Duke University, Pratt School of Engineering

Duke University, Pratt School of Engineering

Department of Mechanical Engineering and Materials Science

Autonomous Underwater Vehicle Mechanical Design

ME160

Dr. Robert Kielb

Spring 2008

Advisor: Marc S. Verdiel

Mark Peterson

Brandon Guard

Andrew Camacho

Brayden Glad

David Klein

Abstract

The goal of this project was to design and fabricate multiple components to be utilized in an Autonomous Underwater Vehicle (AUV). It is our intent that with the help of other members of the Duke Robotics Club from the ECE/CS departments the AUV will compete in San Diego at the end of July. The scope of the project included the chassis, mechanical claw, and marker dropper. It was our aim to improve the previous designs of the chassis and marker dropper and the mechanical claw was incorporated as a new task for the upcoming competition. The tasks that must be completed during the competition involve both vision as well as acoustic software in concert with a durable mechanical structure. A layout of the testing pool is shown in Appendix 5.

Problem Definition - Chassis

Purpose

The AUV competition has been attended by the Duke Robotics club for the last eight years. Along the way, three vehicles have been designed and manufactured. The chassis that was used most recently is still fully functional and only requires the replacement of a couple cards on the computer stack. However, the cylindrical shape of the electronics housing on the previous model did not lend itself to the installation of the newly acquired Doppler Velocity Log (DVL). The DVL is a key component of the AUV providing it with the ability to move in an x-, y-, z-coordinate system once a point of origin is established in the water. The top AUVs in the competition are getting lighter and smaller so reducing the weight of the vehicle was also important. Past chassis designs used a larger, heavier DVL so incorporating the new DVL was crucial in our decision to build a new chassis.

Current Standard

The goal of this project is not to produce an AUV that is up to the industry standard since most of these vehicles are designed by the military or large engineering firms and are far beyond the scope of any undergraduate project. The current standards that our design team is basing the project off of are the previous AUVs built by the Duke Robotics Club as well as the other top competition from schools like The University of Florida (UF) and École de Technologie Supérieure (ETS). A CAD of the previous year’s vehicle can be seen below:

Figure 1: CAD of previous AUV

Expert Consultation

For the AUV, most of the expert consultation was received from the professional machinists. One of the things that must be understood when going into a design project involving components that must be water tight is the difference between the ability to design and the ability to fabricate that component. Past vehicles had made use of cylindrical plastic tubes which were easily made water tight using a plastic weld and/or an end cap sealed with o-rings. The new chassis would be rectangular in shape so it was necessary to consult with Don Pearce from the medical machine shop to determine how this could be accomplished.

Preliminary Design Work - Chassis

Product Design Specifications

The physical requirements of the chassis are that it remains water tight up to approximately two atmospheres of pressure (approximately 30 ft), it must avoid entangling with the external environment of the testing pool, it must be able to operate in chlorinated and saline water, it must fit in a 1.83x0.91x0.91 meter box, and it must be at least 5% positively buoyant. It is also a goal that the overall weight of the vehicle be minimized from the previous year’s model which weighed 36 kg.

Quality Function Deployment (QFD)

The QFD performed on the chassis can be viewed in Appendix two. The information gathered from this calculation was vital in determining the shape of the chassis that will be housing the computer, DVL, and various other electronics. The three shapes that were compared in the QFD were a rectangular box design, the cylindrical electronics tube used last year, and the flooded parabolic shell design that had been employed in the AUV from two years ago. Some of the key parameters were the relative shape of the components stored inside the chassis to the chassis itself. Since most of these components are rectangular in shape, optimal space usage would favor the rectangular chassis. Also of importance was the weight of the vehicle. The competition disqualifies all vehicles weighing over 50 kg and gives a bonus to those vehicles that weigh less than 38 kg. Thus, a design that optimizes the volume of the chassis is ideal. Given that we have already noted the rectangular shape provides optimal packing, the rectangular chassis is again favored. The last parameter that is important to mention is the ease by which the new lightweight DVL can be attached. The cylindrical shell does not offer a conceivable solution to this problem nor does the parabolic shell design.

Design Decision

Given the results found in the QFD exercise, it was determined that the rectangular chassis design would be optimal for the continuing improvements that have been made to the AUV project in the last few years. The chassis will be less bulky and more lightweight with the incorporation of the new DVL and the increased space efficiency. Further additions will need to be made to the chassis to improve the hydrodynamics of the AUV however this is beyond the scope of this design project.

System Modeling - Chassis

Stability and Buoyancy Calculations

One of the most important calculations required for the AUV is the buoyancy and stability of the vehicle. In order to determine these two values, an excel spreadsheet was first set up that determined the total weight and buoyancy of each component being attached to the chassis. The results of this can be seen below. A buoyancy force of zero indicates that a component is located inside the chassis and therefore is already being accounted for in the total buoyancy of the chassis.

Table 1: Totaled Weight and Buoyancy for Chassis Assembly

By performing these calculations it was possible to determine the total weight as well as the net buoyancy of the vehicle. Given that the dry weight of the vehicle at the competition last year was 36 kg, the new chassis design has stripped approximately 6.5 kg off of the previous model. Also, there is a net buoyancy force of 6.25 N ensuring that the AUV will return to the surface if there happened to be a loss of power.

In order to perform the stability calculations for the AUV in terms of the pitch and roll degrees of freedom, an (x,y) origin was set at the center point of the bottom plate as seen in the drawing below. The +Y axis runs vertically upward and the +X axis runs horizontally to the right from the origin. After determining the center of mass for each individual component and multiplying that by the weight of the component, the total X- and Y-moments can calculated. Next, by dividing the moments by the total weight of the vehicle the X and Y center of mass for the AUV is found. Considering the origin is at the center of the base plate, X and Y center of mass values close to zero will ensure the vehicle’s stability with respect to pitch and roll.

Figure 2: Top View of Chassis Assembly

Table 2: X- and Y- center of mass for chassis assembly

The stability of an underwater vessel is also determined by the relative locations of the center of mass and the center of buoyancy in the Z-direction. Another spreadsheet was used to determine these values with the origin set at the top of the base plate as seen below. The +Z axis runs vertically upward and the +Y runs horizontally to the left from the origin.

:

Figure 3: Front View of Chassis Assembly

Table 3: Z-center of mass and Z-center of buoyancy

The calculations show that the Z-center of mass is approximately 3.2 inches below the Z-center of buoyancy. This ensures that the AUV remains stable and is not prone to rolling.

Finite Element Analysis

A fluid dynamics analysis using Solidworks FloWorks was conducted on the entire chassis, using a gliding (motors off) velocity of 1 m/s forward in water. As is visible from the table below, the pressure change over the body is negligible at this speed, which represents more than the maximum velocity of the vehicle. The maximum pressure increase is about 2 kPa, and the wake area generates a pressure drop of a further 7 kPa. From the images, recirculation effects are noticeable but are apparently minor.

Table 4 Minimum and Maximum Values for Chassis gliding at 1 m/s in water

Figure 4:Velocity Profile: A front view demonstrates the effect of the forward-facing fins in forcing fluid over the top of the craft.

Velocity Profile: A side view illustrates the flow over the claw assembly and along a thruster.

Prototype Construction

Because the tolerances involved in fabricating a water tight vessel are so small, our design team found it necessary to have the chassis machined by a professional machinist. Features of the chassis include two view ports machined from polycarbonate, a hole through which the DVL is mounted and made water tight, flanges that run along the base plate as well as the front face of the chassis to which external components are mounted, and a top plate that will be latched to the sides of the chassis using nonlocking screw latches allowing the tension in the clamp to be regulated. An image of this mechanism is shown below:

Figure 5: Nonlocking Latch From mcmaster.com

Aluminum was chosen as the material used for the exterior of the chassis because of its durability and light weight. Other than the low tolerances required for an underwater vessel, it is also difficult to weld aluminum thus providing us with another reason to have the chassis machined professionally. There was an unforeseeable delay in the completion of the chassis so it will be necessary to complete the assembly over the summer.

Problem Definition – Marker Dropper

Purpose

As stated in the AUVSI and ONR’s 11th International Autonomous and Underwater Vehicle Competition, one of the challenges that the AUV must complete is to place a marker within a 12” by 24” box (See Figure 1). There are four possible target bins that the markers can be dropped into, but only two markers may be dropped during the competition. Each bin will have a different point value associated with it, depending on its color or another distinguishing characteristic.[1]

Figure 6 - Marker Box Diagram

The competition rules further state that the marker must fit within a 1.5” by 1.5” by 6” box and weigh no more than 1.5 pounds while in air. Each marker must bear the team name or marking to be differentiated from the other markers that may be in the target bin. Any marker that exceeds these limits by less than 10% will incur a significant point penalty, while any marker that exceeds the limits by 10% will be disqualified.[2]

Current Solution

Last year’s AUV was equipped with a single marker dropper system. It operated through use of a permanent magnet that would be jolted with a pulse of electricity to release a metal ball from its attractive power. This mechanism affected the AUV’s positioning sensor.

It weighed approximately one kilogram and the solid steel ball used as a marker weighed approximately 20 grams.

Proposed Solution

The new competition rules meant that the old marker dropper could not fulfill all the requirements necessary to receive full points in the contest. The proposed solution not only improves accuracy and precision over last year’s model, but also contains sufficient room for additional markers if needed in future competitions.

In the new marker dropper, a cartridge with six holes for markers, 60o apart, is rotated by a stepper motor that turns 7.5o each pulse. Each step rotates the marker cartridge towards a stationary hole in the base of the marker dropper, with eight steps needed to go from one hole to the next. Once the cartridge hole is over the stationary hole, the marker in the cartridge hole will fall through and travel through the water and land in the target bin.

This solution should weigh less than the previous model due to the lighter materials needed for this proposed design.

Information Gathered – Marker Dropper

Existing Products

While there are few, if any marker droppers that have been designed with the specific needs that the International Autonomous and Underwater Vehicle Competition have set out, there are many products out on the market that drop material to a central point or area. Examples include gumball machines and vending machines. However, for this ME 160, project, these products would not efficiently fulfill the contest regulations.

Figure 7 - Gumball Machine and Vending Machine[3][4]

A design based on either a gumball machine or a vending machine would have the advantage of being able to hold many markers. However, a gumball machine does not pack gumballs in an organized or efficient manner that this competition demands. Also, a gumball seems to work best with spherical markers, whose path through water can be hard to predict. A vending machine packs its internal products in a very ordered manner, but its drop point is very irregular, which leads to poor accuracy.

More applicable and practical existing products are ones that focus primarily on accuracy and precision. A very recognizable example of this currently on the market is the revolver. Every bullet is shot from the same position in a repeatable motion. A revolver also loads the bullets in a very ordered and efficient manner.

Figure 8 – Revolver[5]

Preliminary Design Work- Marker Dropper

Product Design Specifications (PDS)

In order to conform to the rules set out by the International Autonomous and Underwater Vehicle Competition, as well as to try and be as efficient as possible in design, some overall goals were established.

Performance Goals:

The marker dropper must drop at least two markers in a simple, repeatable motion; however, if the dropper can release more than two markers, it is more advantageous. The markers must be dropped from the same point each time to maximize accuracy.

Physical Requirements:

In accordance with the International Autonomous and Underwater Vehicle Competition rules, the markers must be able to a 1.5” by 1.5” by 6” box and weigh no more than 1.5 pounds. The marker dropper should also be neutrally buoyant and small, so that it can be placed anywhere on the AUV.

The marker dropper must also be waterproof to at least 20 meters and must function for a minimum period that will last for the entire contest (approximately 10 hours). However, a long lifetime, of four years is preferred.

Quality Function Deployment

The QFD is a tool used to compare customer requirements with the engineering characteristics of a design. Results from this tool are used to weigh the importance of consumer concerns and how they are correlated to considered design elements. In the QFD used for the marker dropper, the design options included: a circle of compartments which turn (six-shooter); spiral rotation flips a catch (vending machine); spring action loads next round (pistol clip). These three designs were compared using the following criteria: ability to drop different sized markers; ability to drop multiple markers; weight; size; cost; style; hydrodynamics. Correlations between the design options were weighted on a -3, -1, 0, 1, 3 scale (-3=terrible, -1=weak, 0=average, 1=good, 3=excellent).

The six-shooter design focuses on rotating a hole to each marker, allowing it to fall through the hole towards the target bin. Depending on its construction, it could handle many different sizes and numbers of markers, but once built, it would be difficult to change the design to accommodate a change in marker design. It would be cheap and lightweight depending on the materials used, but would probably not look very nice and would not be very hydrodynamic.

The vending machine-inspired design would contain a rotating screw that would flip a catch at specific intervals to drop the marker into the water. This seemed very difficult to create and while very versatile, the difficulty in creating it outweighed any advantages it might have.

The pistol design would have a clip of markers that would be pushed out one by one from behind by a piston. The next marker would move into place immediately due to a spring that would be pushing all the markers towards the opening. This design could hold a large amount of markers, but the size and shape of the marker would be limited.

The QFD results show that the simpler design of the six-shooter pushed it over the pistol clip design, as it would be easier to control a stepper motor rotating a hole from place to place over a piston pushing a marker through a hole. The uncertainty of how a marker would perform under the compressing stress of a piston also made a gravity-induced fall more preferable.