You’ll Be Drawn to This Digital Pen

A high-quality optical mouse is capable of tracking very fine, pixel-perfect hand movements. This makes them quite adept at tasks ranging from graphic design and digital art to gaming and document editing. But sensor accuracy aside, the way in which we physically interact with a mouse leads to some limitations. I mean, have you ever had to digitally sign a document online? How closely does the result match your signature on pen and paper? If you are like most people, it is probably not very close. Trying to replicate the same fine hand and wrist motions with a mouse often produces little more than a squiggle.

For those that frequently need the kind of accuracy that can be had with a pen and paper, there are digital pens. Existing options are all flawed in one way or another, however. Some devices rely on external input pads that take up a lot of desk real estate and limit mobility. Portable digital pens, on the other hand, are known to suffer from motion artifacts and other issues that limit their accuracy, which, if you think about it, kind of defeats the whole purpose of a digital pen.

An interesting new concept in digital pen technology has been recently reported on by a pair of researchers at the University of Stuttgart and the University of Duisburg-Essen. Their prototype, called OptiBasePen (engineers are not known for their marketing skills!), is a portable digital pen that can be used on any surface while maintaining the accuracy of a pen with a dedicated input pad. This was made possible by a very unique two-part design.

OptiBasePen consists of a movable base that the user rests their hand on, as well as what looks like a traditional digital pen. The base is equipped with an optical mouse sensor that tracks its relative motion across a surface. Attached to the pen tip is an LED, with two small image sensors on the base to track it within a defined area of about 6 cm x 6 cm. The base’s sensors detect both the position and surface contact of the LED, allowing the pen’s movements to be tracked accurately in relation to the base, rather than relying solely on the pen’s relative position to a static surface.

Near the pen tip, an infrared LED emits a signal that is detected by the base, which houses all the sensing components on a custom PCB. This base includes two PAJ7025R3 infrared image sensors that process the LED’s position as a bright blob in image space at a rate of up to 200 Hz. Though each image sensor has a native resolution of 98x98 pixels, the team upscales each blob’s location to a larger 4095x4095-unit coordinate system for finer precision. The two sensors are positioned 9 cm apart at a 90-degree angle, minimizing occlusion from the user’s fingers resting between them.

A mouse sensor on a separate breakout board detects the base’s overall motion. Processing is handled by an ESP32-WROOM-32 microcontroller, which makes for easy development and debugging as the technology is refined. The hardware is encased in a 3D-printed shell with a detachable cover for hand comfort. The base also features commercial mouse feet on its underside, which reduce friction as it slides along surfaces.

While the OptiBasePen is a more mobile digital pen option, the base, which is essential to the design, does itself limit mobility to some extent. Perhaps a more refined version of the hardware design could reduce its size and make it more suitable for use on the go. Furthermore, the infrared sensing solution may be susceptible to noise emitted from other light sources, which could cause some trouble. But if these issues can be solved, OptiBasePen may be worth looking at in the future.