Abstract:
A mobile robot includes a robot chassis having a forward end, a rearward end and a center of gravity. The robot includes a driven support surface to propel the robot and first articulated arm rotatable about an axis located rearward of the center of gravity of the robot chassis. The arm is pivotable to trail the robot, rotate in a first direction to raise the rearward end of the robot chassis while the driven support surface propels the chassis forward in surmounting an obstacle, and to rotate in a second opposite direction to extend forward beyond the center of gravity of the robot chassis to raise the forward end of the robot chassis and invert the robot endwise.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation (and claims the benefit of priority under 35 USC 120) of U.S. application Ser. No. 13/721,918 filed Dec. 20, 2012, which is a continuation of U.S. application Ser. No. 13/323,019 filed Dec. 12, 2011, which is a continuation of U.S. application Ser. No. 13/078,618, filed Apr. 1, 2011, which is a continuation of U.S. application Ser. No. 12/331,380, filed Dec. 9, 2008. The disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application. 
     
    
     STATEMENT AS TO FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made in part with Government support under contract #FA8650-08-C-7815 entitled “Ember: a Small Mobile Communnications Relay Platform” awarded by the DARPA IPTO LANdroids program. The government may have certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    The invention relates generally to robotic mobile platforms. 
         [0004]    Robots are useful in a variety of civilian, military, and law enforcement applications. For instance, a robotically controlled mobility platform can be used to inspect or search buildings under hazardous or hostile conditions. Dangerous situations can be improved by providing detailed information about the location, activities, and capabilities of opponents. Military applications can include reconnaissance, surveillance, bomb disposal and security patrols. 
         [0005]    Advances are sought in the miniaturization of robots and the ability of robots to surmount obstacles. 
       SUMMARY 
       [0006]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
         [0007]    One aspect of the invention features a mobile robot including a robot chassis having a forward end, a rearward end and a center of gravity. The robot further includes a driven support surface connected to the chassis and configured to propel the robot chassis forward and rearward. A first articulated arm is rotatable about an axis located rearward of the center of gravity of the robot chassis and is configured to: trail the robot, rotate in a first direction, raise the rearward end of the robot chassis while the driven support surface propels the chassis forward in surmounting an obstacle, and rotate in a second opposite direction to extend forward beyond the center of gravity of the robot chassis to raise the forward end of the robot chassis and invert the robot endwise. 
         [0008]    In some embodiments, the driven support surface includes a flexible track trained about a pair of wheels. 
         [0009]    In some embodiments, the axis of the arm is coaxial with an axis of a one of the wheels. 
         [0010]    In some embodiments, the robot includes a second articulated arm rotatable about the axis with the first articulated arm. 
         [0011]    In some embodiments, the first and second arms are located outward of the driven support surface and are continuously rotatable in either direction. In some embodiments, an axle connects the first and second arms and an idler wheel is freely rotatable about the axle. 
         [0012]    In some embodiments, the first arm is located substantially along a central longitudinal axis of the robotic chassis. In some cases, the first arm is configured with a camera. 
         [0013]    In some embodiments, the first arm is continuously rotatable to provide a swimming-type propulsion. 
         [0014]    In some embodiments, the first arm is configured to rotate as a function of an angle of incline of the robot chassis. 
         [0015]    In some embodiments, a duration of rotation of the first arm is predetermined as a function of an angle of incline of the robot chassis. 
         [0016]    In some embodiments, the robot includes a radio transceiver and the first arm is rotatable to raise the robot chassis to elevate the transceiver. In some cases, the robot is a mobile mesh network node robot. 
         [0017]    In some embodiments, the robot includes a cliff detector at the forward end and a proximity sensor on a side of the robot. In some cases, the robot is configured to maintain a fixed proximity to a detected obstacle. In some cases, the fixed proximity is maintained by comparing readings from the first proximity sensor and a second proximity sensor and turning the vehicle to substantially equilibrate the readings. 
         [0018]    In some embodiments, the robot includes an angular rate sensor. configured to detect an impact of the robot with an obstacle and an angle of incline of the robot. 
         [0019]    In some embodiments, the robot includes a slip clutch between the first arm and a first arm drive motor. 
         [0020]    In some embodiments, a pull pin is removably received by the chassis. In some cases, the pull pin locks the first arm in a stowed position. In some cases, the pull pin immobilizes the first arm. In some cases, the pull pin is configured to power-on the vehicle upon removal from the chassis. In some cases, the pull pin is configured to release an antenna from a stowed position and to unlock the first arm from a stowed position. 
         [0021]    In some embodiments, the vehicle substantially fits within a bounding volume approximately 7 inches long, 5 inches wide and 2 inches tall. 
         [0022]    In some embodiments, the first arm is rotatable to align with a horizontal axis of the vehicle. In some cases, the vehicle is configured to fit in a combat uniform cargo pants pocket. 
         [0023]    Another aspect of the invention features a method for operating a mobile robotic vehicle having a driven support surface and a first pivoting trailing arm to surmount obstacles including a series of stair risers. The method includes driving the support surface to propel the vehicle to contact the riser of a first stair with a forward end of the vehicle, and driving the support surface to cause the forward end of the vehicle to ascend the riser of the first stair. The method further includes pivoting the first arm to raise a rearward end of the vehicle as the forward end of the robot approaches the top of the riser of the first stair; driving the support surface to advance the forward end of the robot over the top of the stair riser; and pivoting the arm to further raise the rearward end of the vehicle such that the forward end of the vehicle rotates downward beyond the top of the riser of the first stair. 
         [0024]    In some applications, the method includes repeating the driving and pivoting to surmount a second stair. 
         [0025]    In some applications, the method includes pivoting the arm to raise the forward end of the vehicle above an underlying surface. 
         [0026]    In some applications, the method includes further pivoting the arm to flip the robot endwise. 
         [0027]    In some applications, raising the forward end of the vehicle raises a radio transceiver on the vehicle. 
         [0028]    In some applications, the method includes supporting the vehicle in a first substantially horizontal orientation on an underlying surface, pivoting the arm to contact the underlying surface to raise the forward end of the vehicle and further pivoting the arm to rotate the forward end of the vehicle past a stable point and allowing the vehicle to topple over to a second orientation inverted with respect to the first orientation. 
         [0029]    In some applications, the first arm is pivoted to raise the rear end of the chassis to maintain a chassis angle of incline of less than about  45  degrees as the forward end of the chassis surmounts a stair riser. 
         [0030]    In some cases, the stair risers comprises spaced apart obstacles including a rise in elevation. 
         [0031]    In some applications, pivoting the first arm and driving the support surface are performed simultaneously. In some applications, the pivoting the first arm and driving the support surface are performed substantially asynchronously. 
         [0032]    In some applications, the support surface propels the vehicle to a predetermined angle of incline before the first arm is initially pivoted to raise the rearward end of the vehicle. 
         [0033]    In some applications, the support surface is further driven while the first arm is further pivoted to surmount the riser. 
         [0034]    In some applications, the first arm is pivoted at a predetermined rate for a predetermined period upon detection of a predetermined angle of incline of the vehicle. 
         [0035]    In some applications, the first arm is pivotally retracted to a point above the bottom of the driven support surface upon the vehicle surmounting the riser. 
         [0036]    In some applications, the first arm is pivotally retracted to a predetermined angle to provide clearance for the vehicle to ascend a second riser to a predetermined angle of incline before the first arm contacts an underlying surface. 
         [0037]    In some applications, the method includes detecting when the robot hits an obstacle and generating a random recoil turn rate command. 
         [0038]    In some applications, the method includes propelling the vehicle backwards at a preset speed while turning at the random recoil turn rate for a fixed period of time or until the vehicle hits another obstacle moving backwards. 
         [0039]    In some applications, the method includes, upon detection of impact of the vehicle with an obstacle while moving backwards, generating a second random recoil turn rate and propelling the vehicle forward at a preset speed while turning at the second turn rate. 
         [0040]    In some applications, the method includes detecting an obstacle at a distance and turning the vehicle to avoid the vehicle. 
         [0041]    In some applications, the method includes propelling the vehicle in a spiral pattern at a preset speed and a preset diminishing turn rate until an obstacle is detected. 
         [0042]    Another aspect of the invention features a robot including a body and a differential drive supporting the body. The differential drive includes a left drive motor that turns a left drive wheel and a right drive motor that turns a right drive wheel, each of the left drive wheels and right drive wheel are turned about a common first axis. A flipper arm includes a pivot end and a distal end, the flipper arm being supported with respect to the body to pivot about a second axis parallel to the common drive axis to revolve the distal end about the second axis. A flipper drive, including a flipper drive motor, is connected to the flipper arm to drive the flipper arm through a continuous 360 degrees of revolution about the pivot axis. A left motor circuit controls the left motor, a right motor circuit controls the right motor and a flipper motor circuit controls the flipper motor. A body attitude sensor measures tilt of the body from the direction of gravity, about a third axis parallel to the first axis. A flipper angle sensor measures angular position of the flipper about the second axis. A microcontroller commands the left motor circuit, right motor circuit, and flipper motor circuit and includes an obstacle climbing routine that monitors the body attitude sensor and the flipper angle sensor. The obstacle climbing routine commands the flipper motor circuit to (a) move the flipper to revolve to a position substantially extending along the ground as the differential drive climbs a face of an obstacle, and (b) move the flipper to revolve between the position extending along the ground to a position extending below the body as the differential drive drives forward and overcomes a top of the face of the obstacle, thereby tipping the body over the top of the face of the obstacle. 
         [0043]    In some cases, the flipper is moved to extend along the ground when the body passes more than a certain acute angle, e.g., 45 degrees, from horizontal. For example, the flipper is moved to be positioned about the same acute angle from a direction normal to the top of the body. 
         [0044]    In some cases, the body includes a frame chassis, monocoque or unibody hull. 
         [0045]    Another aspect of the invention features a method of robot obstacle climbing. The method includes monitoring an attitude of a robot body having a differential drive, monitoring an angle of a pivoting flipper with respect to the robot body; pivoting the flipper to a position extending along the ground as the differential drive climbs a face of an obstacle. The flipper is pivoted from the position extending along the ground to a position extending below the body as the differential drive drives forward and overcomes a top of the face of the obstacle, in order to tip the body over the top of the face of the obstacle as the distal end of the flipper supports the body below the top of the face of the obstacle. 
         [0046]    Another aspect of the invention features a ground robot including a substantially box-like rectangular body no more than two inches in height, ten inches in length, and ten inches in width, and having a leading end and a trailing end, and having no more than 1 kg mass. A differential drive supports the body and includes a left driven track and a right driven track each no less than ½ inch in width, differentially driven about a drive axis. A flipper arm having a pivot end and a distal end is supported at its pivot end from the body no more than three inches from the trailing end of the body to pivot about a flipper axis parallel to the drive axis. The flipper arm is rotatable throughout a continuous 360 degrees of revolution about the pivot axis. 
         [0047]    In some cases the differential drive includes tracks with sufficient traction to climb the leading end of an obstacle composed of common materials such as wood, asphalt and concrete by driving the leading end of the tracks up a face of an obstacle while the trailing end tracks drives along the ground. 
         [0048]    In some applications, the robot is configured to prevent the robot from sliding back down the obstacle when the leading end of the track clears the top of the obstacle, and to hold and advance the robot at the top of the obstacle as the trailing end of the tracks are lifted off the ground. 
         [0049]    In some applications, the differential drive propels the robot forward to drag the distal end of the flipper arms along the ground as the flippers are rotated in to swing the distal ends of the flippers in the same direction as the robot is advancing. 
         [0050]    In some cases, the flippers are less than 3% of the total weight of the robot and do not appreciably move the center of gravity of the robot during rotation of the flippers. In some cases, the distal ends of the flippers are sized to partially penetrate into certain loose terrains such as snow, sand and gravel. The flippers are substantially rigid to resist bending. 
         [0051]    In some case the distal ends of the flippers are rounded or tapered to slide along normal surfaces such as wood, asphalt, and concrete as the robot advances over the top of an obstacle using just the traction of the leading ends of the tracks. 
         [0052]    In some cases the distal ends of the flippers are rounded to permit the distal end of the flipper to slip through and past a bottom dead center position in order to push the trailing end of the robot as high as possible before the robot is finally driven to overcome the obstacle. 
         [0053]    In some embodiments, the robot is sized to be readily portable, and to substantially fit within a bounding volume approximately 8 cm (7 in.) long, 12 cm (5 inches) wide and 5 cm (2 inches) tall. A trailing pivoting arm allows the compact robot to climb obstacles as big as itself, including stairs. A particular small robot embodiment is about 13 cm (5″) axle to axle, 3 cm (1″) diameter wheels with a 15 cm (6″) overall length, capable of climbing about obstacles up to about 13 cm (5″). Such compact embodiments can be carried in the various pockets and poaches contained in the law enforcement or militia uniforms. Another embodiment is scaled large enough to climb standard stairs 17-20 cm (7″-8″). These robots are well-suited to urban settings such as on rooftops, sidewalks, stair wells, streets and in indoor residential and office environments. 
         [0054]    In use, the robot is removed from a pocket, a deactivation plug or pin is removed from the side of the robot and the robot is placed or even tossed in a suitable location. This process can be repeated to create a multi-node mesh communications network. Multiple robots can act as radio relays, forming multi-hop communications paths that allow operations over greater ranges. Relay chains are particularly useful for missions in urban terrain or to extend communications around corners, and into cave/tunnel complexes and bunkers, allowing access to more remote areas that a single robot could not access. 
         [0055]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0056]      FIG. 1  illustrates a robot and remote control system. 
           [0057]      FIGS. 2 a - d    are perspective, front, side and top views of a robot having flippers. 
           [0058]      FIGS. 3 a - p    are side views of a sequence of stair surmounting maneuvers. 
           [0059]      FIG. 4  is flowchart of an obstacle surmounting control routine. 
           [0060]      FIGS. 5 a - r    are side views of another sequence of stair surmounting maneuvers 
           [0061]      FIG. 6  is flowchart of another obstacle surmounting control routine. 
           [0062]      FIG. 7  is a perspective view of a robot in an elevated position during a self-righting maneuver. 
           [0063]      FIG. 8  is a perspective view of a robot in a partially elevated position. 
           [0064]      FIG. 9  is a partially exploded view of a robot chassis. 
           [0065]      FIG. 10  is a perspective view of a flipper drive system. 
           [0066]      FIG. 11  is a perspective view of opposing sprockets of a slip clutch. 
           [0067]      FIG. 12  is a cross-sectional view of the opposing sprockets of the slip clutch assembled on a drive axle. 
           [0068]      FIG. 13  is a perspective view of a robot including sensor zones 
           [0069]      FIGS. 14 a - c    are perspective and like views of an electrical subassembly of a robot. 
           [0070]      FIG. 15  is a functional block diagram of system components of a robot. 
       
    
    
       [0071]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0072]    Various tracked robotic vehicles have been developed that are the subject of, for example, U.S. Pat. Nos. 6,431,296, 6,263,989, 6,668,951 and 6,615,885. These patents are instructive on the construction of tracked robotic vehicles having driven flippers, and means of articulation of robotic components, and are hereby incorporated by reference in their entirety into this application. 
         [0073]    Autonomous control routines and control systems useful, for example, to avoid an obstacle, escape an area, or navigate an area, optimize communications or coverage or seek radio performance are disclosed in U.S. patent application Ser. No. 11/633,869 filed Dec. 4, 2006 and titled “AUTONOMOUS COVERAGE NAVIGATION SYSTEM,” and U.S. patent application Ser. No., 12/100,782, filed Apr. 10, 2008 and titled “ROBOTICS SYSTEMS,” the entire contents of which are incorporated herein in their entirety by reference. For example, to avoid an obstacle, a control routine can cause the robot to travel in an outward spiral or to bounce and recoil from obstacles making random turns. 
         [0074]    With reference to  FIG. 1 , a mobile robotic vehicle or robot  2  is operable via an Operator Control Unit (OCU) or remote control system  4 . Remote communications can be transmitted via radio signal, infra red signal, wi-fi signal, cellular signal, or other suitable signal. In some circumstances, robot  2  can perform automated routines without remote use intervention. In other cases, a combination of automated routines and user controls are used to maneuver robot  2 . Remote control system  4  can include any number of radio transceivers or other communications links and can interface with any number of robots or though any number of robots as mesh network nodes or other parts of a communications network. 
         [0075]    This version of robot  2  is sized to be portable, and to substantially fit within a bounding volume approximately 18 cm (7 in.) long, 12 cm (5 inches) wide and 6 cm (2 inches) tall. The overall dimensions of a particular embodiment are about 16 cm×12 cm×6 cm excluding the antenna and camera protruding from the chassis, with a total mass of about 0.5 kg and a top speed of about 0.6 m/s over a smooth surface. In some embodiments, the vehicle is configured to fit in a combat uniform cargo pants pocket. Multiple robots  2  can be stacked to fit in a backpack. Compactness and portability allow robot  2  to be easily transported by an individual and to be deployed into an area by simply tossing the robot. 
         [0076]    The robot is removed from a carrying compartment, is activated by pulling a grenade-style pull pin, and is then tossed down a corridor, up a stairwell or into a window. The platform can be thrown by a single person or launched into an upper window or rooftop using an improvised slingshot. Landing on a top floor, the remotely operated platform may be able to bypass some typical obstacles and travel down stairs. The operator can evaluate the resultant video before determining the next course of action. The robot can also be used to search for and assess booby traps, enemy personnel, and improvised explosive devices (IEDs). 
         [0077]    In a particular embodiment, each of the robot  2  and remote controller  4  and weight less than eight pounds. In some cases the robot is between 0.5 and 1.0 Kg and the remote control  4  is a small PDA with a mass less than 0.5 kg. A separate charging station can be used for both the remote control  4  and robot  2 . 
         [0078]    Remote control system  4  allows an operator to control robot  2  from a distance. The operator can select different levels of human control over the robot, ranging from a teleoperation mode, in which the operator directly controls the motors and actuators on the robot, to autonomous operation, in which the operator passes higher-level command to the robot. In partially autonomous operation, robot  2  can perform tasks such as following a wall, avoiding an obstacle, surmounting an obstacle, avoiding a drop off or “cliff,” avoiding becoming high centered, evading a moving object, positioning a transceiver, self-righting, repositioning to optimize communication network coverage, and the like. 
         [0079]    Alternative versions of the remote control system  4  support teleoperation as well as a means of switching between teleoperation and autonomous control. The user can interrupt autonomous operation of the robot at any time to give commands and direction, and the robot can operate autonomously when not receiving particular directions from the user. The system provides predetermined warning signals to the operator, for instance if it is unable to operate autonomously, possibly by means of a vibrating unit that could be worn by the operator and which would be effective in a noisy environment. In addition, the user can add additional tasks to the robot&#39;s mission and request notification from the robot when milestone tasks have been achieved. 
         [0080]    Versions of the robot can perform various autonomous tasks which can be initiated by the operator from remote control system  4 . These include obstacle avoidance, wall following, climbing stairs, recovery from high centering, self-righting, returning “home,” searching for a designated object, mapping, and establishing a communications network. The robot can use the various mobility modes described above in these autonomous operations, and if necessary, can call for operator assistance during its execution of a task. Alternative configurations of pivotal arms can be used. For example, a single central “arm” can be used. 
         [0081]    With reference to  FIGS. 2 a - d   , robot  2  includes a body portion or chassis  6  housing, inter alia, drive components, a power supply, control system and communication module. A pair of drive wheels  8  is positioned at a forward end of chassis  6  and a pair of idler wheels  10  is positioned at a rearward end of chassis  6 . Wheels  8  and  10  can include spiral spokes to provide additional impact resistance. 
         [0082]    A pair of resilient tracks  12  is trained about wheels  8  and  10  on opposing sides of chassis  6  extending from the sides of the chassis to provide a driven support surface for robot  2 . Tracks  12  are continuous flexible belts with interior surface features for engaging drive wheels  8  and exterior surface features for gaining traction over various terrains. 
         [0083]    First and second rotatable arms or flippers  14  extend radially from a common drive axle  16 . In the illustrated embodiment, idle wheels  10  free spin about flipper drive axle  16 . Flippers  14  are configured to extend through a 360 degree range of motion to allow robot  2  to perform various positioning, obstacle surmounting and self-righting maneuvers. In other embodiments, a single rotatable arm can accomplish these functions. In a particular embodiment, flippers  14  are made from a flame rated advanced formula polymer with 85D Shore hardness available from Quantum Cast, part number AFP3100FR, UL 94 FR and FAR 25.853. Through holes or other features can be provided on flippers  14  for attachment of flipper accessories, e.g., cameras, sensors, or wheels positioned at the flipper tip or along the flipper length. For example, a camera, antenna or sensor can be mounted on the end of the arm to provide better exposure or higher vantage point. 
         [0084]    Flippers  14  can be rotated to any desired angle relative to chassis  6 . Robot  2  is designed to move about in a variety of environments, including an urban environment of buildings (including staircases), streets, underground tunnels, as well as in vegetation, such as through grass and around trees. Robot  2  has a variety of features which provide robust operation in these environments, including impact resistance and tolerance of debris entrainment. Wheels  8  and  10  are positioned on chassis  6  to provide greater ground clearance when right side up, yet can provide sufficient clearance in some embodiments for operation when robot  2  is inverted. In the present embodiments, robot  2  preferably recovers from a tumble or fall in which it is inverted by a self-righting function. 
         [0085]    Chassis  6  and other rigid robot components are designed for strength and low weight and are made from durable plastic, polymer, composites, 7075-T6 aluminum or other suitable lightweight, impact resistant materials. Tracks  12 , wheels  8  and  10  and flippers  14  are also configured to be impact resistant. For example, wheels  8  and  10  can be a pliable material and can include spiraled spokes to provide a degree of resilience. Impact resistance is accomplished, in part, by surrounding much of the vehicle with compliant tracks  12  with pliable cleats. Tracks  12  and cleats provide a first layer of impact protection. 
         [0086]    Tracks  12  are configured to provide skid steering and include compliant belts made of polyurethane or a similar flexible material. The belts are abrasion resistant and have high strength and minimal stretch due to internal steel or fiber cording. Tracks  12  define a left-right alternating tread to smooth successive impacts on most surfaces with a spacing between successive edges on the sides to catch larger terrain features for traction. Without a bogey rail, the robot tends to tread on the portion of the tracks at wheels&#39; bottom dead center. 
         [0087]    Tracks  12  can be stretched over wheels  8  and  10  and driven primarily by friction. The surfaces of wheels  8  and  10  contacting tracks  12  can be provided with a fine knurl pattern to enhance friction with tracks  12  and prevent slippage from formation of water films between wheels  8  and  10  and tracks  12 . Alternatively, tacks  12  and drive wheels  8  can be formed with complementary features to provide positive drive engagement. For example, wheels  8  and  10  can have V-shaped grooves around their circumference to receive an integral V-shaped rib on the inside of track  12 . 
         [0088]    Alternative embodiments of the robot can use other types of tracks, such as tracks made up of discrete elements. However, debris may be caught between elements and such tracks are generally heavier than flexible belts. Other flexible materials can also be used for continuous belt tracks. Tracks  12  can include cleats, ridges, or other projections for additional traction. Such cleats can be angled to divert debris away from chassis  6 . 
         [0089]    Flippers  14  can be can be continuously rotated around axle  16 . Flippers  14  can be rotated to a forward “stowed” position next to chassis  6 . Alternatively, flippers  14  can be rotated to a rearward trailing position to prevent catching of the ends of flippers  14  on terrain, for example in tall grass. In some embodiments, to prevent possible damage, flippers  14  can automatically return to a stowed position when robot  2  detects that it is in free fall. 
         [0090]    With reference to  FIGS. 3 a - p    and  FIG. 4 , flippers  14  are further configured to be driven to rotate at predetermined intervals in stair or obstacle surmounting maneuvers. Flipper positioning angles are stated with reference to the horizontal axis of chassis  6  as shown in  FIG. 4  with 0 degrees being the stowed flipper position, 90 degrees being a vertical position, 180 being a trailing position and 270 being a downward position. An obstacle surmounting control routine is initiated by detection of an obstacle or predetermined scenario or by operator input. In state  1  of the routine, shown in  FIG. 3A , robot  2  approaches a stair with flippers  14  is a forward stowed position substantially parallel to the ground at 0 degrees. 
         [0091]    During obstacle surmounting maneuvers, a main obstacle surmounting control routine  100  is run on a controller at 64 Hz while sampling accelerometer data at 16 Hz and updating the flipper position at 16 Hz. Upon detection of the stair or other obstacle, the control routine enters state  2 , in which flippers  14  are rotated upward and rearward between approximately 45 and 90 degrees, as shown if  FIG. 3B . In state  3  shown in  FIG. 3C , the forward end of robot  2  begins to ascend the front face or riser of a stair. Once the robot has ascended to a predetermined position shown in  FIG. 3C  as detected by an accelerometer, e.g., between about 15 and 45 degrees or a sensor reading of about 0.75 g, the routine enters state  4 . Passage of a predetermined time since entering state  3 , e.g., 3 seconds, can also trigger the fourth state. 
         [0092]    In the fourth state, flippers  14  are rotated further counterclockwise or rearward between the positions shown in  FIGS. 3D-3G , e.g., between about 90-125 degrees, as tracks  12  are further driven such that the forward end continues to ascends the stair riser and rearward end of robot  2  approaches the stair riser. After a predetermined time, e.g.,  1  second, and as robot  2  approaches a substantially vertically position shown in  FIG. 3G , flippers  14  contact the underlying surface at approximately 125 degrees and the routine enters a fifth state. 
         [0093]    In the fifth state, flippers  14  are rotated quickly counterclockwise to lift the rearward end of robot  2  through the range shown in  FIGS. 3G-M , e.g., between about 125 and 275 degrees, while tracks  12  are driven until the center of gravity of robot  2  clears the nose of the stair as shown in  FIG. 3N . Flippers  14  further serve to resist back sliding or any wheelie tendency as robot  2  clears the stair nose. 
         [0094]    In an optional sixth state, if flippers reach about 275 degrees and the accelerometer has not detected that the center of gravity of the robot has cleared the nose of the stair, flippers  14  are kicked backward from about 275 degrees, e.g., to less than about 235 degrees, in an effort to topple robot  2  forward from a possible teetering position. 
         [0095]    Once the accelerometer detects that the center of gravity of robot  2  has cleared the nose of the stair, the routine enters a seventh state. In state  7 , robot  2  overcomes the stair and tips forward as the center of gravity clears the stair nose as shown in  FIG. 3M . Upon entering state  7 , or after a short time delay, e.g., 1 second, flippers  14  are rotated clockwise towards a trailing position as shown in  FIG. 30 . This prevents flippers  4  from catching on the surmounted obstacle as the robot is driven forward. 
         [0096]    Once the accelerometer detects that the robot has settled atop the stair or after a predetermined period of state  7 , e.g., 1 second, an eighth routine state causes flippers  14  to return to a default position, e.g., substantially vertical, to prepare to surmount a second stair. Flipper position is determined in the different states using a flipper position sensor. 
         [0097]    States  1 - 8  and the various maneuvers shown in  FIGS. 3B-P  are then repeated as needed to surmount successive stairs or other obstacles. 
         [0098]    Another obstacle surmounting routine  200  is described with reference to  FIGS. 5A-R  and  FIG. 6 . When the front wheels encounter a vertical stair riser, there may not be sufficient ground friction to allow the wheel/tracks to climb the riser. This is particularly true in dusty or sandy environments where the floor friction can be substantially less than that of the cleaner stair riser. To assist the front wheels in initiating climbing, flippers  14  are used to initially raise the forward end of the robot  2 . This can also be particularly helpful if a stair riser is angled outward. 
         [0099]    In state  1 , the robot advances forward towards the stair riser as shown in  FIG. 5A . Tracks  12  are driven forward until the robot reaches the riser. 
         [0100]    In state  2 , flippers  14  are rotated to the stowed position, e.g., 0 degrees in preparation for lifting the nose of the robot. 
         [0101]    In state  3 , flippers  14  are rotated “clockwise” downward such that the distal ends of flippers  14  contact the underlying surface forward of the center of gravity of robot  2  as shown in  FIG. 5B . Tracks  12  are advanced to ascend the stair riser. 
         [0102]    In state  4 , continued clockwise rotation of flippers  14  causes the forward end of robot  2  to raise up off the underlying surface at to a predetermined angle, e.g., about 15-45 degrees, as shown in  FIG. 5C . If the angle is not reached within a preset time, e.g., 1.5 seconds, the routine advances to the next state. 
         [0103]    The predetermined angle is selected to approximate the angle at which the frictional forces between the tracks and the floor and the tracks and the stair riser are sufficiently balanced to prevent back-sliding of the robot. Balance of the frictional forces between the track the riser and underlying surface enables the track to ascend the riser without continued clockwise rotation of flippers  14 . The routine can periodically test to see if the friction balance point has been achieved by slightly lifting the flippers and using the accelerometers to detect backsliding. Once it is determined that the balance point has been reached or passed, the flipper is no longer needed to raise the forward end of the robot. If the accelerometer detects slippage or backsliding of the robot, previous states can be repeated as needed. In some embodiments, the track velocity is coordinated with the flipper motions to help maintain traction and frictional balance. 
         [0104]    In state  5 , flippers  14  are rotated clockwise to an “upwards” position as shown in  FIGS. 5  E-F, e.g., to 90 degrees, to prevent the robot from flipping over backwards as the wheels continue to climb. This movement is preferably performed without substantially shifting the center of gravity or introducing disturbances that would upset the frictional balance. 
         [0105]    It is advantageous for flippers  14  to be long enough to extend forward of the center of gravity, yet short enough to not get caught under a stair nose when later rotating counterclockwise, as shown in  FIGS. 5D-E  to prepare to raise the rearward end of the robot. An estimated maximum flipper length is calculated by adding the wheel radius to the product of the length of the robot chassis and cosine of the angle at which the frictional forces are sufficiently balanced to enable continued climbing by the tracks. This flipper length provides sufficient clearance for retraction of the flippers from a forward to a rearward position after partial ascend of a stair rise by the robot. Of course, flipper length can be dictated by anticipated obstacle profiles including more aggressive forward riser angles. 
         [0106]    In state  6 , tracks  12  are advanced to position the chassis substantially vertically against the stair riser with flippers rotated counterclockwise to a point adjacent to or contacting the ground as shown in  FIGS. 5G-J . 
         [0107]    In state  7 , flippers  14  are extended, e.g., from about 100 degrees to 175 degrees, while tracks  12  are driven at a “matched” velocity as shown in  FIGS. 5J-O , or slightly faster as dictated by the geometry of the problem, to allow the track to evenly surmount the stair nose as the robot is pushed upward by flippers  14 . Matching of the track velocity to the flipper rotation means that the tracks are advancing a distance equal to the amount of extension provided by rotation of flippers  14 . Velocity matching can also be used to maintain an angle of incline of chassis  6  as the robot surmounts the stair nose. 
         [0108]    In state  8 , tracks  12  are driven while flippers  14  simply drag behind to prevent backsliding or wheelies as shown in  FIG. 5N . Flippers  14  are paused momentarily at the point of maximum extension, e.g., when the flipper tip is farthest away from the track/step corner, while the tracks continue to be driven forward. As the track cleats bounce over the stair nose, the flipper tips will bounce and drag along the ground closer to the step wall. The track speed can be varied to achieve a desire bouncing pattern. 
         [0109]    In state  9 , flippers  14  are again rotated counterclockwise to provide an extra extension to slightly level out the robot. This “over extension” of the flippers can also help tip the robot center of gravity over the stair nose. 
         [0110]    In state  10 , tracks  12  are driven quickly while flippers  14  are slowly rotated clockwise back to the full extension point to climb onto the top of the step as shown in  FIGS. 5P-R . In this position, the flippers can catch the robot should it happen to back slide or tumble backwards. Once the center of mass of the robot extends forward of the stair nose, the robot falls forward on top of the step. 
         [0111]    Once the vehicle tips forward of the step nose (as indicated by the accelerometers showing the tilt angle going back to level) flippers  14  are restored to a default driving or stowed position. 
         [0112]    In another control routine, flippers  14  can be continuously rotated to overcome a high centered position. A high centered position can be detected in multiple ways. For example, monitoring of video data, monitoring accelerometer data, comparing odometer and navigational data, GPS data discrepancies. Track motions can be coordinated with flipper motions to pull the vehicle forward, e.g., by driving the tracks when the flipper is in contact with the surface at the same rate that the flipper is expected to pull the vehicle forward. Flipper rotation rates can depend on the expected or detected terrain, e.g., whether the flipper tips will penetrate the terrain surface. The effective flipper radius can be dynamically determined by signal processing the accelerometer signals after repeated rotations of the flipper as a function of flipper tip penetration into the underlying surface. To prevent “digging in,” the tracks can be driven when the flippers are in contact with the underlying surface. The flipper rotational rate can be selected as a function of surface penetration and movement of the robot over the terrain and baseline data for behavior of the robot driving over different terrains. 
         [0113]    In the depicted embodiment, the flippers extended substantially the distance between the drive wheel axle and the idler wheel axle. In some cases, the flipper length is selected to fit entirely within the length of the chassis and to extend forward of the robot center of gravity. In some cases the flippers are at least as long as the idler wheel radius. The flippers or flipper length can be selected based on the dimensions of anticipated obstacles. 
         [0114]    With reference to  FIG. 7 , flippers  14  are configured to extend from axle  16  centrally to a point beyond the center of gravity of robot  2 . This allows robot  2  to be inverted or self-righted simply by rotation of flipper  14  through an arc of 90 degree beyond contact with an underlying surface. 
         [0115]    Rotation past a vertical stability point causes the robot to fall over completing the inversion. Self-righting is often required after tumbling down stairs or other inclines, or from atop other obstacles. Robot  2  can descend stairs forwards or backwards with flippers  14  in a stowed position, driving tracks  12  either direction and tumbling or rolling to a resting position. 
         [0116]    In some embodiments, robot  2  has more ground clearance in one orientation than another. In some cases, a camera, antenna, sensor or robot accessory may need to be reoriented upward if robot  2  lands upside down after a descent from an obstacle. 
         [0117]    With reference to  FIG. 8 , flippers  14  can also be rotated to partially elevate the rearward end of robot  2 . In some cases, flippers  14  can be used to raise or upend robot  2  to position a camera, antenna, sensor, munitions or the like at a desired height or angle or for helping to pull robot  2  from a high centered position. 
         [0118]    Flippers  14  can be repeatedly or continuously rotated in either direction to provide a “swimming” motion to help propel robot  2  through loose debris, gravel, sand and the like. Flippers  14  can raise the nose of the vehicle, to both help start a climb and to elevate a fixed camera. 
         [0119]    With reference to  FIG. 9  robot chassis  6  houses, inter alia, wheel drive motors  20  and  22  for powering drive wheels  8  and a flapper drive motor  24 . Chassis  6  also houses an electrical subassembly  26  and battery (not shown) positioned below electrical subassembly. The battery is a significant portion of the total weight of robot  2  and is positioned substantially centrally front to back and towards the bottom of chassis  6 . A clamshell chassis body design allows provides sufficient volume for electronics and mechanical drive mechanisms within a protective cover. 
         [0120]    Flipper drive motor  24  is used to control the angle between flappers  14  and chassis  6 . Flipper drive motor  24  is coupled via a gear reduction train to axle  16 . A slip clutch can be used to transfer output torque from flipper drive motor  24  to axle  16 . A slip clutch can be adjustable to set a predetermined slip torque. Flippers  14  are connected via solid axle  16  and an optic sensor on axle  16  provides for detection of the position of flippers  14  regardless of clutch slippage. One clutch embodiment includes two beveled gears engaged with a spring, similar to a cordless drill clutch. Axle  16  passes through a central opening in idler wheels  10  and fixedly connects to flippers  14 . 
         [0121]    At the rear of the robot are two flippers  14  with the ability to rotate 360° continuously to flip the robot over when inverted. The flippers also assist the robot in climbing and negotiating small obstacles. Also integrated into the flipper mechanism is a slip clutch to protect the gearing in case of impact. 
         [0122]    Drive motors  20 , and  22  are 1 watt DC brushed motors. In other versions of the robot, brushless motors can be used. Drive motors  20  and  22  turn output drive gears that attach to the wheels via integral splines. Output drive gears are retained via brass or Delrin bushings that register and align complementary portions of the chassis body. Drive motors  20  and  24  are geared down 29:1 to drive wheels  8 . 
         [0123]    Steering is accomplished using differential speed of the tracks  14  on either side of the robot by varying the speed of drive motors  20  and  22 . The robot will, in principle, skid around the center of chassis  6  approximately at the midpoint of the length of tracks allowing complete turning with the extremes of the robot staying within a 23 cm (9″) diameter circle. 
         [0124]    In some cases, tracks  14  can be driven while flippers  14  maintain an end of robot  2  elevated above an underlying surface, for example to reposition an elevated antenna or camera. Other preprogrammed flipper or robot positions can include fully extended, stowed, inclined, upright, and “wheelie.” In addition, robot  2  can perform several maneuvers including self righting, stair climbing, and recovery from high centering. 
         [0125]    The chassis body can further serve to retain bushings for moving parts and as a mounting surface for an antenna, camera, microphone, sensors and the like. Dust and moisture seals can be provided where axles or other components pass through the chassis body. For example, brass bushing securing at openings around chassis body  6  serve to support axle  16  and the idler wheel axles. Chassis body  6  can also carry an antenna connector base (e.g., standard SMA antenna connector). 
         [0126]    With reference to  FIG. 10 , flippers  14  are rotated by drive motor  24  (0.4 watt DC brushed motor) via gear train  26 . Flipper drive motor  24  is geared down 298:1 to axle  16  to provide a torque of approximately 400 mNm (˜2× required to lift vehicle weight). Clutch can be adjusted to provide up to 700 mNm of slip torque. A slip clutch prevents overloading of flipper drive motor  24  and gearing, for example due to an impact on the arms. 
         [0127]    Flippers  14  can be stowed parallel to chassis  6  and tracks  12  when it is deployed by tossing or dropping it through a window or door or when the robot tumbles. In some embodiments, a mechanical energy storage provides for sudden release to move the flippers to allow the robot to perform a small leap motion. An example energy storage system can be a spring, flywheel or other mechanical energy storage mechanism. 
         [0128]    With reference to  FIGS. 11 and 12 , a slip clutch  70  is provided on axle  16  between drive motor  24  and flippers  14 . Slip clutch  70  includes a first sprocket  72  carrying a series of drive teeth  74  on a first rotary surface  76 . First sprocket  72  is fixedly attached to axle  16 . A second sprocket  78  defines a series of slots  80  in a second rotary surface  82  for receiving the drive teeth  74  of first sprocket  72 . Drive teeth  74  and slots  80  remain engaged so long as first and second rotary surfaces  76  and  82  remain substantially in contact. 
         [0129]    Under sufficient toque, the tapered surfaces of teeth  74  cam rotary surfaces  76  and  82  apart allowing teeth  74  to slip one or more slots. Sprockets  72  and  78  are biased towards engagement via a spring  84 retained on axle  16 . Spring  84  provides an axial force to slip clutch  70  to resist separation of surfaces  76  and  82 . First sprocket  72  is connected to axle  16  while second sprocket  78  spins freely about axle  16  when disengaged from sprocket  72 . Second sprocket  78  includes gear teeth about its circumference to engage drive motor  24 . 
         [0130]    During obstacle surmounting maneuvers, drive motor  24  turns second sprocket  78  which in turn rotates first sprocket  72  and axle  16  to rotate flippers  14 . 
         [0131]    Alternatively, a slip clutch can be formed of sufficiently pliable material to allow flexure of rotary surfaces  76  and  82  under sufficient torque. Any number of frictional or cammed surfaces or other known types of slip clutches can be substituted for slip clutch  70 . 
         [0132]    With reference to  FIG. 13 , robot  2  is provided with a pair of end sensors  28  and side sensors  30 . Sensors  28  can be positioned on one or both ends of robot  2  and sensors  30  can be positioned on one or more sides of robot  2 . Sensors  28  and  30  include IR emitter/detector pairs. Sensors  28  are directed substantially parallel in front of tracks  12  to act as cliff detectors to detect and avoid falls and sensors  30  are directed outward from chassis  6  to act as wall detectors. Sensors  28  and  30  can include filtering features to accommodate ambient sunlight. Sensors  28  and  30  provide feedback that is used by robot  2 , for example, to follow a wall or avoid a drop off. Sensors  28  and  30  can include sonar, infra red, proximity, impact or other sensor suitable to detect the presence or absence of an object in the sensor range. Additional sensor based autonomous robot behavior routines are disclosed in U.S. Pat. No. 6,883,201, titled “ROBOT OBSTACLE DETECTION SYSTEM” which is incorporated herein by reference in its entirety. 
         [0133]    Additional autonomous behavior routines and control systems are disclosed in U.S. Pat. No. 6,809,490 titled “METHOD AND SYSTEM FOR MULTI-MODE 
         [0134]    COVERAGE FOR AN AUTONOMOUS ROBOT” and U.S. Pat. No. 7,459,871 titled “DEBRIS SENSOR FOR CLEANING APPARATUS,” which are incorporated herein by reference in their entirety. The routines include motion control and coverage behaviors such as spiral coverage, cruising, bounce and recoil from an obstacle, wall following, self-alignment, and escape behaviors as selected by an arbiter according to principles of behavior based robotics. Additional reactive controls and behavior routines are provided for reacting to and concentrating on a point of interest in the coverage space. Similar behaviors can be used to seek out a peak signal strength peak or radio hot spots or to reposition a robot as a node in a mesh network. 
         [0135]    Sensors can be shielded within the track volume, within the protective shell of chassis  6  or positioned on the front and rearward ends of the vehicle. The top and bottom portions of chassis  6  can be fitted with any number of sensors, cameras, antennae, chemical sensors, bio-sensors, radiation sensors and the like. 
         [0136]    Additional robot sensors provide input regarding flipper rotation position, connector to a charging station, presence of a deactivation plug (pull pin). For example, robot  2  can be powered off if sensors detect that drive motors  20  or  22  have stalled of if the robot is otherwise stuck. 
         [0137]    Chassis  6  also supports a camera  32  and antenna  34  to provide video telemetry and other communications data. Camera  32  is depicted positioned slightly rear of center, with the lens angled up to minimize the field of view obstructed by the robot vehicle itself. Flippers  14  can be rotated to raise the nose of the vehicle further if the camera view is insufficiently high. To look over an edge, flippers  14  can be used to raise the rear of the vehicle to depress the camera view angle. 
         [0138]    Transmission of video telemetry data or other sensor data from within a building can enable a small force to quickly and safely assess a location or situation. For example, a camera can be used to quickly and safely determine the presence and location of an adversary or explosive in a building 
         [0139]    Robot  2  includes the capability of carrying a variety of accessories or sensors, including cameras, sonar sensors, infra-red detectors, inertial sensors, motor position, velocity and torque sensors, inclinometers, a magnetic compass, microphones, sound generator, or small weapon. Sensors can be placed on all surfaces of the robot. For example, night time or low light operation can be performed using onboard light such as an infra-red (IR) array with a useful range of several meters. A small white light can also be provided for up close color identification of objects. 
         [0140]    A multi camera array can provide stereoscopic vision for navigation and video transmission back to remote control system  4 . For example, multiple cell phone style cameras, each with multi-megapixel accuracy and a 90° field of view, to provide full 360° field of view. The robot can be configured to monitor for motion and alert the operator if motion is detected. Similarly, an onboard microphone can enable an alert to be sent to the operator if sound above a designated threshold is detected. 
         [0141]    Onboard computing coupled with a multi megapixel imager can provide high resolution image capture and digital pan tilt zoom of the digitally compressed and encrypted video stream. This minimizes the mechanical complexity of the system by eliminating the need for a mechanical pan-tilt assembly, and allows the use of image processing for unattended operation such as change detection and digital video recording of motion. Integrated infrared illuminators can provide sufficient illumination for navigation in an urban environment, while white light illuminators can be used to identify targets up close. One example is a 1.3 Megapixel camera with mpeg4 compression capabilities. 
         [0142]    With reference to  FIGS. 14 a - b   , electrical subassembly  26  is mounted between the flipper drive motor  24  and wheel drive motors  20 ,  22  above batteries  52 . Electrical subassembly includes a main printed circuit board (PCB)  40  to which are electrically connected a removable mass memory  44 , USB communication module  46 ; SDIO communication module  48 , SDIO port  50 . The battery, drive motors  20 ,  22  and  24 , camera  32  and antenna  34  are operably coupled to electrical subassembly  26 . PCB  40  also carries end sensors  28  and side sensors  30 . PCB  40  can include rigid circuit boards, flexible polyimide circuits, or other circuit modules or combinations thereof, and may provide power regulation, motion control, sensors, and other functions. 
         [0143]    Battery  52  includes a lithium ion battery pack with three 18650 cells in series. Each cell has a capacity of 2.6 AHr, and contains 0.78 grams/cell of lithium, or 2.3 g of lithium per assembled robot. Internal rechargeable Li-Ion battery pack  52  has a two hour charge time via 110 v or 220 v circuits. Both robot  2  and the OCU remote control  4  can be charged by a single adapter capable of accepting universal power (100-240 VAC 50/60 Hz). Optional charge adapters can be used for charging at 12-24 VDC. Battery  52  can be attached directly to PCB  40  via VHB tape. Solar power can be used to charge the battery or provide for extended duration low power surveillance. 
         [0144]    A substantial capacitor bank is used to minimize the ripple in the battery draw in powering the drive motors. It may be desirable in some cases to destroy internal circuits by reversing the polarity of the capacitor bank into the lithium batteries to ignite the batteries. A sudden reversal of the energy from the capacitors creates a large current surge sufficient to cause an electrical fire. This would help frustrate the ability of hostile warfighters from re-using any of the components. 
         [0145]    PCB  40  includes one or more computer processors and associated memory systems. PCB  40  is coupled to communication modules  46 ,  48 , which include, for example, a radio for exchanging control and feedback information with remote control system  4 . Communications range with USB and SDIO radios was experimentally found to be approximately 40 meters of open area or through two cinderblock walls of a building. 
         [0146]    Odometry sensors detect a pattern referenced to axle  16 , such as a slotted or patterned strip secured to an axle, e.g., via a piece of clear heatshrink tubing, or a slotted disc attached to an axle or driven wheel  8 . The odometery sensor is located on the idler wheel to account for track slippage on drive wheel  8 . Odometry reading accuracy may be increased by harder turns as opposed to sweeping turns. 
         [0147]    Additional sensors determine the angle between flippers  14  and chassis  6  and the rate of rotation of flippers  14  or wheels  8  or  10 . An angular rate sensor is placed near the center of gravity of the robot  2  to track the bearing of the robot and provide increased positioning accuracy, facilitating movement in areas with few visual landmarks. Optional accelerometers can be located near the angular rate sensor. These inputs are used during full or partial autonomous robot operation. 
         [0148]    With reference to  FIG. 15 , a functional block diagram of electrically connected system components of an embodiment of a robot is shown. PCB  40  is electrically connected to an iMX31 processor  54 , USB communication module  46 , SDIO communication module  48 , flash memory  44 , infrared sensors  28 , proximity sensors  30 , STmicro LIS344ALH three axis accelerometer  56 , flipper rotary position sensor (RPS)  58 , SDIO/USB Payload PCB  42 , and STmicro LISY300AL angular right sensor, camera  32 , battery  52 , drive motors  20 ,  22  and  24  with appropriate interfaces, controllers and the like. 
         [0149]    The electrical components may also include one or more of the following: microphone, yaw sensor, active/passive analog IR LED and phototransistors, SDIO radios, 802.11b/g/n radio, satellite phone, EVDO cellular phone, USB peripherals, additional batteries, Bluetechnix IMX, GPS transponders and the like. 
         [0150]    In various embodiments, communications modules  46  and  48  serve to provide multi-hop style communications chains, to extend the usefulness of the robots deep into radio frequency (RF) denied areas using standard Optimized Link State Routing daemon (OLSRd) software mesh networking. The robots can be repositioned to maintain a self healing communications network 
         [0151]    According to one method of establishing a mesh network, the deactivation plug is removed to activate each robot. The robots are then placed in approximate locations for autonomous mesh networking The robots automatically reposition to maintain the mesh network. 
         [0152]    Advantageous Mesh Network Capabilities are disclosed generally in the LANdroids Bidders Day Briefing, Document Number BAA 07-46, released Jul. 6, 2007 and available from DARPA. 
         [0153]    When the mission is over, the robots can be recovered as required and charged to be ready for the next mission. When the robots are not in use and not being charged, the deactivation plugs are installed into the charge connectors to power off the robots and keep the batteries from draining prior to the next mission. 
         [0154]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, alternative embodiments can include four or six driven wheels and a single or multiple trailing pivoting arms. Accordingly, other embodiments are within the scope of the following claims.