Could anyone tell what are the sensors used in a digital pen which or specifically Equil smart pen and smart marker which can track hand writings. Is it MEMS based??If Yes, Is it MEMS accelerometer only or a combination of MEMS sensors like gyroscope and magnetometer and acceleometer? What algorithms are used here?
Classical odometry sensors:
- IMU:gyro, accelerometer, compass (Wiimote IMU - not infrared)
- Optical flow (optic mouse)
- encoders (like mouse marble)
These systems are cheap and easy to apply. However these sensors needs sensor fusion and filtering because of error cumulation. These sensors are not suitable for long processes.
Reference based systems:
- sensor surfaces (touchscreen, electromagnetic/resistive/capacitve etc.)
- Infrared reference tracking (wiimote IR and white board)
- Multi point reference: (ultrasonic transmitter and receivers) source emits ultrasonic waves and triliteration/triangulaion made accoridng to 3 or more receivers
- Optical tracking (VICON, multiple cameras placed allover the target/pen and according trace of pointers which placed on target motions are modelled )
these systems are more accurate but needs computational power and a lot of hardware. Again, you should use sensor fusion and filtering. You can also search triangulation and trileteration methods and algorithms.
For sensor fusion and filtering algorithms please search:
- Kalman filter/ Extended Kalman Filter
- Complementary Filter
- Particle Filer
Most smart pens use a number of differenst sensors different types of MEMS but more importanly they use optical sensors. Pen tip motions are recreated based on measurements of all sensors using sensor fusion.
Optical sensors (Ir cameras or visible cameras) and ultrasonic sensors are used for pose (position and orientation measurment). Some pens, as the Equil use an external sensing (with infrared and ultrasound sensors) unit which tracks the pen relative to the sensor. Other solution include small cameras near the tip of the pen which identify special markers on the paper and so can calculate the relative position of the pentip on the paper. These directly measure pen relative to the paper.
The optical pose sensing is more reliable then the MEMS sensors since they directly measure positions and so they do not suffer from drift. However, MEMS sensors help to recreate motions using sensor fusion. Both giros and acceleromaters offer usefull input to the sensor fusion process, using both of them if just one type may be an economic decision and not a technical one.
I am not sure to what extent magnetometers are/can be used since magnetic field vector fo the earth, which they can sense is dependent on geographical location. Also they are effected by nearby electro-magnetic fields (e.g. motors, maybe large LCD screens due to their inverter).