[HELP]Moving on const speed on rise and fall terrain

Hi.
I've read almost every topic about moving on constant speed.
And tried a long time to implement it.
But still it doesn't seem working...

I am now coding like this in my forceCallback

pNewtonBody = m_pCharacter->getBody();
Ogre::Vector3 currVelocity = pNewtonBody->getVelocity();
Ogre::Vector3 delta, force;

force = quat _desireDir * m_MoveForce * mass; //to rotate to desire direction m_MoveForce is Vector3(0,0,200), if moving forward; Vector3(0,0,-200), backward, for example.
delta = (force - currVelocity ); // the lack force since last forceCallback
force += delta ; // add up the lack force since last forceCallback
m_pNewtonBody ->setLocalForce( force );


And I have my character Friction with terrain set "setDefaultFriction( 0.4, 0.3);".

Could someone give me some suggestion?
THANKS