Question

Please use at least 200 words to describe each of the following questions:

To minimize the uncertainty in a new instrument for measuring distance, would it be better to calibrate the new instrument with the PDT or the Ultra-sonic Sensor?

Also, if possible, discuss how to use the value of the range, total uncertainties, and full-scale error of PDT and The Ultra-Sonic Sensor to select a sensor for a specific application

Answer 1

Ultrasonic distance sensors are designed for non-
contact distance measurement and these types consist of
transmitter and receiver or transceiver which is able to
transmit and to receive ultrasonic sound (Figure 1). Main
idea is to measure time to fly of ultrasonic sound wave
from sensor to detected object. An ultrasonic transmitter
sends a sound frequency of above 18 kHz in the air at the
speed of 344 meter per second (at 20°C) and the receiver
receives the reflected sound from the object. Distance
between the transmitter and the object can be calculated
by simple calculation by considering the time taken by the
ultrasonic wave to travel from transmitter and received
back (reflected) by the receiver. Measurement range is up
to several meters.
Figure 1. Ultrasonic sensor working principle
Almost all materials reflect sound waves, so ultrasonic
sensors are a fine choice for many tasks. Excellence in the
detection and measurement of films, transparent objects,
and liquids separate these sensors from their photoelectric
counterparts. Target color or frequent color changes also
have no effect on ultrasonic sensors.
Due to their use of sound waves, ultrasonic sensors also
perform well in dusty, dirty environments. However, they
do not operate well with small targets against large
backgrounds or targets such as foam batting that are
excellent for absorbing sound waves [1,2,3,4].
A typical ultrasonic sensor (Figure 2) comprises a clock
(signal) generator and a controller to excite the transducer,
then a processor and output amplifier to handle the returnUltrasonic sensors have variety application as distance
measurement, obstacle avoiding and anti-collision
detection, robot navigation, measurement in automotive
parking assistance systems, measurement of air flow
velocity - anemometer, medical ultrasonography, non-
destructive testing, piezoelectric transducers, level
measurement, pallet detection on forklifts, vehicle
detection in barrier systems etc.
Ultrasonic sensors are non-intrusive in that they do not
require physical contact with their target, and can detect
certain clear or shiny targets otherwise obscured to some
vision-based sensors. On the other hand, their
measurements are very sensitive to temperature and to the
angle of the target. Temperature and humidity affect the
speed of sound in air. Therefore, range finders may need
to be recalibrated to make accurate measurements in a
new environment. Temperature variations and air currents
can create invisible boundaries that will reflect ultrasonic
waves, so care must be taken to avoid these. For the
transmitted wave to echo back to the receiver, the target
surface must be perpendicular to the transmitter. Round
objects are therefore most easily sensed since they always
show some perpendicular face. When targeting a flat
object, care must be taken to ensure that its angle with
respect to the sensor does not exceed a particular range.
Ultrasonic sensors typically have a “dead zone”
immediately in front of them in which objects cannot be
detected because they deflect the wave back before the
receiver is operational. (This is because reverberations
from the transmitter force the receiver to pause a moment
before beginning to listen for the echo). Some materials
are more absorbent than others, and these will reflect less
ultrasound. This complicates using the attenuation method
to measure the distance of arbitrary object

This section addresses the problem of how to select a sensor to continue keep-
ing track of the target. We assume that we have redundant sensor coverage
i.e. the sensors are deployed in such a way that at any given time, the target
is always in the range of at least two sensors. We are therefore interested in
determining how to select sensors to keep track of the target. Figure 4 presents
a scenario of sensor deployment, that illustrates the need for sensor selection.
Given a set of sensors S={S1, S2....Sn} all deployed in one room, if the target
tends to go out of range of one sensor or when the tracking quality is decreas-
ing, a hand-over algorithm is initiated upon a neighboring sensor which has
better coverage of the target. This algorithm also includes a “wake-up and
synchronize procedure”. In this manner, energy conservation is accomplished
by ensuring that at any given time, at most two sensors are actively tracking
the target.

The sensor candidate set is the collection of sensors which can detect the
target at a given time. This set changes dynamically for each position of the
target in the coordinate system. Below we present a mathematical model to
generate a sensor candidate set corresponding to a specific target’s location.
This mathematical model is based on the coordinates of the target and the
parameters of the sensors as described in section 2.2.1.
To determine the relative position between target and sensor, we need to
calculate the distance between target and sensor and the angle made by the
line joining target to sensor with the X-axis. The simplicity of calculating

distance leads us to focus on the angle determination.
θ = cos−1
(
Ox − Sx
|
−→Si0|
) = cos−1
(
Ox − Sx q
(Ox − Sx)
2 + (Oy − Sy)
2
)
If we call the angle made by the line joining sensor to target with the X-axis
φ, then we have the following two cases:
φ =
θ if 0 ≤ Siα < 180
360 − θ otherwise
(3)
The object is in range of a sensor Si