t_frog:
  # Publish all joint states -----------------------------------
  joint_state_controller:
    type: joint_state_controller/JointStateController
    publish_rate: 50  

  # Position Controllers ---------------------------------------
  joint1_position_controller:
    type: effort_controllers/JointPositionController
    joint: joint1
    pid: {p: 100.0, i: 0.01, d: 10.0}
  joint2_position_controller:
    type: effort_controllers/JointPositionController
    joint: joint2
    pid: {p: 100.0, i: 0.01, d: 10.0}

t_frog:
  # Publish all joint states -----------------------------------
  joint_state_controller:
    type: joint_state_controller/JointStateController
    publish_rate: 50  

  # Position Controllers ---------------------------------------
  joint1_position_controller:
    type: velocity_controllers/JointVelocityController
    joint: left_wheel_hinge
    pid: {p: 100.0, i: 0.01, d: 10.0}
  joint2_position_controller:
    type: velocity_controllers/JointVelocityController
    joint: right_wheel_hinge
    pid: {p: 100.0, i: 0.01, d: 10.0}

Change the type of joint to JointVelocity from JointPosition
Change the type of joint to JointVelocity from JointPosition
