Flapping Wing Miniature Air Vehicles
S.K. Gupta, Arvind Ananthanarayanan, Wojciech Bejgerowski, Hugh Bruck, Li-Jen Chang, John Gerdes, Tobias Karch, Dominik Mueller, Guru Ramu, Philippe Wandji, and Stephen Wilkerson
This project is sponsored by U.S. Army Research Laboratory and SMART scholarship.
Micro air vehicles, flapping wing flight, ornithopter, bird
Miniature air vehicles provide unique benefits relative to larger unmanned aerial vehicles. Several performance properties scale favorably as size is reduced including structural strength and maneuverability. Due to very light weight and small size, miniature air vehicles are easily portable and deployable, while costs are low enough to be considered disposable. Unique flight capabilities such as hover, very low speed flight, small radius turns, and short take-off and landing are possible with miniature air vehicles. A key benefit of flapping wing flight is low frequency wing flapping, enabling very quiet flight relative to propeller-driven aircraft. Several applications can benefit from a miniature air vehicle including search and rescue, military, and scientific research. In order to use miniature air vehicles in consumer applications, the following challenges need to be addressed:
• The production process needs fewer assembly operations that add time and cost to the MAV
• Payload capacity is very low in most MAV systems, resulting in fewer potential missions
• Flight endurance is highly limited by battery storage capacity relative to weight, thus requiring an efficient flapping mechanism
We believe that addressing these challenges will make flapping wing MAVs more suitable for a wider range of applications. By combining the best features of fixed-wing MAVs and rotorcraft, flapping wing MAVs will offer an attractive combination of beneficial properties including maneuverability, stealth, payload capacity, and ruggedness.
The objectives of this project are:
1. Develop a simulation based optimization methodology to design an optimal compliant drive mechanism: Compliant drive mechanisms offer promise for improved efficiency of power transmission by minimizing inertial losses. By using simulation tools in the optimization process, the need for exhaustive physical testing is reduced, accelerating the design progress.
2. Develop advanced mold design concepts to realize novel lightweight drive mechanism: Injection molding offers the potential for manufacturing automation, thus reducing the number of costly assembly operations. Using advanced injection molding concepts such as multi piece mold designs and multi-material molding, the part count and complexity of the mechanism is reduced, proving improved functionality, more simplistic design, and faster production.
3. Characterize performance of drive mechanism by conducting flight tests: A key part of the optimization loop is to verify the modeling and simulation results with real world flight test results. This provides an opportunity to tune the models, and verify predicted performance metrics.
Overview of Approach
The drive mechanism is a critical component of any MAV. The drive mechanism of a MAV must convert the input energy source to the flapping action of the wings using an actuator. Potential forms of input energy are solar energy, wind energy, or simply a DC battery. Typical examples of actuators are DC motors and servo motors. MAVs are usually constructed using the following concept. First, an energy input is supplied to an actuator. Next, the actuator motion is converted to the flapping action of the wing using a drive mechanism. This flapping action of the wings produces lift and thrust forces that sustain MAV flight. The design of the drive mechanism depends on the source of input energy, the actuator used, and the amount of power that has to be transmitted from the actuator to the wings. This section will present a simulation based design framework for realizing the design goal of a MAV drive mechanism, which converts the input energy to flapping action of the wings. The overall approach for realizing this design goal is illustrated in Figure 1.
Simulation based design of drive mechanism: In the first step, the designer identifies several design concepts for the drive mechanism. These design concepts can be selected from a design repository after performing a feasibility check for the desired flapping action of the MAV, the input energy source and the actuator type to be used. Subsequently, the design concept suitability has to be evaluated. The mechanism is evaluated based on the power transmission capability of the input energy source, the actuators to be used, and geometric constraints. The mechanism model also depends on the properties of the materials used to realize the mechanism, such as density, structural rigidity etc. Currently good computational tools do not exist for estimating lift and thrust force generated by flapping wings. Hence, an oversized and hence safe mechanism prototype need to be used to experimentally measure the lift and thrust forces generated during the flapping motion. The experimentally measured lift and thrust forces are used in a detailed kinematic and dynamic analysis to develop an understanding of the forces acting on the structural members of the mechanism. Subsequently, the mechanism design is evaluated for performance using finite element analysis to evaluate the stresses on the structural members and manufacturability analysis to impose size constraints on the structural members. The manufacturability analysis on the MAV drive mechanism involves: 1) selection of an appropriate manufacturing process that can be used to fabricate the MAV drive mechanism and 2) development of a detailed manufacturing framework based on the manufacturing constraints. The final design of the MAV drive mechanism is obtained using an iterative process involving the steps outlined above.
Development of advanced mold design concepts: As part of our research at the Advanced Manufacturing Lab, we have utilized advanced injection molding concepts such as multi material molding and multi-piece mold designs to several different generations of MAV drive mechanisms. Use of multi-piece mold designs has enabled us to realize high strength to weight ratio drive mechanisms by significantly reducing the overall part count. Use of multi piece molds to fabricate complex geometries for the drive mechanism enables elimination of time consuming assembly operations. We have utilized a simulation based design framework to optimize the multi piece mold design process to minimize the number of mold pieces and maximize the structural rigidity of critical components of the MAV drive mechanism. We have also developed multi-material molding methods to allow for utilization of light-weight polymer grades to realize various functions in the single body mechanism, such as rigid links and flexible joints of a mechanism. Therefore, multi-material molding is a promising manufacturing method, allowing for process automation through reduction of assembly operations, which lowers cost. We have developed a detailed simulation based design framework for development of mold designs to realize multi material molding. This process can be used to concurrently optimize 1) the material combination used for multi material molding, 2) the geometry of the interface between the molded materials to ensure robust bonding and 3) the runner and gating design to ensure proper filling of the mold during injection molding and eliminating weld-lines from structurally demanding locations.
Evaluation of drive mechanism using flight tests: We have developed several different generations of successfully flying flapping wing MAVs in the Advanced Manufacturing lab. Figure 9 illustrates the small bird. The drive mechanism for the small bird was manufactured using the multi piece molding method. The performance attributes of the small bird are listed in table 1. Figure 3 illustrates the jumbo bird that was manufactured using the multi-material molding technology. The performance attributes of the jumbo bird are listed in table 2.
Table 1 Performance attributes of the Small Bird MAV
|Overall Max. Weight||15.4 g|
|Payload Capability||5.7 g|
|Max. Flapping Frequency||12.1 Hz|
|Flight Duration||5 min.|
|Flight Velocity||4.4 m/s|
Table 2 Performance attributes of the Jumbo Bird MAV
|Overall Max. Weight||71.0 g|
|Payload Capability||33.0 g|
|Max. Flapping Frequency||6.1 Hz|
|Flight Duration||15 min.|
|Flight Velocity||3.04 m/s|
For additional information please contact:
Dr. Satyandra K. Gupta
Department of Mechanical Engineering and Institute for Systems Research
3143 Martin Hall
University of Maryland
College Park, MD-20742
Project Website: http://terpconnect.umd.edu/~skgupta/UMdBird.html