Research On Braking Mechanism Of Hydraulic Retarder Based On Magnetorheological Fluid Transmission

As an effective auxiliary brake device,hydraulic retarder can provide enough braking torque through the interaction of the working medium and the impellers.In general,the adjustment of braking torque depends on the liquid filling rate in the working chamber controlled by the driving air pressure.However,the high performance solenoid valves are demanded to meet the requirements of the frequent fluid charging/discharging caused by high precision torque control,and the torque control ability will be influenced by torque fluctuation caused by temperature change.So a MRFHR scheme is proposed in this thesis to solve the above problems.In this scheme,the traditional transmission medium is replaced by the magnetorheological fluid(MRF),and the braking torque can be adjusted by the external magnetic field.This scheme can take full advantage of the controllable rheological properties of MRF to achieve braking torque accurate adjustment.Meanwhile,the variation of the braking torque caused by the temperature will be compensated by the magnetic field,which effectively improves the performance of the hydraulic retarder.Some major points of the MRFHR are studied in this thesis.The flow characteristics and braking mechanism of the traditional hydraulic retarder are analyzed in this thesis,based on that,the braking mechanism and torque modulation method of the MRF hydraulic retarder are obtained combined with the rheological properties,working mode of the MRF and the excitation magnetic field.The MRF constitutive model was established,and the influence of temperature and magnetic field on the MRF was analyzed based on the test data.Combining with the two working modes and energy loss equations of MRF in the hydraulic retarder,a mathematical model of braking torque is established and the characteristics of braking torque with revolution and magnetic field are obtained.It is found that the braking torque decreases with the increase of the circumferential magnetic field,but grows with the axial magnetic field.On this basis,the adjusting ability of different magnetic field is compared and the results show that circumferential magnetic field has a much better performance than axial magnetic field.To maintain the stability of the braking torque,the fine tuning of the magnetic field is adopted in this thesis.The influences of temperature on the viscosity,density and rheology of MRF are analyzed to get the fluctuation rule of braking torque.It is found that the farther the temperature deviates from the reference temperature,the greater the compensation magnetic field is needed.And in the low magnetic field,the compensation magnetic field increases linearly with the original magnetic field,while in the high magnetic field area,the compensated magnetic field maintains stable trend.The prototype and excitation control system of MRFHR are designed,and the experimental platform is built to test characteristic of the braking torque versus the magnetic field.Results reflect that the braking torque declines with the enhance of the circumferential magnetic field,but grows with the axial magnetic field,which is in accordance with the simulation results.So,adjusting the braking torque through magnetic field is feasible.