EMR

Overview

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.

EMR key features

• Power

  • Passive EMR: The pen gets power from proximity to the tablet

  • Active EMR: The pen gets power from a battery inside

• Hover (i.e. proximity detection)

• Position detection

• Pressure detection

• Tilt detection

• Barrel rotation detection - very rare

• Communication of button information.

Explanation of how EMR works

Pressure detection

See this document for details on EMR pressure detection. It also contains a clarification of pressure detection in the video which depicts a very old way of doing pressure detection.

Concepts vs Implementation

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.

Active EMR vs Passive EMR

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

Active EMR battery type - The battery may be replaceable or rechargeable.

Mixing and matching? - 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.

Resonant frequency

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

How fast does the tablet switch between sensing the pen and sending it power?

We don't know.

We suspect this is MUCH faster than a typical pen report rate of 200Hz.

Other pen technologies

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

General resources

• Sabins - Understanding Inductors

• Lesics - The beauty of LC Oscillations!

• The Organic Chemistry Tutor - LC Oscillator Tank Circuit

• Digital Art Tablet Guides - The Insides of Some Tablet Pens!

• rukozhop - This video shows the internals of a Huion PW100 pen

• Satoshi Uchino, et al - A full integration of electromagnetic resonance sensor and capacitive touch sensor into LCD

Wacom resources

• 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

Scanlime resources

• 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."

Apple Pencil

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

Circuit simulations

The companion video contains several circuit simulations.

I used the Falstad tool https://www.falstad.com/circuit/ to create those simulations