Proceedings of the 23rd

CANCAM

DEVELOPMENT OF A DYNAMIC VARIABLE MEASUREMENT SYSTEM FOR USE IN WIND POWERED YACHTS.

Alexandre Bergeron and Natalie Baddour Department of Mechanical Engineering

University of Ottawa Ottawa, Ontario

E-mail: aberg098@uottawa.ca, nbaddour@uottawa.ca

ABSTRACT

The present study is on the design process of an

inertial measurement data acquisition system intended for use

in sailboats. The variables of interest are 3-axis acceleration,

3-axis rotation, GPS position/velocity, magnetic compass

bearing and wind speed/direction. The prototype is then

submitted to a basic functionality test successfully.

INTRODUCTION

An inertial measurement unit (IMU) is a device

which is used primarily to assess the movement of an object

with relation to the Earth. It is a combination of

accelerometers and gyroscopes, typically arranged

orthogonally along three axes so as to measure the inertial

acceleration of the unit.

Typical applications of IMU devices are for inertial

navigation in various vehicles such as aircraft, UAVs,

missiles and land based craft. An extension of inertial

navigation is autonomous control of these vehicles. Other

uses include motion capture of vehicles, various objects and

the human body.

The majority of commercial off the shelf (COTS)

IMUs targeted at the average consumer are intended for the

motorsports and video gaming markets. The aim of this

project is to explore the possibility of applying this level of

technology to the competitive sailing realm.

BACKGROUND

A. Sailboats

Sailboats comprise any type of vessel using the

wind as its primary method of propulsion. Traditionally, this

is accomplished through a vertical cloth aerofoil that can be

trimmed depending on intended direction of travel and wind

conditions. This requires a fair amount of operator skill and,

depending on the size of the boat, teamwork. As such,

throughout history there has been a long tradition of contests

amongst sailors and their boats.

Modern sailing races include the “America’s Cup”

and the “Volvo Ocean Race”. These events can have budgets

running above tens of millions of dollars [1]. This vast

investment at the higher levels has not quite yet filtered down

to the lower “club” levels of racing or the consumer level.

This trend is unlike what is seen in the automotive industry,

where the innovations seen in racing can and are applied to

the mass produced models. Many of these technical

developments are not shared with the larger sailing

community as they contribute to the competitive edge of one

team over another; therefore few of these innovations are

available to an average club sailor.

Potential benefits of integrating IMUs to sailboats

and reducing the overall cost to the consumer are enormous.

This technology could allow the same level of performance

analysis to be made for a wide variety of teams, facilitating

crew training and providing the skipper with better “real-

time” information. All of which could be used to improve the

overall performance of a given boat and crew combination.

Inertial measurement data is also critical in

improving archaic sailboat design methods, especially in the

area of rigging and mast design. Traditional methods such as

Skene’s method [2] or the Nordic Boat Standard [3] employ a

great deal of arbitrary and empirical factors, as well as rules

of thumb to accomplish their goals. Armed with true inertial

data, a designer could better assess the given loads on a full

size prototype and further refine it. This would lead to a

better optimization of the boat’s design and construction, as

well as greater refinement of existing design methodology

and standards.

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B. Low-Cost IMUs and Commercial off the Shelf

There exists several consumer devices integrating

IMUs designed for use in vehicles. These are however,

primarily intended for use in automotive applications, such as

racing cars. Devices from manufacturers such as Traqmate or

VBox integrate accelerometers with a GPS device to provide

the user with replay capability. These devices are meant to

compare lap times around a given circuit and analysing

cornering and braking forces: These are essentially two-

dimensional.

As far as devices intended for sailboats, most marine

electronics manufacturers supply devices which are meant to

indicate or display a certain variable, such as heading or

speed. There is no indication of a commercially available

IMU recording device tailored specifically to sailboats.

MATERIALS AND METHODS

A proof of concept device was designed in order to

ascertain the feasibility, use and validity of transferring

existing IMU technology to a sailboat application. The

primary concerns during the design of this prototype were

maximising the use of COTS components, reducing cost and

ease of assembly. This last point is favoured by a large

community support for each of the devices and the

accessibility of information from the vendors, designers and

other users.

A. Variables of interest

A proper IMU must measure linear motion and

rotational motion along at least three axes. Most IMUs also

include GPS technology to validate some of their

measurements and also for additional data. This includes

global position, heading and velocity along the Earth’s

surface.

Useful data that is more specific to sailboats

includes the traditional magnetic compass heading and wind

speed/direction. This is the traditional information that a

sailor would use to set and maintain a given course, as well

as adjust the sails.

A summary of the variables of interest is given in

Table 1: Variables of Interest, along with the necessary sensor

to measure it.

Table 1: Variables of Interest

Variable Sensor

3-axis linear acceleration Accelerometer

3-axis rotational acceleration Gyroscope

Magnetic heading Compass

Wind speed Anemometer

Wind direction Weather Vane

Global position GPS

Global heading GPS

Planar velocity GPS

B. Microcontroller

The chosen microcontroller to build the proof of

concept setup is the Society of Robot’s AxonII [4]. It is based

around Atmel’s ATmega640 8-bit processor which

incorporates 64KB of programmable memory and 16

channels of 10 bit A/D conversion.

The AxonII can interface with devices using an I2C

protocol port and through 3 standard universal asynchronous

receiver/transmitter (UART) ports. The UART ports function

like a computer’s traditional COM ports and have a

selectable BAUD rate for data transmission. A fourth UART

port is used to communicate with a PC through a USB cable.

The integrated development environment of choice

for the AxonII is AVR Studio, which is Atmel’s

complimentary product for their microprocessors. The

programming language and structure is similar to C.

Specifically, the open-source library webbotlib is used for

robotics applications. It contains the low-level

communication routines used to interface a large number of

devices with AVR processors and also has a portion tailored

specifically to the AxonII.

C. Accelerometer and Gyroscopes

All gyroscopes and accelerometers used are

inexpensive micro electromechanical systems (MEMS).

These devices are compact and draw very little power.

A 3-axis accelerometer in the form of the Analog

Devices ADXL335 provides the required linear

measurements. It has a range of +/-3G and is intended for use

in cost sensitive motion sensing applications [5].

Two gyroscopes provide angular measurements, a 2-

axis LPR530AL and a single axis LY530ALH, both

manufactured by STMicroelectronics. There are currently no

inexpensive 3-axis MEMS devices available, hence the need

for at least two separate devices. Both these gyroscopes have

a +/- 300 degree/second measuring range [6].

Conveniently, the above sensors are mounted on a

combination IMU board sold by Sparkfun Electronics under

the name “Razor 6DOF IMU” The output signals are in the

form of voltages.

Interfacing with the AxonII is through A/D

converter ports. The Webbotlib library provides the necessary

software to the microcontroller which allows it to interpret

the signals and convert the voltage to the proper units, m/s or

degrees/s.

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D. GPS

Due to the physical layout of a sailboat and the

possibility of mounting the data acquisition package below

deck, GPS signal reception is of great importance. For this

reason, a GPS device with external antenna connection was

chosen. The package used is the GPS Micro-Mini with SMA

connector, sold by Sparkfun Electronics. It is based on the

Micro Modular Technologies MN5010HS GPS receiver chip,

which uses the SiRF III chipset.

The SMA connector packaged with this sensor

allows the connection of an external antenna, in this case one

with a magnetic mount and 5m cable. The antenna has a gain

of 26dB and a voltage standing wave ratio (VSWR) of less

than 2.0 [7].

The GPS receiver returns several values in the

format of a standardised string, defined by the National

Marine Electronics Association (NMEA) standards. The

information of use in this case will be latitude and longitude

positions, time, course over ground (bearing) and speed over

ground.

E. Magnetic Compass

The use of a magnetic compass was deemed

necessary to account for the possible inaccuracies of the GPS

bearing measurement. In fact, GPS bearing measurement

requires the object to be in motion in order to ascertain the

heading from positional changes; a given sailboat may

sometimes move too slowly for the GPS to accomplish this

properly. Additionally, the difference between magnetic

compass heading and GPS heading can be used to determine

the boat's leeway angle.

The magnetic compass implementation uses a

Honeywell HMC 6352 solid state magneto-resistive based

device. The integrated circuit does all of the necessary

interpretation and converts the output to a magnetic heading

directly. Sensor resolution is 0.5 degrees with 1 degree of

repeatability [8].

The actual sensor itself changes its resistance

depending on its orientation with the earth’s magnetic field.

Some further processing is done internally to determine the

direction of the strongest magnetic reading, which should

correspond to magnetic north. This value is outputted as an

integer corresponding to the bearing in degrees. The outgoing

signal is relayed to the AxonII through an I2C connection.

This solid state magnetic compass is subject to the

usual limitations of the traditional needle compass, such as

interference effects from large metal structures or nearby

magnetic fields.

F. Anemometer and Weather Vane

Both weather instruments used are originally from a

kit imported by Argent Data Systems as the “Weather Sensor

Assembly p/n 80422 [9]. The basic unit comes with a rain

gauge which will not be used.

The wind vane uses a voltage divider type of sensor

and measures wind direction in increments of 22.5 degrees.

The anemometer uses a reed switch which is

activated by a magnet on the rotating cup assembly once

every revolution. A simple frequency measurement by the

AxonII converts the number of pulses into wind speed.

G. Physical Setup and Wiring

One major concern for anything operating on water

is waterproofing and resistance to corrosion. To remedy this,

most of the electronics are housed in a waterproof Pelican

1120 Case, with the exception of the GPS antenna and wind

instrumentation. Figure 1: IMU data acquisition

packageshows a view of the assembled device.

Figure 1: IMU data acquisition package

H. Computer interface

The AxonII uses a Silicon Labs CP210x USB to

UART bridge chip to convert its lower level UART signals to

a more convenient USB format. On the computer end, drivers

create a virtual COM port, which Windows PCs treat like an

older style 9-pin (RS-232) serial port [4].

Currently, the AxonII handles the conversion of all

input signals to their respective engineering units. At a

chosen interval, it samples all signals and outputs them in a

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string format to the USB to UART bridge and ultimately to

the PC.

The current testing setup uses Microsoft

HyperTerminal to open a connection with the AxonII and

displays the output strings.

RESULTS

A simple functionality test was devised where the

GPS antenna was set in a fixed position while the IMU was

rotated about all three axes. This established that the

accelerometer can properly track the direction of gravity and

the gyroscopes detect rotation. The magnetic compass

reading was also compared to a traditional compass and

results were shown to be accurate.

Sample results outputted by the AxonII

microcontroller as received by Microsoft HyperTerminal are

shown inbelow.

Table 2: Sample Results

Time:+162803.0000

Latitude(radians):+0.794115082

Longitude(radians):-1.316378939

Speed(knots):+0.030000000

Track(Deg):+194.7899933, Bearing= 215 Deg

Ax= -61 mG, Ay= -2 mG, Az= 976 mG

Gx= 402 Deg/s, Gy= 1157 Deg/s, Gz= -802 Deg/s

Time, latitude, longitude, speed and track are given

in the raw GPS NMEA format. The units for time are seconds

according to the GPS satellite clocks. The bearing is from the

magnetic compass, while the track is from the GPS device.

The A and G values are both linear acceleration and angular

velocity respectively. The units for acceleration are shown as

thousandths of the gravitational constant (mG).

As previously stated, the GPS device was stationary

for the test; therefore its speed reading is more indicative of

drift/measurement errors between readings. The same is true

of the track measurement and also accounts for the large

discrepancy between it and the magnetic compass bearing.

DISCUSSION

At this stage in the project, the data acquisition

package successfully relays the data from all the required

sensors to a computer and seems to fulfill its intended

function. Expanding on this concept is the development of a

computer interface which incorporates a recording feature.

Other possibilities are display features to allow the user to

visualise the incoming data graphically.

Validation testing is also planned for all of the

sensors, to compare their readings against a known value.

This will determine the accuracy and inherent errors for all of

the instruments.

CONCLUSION

A data acquisition capable IMU device can be an

invaluable tool to the sailor as a performance enhancing

device. Other engineering applications of such a device are

widespread in the fields of hull and rigging design as well as

materials optimisation. This study has shown that it is

possible to construct such a device using low cost

commercial off-the shelf components and capture

measurements data successfully.

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