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
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.
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.
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.
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.
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 “realtime” information. All of which could be used to improve the overall performance of a given boat and crew combination.
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.
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.
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
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Global position Global heading Planar velocity
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 twodimensional.
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.
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
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.
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.
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. 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.
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].
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.
Two gyroscopes provide angular measurements, a 2axis 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].
A summary of the variables of interest is given in Table 1: Variables of Interest, along with the necessary sensor to measure it.
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.
Table 1: Variables of Interest
Variable 3-axis linear acceleration 3-axis rotational acceleration Magnetic heading Wind speed Wind direction
GPS GPS GPS
Sensor Accelerometer Gyroscope Compass Anemometer Weather Vane
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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Assembly p/n 80422 [9]. The basic unit comes with a rain gauge which will not be used.
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 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.
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].
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.
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.
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].
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
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.
This will determine the accuracy and inherent errors for all of the instruments.
The current testing setup uses Microsoft HyperTerminal to open a connection with the AxonII and displays the output strings.
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.
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.
REFERENCES
Sample results outputted by the AxonII microcontroller as received by Microsoft HyperTerminal are shown inbelow.
[1] Story Lead: Financial. Volvo Ocean Race Offical Press Information. [Online] 09 2008. [Cited: 11 17, 2010.] http://press.volvooceanrace.com/?p=142#more-142. [2] Skene, N. L. Elements of Yacht Design. 6th Edition. s.l. : Sheridan House, 2001. [3] Larsson, L. & Eliasson, R. Principles of Yacht Design. 3rd Edition. s.l. : McGraw-Hill, 2007. [4] Palmisano, John: Society of Robots . AxonII. Society of Robots. [Online] [Cited: 11 24, 2010.] http://www.societyofrobots.com/axon2/. [5] Analog Devices. ADXL335: Small, Low Power, 3-Axis Âą3 g Accelerometer. Analog Devices | Mixed-signal and Digital Signal Processing ICs. [Online] 01 2010. [Cited: 11 24, 2010.] http://www.analog.com/static/importedfiles/data_sheets/ADXL335.pdf. [6] Sparkfun.com. IMU 6DOF Razor - Ultra-Thin IMU. Sparkfun Electronics. [Online] [Cited: 11 24, 2010.] http://www.sparkfun.com/products/9431. [7] onshine.com.tw. GPS Active Antenna ANT-555 Datasheet. http://php2.twinner.com.tw. [Online] [Cited: 11 24, 2010.] http://php2.twinner.com.tw/files/onshine/ANT5552006-NEW.pdf. [8] SSEC.Honeywell. Honeywell - Magnetic Sensors Data Sheets. Honeywell Microelectronics and Precision Sensors. [Online] 01 2006. [Cited: 11 24, 2010.] http://www.ssec.honeywell.com/magnetic/datasheets/HMC63 52.pdf. [9] Argent Data Systems. Weather Sensor Assembly p/n 80422. Argent Data Systems. [Online] [Cited: 11 24, 2010.] http://www.sparkfun.com/datasheets/Sensors/Weather/Weath er%20Sensor%20Assembly..pdf.
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.
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