An essential issue when using pens as an input device is the detection of the ``inking'' condition. This is not a trivial problem. In a normal writing implement, such as the ballpoint, the inking process starts when the pen tip touches the paper and the ball starts to roll, smearing the paper surface continually with ink. In electronic pens, we can make a distinction between two dimensions: the height of the pen tip above the writing plane, and the force exerted by the pen on the writing plane after contact is made. If the height of the pen tip is within a threshold, say 1 mm above the paper, this is usually identified as the ``pen-near'' condition. However, height is not a reliable identifier for inking and leads to annoying visible hooks in the measured trajectory at pen-down and pen-up stages.
A more reliable method is the measurement of the normal force. Direct measurement of the normal force can be done by touch tablets or by force transducers under the writing surface. Another technique is using the axial pen force. The pen may be equipped with a force transducer, which measures the force component exerted in the longitudinal direction of the stylus. Either the transducer is positioned at the end of the stylus within the barrel, or it is integrated within the stylus. If analog axial force is used to identify the ``inking'' condition, a force threshold must be used, above which we may safely assume that the pen is on the paper. However, analog amplifier drift and mechanical problems like stiction an hysteresis pose a problem. Writers have their typical average pen force (0.5 N -- 2.5 N), such that the threshold cannot be fixed to a single small value. In practice, thresholds between 0.15 N and 0.4 N have been used for pen-up/pen-down identification. If necessary, the normal force may be calculated from the axial pen force if the pen angle with respect to the writing surface is known. Pen force is a useful signal for writer identification, but the force variations are mostly unrelated to character shape [302].
However, the most-often used technique for identifying the inking condition is an axial on/off switch inside the barrel. In the case of writing with an electronic pen with ballpoint refill on paper, there may be an inconsistency between the actual ink path on paper and the recorded pen-up/pen-down because the writer did not produce sufficient force to activate the switch. In such a case, pieces of the trajectory are misidentified as a pen-up sequence. Another problem with binary tip switches is that their axial movement is noticeable at each pen-down event, which disturbs the writing, drawing or menu selection movements by the user. Also, axial force transducers which infer axial force from the amount of compression (axial displacement) of the transducer itself suffer from this problem. An acceptable axial displacement at time of contact is 0.2 mm, but larger displacements are unacceptable for serious use. The use of pens with an axial displacement which is larger than 0.2 mm is restricted to 'sketching'-like drawing.
Esprit Project 8579/MIAMI (Schomaker et al., '95)