This is a classical management downside. You wish to create a suggestions loop that takes the divergence from optimum place and applies the suitable torque to nudge it again into place.

btQuaternion targetOrientation  = // no matter you want
btQuaternion currentOrientation = myObject->getOrientation();

Getting the delta orientation is kind of easy, you might want to discover the “distinction” between two the quaternions goal and present orientation.

btQuaternion deltaOrientation = targetOrientation *  currentOrientation.inverse();

Unfortunately this quaterion doesn’t say a lot about any divergence that we are able to use.
But in case you convert this delta orientation into euler angles.

btVector3 deltaEuler = QuaternionToEulerXYZ(deltaOrientation );

You mainly get the scaled inverse of the torque you wish to apply. Now you “simply” want to seek out an acceptable quantity to ease it in.

btVector torqueToApply = management(-deltaEuler);

The easy answer is to multiply it with an element. You might have to mess around with the issue till you discover a good worth.

// easy proportional controller
btVector management(btVector in) 
{
    // mess around with the issue till you discover a matching one
    static Kp = 0.5;
    return in * Kp;
}

You may additionally wish to look right into a PID-Controller to get a smoother extra responsive easing.