| Free-piston linear generator is a new type of power plant composed of a free piston engine and a linear motor.The mover of the linear motor is connected with the connecting rod of the engine,so the ignition on each side of the cylinder pushes the connecting rod to reciprocate continuously,during which electrical power is generated by the linear motor.In recent years,free-piston linear generator has gained more research interests due to such potential advantages as simple structure,low friction loss,high energy efficiency,high power density and multi-fuel adaptability.The purpose of this paper is to study on the modeling techniques and control method of the electrical system of the free-piston linear generator including simulation and model validation,experimental system design,parameter analysis and control strategy research.The main contents and major conclusions are as follows:(1)The simulation model of the linear motor with three-phase load is established using SimPowerSystems toolbox of MATLAB,before which the principle of the linear motor is analyzed,the mathematical model is established with equivalent to rotary motor.Prepare to carry out the prototype test in order to verify the accuracy of model.(2)The operation control scheme of the free-piston linear generator is proposed,aiming to meet the control requirements of different operating conditions.Based on the design scheme,detailed model and control strategy is developed.Firstly,the vector control strategy in electric mode of motor is designed out,that is,the feedback loop method of displacement loop,velocity loop and current loop is adopted,based on the extensive study on the coordinate transformation approach,vector control theory and SVPWM technique.Then,based on the control equations of rectification in two-phase coordinate system,the simulation model of voltage-current loop control strategy is built.The operation performance of system in electric mode bas been analyzed to prove the validity of control strategy.(3)The influence of different factors on the operation performance is discussed in detail,such as the structural parameters,the operating parameters and subsystem design and matching.The simulation results reveal the problem of unreasonable design and matching of the prototype by means of optimizing the motor efficiency,varying the loads,adjusting parameters like mover quality,gas leakage amount,spark-ignition time and so on.(4)On the basis of the existing control strategies and the response characteristics of the control object,Long-time matching control scheme and short-time stability control scheme is proposed.Control schemes are given to a different degree such as the control plan of optimizing the main engine parameters,the plan of the motor control as a primary objective while engine ignition control as a supplement,the plan of optimizing trajectory of mover and the plan of best performance of power output.As a result,the system is sensitive to the change of the exothermic heat quantity and the heat factor is necessary for control.As for power constant control strategy,the phenomenon of energy backflow is key to keep system continuous,but it also leads to speed and the cylinder pressure change.In response,the control strategy is optimized.In addition,the feasibility of the trajectory tracking control strategy is verified through the sinusoidal displacement command.Since the sinusoidal curve does not conform to the spontaneous motion curve of the system,the power flow direction can be changed,which can correct the instruction trace to improve the efficiency.However,the difficulty of system design and match is increased.(5)The construction of the test rig and the experimental study are carried out,aiming to verify model correctness.By analyzing the experimental data,the major restrictive factors of test rig have been found out which result in air leakage and cyclic variation,including the unreasonable matching of the sub-models,scavenging,air organization,combustion condition,air leakage,friction and other test conditions. |
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