Patent Publication Number: US-2011076918-A1

Title: Vibration Powered Toy

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Patent Application No. 61/246,023, entitled “Vibration Powered Vehicle,” filed Sep. 25, 2009, which is incorporated herein by reference in its entirety. This application also is a continuation-in-part and claims the benefit under 35 U.S.C. §120 of U.S. patent application Ser. No. 12/860,696, entitled “Vibration Powered Vehicle,” filed Aug. 20, 2010, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     This specification relates to devices that move based on oscillatory motion and/or vibration. 
     One example of vibration driven movement is a vibrating electric football game. A vibrating horizontal metal surface induced inanimate plastic figures to move randomly or slightly directionally. More recent examples of vibration driven motion use internal power sources and a vibrating mechanism located on a vehicle. 
     One method of creating movement-inducing vibrations is to use rotational motors that spin a shaft attached to a counterweight. The rotation of the counterweight induces an oscillatory motion. Power sources include wind up springs that are manually powered or DC electric motors. The most recent trend is to use pager motors designed to vibrate a pager or cell phone in silent mode. Vibrobots and Bristlebots are two modern examples of vehicles that use vibration to induce movement. For example, small, robotic devices, such as Vibrobots and Bristlebots, can use motors with counterweights to create vibrations. The robots&#39; legs are generally metal wires or stiff plastic bristles. The vibration causes the entire robot to vibrate up and down as well as rotate. These robotic devices tend to drift and rotate because no significant directional control is achieved. 
     Vibrobots tend to use long metal wire legs. The shape and size of these vehicles vary widely and typically range from short 2″ devices to tall 10″ devices. Rubber feet are often added to the legs to avoid damaging tabletops and to alter the friction coefficient. Vibrobots typically have 3 or 4 legs, although designs with 10-20 exist. The vibration of the body and legs creates a motion pattern that is mostly random in direction and in rotation. Collision with walls does not result in a new direction and the result is that the wall only limits motion in that direction. The appearance of lifelike motion is very low due to the highly random motion. 
     Bristlebots are sometimes described in the literature as tiny directional Vibrobots. Bristlebots use hundreds of short nylon bristles for legs. The most common source of the bristles, and the vehicle body, is to use the entire head of a toothbrush. A pager motor and battery complete the typical design. Motion can be random and directionless depending on the motor and body orientation and bristle direction. Designs that use bristles angled to the rear with an attached rotating motor can achieve a general forward direction with varying amounts of turning and sideways drifting. Collisions with objects such as walls cause the vehicle to stop, then turn left or right and continue on in a general forward direction. The appearance of lifelike motion is minimal due to a gliding movement and a zombie-like reaction to hitting a wall. 
     SUMMARY 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the eccentric load, and a plurality of legs. Each leg includes a leg base and a leg tip at a distal end relative to the leg base. The legs are coupled to the housing at the leg base and include at least one driving leg constructed from a flexible material and configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. At least one leg is adapted to drag. 
     These and other embodiments can each optionally include one or more of the following features. The apparatus includes fewer than twenty legs that contact a support surface as the at least one driving leg causes the apparatus to move. The apparatus includes fewer than twenty legs that provide support when the apparatus is in an upright position. The legs are sufficiently stiff that four or fewer legs are capable of supporting the apparatus without substantial deformation when the apparatus is in an upright position. A coefficient of friction of a portion of legs that contact a support surface is sufficient to substantially eliminate drifting in a lateral direction (i.e., substantially perpendicular to the direction of movement). The legs are molded from a elastomer. The legs are co-molded with at least a portion of the body. The legs are injection molded. Multiple legs are molded simultaneously. Multiple legs and at least a portion of the body are simultaneously integrally injection molded from an elastomer. Multiple legs are co-molded with a portion of the housing, wherein the portion of the housing includes a nose section. The legs are tapered. The housing includes at least a nose and two lateral sides and each leg is coupled to the housing in a vicinity of one of the lateral sides. A diameter of each driving leg is at least 5% of the length of the leg. The legs are curved. The legs are constructed from an elastomeric material. The flexible material includes rubber. The flexible material includes an elastomer. The at least one driving leg is configured to cause the apparatus to repeatedly hop as the rotational motor rotates the eccentric load. The at least one driving leg is curved between the leg base and the leg tip. The eccentric load is configured to be located toward a front end of the apparatus relative to the driving legs, wherein the front end of the apparatus is defined by an end in the direction of movement. The repeated hopping causes the apparatus to move in the direction generally defined by an offset between the leg base and the leg tip. The legs include at least two legs adapted to cause the apparatus to move. The leg tip of the at least one leg adapted to drag has a lower coefficient of friction than the at least one driving leg. The at least one leg that is adapted to drag is configured to have a lesser stiffness than the at least one driving leg. The at least one driving leg includes a durometer in the range of approximately 55-75, based on the Shore A scale. The eccentric load includes an inertial load adapted, when the eccentric load is rotated by the rotational motor, to cause the at least one driving leg to hop off a flat support surface. The plurality of legs are adapted to allow the apparatus to turn when the at least one driving leg hops off a flat support surface. The at least one driving leg is constructed from polystyrene-butadiene-styrene. The at least one driving leg has a ratio of a leg length to a leg diameter in the range of 2.0 to 10.0. The thickness of the legs is defined by a diameter of approximately 5.25 times less than the length of the leg. A curvature of the legs is adapted to enhance a tendency of the apparatus to move in the direction generally defined by the offset between the leg base and the leg tip. The curvature of the legs in combination with a resiliency of the legs are adapted to allow the legs to maintain an approximately neutral position when the rotational motor is not rotating the eccentric load and to bend in a direction of the curvature when a rotational movement of the eccentric load introduces a downward force on the apparatus. The neutral position is defined by a shape of the legs when not supporting a load. At least one driving leg has a ratio of radius of curvature to leg length in a range of 2.5 to 20. The curvature of the legs is approximately consistent from the leg base to the leg tip. The curvature of the legs is defined by a radius of curvature of approximately 3 to 6 times the length of the leg. A relative stiffness of at least two specific legs of the plurality of legs is configured to alter a tendency of the apparatus to turn. The plurality of legs are arranged in two rows, with each row having at least two legs, the leg base of the legs in each row being aligned along each lateral side of the housing. The plurality of legs are arranged in two rows, with each row having at least four legs, the leg base of the legs in each row being aligned along each lateral side of the housing. The plurality of legs are arranged in two rows, with each row having at least six legs, the leg base of the legs in each row being aligned along each lateral side of the housing. At least one of the legs in a first one of the rows is longitudinally offset from a corresponding leg in a second one of the rows to alter a tendency of the apparatus to turn as a result of a rotation of the eccentric load. A lateral distance between the eccentric load and the leg tip of the at least one driving leg is within a range of 50-150% of a length of the at least one driving leg. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. The legs are constructed from a flexible material, integrally coupled to the housing at the leg base, arranged in two rows with the leg base of the legs in each row coupled to the housing substantially along a lateral edge of the housing, and include at least one driving leg configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. 
     These and other embodiments can each optionally include one or more of the following features. At least one leg is adapted to drag. As stated above, the flexible material can include an elastomer and can be rubber. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. The legs are coupled to the housing at the leg base and include at least one driving leg configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. A relative stiffness of at least two specific legs of the plurality of legs is configured to alter a tendency of the apparatus to turn. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. The legs are coupled to the housing at the leg base and include at least one driving leg configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. A relative position of at least two specific legs of the plurality of legs is configured to alter a tendency of the apparatus to turn. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. At least one leg is situated on a first lateral side of the apparatus and at least one leg is situated on a second lateral side of the apparatus. The legs are coupled to the housing at the leg base and include at least one driving leg configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. A distance between a plane defined by the leg tips and a longitudinal center of gravity of the apparatus is less than a distance between a leg tip of the at least one leg on the first lateral side of the apparatus and a leg tip of the at least one leg on the second lateral side of the apparatus. 
     These and other embodiments can each optionally include one or more of the following features. At least a portion of the rotational motor is located between at least a portion of at least two of the legs. The apparatus includes a switch for controlling the rotational motor wherein at least a portion of the switch is located between at least a portion of each of at least two of the legs. The apparatus includes a battery for powering the rotational motor wherein at least a portion of the battery is located between at least a portion of at least two of the legs. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. The legs are coupled to the housing at the leg base and include at least one driving leg configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. The axis of rotation of the rotational motor passes approximately through a center of gravity of the apparatus. 
     These and other embodiments can each optionally include one or more of the following features. The axis of rotation passes within 20% of the center of gravity of the apparatus as a percentage of the height of the apparatus. The axis of rotation passes within about 6% of the center of gravity of the apparatus as a percentage of the height of the apparatus. The axis of rotation of the rotational motor passes sufficiently close to the center of gravity of the apparatus to induce a substantially constant tendency for the apparatus to roll about the longitudinal center of gravity. The housing is configured to facilitate rolling of the apparatus about the longitudinal center of gravity, based on a rotation of the eccentric load, when apparatus is on a substantially flat surface with the legs oriented in an upward direction. The apparatus is configured to prevent the apparatus from resting in an inverted position on the substantially flat surface, wherein the inverted position is defined by the apparatus being in a position where the legs point in substantially an opposite direction from when the legs rest on the substantially flat surface. The housing includes a shoulder on each lateral side and a top side that includes a protruding surface that extends above the shoulder on each lateral side when the apparatus is in an upright position. A distance between the substantially flat surface and the longitudinal center of gravity is approximately the same as a distance between the protruding surface and the longitudinal center of gravity. The distance between the center of gravity and the substantially flat surface is in a range of 50-80% of the value of a lateral stance, wherein the lateral stance is defined by a distance between outermost left and right legs. A lateral distance between the eccentric load and the leg tip of the at least one driving leg is within a range of 50-150% of a length of the at least one driving leg. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. The housing includes a top side and a bottom side. The top side includes a shoulder on each lateral side of the housing and a protruding surface extending above each shoulder when the apparatus is oriented with the top side facing up. The rotational motor includes an axis of rotation. The legs extend from the bottom side of the housing and are coupled to the housing at the leg base. The legs include at least one driving leg configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. A center of gravity of the apparatus is within a range of 40-60% of the distance between a plane that passes through the leg tips of the plurality of legs and the protruding surface on the top side of the housing. 
     These and other embodiments can each optionally include one or more of the following features. The leg base for each of the plurality of legs is above the center of gravity of the apparatus when the apparatus is oriented with the top side facing up. The axis of rotation of the rotational motor passes within approximately 6% of a center of gravity of the apparatus as a percentage of the height of the apparatus. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. The housing includes a front end, rear end, top side, bottom side, and lateral sides. The front end includes a nose adapted to contact obstacles as the apparatus moves in a forward direction and to have increased deformable resilience relative to the lateral sides of the housing. The rotational motor includes an axis of rotation. The legs are coupled to the housing at the leg base and include at least one driving leg configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. 
     These and other embodiments can each optionally include one or more of the following features. The nose is further adapted to cause the apparatus to deflect off of obstacles at an angle as the apparatus moves in a forward direction. The nose includes a first surface extending toward a first lateral side of the nose and a second surface extending toward a second lateral side of the nose, wherein each of the first surface and the second surface are angled away from a forward direction of motion as the first surface and the second surface extend toward the lateral sides of the nose. The first surface and the second surface substantially meet at a point at approximately a centerline of the nose. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. The legs are coupled to the housing at the leg base and include at least one driving leg configured to cause the apparatus to move in a forward direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. Forces from rotation of the eccentric load interact with a resilient characteristic of the at least one driving leg to cause the at least one driving leg to leave a supporting surface as the apparatus translates in the forward direction. 
     These and other embodiments can each optionally include one or more of the following features. Translation in the forward direction results from a bending of the at least one driving leg in a direction generally opposite the forward direction that is induced at least in part by the rotation of the eccentric load. A coefficient of friction of a portion of at least a subset of the legs that contact a support surface is sufficient to substantially eliminate drifting in a lateral direction. Legs from at least a subset of the plurality of legs are constructed from an elastomeric material. Legs from at least a subset of the plurality of legs are molded from a moldable material. Legs from at least a subset of the plurality of legs are substantially simultaneously integrally injection molded from the moldable material. The moldable material includes an elastomer. The legs that are substantially simultaneously integrally injection molded from the moldable material are co-molded with at least a portion of the housing. Forces from rotation of the eccentric load interact with the resilient characteristic of the at least one driving leg to cause the plurality of legs to leave the supporting surface as the apparatus translates in the forward direction. Forces from rotation of the eccentric load interact with the resilient characteristic of at least a subset of the plurality of legs to cause the plurality of legs to leave the supporting surface as the apparatus translates in the forward direction. The forces from rotation of the eccentric load interact with the resilient characteristic of at least a subset of the plurality of legs to cause the at least one driving leg to leave the supporting surface by a greater distance than others in the plurality of legs as the apparatus translates in the forward direction. At least one leg is adapted to drag, and the at least one leg adapted to drag includes a leg that is in contact with the supporting surface a greater relative amount of time than the at least one driving leg as forces from rotation of the eccentric load interact with the resilient characteristic of at least a subset of the plurality of legs to cause the plurality of legs to leave the supporting surface. A coefficient of friction of a portion of at least a subset of the legs that contact a support surface is sufficient to substantially eliminate drifting in a lateral direction. The at least one driving leg is configured to tend to bend, in a direction opposite the direction of movement, without substantial slippage on a support surface when a net downward force exists between the one or more driving legs and the support surface, where bending of the at least driving leg induces the movement in the forward direction. The at least one leg is configured to tend to return to a neutral position without inducing a sufficient force opposite the direction of movement to overcome a momentum of the apparatus resulting from the movement in the forward direction and/or to overcome a frictional force between one or more other legs of the plurality of legs and the support surface when a net upward force exists between the at least one driving leg and the support surface. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of molded legs each having a leg base and a leg tip at a distal end relative to the leg base. The legs are coupled to the housing at the leg base and include at least one driving leg configured to cause the apparatus to move in a forward direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. The at least one driving leg is configured to tend to bend, in a direction opposite the direction of movement, without substantial slippage on a support surface when a net downward force exists between the at least one driving leg and the support surface. The at least one driving leg is also configured to tend to return to a neutral position without inducing a sufficient force opposite the direction of movement to overcome a momentum in the forward direction. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. The legs are coupled to the housing at the leg base and include at least one driving leg constructed from a flexible material and configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. Fewer than twenty legs contact a support surface as the at least one driving leg causes the apparatus to move. 
     These and other embodiments can each optionally include one or more of the following features. Fewer than twenty legs provide support when the apparatus is in an upright position. The legs that provide support when the apparatus is in an upright position are sufficiently stiff that four or fewer legs capable of supporting the apparatus without substantial deformation when the apparatus is in an upright position. The legs that provide support deform less than five percent relative to the height of the device under the weight of the device. A coefficient of friction of a portion of legs that contact a support surface is sufficient to substantially eliminate drifting in a lateral direction as the at least one driving leg causes the apparatus to move. The legs that provide support are molded from a elastomeric material. At least a subset of the legs that provide support are molded from an elastomeric material. The legs that provide support are injection molded. The legs that are molded from an elastomeric material are substantially simultaneously integrally injection molded. The legs that are substantially simultaneously integrally injection molded from the elastomeric material are co-molded with at least a portion of the housing. At least a portion of the legs that provide support are curved. The legs that provide support are tapered. The housing includes at least a nose and two lateral sides and each leg is coupled to the housing in a vicinity of one of the lateral sides. A diameter of the at least one driving leg is at least five percent of the length of the leg. A diameter of the at least one driving leg is at least ten percent of the length of the leg. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. The legs are coupled to the housing at the leg base and include at least one driving leg constructed from a flexible material and configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. A coefficient of friction of a portion of at least a subset of the plurality of legs that contact a support surface is sufficient to substantially eliminate drifting in a lateral direction. 
     These and other embodiments can each optionally include one or more of the following features. The plurality of legs are constructed from an elastomeric material. The plurality of legs are molded from the elastomeric material. At least a subset of the legs and at least a portion of the housing are co-molded from an elastomeric material. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of molded legs each having a leg base and a leg tip at a distal end relative to the leg base. The molded legs are coupled to the housing at the leg base and include at least one driving leg constructed from a flexible material and configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. 
     These and other embodiments can each optionally include one or more of the following features. A coefficient of friction of at least the driving leg is sufficient to substantially eliminate slipping on a support surface when rotation of the eccentric load causes a net downward force on the at least one driving leg. The plurality of molded legs are co-molded with at least a portion of the housing. The molded legs are injection molded. The plurality of molded legs are integrally molded. The plurality of molded legs are integrally molded with at least a portion of the housing. The integrally molded plurality of molded legs and portion of the housing are molded from an elastomeric material. The portion of the housing includes a nose section of the housing. The plurality of molded legs are curved. The plurality of molded legs are tapered. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of tapered legs each having a leg base and a leg tip at a distal end relative to the leg base. The tapered legs are coupled to the housing at the leg base and include at least one driving leg constructed from a flexible material and configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. 
     These and other embodiments can each optionally include one or more of the following features. The plurality of tapered legs are injection molded. At least a portion of the plurality of tapered legs are curved in a direction from the leg base to the leg tip. A diameter of the at least one driving leg is at least five percent of the length of the driving leg. A diameter of each of the plurality of tapered legs is at least five percent of the length of the leg. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of curved legs each having a leg base and a leg tip at a distal end relative to the leg base. The curved legs are coupled to the housing at the leg base and include at least one driving leg constructed from a flexible material and configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. The plurality of curved legs are curved in the direction generally defined by the offset between the leg base and the leg tip. 
     These and other embodiments can each optionally include one or more of the following features. The housing includes at least a nose and two lateral sides and each leg is coupled to the housing in a vicinity of one of the lateral sides. A diameter of each of the plurality of legs is at least five percent of the length of the leg. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base and each having a diameter of at least five percent of a length of the leg between the leg base and the leg tip. The legs are coupled to the housing at the leg base and include at least one driving leg constructed from a flexible material and configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. 
     These and other embodiments can each optionally include one or more of the following features. Each of the plurality of legs includes a diameter of at least ten percent of the length of the leg. 
     In general, another aspect of the subject matter described in this specification can be embodied in apparatus that include a housing, a rotational motor situated within the housing, an eccentric load adapted to be rotated by the rotational motor, and a plurality of legs each having a leg base and a leg tip at a distal end relative to the leg base. The legs are coupled to the housing at the leg base and include at least one driving leg constructed from an elastomeric material and configured to cause the apparatus to move in a direction generally defined by an offset between the leg base and the leg tip as the rotational motor rotates the eccentric load. 
     In general, another aspect of the subject matter described in this specification can be embodied in methods that include the acts of supporting a device on a substantially flat surface and inducing vibration of the device to cause the device to move across the substantially flat surface in a forward direction. The device includes a housing and a plurality of molded legs each having a leg base and a leg tip at a distal end relative to the leg base, and the legs are coupled to the housing at the leg base and include at least one elastomeric driving leg. The forward direction is generally defined by an offset between the leg base and the leg tip of the at least one driving leg as the device vibrates. Vibration of the device causes the at least one driving leg to deflect in a direction opposite the forward direction without substantial slipping of the at least one driving leg on the surface when net forces on the at least one driving leg are downward, and resiliency of the at least one elastomeric driving leg causes the at least one driving leg to deflect in the forward direction when net forces on the at least one driving leg are upward. 
     These and other embodiments can each optionally include one or more of the following features. Forces induced by the vibration of the device cause the at least one driving leg to leave the substantially flat surface during at least a portion of intervals in which the net forces on the at least one driving leg are upward. The forces induced by the vibration of the device cause the at least one driving leg to leave the substantially flat surface by differing amounts depending on varying upward forces resulting from the resiliency of the at least one driving leg. A subset of the plurality of legs tend to be in contact with the surface for a greater proportion of time than the at least one driving leg and legs in the subset of legs on each lateral side of the device include different drag characteristics. Greater drag forces can be generated, based on the different drag characteristics, with legs from the subset of legs on one lateral side of the device than on another lateral side of the device as the device moves in the forward direction. The legs on each lateral side of the device are arranged in a row. The vibration is induced by a rotational motor rotating an eccentric load. The method further includes the act of inducing rolling of the device to an upright position based on the rotation of the eccentric load in combination with an outer shape of the device generally along a longitudinal dimension that is substantially parallel to an axis of rotation of the rotational motor. The plurality of legs are arranged in two rows along each lateral side of the device and the rows are substantially parallel to the axis of rotation of the rotational motor, and the method can further include the act of stopping rolling of the device when the device reaches an upright position based on a spacing of the two rows of legs. The device includes an outer perimeter including a nose, a first shoulder on a first lateral side, and a second shoulder on a second lateral side. The nose, the first shoulder, and the second shoulder are constructed from a resilient material and the nose has increased elasticity relative to the first shoulder and the second shoulder, and the method further includes the act of inducing the device to bounce off an obstacle using the resilient material at the nose of the device. The vibration is induced by a rotational motor rotating an eccentric load and at least a subset of the plurality of legs include a sufficient coefficient of friction to substantially reduce lateral drifting, when the legs are in contact with the surface, resulting from lateral forces induced by the rotation of the eccentric load. 
     In general, another aspect of the subject matter described in this specification can be embodied in methods that include the acts of molding an undercarriage for a device, molding an upper shell having low elasticity, co-molding the upper shell and an elastomeric material to form an upper body, and attaching the upper body to the undercarriage to form a device housing. The upper body includes a plurality of molded legs each having a leg base and a leg tip at a distal end relative to the leg base, and the molded legs are coupled to the housing at the leg base and include at least one driving leg. The device housing encloses an eccentric load, a rotational motor adapted to rotate the eccentric load, and a power source electrically coupled to the rotational motor, wherein the at least one driving leg is configured to cause the device to move in a direction generally defined by an offset between the leg base and the leg tip when the rotational motor rotates the eccentric load. 
     These and other embodiments can each optionally include one or more of the following features. Co-molding the upper shell and the elastomeric material includes injection molding at least the elastomeric material. At least the legs of the upper body and a shoulder on each lateral side of the upper body are integrally molded. The at least one driving leg is curved. The plurality of molded legs are tapered. The plurality of molded legs each have a diameter of at least five percent of a length of the leg between the leg base and the leg tip. 
     The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram that illustrates an example vibration powered device. 
         FIGS. 2A through 2D  are diagrams that illustrate example forces that are involved with movement of the vibration powered device of  FIG. 1 . 
         FIGS. 3A through 3C  are diagrams that show various examples of alternative leg configurations for vibration powered devices. 
         FIG. 4  shows an example front view indicating a center of gravity for the device. 
         FIG. 5  shows an example side view indicating a center of gravity for the device. 
         FIG. 6  shows a top view of the device and its flexible nose. 
         FIGS. 7A and 7B  show example dimensions of the device. 
         FIG. 8  shows one example configuration of example materials from which the device can be constructed. 
         FIGS. 9A and 9B  show example devices that include a shark/dorsal fin and a pair of side/pectoral fins, respectively. 
         FIG. 10  is a flow diagram of a process for operating a vibration-powered device. 
         FIG. 11  is a flow diagram of a process for constructing a vibration-powered device. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Small robotic devices, or vibration-powered vehicles, can be designed to move across a surface, e.g., a floor, table, or other relatively flat surface. The robotic device is adapted to move autonomously and, in some implementations, turn in seemingly random directions. In general, the robotic devices include a housing, multiple legs, and a vibrating mechanism (e.g., a motor or spring-loaded mechanical winding mechanism rotating an eccentric load, a motor or other mechanism adapted to induce oscillation of a counterweight, or other arrangement of components adapted to rapidly alter the center of mass of the device). As a result, the miniature robotic devices, when in motion, can resemble organic life, such as bugs or insects. 
     Movement of the robotic device can be induced by the motion of a rotational motor inside of, or attached to, the device, in combination with a rotating weight with a center of mass that is offset relative to the rotational axis of the motor. The rotational movement of the weight causes the motor and the robotic device to which it is attached to vibrate. In some implementations, the rotation is approximately in the range of 6000-9000 revolutions per minute (rpm&#39;s), although higher or lower rpm values can be used. As an example, the device can use the type of vibration mechanism that exists in many pagers and cell phones that, when in vibrate mode, cause the pager or cell phone to vibrate. The vibration induced by the vibration mechanism can cause the device to move across the surface (e.g., the floor) using legs that are configured to alternately flex (in a particular direction) and return to the original position as the vibration causes the device to move up and down. 
     Various features can be incorporated into the robotic devices. For example, various implementations of the devices can include features (e.g., shape of the legs, number of legs, frictional characteristics of the leg tips, relative stiffness or flexibility of the legs, resiliency of the legs, relative location of the rotating counterweight with respect to the legs, etc.) for facilitating efficient transfer of vibrations to forward motion. The speed and direction of the robotic device&#39;s movement can depend on many factors, including the rotational speed of the motor, the size of the offset weight attached to the motor, the power supply, the characteristics (e.g., size, orientation, shape, material, resiliency, frictional characteristics, etc.) of the “legs” attached to the housing of the device, the properties of the surface on which the device operates, the overall weight of the device, and so on. 
     In some implementations, the devices include features that are designed to compensate for a tendency of the device to turn as a result of the rotation of the counterweight and/or to alter the tendency for, and direction of, turning between different robotic devices. The components of the device can be positioned to maintain a relatively low center of gravity (or center of mass) to discourage tipping (e.g., based on the lateral distance between the leg tips) and to align the components with the rotational axis of the rotating motor to encourage rolling (e.g., when the device is not upright). Likewise, the device can be designed to encourage self-righting based on features that tend to encourage rolling when the device is on its back or side in combination with the relative flatness of the device when it is upright (e.g., when the device is “standing” on its leg tips). Features of the device can also be used to increase the appearance of random motion and to make the device appear to respond intelligently to obstacles. Different leg configurations and placements can also induce different types of motion and/or different responses to vibration, obstacles, or other forces. Moreover, adjustable leg lengths can be used to provide some degree of steering capability. In some implementations, the robotic devices can simulate real-life objects, such as crawling bugs, rodents, or other animals and insects. 
       FIG. 1  is a diagram that illustrates an example device  100  that is shaped like a bug. The device  100  includes a housing  102  (e.g., resembling the body of the bug) and legs  104 . Inside (or attached to) the housing  102  are the components that control and provide movement for the device  100 , including a rotational motor, power supply (e.g., a battery), and an on/off switch. Each of the legs  104  includes a leg tip  106   a  and a leg base  106   b . The properties of the legs  104 , including the position of the leg base  106   b  relative to the leg tip  106   a , can contribute to the direction and speed in which the device  100  tends to move. The device  100  is depicted in an upright position (i.e., standing on legs  104 ) on a supporting surface  110  (e.g., a substantially planar floor, table top, etc. that counteracts gravitational forces). 
     Overview of Legs 
     Legs  104  can include front legs  104   a , middle legs  104   b , and rear legs  104   c . For example, the device  100  can include a pair of front legs  104   a  that may be designed to perform differently from middle legs  104   b  and rear legs  104   c . For example, the front legs  104   a  may be configured to provide a driving force for the device  100  by contacting an underlying surface  110  and causing the device to hop forward as the device vibrates. Middle legs  104   b  can help provide support to counteract material fatigue (e.g., after the device  100  rests on the legs  104  for long periods of time) that may eventually cause the front legs  104   a  to deform and/or lose resiliency. In some implementations, device  100  can exclude middle legs  104   b  and include only front legs  104   a  and rear legs  104   c . In some implementations, front legs  104   a  and one or more rear legs  104   c  can be designed to be in contact with a surface, while middle legs  104   b  can be slightly off the surface so that the middle legs  104   b  do not introduce significant additional drag forces and/or hopping forces that may make it more difficult to achieve desired movements (e.g., tendency to move in a relatively straight line and/or a desired amount of randomness of motion). 
     In some implementations, the device  100  can be configured such that only two front legs  104   a  and one rear leg  104   c  are in contact with a substantially flat surface  110 , even if the device includes more than one rear leg  104   c  and several middle legs  104   b . In other implementations, the device  100  can be configured such that only one front leg  104   a  and two rear legs  104   c  are in contact with a flat surface  110 . Throughout this specification, descriptions of being in contact with the surface can include a relative degree of contact. For example, when one or more of the front legs  104   a  and one or more of the back legs  104   c  are described as being in contact with a substantially flat surface  110  and the middle legs  104   b  are described as not being in contact with the surface  110 , it is also possible that the front and back legs  104   a  and  104   c  can simply be sufficiently longer than the middle legs  104   b  (and sufficiently stiff) that the front and back legs  104   a  and  104   c  provide more support for the weight of the device  100  than do the middle legs  104   b , even though the middle legs  104   b  are technically actually in contact with the surface  110 . In some implementations, even legs that have a lesser contribution to support of the device may nonetheless be in contact when the device  100  is in an upright position, especially when vibration of the device causes an up and down movement that compresses and bends the driving legs and allows additional legs to contact the surface  110 . Greater predictability and control of movement (e.g., in a straight direction) can be obtained by constructing the device so that a sufficiently small number of legs (e.g., fewer than twenty or fewer than thirty) contact the support surface  110  and/or contribute to the support of the device in the upright position when the device is either at rest or as the rotating eccentric load induces movement. In this respect, it is possible for some legs to provide support even without contacting the support surface  110  (e.g., one or more short legs can provide stability by contacting an adjacent longer leg to increase overall stiffness of the adjacent longer leg). Typically, however, each leg is sufficiently stiff that four or fewer legs are capable of supporting the weight of the device without substantial deformation (e.g., less than 5% as a percentage of the height of the leg base  106   b  from the support surface  110  when the device  100  is in an upright position). 
     Different leg lengths can be used to introduce different movement characteristics, as further discussed below. The various legs can also include different properties, e.g., different stiffnesses or coefficients of friction, as further described below. Generally, the legs can be arranged in substantially parallel rows along each lateral side of the device  100  (e.g.,  FIG. 1  depicts one row of legs on the right lateral side of the device  100 ; a corresponding row of legs (not shown in  FIG. 1 ) can be situated along the left lateral side of the device  100 ). 
     In general, the number of legs  104  that provide meaningful or any support for the device can be relatively limited. For example, the use of less than twenty legs that contact the support surface  110  and/or that provide support for the device  100  when the device  100  is in an upright position (i.e., an orientation in which the one or more driving legs  104   a  are in contact with a support surface) can provide more predictability in the directional movement tendencies of the device  100  (e.g., a tendency to move in a relatively straight and forward direction), or can enhance a tendency to move relatively fast by increasing the potential deflection of a smaller number of legs, or can minimize the number of legs that may need to be altered to achieve the desired directional control, or can improve the manufacturability of fewer legs with sufficient spacing to allow room for tooling. In addition to providing support by contacting the support surface  110 , legs  104  can provide support by, for example, providing increased stability for legs that contact the surface  110 . In some implementations, each of the legs that provides independent support for the device  100  is capable of supporting a substantial portion of the weight of the device  100 . For example, the legs  104  can be sufficiently stiff that four or fewer legs are capable of statically (e.g., when the device is at rest) supporting the device without substantial deformation of the legs  104  (e.g., without causing the legs to deform such that the body of the device  100  moves more than 5% as a percentage of the height of the leg base  106   b  from the support surface). 
     As described here at a high level, many factors or features can contribute to the movement and control of the device  100 . For example, the device&#39;s center of gravity (CG), and whether it is more forward or towards the rear of the device, can influence the tendency of the device  100  to turn. Moreover, a lower CG can help to prevent the device  100  from tipping over. The location and distribution of the legs  104  relative to the CG can also prevent tipping. For example, if pairs or rows of legs  104  on each side of the device  100  are too close together and the device  100  has a relatively high CG (e.g., relative to the lateral distance between the rows or pairs of legs), then the device  100  may have a tendency to tip over on its side. Thus, in some implementations, the device includes rows or pairs of legs  104  that provide a wider lateral stance (e.g., pairs of front legs  104   a , middle legs  104   b , and rear legs  104   c  are spaced apart by a distance that defines an approximate width of the lateral stance) than a distance between the CG and a flat supporting surface on which the device  100  rests in an upright position. For example, the distance between the CG and the supporting surface can be in the range of 50-80% of the value of the lateral stance (e.g., if the lateral stance is 0.5 inches, the CG may be in the range of 0.25-0.4 inches from the surface  110 ). Moreover, the vertical location of the CG of the device  100  can be within a range of 40-60% of the distance between a plane that passes through the leg tips  106   a  and the highest protruding surface on the top side of the housing  102 . In some implementations, a distance  409   a  and  409   b  (as shown in  FIG. 4 ) between each row of the tips of legs  104  and a longitudinal axis of the device  100  that runs through the CG can be roughly the same or less than the distance  406  (as shown in  FIG. 4 ) between the tips  106   a  of two rows of legs  104  to help facilitate stability when the device is resting on both rows of legs. 
     The device  100  can also include features that generally compensate for the device&#39;s tendency to turn. Driving legs (e.g., front legs  104   a ) can be configured such that one or more legs on one lateral side of the device  100  can provide a greater driving force than one or more corresponding legs on the other lateral side of the device  100  (e.g., through relative leg lengths, relative stiffness or resiliency, relative fore/aft location in the longitudinal direction, or relative lateral distance from the CG). Similarly, dragging legs (e.g., back legs  104   c ) can be configured such that one or more legs on one lateral side of the device  100  can provide a greater drag force than one or more corresponding legs on the other lateral side of the device  100  (e.g., through relative leg lengths, relative stiffness or resiliency, relative fore/aft location in the longitudinal direction, or relative lateral distance from the CG). In some implementations, the leg lengths can be tuned either during manufacturing or subsequently to modify (e.g., increase or reduce) a tendency of the device to turn. 
     Movement of the device can also be influenced by the leg geometry of the legs  104 . For example, a longitudinal offset between the leg tip (i.e., the end of the leg that touches the surface  110 ) and the leg base (i.e., the end of the leg that attaches to the device housing) of any driving legs induces movement in a forward direction as the device vibrates. Including some curvature, at least in the driving legs, further facilitates forward motion as the legs tend to bend, moving the device forward, when vibrations force the device downward and then spring back to a straighter configuration as the vibrations force the device upward (e.g., resulting in hopping completely or partially off the surface, such that the leg tips move forward above or slide forward across the surface  110 ). 
     The ability of the legs to induce forward motion results in part from the ability of the device to vibrate vertically on the resilient legs. As shown in  FIG. 1 , the device  100  includes an underside  122 . The power supply and motor for the device  100  can be contained in a chamber that is formed between the underside  122  and the upper body of the device, for example. The length of the legs  104  creates a space  124  (at least in the vicinity of the driving legs) between the underside  122  and the surface  110  on which the device  100  operates. The size of the space  124  depends on how far the legs  104  extend below the device relative to the underside  122 . The space  124  provides room for the device  100  (at least in the vicinity of the driving legs) to move downward as the periodic downward force resulting from the rotation of the eccentric load causes the legs to bend. This downward movement can facilitate forward motion induced by the bending of the legs  104 . 
     The device can also include the ability to self-right itself, for example, if the device  100  tips over or is placed on its side or back. For example, constructing the device  100  such that the rotational axis of the motor and the eccentric load are approximately aligned with the longitudinal CG of the device  100  tends to enhance the tendency of the device  100  to roll (i.e., in a direction opposite the rotation of the motor and the eccentric load). Moreover, construction of the device housing to prevent the device from resting on its top or side (e.g., using one or more protrusions on the top and/or sides of the device housing) and to increase the tendency of the device to bounce when on its top or side can enhance the tendency to roll. Furthermore, constructing the legs of a sufficiently flexible material and providing clearance on the housing undercarriage that the leg tips to bend inward can help facilitate rolling of the device from its side to an upright position. 
       FIG. 1  shows a body shoulder  112  and a head side surface  114 , which can be constructed from rubber, elastomer, or other resilient material, contributing to the device&#39;s ability to self-right after tipping. The bounce from the shoulder  112  and the head side surface  114  can be significantly more than the lateral bounce achieved from the legs, which can be made of rubber or some other elastomeric material, but which can be less resilient than the shoulder  112  and the head side surface  114  (e.g., due to the relative lateral stiffness of the shoulder  112  and the head side surface  114  compared to the legs  104 ). Rubber legs  104 , which can bend inward toward the body  102  as the device  100  rolls, increase the self-righting tendency, especially when combined with the angular/rolling forces induced by rotation of the eccentric load. The bounce from the shoulder  112  and the head side surface  114  can also allow the device  100  to become sufficiently airborne that the angular forces induced by rotation of the eccentric load can cause the device to roll, thereby facilitating self-righting. 
     The device can also be configured to include a degree of randomness of motion, which can make the device  100  appear to behave like an insect or other animate object. For example, vibration induced by rotation of the eccentric load can further induce hopping as a result of the curvature and “tilt” of the legs. The hopping can further induce a vertical acceleration (e.g., away from the surface  110 ) and a forward acceleration (e.g., generally toward the direction of forward movement of the device  100 ). During each hop, the rotation of the eccentric load can further cause the device to turn toward one side or the other depending on the location and direction of movement of the eccentric load. The degree of random motion can be increased if relatively stiffer legs are used to increase the amplitude of hopping. The degree of random motion can be influenced by the degree to which the rotation of the eccentric load tends to be either in phase or out of phase with the hopping of the device (e.g., out of phase rotation relative to hopping may increase the randomness of motion). The degree of random motion can also be influenced by the degree to which the back legs  104   c  tend to drag. For example, dragging of back legs  104   c  on both lateral sides of the device  100  may tend to keep the device  100  traveling in a more straight line, while back legs  104   c  that tend to not drag (e.g., if the legs bounce completely off the ground) or dragging of back legs  104   c  more on one side of the device  100  than the other can tend to increase turning. 
     Another feature is “intelligence” of the device  100 , which can allow the device to interact in an apparently intelligent manner with obstacles, including, for example, bouncing off any obstacles (e.g., walls, etc.) that the device  100  encounters during movement. For example, the shape of the nose  108  and the materials from which the nose  108  is constructed can enhance a tendency of the device to bounce off of obstacles and to turn away from the obstacle. Each of these features can contribute to how the device  100  moves, and will be described below in more detail. 
       FIG. 1  illustrates a nose  108  that can contribute to the ability of the device  100  to deflect off of obstacles. Nose left side  116   a  and nose right side  116   b  can form the nose  108 . The nose sides  116   a  and  116   b  can form a shallow point or another shape that helps to cause the device  100  to deflect off obstacles (e.g., walls) encountered as the device  100  moves in a generally forward direction. The device  100  can includes a space within the head  118  that increases bounce by making the head more elastically deformable (i.e., reducing the stiffness). For example, when the device  100  crashes nose-first into an obstacle, the space within the head  118  allows the head of the device  100  to compress, which provides greater control over the bounce of the device  100  away from the obstacle than if the head  118  is constructed as a more solid block of material. The space within the head  118  can also better absorb impact if the device falls from some height (e.g., a table). The body shoulder  112  and head side surface  114 , especially when constructed from rubber or other resilient material, can also contribute to the device&#39;s tendency to deflect or bounce off of obstacles encountered at a relatively high angle of incidence. 
     Wireless/Remote Control Embodiments 
     In some implementations, the device  100  includes a receiver that can, for example, receive commands from a remote control unit. Commands can be used, for example, to control the device&#39;s speed and direction, and whether the device is in motion or in a motionless state, to name a few examples. In some implementations, controls in the remote control unit can engage and disengage the circuit that connects the power unit (e.g., battery) to the device&#39;s motor, allowing the operator of the remote control to start and stop the device  100  at any time. Other controls (e.g., a joy stick, sliding bar, etc.) in the remote control unit can cause the motor in the device  100  to spin faster or slower, affecting the speed of the device  100 . The controls can send the receiver on the device  100  different signals, depending on the commands that correspond to the movement of the controls. Controls can also turn on and off a second motor attached to a second eccentric load in the device  100  to alter lateral forces for the device  100 , thereby changing a tendency of the device to turn and thus providing steering control. Controls in a remote control unit can also cause mechanisms in the device  100  to lengthen or shorten one or more of the legs and/or deflecting one or more of the legs forward, backward, or laterally to provide steering control. 
     Leg Motion and Hop 
       FIGS. 2A through 2D  are diagrams that illustrate example forces that induce movement of the device  100  of  FIG. 1 . Some forces are provided by a rotational motor  202 , which enable the device  100  to move autonomously across the surface  110 . For example, the motor  202  can rotate an eccentric load  210  that generates moment and force vectors  205 - 215  as shown in  FIGS. 2A-2D . Motion of the device  100  can also depend in part on the position of the legs  104  with respect to the counterweight  210  attached to the rotational motor  202 . For example, placing the counterweight  210  in front of the front legs  104   a  will increase the tendency of the front legs  104   a  to provide the primary forward driving force (i.e., by focusing more of the up and down forces on the front legs). For example, the distance between the counterweight  210  and the tips of the driving legs can be within a range of 20-100% of an average length of the driving legs. Moving the counterweight  210  back relative to the front legs  104   a  can cause other legs to contribute more to the driving forces. 
       FIG. 2A  shows a side view of the example device  100  shown in  FIG. 1  and further depicts a rotational moment  205  (represented by the rotational velocity ω m  and motor torque T m ) and a vertical force  206  represented by F v .  FIG. 2B  shows a top view of the example device  100  shown in  FIG. 1  and further shows a horizontal force  208  represented by F h . Generally, a negative F v  is caused by upward movement of the eccentric load as it rotates, while a positive F v  can be caused by the downward movement of the eccentric load and/or the resiliency of the legs (e.g., as they spring back from a deflected position). 
     The forces F v  and F h  cause the device  100  to move in a direction that is consistent with the configuration in which the leg base  106   b  is positioned in front of the leg tip  106   a . The direction and speed in which the device  100  moves can depend, at least in part, on the direction and magnitude of F v  and F h . When the vertical force  206 , F v , is negative, the device  100  body is forced down. This negative F v  causes at least the front legs  104   a  to bend and compress. The legs generally compress along a line in space from the leg tip to the leg base. As a result, the body will lean so that the leg bends (e.g., the leg base  106   b  flexes (or deflects) about the leg tip  106   a  towards the surface  110 ) and causes the body to move forward (e.g., in a direction from the leg tip  106   a  towards the leg base  106   b ). F v , when positive, provides an upward force on the device  100  allowing the energy stored in the compressed legs to release (lifting the device), and at the same time allowing the legs to drag or hop forward to their original position. The lifting force F v  on the device resulting from the rotation of the eccentric load combined with the spring-like leg forces are both involved in allowing the vehicle to hop vertically off the surface (or at least reducing the load on the front legs  104   a ) and allowing the legs  104  to return to their normal geometry (i.e., as a result of the resiliency of the legs). The release of the spring-like leg forces, along with the forward momentum created as the legs bend, propels the vehicle forward and upward, based on the angle of the line connecting the leg tip to the leg base, lifting the front legs  104   a  off the surface  110  (or at least reducing the load on the front legs  104   a ) and allowing the legs  104  to return to their normal geometry (i.e., as a result of the resiliency of the legs). 
     Generally, two “driving” legs (e.g., the front legs  104   a , one on each side) are used, although some implementations may include only one driving leg or more than two driving legs. Which legs constitute driving legs can, in some implementations, be relative. For example, even when only one driving leg is used, other legs may provide a small amount of forward driving forces. During the forward motion, some legs  104  may tend to drag rather than hop. Hop refers to the result of the motion of the legs as they bend and compress and then return to their normal configuration—depending on the magnitude of F v , the legs can either stay in contact with the surface or lift off the surface for a short period of time as the nose is elevated. For example, if the eccentric load is located toward the front of the device  100 , then the front of the device  100  can hop slightly, while the rear of the device  100  tends to drag. In some cases, however, even with the eccentric load located toward the front of the device  100 , even the back legs  104   c  may sometimes hop off the surface, albeit to a lesser extent than the front legs  104   a . Depending on the stiffness or resiliency of the legs, the speed of rotation of the rotational motor, and the degree to which a particular hop is in phase or out of phase with the rotation of the motor, a hop can range in duration from less than the time required for a full rotation of the motor to the time required for multiple rotations of the motor. During a hop, rotation of the eccentric load can cause the device to move laterally in one direction or the other (or both at different times during the rotation) depending on the lateral direction of rotation at any particular time and to move up or down (or both at different times during the rotation) depending on the vertical direction of rotation at any particular time. 
     Increasing hop time can be a factor in increasing speed. The more time that the vehicle spends with some of the leg off the surface  110  (or lightly touching the surface), the less time some of the legs are dragging (i.e., creating a force opposite the direction of forward motion) as the vehicle translates forward. Minimizing the time that the legs drag forward (as opposed to hop forward) can reduce drag caused by friction of the legs sliding along the surface  110 . In addition, adjusting the CG of the device fore and aft can effect whether the vehicle hops with the front legs only, or whether the vehicle hops with most, if not all, of the legs off the ground. This balancing of the hop can take into account the CG, the mass of the offset weight and its rotational frequency, F v  and its location, and hop forces and their location(s). 
     Turning of Device 
     The motor rotation also causes a lateral force  208 , F h , which generally shifts back and forth as the eccentric load rotates. In general, as the eccentric load rotates (e.g., due to the motor  202 ), the left and right horizontal forces  208  are equal. The turning that results from the lateral force  208  on average typically tends to be greater in one direction (right or left) while the device&#39;s nose  108  is elevated, and greater in the opposite direction when the device&#39;s nose  108  and the legs  104  are compressed down. During the time that the center of the eccentric load  210  is traveling upward (away from the surface  110 ), increased downward forces are applied to the legs  104 , causing the legs  104  to grip the surface  110 , minimizing lateral turning of the device  100 , although the legs may slightly bend laterally depending on the stiffness of the legs  104 . During the time when the eccentric load  210  is traveling downward, the downward force on the legs  104  decreases, and downward force of the legs  104  on the surface  110  can be reduced, which can allow the device to turn laterally during the time the downward force is reduced. The direction of turning generally depends on the direction of the average lateral forces caused by the rotation of the eccentric load  210  during the time when the vertical forces are positive relative to when the vertical forces are negative. Thus, the horizontal force  208 , F h , can cause the device  100  to turn slightly more when the nose  108  is elevated. When the nose  108  is elevated, the leg tips are either off the surface  110  or less downward force is on the front legs  104   a  which precludes or reduces the ability of the leg tips (e.g., leg tip  106   a ) to “grip” the surface  110  and to provide lateral resistance to turning. Features can be implemented to manipulate several motion characteristics to either counteract or enhance this tendency to turn. 
     The location of the CG can also influence a tendency to turn. While some amount of turning by the device  100  can be a desired feature (e.g., to make the device&#39;s movement appear random), excessive turning can be undesirable. Several design considerations can be made to compensate for (or in some cases to take advantage of) the device&#39;s tendency to turn. For example, the weight distribution of the device  100 , or more specifically, the device&#39;s CG, can affect the tendency of the device  100  to turn. In some implementations, having CG relatively near the center of the device  100  and roughly centered about the legs  104  can increase a tendency for the device  100  to travel in a relatively straight direction (e.g., not spinning around). 
     Tuning the drag forces for different legs  104  is another way to compensate for the device&#39;s tendency to turn. For example, the drag forces for a particular leg  104  can depend on the leg&#39;s length, thickness, stiffness and the type of material from which the leg is made. In some implementations, the stiffness of different legs  104  can be tuned differently, such as having different stiffness characteristics for the front legs  104   a , rear legs  104   c  and middle legs  104   b . For example, the stiffness characteristics of the legs can be altered or tuned based on the thickness of the leg or the material used for the leg. Increasing the drag (e.g., by increasing a leg length, thickness, stiffness, and/or frictional characteristic) on one side of the device (e.g., the right side) can help compensate for a tendency of the device to turn (e.g., to the left) based on the force F h  induced by the rotational motor and eccentric load. 
     Altering the position of the rear legs  104   c  is another way to compensate for the device&#39;s tendency to turn. For example, placing the legs  104  further toward the rear of the device  100  can help the device  100  travel in a more straight direction. Generally, a longer device  100  that has a relatively longer distance between the front and rear legs  104   c  may tend to travel in more of a straight direction than a device  100  that is shorter in length (i.e., the front legs  104   a  and rear legs  104   c  are closer together), at least when the rotating eccentric load is located in a relatively forward position on the device  100 . The relative position of the rearmost legs  104  (e.g., by placing the rearmost leg on one side of the device farther forward or backward on the device than the rearmost leg on the other side of the device) can also help compensate for (or alter) the tendency to turn. 
     Various techniques can also be used to control the direction of travel of the device  100 , including altering the load on specific legs, adjusting the number of legs, leg lengths, leg positions, leg stiffness, and drag coefficients. As illustrated in  FIG. 2B , the lateral horizontal force  208 , F h , causes the device  100  to have a tendency to turn as the lateral horizontal force  208  generally tends to be greater in one direction than the other during hops. The horizontal force  208 , F h  can be countered to make the device  100  move in an approximately straight direction. This result can be accomplished with adjustments to leg geometry and leg material selection, among other things. 
       FIG. 2C  is a diagram that shows a rear view of the device  100  and further illustrates the relationship of the vertical force  206  F v  and the horizontal force  208  F h  in relation to each other. This rear view also shows the eccentric load  210  that is rotated by the rotational motor  202  to generate vibration, as indicated by the rotational moment  205 . 
     Drag Forces 
       FIG. 2D  is a diagram that shows a bottom view of the device  100  and further illustrates example leg forces  211 - 214  that are involved with direction of travel of the device  100 . In combination, the leg forces  211 - 214  can induce velocity vectors that impact the predominant direction of travel of the device  100 . The velocity vector  215 , represented by T load , represents the velocity vector that is induced by the motor/eccentricity rotational velocity (e.g., induced by the offset load attached to the motor) as it forces the driving legs  104  to bend, causing the device to lunge forward, and as it generates greater lateral forces in one direction than the other during hopping. The leg forces  211 - 214 , represented by F 1 -F 4 , represent the reactionary forces of the legs  104   a   1 - 104   c   2 , respectively, that can be oriented so the legs  104   a   1 - 104   c   2 , in combination, induce an opposite velocity vector relative to T load . As depicted in  FIG. 2D , T load  is a velocity vector that tends to steer the device  100  to the left (as shown) due to the tendency for there to be greater lateral forces in one direction than the other when the device is hopping off the surface  110 . At the same time, the forces F 1 -F 2  for the front legs  104   a   1  and  104   a   2  (e.g., as a result of the legs tending to drive the device forward and slightly laterally in the direction of the eccentric load  210  when the driving legs are compressed) and the forces F 3 -F 4  for the rear legs  104   c   1  and  104   c   2  (as a result of drag) each contribute to steering the device  100  to the right (as shown). (As a matter of clarification, because  FIG. 2D  shows the bottom view of the device  100 , the left-right directions when the device  100  is placed upright are reversed.) In general, if the combined forces F 1 -F 4  approximately offset the side component of T load , then the device  100  will tend to travel in a relatively straight direction. 
     Controlling the forces F 1 -F 4  can be accomplished in a number of ways. For example, the “push vector” created by the front legs  104   a   1  and  104   a   2  can be used to counter the lateral component of the motor-induced velocity. In some implementations, this can be accomplished by placing more weight on the front leg  104   a   2  to increase the leg force  212 , represented by F 2 , as shown in  FIG. 2D . Furthermore, a “drag vector” can also be used to counter the motor-induced velocity. In some implementations, this can be accomplished by increasing the length of the rear leg  104   c   2  or increasing the drag coefficient on the rear leg  104   c   2  for the force vector  804 , represented by F 4 , in  FIG. 2D . As shown, the legs  104   a   1  and  104   a   2  are the device&#39;s front right and left legs, respectively, and the legs  104   c   1  and  104   c   2  are the device&#39;s rear right and left legs, respectively. 
     Another technique for compensating for the device&#39;s tendency to turn is increasing the stiffness of the legs  104  in various combinations (e.g., by making one leg thicker than another or constructing one leg using a material having a naturally greater stiffness). For example, a stiffer leg will have a tendency to bounce more than a more flexible leg. Left and right legs  104  in any leg pair can have different stiffnesses to compensate for the turning of the device  100  induced by the vibration of the motor  202 . Stiffer front legs  104   a  can also produce more bounce. 
     Another technique for compensating for the device&#39;s tendency to turn is to change the relative position of the rear legs  104   c   1  and  104   c   2  so that the drag vectors tend to compensate for turning induced by the motor velocity. For example, the rear leg  104   c   2  can be placed farther forward (e.g., closer to the nose  108 ) than the rear leg  104   c   1 . 
     Leg Shape 
     Leg geometry contributes significantly to the way in which the device  100  moves. Aspects of leg geometry include: locating the leg base in front of the leg tip, curvature of the legs, deflection properties of the legs, configurations that result in different drag forces for different legs, including legs that do not necessarily touch the surface, and having only three legs that touch the surface, to name a few examples. 
     Generally, depending on the position of the leg tip  106   a  relative to the leg base  106   b , the device  100  can experience different behaviors, including the speed and stability of the device  100 . For example, if the leg tip  106   a  is nearly directly below the leg base  106   b  when the device  100  is positioned on a surface, movement of the device  100  that is caused by the motor  202  can be limited or precluded. This is because there is little or no slope to the line in space that connects the leg tip  106   a  and the leg base  106   b . In other words, there is no “lean” in the leg  104  between the leg tip  106   a  and the leg base  106   b . However, if the leg tip  106   a  is positioned behind the leg base  106   b  (e.g., farther from the nose  108 ), then the device  100  can move faster, as the slope or lean of the legs  104  is increased, providing the motor  202  with a leg geometry that is more conducive to movement. In some implementations, different legs  104  (e.g., including different pairs, or left legs versus right legs) can have different distances between leg tips  106   a  and leg bases  106   b.    
     In some implementations, the legs  104  are curved (e.g., leg  104   a  shown in  FIG. 2A , and legs  104  shown in  FIG. 1 ). For example, because the legs  104  are typically made from a flexible material, the curvature of the legs  104  can contribute to the forward motion of the device  100 . Curving the leg can accentuate the forward motion of the device  100  by increasing the amount that the leg compresses relative to a straight leg. This increased compression can also increase vehicle hopping, which can also increase the tendency for random motion, giving the device an appearance of intelligence and/or a more life-like operation. The legs can also have at least some degree of taper from the leg base  106   b  to the leg tip  106   a , which can facilitate easier removal from a mold during the manufacturing process. 
     The number of legs can vary in different implementations. In general, increasing the number of legs  104  can have the effect of making the device more stable and can help reduce fatigue on the legs that are in contact with the surface  110 . Increasing the number of legs can also affect the location of drag on the device  100  if additional leg tips  106   a  are in contact with the surface  110 . In some implementations, however, some of the legs (e.g., middle legs  104   b ) can be at least slightly shorter than others so that they tend not to touch the surface  110  or contribute less to overall friction that results from the leg tips  106   a  touching the surface  110 . For example, in some implementations, the two front legs  104   a  (e.g., the “driving” legs) and at least one of the rear legs  104   c  are at least slightly longer than the other legs. This configuration helps increase speed by increasing the forward driving force of the driving legs. In general, the remaining legs  104  can help prevent the device  100  from tipping over by providing additional resiliency should the device  100  start to lean toward one side or the other. 
     In some implementations, one or more of the “legs” can include any portion of the device that touches the ground. For example, the device  100  can include a single rear leg (or multiple rear legs) constructed from a relatively inflexible material (e.g., rigid plastic), which can resemble the front legs or can form a skid plate designed to simply drag as the front legs  104   a  provide a forward driving force. The oscillating eccentric load can repeat tens to several hundred times per second, which causes the device  100  to move in a generally forward motion as a result of the forward momentum generated when F v  is negative. 
     Leg geometry can be defined and implemented based on ratios of various leg measurements, including leg length, diameter, and radius of curvature. One ratio that can be used is the ratio of the radius of curvature of the leg  104  to the leg&#39;s length. As just one example, if the leg&#39;s radius of curvature is 49.14 mm and the leg&#39;s length is 10.276 mm, then the ratio is 4.78. In another example, if the leg&#39;s radius of curvature is 2.0 inches and the leg&#39;s length is 0.4 inches, then the ratio is 5.0. Other leg  104  lengths and radii of curvature can be used, such as to produce a ratio of the radius of curvature to the leg&#39;s length that leads to suitable movement of the device  100 . In general, the ratio of the radius of curvature to the leg&#39;s length can be in the range of 2.5 to 20.0. The radius of curvature can be approximately consistent from the leg base to the leg tip. This approximate consistent curvature can include some variation, however. For example, some taper angle in the legs may be required during manufacturing of the device (e.g., to allow removal from a mold). Such a taper angle may introduce slight variations in the overall curvature that generally do not prevent the radius of curvature from being approximately consistent from the leg base to the leg tip. 
     Another ratio that can be used to characterize the device  100  is a ratio that relates leg  104  length to leg diameter or thickness (e.g., as measured in the center of the leg or as measured based on an average leg diameter throughout the length of the leg and/or about the circumference of the leg). For example, the length of the legs  104  can be in the range of 0.2 inches to 0.8 inches (e.g., 0.405 inches) and can be proportional to (e.g., 5.25 times) the leg&#39;s thickness in the range of 0.03 to 0.15 inch (e.g., 0.077 inch). Stated another way, legs  104  can be about 15% to 25% as thick as they are long, although greater or lesser thicknesses (e.g., in the range of 5% to 60% of leg length) can be used. Leg  104  lengths and thicknesses can further depend on the overall size of the device  100 . In general, at least one driving leg can have a ratio of the leg length to the leg diameter in the range of 2.0 to 20.0 (i.e., in the range of 5% to 50% of leg length). In some implementations, a diameter of at least 10% of the leg length may be desirable to provide sufficient stiffness to support the weight of the device and/or to provide desired movement characteristics. 
     Leg Material 
     The legs are generally constructed of rubber or other flexible but resilient material (e.g., polystyrene-butadiene-styrene with a durometer near 65, based on the Shore A scale, or in the range of 55-75, based on the Shore A scale). Thus, the legs tend to deflect when a force is applied. Generally, the legs include a sufficient stiffness and resiliency to facilitate consistent forward movement as the device vibrates (e.g., as the eccentric load  210  rotates). The legs  104  are also sufficiently stiff to maintain a relatively wide stance when the device  100  is upright yet allow sufficient lateral deflection when the device  100  is on its side to facilitate self-righting, as further discussed below. 
     The selection of leg materials can have an effect on how the device  100  moves. For example, the type of material used and its degree of resiliency can affect the amount of bounce in the legs  104  that is caused by the vibration of the motor  202  and the counterweight  210 . As a result, depending on the material&#39;s stiffness (among other factors, including positions of leg tips  106   b  relative to leg bases  106   a ), the speed of the device  100  can change. In general, the use of stiffer materials in the legs  104  can result in more bounce, while more flexible materials can absorb some of the energy caused by the vibration of the motor  202 , which can tend to decrease the speed of the device  100 . 
     Frictional Characteristics 
     Friction (or drag) force equals the coefficient of friction multiplied by normal force. Different coefficients of friction and the resulting friction forces can be used for different legs. As an example, to control the speed and direction (e.g., tendency to turn, etc.), the leg tips  106   a  can have varying coefficients of friction (e.g., by using different materials) or drag forces (e.g., by varying the coefficients of friction and/or the average normal force for a particular leg). These differences can be accomplished, for example, by the shape (e.g., pointedness or flatness, etc.) of the leg tips  106   a  as well as the material of which they are made. Front legs  104   a , for example, can have a higher friction than the rear legs  104   c . Middle legs  104   b  can have yet different friction or can be configured such that they are shorter and do not touch the surface  110 , and thus do not tend to contribute to overall drag. Generally, because the rear legs  104   c  (and the middle legs  104   b  to the extent they touch the ground) tend to drag more than they tend to create a forward driving force, lower coefficients of friction and lower drag forces for these legs can help increase the speed of the device  100 . Moreover, to offset the motor force  215 , which can tend to pull the device in a left or right direction, left and right legs  104  can have different friction forces. Overall, coefficients of friction and the resulting friction force of all of the legs  104  can influence the overall speed of the device  100 . The number of legs  104  in the device  100  can also be used to determine coefficients of friction to have in (or design into) each of the individual legs  104 . As discussed above, the middle legs  104   b  do not necessarily need to touch the surface  110 . For example, middle (or front or back) legs  104  can be built into the device  100  for aesthetic reasons, e.g., to make the device  100  appear more life-like, and/or to increase device stability. In some implementations, devices  100  can be made in which only three (or a small number of) legs  104  touch the ground, such as two front legs  104   a  and one or two rear legs  104   c.    
     The motor  202  is coupled to and rotates a counterweight  210 , or eccentric load, that has a CG that is off axis relative to the rotational axis of the motor  202 . The rotational motor  202  and counterweight  210 , in addition to being adapted to propel the device  100 , can also cause the device  100  to tend to roll, e.g., about the axis of rotation of the rotational motor  200 . The rotational axis of the motor  202  can have an axis that is approximately aligned with a longitudinal CG of the device  100 , which is also generally aligned with a direction of movement of the device  100 . 
       FIG. 2A  also shows a battery  220  and a switch  222 . The battery  220  can provide power to the motor  202 , for example, when the switch  222  is in the “ON” position, thus connecting an electrical circuit that delivers electric current to the motor  202 . In the “OFF” position of the switch  222 , the circuit is broken, and no power reaches the motor  202 . The battery  220  can be located within or above a battery compartment cover  224 , accessible, for example, by removing a screw  226 , as shown in  FIGS. 2A and 2D . The placement of the battery  220  and the switch  222  partially between the legs of the device  100  can lower the device&#39;s CG and help to prevent tipping. Locating the motor  202  lower within the device  100  also reduces tipping. Having legs  104  on the sides of a vehicle  100  provides a space (e.g., between the legs  104 ) to house the battery  220 , the motor  204  and the switch  222 . Positioning these components  204 ,  220  and  222  along the underside of the device  100  (e.g., rather than on top of the device housing) effectively lowers the CG of the device  100  and reduces its likelihood of tipping. 
     The device  100  can be configured such that the CG is selectively positioned to influence the behavior of the device  100 . For example, a lower CG can help to prevent tipping of the device  100  during its operation. As an example, tipping can occur as a result of the device  100  moving at a high rate of speed and crashing into an obstacle. In another example, tipping can occur if the device  100  encounters a sufficiently irregular area of the surface on which it is operating. The CG of the device  100  can be selectively manipulated by positioning the motor, switch, and battery in locations that provide a desired CG, e.g., one that reduces the likelihood of inadvertent tipping. In some implementations, the legs can be configured so that they extend from the leg tip  106   a  below the CG to a leg base  106   b  that is above the CG, allowing the device  100  to be more stable during its operation. The components of the device  100  (e.g., motor, switch, battery, and housing) can be located at least partially between the legs to maintain a lower CG. In some implementations, the components of the device (e.g., motor, switch and battery) can be arranged or aligned close to the CG to maximize forces caused by the motor  202  and the counterweight  210 . 
     Self-Righting 
     Self-righting, or the ability to return to an upright position (e.g., standing on legs  104 ), is another feature of the device  100 . For example, the device  100  can occasionally tip over or fall (e.g., falling off a table or a step). As a result, the device  100  can end up on its top or its side. In some implementations, self-righting can be accomplished using the forces caused by the motor  202  and the counterweight  210  to cause the device  100  to roll over back onto its legs  104 . Achieving this result can be helped by locating the device&#39;s CG proximal to the motor&#39;s rotational axis to increase the tendency for the entire device  100  to roll. This self-righting generally provides for rolling in the direction that is opposite to the rotation of the motor  202  and the counterweight  210 . 
     Provided that a sufficient level of roll tendency is produced based on the rotational forces resulting from the rotation of the motor  202  and the counterweight  210 , the outer shape of the device  100  can be designed such that rolling tends to occur only when the device  100  is on its right side, top side, or left side. For example, the lateral spacing between the legs  104  can be made wide enough to discourage rolling when the device  100  is already in the upright position. Thus, the shape and position of the legs  104  can be designed such that, when self-righting occurs and the device  100  again reaches its upright position after tipping or falling, the device  100  tends to remain upright. In particular, by maintaining a flat and relatively wide stance in the upright position, upright stability can be increased, and, by introducing features that reduce flatness when not in an upright position, the self-righting capability can be increased. 
     To assist rolling from the top of the device  100 , a high point  120  or a protrusion can be included on the top of the device  100 . The high point  120  can prevent the device from resting flat on its top. In addition, the high point  120  can prevent F h  from becoming parallel to the force of gravity, and as a result, F h  can provide enough moment to cause the device to roll, enabling the device  100  to roll to an upright position or at least to the side of the device  100 . In some implementations, the high point  120  can be relatively stiff (e.g., a relatively hard plastic), while the top surface of the head  118  can be constructed of a more resilient material that encourages bouncing. Bouncing of the head  118  of the device when the device is on its back can facilitate self-righting by allowing the device  100  to roll due to the forces caused by the motor  202  and the counterweight  210  as the head  118  bounces off the surface  110 . 
     Rolling from the side of the device  100  to an upright position can be facilitated by using legs  104  that are sufficiently flexible in combination with the space  124  (e.g., underneath the device  100 ) for lateral leg deflection to allow the device  100  to roll to an upright position. This space can allow the legs  104  to bend during the roll, facilitating a smooth transition from side to bottom. The shoulders  112  on the device  100  can also decrease the tendency for the device  100  to roll from its side onto its back, at least when the forces caused by the motor  202  and the counterweight  210  are in a direction that opposes rolling from the side to the back. At the same time, the shoulder on the other side of the device  100  (even with the same configuration) can be designed to avoid preventing the device  100  from rolling onto its back when the forces caused by the motor  202  and the counterweight  210  are in a direction that encourages rolling in that direction. Furthermore, use of a resilient material for the shoulder can increase bounce, which can also increase the tendency for self-righting (e.g., by allowing the device  100  to bounce off the surface  110  and allowing the counterweight forces to roll the device while airborne). Self-righting from the side can further be facilitated by adding appendages along the side(s) of the device  100  that further separate the rotational axis from the surface and increase the forces caused by the motor  202  and the counterweight  210 . 
     The position of the battery on the device  100  can affect the device&#39;s ability to roll and right itself. For example, the battery can be oriented on its side, positioned in a plane that is both parallel to the device&#39;s direction of movement and perpendicular to the surface  110  when the device  100  is upright. This positioning of the battery in this manner can facilitate reducing the overall width of the device  100 , including the lateral distance between the legs  104 , making the device  100  more likely to be able to roll. 
       FIG. 4  shows an example front view indicating a center of gravity (CG)  402 , as indicated by a large plus sign, for the device  100 . This view illustrates a longitudinal CG  402  (i.e., a location of a longitudinal axis of the device  100  that runs through the device CG). In some implementations, the vehicle&#39;s components are aligned to place the longitudinal CG close to (e.g., within 5-10% as a percentage of the height of the vehicle) the physical longitudinal centerline of the vehicle, which can reduce the rotational moment of inertia of the vehicle, thereby increasing or maximizing the forces on the vehicle as the rotational motor rotates the eccentric load. As discussed above, this effect increases the tendency of the device  100  to roll, which can enhance the self-righting capability of the device.  FIG. 4  also shows a space  404  between the legs  104  and the underside  122  of the vehicle  100  (including the battery compartment cover  224 ), which can allow the legs  104  to bend inward when the device is on its side, thereby facilitating self-righting of the device  100 .  FIG. 4  also illustrates a distance  406  between the pairs or rows of legs  104 . Increasing the distance  406  can help prevent the vehicle  100  from tipping. However, keeping the distance  406  sufficiently low, combined with flexibility of the legs  104 , can improve the vehicle&#39;s ability to self-right after tipping. In general, to prevent tipping, the distance  406  between pairs of legs needs to be increased proportionally as the CG  402  is raised. 
     The vehicle high point  120  is also shown in  FIG. 4 . The size or height of the high point  120  can be sufficiently large enough to prevent the device  100  from simply lying flat on its back after tipping, yet sufficiently small enough to help facilitate the device&#39;s roll and to force the device  100  off its back after tipping. A larger or higher high point  120  can be better tolerated if combined with “pectoral fins” or other side protrusions to increase the “roundness” of the device. 
     The tendency to roll of the device  100  can depend on the general shape of the device  100 . For example, a device  100  that is generally cylindrical, particularly along the top of the device  100 , can roll relatively easily. Even if the top of the device is not round, as is the case for the device shown in  FIG. 4  that includes straight top sides  407   a  and  407   b , the geometry of the top of the device  100  can still facilitate rolling. This is especially true if distances  408  and  410  are relatively equal and each approximately defines the radius of the generally cylindrical shape of the device  100 . Distance  408 , for example, is the distance from the device&#39;s longitudinal CG  402  to the top of the shoulder  112 . Distance  410  is the distance from the device&#39;s longitudinal CG  402  to the high point  120 . Further, having a length of surface  407   b  (i.e., between the top of the shoulder  112  and the high point  120 ) that is less than the distances  408  and  410  can also increase the tendency of the device  100  to roll. Moreover, if the device&#39;s longitudinal CG  402  is positioned relatively close to the center of the cylinder that approximates the general shape of the device  100 , then roll of the device  100  is further enhanced, as the forces caused by the motor  202  and the counterweight  210  are generally more centered. The device  100  can stop rolling once the rolling action places the device  100  on its legs  104 , which provide a wide stance and serve to interrupt the generally cylindrical shape of the device  100 . 
       FIG. 5  shows an example side view indicating a center of gravity (CG)  502 , as indicated by a large plus sign, for the device  100 . This view also shows a motor axis  504  which, in this example, closely aligns with the longitudinal component of the CG  502 . The location of the CG  502  depends on, e.g., the mass, thickness, and distribution of the materials and components included in the device  100 . In some implementations, the CG  502  can be farther forward or farther back from the location shown in  FIG. 5 . For example, the CG  502  can be located toward the rear end of the switch  222  rather than toward the front end of the switch  222  as illustrated in  FIG. 5 . In general, the CG  502  of the device  100  can be sufficiently far behind the front driving legs  104   a  and the rotating eccentric load (and sufficiently far in front of the rear legs  104   c ) to facilitate front hopping and rear drag, which can increase forward drive and provide a controlled tendency to go straight (or turn if desired) during hops. For example, the CG  502  can be positioned roughly halfway (e.g., in the range of roughly 40-60% of the distance) between the front driving legs  104   a  and the rear dragging legs  104   c . Also, aligning the motor axis with the longitudinal CG can enhance forces caused by the motor  202  and the counterweight. In some implementations, the longitudinal component of the CG  502  can be near to the center of the height of the device (e.g., within about 3% of the CG as a proportion of the height of the device). Generally, configuring the device  100  such that the CG  502  is closer to the center of the height of the device will enhance the rolling tendency, although greater distances (e.g., within about 5% or within about 20% of the CG as a proportion of the height of the device) are acceptable in some implementations. Similarly, configuring the device  100  such that the CG  502  is within about 3-6% of the motor axis  504  as a percentage of the height of the device can also enhance the rolling tendency. 
       FIG. 5  also shows an approximate alignment of the battery  220 , the switch  222  and the motor  202  with the longitudinal component of the CG  502 . Although a sliding switch mechanism  506  that operates the on/off switch  222  hangs below the underside of the device  100 , the overall approximate alignment of the CG of the individual components  220 ,  222  and  202  (with each other and with the CG  502  of the overall device  100 ) contributes to the ability of the device  100  to roll, and thus right itself. In particular, the motor  202  is centered primarily along the longitudinal component of the CG  502 . 
     In some implementations, the high point  120  can be located behind the CG  502 , which can facilitate self-righting in combination with the eccentric load attached to the motor  202  being positioned near the nose  108 . As a result, if the device  100  is on its side or back, the nose end of the device  100  tends to vibrate and bounce (more so than the tail end of the device  100 ), which facilitates self-righting as the forces of the motor and eccentric load tend to cause the device to roll. 
       FIG. 5  also shows some of the sample dimensions of the device  100 . For example, a distance  508  between the CG  502  and a plane that passes through the leg tips  106   a  on which the device  100  rests when upright on a flat surface  110  can be approximately 0.36 inches. In some implementations, this distance  508  is approximately 50% of the total height of the device (see  FIGS. 7A &amp; 7B ), although other distances  508  may be used in various implementations (e.g., from about 40-60%). A distance  510  between the rotational axis  504  of the motor  202  and the same plane that passes through the leg tips  106   a  is approximately the same as the distance  508 , although variations (e.g., 0.34 inches for distance  510  vs. 0.36 inches for distance  508 ) may be used without materially impacting desired functionality. Greater variations (e.g., 0.05 inches or even 0.1 inches) may be used in some implementations. 
     A distance  512  between the leg tip  106   a  of the front driving legs  104   a  and the leg tip  106   a  of the rearmost leg  104   c  can be approximately 0.85 inches, although various implementations can include other values of the distance  512  (e.g., between about 40% and about 75% of the length of the device  100 ). In some implementations, locating the front driving legs  104   a  behind the eccentric load  210  can facilitate forward driving motion and randomness of motion. For example, a distance  514  between a longitudinal centerline of the eccentric load  210  and the tip  106   a  of the front leg  104   a  can be approximately 0.36 inches. Again, other distances  514  can be used (e.g., between about 5% and about 30% of the length of the device  100  or between about 10% and about 60% of the distance  512 ). A distance  516  between the front of the device  100  and the CG  502  can be about 0.95 inches. In various implementations, the distance  516  may range from about 40-60% of the length of the device  100 , although some implementations may include front or rear protrusions with a low mass that add to the length of the device but do not significantly impact the location of the CG  502  (i.e., therefore causing the CG  502  to be outside of the 40-60% range). 
       FIGS. 9A and 9B  show example devices  100   y  and  100   z  that include, respectively, a shark/dorsal fin  902  and side/pectoral fins  904   a  and  904   b . As shown in  FIG. 9A , the shark/dorsal fin  902  can extend upward from the body  102  so that, if the device  100   y  tips, then the device  100   y  will not end up on its back and can right itself. The side/pectoral fins  904   a  and  904   b  shown in  FIG. 9B  extend partially outward from the body  102 . As a result, if the device  100   z  begins to tip to the device&#39;s left or right, then the fin on that side (e.g., fin  904   a  or fin  904   b ) can stop and reverse the tipping action, returning the device  100   z  to its upright position. In addition, the fins  904   a  and  904   b  can facilitate self-righting by increasing the distance between the CG and the surface when the device is on its side. This effect can be enhanced when the fins  904   a  and  904   b  are combined with a dorsal fin  902  on a single device. In this way, fins  902 ,  904   a  and  904   b  can enhance the self-righting of the devices  100   y  and  100   z . Constructing the fins  902 ,  904   a  and  904   b  from a resilient material that increases bounce when the fins are in contact with a surface can also facilitate self-righting (e.g., to help overcome the wider separation between the tips of the fins  902 ,  904   a  and  904   b ). Fins  902 ,  904   a  and  904   b  can be constructed of light-weight rubber or plastic so as not to significantly change the device&#39;s CG. 
     Random Motion 
     By introducing features that increase randomness of motion of the device  100 , the device  100  can appear to behave in an animate way, such as like a crawling bug or other organic life-form. The random motion can include inconsistent movements, for example, rather than movements that tend to be in straight lines or continuous circles. As a result, the device  100  can appear to roam about its surroundings (e.g. in an erratic or serpentine pattern) instead of moving in predictable patterns. Random motion can occur, for example, even while the device  100  is moving in one general direction. 
     In some implementations, randomness can be achieved by changing the stiffness of the legs  104 , the material used to make the legs  104 , and/or by adjusting the inertial load on various legs  104 . For example, as leg stiffness is reduced, the amount of device hopping can be reduced, thus reducing the appearance of random motion. When the legs  104  are relatively stiff, the legs  104  tend to induce hopping, and the device  100  can move in a more inconsistent and random motion. 
     While the material that is selected for the legs  104  can influence leg stiffness, it can also have other effects. For example, the leg material can be manipulated to attract dust and debris at or near the leg tips  106   a , where the legs  104  contact the surface  110 . This dust and debris can cause the device  100  to turn randomly and change its pattern of motion. This can occur because the dust and debris can alter the typical frictional characteristics of the legs  104 . 
     The inertial load on each leg  104  can also influence randomness of motion of the device  100 . As an example, as the inertial load on a particular leg  104  is increased, that portion of the device  100  can hop at higher amplitude, causing the device  100  to land in different locations. 
     In some implementations, during a hop and while at least some legs  104  of the device  100  are airborne (or at least applying less force to the surface  110 ), the motor  202  and the counterweight  210  can cause some level of mid-air turning and/or rotating of the device  100 . This can provide the effect of the device landing or bouncing in unpredictable ways, which can further lead to random movement. 
     In some implementations, additional random movement can result from locating front driving legs  104   a  (i.e., the legs that primarily propel the device  100  forward) behind the motor&#39;s counterweight. This can cause the front of the device  100  to tend to move in a less straight direction because the counterweight is farther from legs  104  that would otherwise tend to absorb and control its energy. An example lateral distance from the center of the counterweight to the tip of the first leg of 0.36 inches compared to an example leg length of 0.40 inches. Generally, the distance  514  from the longitudinal centerline of the counterweight to the tip  106   a  of the front leg  104   a  may be approximately the same as the length of the leg but the distance  514  can vary in the range of 50-150% of the leg length. 
     In some implementations, additional appendages can be added to the legs  104  (and to the housing  102 ) to provide resonance. For example, flexible protrusions that are constantly in motion in this way can contribute to the overall randomness of motion of the device  100  and/or to the lifelike appearance of the device  100 . Using appendages of different sizes and flexibilities can magnify the effect. 
     In some implementations, the battery  220  can be positioned near the rear of the device  100  to increase hop. Doing so positions the weight of the battery  220  over the rearmost legs  104 , reducing load on the front legs  104   a , which can allow for more hop at the front legs  104   a . In general, the battery  220  can tend to be heavier than the switch  222  and motor  202 , thus placement of the battery  220  nearer the rear of the device  100  can elevate the nose  108 , allowing the device  100  to move faster. 
     In some implementations, the on/off switch  222  can be oriented along the bottom side of the device  100  between the battery  220  and the motor  204  such that the switch  222  can be moved back and forth laterally. Such a configuration, for example, helps to facilitate reducing the overall length of the device  100 . Having a shorter device can enhance the tendency for random motion. 
     Speed of Movement 
     In addition to random motion, the speed of the device  100  can contribute to the life-like appearance of the device  100 . Factors that affect speed include the vibration frequency and amplitude that are produced by the motor  202  and counterweight  210 , the materials used to make the legs  104 , leg length and deflection properties, differences in leg geometry, and the number of legs. 
     Vibration frequency (e.g., based on motor rotation speed) and device speed are generally directly proportional. That is, when the oscillating frequency of the motor  202  is increased and all other factors are held constant, the device  100  will tend to move faster. An example oscillating frequency of the motor is in the range of 7000 to 9000 rpm. 
     Leg material has several properties that contribute to speed. Leg material friction properties influence the magnitude of drag force on the device. As the coefficient of friction of the legs increases, the device&#39;s overall drag will increase, causing the device  100  to slow down. As such, the use of leg material having properties promoting low friction can increase the speed of the device  100 . In some implementations, polystyrene-butadiene-styrene with a durometer near 65 (e.g., based on the Shore A scale) can be used for the legs  104 . Leg material properties also contribute to leg stiffness which, when combined with leg thickness and leg length, determines how much hop a device  100  will develop. As the overall leg stiffness increases, the device speed will increase. Longer and thinner legs will reduce leg stiffness, thus slowing the device&#39;s speed. 
     Appearance of Intelligence 
     “Intelligent” response to obstacles is another feature of the device  100 . For example, “intelligence” can prevent a device  100  that comes in contact with an immovable object (e.g., a wall) from futilely pushing against the object. The “intelligence” can be implemented using mechanical design considerations alone, which can obviate the need to add electronic sensors, for example. For example, turns (e.g., left or right) can be induced using a nose  108  that introduces a deflection or bounce in which a device  100  that encounters an obstacle immediately turns to a near incident angle. 
     In some implementations, adding a “bounce” to the device  100  can be accomplished through design considerations of the nose and the legs  104 , and the speed of the device  100 . For example, the nose  108  can include a spring-like feature. In some implementations, the nose  108  can be manufactured using rubber, plastic, or other materials (e.g., polystyrene-butadiene-styrene with a durometer near 65, or in the range of 55-75, based on the Shore A scale). The nose  108  can have a pointed, flexible shape that deflects inward under pressure. Design and configuration of the legs  104  can allow for a low resistance to turning during a nose bounce. Bounce achieved by the nose can be increased, for example, when the device  100  has a higher speed and momentum. 
     In some implementations, the resiliency of the nose  108  can be such that it has an added benefit of dampening a fall should the device  100  fall off a surface  110  (e.g., a table) and land on its nose  108 . 
       FIG. 6  shows a top view of the vehicle  100  and further shows the flexible nose  108 . Depending on the shape and resiliency of the nose  108 , the vehicle  100  can more easily deflect off obstacles and remain upright, instead of tipping. The nose  108  can be constructed from rubber or some other relatively resilient material that allows the device to bounce off obstacles. Further, a spring or other device can be placed behind the surface of the nose  108  that can provide an extra bounce. A void or hollow space  602  behind the nose  108  can also contribute to the device&#39;s ability to deflect off of obstacles that are encountered nose-first. 
     Alternative Leg Configurations 
       FIGS. 3A-3C  show various examples of alternative leg configurations for devices  100   a - 100   k . The devices  100   a - 100   k  primarily show leg  104  variations but can also include the components and features described above for the device  100 . As depicted in  FIGS. 3A-3C , the forward direction of movement is left-to-right for all of the devices  100   a - 100   k , as indicated by direction arrows  302   a - 302   c . The device  100   a  shows legs connected with webs  304 . The webs  304  can serve to increase the stiffness of the legs  104  while maintaining legs  104  that appear long. The webs  304  can be anywhere along the legs  104  from the top (or base) to the bottom (or tip). Adjusting these webs  304  differently or on the device&#39;s right versus the left can serve to change leg characteristics without adjusting leg length and provide an alternate method of correcting steering. The device  100   b  shows a common configuration with multiple curved legs  104 . In this implementation, the middle legs  104   b  may not touch the ground, which can make production tuning of the legs easier by eliminating unneeded legs from consideration. Devices  100   c  and  100   d  show additional appendages  306  that can add an additional life-like appearance to the devices  100   c  and  100   d . The appendages  306  on the front legs can resonate as the devices  100   c  and  100   d  move. As described above, adjusting these appendages  306  to create a desired resonance can serve to increase randomness in motion. 
     Additional leg configurations are shown in  FIG. 3B . The devices  100   e  and  100   f  show leg connections to the body that can be at various locations compared to the devices  100   a - 100   d  in  FIG. 3A . Aside from aesthetic differences, connecting the legs  104  higher on the device&#39;s body can serve to make the legs  104  appear to be longer without raising the CG. Longer legs  104  generally have a reduced stiffness that can reduce hopping, among other characteristics. The device  100   f  also includes front appendages  306 . The device  100   g  shows an alternate rear leg configuration where the two rear legs  104  are connected, forming a loop. 
     Additional leg configurations are shown in  FIG. 3C . The device  100   h  shows the minimum number of (e.g., three) legs  104 . Positioning the rear leg  104  right or left acts as a rudder changing the steering of the device  100   h . Using a rear leg  104  made of a low friction material can increase the device&#39;s speed as previously described. The device  100   j  is three-legged device with the single leg  104  at the front. Steering can be adjusted on the rear legs by moving one forward of the other. The device  100   i  includes significantly altered rear legs  104  that make the device  100   i  appear more like a grasshopper. These legs  104  can function similar to legs  104  on the device  100   k , where the middle legs  104   b  are raised and function only aesthetically until they work in self-righting the device  100   k  during a rollover situation. 
     In some implementations, devices  100  can include adjustment features, such as adjustable legs  104 . For example, if a consumer purchases a set of devices  100  that all have the same style (e.g., an ant), the consumer may want to make some or all of the devices  100  move in varying ways. In some implementations, the consumer can lengthen or shorten individual leg  104  by first loosening a screw (or clip) that holds the leg  104  in place. The consumer can then slide the leg  104  up or down and retighten the screw (or clip). For example, referring for  FIG. 3B , screws  310   a  and  310   b  can be loosened for repositioning legs  104   a  and  104   c , and then tightened again when the legs are in the desired place. 
     In some implementations, screw-like threaded ends on leg bases  106   b  along with corresponding threaded holes in the device housing  102  can provide an adjustment mechanism for making the legs  104  longer or shorter. For example, by turning the front legs  104   a  to change the vertical position of the legs bases  106   b  (i.e., in the same way that turning a screw in a threaded hole changes the position of the screw), the consumer can change the length of the front legs  104   a , thus altering the behavior of the device  100 . 
     In some implementations, the leg base  106   b  ends of adjustable legs  104  can be mounted within holes in housing  102  of the device  100 . The material (e.g., rubber) from which the legs are constructed along with the size and material of the holes in the housing  102  can provide sufficient friction to hold the legs  104  in position, while still allowing the legs to be pushed or pulled through the holes to new adjusted positions. 
     In some implementations, in addition to using adjustable legs  104 , variations in movement can be achieved by slightly changing the CG, which can serve to alter the effect of the vibration of the motor  202 . This can have the effect of making the device move slower or faster, as well as changing the device&#39;s tendency to turn. Providing the consumer with adjustment options can allow different devices  100  to move differently. 
     Device Dimensions 
       FIGS. 7A and 7B  show example dimensions of the device  100 . For example, a length  702  is approximately 1.73 inches, a width  704  from leg tip to leg tip is approximately 0.5 inches, and a height  706  is approximately 0.681 inches. A leg length  708  can be approximately 0.4 inches, and a leg diameter  710  can be approximately 0.077 inches. A radius of curvature (shown generally at  712 ) can be approximately 1.94 inches. Other dimensions can also be used. In general, the device length  702  can be in the range from two to five times the width  704  and the height  706  can be in the approximate range from one to two times the width  704 . The leg length  708  can be in the range of three to ten times the leg diameter  710 . There is no physical limit to the overall size that the device  100  can be scaled to, as long as motor and counterweight forces are scaled appropriately. In general, it may be beneficial to use dimensions substantially proportional to the illustrated dimensions. Such proportions may provide various benefits, including enhancing the ability of the device  100  to right itself after tipping and facilitating desirable movement characteristics (e.g., tendency to travel in a straight line, etc.). 
     Construction Materials 
     Material selection for the legs is based on several factors that affect performance. The materials main parameters are coefficient of friction (COF), flexibility and resilience. These parameters in combination with the shape and length of the leg affect speed and the ability to control the direction of the device. 
     COF can be significant in controlling the direction and movement of the device. The COF is generally high enough to provide resistance to sideways movement (e.g., drifting or floating) while the apparatus is moving forward. In particular, the COF of the leg tips (i.e., the portion of the legs that contact a support surface) can be sufficient to substantially eliminate drifting in a lateral direction (i.e., substantially perpendicular to the direction of movement) that might otherwise result from the vibration induced by the rotating eccentric load. The COF can also be high enough to avoid significant slipping to provide forward movement when F v  is down and the legs provide a forward push. For example, as the legs bend toward the back of the device  100  (e.g., away from the direction of movement) due to the net downward force on the one or more driving legs (or other legs) induced by the rotation of the eccentric load, the COF is sufficient to prevent substantial slipping between the leg tip and the support surface. In another situation, the COF can be low enough to allow the legs to slide (if contacting the ground) back to their normal position when F v  is positive. For example, the COF is sufficient low that, as the net forces on the device  100  tend to cause the device to hop, the resiliency of the legs  104  cause the legs to tend to return to a neutral position without inducing a sufficient force opposite the direction of movement to overcome either or both of a frictional force between one or more of the other legs (e.g., back legs  104   c ) in contact with the support surface or momentum of the device  100  resulting from the forward movement of the device  100 . In some instances, the one or more driving legs  104   a  can leave (i.e., hop completely off) the support surface, which allows the driving legs to return to a neutral position without generating a backward frictional force. Nonetheless, the driving legs  104   a  may not leave the support surface every time the device  100  hops and/or the legs  104  may begin to slide forward before the legs leave the surface. In such cases, the legs  104  may move forward without causing a significant backward force that overcomes the forward momentum of the device  100 . 
     Flexibility and resilience are generally selected to provide desired leg movement and hop. Flexibility of the leg can allow the legs to bend and compress when F v  is down and the nose moves down. Resilience of the material can provide an ability to release the energy absorbed by bending and compression, increasing the forward movement speed. The material can also avoid plastic deformation while flexing. 
     Rubber is an example of one type of material that can meet these criteria, however, other materials (e.g., other elastomers) may a have similar properties. 
       FIG. 8  shows example materials that can be used for the device  100 . In the example implementation of the device  100  shown in  FIG. 8 , the legs  104  are molded from rubber or another elastomer. The legs  104  can be injection molded such that multiple legs are integrally molded substantially simultaneously (e.g., as part of the same mold). The legs  104  can be part of a continuous or integral piece of rubber that also forms the nose  108  (including nose sides  116   a  and  116   b ), the body shoulder  112 , and the head side surface  114 . As shown, the integral piece of rubber extends above the body shoulder  112  and the head side surface  114  to regions  802 , partially covering the top surface of the device  100 . For example, the integral rubber portion of the device  100  can be formed and attached (i.e., co-molded during the manufacturing process) over a plastic top of the device  100 , exposing areas of the top that are indicated by plastic regions  806 , such that the body forms an integrally co-molded piece. The high point  120  is formed by the uppermost plastic regions  806 . One or more rubber regions  804 , separate from the continuous rubber piece that includes the legs  104 , can cover portions of the plastic regions  806 . In general, the rubber regions  802  and  804  can be a different color than plastic regions  806 , which can provide a visually distinct look to the device  100 . In some implementations, the patterns formed by the various regions  802 - 806  can form patterns that make the device look like a bug or other animate object. In some implementations, different patterns of materials and colors can be used to make the device  100  resemble different types of bugs or other objects. In some implementations, a tail (e.g., made of string) can be attached to the back end of the device  100  to make the device appear to be a small rodent. 
     The selection of materials used (e.g., elastomer, rubber, plastic, etc.) can have a significant effect on the vehicle&#39;s ability to self-right. For example, rubber legs  104  can bend inward when the device  100  is rolling during the time it is self-righting. Moreover, rubber legs  104  can have sufficient resiliency to bend during operation of the vehicle  100 , including flexing in response to the motion of (and forces created by) the eccentric load rotated by the motor  202 . Furthermore, the tips of the legs  104 , also being made of rubber, can have a coefficient of friction that allows the driving legs (e.g., the front legs  104 ) to push against the surface  110  without significantly slipping. 
     Using rubber for the nose  108  and shoulder  112  can also help the device  100  to self-right. For example, a material such as rubber, having higher elasticity and resiliency than hard plastic, for example, can help the nose  108  and shoulder  112  bounce, which facilitates self righting, by reducing resistance to rolling while the device  100  is airborne. In one example, if the device  100  is placed on its side while the motor  202  is running, and if the motor  202  and eccentric load are positioned near the nose  108 , the rubber surfaces of the nose  108  and shoulder  112  can cause at least the nose of the device  100  to bounce and lead to self-righting of the device  100 . 
     In some implementations, the one or more rear legs  104   c  can have a different coefficient of friction than that of the front legs  104   a . For example, the legs  104  in general can be made of different materials and can be attached to the device  100  as different pieces. In some implementations, the rear legs  104   c  can be part of a single molded rubber piece that includes all of the legs  104 , and the rear legs  104   c  can be altered (e.g., dipped in a coating) to change their coefficient of friction. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. Other alternative embodiments can also be implemented. For example, some implementations of the device  100  can omit the use of rubber. Some implementations of the device  100  can include components (e.g., made of plastic) that include glow-in-the-dark qualities so that the device  100  can be seen in a darkened room as it moves across the surface  110  (e.g., a kitchen floor). Some implementations of the device  100  can include a light (e.g., an LED bulb) that blinks intermittently as the device  100  travels across the surface  110 . 
       FIG. 10  is a flow diagram of a process  1000  for operating a vibration-powered device  100  (e.g., a device that includes any appropriate combination of the features described above). The device can include any appropriate combination of features, as described above. In various embodiments, different subsets of the features described above can be included. 
     Initially, a vibration-powered device is placed on a substantially flat surface at  1005 . Vibration of the device is induced at  1010  to cause forward movement. For example, vibration may be induced using a rotational motor (e.g., battery powered or wind up) that rotates a counterweight. The vibration can induce movement in a direction corresponding to an offset between the leg bases and the leg tips of one or more driving legs (i.e., the forward direction). In particular, this vibration can cause resilient legs to bend in one direction, at  1015 , as the net downward forces cause the device to move downward. This bending, along with using a material with a sufficiently high coefficient of friction to avoid substantial slipping, can cause the device to move generally forward. 
     As the vibration causes net upward forces (e.g., due to the vector sum of the forces induced by the rotating counterweight and the spring effect of the resilient legs) that cause the driving legs to leave the surface or to come close to leaving the surface, the tips of the one or more driving legs move in the forward direction (i.e., the leg deflects in the forward direction to return to a neutral position) at  1020 . In some implementations, the one or more driving legs can leave the surface at varying intervals. For example, the driving legs may not leave the surface every time the net forces are upward because the forces may not overcome a downward momentum from a previous hop. In addition, the amount of time the driving legs leave the surface may vary for different hops (e.g., depending on the height of the hop, which in turn may depend on the degree to which the rotation of the counterweight is in phase with the spring of the legs). 
     During the forward motion of the device, different drag forces on each lateral side of the device can be generated at  1025 . Generally, these different drag forces can be generated by rear legs that tend to drag (or at least that drag more than front driving legs) and alter the turning characteristics of the device (e.g., to counteract or enhance turning tendencies). Typically, the legs can be arranged in (e.g., two) rows along each lateral side of the device, such that one or more of the legs in one row drag more than corresponding legs in another row. Different techniques for causing the device to generate these different drag forces are described above. 
     If the device overturns, rolling of the device is induced at  1030 . In general, this rolling tendency can be induced by the rotation of the counterweight and causes the device to tend to independently right itself. As discussed above, the outer shape of the device along the longitudinal dimension (e.g., substantially parallel to the axis of rotation and/or the general forward direction of movement of the device) can be shaped to promote rolling (e.g., by emulating longitudinal “roundness”). Rolling of the device can also be stopped by a relatively wide spread between the rows of legs at  1035 . In particular, if the legs are wide enough relative to the COG of the device, the rotational forces generated by the rotating counterweight are generally insufficient (absent additional forces) to cause the device to roll over from the upright position. 
     At  1040 , resiliency of the nose of the device can induce a bounce when the device encounters an obstacle (e.g., a wall). This tendency to bounce can facilitate changing directions to turn away from an obstacle or toward a higher angle of incidence, particularly when combined with a pointed shaped nose as discussed above. The resilient nose can be constructed from a elastomeric material and can be integrally molded along with lateral shoulders and/or legs using the same elastomeric material. Finally, lateral drifting can be suppressed at  1045  based on a sufficiently high coefficient of friction at the leg tips, which can prevent the legs from tending to slide laterally as the rotating counterweight generates lateral forces. 
       FIG. 11  is a flow diagram of a process  1100  for constructing a vibration-powered device  100  (e.g., a device that includes any appropriate combination of the features described above). Initially, the device undercarriage is molded at  1105 . The device undercarriage can be the underside  122  shown in  FIG. 1  and can be constructed from a hard plastic or other relatively hard or stiff material, although the type of material used for the underside is generally not particularly critical to the operation of the device. An upper shell is also molded at  1110 . The upper shell can include a relatively hard portion of the upper body portion of the housing  102  shown in  FIG. 1 , including the high point  120 . The upper shell is co-molded with an elastomeric body at  1115  to form the device upper body. The elastomeric body can include a single integrally formed piece that includes legs  104 , shoulders  112 , and nose  108 . Co-molding a hard upper shell and a more resilient elastomeric body can provide better constructability (e.g., the hard portion can make it easier to attach to the device undercarriage using screws or posts), provide more longitudinal stiffness, can facilitate self-righting (as explained above), and can provide legs that facilitate hopping, forward movement, and turning adjustments. The housing is assembled at  1120 . The housing generally includes a battery, a switch, a rotational motor, and an eccentric load, which may all be enclosed between the device undercarriage and the upper body. 
     Thus, particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims.