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In a video I published in 2022 (How do EMR pens work), a specific method to detect pressure is shown. This method involves moving the ferrite core which then alters the resonant frequency of the LC circuit. This design is from a very, very old EMR pen design.
Modern designs use a separate pressure sensor which is more reliable for detecting pressure.
The OLD EMR pen design can be seen in this teardown of the Huion PEN80 pen.
This pressure detection mechanism is discussed in great detail here:
EMR pens do NOT report the pens tilt to the tablet. For example it doesn't tell the tablet "I am tilted by 5 degrees"
Instead, an EMR digitizer can detect the tilt of the pen just by examining the strength and shape of the signal the digitizer coils receive from the pen.
When the pen is perpendicular, the digitizer detects a single perpendicular shape.
As the pen tilts, it produces two shapes. And the relationship between the two shapes indicates the tilt.
For example, in the diagram below, the pen is pointing to the lower left - meaning the top part of the pen is "falling" toward the lower left of the tablet.
The work for the EMR sensor and tablet firmware is to detect that there are two signal peaks and then disambiguate them.
Once two peaks are established and their relative strengths and positions are known, then the direction of tilt and amount of tilt can be established and then reported to the computer.
The specific algorithm to do this has, to my knowledge, never been exactly revealed. But those knowledgeable in how how signals like this are processed will probably have a clearer idea of how this tilt detection be done.
The tablet measures the tilt in both the x and y directions
Measuring as x tilt and y tilt is also equivalent to measuring as azimuth and altitude. Think of azimuth as an angle from the tablet measured from "north" and the altitude as how high the far end of the pen is from the tablet surface.
In the example below the orange lines indicator the azimuth. And the purple line indicates the altitude (also known as elevation).
EMR technology was introduced to drawing tablets by Wacom. And they held all the core key patents to EMR. Those core patents have expired and now other manufacturers can build increasingly sophisticated EMR designs which are begin to rival Wacom products.
Powering the pen - With Passive EMR the pen gets power from proximity to the tablet. However with Active EMR: The pen gets power from a battery inside. More here: Active EMR vs Passive EMR
Position detection - The basics of how the tablet detects position are described here: EMR position detection.
Hover (i.e. proximity detection) -
Pressure detection - See this document for details on EMR pressure detection. It also contains a clarification of pressure detection in the video below (which depicts a very old way of doing pressure detection).
Tilt detection - more here: EMR tilt detection
Barrel rotation detection - very rare in EMR pens
Communication of button press information
Think of the EMR design illustrated in the video as a baseline example that demonstrates the fundamental concepts, different manufacturers can tweak this design in their implementation.
The exact resonant frequency used by the pen will vary.
In the case of the Wacom Bamboo Fun tablet (CTH-661) the frequency is around 750KHz. Source: the last 10 seconds of this scanlime video: https://www.youtube.com/watch?v=oKVCwPn6OPI
We don't know.
We suspect this is MUCH faster than a typical pen report rate of 200Hz.
Even though EMR is used in drawing tablets, there are many other pen technologies in the market, such as AES, Apple Pencil, etc. More here: Digital pen tech
The Apple Pencil does not use EMR. Apple uses a proprietary protocol for their pen. If you are curious about what is inside an Apple pencil, see this video: https://youtube.com/shorts/M9sArtVjRps?feature=share
More here: EMR technical resources
I often get asked if it is possible to build an EMR pen. The short answer is: YES, but it may be challenging.
If you watched my how do ER pens work video you'd probably know enough if you were electronically inclined to acquire the basic components and put them together in a way that would work if you were sufficiently motivated.
Understanding how the pen and tablet talk. You’ll need an oscilloscope and learn how to use it correctly. See the scanline youtube channel which has a couple of videos that explores EMR tablets. You should learn a lot from there.
Components.
Most components are things you can buy: ferritr core, copper coil, spring, piezoelectric sensor.
Some components you may need to 3d print: the pen shell, nibs, assorted bits of plastic that hold everything together inside
The PCB will require you to make something small enough to fit in the case. And it has to use very little power and know how to communicate certain kinds of data to the tablet.
Powering the pen. You need to construct the indictor and PCB to get energy from the pen and briefly power the pen to transmit a signal back.
Position and tilt are done by the tablet simply by sensing the analog signal produced by the pen.
Pressure and button presses require extra work. After studying the tablet and a pen, you will need to transmit the button presses in the same way with components on the PCB. Some pens transmit these in an analog way - by modifying the resonant frequency of the signal from the pen. Some pens transmit these in a digital way by encoding the information digital in the signal.
Modern EMR pens are Passive EMR. DO NOT Buy a tablet that uses Active EMR.
Active vs Passive indicates how an EMR gets power.
Passive EMR - The tablet wirelessly powers the pen
Active EMR - A battery inside the pen powers the pen
The battery may be replaceable or rechargeable.
It doesn't work.
You cannot use an Active EMR pen with a Passive EMR tablet
You cannot use a Passive EMR pen with Active EMR tablet.
Drawing tablets
Wacom: EMR Stylus (Electro-magnetic Resonance): How Wacom Pens work https://community.wacom.com/us/emr-stylus-electro-magnetic-resonance-how-wacom-pens-work/
Wacom: EMR (Electro-Magnetic Resonance) Technology https://archive.is/mhHkP
Wacom: How the Wacom cordless, batteryless pen work https://quietpc.sk/instructions/wacom/tech_bam_en.pdf
Wacom Patents https://patents.google.com/patent/US4786765A/en https://patents.google.com/patent/US4878553B1/en
Wacom: Wacom feel EMR https://wcm-cdn.wacom.com/-/media/graveyard/wacomdotcom/archived%20images/enterprise/technology-solutions/2015-12-21/f_emr_datasheet_12192015.pdf?la=en&rev=5973a6a064ce4f57b20a049410aed106&hash=A479EA340EE48BD510113192CCC3D271
The Scanlime videos are the deepest examination on drawing tablet tech I have found.
Scanlime: Your Wacom pen is an Electric Pendulum (https://youtu.be/oKVCwPn6OPI)
1:10 a tuned LC circuit can act as an EMR pen
Scanlime 013: Graphics Tablet Primer for Hackers (https://youtu.be/nPab7pbOhBY)
Accompanying write up: https://scanlime.org/2016/08/scanlime013-wacom-teardown-and-schematic/
This video shows an Huion H610PRO tablet and Huion PEN80 (rechargable) battery-powered pen.
Key points at around 24:20
24:22 buttons lower the frequency of the oscillation
24:41 the nib is plastic and does not affect the inductance of the coil
24:44 the nib moves a small ferrite core that changes the inductance of the coil.
24:50 Pressure decreases inductance which decreases energy stored in the magnetic field and slightly increases the oscillation
Other points
25:35 Shielding freom electromagnetic interference
Scanlime: Wacom Teardown and Schematic - (https://youtu.be/j4AKwJERxOw)
What’s Inside those Wacoms, And How Can You Use Them In Projects?(https://makezine.com/article/maker-news/whats-inside-wacoms-can-use-projects/) (archive: https://archive.is/wip/wr9mn)
Key points:
"At this point the Wacom and Huion designs diverge. Huion’s pen [PEN80] is a single-transistor oscillator. Pressure on the nib changes the frequency from 255 to 266kHz by tuning the inductor, and the two buttons switch to 235 or 245kHz with additional capacitors. The simplest Wacom pen would be a resonant LC circuit tuned to 750kHz. To transmit button and pressure status, an additional digital circuit modulates the resonant damping to send out individual bits of sensor data on each carrier burst."
The starting point for understanding EMR is learning how the tablet detects the position of the pen. Once this is known, many other aspects of the tablet can be understand.
The tablet and pen are communicating with each other. They swap between listening and transmitting an electromagnetic signal many times a second. This document focuses on the pen producing an electromagnetic signal that the tablet detects.
Please keep in mind, this is a simplified, conceptual explanation. Naturally, the description leaves out many details.
The fundamental component of a drawing tablet the EMR sensor. The more common name for this is digitizer.
The digitizer is a printed circuit board (PCB) that contains:
Some chips
Some firmware code running in some of this chips
a set of coils (wires) laid out on the PCB. The coils that are laid out vertically are clearly visible. The horizontal coils are partially visible as darker regions on the green PCB.
This is the top of the Wacom digitizer used in the Wacom Intuos Pen Small (CTL-480). This is a smaller version of a much larger image on Wikipedia commons.
The bottom of the digitizer is below. You can see the larger version here.
The digitizer has something underneath it (on top on the photo) - this is probably a thin piece of something metallic to prevent electromagnetic interference from this device to others from the bottom of the tablet.
In this view you can see chips on the digitizer PCB attached to other components inside the tablet.
Now that we know what the digitizer looks like, by exploring position detection, we can understand what we are seeing.
The coil is a piece of wire that extends from a digitizer chip.
NOTE:
A real digitizer will have multiple chips, but in these diagrams only 1 or 2 chips are shown.
The wire does not look like what you might think of as a "coil". If you look up an electromagnetic component called an "inductor" you'll realize why it has this name. Another doc covering the LC Circuit in EMR pens will explain it in more detail.
If the pen is sending an electromagnetic signal and is near the coil. The signal will induce a current in the coil. This is called electromagnetic induction - meaning a current in one thing is causing a current in something else even though they are not touching. The chip registers the strength of this signal - I draw it as a light blue bar near the detector. The closer the pen is the stronger the signal. If the pen is far enough away, there really isn't any current detected on the coil.
Key points
At this stage, the signal on the coil tells us something about how far away the pen is, but nothing else.
The coil is oriented vertically but it doesn't know the vertical position of the pen. The pen could be on the top or the middle or near the bottom and the coil wouldn't know it.
Lets have multiple coils next to each other in a row horizontally. Each coil independently detects the signal from the pen. So the signal strength is different for each coil - and depends on how far away the pen is from that specific coil.
The pen is right on top of one coil so it has the strongest signal. As coils are further from the pen the strength diminishes. And of course some coils essentially don't detect any signal.
The coils are going up and down, but the set of coils are arranged horizontally. This arrangement means that the coils can detect the horizontal location of the pen, but again the coils have now idea were the pen is vertically.
Before we continue, I'm going to draw the coils in a simpler way. It will look like I am drawing them as a wire just sticking out of the detector, but really think of them as looping back into the detector. Drawing it this way will make looking at the later diagrams a bit easier.
Now we are going to have two arrays of coils. One is a horizontal array like we had above. But the other array is oriented vertically.
Now we are going but these arrays on top of each other.
I must stress this, even the diagram makes it look like the horizontal and vertical coils are touching - they are not touching at all. They are on two separate layers. None of the coils actually touch another coil.
With the coils arranged horizontally and vertically, the coils can detect the position of the pen. Th e vertical coils detect the horizontal (x) position. The horizontal coils detect the vertical (y) position.
Again notice that multiple coils in each dimension are detecting the signal for the pen.
In this diagram below, the pen coincidentally is right on top of one vertical coil and one horizontal coil. As a result, a single strong peak signal for the both the horizontal and vertical components.
Drawing tablets do not have a large number of coils. Not thousands. Not hundreds. They have more like tens of coils for each of the horizontal and vertical dimensions.
The consequence of this very sparse arrangement of coils is that the vast majority of the time, the pen is NOT exactly on top of a specific coil. It's almost always between two coils.
Also only a small number of coils detect the pen. For example in in any given dimension it could be only 4 coils that detect the pen.
In this case above the pen is exactly in-between two horizontal coils and exactly in-between two vertical coils. So now in both the horizontal and vertical directions, there isn't a single strong signal, two signals in each direction have the same value. So the tablet, can infer that the pen is exactly between coils in both directions.
And of course the pen may not not be exactly on or exactly in-between coils as shown below.
So, a drawing tablet has to carefully look at the signal strengths to handle all these cases of the pen's position. Fortunately EMR tablets are very fast and accurate at doing this. In terms of resolution, a typical Wacom EMR tablet can identify 5080 different positions inside every inch - that's 200 different positions for every millimeter! As you can see, the resolution of detection is surprisingly good.
When the pen is in the middle of the tablet, there are lots of coils that detect the pen.
But the pen can also be at the edge or corner of the grid of coils. In this case there are fewer coils detecting the pen.
With fewer coils detecting the pen in this scenario the tablet has less data to work with to estimate the position of the pen. You will notice this in almost all tablets - that as you get within about 3mm of the edges or corners the pointer will start to drift away from the tip of the pen.
This loss of accuracy is typical and is present in all tablets - even the most high-end professional ones.
Tablet manufacturers try to counteract this a little bit. The surface of the tablet will usually mark out the corners of the active area.
We represent the active area as a red box in this diagrams, you'll notice that the active area is not the full size of the grid. Instead it is inset a little bit. This helps the tablet identify the pens position at what you see as the active area.
