Originally, pens provided with digitizer tablets had a diameter of 10-14 mm and were connected to the tablet by a wire. Such a pen thickness is unusable for handwriting, especially if the pen tip is not visible while writing. From the ergonomical point of view, a good guideline for the pen thickness is a diameter of 8 mm [194]. Fortunately, current technology allows for wireless pens, since the wire was a continual nuisance (curling, risk of connection damage, etc.). In the case of touch-sensitive technology, any object with a sharp tip may be used (as long it does not scratch the surface). In case of electromagnetic tablets, a transponder technique is used. Formerly, either the tablet grid or the pen coil was the transmitter, and the other component was the receiver of a high-frequency carrier (50-150 kHz). The peak of the electromagnetic field strenght corresponds to the (estimated) pen-tip position. With the transponder technique, modern tablets transmit in short bursts and receive the emitted resonant signal from a coil and capacitor circuit which is fitted inside the pen and tuned to the carrier signal. The tablet thus alternates between transmission and reception. Axial pen force can be coded by, e.g., capacitively, de-tuning the circuit in the pen. A disadvantage of the wireless pen in electromagnetic devices, is that it is expensive and may be easily lost, or mixed up with regular pens. For this reason, pen-based Personal Digital Assistents are equipped with touch-sensitive tablets, on which any pen-shaped object may be used.
In addition to the internal ``inking'' switch, electronic pens may have a barrel switch to the side of the stylus. It is still unclear whether this unnatural addition, as compared to the normal pen-and-paper setup, is ergonomically acceptable. Two problems are present. First, the index finger is normally used for ensuring a stable pen grip. Applying force to the barrel button disturbs this posture-stability function of the index finger. Especially the release movement from the button is incompatible with the normal pen manipulation. The finger must 'find' a free area on the pen after the switching action. The second problem is more on the software and application side. Since there is no standard approach for using the side switch over several application, the user may forget the function of the side switch, much as mouse users often forget the function of the second (and/or third) mouse button.
Esprit Project 8579/MIAMI (Schomaker et al., '95)