The ultrasonic positioning system uses sound to accurately determine the
position of a transmitter. Using multiple receivers, the phase
difference is measured to obtain the movement of the transmitter from an
arbitrary point in space. The
base station produces a 40 kHz sound wave, a frequency outside human
hearing, and measures the difference between the reference and the
signal that has been transmitted through air.
Because the wavelength is 8.6 mm in air, movement of 8.6mm will
produce a phase difference of 2pi, and measuring the phase shift will
give the distance traveled.
Using a 555 timer working at 40kHz and a MAXIM 264 Universal Filter, the
transmitter can send a sine wave over six feet without amplification.
The square wave from the timer circuit gets converted to a sinusoidal
wave thanks to a bandpass filter from the MAX264 chip. This filter
circuit uses a bandwidth of 10 kHz centered at 40 kHz and is driven by a
2.45 MHz crystal to produce an alias free signal. Finally using a 10
gain operational aplifier the signal is boosted to use the maximum
output level of the 17 volt transducer.
Small Signal Amplifier
In order for the receiver circuit to operate
properly, it requires the received signal to sufficiently large. The
small signal amplifier serves as the front-end to the receiver
circuit, providing large amplification (approximately 200x) of the
received signal to achieve the sufficient level needed by the rest
of the receiving circuit.
Zero Crossing Detector
The zero crossing detector converts
the amplified sound wave into a square wave to be used by
the digital phase detector.
Using a NPN transistor amplifier and a digital
inverter allows for a clean digital signal.
Phase Detector Diagram
The phase detector measures the difference in phase
between the reference signal and the signal received
from the transmitter.
By using digital components, the phase
detector outputs a square wave with a duty cycle
proportional to phase difference.
processes the output of the digital phase
detector in two stages: first, a lowpass
filter is applied to convert the phase
detector output into a suitable DC voltage.
A simple gain stage is then applied so that
the filtered signal lies in the range of 0
to 5 volts.
The major computing takes place in a
microcontroller. The PIC16F688 from
Microchip was chosen for its
availability, program memory size
and low cost. The analog to digital
converter can convert the output of
the phase converter to a 0-255
value, giving an angular resolution
of 1.4º. The LCD displays the
results of the calculations, but
takes large amounts of programming
time. This only allows approx. 8
readings per second, and limits the
temporal resolution of the
measurements, as the sampled signal
will experience aliasing. The
maximum speed that can be resolved
is 34mm/per second, or
3.4cm/second. The PIC16F688 has the
capability to measure edges in a 16
bit register, and if implemented,
would increase the temporal
resolution to 65535 full wavelength
changes per reading. This would
allow 563 meters/second for a single
reading per second, or more likely,
causing the temporal limitation to
occur in another component of the
system (such as the low pass filter,
with a time constant of 33 Hz).