Study On Boom Energy Recovery Element And System Of Hybrid Excavator

Boom energy recovery is an effective approach to further reduce the fuel consumption and emission of hybrid excavators.It is helpful for the low efficiency and heavy pollution of excavators in current. The involved results can also provide the energy recovery research of other construction machinery with references.Since the control mode of the boom with energy recovery in a hybrid excavator is changed in comparison with a conventional machine, how to obtain significant energy saving and good operability simultaneously becomes the key problem which restricts the practical application of boom energy recovery. To solve the problem, this dissertation proposes a novel boom energy recovery scheme and working principle. The system structure is composed of a hydraulic motor-generator element and a throttle in series. Using pressure compensation principle, the electromagnetic torque of the generator is adaptive to the loaded pressure. Therefore, on the one hand, the pressure drop over the throttle is guaranteed to a small constant and then the velocity of the boom cylinder can be governed effectively by controlling the opening of the throttle; on the other hand, boom energy can be recovered and reused by the hydraulic motor-generator element. With the speed information of the vector-controlled generator and the relationship between the flow rate and pressure drop over the throttle, the pressure drop can be estimated and a control scheme without pressure sensors is realized to avoid additional hardware and reduce complexity. In addition, the dissertation investigates the hydraulic motor-generator element which has a profound influence on the system performance. To satisfy the requirements of high efficiency and low torque ripple under dimensional restraints of the energy recovery generator, a design method which takes both of design efficiency and accuracy into account is proposed to optimize the stator and rotor stepwise based on the cooperation of an analytical parametric model and finite element analysis. In terms of the element control, a current controller which combines feedforward and feedback together and a speed controller with disturbance compensation are proposed respectively to ensure the dynamic performance of the element either in torque mode or speed mode. The design and control methods of boom energy recovery proposed in this dissertation, which can realize energy recovery with good boom operability, strongly push forward the practical application of boom energy recovery system in hybrid excavators. The dissertation is organized as follows.In Chapter1, the significance of study on boom energy recovery of excavators under the background of energy crisis and environmental pollution is discussed.In the introduction of the state-of-art of boom energy recovery, the characteristics and existing problems of several schemes are analyzed in detail. For electrical energy recovery which is the suitable scheme of hybrid excavators, the type selection of the hydraulic motor-generator is discussed and the permanent magnet generator is reviewed in terms of design and control. Finally, the main research contents of the dissertation are presented.In Chapter2, the boom energy recovery system of hybrid excavators is researched in whole. The working conditions are analyzed and the evaluation indexes are summarized as energy saving and operability. The structure scheme which consists of a hydraulic motor-generator element and a throttle in series is proposed. Based on the mathematical models of main components, the losses in every energy conversion are calculated and the design of key parameters in the system is discussed. The aforementioned work also establishes the basis of component research. In Chapter3,as a key component, the energy recovery generator is designed and optimized. A design method which optimizes the stator and rotor stepwise is proposed to satisfy the requirements of high efficiency and low torque ripple under dimensional restraints of the energy recovery generator. Firstly, the stator structural parameters and flux density distribution are optimized with the goal of minimum losses by an analytical parametric model and the particle swarm algorithm. Secondly, the rotor structural parameters are optimized with the goal of minimum waveform distortion in airgap flux density distribution, and the practical distribution should be the same as the predetermined result. Armature reaction, demagnetization and temperature rise in intermittent working condition are calculated and checked. A prototype of the generator is fabricated and the test results verify the effectiveness of the design,In Chapter4, the control of the hydraulic motor-generator element is discussed.In the current control of the energy recovery generator, a proportion-integration current controller with feedforward compensation is designed to reduce the effect of back electromotive force.In the rotational speed control of the element, since the pressure in the inlet of the hydraulic motor is fluctuant, disturbance compensation is introduced to improve the dynamic performance of the element. The control systems are researched by simulation. Then, an experimental platform is built to test the energy recovery element. The results show that good control performance of the element can be achieved either in torque mode or speed mode, which promises the feasibility of the element’s applications in the boom energy recovery system.In Chapter5, three control methods are proposed and researched according to the structure and characteristics of the energy recovery system. Direct speed control method governs the boom velocity by controlling the rotational speed of the hydraulic motor-generator element directly and the throttle is almost kept opening totally. Load pressure control method governs the boom velocity by controlling the opening of the throttle and the object electromagnetic torque of the generator is given by the feedback of loaded pressure. Pressure drop control method also governs the boom velocity with the throttle, and the generator torque is determined by the feedback of pressure drop over the throttle. The transfer functions of the system under different control methods are derived and the pressure drop control method has the best frequency response and damping ratio by comparing and analyzing their dynamic performances. Performance tests are implemented on an experimental platform of boom energy recovery for hybrid excavators, and the results are in accordance with the analytical analysis.In Chapter6, the energy recovery system with pressure drop control method is further investigated. Analogized to a conventional pressure compensator, the principle of energy recovery is extended to the whole boom motion which includes the lowering and lifting process. The recoverable energy under the two conditions is evaluated. A sensorless control approach, which uses the speed information of the vector-controlled generator and the relationship between the flow rate and pressure drop over the throttle, is proposed to avoid additional hardware and reduce complexity. The equivalence of the control methods with and without pressure drop sensors is proved at both steady and dynamic states. A large number of experiments are carried out in terms of energy saving and operability. The results show that the proposed boom energy recovery system and control method have good control performance in various operations. The recovered energy in experiments is in accordance with analytical value and the recovery efficiency is in the range between40%and50%, which can be further improved in practical application. In Chapter7, the main conclusions and achievements are summarized and the further research work is put forward.