IEEE Robotics & Automation Magazine - September 2015 - 75

Positioning (PlaDyPos) acts as an ASV and carries the international flag marking underwater activity. It has four thrusters in an "X"-shaped configuration, allowing omnidirectional
motion, i.e., motion in the horizontal plane under any orientation. PlaDyPos, shown in Figure 3, has been developed at
the Laboratory for Underwater Systems and Technologies at
the University of Zagreb Faculty of Electrical Engineering and
Computing, Croatia, and it is 0.35-m high, 0.707-m wide and
long, and weighs approximately 25 kg. The control computer
(isolated from environmental disturbances inside the platform hull) is in charge of performing control and guidance
tasks (dynamic positioning, path following, and diver following) and all the data processing. Apart from the compass, batteries, and central processing units the PlaDyPos payload
relevant to the diver-tracking experiments consists of
● a ublox Neo 6P GPS for determining position and, indirectly, the diver position in the horizontal plane
● a Tritech MicroNav ultrashort baseline (USBL) used to determine the position of the diver relative to the vehicle,
with integrated acoustic modem
● a Bullet M2 wireless modem used for two-way communication with the ground station, thus making PlaDyPos a
router from the diver to the surface station where the diving supervisor is stationed.
The USBL, shown in Figure 3(b), is used simultaneously for
localization and two-way data transmission via an acoustic
link (the second modem is mounted on the diver). While
diver localization is the main topic of interest in this article, it
should be mentioned that the acoustic link can be used to

Wi-Fi Link

transmit messages as well as diver position based on USBL
and GPS measurements from the vehicle.
Diver
The diver is mounted with an acoustic modem that is used for
localization on board the surface vehicle as well as to communicate with the modem on the surface vehicle. As shown in
Figure 2, the diver-mounted acoustic modem is
The overall diver-tracking
connected to the RS232Bluetooth converter,
system presented in this
which allows transmission
of the data via Bluetooth
article consists of three
link. Our experiments
have shown that Bluemain components.
tooth communication has
a range of about 15 cm
underwater, which allows
the diver-mounted Bluetooth module to establish a connection with another Bluetooth device in close proximity, such as
the one on the tablet in the waterproof casing.

(a)

GPS

USBL +
Acoustic
Modem

ASV

Acoustic
Link

Acoustic
Modem

Diver
RS 232

UW Tablet
Bluetooth
Modem

Bluetooth
Link

Bluetooth
Modem

(b)
Figure 2. A schematic representation of the communication
structure between the surface robot, the diver, and the
underwater tablet. The ASV is linked to the diver-mounted
modem via acoustic link, while the modem communicates
via RS232 with the Bluetooth modem that connects to the
underwater tablet through the Bluetooth connection that has
proved to work underwater at short distances.

Figure 3. The diver-tracking ASV PlaDyPos viewed from (a) above
and (b) below. The USBL mounted on the bottom of the hull is
used to communicate and determine the position of the divermounted modem via acoustic link. This configuration makes
PlaDyPos a router between the diver and the surface station
where the diving supervisor is stationed.

September 2015

IEEE ROBOTICS & AUTOMATION MAGAZINE

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2015