Writers acquire their technique by spotting, savoring, and reverse-engineering examples of good prose.”
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ROLLING OFF THE TABLET
this venture in designing a stylus that magnetically adheres to the Note Air started by dusting off our high school notebooks on magnetism. We all come into contact with (permanent) magnets many times in the course of our life. Because of their easiness, we tend to think that we understand these objects: they attract or repel each other, and they stick to some metals. However, this simplicity hides complex physical attributes. We aren’t physicists, nor do we pretend to become. The key was then to apply reverse engineering! By analyzing the design features of both Boox magnetic pen and Samsung pen for the Galaxy Tab S6 Lite (hereafter referred as S-pen) we were able to understand how to device an effective arrangement of magnets for our own stylus with very little insights into exactly how magnets work.
Magnets: That’s all we needed to know
As you all know, magnets have north and south poles. Opposite poles are attracted to each other, while equal poles repel each other. The force of magnetic attraction (i.e., the magnetism) is mediated by a set of invisible lines called magnetic lines of force or flux. Each line leaves the north pole and enters the south pole as illustrated in Figure 1. This ensemble of lines is called the magnetic field. When an iron object is near a magnet, but not within the magnetic field, the object will not be attracted to the magnet. When the object enters the magnetic field, the force of the magnet acts, and the object is attracted. The density of magnetic lines determines the strength of the magnet (at a particular point): the higher the number of magnetic lines, the stronger the attraction force. A field’s strength is measured in Tesla (T) or Gauss (G). One Tesla equals to 10.000 Gauss.
As you all know, magnets have north and south poles. Opposite poles are attracted to each other, while equal poles repel each other. The force of magnetic attraction (i.e., the magnetism) is mediated by a set of invisible lines called magnetic lines of force or flux. Each line leaves the north pole and enters the south pole as illustrated in Figure 1.
This ensemble of lines is called the magnetic field. When an iron object is near a magnet, but not within the magnetic field, the object will not be attracted to the magnet. When the object enters the magnetic field, the force of the magnet acts, and the object is attracted. The density of magnetic lines determines the strength of the magnet (at a particular point): the higher the number of magnetic lines, the stronger the attraction force. A field’s strength is measured in Tesla (T) or Gauss (G). One Tesla equals to 10.000 Gauss.
Magnets Inside the Note Air
Before analyzing the magnetic properties of the different styli, we needed first to study the magnets inside the Note Air. The Note Air has a series of 5 magnets arranged vertically over the right edge of the tablet, as shown in Figure 2. This can be easily verified by placing magnets along the edge of the tablet. The magnets are spaced 13mm apart and we could estimate that each magnet is about 1cm large. Using a magnetometer, we found that the density flux of the magnets inside the Note Air is about 3.5G. In comparison, refrigerator magnets are about 50G.
Digging into the Boox Pen
Disassembling the Boox pen is not an easy task, as its components are tightly integrated into the barrel of the stylus. However, as the old Chinese proverb says: Be resolved and the thing is done! Thus, after various minutes of delicate surgery, voilà the Boox pen interior shown in Figure 3. The Boox pen shaft (also called refill) consists of two parts: the Shimononome GII resonant inductive coil (i.e., the EMR device) and a stem that holds the magnet.
As it can be seen, the Boox pen contains one single magnet. This magnet is 45mm large and 3.5mm wide. It’s a very strong magnet of about 33G; though, it can only adhere simultaneously to at most two (out of the five) magnets on the table. One may think that a strong magnetic force offsets the limited number of adhesion points. Yet, other factors must also be considered. In the first place, the Boox pen weighs 15 grams (g). This is one of the heaviest EMR styli in the market. For a comparison, Table 1 summarizes the weight of some popular styli. Because of the significant weight, the few points of adhesion face difficulties to hold the pen attached to the tablet.
In addition, the magnetic side of the pen is flat, compared to the Note Air’s curved edge contour. This means that the contact surface between the tablet and the stylus is minimal, which substantially reduces the mutual adherence.
On the other hand, when attached to the Note Air, part of the Boox pen magnet is inside the magnetic field produced by other magnets on the tablet to which it cannot stick. This generates additional attractive forces that favor the pencil to slide along the edge of the tablet. To better understand this issue, we have simulated the magnetic field generated by the interaction between the magnets of the Note Air and the Boox pen. The large box on left-hand side in Figure 4 corresponds to Boox pen magnet. Similarly, the three small boxes on the right-hand side represent Note Air’s magnets. As we have previously commented, the stylus’ magnet can only stick to two magnets in the tablet. A third magnet, however, lies on the vicinity of the stylus’ magnet. The proximity between these two magnets generates a flux that pulls the magnets together. This is illustrated in Figure 4 by the line of force highlighted in yellow. Because of the additional forces, the stylus is pulled downward, which in conjunction with the force of gravity cause the stylus to easily glide along the edge of the tablet.
On the other hand, when attached to the Note Air, part of the Boox pen magnet is inside the magnetic field produced by other magnets on the tablet to which it cannot stick. This generates additional attractive forces that favor the pencil to slide along the edge of the tablet. To better understand this issue, we have simulated the magnetic field generated by the interaction between the magnets of the Note Air and the Boox pen. The large box on left-hand side in Figure 4 corresponds to Boox pen magnet. Similarly, the three small boxes on the right-hand side represent Note Air’s magnets.
As we have previously commented, the stylus’ magnet can only stick to two magnets in the tablet. A third magnet, however, lies on the vicinity of the stylus’ magnet. The proximity between these two magnets generates a flux that pulls the magnets together. This is illustrated in Figure 4 by the line of force highlighted in yellow. Because of the additional forces, the stylus is pulled downward, which in conjunction with the force of gravity cause the stylus to easily glide along the edge of the tablet.
Remark: It should be noted that Boox has recently modified the magnet arrangement on both the Note Air and the Boox pen. As currently advertised in their official store, the Note Air and the Boox pen now have a symbol that identifies the location of the magnet (see Figure 5). While we do not know the exact features of this new magnetic system, we conjecture that Boox has simply decided to replace the multiple magnet arrangement with a single magnet. Therefore, this article and the ensuing editions of the blog will only refer to the first models of the Note Air and the Boox pen.
Stripping the S-pen
Remark: It should be noted that Boox has recently modified the magnet arrangement on both the Note Air and the Boox pen. As currently advertised in their official store, the Note Air and the Boox pen now have a symbol that identifies the location of the magnet (see Figure 5). While we do not know the exact features of this new magnetic system, we conjecture that Boox has simply decided to replace the multiple magnet arrangement with a single magnet. Therefore, this article and the ensuing editions of the blog will only refer to the first models of the Note Air and the Boox pen.
Stripping the S-pen
By stripping the S-pen of its outer casing, we have discovered a much better, though not perfect design. The S-pen shaft is shown in Figure 6. It is made up of two stems. The first contains the resonant inductive coil and one magnet. The second possesses two additional magnets and the eraser button. Oddly, the three magnets are irregularly spaced. The two magnets closest to the coil are spaced 17mm apart, while the third magnet is distanced from the second by 42mm. Given this particular arrangement of magnets, and depending on where we snap the stylus along the edge of the Note Air, no more than two magnets stick to the tablet.
The careful reader will have noticed that despite the above, it is physically possible for the three magnets on the stylus to match the location of three other magnets on the tablet. The question arises: How is it possible that only two (out of three) magnets exert an attraction? To answer this question, we need to look at the possible magnetizations of a magnet.
The magnetization of a magnet corresponds to the orientation of the poles with respect to the geometry of the magnet. Consider a cuboidal magnet. The typical magnetization is through the diameter of the magnet, that is, the north and south poles are on the two opposite larger surfaces, as illustrated in Figure 7. This is the case of the magnets inside the Note Air, whose south poles point towards outside the tablet.
Another alternative orientation is through the length, which places the north and south poles on the opposite smaller faces, as exemplified in Figure 8. This unusual orientation was adopted for the S-pen with the north pole pointing towards the cap of the stylus.
With this in mind, we can now provide an answer to our question. It is enough to remember that opposite poles attract, while equal poles repel.
Figure 9 shows the typical position of the S-pen when attached to the Note Air. As we can see, the north pole of the first and second magnets on the stylus (from the cap to the nib) match the south pole of the magnets inside the tablet. Therefore, each pair of those magnets attract each other. In contrast, the south pole of the third magnet on the pen is exposed to the south pole of the magnet inside the Note Air. Consequently, this pair of magnets repel each other.
Remarkably, despite the repulsive force of the third magnet, the S-pen adheres quite well to the tablet with only the help of the other two magnets.
It is also worth noting that the S-pen magnets are not very strong. In fact, their flux density is about 1.5G, which is half the density of the magnets inside the tablet, and 20 times less than the density of the magnet inside the Boox pen. In spite of this, the S-pen adheres to the Note Air much better than the Boox pen. This is because the design of the S-pen shaft allows a better coupling between the magnetic fields of the magnets inside the pen and those inside the Note Air.
It is also worth noting that the S-pen magnets are not very strong. In fact, their flux density is about 1.5G, which is half the density of the magnets inside the tablet, and 20 times less than the density of the magnet inside the Boox pen. In spite of this, the S-pen adheres to the Note Air much better than the Boox pen. This is because the design of the S-pen shaft allows a better coupling between the magnetic fields of the magnets inside the pen and those inside the Note Air.
Our Shaft
We have discovered that the magnetic arrangement of the Boox pen is simply not compatible with the Note Air. That is not to mention the pen’s significant weight and flat magnetic surface, which do not adapt well to the tablet. On the other hand, the shaft of the S-pen has a design that allows for a better magnetic adhesion. However, the location of the magnets along the shaft is not compatible with the Note Air. In addition, their magnetization doesn’t match the orientation of the Note Air’s magnets.
Inspired by the internal design of the S-pen, we have devised our own shaft, depicted in Figure 10. The arrangement of the magnets along our shaft separates the magnets from each other following the same pattern inside the Note Air. Therefore, each magnet on the stylus matches a magnet on the tablet. The shaft contains four magnets. Thus, depending on where we snap the stylus along the edge of the Note Air, up to four magnets ensure its adhesion to the tablet.
The next step consisted of manufacturing our shaft. After several dozen 3D-prints, each one correcting minor design imperfections, we finally obtained the functional version shown in Figure 11. We have completed our design with neodymium magnets of the same size, flux density and magnetization as the Note Air’s magnets.
We have wanted to share with you a little of our experience in this first step of our creative process. Conceptualizing ideas and turning them into tangible objects requires acquiring knowledge, skills, and training. The physical materialization of our shaft is a rather banal and simple piece of plastic. However, it hides countless hours of work to ensure that our stylus won’t roll off the Note Air.
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Audrey & Andrés
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