IEEE Robotics & Automation Magazine - December 2016 - 36

and real-time applications. A custom interface printed circuit
board was designed and fabricated in house to provide the
necessary translations between the XMOS XC-2 board and
Icefin relays, actuators, and sensors. Most sensors on the
vehicle use RS-232, RS-422, or RS-485 for serial communication, which can be translated to transistor-transistor logic
levels using the XMOS interface board or translated directly
to USB protocol with a Future Technology Devices International, Ltd. (FTDI) converter. Mechanical relays are used to
switch ON-OFF high-voltage (24-48 V) power to each of
the vehicle sensors and thrusters independently as a powersaving technique.
Ethernet is used for communication among the vehicle's
onboard computer, XMOS board, BlueView forward-looking sonar, and tether connection to the surface. Because the
tether between the vehicle and surface is composed of a single strand of optical fiber, all data signals are multiplexed
using an optical multiplexer and then demultiplexed at the
surface in a similar manner. For this purpose, a Moog 907E
mux board is located both inside the electronics module
pressure housing and at the surface control station. A KVH
Industries 1,750 fiber-optic gyro inertial measurement unit
(IMU) is used with a NavQuest 600-micro DVL to provide
inertial navigation data in both the translational and rotational directions. Depth (the z position in vehicle coordinates) is known with 0.01% accuracy and zero drift from
the Valeport mini IPS (intelligent pressure sensor). Distance to the ice cover is obtained with an upward-looking
OceanTools altimeter. Because compass sensors are ineffective at the poles, a relative coordinate reference system is
used based on the initial state (position sensors are zeroed)
of the vehicle at the surface before deploying beneath the
ice. This can be translated to an absolute coordinate system
by using the GPS coordinates of the deployment hole and
initial rotational state of the vehicle at the surface. The navigational sensor outputs (IMU, DVL, depth, and altimeter)
are processed directly by the vehicle's onboard computer to
enable a real-time navigation solution. The conductivitytemperature sensor data and battery status data are not
needed for real-time processing and are both relayed
directly to the surface control station using serial ports
present on the Moog optical multiplexer. The four directional thrusters are each controlled independently using the
vehicle's main computer via communication over a single
FTDI USB to an I2C adaptor cable.
To provide power to the electronics, sensors, and thrusters,
15 Inspired Energy NH2054HD31 14.4-V 6.8-Ah rechargeable lithium-ion batteries (for a total of 1,470 Wh) are mounted in the lower portion of the electronics module, which also
provides some robustness to roll disturbances. Peltier plates
were considered in the design of the electronics module to
provide additional heat to keep the temperature of the batteries within the optimal operating range (above 10 °C). However, this additional heat was not required due to the placement
of heat-generating power conversion boards in close proximity to the batteries. The battery temperature during Antarctic
36

IEEE ROBOTICS & AUTOMATION MAGAZINE

DECEMBER 2016

field deployment remained above the required 10 °C, despite
subzero exterior conditions; thus, battery performance was
not impacted. These 15 batteries are divided into three independent banks: an electronics bank providing 3.3-, 5-, 12-,
and -12-V dc; a thruster bank providing 48-V dc to the rear
thruster; and a high-voltage bank providing 28- and 24-V dc
for the remaining thrusters and sensors. Separate battery
banks isolate the noisy thruster and sensor power from the
sensitive electronics. The electronics bank was estimated by
the corresponding power controller to have a battery life of
approximately 8 h under a nominal computational load in
low-temperature under-ice mission conditions. However, the
sensor and thruster banks encounter drastically varying battery life depending on the amount of actuator and sensor
engagement. The actual operating duration capability of the
vehicle during deployment exceeded 8 h between under-ice
mission time and idle time (thrusters unengaged). The main
electronics bank is commanded to switch power ON-OFF
using an external bulkhead dummy plug. The higher-voltage
sensor and thruster power banks are designed to be switched
ON-OFF through software during the mission for power
reduction and safety.
Control Architecture and Software Design
The main software component of the Icefin system is composed of a customized version of Greensea's Balefire software.
This software provides a framework for streaming sensor data
and vehicle status to a control station, calculating inertial navigation estimates onboard the vehicle in real time, controlling
the vehicle actuators in real time, and transferring high-level
control signals from the surface control station to the vehicle
during mission operation. The control software provides the
capability for both real-time human operator control as well
as autonomous control of the vehicle. Vehicle state and estimated position information is presented through a graphical
interface to the operator at the surface control station
(Figure 6). The actuator control architecture consists of a
low-level front seat driver application running on the vehicle's
onboard computer, whereas a higher-level back seat driver
application is run at the surface control station. The front seat
driver application controls the actuators directly through lowlevel hardware commands. The back seat driver application
uses input from the autopilot or human operator to provide
set points for the front seat driver's low-level mixing functionality and proportional-integral-derivative controllers. For
human operation of the vehicle, two joysticks are incorporated into the surface station for control of the thrusters: one
three-axis joystick for control of surge, sway, and yaw and one
two-axis joystick for depth control (Figure 6). Video streams
from both vehicle cameras, and a visual representation of the
sonar data is also presented at the control station in real time
using an external host computer.
Because recovery of the Icefin vehicle through the thick
Antarctic ice shelves requires the use of a tether, the current
configuration of the vehicle is designed to remain tethered
with constant communication and control from the surface.

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2016