A Good Pen is Hard to Find

The give and take of give-and-receive technology

As simple as they seem, affordable, reliable pens and digitizer pads for pen-based computers aren’t easy to make. For instance, if the pen doesn’t provide a “natural” touch and deliver reliable, responsive interaction, users won’t use it — and if it isn’t produced at an affordable price, they won’t buy it.

Pens range from the non-electronic stylus used by the UPS DIAD system (see story on UPS in this issue) to the state-of-the-art “electromagnetic give and receive” (also called electromagnetic resonance) technology patented by Wacom. In general, the main differences amongst the various pens are those that have:

• an “active” power source provided by an internal battery or external power cord connected to the computer;
• a “passive” power source that have neither a battery or power cord;
• no electricity at all.

Wacom, a Japanese company with a US branch in Paramus, New Jersey (with additional outposts in Korea and Germany), builds the pen/pad used in the Go prototype systems and the pen recommended by Microsoft for the Pen Windows environment. In addition, Wacom produces standalone peripheral pen/pad systems for the Macintosh and PC. Wacom’s pen/pad has also been adopted by researchers at MIT Media Lab for some of their projects. And in the near future, we can expect to see a number of other pen/pad developers — companies like Kurta, CalComp, and Summagraphics that currently have peripheral pen/pads for desktop computers — get into the pen-based computer arena, too.

The requirements for pen-based computers are more stringent, however, and therein lies the challenge.

The Give and Take of Give-and-Receive Technology

Give-and-receive technology (the pad gives signals to and receives signals from the pen electromagnetically), developed by Wacom nearly 10 years ago, is the approach most pen developers are looking at the closest. Electromagnetic resonance works by emitting electrical energy from both the pen and the digitizing pad. This enables what’s referred to as “proximity awareness” whereby pen motion is tracked even before the stylus makes contact with the surface, and tracked whenever the pen enters and leaves a zone of proximity to the surface.

Inside a give-and-receive pen is an inductive coil, capacitor, and passive resonant circuitry. The pad, on the other hand, consists of an electronic matrix (or grid) of flat-wound coils usually spaced from 0.02 to 0.001 inch apart, leading to an effective resolution of more than 1200 lines per inch. Speed is measured in how many points per second (pps) the pen/pad can read; systems like Wacom have reading speeds in excess of 200 pps with accuracy levels of 0.06 inch. Pen/pad speeds are likely to remain at this level in the near future because, say Wacom officials, customers haven’t asked for anything faster.

As stated, the pen is passive; that is, it doesn’t need batteries or other external power. Instead, the pad — powered by the computer’s power system — emits electronic signals that “excite” the pen, inducing it to emit signals of its own. It doesn’t take much power to operate the pen — usually no more than 20 milliamps at 5 volts or less.

The pad is both a transmitter and receiver. It constantly “gives” signals to the pen, then immediately readies itself to “receive” the resulting signal from the pen — if it is in the proximity. If the pen isn’t near the pad, it gives out another signal and so on. The pen’s position is determined by the intersection points on the grid that receive the signals or by which intersections receive the strongest signals (if the pen isn’t touching the surface). One obvious drawback of this continuous process is constant, albeit slight, power consumption. Lower power requirements are therefore high on pen/pad developers’ list of priorities. This is compensated to a large degree by efficient, intelligent power management systems. It also makes a multitasking operating system a virtual requirement.

As stated above, a benefit of the technique is that since the pad is always searching, the pen’s location can be determined even before it makes contact with the pad’s surface (“proximity awareness”). Typical proximity awareness is from 1/4 to 1/2 inch from the pad. This leads to faster system responsiveness and, in some systems, proximity data can be used to enhance user interaction and assist in text recognition.

The Pen Is the Point

As stated, the electrically-passive stylus derives its power from the digitizer pad. The stylus is usually about 130 mm in length, 1/4 to 1/2 inch in diameter, and weighs about 55 grams. In short, they’re about the size of a fine-quality ink pen. Cordless pens may in fact have a pocket clip as do familiar pens.

Pressure-sensitivity is an important quality for pens used in stylus-based computers. Pressure-sensitive pens will react in much the same way as felt-tip pens; that is, the more pressure you apply while writing, the more “ink” will be laid down on the pad, particularly in terms of the width of the “ink” flow. This is possible because of a movable ferrite core inside the pen. Pressure on the pen’s point alters the inductance of the resonance coil, affecting electrical frequency.

Pressure-sensitivity is an important feature — some people like a “stiff” pen, others a “soft” one. Furthermore, it’s possible for application programs to interrupt pressure data. (PenCept is one software developer that incorporates pressure data into its database of prototypes.) In many ways, pressure-sensitivity isn’t really new to electronic devices; electronic musical keyboards for years have provided a similar feature called “aftertouch” whereby each key has the ability to sense how hard it’s being pressed.

The Digitizing Pad

There’s nothing really new about the base technology for digitizing tablets that’ll be used in pen-based systems (we had our first contact with digitizing pads nearly 10 years ago, in fact, when we produced the documentation for a Kurta-produced tablet). It is simply a fine grid of thin wires that first transmit, then receive signals, thereby providing stylus location information.

But pen-based computers will challenge established tablet developers as never before because of the mobility required by pen-based systems. For instance, desktop digitizing pads that Wacom manufactures range in weight from 92 pounds for the top-of-the-line professional CAD/CAM model to nearly 4.5 pounds for the much smaller personal unit. Consequently, most of the weight in a pen-based notebook computer can be directly attributed to the weight of the digitizer pad itself.

Secondarily, the digitizer tablets are fragile — the glass surface can break, scratch, or crack, and shocks to the sensitive grid can lead to inaccuracies. (Note that the UPS DIAD unit must sustain vertical drops of 6 feet onto concrete.) Wacom claims its design is more rugged than that of competitors like Scriptel or Microtouch because Wacom puts its digitizer behind the LCD.

Handwriting burdens the system with the additional requirements of speed and accuracy. The pad, for instance, has to be able to provide good data without regard to how much the user is tilting the stylus and it has to provide good accuracy over the entire surface of the pad. Close isn’t good enough.

The Challenge of Portable Pen/Pads

In short, the special requirements for pen/pads used in stylus-based systems are high-speed, optimization for handwriting (particularly in terms of resolution), small footprint, low power, and ruggedness.

Fortunately, most of these issues are problems of refinement and implementation, not necessarily of new invention. However, with its patent, Wacom clearly leads the pack and other pen/pad developers will have to license Wacom technology or develop workable, affordable alternatives. Still, Wacom doesn’t give any indication that they’re ready to relinquish their pole position. The company is currently working at implementing much of its circuitry into silicon in the form of ASIC (“application specific integrated circuits”) chips. While this approach won’t necessarily provide higher speed (but it doesn’t look like we’ll need faster pen/pads in the near future), it is a step to developing smaller, more shock resistant pen and digitizer pads at a lower cost.