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Wacom brought EMR technology to drawing tablets decades ago. In the process they held the original patents (long since expired) and have many new ones.
Wacom lists all their patents here: https://www.wacom.com/en-us/patents
If you have time for some technical reading, these patents below describe some key aspects of EMR technology.
US Patent 4,878,553 - POSITION DETECTING APPARATUS
Date Filed: 1987/09/14
Date of Patent: 1989/11/07
Status: EXPIRED
US Patent 4,999,461 - COORDINATES INPUT APPARATUS (LINK TO PATENT)
Date Filed: 1989/05/19
Date of Patent: 1991/03/12
Status: EXPIRED
US Patent 5,134,689 COORDINATE INPUT SYSTEM AND INPUT IMPLEMENT USED IN THE SYSTEM
Date Filed: 1988/08/24
Date of Patent: 1992/07/28
US Patent 5,691,513 SCANNING METHOD FOR SENSOR COILS IN COORDINATE DATA INPUT DEVICE
Date Filed: 1995/09/25
Date of Patent: 1997/11/25
Status: EXPIRED
US Patent Reissue 4,786,765 COORDINATES INPUT SYSTEM
Date Filed: 1987/07/23
Date of Patent: 1988/11/22
Status: Ceased
US Patent Reissue 5,023,408 ELECTRONIC BLACKBOARD AND ACCESSORIES SUCH AS WRITING TOOLS
Date Filed: 1989/06/13
Date of Patent: 1991/06/11
Status: Ceased
Wacom invented the AES technology also. It works in a fundamentally different way than EMR.
We believe that the Apple Pencil tech is very very similar to AES technology.
Wacom's AES page https://www.wacom.com/en-us/for-business/technologies/aes
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.
1:10 a tuned LC circuit can act as an EMR pen
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
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.
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.
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.
Hover (i.e. proximity 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.
We don't know.
We suspect this is MUCH faster than a typical pen report rate of 200Hz.
The companion video contains several circuit simulations.
I used the Falstad tool https://www.falstad.com/circuit/ to create those simulations
Scanlime: Your Wacom pen is an Electric Pendulum ()
Scanlime 013: Graphics Tablet Primer for Hackers ()
Accompanying write up:
Scanlime: Wacom Teardown and Schematic - ()
What’s Inside those Wacoms, And How Can You Use Them In Projects?() (archive: )
This is the top of the Wacom digitizer used in the Wacom Intuos Pen Small (CTL-480). This is a smaller version of a on Wikipedia commons.
The bottom of the digitizer is below. You can see the .
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:
Position detection - The basics of how the tablet detects position are described here: .
Pressure detection - See this document for details on . 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:
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:
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:
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:
More here:
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.
MPP (Microsoft Pen Protocol) is pen tech invented by Microsoft. We believe this is also very similar to AES.
Originally Microsoft used a tech called N-TRIG in their Microsoft Surface devices, but switched to MPP.
It is still used in many Microsoft Surface devices and a few other mobile products.
Microsoft Surface Pen - https://en.wikipedia.org/wiki/Surface_Pen
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 upper right of the tablet.
The tablet reported tilt as as an X tilt and a Y tilt number
Below is what it looks like in the Diagnostics UI of the Wacom Driver for the Wacom Intuos Pro Large PTH-860
The X tilt reported by Wacom ranges from -64 to 63
a negative X value means that the pen is "falling" to the left of the tablet
a positive X value means the pen is "falling" to the right of the tablet
The Y tilt reported by Wacom ranges from -64 to 63
a negative Y value means that the pen is "falling" to the top of the tablet
a positive Y value means the pen is "falling" to the bottom of the tablet
If the X & Y are both zero, then the pen is perfectly perpendicular to the tablet
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:
There are a lot of digital pen technologies out there. Drawing tablets use EMR but there are technologies that offer a similar feature set
Traditional drawing tablets ALL use EMR (Electromagnetic Resonance) More here: EMR. It is also used prominently by some mobile devices such as Samsung Galaxy Tab S series tablets. We believe that EMR overall offers the best drawing experience.
Active Electrostatic (AES) - Used by many mobile devices. More here: AES
Apple Pencil tech - Used only by Apple devices: More here: Apple Pencil tech
Microsoft Pen Protocol (MPP) - Used by Microsoft surface devices and a few others: MPP
N-Trig - Used by some older Microsoft surface models
r/stylus: Wacom AES vs EMR vs N-Trig
r/stylus: MPP vs AES? For digital drawing