Pen Drawing Tablet

XP-Pen Star G640 drawing tablet, 6x4", 8192 pressure levels, battery free stylus with two push buttons, resolution of 5080 LPI, for $25

It works on the electromagnetic induction principle.  

From the teardowns that other people have shown on the internet, the stylus is entirely passive with a coil and capacitors, that would exclude any encoding scheme.  The working hypothesis is that the tablet will transmit a signal to excite the LC tank, the LC tank oscillates and signals get picked up by the tablet.  The working area of the tablet is a PCB with horizontal and vertical traces.  The position of the stylus is calculated perhaps by centroiding on the signal strength.

The pressure level is transmitted probably by frequency modulation.  There is a ferrite on the rod that the nib is attached and the rod is pushed on a spring.  The pressure on the nib moves the ferrite relative to the coil, changing the resonant frequency.  And pressing the button connects additional capacitor to the tank, also altering the frequency.

We will try to verify the working principle without disassembling the stylus or the tablet.  We will use a simple wire loop to pick up the signals.

Without the stylus on the tablet, we pick up some signal from the tablet.   It is possibly just a 500KHz square wave ac-coupled.

 When the pen is on the tablet, we see the oscillating LC signal. 

We can take a close look at the frequency of the oscillation,

Zooming in at the peak, we can see the difference in the rising envelop when the tablet is driving and the falling envelop when the stylus is free oscillating.

At pressure level 0, the frequency is about 506KHz, and at the max pressure of 8192, the frequency is about 530KHz.  So the circuitry has a resolution of 3Hz.  When the upper button is pressed, the frequency is 484KHz, and the lower button 464KHz.  And the frequency changes from there if pressure is applied while the button is pressed.

We can make a circuit model and run simulation to compare with the measurements.

We will design a frequency to voltage conversion circuit to extract the pressure level in a future blog.

The stylus is detected with the tip about 1cm above the tablet; there is some hysteresis on that distance.  Placing the stylus flat lengthwise on the tablet  does not work, but vertically flat widthwise works.  This is a little curious; perhaps the excitation is only generate by traces in one direction.  It may be a deliberate design choice, because the stylus can be left on the tablet without affecting other pointing devices.

When we place a ferrite toroid around the stylus, the tablet can register button clicks with buttons being click.  This implies that the presence of the ferrite toroid lowers the resonant frequency to be in the range of that of button click.