Mechanism for biaxial rotation of a wing and vehicle containing such mechanism

A vehicle with wings and a mechanism for causing a flapping motion in wings. Each wing structure comprises a wing and a wing spar coupled to a follower via a resilient member. Each wing carrier is pivotally connected to the body and is configured to restrain lateral movement and permit rotation of the wing spar about a feathering axis. A biasing member provides torsional bias to each wing spar. A linkage, driven by an actuator, transmits cyclic motion that rotates the wing carrier about a flapping axis, which moves the follower along a follower path. A guide attached to the vehicle body lies in the path of each follower, and the follower and guide are shaped such that each wing spar has a first rotational position about its axis along a first portion of the follower path and a second rotational position along a second portion of the follower path.

BACKGROUND OF THE INVENTION

In recent years, there has been a demand to reduce the size of unmanned air vehicles. The Defense Advanced Research Projects Agency (DARPA) defines the class of air vehicles measuring 15 cm or less in any dimension as micro air vehicles (MAVs). Possible applications of these vehicles range from civilian and military surveillance to search and rescue operations. Numerous research groups are actively developing designs of new MAVs. Examples of MAVs include the Black Widow, developed by AeroVironment Inc., and the University of Florida's flexible wing design. Like the majority of MAVs, the above examples comprise scaled-down versions of larger traditional flying vehicles with fixed or rotary wings. Unfortunately, the Reynolds number and the aerodynamic lift of traditional flying vehicles decreases substantially as the wing length of the vehicles decreases.

Numerous research groups are attempting to solve the problem of low aerodynamic lift in MAVs by developing a class of MAVs known as ornithopters, or flapping wing vehicles. The flapping wings of animals, and potentially new flapping wing MAVs, rely on translational motion, referred to as “flapping,” and rotational motion, called “feathering,” of their wings to develop unusually high lift. Thus, there is an interest in designing a lightweight, compact mechanism that enables wing flapping in an ornithopter that is inspired from the wing motion of hummingbirds and hovering insects.

SUMMARY OF THE INVENTION

One aspect of the invention comprises a mechanism for flapping a wing attached to a body. The mechanism comprises a wing structure comprising the wing and a wing spar having a feathering axis and coupled to a follower via a flexible member. A wing carrier is pivotally connected to the body about a flapping axis and configured to receive the wing spar, to restrain axial movement of the wing spar, and to permit rotation of the wing spar about the feathering axis. A biasing member having a first end attached to the wing spar and a second end attached to the wing carrier provides torsional bias to the wing spar. A guide attached to the body is positioned to lie in a path of the follower. A linkage, driven by an activator, transmits cyclic motion to propel the follower along the follower path. The follower and guide are shaped to interface with one another such that the wing spar has a first rotational position about the feathering axis along a first portion of the path and a second rotational position about the feathering along a second portion of the path. The interaction of the guide and the follower ideally provides the wing with a feathered configuration on an upstroke of the wing and a pronated configuration on a downstroke of the wing. In a vehicle having two wings, the linkage is adapted to drive both wings symmetrically and simultaneously.

Another aspect of the invention comprises a flying vehicle comprising a body, a pair of wings, and a mechanism for flapping the wings, the mechanism comprising a pair of wing structures, a pair of wing carriers, a pair of biasing members, and a pair of guides as described above, a linkage for transmitting cyclic motion to propel each wing follower along its respective follower path, and an activator for driving the linkage, wherein each follower and respective guide are shaped to interface with one another as described above. The linkage may comprise a four-bar linkage for driving both wings simultaneously.

The mechanism may be used to generate biaxial wing rotation for ornithopter applications using a single actuator to create rotation of each wing about two orthogonal axes (flapping and feathering). It may also have applications in other engineering areas. In one embodiment, the mechanism combines a four-bar mechanism with a novel spring loaded cam/follower mechanism to achieve this task, actuated by a single motor.

DETAILED DESCRIPTION OF THE INVENTION

In an attempt to mimic the wing motion of smaller birds and insects, designs of ornithopters that can rotate the wings about two orthogonal axes are described. An exemplary ornithopter10is shown inFIG. 1. The ornithopters described herein have an aerodynamically advantageous wing motion; a light-weight and compact mechanism design; and a minimum number of actuators. The designs described herein advantageously comprise a flapping mechanism that creates wing rotation about two orthogonal axes, using a single actuator to create rotations about both axes for both wings.

The mechanism creates a motion similar to insects and hummingbirds by quickly rotating the wing at the top and bottom of the up and down stroke.FIGS. 2A and 2Billustrate this motion. While wing12is translating downwards during flapping, the wing's flat surface14is perpendicular to the direction of the wing motion, pulling the air along with it generating positive lift, as shown inFIG. 2B. At the bottom of the stroke, the wing quickly rotates, or feathers, by approximately 45 degrees about the feathering axis which is coaxial with wing spar16. During the upstroke, the wing generates a negative lift. Since the wing has feathered, its effective surface area is reduced. Consequently, the magnitude of the negative lift during the upstroke is less than the positive lift generated during the down stroke. As a result, a net positive lift is generated. At the top of the stroke, the wing quickly rotates to its initial position and the process repeats during the next cycle.

The exemplary mechanism as shown inFIGS. 3,4and5, consists of a body20, a motor22, two gear wheels24aand24b, two connecting rods26, two wing carriers28, two wing spars16with a wing12and a follower30attached to opposite ends. Two guides32are fixed to the body. Two torsion springs34and two bending springs36provide bias to the wing spars. The bending springs may comprise, for example, helical compression springs.

Wing carrier28supports wing spar16and attaches to body20by a pin joint38. Wing spar16rotates freely within the housing of wing carrier28but stops40prevent it from sliding laterally. Connecting rod26attaches to wing carrier28and gear wheel24aor24bby two pin joints38b,38cat opposite ends. Gear wheels24aand24bare pinned to the body at pin joints38d. While the motor gear42attached to motor22drives right gear wheel24adirectly, it indirectly drives left gear wheel24bthrough an idler gear44. This system ensures symmetric and simultaneous flapping of the wings by rotating the gear wheels in opposite directions.

The wing spar assembly has three degrees-of-freedom: rotation about pin joint38athat connects wing carrier28to body20(flapping axis), rotation about axis W of wing spar16(feathering axis), and deflection of bending spring36moving follower30out of alignment with wing spar16. Bending spring36acts as a coupler connecting follower30to wing spar16. Bending spring36creates a rigid connection in torsion but allows for some misalignment of the wing spar (feathering) axis W and follower axis F, as shown inFIG. 5.

The exemplary mechanism shown inFIG. 4that creates the major flapping of the wings comprises a simple four-bar linkage. Motor22turns two gear wheels24a,24b(input crank), which in turn move connecting rods26(couplers) up and down. Connecting rods26are pinned to wing carriers28, thus rotating the wing carriers and wing spars16(rocker) in a reciprocating manner about the flapping axis.

The mechanism that creates the feathering motion is a cam-follower system. It comprises torsion spring34, bending spring36, follower30, and guide32. Torsion spring34is located near the center of wing carrier28between the two bearings46a,46bthat support the wing spar. Wing spar16passes through the center of torsion spring34. In one embodiment, shown inFIG. 6, one end of the torsion spring attaches to a small pin48that protrudes from the wing spar, while the other end51attaches to wing carrier28. In this way, as wing spar16rotates relative to the wing carrier28, torsion spring34is compressed and creates a moment about the wing spar axis. When the external force causing the rotation of the wing spar is removed, the torsion spring returns the wing spar to its original pronated position.

Bending spring36acts as a coupler connecting follower30to wing spar16. Bending spring36allows for a slight misalignment of follower30while still transferring a moment from follower30to the wing spar16. Bending spring36has a second function of creating a moment that keeps follower30in contact with guide32. Bending spring36is unstretched when follower axis F is in line with the wing spar axis W. Although, shown and described as a bending spring, any resilient member that biases the follower axis to align with the wing spar axis may be used.

Guide32comprises a narrow ridge attached to body20of the ornithopter and is positioned so that it lies in the path of follower30, as illustrated inFIG. 7. The top edge50of guide32lies slightly in front of the follower axis and its bottom edge52lies slightly behind the follower axis. The slightly slanted portions of the guide at its opposite ends force follower30to move along the back side54of guide32while moving downwards in the direction of arrow D and to move along the front side56of guide32while moving upwards in the direction of arrow U.

The follower motion is illustrated inFIG. 7by showing the follower in various positions a-h along follower path illustrated by arrows T, D, B, and U. The follower begins slightly above top edge50of guide32as in position a. As the follower moves down, it contacts the guide in positions b, c, and d. This contact occurs on flange58(the pointed end) of the follower, forcing the follower to rotate about the feathering axis, consequently rotating the entire wing. Follower30then slides down back edge54of guide32with torsion spring34compressed. Bending spring36forces follower30to remain in contact with guide32until the follower moves below bottom edge52of guide32in position e. At this point, the forces acting on bending spring36and torsion spring34are released, and the spring biases cause wing12to rotate into its original, fully-pronated position, realigning follower axis F with wing spar axis W. Next, follower30reverses direction and contacts bottom edge52of guide32in position f. This time follower30is forced to move along front edge56of guide32through positions g and h, but with no flange on the back side of the follower, the wing remains fully pronated.

To test the described mechanism, three individual prototypes were created, named MHP I, MHP II, and MHP III. MHP II had a wingspan of 60 cm, a weight of 40 grams, and a flap rate of approximately 1.5 Hz. MHP III had a wingspan of only 48 cm, a weight of 50 grams (the extra weight is due to added sensors on the machine), and a flap rate of 4 Hz. These dimensions are merely exemplary, however, and larger or smaller dimensions can be chosen, as desired.

The prototypes comprised a body formed of DELRIN® acetal resin, with integral DELRIN® acetal resin guides, nylon fabric wings attached to carbon fiber wing spars, a DELRIN® acetal resin follower attached to the wing spar by a small compression spring acting as the bending spring, DELRIN® acetal resin connecting rods, a DELRIN® acetal resin wing carrier, nylon gear wheels, and a small DC motor. The materials discussed above are merely exemplary materials, however, and similar mechanisms and vehicles may be created using any materials known in the art.

The only significant design difference between the MHP II and MHP III prototypes, illustrated inFIGS. 8A and 8B, was the way in which torsion spring34was attached to wing spar16. In the MHP II prototype shown inFIG. 8A, and one end62of torsion spring34was attached to wing spar16via pin48in the wing spar, and the other end64of torsion spring34was attached to wing carrier28through a hole (not shown) drilled into the wing carrier. In MHP III, one end66of the torsion spring34was attached to wing spar16via a small hole in hub60attached to the wing spar as shown inFIG. 8B, and the opposite end68was attached to wing carrier28though a hole (not shown). The differences between these connections is believed to have had a drastic impact on the generated lift, as discussed herein later.

Theoretical and Experimental Results

A computer program was developed conforming to a dynamic and aerodynamic model developed for the exemplary ornithopter. The model assumed a wing shape of a quarter ellipse with its minor axis measuring 0.14 meters and major axis 0.30 meters. The wing kinematics examined lift and flap rates consistent with those achievable by MHP II and MHP III.

In order to correlate with the theoretical results, a series of experiments were performed on MHP II to measure the actual force generated by the wings at various flap rates and the necessary power to run the mechanism. Similar experiments were run on MHP III. Detailed information regarding the dynamic and aerodynamic models, and the experimental results, can be found in “Design of A Mechanism for Biaxial Rotation of a Wing for a Hovering Vehicle, IEEE Trans. on Mechatronics,” April 2006, vol. 2, NO. 2 incorporated herein by reference.

Forces in the x, y, and z directions were measured at 6 different voltage levels corresponding to 6 different flap rates for MHP II, with the force in the z direction (lift) being the measurement of most interest. As predicted by the theoretical model, the experimental data showed that lift increases with the flap rate, as shown in Table 1. Forces generated in the x and y directions were small compared to the z direction, but they were sizable enough to potentially cause perturbations in the lateral motion of a flying prototype.

Data collection for the MHP III showed that although MHP III was able to flap its wings much faster then the MHP II, it did not generate nearly as much lift. In fact, the majority of the forces generated with this prototype were in the x and y directions. This result is believed to have been caused by a slight variation in the feathering motion of the wing in the MHP III design as compared to the MHP II design, caused by the differences in how torsion spring34was connected to wing spar16. During the top of the upstroke in the MHP II prototype illustrated inFIG. 8A, pin48prevented wing12from over rotation. Pin48is not present in the MHP III design, so consequently, the wing was able to over-pronate at the end of the upstroke, reducing the wing's effective surface area during the down stroke. The respective wing motions of MHP II and MHP III are shown inFIGS. 9A and 9B, for comparison.

Thus, although both the MHP II and MHP III embodiments provided acceptable lift, the differences in these embodiments highlights the benefits of providing a structure that prevents over-pronation of the wing on the end of the upstroke. Although pin48protruding from wing spar16provides one embodiment that prevents over pronation, any structure that provides a mechanical stop to prevent overpronation may be used for providing this design advantage.

Thus, the claimed invention provides a novel mechanism to actuate the wings of a hovering micro air vehicle (MAV). The mechanism uses a single actuator, but each wing can rotate about two orthogonal axes. The light-weight, compact mechanism for flapping the wings, inspired from the wing motion of hummingbird and hovering insects, is capable of generating enough lift for a vehicle to hover.

It should be noted that although ideal for designing a flying vehicle or ornithopter having two wings, the mechanisms described herein are not limited to use in flying vehicles. Any application in which it is desired to create a flapping motion of one or two wings may benefit from the teachings of this invention. For example, such a flapping motion may be used to provide propulsion for an underwater vehicle, or, if the flapping wing is attached to a fixed body, to provide an air current.