Miniature Underwater Glider: Design, Modeling, and Experimental Results Feitian Zhang, John Thon, Cody Thon and Xiaobo Tan Abstract— The concept of gliding robotic fish combines glid- ing and fin-actuation mechanisms to realize energy-efficient locomotion and high maneuverability, and holds strong promise for mobile sensing in versatile aquatic environments. In this paper we present the modeling and design of a miniature fish-like glider, a key enabling component for gliding robotic fish. The full dynamics of the glider is first derived and then reduced to the sagittal plane, where the lift, drag, and pitch moment coefficients are obtained as linear or quadratic functions of the attack angle based on computational fluid dynamics (CFD) analysis. The model is used to design the glider by accommodating stringent constraints on dimensions yet meeting the desired specification on speed. A fully untethered prototype of underwater glider is developed, with a weight of 4 kg and length of 40 cm. With a net buoyancy of 20 g, it realizes a steady gliding speed of 20 cm/s. The volume and net buoyancy of this glider are less than 10% and 5%, respectively, of those of reported gliders in the literature, and its speed per unit net buoyancy is over 9 times of those other vehicles. Experimental results have shown that the model is able to capture well both the steady glide behavior under different control inputs, and the dynamics during transients. I. I NTRODUCTION There is a growing interest in monitoring aquatic envi- ronments with autonomous underwater robots. Application examples include patrolling seaports, tracking oil spills, and monitoring harmful algal blooms, to name just a few. To be feasible for such applications, the robots need to be highly energy-efficient to maintain sustained field operation, and at the same time be highly maneuverable to navigate versatile environments such as ponds, lakes and rivers [1]. Underwater seagliders are known for their great energy-efficiency and long-duration operation in oceanographic applications [2]. An underwater glider utilizes its buoyancy and gravity to enable motion without any additional propulsion, and ad- justs its center of gravity to achieve certain attitude, which results in glide and thus horizontal travel. Since energy is needed only for buoyancy and center-of-gravity adjustment when switching the glide profile, underwater gliders are very energy-efficient, as proven by the great success of the Seaglider [3], Spray [4] and Slocum [5]. The maneuverability of underwater gliders, however, is quite limited. The large size (1– 2 m long), weight (50 kg and above), and cost of This work was supported in part by NSF (IIS 0916720, ECCS 1050236) and ONR (Grant N000140810640). F. Zhang, J. Thon, C. Thon and X. Tan are with the Smart Mi- crosystems Laboratory, Department of Electrical and Computer Engineer- ing, Michigan State University, East Lansing, MI 48824, USA. Email: zhangzft@msu.edu (F. Z.), jthon@hpsk12.net (J. T.), cdthon@gmail.com (C. T.), xbtan@egr.msu.edu (X. T.). Send correspondence to X. Tan. Tel: 1-517-432-5671; Fax: 1-517-353- 1980. these vehicles also impede their adoption in environments such as ponds and inland lakes. On the other hand, over the past two decades, there has been significant interest in developing robots that propel and maneuver themselves like real fish do. Often called robotic fish, they accomplish swim- ming by deforming the body and fin-like appendages [6]– [12]. While robotic fish typically have high maneuverability (e.g., small turning radius), they require constant actuation for swimming and cannot work for extended periods of time without battery recharge. The concept of gliding robotic fish [1] combines the desir- able features of both an underwater glider and a robotic fish. Such a robot would realize most of its locomotion through gliding and thus be energy-efficient. However, it would be much smaller than a traditional underwater glider, and use actively controlled fins to achieve high maneuverability. Of course, fins can also provide additional propulsive power during locomotion, if needed. Given that fin-actuated robotic fish have been demonstrated by a number of researchers, the key challenge in developing gliding robotic fish is to realize a miniature underwater glider that can be readily integrated with fin-actuation mechanisms. In this paper, we present the design and modeling of a miniature underwater glider, and report, to our best knowl- edge, the smallest untethered glider that has been demon- strated in the literature. The glider has two independent actuators to realize the shift of center of gravity and the change of net buoyancy, by displacing a movable mass and pumping fluids, respectively. The dynamic model and the subsequent steady glide model are used to design and select the actuation mechanisms to meet the size and weight constraints, and to meet performance specifications. For example, from the model, we find that, if the center of gravity is just slightly below the center of buoyancy, the required pitch can be achieved with minimal movement of the internal mass, contributing to energy saving. Based on the design, we have successfully developed a prototype of miniature underwater glider. Measuring only 40 cm long and weighing 4 kg, the glider has demonstrated desired glide profile and speed perofrmance. For example, with a net buoyancy of 20 g only, it achieves a glide speed of about 20 cm/s. We have conducted extensive experiments to investigate the steady gliding performance and transient dynamics of the glider, and both have been shown to match the mdoel predictions well. 2012 IEEE International Conference on Robotics and Automation RiverCentre, Saint Paul, Minnesota, USA May 14-18, 2012 978-1-4673-1404-6/12/$31.00 ©2012 IEEE 4904