Work vehicle and control method thereof

A work vehicle, comprising:

• an engine;
• a hydraulic pump configured to be driven by the engine;
• a work implement configured to be driven by a hydraulic fluid discharged from the hydraulic pump;
• a travel device configured to be driven by the engine;
• a power transmission device configured to transmit a driving force from the engine to the travel device;
• a control unit configured to control the power transmission device;
• a forward/rearward movement operating member for switching between forward movement and rearward movement of the vehicle, the forward/rearward movement operating member being configured to be selectively switched at least between a forward movement position and a rearward movement position;
• an accelerator operating member;
• an accelerator operation detecting part configured to detect an operating amount of the accelerator operating member;
• and
• a vehicle speed detecting part configured to detect a vehicle speed,
• the power transmission device including
• an input shaft,
• an output shaft,
• a gear mechanism having a planetary gear mechanism and configured to transmit a rotation of the input shaft to the output shaft, and
• a motor connected to a rotary element of the planetary gear mechanism,
• the power transmission device being configured to change a rotational speed ratio of the output shaft to the input shaft by changing a rotational speed of the motor,
• the control unit including
• a storage part storing a required traction force characteristic, the required traction force characteristic defining a relation between a required traction force and an output rotational speed, the output rotational speed being a rotational speed of the output shaft of the power transmission device, the required traction force characteristic being set such that the required traction force increases with an increase in the operating amount of the accelerator operating member,
• a required traction force determining part configured to determine the required traction force corresponding to the output rotational speed based on the required traction force characteristic,
• a motor command determining part configured to determine a command torque to be transmitted to the motor to obtain the required traction force,
• an opposite movement determining part configured to determine that the vehicle is oppositely moving when the vehicle speed becomes a predetermined speed threshold or greater in a direction opposite to a moving direction corresponding to a position of the forward/rearward movement operating member, and
• a traction force assisting part configured to perform a traction force assisting control of increasing the required traction force in the moving direction corresponding to the position of the forward/rearward movement operating member when it is determined that the vehicle is oppositely moving.