AquaMonkey: A Novel Multi-Mode Robotic Vehicle

Christopher M. Keegan Bradley E. Bishop

Weapons & Systems Engineering Department United States Naval Academy Annapolis, MD 21412 USA

bishop@usna.edu Abstract—This video presents results from an undergraduate research project involving the design and analysis of a lightweight multimode vehicle capable of driving on solid ground, swimming on the surface of the water, climbing up magnetic surfaces and easily transitioning between the three. Locomotion in and out of the water is provided by buoyant paddlewheels on each side of the vehicle. The climbing capability is achieved by the use of strip magnets mounted to each wheel. The physical design of the robot provides efficiency in and out of the water while considering a necessity for stealth, compactness, and minimal weight. This project has been designed with the expectation of adding autonomy to the vehicle.

I. INTRODUCTION

Novel locomotion schemes in mobile robot systems have seen a great deal of work in recent years. From novel land-based locomotion to systems with climbing abilities, robots are being designed to carry sensor packages in an expanding array of new, challenging environments. The focus of this work is to consider extending new locomotive capabilities to a device that can transition on the surface of the water.

A great deal of effort has been devoted to developing underwater robotic systems, but autonomous surface vessels lag far behind their submersible counterparts, as well as land- and air-based systems. While autonomous boats are becoming more common, little effort has been devoted to considering novel locomotive capability on the surface of the water. In this video, we demonstrate the AquaMonkey, a two-wheeled robotic system designed to travel on the surface of the water, overland, and even to climb on ferrous surfaces. We envision eventual use of these robots for reconnaissance, security, etc. using techniques from swarm control research [1].

II. BACKGROUND

The AquaMonkey’s basic two-wheel differentially-

driven body design is inspired to a great extent by the work done at the University of Minnesota. The “Scout” has been developed at the University of Minnesota in conjunction with MTS, Honeywell, and ATC [2]. These small versatile robots have cylindrical bodies with differentially-driven wheels on either end, and are equipped with a stabilizing tail that is crucial to locomotion. The Scouts have been

designed to carry out a number of tasks using an assortment of sensors, and have been programmed work alone or to communicate with one another in a cooperative team [2].

A wide array of Scout configurations have been developed, but no amphibious or climbing Scouts have been produced. The AquaMonkey has a unique design in that it will be operating in both land and aquatic environments in addition to climbing ferrous surfaces, even though it uses the same basic two-wheel differential morphology as the Scout.

In order to employ magnetic climbing, as shown in Fig. 1, AquaMonkey uses a design inspired by the Climbing Mini-Whegs [3] of Case Western Reserve University. The Climbing Mini-Whegs robot uses the wheg wheel/leg concept together with adhesive strips to climb up vertical and angled surfaces. Instead of adhesive strips, the AquaMonkey uses strip magnets for climbing, and has wheels designed to be used as paddles in the water, even with the magnets attached.

Fig 1. AquaMonkey climbing

III. STRUCTURAL DESIGN

The AquaMonkey design was intended to be lightweight, simple and robust, even with the added challenges of implementing amphibious operation and climbing. The body of AquaMonkey is made of lightweight plastic tubing containing the electronics and motors. An end cap fits snugly onto ether side of the tube, creating a watertight seal. The actuators used for the robot are high torque, low weight R/C servomotors that have been modified for continuous rotation.

Proceedings of the 2007 IEEE/RSJ InternationalConference on Intelligent Robots and SystemsSan Diego, CA, USA, Oct 29 - Nov 2, 2007

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The wheel design for the AquaMonkey is an extremely important component of the overall capability. Besides regular land-based locomotion, the wheels provide the buoyancy required for AquaMonkey to float and also provide sufficient motive force, through paddling action, for movement across the water, as shown in Fig. 2. The paddle wheels are made from high density foam, providing excellent buoyancy and strength with very little weight. The paddle wheels have a diameter of 7.95 inches and are 4.00 inches thick. Each of the eight paddles on the wheel has a width of 1.50 inches. The design of the paddle wheels was the result of extensive testing and recourse to texts on paddle wheel design [4].

Fig 2. AquaMonkey Swimming Extensive research has been done regarding the

performance of the servo motors and paddle wheels without the magnets attached [5]. With the addition of the magnet system, additional analyses were required.

The quantity that defines the efficiency of a paddle wheel is called slip. Slip is essentially the difference in efficiency of the wheel on land and in the water. Generally, a slip of 20% or less is considered good for a well designed paddle wheel. The slip for AquaMonkey’s wheels without the strip magnets is 59.3% [5]. With the magnets, the efficiency is better if paddling in the direction in which the magnets point (~35% slip), and slightly worse (~65% slip) if paddling so that the magnets lie along the surface of the wheel (as they would when climbing). Fig. 2 shows the AquaMonkey in the best pose for water-based locomotion, away from the tail as seen in the figure.

IV. CLIMBING

As mentioned, the AquaMonkey uses highly saturated

strip magnets for climbing. These magnets have a pull strength of ~20 lbs/foot normal to the contact surface. Magnetic adhesion was selected for the AquaMonkey in order to provide a climbing methodology that would not be impeded by wetting of the climbing surfaces or fouling caused by sand, dirt, or other loose material through which the robot may transition. Of course, the actual holding force tangential and normal to the surface will involve a variety of factors, including frictional characteristics, quality of magnetic conformance to the surface, amount of magnet

contact, etc. In practice, the strip magnets proved very capable in holding the system in a variety of conditions.

As discovered in the development of the Climbing Mini-Whegs [3], tail length is important for climbing capability. In the two-wheel configuration of the AquaMonkey, this is even more true. Analysis of the forces acting on the device indicates that the length of the tail is inversely proportional to the ‘rear-off’ effect caused by the tail’s counteraction of the torque generated for the wheels. All else being equal, a longer tail will allow the device to climb a steeper slope without the magnets breaking free of the surface. Unfortunately, the tail needs to be kept short to limit weight and provide agility in cluttered terrain. The designed length, approximately one foot, currently provides the best simultaneous solution to these two problems.

V. CONTROL

Currently, the AquaMonkey is controlled remotely by a

human operator, but the final AquaMonkey prototype will include some autonomous capabilities. Currently, work is being done to implement GPS-based navigation. Additionally, a digital accelerometer will be used to determine orientation. to allow the system to take full advantage of its locomotive capabilities in and out of the water, as discussed above.

VI. CONCLUSIONS

The AquaMonkey is an ongoing project. Although, the robot can paddle through the water, drive on rough land, and climb up ferrous surfaces, as well as easily transition between these modes, there is still work to be done with its climbing and control system. Currently, AquaMonkey can only climb an angle as steep as 80o. For optimal mission capabilities, it should be able to climb up perfectly vertical surfaces, or even surfaces with an acute angle in relation to the water (the hull of a ship, for instance). Obtaining stronger magnets or modifying the coating on the strips may help to reach this objective.

VII. REFERENCES

[1] B. E. Bishop, “Control of Platoons of Nonholonomic Vehicles Using

Redundant Manipulator Analogs,” ASME Journal of Dynamic Systems, Measurements and Control, Special Issue on Novel Robotics and Control, Vol. 128, No. 1, March 2006, pp. 171 – 175.

[2] S. A. Stoeter, I. T. Burt and N. Papanikolopouslos, “Scout robot motion model,” Proceedings of the 2003 IEEE International Conference on Robotics and Automation,Sept. 2003, pp 90 – 95.

[3] K. A. Daltorio, A. D. Horchler, S. Gorb, R. E. Ritzmann, R. D. Quinn. A Small Wall-Walking Robot with Compliant, Adhesive Feet. Proceedings of the 2005 International Conference on Intelligent Robots and Systems. Aug. 2005, pp. 3648 - 3653.

[4] J. H. Biles, The Design and Construction of Ship, London, Charles Griffin and Company, 1911.

[5] C. M. Keegan, B. E. Bishop. The Sea Scout: A Novel Multi-Mode Autonomous Vehicle. Proceedings of the 39th Southeastern Symposium on Systems Theory, March 2007, pp. 11 – 15.

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