Touch screens have pervaded our lives as the most widely used human-machine interface, and much research has focused recently on producing vivid tactile sensations on these flat panels. One of the main methods used for this purpose is based on ultrasonic vibration to controllably reduce the friction experienced by a finger touching a glass plate. Typically, these devices modulate the amplitude of the vibration in order to control the frictional force that the finger experiences without monitoring the actual output. However, since friction is a complex physical process, the open-loop transfer function is not stationary and varies with a wide range of external parameters such as the velocity of exploration or the ambient moisture. The novel interface we present here incorporates a force sensor which measures subtle changes of the frictional force on a wide frequency bandwidth including static forces. This force sensor is the basis for real time control of the frictional force of the finger, which reduces significantly the inherent variability of ultrasonic friction modulation while maintaining a noise level below human perception thresholds. The interface is able to render of precise and sharp frictional patterns directly on the user's fingertip.