Research On The Dynamic Characteristics Of High Speed Motor-Pump Unit

High speed motor-pump unit is a compact electro-hydraulic power transmission unit that integrates a multiphase fault-tolerant permanent-magnet motor and a high speed swash-plate piston pump.Due to their advantages in power-density and reliability,high speed motor-pump units have been widely applied in more electric aircrafts.The development of the motor-pump unit has become a multi-disciplinary research topic,which is related to the areas of fluid power,electric-machines,system dynamics and relevant control techniques.To design and manipulate the motor-pump unit in a time-efficient manner,a unified model has been established in this thesis to describe effectively the dynamic characteristics of the electric-motor and hydraulic pump.The topic of the thesis is on the key theories and related technologies of the motor-pump unit with strong engineering application background and academic research value.The highlrghts of the thesis are summarized as follows1.A unified mathematical model that can be used in the simultaneous time-and frequency-domain dynamic characteristics analysis of a high speed motor-pump unit has been developed.In previous research,the pump and motor were investigated separately.In this thesis,the major physical parameters and key performance characteristics of the motor-pump unit(both the motor and pump are rotational machines with periodicities existing during their operations)have been formulated as harmonic superposition form in a unified model.Within the harmonic superposition form,the steady-state component is determined by the operating point and obtained from steady-state calculations while the harmonic kernel function is defined by the structural parameters,and the harmonic coefficients can be properly adjusted to meet the performance requirements.Based on the harmonic superposition form,analytical expressions of the motor-pump unit torque ripples and excitation forces/torques on the swash-plate as well as the instability critical speed of the pump rotational assembly have been derived(accounting for arbitrary phase-coil arrangement and valve-plate pressure transition angle).It is shown in the calculated results that the pump torque pulsation-rate is always less than 3%with a reasonable variation range of the pressure transition angle;the torque ripples of the motor-pump unit are dominated by the motor while the motor torque pulsations can be reduced by the adoption of odd phase number;the critical speed of the rotational assembly is inversely proportional to a dimensionless ratio of the centrifugal force of the piston-slipper assembly to the piston chamber average operating pressure.2.A closed-form harmonic analytical model of the swash-plate high-frequency oscillations has been established for the time-and frequency-domain analysis of swash-plate vibration characteristics under different operating speeds.In the pressure-compensated system of a variable-displacement piston pump,the swash-plate oscillates around its steady-state position in high-frequency because of the discharge pressure pulsations(as the feedback-signal noises)and the net-torque from the piston-slipper assemblies(as the external disturbances).The increase of the pump power loss and vibrations are resulted.Due to the large number of state-variables and nonlinearities of valve-orifice flow in the pressure-compensated system,the conventional time-domain model solved with numerical calculation method is computationally time-inefficient,while the nonlinearities can not be effectively handled in the frequency-domain model based on Laplace transform.With the dynamic governing equations of the pressure-compensated system formulated in harmonic forms,a closed-form harmonic analytical model has been developed to describe the swash-plate high-frequency oscillations.In terms of the harmonic amplitudes,the influences of the external disturbances and(easily-measured)feedback pressure on the swash-plate high-frequency oscillations have been intuitively demonstrated in the analytical model via operating speed-dependent characteristic matrices.Besides,a much less computational time is consumed by the analytical model,which can be applied in the time-and frequency-domain characteristics analysis of the swash-plate high-frequency vibrations.An experimental test rig has been set up to validate the derived model,where the high-frequency discharge pressure and the swash-plate angle are simultaneously measured.Good agreements have been found between the experimental and calculated results for the validation of the harmonic analytical model.The effectiveness of the analytical model in the description of the swash-plate high-frequency oscillations is also demonstrated.It is shown that the swash-plate oscillations are dominated by their fundamental components,and the amplitudes of the fundamental harmonics are not decreased strictly with the increasing speed due to different pressure pulsations and disturbance torque.3.An inverse harmonic model has been developed to compute the optimal currents in real-time for the suppression of the torque ripples and unbalanced forces in a multiphase fault-tolerant permanent-magnet motor.To reduce the torque ripples under healthy mode and large unbalanced forces on the rotor under one phase open-circuit(the most probable fault)of the motor,an inverse model is highly desired to compute the optimal(input)currents for a specified ripple-free(output)torque.Due to the operations of the matrix inversion and interpolation fitting,large computational time and high-performance controller are required in the conventional time-domain inverse model.While in the inverse harmonic model,the computation of the optimal currents is divided into two parts:one part can be computed offline;another part must be completed online to multiply the current harmonic amplitudes by the corresponding harmonic kernel functions.Hence,the computational time required to update the optimal currents is significantly reduced(from hundreds of microseconds using a conventional time-based method to 2 ?seconds).The delay caused by the computational time and the requirements for the controller calculation capacity can be reduced accordingly.The above findings are essential to guarantee the implementation performance of the motor control systems operated under higher speed.It is concluded from literature that the unbalanced forces on the rotor are considered in the remedy strategy of the motor(under the one phase open-circuit fault)for the first time in the developed model,which can be extended to multiphase fault-tolerant motors with any phase number.The thesis is organized as follows:Chapter 1,Introduction.The research and development status of the high speed motor-pump unit are summarized in terms of multiphase fault-tolerant permanent magnet motor,high-speed swash-plate piston pump,the integrated design and compound control of variable displacement and variable speed of motor/pump according to a comprehensive literature review.The research objectives and contents are outlined.The difficulty in conducting the research is also introduced.Chapter 2,Unified mathematical model of a high speed motor-pump unit.A unified mathematical model is developed to describe effectively the dynamic characteristics of the motor and pump.The influences of the motor and pump key parameters on the the dynamic characteristics of the motor and pump are investigated to demonstrate the effectiveness of the unified model in the description of the high speed motor-pump unit dynamic characteristics.Chapter 3,Harmonic-based design method for a multiphase fault-tolerant permanent-magnet motor.Based on the high speed motor-pump unit unified model,the selection principle of the motor structural parameters as well as the mapping between the performance parameters and the harmonics/coefficients in the static magnetic field of the motor are established.A design example is presented to illustrate and validate the proposed design method.Chapter 4,Characteristics analysis of the swash-plate high-frequency oscillations for variable-displacement piston pumps.From the view of control system,the mechanism of the swash-plate high-frequency oscillations is revealed,and a close-form harmonic analytical model is established.Numerical and experimental investigations are conducted for the validation of the proposed model and the analysis of the high-frequency vibrations characteristics of the swash-plate under differernt operating speed.Chapter 5,Harmonic compensations methods for the vibration suppression of the high speed motor-pump unit.Firstly,the sources of vibrations/noises for the high speed motor-pump unit are summarized and explained.To reduce vibrations,the harmonic compensations methods are proposed to reduce the torque ripples and possible unbalanced forces(acting on the rotor)of multiphase fault-tolerant motor,and attenuate the discharge pressure pulsations of the piston pump.Specifically,an inverse harmonic model is developed to compute the optimal currents in real-time for the reductions of the torque ripples and possible unbalanced forces of the motor,while a network-topology resonant-type pulsation attenuator is designed to accommodate with the strict install space.Numerical simulations and experiments are carried out to confirm the harmonic compensations methods as effective ways to suppress the vibrations/noises emitted by the motor-pump unit.Chapter 6,Summary and Perspective.The achievements and innovational research are highlighted.Suggestions are also provided for the future work.