance and good fuel economy. Experiment results show that SOC is controlled to approach the middle level under the contant SOC stategy, so that capacitor has enough energy and capacity for the electric swing system and hybrid power train. Both of them are operated independently, however, under the theremostat strategy the engine is overloaded and a large reduction in speed is caused.In Chapter 6, the energy saving is investigated of the electric driving swing system. The energy conversion and component efficiency are analyzed and the four key factors to the energy consumption are obtained:inertia of platform, driving efficiency, energy recovery efficiency, and maximum kinetic energy. The key factors to driving efficiency and energy recovery efficiency are the wide high efficiency region and the motor braking torque. Efficiency model of electric swing system is constructed and verified. Results show driving efficiency and energy recovery efficiency are between 70% and 80%. Both of them are lowered to 40%-50% at lower motor speed. The most important factor to energy consumption of electric driving swing system is rotaray displacement, and the second is the inertia of platform. The energy saving amount is increased with the rotaray displacement, the inertia of platform and the engine set speed compared to the valve-controlled hydraulic motor swing system. Energy saving ratio is about 70%-90%, increased with the inertia, but decresed with rotaray displacement. It is conluded that the energy saving is obvious of electric driving swing system, and it only accounts for a little proportion of the valve-controlled hydraulic motor swing system.In Chapter 7, the main conclusions and achievements are summarized and the further research work is also put forward.