Final Design

The SPAWAR GPS Tracking Buoy project centered on the incorporation of a surface buoy and a large underwater structure (called a drogue) designed to be carried with ocean currents. These two elements together simulate the descent characteristics of contaminants in open water and record experimental data. The drogue lowers itself from the surface buoy at a descent rate identical to the contaminants under study. Upon reaching the ocean floor, the drogue records its position and ascends until it reaches the surface. The buoy, which is equipped with a satellite GPS transmitter, then broadcasts its location to identify the area where the contaminant plume has settled.

This project focused on the development of the winching mechanism that lowers the drogue from the surface buoy and the sensors that determine when the drogue has reached the ocean floor. The robotic winching system consists of a mechanical line-spooling system and a waterproof pressure case that will house an actuator, a controller, batteries, primary sensors, and auxiliary sensors. The purpose of this winching mechanism is to autonomously control the descent speed of the drogue.

Our final design consists of four main sections, as seen in Figure 1.

Figure 1: Final Winch Design

Figure 2 displays the sensor endcap chassis, which is located on the bottom end of the housing. It contains the sonar sensor as well as the batteries and microcontroller. The two lithium-ion batteries provide 7.4 V and 20.8 Ah of power for the entire system. The Arduino microcontoller stack consists of an Arduino Uno, microSD card shield, and a motor shield. The microSD shield stores data from the motor encoder, sonar, and any additional sensors. The motor encoder data is used to calculate the depth of the drogue by relating the shaft position to the amount of line released. The sonar sensor detects the bottom and tells the system to begin ascending. After retrieval, the encoder data can be compared with the GPS data to pinpoint the final settling location.

Figure 2: Sensor Endcap Chassis Design

Figure 3 shows the motor endcap chassis, which is located on the top end of the housing. The geared motor is attached by means of a rigid aluminum coupler to a 316 stainless steel shaft, which also functions as the spool for the line. The rotary shaft seal prevents water from leaking around the rotating shaft. The ball bearing carries any lateral load that the motor shaft experiences, thus protecting the motor and keeping it aligned.

Figure 3: Motor Endcap Chassis Design

The level wind system, shown in Figure 4, works like a fishing reel, consistently winding the line to lower or raise the drogue. It is located on top of the motor endcap and spools the line evenly by controlling the linear motion of a line guide. Consistency is important in order to accurately determine the descent speed of the system, which is directly related to the angular velocity as well as the diameter of the shaft and any layers of line. The PID controller then compensates for diameter changes to keep the speed constant.

Figure 4: Level Wind System Major Components

When the winch is placed within the SVP Lagrangian Drifter, the entire system can be modeled as seen in Figure 5.

Figure 5: Drifter System

Our assembly will attach to the drogue on the top and bottom using the PVC pipe coupler and the plastic holding ring, respectively. Because our winch is shorter than the height of the drogue, an extension is necessary to properly secure it.

Major Components:

Housing:

The housing encased all of the electronic components. It consisted of 2 major components, endcaps and the housing. A cylindrical housing is an oceanographic standard so it was the basis for our design. This shape is efficient at distributing load, and allows for the use of O-rings. The endcaps were made out of delrin due to the materials strength. Dual piston o-rings seals were incorporated into the endcap in order to seal against the housing, see figures 2 and 3, and was chosen due its its minimalist design. To secure the endcaps, screws put through the housing and screwed directly into the endcap. This lead to efficient assembly.

Motor:

The Pololu 172:1 DC Geared Motor, depicted in Figure 6, was selected over other motors because of the attached encoder, its high torque rating, and its low power usage. The benefit of having an attached quadrature encoder allows the accurate reading of the rotation of the output shaft. With a 48 CPR (cycles per revolution) rating, the encoder is able to translate one revolution of the output shaft to 8246 bits thus providing an extremely high resolution of the output shaft's cycles. The motor is rated for a torque of 1.2 Nm, relatively close to our previously assumed maximum torque allowed of 2.2 Nm. Compared to other motors of similar torque output, the Pololu DC Geared motor consumed significantly less power. The only other motor with similar torque outputs and accurate tracking was the Pololu Stepper motor that required 5.6 A compared the the DC geared motor's required 2.2 A. The DC geared motor was the logical choice for the final design of the winch.

Figure 6: Pololu 172:1 DC Geared Motor

A PID control system is utilized to control the rotation of the shaft by taking advantage of the 8246 bits per revolution quadrature encoder attached to the back of the motor. A continuously changing variable was first created to be the target revolution of the output shaft. The motor, via the PID controller, then attempts to reach the target as closely as possible as it changes over time. With the motor alone, the PID controller has been able to track the desired target accurately to an average error of 1.16 bits.

Battery:

The final choice for the battery was a Tenergy 7.4V 10,400mAh lithium-ion battery priced at $57.99, which can be seen in Figure 7. A lithium-ion chemistry was chosen since it was the most energy dense of all options, and considering the high capacity requirements of the system, would be ideal for maintaining a smaller pressure case since the battery was the biggest contributor to the mass and size of the system. Additionally, this battery’s dimensions (56x56x68mm) were ideal for the pressure case, and two batteries could be stacked lengthwise and connected in parallel in order to double the capacity to 20,800mAh for a factor of safety of approximately 2 for total available energy.

Figure 7: Tenergy lithium-ion battery

Microprocessor:

The Arduino Uno was chosen as the motor controller and data acquisition. An Arduino was chosen because of its simplicity, its open-source design, and the large amount of community support. Arduino products have been used for prototype designs and by hobbyists of all levels. It allows for new users to learn quickly and old users to push the limit of the microcomputer. The motor shield was necessary to provide enough power to the motor and the microSD card shield will be used during the actual runs to store sensor data.

Figure 8: From left to right - Arduino Uno, motor shield, and microSD shield

Line:

To connect the surface buoy to the underwater drogue, we chose a braided spectra line made by Spiderwire, which has a break strength of 356 N and a diameter of .43 mm. We originally planned to have a line with a .84 mm diameter, which was the largest diameter that maintained the 40:1 drag area ratio. From this, we designed the level wind gear ratio so that every complete revolution moved the guide .84 mm along the shaft. However, testing showed that the line would flatten under tension, causing the line to overlap and not wind consistently. To account for this, we changed to the smaller line shown in Figure 7. Even so, this was sufficient for our project because the line was not meant to bear a large load; it was only there to connect the drogue to the buoy. The buoy followed the drogue with little resistance and subjected the line to minimal tension, especially considering that the drogue was close to neutrally buoyant at around 8.9 N of wet weight. Clearly, the line rated for 356 N is strong enough for the average operating conditions. However, the extra strength is necessary to deal with any impulse forces that the system may encounter in the ocean.

Figure 9: SpiderWire Braided Spectra Line