10-05-2014, 12:44 PM
Handwriting Input System Based on Ultrasonic Transducers
Handwriting Input System.pdf (Size: 1.76 MB / Downloads: 19)
Abstract:
A handwriting input system was developed using three collinear ultrasonic transducers. These col-
linear polyvinylidene fluoride (PVDF) transducers were specially designed for the handwriting input system
to give a large writeable area with writing in any direction. Driver and detection circuits were developed for
the handwriting system. This handwriting input system based on 2-dimensional position tracing has large
writeable area (A4 paper), low drive voltage (5 V), and is independent of the handwriting pad or the pen.
Introduction
Since humans have long used pen and paper, hand-
writing is a natural way to store and transfer informa-
tion. Pen-like devices that digitize the user's handwrit-
ing trajectory have attracted much attention as they
provide a more user-friendly man machine interface,
especially for mobile applications.
Handwriting input systems (HIS) involve the auto-
matic conversion of text as it is written on a special
digitizer or PDA, where a sensor picks up the pen-tip
movements X(t) and Y(t) as well as the pen-up/pen-
down switching[1]. This data is known as digital ink
and is a dynamic representation of the handwriting.
The signal is then converted into letter codes for use in
graphical and text-processing applications.
Handwriting Input System
The HIS has a sending module using a PVDF trans-
mitter to transmit an ultrasound signal and a receiving
module with two PVDF receivers. The receivers con-
nect to a signal processing module to interpret the
movements of the pen across the writing surface,
translating the resulting curves into digital text on an
output display. Figure 1 shows a schematic of the HIS
working principle. The system is placed in a coordinate
plane as in the figure[9]. The transmitter labeled Γ(0, 0)
emits omnidirectional ultrasonic waves (80 kHz) when
the HIS is working. The ultrasonic signal generated by
the transducer hits the pen or the finger and bounces
back to the receivers. The two receivers at positions
R\(R, 0) and Ρ2(-Ρ, 0) in the same line with the trans-
mitter receive the reflected ultrasound signal at time th
the time for the ultrasound waves to transmit from T to
R\ and at t2, the time for the ultrasound waves to
transmit from T to R2. The PVDF transducers trans-
form the ultrasonic signals into voltage signals based
on the piezoelectric effect and send them to a module
that calculates the times t\ and t2 which are then sent to
the signal processing module.
Receiver Circuit Design
The receiver circuit shown in Fig. 4 has two PVDF
receivers to detect the very weak echoed ultrasonic
signals and transform them into electrical signals. A
dual-stage bandpass amplifier is used to reduce the
circuit noise and provide high gain amplification. The
detection circuit detects the signal envelopes and sends
them to the voltage comparator to obtain the echo time
which can be easily processed by the MCU to calculate
the positions.