Speed Ratio Control Strategy of Wheel Loader with HMT Based on PTO Torque Estimation

Hydro-mechanical continuously variable transmission（HMT） has become one of the development directions of wheel loader transmission system, because of its ability to achieve high-power stepless speed changing and high-efficiency transmission.However, the resistance of the working device changes frequently during the operation, which will reduce the working efficiency and economy. To solve this problems, a ZL50 type wheel loader equipped with HMT is taken as the research object. The characteristics of HMT's efficiency, torque and power flow are analyzed, and a Power take-off（PTO） torque estimation model is established with the key parameters such as hydraulic circuit pressure, engine speed, torque, etc. The influence of PTO torque on the target speed ratio is analyzed, a method to solve the target speed ratio considering PTO torque is proposed, and the speed ratio control strategy is formulated. In order to verify the effectiveness of this strategy, the V-type working cycle is constructed with reference to the typical operation cycle resistance and distance, and the simulation platform of the loader equipped with HMT is built in Matlab/Simulink.The results show that, the maximum error of the torque estimation is 9.18% except for transient conditions such as range changing;Compared with the traditional control strategy, the speed ratio control strategy based on PTO torque can increase the speed ratio,vehicle speed and engine load rate when the PTO demand torque is small; when the PTO torque demand becomes larger, it can reduce the speed ratio, distribute more power to the working system; the work efficiency is increased by 6.29% and the total fuel consumption is saved by 3.95% in a V-type working cycle.