From Studying Real Hummingbirds to Designing Hummingbird-Like Robots—A Literature Review <s> III. THE SKELETON STRUCTURE OF HUMMINGBIRDS <s> AlsTRACT.--Intensity patterns of electromyographic (EMG) signals from selected muscles of the wing were studied during different modes of flight in trained Rock doves (Columba livia). Shoulder muscles exhibited a stereotypic pattern producing maximal EMG intensity during the deceleration phases of the upstroke and the downstroke, whereas the muscles of the brachium and antebrachium acted primarily as joint stabilizers during level flapping flight. During nonsteady flight (e.g. takeoff, landing, vertical ascending flight), the distal forelimb muscles exhibited maximal EMG intensity; their primary function appears to be associated with changing the camber and planform of the wing during rapid oscillation. During steady flight, an automatic linkage system consisting of forelimb skeletal elements and ligamentous attachments is thought to permit proper excursion of the wing as a result of forces generated solely by proximal muscles of the wing. To test this hypothesis, the medianoulnaris and radialis nerves were cut in five animals, thus eliminating the contribution of the forearm muscles, and flight tests were performed. Even though forearm muscles were incapable of contracting, the birds were capable of sustained level flapping flight. They were unable to take off independently or perform controlled landings. Received 3 October 1991, accepted 29 March 1992. DESPITE THE large number of bird species, the wide range of wing shapes (Savile 1957), and variation in flight styles or wing-beat gaits (Ray- net 1988), natural selection has acted to retain the basic musculoskeletal design of the avian forelimb. Few data exist on the functional re- lationship between a species' flying capabilities and its forelimb musculoskeletal architecture. Previous studies of the musculoskeletal system document structural variation, but few studies (see Brown 1948, Fisher 1946, Sy 1936) address the functional aspects of forelimb components. Compared with terrestrial locomotion, flying is metabolically efficient per unit distance trav- elled, but energetically expensive per unit time (Tucker 1968, Schmidt-Nielsen 1984); this is due to the muscular demands associated with gen- erating lift using a rapidly oscillating append- age. Consequently, the musculoskeletal appa- ratus of the avian forelimb should be subject to considerable selective pressures. One way to minimize the moment of inertia of a rapidly moving appendage is to distribute the mass closer to the pivot (Hildebrand 1988). This phe- nomenon is evident among birds as the bulk of the wing's mass is positioned proximally. In <s> BIB001 </s> From Studying Real Hummingbirds to Designing Hummingbird-Like Robots—A Literature Review <s> III. THE SKELETON STRUCTURE OF HUMMINGBIRDS <s> X-Ray Reconstruction of Moving Morphology (XROMM) comprises a set of 3D X-ray motion analysis techniques that merge motion data from in vivo X-ray videos with skeletal morphology data from bone scans into precise and accurate animations of 3D bones moving in 3D space. XROMM methods include: (1) manual alignment (registration) of bone models to video sequences, i.e., Scientific Rotoscoping; (2) computer vision-based autoregistration of bone models to biplanar X-ray videos; and (3) marker-based registration of bone models to biplanar X-ray videos. Here, we describe a novel set of X-ray hardware, software, and workflows for marker-based XROMM. Refurbished C-arm fluoroscopes retrofitted with high-speed video cameras offer a relatively inexpensive X-ray hardware solution for comparative biomechanics research. Precision for our biplanar C-arm hardware and analysis software, measured as the standard deviation of pairwise distances between 1 mm tantalum markers embedded in rigid objects, was found to be ±0.046 mm under optimal conditions and ±0.084 mm under actual in vivo recording conditions. Mean error in measurement of a known distance between two beads was within the 0.01 mm fabrication tolerance of the test object, and mean absolute error was 0.037 mm. Animating 3D bone models from sets of marker positions (XROMM animation) makes it possible to study skeletal kinematics in the context of detailed bone morphology. The biplanar fluoroscopy hardware and computational methods described here should make XROMM an accessible and useful addition to the available technologies for studying the form, function, and evolution of vertebrate animals. J. Exp. Zool. 313A:262–279, 2010. © 2010 Wiley-Liss, Inc. <s> BIB002 </s> From Studying Real Hummingbirds to Designing Hummingbird-Like Robots—A Literature Review <s> III. THE SKELETON STRUCTURE OF HUMMINGBIRDS <s> Hummingbirds (Trochilidae) are widely known for their insect-like flight strokes characterized by high wing beat frequency, small muscle strains and a highly supinated wing orientation during upstroke that allows for lift production in both halves of the stroke cycle. Here, we show that hummingbirds achieve these functional traits within the limits imposed by a vertebrate endoskeleton and muscle physiology by accentuating a wing inversion mechanism found in other birds and using long-axis rotational movement of the humerus. In hummingbirds, long-axis rotation of the humerus creates additional wing translational movement, supplementing that produced by the humeral elevation and depression movements of a typical avian flight stroke. This adaptation increases the wing-to-muscle-transmission ratio, and is emblematic of a widespread scaling trend among flying animals whereby wing-to-muscle-transmission ratio varies inversely with mass, allowing animals of vastly different sizes to accommodate aerodynamic, biomechanical and physiological constraints on muscle-powered flapping flight. <s> BIB003 </s> From Studying Real Hummingbirds to Designing Hummingbird-Like Robots—A Literature Review <s> III. THE SKELETON STRUCTURE OF HUMMINGBIRDS <s> Hummingbirds are very distinctive in their form and behavior, the evolution of which is tightly connected to the evolution of their primary source of energy - floral nectar. About forty million years ago, the practical use of this dense fuel, available only in widely-dispersed, insect-sized aliquots - it was originally intended for insect pollinators - presented a severe test to the avian bauplan. This selective pressure forced broad changes in form and function, affecting anatomical structures ranging from the feeding apparatus to the locomotor system. We describe here how these pressures shaped a bird that flies like a bird into one that flies like a fly. <s> BIB004 </s> From Studying Real Hummingbirds to Designing Hummingbird-Like Robots—A Literature Review <s> III. THE SKELETON STRUCTURE OF HUMMINGBIRDS <s> Physical and aerodynamic characteristics of the bird in flight may offer benefits over typical propeller or rotor driven miniature air vehicle (MAV) locomotion designs in certain types of scenarios. A number of research groups and companies have developed flapping wing vehicles that attempt to harness these benefits. The purpose of this paper is to report different types of flapping wing designs and compare their salient characteristics. For each category, advantages and disadvantages will be discussed. The discussion presented will be limited to miniature-sized flapping wing air vehicles, defined as 10–100 grams total weight. The discussion will be focused primarily on ornithopters which have performed at least one successful test flight. Additionally, this paper is intended to provide a representation of the field of current technology, rather than providing a comprehensive listing of all possible designs. This paper will familiarize a newcomer to the field with existing designs and their distinguishing features. By studying existing designs, future designers will be able to adopt features from other successful designs. This paper also summarizes the design challenges associated with the further advancement of the field and deploying flapping wing vehicles in practice.© 2010 ASME <s> BIB005 </s> From Studying Real Hummingbirds to Designing Hummingbird-Like Robots—A Literature Review <s> III. THE SKELETON STRUCTURE OF HUMMINGBIRDS <s> Flapping-wing micro air vehicles (FWMAVs) are a class of unmanned aircraft that imitate flight characteristics of natural organisms such as birds, bats, and insects, in order to achieve maximum flight efficiency and manoeuvrability. Designing proper mechanisms for flapping transmission is an extremely important aspect for FWMAVs. Compliant transmission mechanisms have been considered as an alternative to rigid transmission systems due to their lower the number of parts, thereby reducing the total weight, lower energy loss thanks to little or practically no friction among parts, and at the same time, being able to store and release mechanical power during the flapping cycle. In this paper, the state-of-the-art research in this field is dealt upon, highlighting open challenges and research topics. An optimization method for designing compliant transmission mechanisms inspired by the thoraxes of insects is also introduced. <s> BIB006 </s> From Studying Real Hummingbirds to Designing Hummingbird-Like Robots—A Literature Review <s> III. THE SKELETON STRUCTURE OF HUMMINGBIRDS <s> Flapping mechanism, as one of important components of a flapping wing Micro Air Vehicle (MAV), is directly related to the flapping motion, and further influences on flight performance when flying. Therefore, designing of an efficient flapping mechanism is an interesting research topic. In this paper, flapping mechanisms of three existing successful flapping wing MAVs with hovering and actively stable flight performances are comparably and analytically studied. Via kinematics' analysis, the results indicate that the designed flapping mechanism of ASL hummingbird-like robot (ASL-Colibri) seems to be better than other two's. However, others' two can be still stable flight, which means that no perfection design on flapping mechanism may be compensated through an active control so as to make them stable flying. <s> BIB007 </s> From Studying Real Hummingbirds to Designing Hummingbird-Like Robots—A Literature Review <s> III. THE SKELETON STRUCTURE OF HUMMINGBIRDS <s> This paper presents compliant transmission mechanisms for a flapping-wing micro air vehicle. The purpose of this mechanism is to reduce power consumption, a critical issue in this kind of vehicles, as well as to minimize the peak input torque required by the driving motor, which helps to maintain flight stability and reduces mechanical shocks of the structure. We first describe the development of pseudo-rigid-body model of the mechanism and the analysis of the corresponding kinematics. Second, we compute the required input torque for driving stable flapping motions, from the perspectives of work and energy. For this computation, two methods are applied, one based on the principle of virtual work and another one based on rigid-body dynamics. Our mathematical analysis demonstrates that both methods are consistent with each other in terms of the resulting input torque from the motor. Finally, according to the results from the input torque analysis, the main parameters characterizing the compliant joints, the torsional stiffness of virtual spring and initial neutral angular position, are optimized. The experimental results carried out with two different mechanical setups, one with rigid components, and another one with flexible components, demonstrate the relationship between the input voltage (that is directly related to flapping frequency) and power saving of the compliant mechanism. The average power consumption is reduced of up to 4%, and peak power consumption is reduced up to 25% using the compliant transmission mechanisms compared to the rigid mechanism. The experiments also show a clear relationship between flapping frequency and power savings. <s> BIB008
Hummingbirds flap their wings in similar arcs and a high wingbeat frequency with large amplitude then achieve remarkable agility in flight. They must convert small muscle strains into large amplitude wing motions, or probably require changing the wing skeleton and flight stroke. Hummingbird's wing has a massive of skeletons shown in Fig. 6 -I, but it can be simply considered that there are three parts (upper arm, forearm, and handwing, shown in Fig. 6 -II (left)). By comparing the morphology of wing in hummingbirds and pigeons, the upper arm and forearm bones are much shorter than pigeons (the pigeon wing scaled to the hummingbird wing length) shown in Fig. 6 -II (right), whereas the ''hand'' part of a wing, called hand wing, is relatively larger: over 75% handwing of entire wing area in hummingbirds to about 50% handwing in most of the birds BIB004 . Obviously, the most probable causes for these differences are the high wing-beat frequency, large stroke amplitude of hummingbird's flight, peculiar physiological structure and supracoracoideus muscles of hummingbirds. The shorter joint bones of elbows can be swift enough to enable hummingbirds to apply the high wing-beat frequency and large stroke amplitude flight pattern in a small muscle's strains dynamics. Compared with pigeons (scaled to hummingbird), the longer wing hand ossature (brown, manus) is applied to achieve extreme agile wing rotation BIB002 , BIB001 . Besides, normal birds articulate their wings from shoulder, elbow, and wrist, whereas all the motion in hummingbirds only comes from the very mobile shoulder. Therefore, the wing rigidity is similar to that of insects and possesses similar flight characteristics. Hummingbirds can hover for a long period since they use symmetric hovering, also called insect stroke. Meanwhile, the hummingbird's wings are fully extended during downstroke to generate most of the lift force (about 70% of total lift), whereas approximately 25%-33% of the total lift is produced in upstroke as well as the drag is reduced by flexing wings in the upstroke. Rotating and twisting wings are placed at the end of each half stroke. In addition, it is interestingly discovered hummingbird wings are slimmer than that of pigeons, shown in Fig. 6 . The details of wing morphologies will be discussed in the following section. Recent studies revealed that most of the wing inversion is produced by supination of the forearm, namely, inverting the bones (and feathers) of handwing BIB003 . The hummingbirds through moving the joints and rotating the three axes (spherical rotation, long-axis rotation, and polar rotation) are to achieve the wing flapping motion and wing rotation. By applying this approach, hummingbirds are able to flap the wings in a path to generate the required aerodynamic forces and to balance the weight in the unique flight patterns BIB003 . In hummingbirds, they drive muscles and bones to flap their wings. However, in order to generate flapping motion, many flapping mechanisms of FWMADs are developed and designed based on the different mechanisms such as dual serial four-bar linkage  and stroke-cam flapping mechanism  . The comparison study on flapping mechanisms of three hovering FWMADs is presented in BIB007 . This paper presented that no perfection design (uncertainty factors) on flapping mechanism may be compensated through an active control to make them stable flying. The flapping mechanism of three existing flapping wing MAVs are presented in Fig. 7 . The flapping mechanism of Colibri as an example is discussed in this paper. There are two mechanism stages in the flapping mechanism: 1) slider crank with a rocker produce oscillation motion 2) a four-bar mechanism is for motion amplification. Angle θ is output angle from crank. Flapping amplitude φ, output angle of output link, can be obtained through following kinematics analysis. Also, compliant transmission mechanisms of FWMAD are summarized and reviewed as well in BIB006 , BIB005 . In addition, the effects of elastic hinges flapping-wing compliant transmission mechanism is studied BIB008 . As for the flapping mechanism of Maryland-hummingbird, it is a novel and interesting flapping mechanism, which the five-bar linkage is applied in this robot. In this flapping mechanism, the four-bar linkage is firstly used to launch flapping motion driven by DC motor, and the fifth bar is then added to amplify the motion to the desired flapping amplitude. And as for the flapping mechanism of KUBeetle, two mechanisms are connected by the link bar and fix on the big pulley mechanism to transform flapping motion. In order to improve the flapping amplitude, the belt pulley mechanism is applied to amplify the flapping amplitude, then to reach desired angle. In this paper, the flapping mechanism weight distributions of hummingbird-like robots are organized in Table 1 . From this table, we can see that the weights of flapping mechanisms for successful hovering hummingbird-like robots are between in 13%∼ 14%. (At present, Colibri, Nano-hummingbird, Maryland-hummingbird and KUBeetle are successful hovering flight.)