Abstract:
A locomotion device, such as a pogo stick, having a housing having a first end portion and a second end portion, a plunger being slideably connected to the housing second end portion, the leaf spring also pivotally attached to the plunger. A method for locomoting using such a device is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to energy storage devices. More particularly, the present invention is directed to energy storage devices taking the form of locomotion machines using springs. 
     2. Description of the Background 
     Pogo sticks have long been enjoyed by children, but the play experience has been limited to bouncing in place, trying to locomote forward, and negotiating small obstacles. Conventional pogo sticks generally employ inefficient steel compression springs. These pogo sticks provide only a few inches of ground clearance and require the rider to exert substantial effort on every bounce to keep hopping. 
     Steel compression springs typically have a specific energy limited to 1200 inch-pounds/pound of mass. A steel spring of one (1) pound mass may store 1200 inch-pounds of elastic energy, and theoretically could lift its own weight. 1200 inches vertically and could lift a 150-pound load, the weight of an average person, a total height of 8 inches above the bottom most point when the steel spring is compressed. Accordingly, a two (2) pound spring would be necessary to exhibit a more desirable pogo stick lift of sixteen (16) inches, which would result in an eight (8) inch rise above the ground. 
     A second disadvantage of the steel compression spring used in conventional pogo sticks is its propensity to buckle laterally. That is, if the spring is held at its two ends and compressed, at some level of compression the middle of the spring will shift sideways and the spring no longer operates in its normal symmetric mode. Coil springs with an aspect ratio of greater than about 6:1 ratio exhibit buckling when substantially compressed, wherein the aspect ratio is defined as the ratio of free length to nominal coil diameter. A typical pogo stick spring has an aspect ratio around 8:1 or higher, so buckling is inevitable. This spring must be guided laterally which can cause significant sliding friction between the spring and the pogo stick plunger. 
     Yet another disadvantage of the steel compression springs used in conventional pogo sticks is that they have linear force/deflection characteristics. This results in the peak force at maximum compression being nearly twice the average force. The average force is defined by the total work on the spring during compression divided by the stroke length. 
     Accordingly, there is a need for a pogo stick having an efficient energy storage device resulting in the pogo stick providing a large amount of ground clearance while requiring a minimal effort to keep hopping. 
     There is a further need for a pogo stick having a configuration that precludes the sliding friction inherent in the conventional pogo sticks having steel coil springs. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a locomotion device using a leaf spring as the energy storage device. 
     The present invention further provides a leaf spring in the form of a high-performance composite spring material such as fiberglass/epoxy or graphite/epoxy that can store at least two times more energy than a conventional steel coil spring having the same weight. 
     The present invention further provides a leaf spring energy storage device that bends and is pivoted at the top and bottom ends of the pogo stick such that energy losses from sliding friction associated with lateral buckling are substantially eliminated. 
     Further, the force/deflection characteristics of the present invention may be tailored for the best performance in terms of energy storage and operator comfort such that one embodiment of the present invention with an adult rider exhibits hopping heights of thirty (30) inches or more as measured from the base of the device to the ground, compared to approximately five (5) inches for commercially available pogo sticks. 
     The present invention further provides a pogo stick that uses rollers to guide the plunger that further improve the overall efficiency and performance of the pogo stick. 
     The present invention further provides a leaf spring that provides nearly constant force, allowing nearly twice the energy storage of a linear spring having the same plunger stroke and maximum force. In some such embodiments of the present invention, the spring force increases by only 20% from its initial to its fully compressed value thus, giving nearly constant force over the full range. 
     Other details, objects and advantages of the present invention will become more apparent with the following description of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     For the present invention to be understood and readily practiced, the present invention will be described in conjunction with the following Figures wherein: 
     FIG. 1 is a sectional side view of the pogo stick employing the present invention wherein the spring in the retracted position is shown in broken lines; 
     FIG. 2 is a sectional rear view of the pogo stick of the present invention shown in FIG. 1; 
     FIG. 3 is a side view of the spring of the pogo stick of the present invention shown in FIG. 1 wherein the spring is shown in the retracted position in broken lines; 
     FIG. 4 is a rear view of the spring of the pogo stick of the present invention shown in FIG. 3 wherein the spring is shown in the retracted position in broken lines; 
     FIG. 5 is a plot of force vs. deflection for the spring shown in FIGS. 3 and 4; 
     FIG. 6 is a side view of alternative flat spring of the present invention wherein the spring is shown in the retracted position in broken lines; 
     FIG. 7 is a plot of force vs. deflection for various lateral offsets for the spring shown in FIG. 6; and. 
     FIG. 8 is a perspective view of another embodiment of a pogo stick employing a flat spring of the present invention and wherein the pogo stick has an offset lever and is shown in the extended position; 
     FIG. 9 is a perspective view of the pogo stick of the present invention shown in FIG. 8 wherein the pogo stick is shown in the retracted position; 
     FIG. 10 is a side view of another embodiment of the present invention wherein a flat foot is pivotably attached to the foot of the pogo stick; 
     FIG. 11 is a cross-sectional view showing an alternative design of a flat foot for the present invention using a ball-and-socket joint. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described below in terms of a pogo stick. It should be noted that describing the present invention in terms of a pogo stick is for illustrative purposes and the advantages of the present invention may be realized using other structures and technologies that have a need for a locomotion machine with an efficient energy storage device. 
     It is to be further understood that the Figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements and/or descriptions thereof found in a typical pogo stick. Those of ordinary skill in the art will recognize that other elements may be desirable in order to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. 
     FIGS. 1-4 illustrate a pogo stick  10  of the present invention wherein the spring in the retracted position is shown in broken lines. The pogo stick  10  of the present invention comprises a housing  12 , a handle  14 , an upper spring pivot  16 , a lower spring pivot  18 , a lower pivot lug  36 , a leaf spring  20 , upper guide rollers  22 , lower guide rollers  24 , a footrest  26 , a foot member  28 , a foot cushion  35 , and a tubular plunger  30 . As with a conventional pogo stick, the user stands ,with each foot on the footrest  26 , one on each side of the housing  12 , holding the handle  14  with both hands. During bouncing, the foot cushion  35  contacts the ground. 
     The housing  12  may be a substantially three-dimensional rectangular body that may be made from reinforced plastic and be forty (40) inches in height. Alternatively, the housing  12  may be made from wood having aluminum reinforcements at high stress points, or primarily of aluminum, steel, or other well known structural material. The handle  14  is fixedly attached to the top of the housing  12 . The handle  14  may be made of steel or aluminum tubing having a one (1) inch diameter, a length of fourteen (14) inches and having bicycle-type handgrips (not shown) mounted thereon. The plunger  30  may be made from steel or aluminum tubing having a 1.25 inch diameter and being thirty-two (32) inches in length. The footrest  26  is fixedly connected to the housing  12 . The footrest  26  may be a wood, metal, or reinforced plastic flat plate with a rough traction surface on the top where the user&#39;s feet rest. 
     The upper guide rollers  22  and the lower guide rollers  24  are spaced relative to one another and are each fixedly connected to the interior of the housing  12  using a steel pin that may have a 0.25 inch diameter. The upper guide rollers  22  and the lower guide rollers  24  each may be two acetal plastic, V-shaped rollers having a 1.5 inch diameter and being approximately 1.5 inches in length. The plunger  30  moves vertically in the housing  12 , guided by guide rollers  22  and  24 . 
     The leaf spring  20  is shown in a retracted position at  32  and in an extended position at  34 . In the present embodiment, the leaf spring  20  may be made from unidirectional fiberglass or graphite strips 0.08 inches thick, 2.5 inches wide, and 36 inches in length. The leaf spring is laminated with epoxy from individual strips each 0.08 inches thick on a curved form to give a constant thickness of 0.48 inches and a moderate curvature of approximately 60 degrees over the 36 inches length of the leaf spring  20 . When in the fully retracted position at  32  the leaf spring  20  bends to about 180 degrees, i.e., a half circle. This yields a total deflection of about 10.7 inches. The width of the leaf spring  20  is 2.5 inches at the mid-length, and contoured in sinusoidal shape to zero at the ends of the leaf spring  20 , except that the ends are made wide enough to resist the shear stresses. It will be understood that the leaf spring may be fabricated from other materials such as metals, thermoplastics, fiber-reinforced thermoplastics such as fiberglass/nylon, or plastics using graphite, glass or other fiber reinforcing materials. 
     The proximal end  15  of the leaf spring  20  is pivotably connected at the top of the housing  12  with upper spring pivot  16 . The upper spring pivot  16  may be a steel rod having 0.25 inches diameter. The distal end  17  of the leaf spring  20  is pivotably connected by the lower spring pivot  18  to the lower pivot lug  36 , which is mounted on the plunger  30  such that the plunger  30  may slide between upper and lower guide rollers  22  and  24 . The leaf spring  20  is mounted to the housing  12  such that when the leaf spring  20  is in the retracted position at  32 , the leaf spring  20  stores elastic energy in bending deflection. Unlike a long coil spring, the leaf spring  20  does not need guides to prevent buckling, thus precluding the sliding friction inherent in steel coil springs. Friction of the plunger moving in the body is minimized by the upper and lower guide rollers  22  and  24  and the overturning moment of the plunger  30  is resisted. 
     The stop cushion (not shown) mounts to the bottom of the lower pivot lug  36 . When the plunger  30  is fully retracted within the housing  12 , the stop cushion contacts the footrest  26 , thereby cushioning the impact of the plunger  30  against the housing  12 . This minimizes noise and wear associated with the impact. 
     In operation, a user mounts the pogo stick  10  by placing his feet on the footrest  26  and then bounces on the pogo stick  10  such that the foot  28  and foot cushion  35  is raised off the ground and thereafter strikes the ground. The foot cushion  35  prevents damage to the foot and ground and provides a traction surface to prevent slipping of the foot  28  on the ground. When the foot  28  impacts the floor, the plunger  30  slides within the housing  12  applying force F to bottom end of the leaf spring  20  and thus, the leaf spring  20  moves to the retracted position at  32 . In the retracted position at  32  energy is stored by the bending deflection of the spring  20 . The user adds energy to the bouncing motion while the foot  28  is in contact with the ground by the user pushing downward with his feet and legs against the footrest  26 , and with his hands and arms against the handle  14 . By adding energy on each bounce, the user causes the bounce height to increase from one bounce to the next, until the energy storage limit of the spring is reached. 
     FIG. 5 is a plot of force vs. deflection for the leaf spring  20  shown in FIGS. 3 and 4. The plot has been normalized such that the maximum force and deflection correspond to values of 1.0 on the plot. For one embodiment of the invention, maximum force is 690 pounds and maximum deflection is 10.7 inches. Maximum bending stress in the outer fibers is about 80,000 PSI; maximum shear (at centerline) is about 2000 PSI. The total energy storage is 4700 pound-inches. While the force increases monotonically with deflection, the stiffness or spring rate decreases with deflection. Compared to the conventional linear spring, also shown in FIG. 5, the prototype spring has a higher average force, and thus greater energy storage for the same deflection and peak force. 
     FIG. 6 is a side view of an alternative flat spring of the present invention wherein the spring is shown in the extended position  34  and the retracted position  32  in broken lines. The stiffness characteristic of the spring  20  of the present invention may be adjusted by various methods. For example, forming the spring  20  such as shown in FIGS. 3 and 4 but without any initial curvature produces a nearly constant force, increasing about 20% from zero to full deflection. Offsetting the pivot points  40  from the spring centerline, as shown in FIG. 6, also affects the spring-rate curve. The flat spring design has a number of advantages including simplicity of fabrication in that the spring  20  can be simply cut from a plate of proper thickness. Also, the monolithic material can sustain higher shear stresses than the bond between laminations, allowing application of higher loads. The difference in shear strength is typically about a 2:1 ratio. The offset lever arms  42  can be easily changed to adjust the spring characteristics, thus accommodating different riders or operating conditions. 
     FIG. 7 is a plot of force vs. deflection for various lateral offsets for the spring shown in FIG.  6 . With zero offset (top curve FIG. 7) the force is nearly constant over the total deflection range, increasing by about 20%. While a constant-force spring yields the greatest energy storage for a given deflection D and peak force, experiments show that the shock due to this sudden change of force on impact is uncomfortable for the rider and may be detrimental to control and efficiency. The ideal would seem to be a curve that rises rapidly to maximum, then flattens-out for most of the deflection range such as the curve with normalized offset of 0.1. Recent experiments have shown that a normalized offset (the offset lever length divided by half the spring length) around 0.2 provides a good compromise between comfort/efficiency and energy storage. 
     While the figure shows the pivots offset at both ends, similar effect can be had by using the offset at one end only; however, a greater offset is needed for comparable effect. The particular offset(s) used affects the spring profile (e.g., width variation) needed for optimal energy storage. Using springs with constant width simplifies manufacturing and precludes splintering. 
     FIGS. 8 and 9 are perspective views of another embodiment of the pogo stick  10  of the present invention wherein the pogo stick  10  has offset levers  42  and a flat, untapered spring  20  and a plate  130  for preventing rotation of the plunger positioned within the housing  12  through which plunger  30  slides. The plunger  30  and spring  20  extended in FIG. 8 retracted in FIG.  9 . This embodiment is similar to the embodiment illustrated in FIGS. 1-4 except that this pogo stick  10  has a flat spring  20  and employs offset levers  42 . The spring  20  has a constant cross-section of 2.5 inches wide and 0.364 inches thick, and is 30.7 inches in length. The spring  20  is a monolithic piece of unidirectional fiberglass reinforced epoxy as used in the archery limb industry. The plunger stroke is 15 inches. The total height of pogo stick  10  is approximately 55 inches. The pivet  40  is offset 2.8 inches from the centerline of the spring  20 , providing a gradually increasing force profile. Hopping heights of about 24 inches have been measured with a 130 pound rider. We have used it to hop up and down steps, made long jumps over 6 feet, and covered 50 feet of lateral distance in 10 hops. This embodiment weighs approximately 10 pounds. The lighter weight makes this unit easier to control and less tiring to operate. 
     The pogo stick of the present invention shown in FIG. 8 and 9 was fitted with a wider tapered spring. The spring was 0.364 inches thick, 3.05 inches wide at the middle, 2.0 inches wide at the ends, and 30.7 inches long. With this spring a 130 pound rider achieved hopping heights over 40 inches. 
     FIG. 10 is a side view of another embodiment of the present invention wherein a flat foot is pivotably attached to the foot of the pogo stick and FIG. 11 is a cross-sectional view showing an alternative design of a flat foot for the present invention using a ball-and-socket joint. The rounded rubber foot cushion  35 , approximately 3 inches in diameter on present prototypes, provides good impact absorption and traction on paved and other hard surfaces. Alternatively, the present invention may have a large substantially flat foot that pivots to accommodate ground contact. FIGS. 10 and 11 illustrate two embodiments of the present invention having a substantially flat foot. In FIG. 10, a flat foot plate  50  is attached to the bottom of the foot  28  and foot cushion  35  with elastic bands  37 . The foot plate  50  can roll against the foot cushion  35  to align to the ground surface, and allow the plunger  30  and pogo stick  10  to rotate about the contact point. This embodiment has a five (5) inch diameter disk of 0.75 inch plywood as the foot plate  50  and was found to function well on grass, gravel, and sand, and reduces the tendency of the foot to sink into the ground. 
     An alternate design shown in FIG. 11 uses a ball pivot  45  that attaches to the end of the plunger  30  and rests in a socket  46 . Springs or elastic bands  47  are used to keep the foot plate  50  nominally perpendicular to the plunger  30  while allowing the foot plate  50  to align itself with the ground surface. The springs  47  may also hold the ball  45  and socket  46  together. Small rubber feet  38  shown in FIG. 10 may be used to increase traction on smooth surfaces. Cleats  39  shown in FIG. 11 may be used to increase traction on soft surfaces. 
     Maximum hopping height depends both on the amount of energy put into the spring and the ability of the user to maintain an upright posture. Spring energy is limited by the maximum compression force the user can exert on the spring. But energy is also proportional to the plunger stroke length so energy and hopping height greater than demonstrated in the present embodiment are likely possible. The maximum stroke length may be limited mainly by the available space and geometry of the device. Experiments have shown that control of upright attitude is increasingly difficult with higher hopping heights and flight times, but training may allow heights much greater than the 40 inches achieved thus far. 
     Another embodiment of the present invention comprises a device similar to the pogo stick used as a leg on a single or multi-legged robot, toy or other locomotion device by attaching the housing of the pogo stick to a hip joint on such a machine. Another embodiment of the present invention may use two of the pogo sticks of the present invention, one for each leg of the user connected to the user&#39;s leg or body, thus allowing a running motion with springy legs. Similarly, a single pogo stick device could be fitted with a pivot at the top to allow the device to swing fore/aft and/or laterally to accommodate rapid fore/aft or sideways motion. 
     The present invention may be used in devices for recreation, exercise, sport and transportation. Lightweight and efficient designs of the present invention enhance control, permit stepping around, over and onto obstacles. 
     Those of ordinary skill in the art will recognize, however, that many modifications and variations of the present invention may be implemented without departing from the spirit and scope of the present invention. The foregoing description and the following claims are intended to cover such modifications and variations.