Design of a Prototype Miniature Autonomous

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PrOceedingS of the 2003 IEEBRSJInn.Conference w1 intelligent Robots and Systems

Las Veaas. Nevada October2003

Design of a Prototyp e Miniature AutonomousUnderwater Vehicle

Adi tya S. Gadre Jared J. Mach Daniel J. Stilwell Carl E. Wick

The Bradley Department of Electrical and Computer EngineeringVirginia Polytechnic Inst itute and State University

Systems Engineering DepartmentUnited States Naval Academy

Blacksburg, VA 24061

{agadre, jmach, stilwell}@ut.edu,

Abstmct- Platoons of cooperating autonomous underwa-

ter vehicles have the potential to contribute significantly

to scientific investigations in the marine environment. Pla-toon s of vehicles can survey large areas, adaptively track and

measure timevarying processes such as tidal fronts and al-

gal blooms, and they ar e robust to single-point failures. We

have developed a prototype miniature low-cost autonomous

underwater vehicle to address the platform requirements ofthese missions. The vehicle is designed as a test-bed for

the development of distributed control and estimation al-gorithms, and for experiments in advanced navigation andcontrol.

I. INTRODUCTION

Platoons of cooperating autonomous vehicles have signif-

icant and well-know advantages over single vehicles oper-

ating independently. Cooperating vehicles can rapidly sur-

vey large areas, they can survey in four dimensions (space

and time), and they are robust to single point failures. Itis also hypothesized that the individual vehicles in a pla-

toon of cooperating vehicles could be less expensive andless sophisticated than vehicles designed to operate inde-

pendently.

Most research on multiple cooperating vehicles is con-

ducted using terrestrial vehicles. Yet the marine and ocean

science communities would benefit significantly from suc-

cessful use of cooperating underwater vehicles. Potential

applications include surveying chemical plumes, mapping

oceanographic processes (e.g., tidal front), and mapping bi-

ological populations. For underwater vehicle applications,

the complexity and cost of deployments with multiple un-

derwater vehicles is a major factor limiting experimental

activities. To facilitate experiments with platoons ofc o o perating autonomous underwater vehicles (AUVs), we have

embarked on a program to develop a miniature low-cost

AUV tha t operates robustly in the field. Our prototype

miniature AUV is described herein.Other small AUVs, such as REMUS [I] and Ranger (21

could be utilized for multi-vehicle research, but their costs

and/or infrastructure requirements are prohibitive. At a

cost of less than $3,000 in parts, the prototype AUV de-

scribed herein is dramatically less expensive than currently

D. J. Stilweli gratefully acknowledges the support of the Na -tional Science Foundation via grant IIS0238092, Office of Naval Re-search via grant N00014-03-1-0444,nd the Woodrow W. Everett,Jr . SCEEE Development Fund in cooperation with the SoutheasternConsortium of Electrical Engineering Mueatom.

Annapolis, MD 21402

wicL&hsn,a. edu

available AUVs.

The challenges of acquiring and deploying a platoon of

AUVs are the principle motivation for developing the AUV

described herein. These challenges are exemplified by the

very limited literature of reported experiments with pla-

toons of underwater vehicles. A notable exception was de-

scribed in (31. Leader-follower control was demonstrated

using Odyssey vehicles with a modified ultra-short baselinetransponder on the lead vehicle. Though the control algo-

rithm for the follower vehicles was somewhat ad hoc, the

system displayed significant robustness. In 141, the devel-

opment of a new autonomous underwater vehicle designed

for laboratory experiments with platoons of vehicles is de-

scribed. Field operations where multiple AUVs operate in

the same area, but are otherwise uncoordinated, have also

been reported [Si, 61.

. , . , ... -I ..

' . j

. . - . ... .

Fig. 1 . filly assembled A W .

Fig. 2. Hull and nose removed.

11. A MlNiATURE AUV

The principal goals of our miniature AUV design effort

is to produce a vehicle that is small, inexpensive, easy to

deploy, and yet fully functional as a platform for marine

mailto:stilwell%[email protected]

mailto:stilwell%[email protected]



antenna. The inside of the nose cone is hollowed out to al-

low for mounting of sensors that require exposure to water,

such as the sonar altimeter. The nose attaches to the hull

via three dowel pins.

Fig. 5 . Hull and Nare Cane

E. Electronics

Power is provided by a battery stack consisting of 4/3

A size NiMH cells. The battery stack provides 80 watt -

hours at 21.6V. Regulated power at 3.3V, 5V, and 12V is

provided by a miniature switching power regulators (Power

Trends 7 8 S b H C ) . A separate 3V lithium cell powers the

GPS receiver's real-time clock and allows it to save almanac

data. A small mechanical relay is used to open and close

the main battery circuit through a simple shunt switch in

a water tight connector in the forward bulkhead.

The AUV control system is centered around a Rabbit

3000 &bit microcontroller (RCM3200 module), running at44.2 MHz, and programmed in C. The microcontroller in-

terfaces directly with a custom-engineered serial port ex-

pansion card and external 8M B FLASH memory. An u p

grade path to a laptop HD for high density data storage

exists. A block diagram showing the data interfaces with

the microcontroller is shown in Figure 6.

Fig. 6. Control System.

All data communication within t he AUV is implemented

The Rabbit microcontrollersing serial communication.

has six built-in serial ports, and a custom engineered

printed circuit board was designed to provide 5 additional

serial ports. One of the Rabbit's serial ports is multiplexed

to provide 1 x 4 serial port expansion using line drivers

and buffers. A dual UART provides two additional I jO-mapped serial ports: making total number of serial ports

11. Out of these, two serial ports are Rs232 while all others

are TTL level.The propeller is driven with a 24V, 40 watt, brushless DC

motor. A custom-engineered printed circuit board (PCB)

was designed to drive and control the brushless motor along

with the three servo motors. It performs electronic com-

mutation of the brushless motor and controls motor v e

locity using a quadrature encoder signal from the motor

and pulse-width modulation of the motor phase voltages.

The motor controller communicates directly with the Rab-

bit microprocessor over a serial link. The motor controller

sources up to 3 amps at 50 V.

C. Sensors

A custom-engineered printed circuit board enables pro-

cessing of analog sensor signals, including depth trans-ducer, accelerometers, and rategyros . Three-axis accel-

eration is measured by MEMS accelerometers (ADXLlOS),

and 3-axis angular rate is measured by miniature rate gy-

ros (Gyration Micrc-Gyro 100). A network of operational

amplifiers provides signal conditioning, including scaling

and shifting. Analog-to-digital conversion is accomplished

with an 8-channel 12-bit serial ADC (Maxim MAX1270).

A PIC microcontroller manages the AD C conversion pro-

cess, provides digital filtering, and interfaces to the Rabbit

microcontroller via the serial bus. The PIC microcontroller

samples all analog signals at 2 KHz, averages the samples,

and provides them to the Rabbit microcontroller at 10 Hz.

Fast sampling and averaging reduces noise by more than

an order of magnitude.

An off-the-shelf electronic compass (PNI TCM2-20) pro-

vides magnetic heading, pitch, roll, and internal temper-

ature . Pitch and roll measurements are also provided by

accelerometers on the analog sensors card that measure the

gravity vector with respect to body fixed coordinates.

A GPS receiver (Motorola M12 OnCore) provides precise

position when the vehicle is on the surface. The GPS re-

ceiver can provide data in NMEA 0183 data format (at

4800 baud) or Motorola binary format (at 9600 baud).

Since hinary data provides more information faster, it is

used instead of NMEA format. Time-to-first-fix is reduced

by synchronizing time with the Rabbit Microcontroller,

which in turn receives time from the host PC.Use of a GPS receiver for precise navigation implies that

the AUV will periodically surface for a GPS fix. This is a

reasonable assumption given the shallow water applications

for which the AUV is intended. Yet an important consid-

eration is the time-to-first& after the AUV has beensub-

merged and returns t o the surface. If time-to-first-fix is too

long, then the AUV will spend an excessive amount of time

on the surface waiting for a GPS&. To determine time-to-

first.& after the AUV has been submerged, we submerged

644



the G PS antenna, brought it, to the surface after 5 or 10

minutes, and recorded the time-to-first-fix. Histograms of

the results of these experiments are shown in Figures 7 and 8. Using a simple Student4 distribution, we deter-

mine that if the AU V has been submerged for 5 minutes,

then time-to-fist -& occurs within 5 *0.61 seconds90% of

the time, and that if the AU V has been submerged for 10

minutes, then time-to-first-fix occurs within 14.67i .0 1seconds 90% of the time. Of course, the applicability of

our results is limited since only a small sample of datawa staken on a single day. These initial experiments suggest

that G PS is a good source of precise navigation data for

shallow-water missions where it is reasonable for the AU V

to periodically surface. The AU V will not need to spend

an excessive amount of time on the surface waiting for a

GPS fix.

I 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4

time 10fix (secs)

Fig. 7. Time-to-first-fix after being submerged for 5 minut-

I

0

1 3 5 7 9 1 1 13 15 17 19 21 23 25 21 29

time to fi x (secs)

Fig. 8. Tim-to-first-fix &er being submerged for 10 minutes

material. The transducer housing, shown in Figure 9, also

contains the transmitter. The receiver is approximately the

same size as the transmitter and is housed inside the AUV.

Fig. 9. Custom-engineered sonar altimeter.

Data communication between AUV and the host PC

is standard Rs232 serial communication over a Freewave

9OOMHz radio modem. The modem operates at 57600 baud

full-duplex (115Khaud total). It implements da ta encoding

and error checking internally, freeing time consuming tasksfrom the Rabbit microcontroller.

111. NEXTGENERATIONINIATURE UV

A next-generation AU V has been designed based on

lessons learned from the design, fabrication, and initial

testing of the prototype AUV MODO. The next-generation

AUV retains most of the features of MODO while incorp-

rating a few key modifications.

A . Mission Sensor Location

Mission sensors for the next generation AUV are

mounted on the exterior of he AU V and the nose is not

flooded. In contrast, mission sensor transducers are placed

inside the flooded nose of the prototype AUV MODO, while

associated electronics are placed inside a water tight bulk-

head immediately behind the nose. By moving mission

sensors to the exterior and removing the forward bulkhead,

the overall length of the AU V is reduced, and significant

flexibility in modifying the mission sensor payload is ob-

tained. Of course, the costs of mounting mission sensors

externally include reduced hydrodynamic efficiency. Fig-ure 10 shows the next generation AUV with a YSI 600XL

TWO custom-engineered miniature sonar altimeters are

mounted on the AUV. One faces down to determine ve-

hicle altitude while the other faces forward for obstacle

avoidance. The altimeter operates at 450 kHz and has a

resolution of approximately 2 cm between 8cm and 9OOcm.

Due to a large transducer surface relative to the operat-

ing frequency, the beamwidth is approximately 5 degrees.

The transducer consists of a small sheet of piezo-electric

CTDlDO sonde.

l?' InCEased Diameter

The prototype A W MODO requires 635 grams of lead

ballast, restricting the weight of components that will be

added in the future. The next generation vehicle is 15 cm

shorter than MODO, and thus its displacement is signif-

icantly reduced. To achieve an additional safety margin

845



hide for complex 3-D sensing,” in Proceedings of he IEEE/MTS

OCEANS, pp. 2043-2045, 2001.

131 H. Singh, J . Catipovic, R. Eastwood, L. Freitag, H. Henricksen,F. Hover, D. Yaerger, J. Bellingham, and B. Moran, “An inte-grated approach to multiple AUV communications, navigation,and docking,” in Proceedings of IEEE/MTS Oceans, (FortLaud.erdale, Florida), pp. 59-64, September 1996.

141 R. Bachmeyer and N. Leonard, “A test-bed for multi AUV exper-iments,” in Proceedings of the the 1Oh International Symposium

on Unmanned Untethered Submersible Technology, (Durham,Ne w Hampshire), August 2001.

Specification

9.6 cm

4 .5 kg (without CTD/DO sonde)

DIMM-PC (133 MH z AMD Elan

SC520)22.2 volt, 142 watt-hour lithium

polymer battery stack

Parameter

Length 76.0 cm

~ i ~ ~ ~ t ~

MassCPU

Power

Design of a Prototype Miniature Autonomous

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