why my wheel joint is spin,when I use Joint::setposition to control it?

edit

How to use Joint::setposition to control a joint? Is there a doc or example? I know how to control joint by SetPositionPID,but this method is not very good performance, the wheel movement is not very accurate.

Thanks advance!

code like this :

void TestS::Update(const common::UpdateInfo &iInfo){

        m_ptrModel->GetJoint("left_wheel_joint")->SetForce(0,3800);
        m_ptrModel->GetJoint("left_wheel_joint")->GetChild()->SetEnabled(true);
        m_ptrModel->GetJoint("left_wheel_joint")->GetParent()->SetEnabled(true);

        m_ptrModel->GetJoint("right_wheel_joint")->SetForce(0,3800);
        m_ptrModel->GetJoint("right_wheel_joint")->GetChild()->SetEnabled(true);
        m_ptrModel->GetJoint("right_wheel_joint")->GetParent()->SetEnabled(true);
        m_ptrModel->GetJoint("left_wheel_joint")->SetPosition(0,update);
        m_ptrModel->GetJoint("right_wheel_joint")->SetPosition(0,update);
       update++;

}