Patent Grant
8219308
The present invention relates to motorized vehicles and, in particular, to a monowheel type vehicle where the outer periphery of a wheel circumscribes and rotates around the motor, and rotation of the wheel over a surface propels the vehicle.
Monowheel vehicles of various sorts exist. An advantage of a monowheel vehicle over multi-wheeled vehicles is that the radius of the wheel is maximized relative to the size of a given vehicle, enabling the vehicle to negotiate larger obstacles than conventional vehicles situated atop smaller wheels. Many of the known monowheel vehicles transport a person, where the person circumscribed by the wheel provides the propulsion power as well as balances the vehicle. An excellent catalog of the history of this seemingly bizarre type of vehicle can be found at the following web addresses: dself.dsl.pipex.com/MUSEUM/TRANSPORT/MOTORWHL/motorwhl.htm; and dself.dsl.pipex.com/MUSEUM/TRANSPORT/MOTORWHL/motorwh12.htm#coat.
A brief chronology of monowheel vehicles is presented below, primarily obtained from the websites identified above.
1869—First monowheel patent, by Bergner, featuring hand crank and foot treadle actuation (Bergner, U.S. Pat. No. 91,510). Teaches dual tires attached to a single wheel.
1869—Greene & Dyer hand crank-driven monowheel patent (Greene et al., U.S. Pat. No. 91,535)
1884—Figure from book, “Victorian Inventions” suggests a monowheel vehicle that appears to indicate dual tires attached to a single wheel like the Bergner patent.
1892—Harper pedal-powered monowheel patent (Harper, U.S. Pat. No. 511,139)
1897—Venable monowheel patent, essentially a bicycle inside a large wheel (Venable, U.S. Pat. No. 611,534). There is a suggestion to design the wheel as the outer race of a giant ball bearing with the frame comprising the inner bearing race.
1904—The Monocycle Garavaglia, the first monowheel with an engine.
1912—Coates propeller-driven monowheel patent (Coates, U.S. Pat. No. 1,046,267)
1917—D'Harlingue propeller-driven monowheel patent (D'Harlingue, U.S. Pat. No. 1,228,100)
1923—Cislaghi monowheel. Cislaghi & Govetosa received a French patent in 1924 (No. 573,801). The patent seems to indicate that the plane of the wheel can be tilted from the plane of the rider, ostensibly to keep the wheel away from the riders head. A photo from around the same time doesn't seem to show such a tilt, but a photo caption suggests retracting stabilizer wheels that extend when the vehicle stops. Cislaghi received a British patent in 1927 (No. 275647). The Motoruota company was founded by Cislaghi in 1927 in an attempt to commercialize this monowheel vehicle.
1923—E. J. Christie monowheel appears in “Everyday Science & Radio News” and features a flywheel on each side of the main, driven wheel that “act as gyroscopic balancers and rudders”.
1926—Gyrocycle pedal-powered monowheel
1932—J. H. Purves' “Dynosphere”. It is a monowheel that is wide enough that static stability provides balance from side-to-side. One photo from Popular Science depicts a smaller version that is electrically-powered.
1933—Gutierrez's 3-wheeled, “monowheel”-like armed and amphibious tank patent (Gutierrez, U.S. Pat. No. 1,915,886).
1937—Rose monowheel patent (Rose, U.S. Pat. No. 2,107,766) includes external wheel to prevent rotation due to excessive acceleration or deceleration.
1940s—Inverted pendulum control problem.
1968—Malick unicycle patent using a gyroscope to measure pitch attitude and provide feedback for control of a planar unicycle.
1994—David Vos designed and implemented a unicycle robot with automatic balance control (Vos, MIT Ph.D. thesis, May 1992).
1997—Chiba University unicycle robot employing gyroscopic actuation to provide dynamic stability about the roll axis.
2000—Owen “di-wheel” with two wheels separated by approximately one wheel diameter to permit two riders to sit in between the wheels.
2003—Jake Lyall introduces the RIOT (Re-Invention Of The) wheel. Instead of using the rider's weight to provide the reaction to the engine's thrust on the wheel, the reaction torque is provided by a lead-weighted scooter engine suspended from the main wheel axle, powered through a sprocket. The rider sits in front of the wheel and a 450-pound internal counterweight keeps the rider hovering above the ground at the front. The rider steers the vehicle by a combination of precessing a 65-pound gyroscope and shifting his weight.
2005—Wheelsurf markets a commercial monowheel vehicle. See following web address: wheelsurft.nl/.
In the monowheel examples, the human driver, circumscribed by the wheel, controls the balance and turns by leaning. The vehicle must have some velocity in order for the driver to maintain balance, using the gyroscopic effect of the spinning wheel to maintain balance, similar to a standard bicycle or motorcycle.
The present inventor realized that a monowheel vehicle that does not transport a rider has some important uses, even though complexity is somewhat increased in order to incorporate an automatic balance control mechanism. For example, a riderless vehicle could be equipped with a camera or other payload and be remotely operated at much higher speeds and over rugged terrain including climbing stairs. A monowheel vehicle presents a minimal frontal cross section, which is useful to navigate narrow entry ways, improve its ability to hide behind objects, and reduce its profile as seen from a distance. If the vehicle were able to turn at zero forward velocity, the vehicle would be able to more easily navigate confined spaces and at slower speeds. Such capability is not possible with the known monowheel vehicles.
A monowheel vehicle is described herein that comprises two wheels close together that enable the vehicle to turn at zero forward velocity by rotating the wheels in opposite directions, rather than relying on leaning the vehicle at some forward velocity to turn it. The center of gravity is located below the wheels' axes of rotation, and the wheels rotate around the propulsion system. When the vehicle is moving straight and forward, the closely spaced wheels act as a single wheel.
An active differential gear mechanism, controlled by a steering motor, is employed to achieve such differential wheel actuation. A single propulsion system provides a drive torque that is shared by the two wheels. A separate steering torque is added to one wheel while being subtracted from the other wheel for turning the wheels at different speeds and directions. This enables the vehicle to turn in any direction at both zero and low speeds, as long as both wheels are in contact with the ground, without relying on tilting the vehicle to turn.
The steering motor can be disengaged when not in use, such as by a clutch, reducing the active differential to a passive differential to allow the wheels to turn at different speeds, such as when turning at a forward velocity.
In one embodiment, immediately prior to the vehicle being powered down, the two wheels are moved apart from each other (e.g., by means of an electric motor) so that the vehicle may remain upright when in the powered down state. When the vehicle is switched to its active state, the two wheels are automatically moved close together with a combined contour resembling a single wheel.
The vehicle employs attitude sensors (for sensing roll, pitch, and yaw) and an automatic balancing system. A flywheel in the vehicle spins at a high rate around a spin axis. The flywheel may be a balanced electric motor that rotates around an axis (a shaft) wherein the shaft comprises the stator and the body of the motor comprises the rotor. The spin axis is connected to a frame of the vehicle by a pivot or gimbal that is rotatable by an electric orientation motor such that the combination of flywheel and actuated gimbal comprise a gyroscopic actuator. The axis angle may be continually adjusted, and/or the flywheel speed is continually adjusted, so that the flywheel provides a torque in any desired direction to maintain the vehicle's balance, whether the vehicle is going straight or turning. Two gyroscopic actuators are provided in one embodiment so that one gyroscopic actuator can be used to cancel out undesired steady torques created by the other gyroscopic actuator or provide net torque vector combinations not possible with a single actuator. In a steady state condition, the flywheels are at opposite 45 degree angles to the vehicle centerline (the {circumflex over (x)} axis) and spin in opposite directions to cancel out any yawing component in the gyroscopic torques applied if only a single flywheel were employed. The two flywheels can also be used to double the torque when needed by changing their angles and speeds. Various other advantages exist using the two flywheels.
Rapid mechanical braking of the flywheels and/or rapid control of the axis angle can be used to create torques that cause the vehicle to right itself from a horizontal, minimum gravitational potential energy position. The flywheel acceleration or deceleration may also be controlled by controlling power to the flywheel motors.
The gyroscopic actuators and their control methods can also be used to dynamically balance a monowheel vehicle employing just a single wheel.
Various other features are described.
The invention is a self-balancing, self-propelled, monowheel vehicle, having its center of mass (COM) located within the diameter of the wheel, where all components of the drive and balancing system are circumscribed by the periphery of the wheel. Possessing only a single wheel (or two wheels close together), the vehicle's posture (or attitude) is inherently statically unstable in its roll orientations and is challenged to maintain its pitch with arbitrary wheel acceleration torques. This general class of vehicles will be referred to as a “monowheel,” even though the vehicle may have two closely spaced wheels.
A right wheel axle 7 (a solid axle, shown in
Housed within the frame 20 is an internal combustion engine and transmission assembly 30 (
Disposed on either side of the engine and transmission assembly 30 is a pair of identical gyroscopic actuator assemblies (
The flywheel orientation motors 11 and 14 are rotated using power from the battery/fuel cell 27.
One object of this invention is to make the monowheel vehicle dynamically stable by incorporating suitable means for providing corrective torques, via the flywheels, in response to perturbations about the pitch and roll axes. Maintenance of such a statically unstable posture or attitude is commonly referred to as balance.
Static instability in the pitch direction (forward and backward tilt in the plane of the wheel), φ, can be desirable to reduce the effort required to reorient the pitch of the vehicle albeit at the expense of increased complexity of control and an aspect of this invention is to provide suitable compensation means. However, as a variant of this invention, passive stability in the pitch direction can be achieved simply by locating the center of mass (COM) of the vehicle (largely the drive system) below the axis of rotation of the wheel assembly, causing the drive system to remain in a natural position below the wheels' axes while the wheels rotate around the drive system. This is shown by the center of mass in
A degree of passive stability in the roll direction, θ, can be achieved by contouring the tire geometry such that each tire's outer radius, r′ (
Although the ground contact between an ideal wheel, defined as a simple disk of infinitesimal thickness, and the ground is a single point, in reality, a real wheel possesses a line contact with the ground, and a typical wheel tire contacts the ground with a finite footprint of finite area. As such, steering a single wheeled vehicle such as a unicycle requires the wheel to “skid” as it twists about the vehicle yaw (ŷ) axis. One object of the present invention is to provide the monowheel vehicle with a pair of slender wheels, with a small separation distance, to substantially reduce the amount of “skid” experienced by the wheel assembly and thereby substantially reduce the energy required to turn the vehicle at low speeds
One aspect of this invention is the use of the flywheels to exert torques on the vehicle. This is accomplished in two ways: as reaction wheels and as gyroscopic actuators. The former case leverages the concept of Newton's third law roughly stated that for every action there is an equal and opposite reaction, such that when a small, rotational inertia rotating at high angular velocity changes rotational velocity, this exerts a reaction torque on the rest of the vehicle about its rotational axis. These changes in angular velocity can be positive or negative, corresponding to increasing or decreasing angular velocity, respectively. It is challenging to create flywheels that can speed up as fast as would be required to produce appreciable torques. However, it is relatively easy to brake quite rapidly. Such braking of the flywheels is particularly useful to right the vehicle from a fallen pose. Preferably, the rotating inertia is of low mass and the angular velocity is very high to minimize impact on overall vehicle weight. Energetically dissipative braking can occur by application of mechanical brakes or by dissipating the power on the electrical side by shorting the leads of the flywheel spin motors through a large regeneration resistor. One suitable type of mechanical brake is an electrically controlled clutch (e.g., a solenoid) mounted on each flywheel yoke 17 that presses a high friction surface against the rotating flywheel when actuated to quickly decelerate the flywheel by converting its kinetic energy into heat. Concurrently, power may be temporarily removed from the spinning flywheel motor to avoid high currents during the braking. If the flywheels are stepper motors, the stepper motors are rapidly stopped by stopping the pulses applied to the motor. Alternatively, mechanical power can be removed by regenerative braking, converting the kinetic energy of the flywheel into electrical energy and storing it in the battery/fuel cell assembly 27.
As gyroscopic actuators, the flywheels actively provide roll attitude control as well as pitch attitude control, especially during acceleration and deceleration of the wheels 2 and 4 when instability often arises. Twisting of each flywheel assembly about the {circumflex over (ψ)} axis (
When negotiating turns, the turns themselves create a yawing motion with respect to inertial space that would create undesirable gyroscopic effects if only a single gyroscopic actuator (flywheel) were employed. Conversely, a single gyroscopic actuator used to compensate for pitch or roll may also generate a yaw component. To address this side effect, a pair of counter rotating (in both spin and twist) gyroscopic actuators should be employed to cancel out the yaw effect of the other. Therefore, the preferred vehicle employs the front and rear flywheel/spin motor assemblies 10 and 13.
The gyroscopic actuators (flywheels) have maximum impact on the roll axis, θ, when they are oriented about ψ (
A practical vehicle of this type should incorporate some means of mechanical suspension to dampen the shocks of abrupt impact with the ground due to a fall or ballistic impact.
Benefits of the proposed arrangement include the following:
Strictly speaking, the wheels need not be coaxial.
Static Balance (YZ Plane):
Static balance in the simplest case consists of maintaining the center of gravity (CG), which is the projection of the center of mass (COM) of an object collinear to the gravity vector, inside the effective contour of support projected onto a plane normal to the gravity vector.
The ground contact (GC) represents an instantaneous point or small region of ground contact, so if the CG remains above this point, then per the definition above, the vehicle is said to remain in a state of static balance. Intuitively, one can appreciate that an object's CG may temporarily fall outside the effective contour of support and still maintain balance. This situation is referred to as dynamic balance.
Strategies for Dynamic Balance:
The gyroscopic action of the large wheels 2 and 4 can produce a correcting action that provides a measure of dynamic stability. At higher velocities {dot over (x)}>>0, the angular momentum of the wheels in the XZ plane (
τ{circumflex over (z)}=Jwωw{dot over (θ)}cos θ
where the torque scales with the angular momentum, Jwωw, about ŷ. But since the yaw dynamics, neglecting friction at the GC, is:
τ{circumflex over (z)}=J{circumflex over (z)}{umlaut over (γ)},
then
J{circumflex over (z)}{umlaut over (γ)}=Jwωw{dot over (θ)}cos θ.
This action is sometimes referred to as “steering into the turn” and is a natural property of gyroscopic effects on monowheels or bicycles. As a result, the wheel(s) no longer tracks along a straight-line trajectory, but rather, an arcing trajectory. If the yaw torque can result in turning the GC back under the CG before the roll dynamics cause the vehicle to tip too far, then the vehicle can be considered dynamically stable in roll.
Note that this phenomena doesn't require that the preferentially high angular momentum come from the wheel alone. The spinning flywheel(s), whose axis of rotation may be largely parallel to the wheel (or within 45 degrees, as previously mentioned) can also contribute:
τ{circumflex over (z)}=(Jwωw+Jfωf cos(ψ−γ)){dot over (θ)}cos θ.
As a result, a monowheel proceeding in the {circumflex over (x)} direction with even a small nonzero velocity sizeable angular momentum about the ŷ axis can produce a sizeable yaw torque due to the due to the spinning flywheel. Thus, due to the contribution of the flywheel, although it is possible to have sizeable angular momentum about the ŷ axis even at zero forward velocity due to the spinning flywheel, the monowheel's forward velocity must be nonzero ({dot over (x)}>0) in order for the GC to have a chance to move out from under the CG and produce a torque to correct the lean.
At intermediate velocities, with a COM (center of mass) far from GC, the vehicle can control roll by navigating a serpentine rather than a straight line trajectory in {circumflex over (x)} which is a common technique when riding a unicycle. This results in an oscillating θ and correcting torques generated by centripetal acceleration in each turn.
Dynamic Balance (YZ plane) at Zero Forward Velocity:
At low velocity, {dot over (x)}≈0, tilt balance can be maintained even if the body at times is instantaneously not in a state of static balance.
Strategies:
The preferred embodiment of the vehicle uses torque against a reaction wheel (the flywheels) to provide balance, especially at low or zero vehicle velocities. This is analogous to a tight rope walker with balance pole. The balance pole, although light-weight, represents a much larger rotational inertia than the tight rope walker himself and provides a reaction inertia. Addition of the large balance pole inertia to his own also substantially reduces his own tipping frequency and thus provides more reaction time. Torques about the axis of the tight rope can be generated by accelerations of the balance pole reaction inertia in the opposite direction. Since the tight rope walker is nominally erect (i.e., oriented parallel to the gravity vector, otherwise he'd fall), the practical range of angles of the pole (relative to horizontal) is limited, and similarly, the achievable correcting torques remain small.
Large torques are possible by braking, but as this is a continuous process, you can't continuously brake although short term braking is practical for impulse corrections. As a result, a large torque actuator (flywheel) would be required and may not be efficient unless the energy could be reharvested.
The limitation requiring a respectable nonzero forward velocity of a wheel as a gyroscope can be overcome by mounting the flywheel onto a pivot as indicated in
τ{circumflex over (x)}=Jfωf{dot over (ψ)}cos(ω−γ)
where ψ is the deflection of the pivot in inertial coordinates. Then, including the effect of the preferred gyroscopic actuator in the simplified vehicle roll dynamics:
JGC{umlaut over (θ)}−Mglθ=−Jfωf{dot over (ψ)}f cos(ψ−γ)
A control law can be applied to servo this twisting motion about ψ to correct vehicle lean. This situation sounds promising when traversing straight trajectories but, in turns, the turns themselves create a yawing motion with respect to inertial space that would create undesirable gyroscopic effects if only a single flywheel were employed, resulting in turns in one direction to be easier and in the other direction much more difficult. To null this side effect, a pair of counter rotating (in both spin and twist) gyroscopic actuator assemblies (flywheel assemblies) with equal rotational inertias should be employed.
Like the unicycle rider balancing in the fore-aft direction, dynamic balance can be maintained with a hysteresis or limit cycle motion wherein the body continuously oscillates about θ=0. We say it is dynamic because at almost any instant in time, the body may be instantaneously unstable, but net stable over time maintaining balance. If the dominant mass is a necessary system component like the wheel motor or batteries/fuel cell, a mechanical oscillator between it and a spring can be used, with a smaller actuator that excites it at this frequency. With a preferentially series elastic actuator (see U.S. Pat. No. 5,650,704) a control scheme that adjusts the neutral position can be employed. The optimal natural frequency of this system should be much higher than (or match) the tipping frequency. In this case, the physical body doesn't actually rotate substantially. When moving, locking the actuator (say, by shorting the leads) prevents inadvertent excitation that might occur if the spatial frequency of bumps and forward speed are such that this frequency is excited. Preferentially, it would be desirable to design it as a dynamic absorber system so that there are no lateral forces at GC.
A monowheel with an omnidirectional wheel could employ passive stability in the X-Z plane and dynamic balance in the Y-Z plane much the same what the unicycle rider does at zero velocity and leverage gyroscopic effect of the wheel at speed. The omnidirectional wheel could also provide a means of instituting “lean” by moving the GC out from underneath the COM. It could also provide a means of turning at zero forward velocity since it would have two GC points on adjacent mini-wheels that could rotate in opposite directions. All the mini wheels could be powered off of a single actuator with alternating wheels rotating in opposite directions. If the transmission ratio of the wheels spinning in one direction was different from the other then a single actuator could cause a “hovering” motion to move the GC with respect to the COM to balance laterally albeit with a slight simultaneous but tolerable yaw motion.
An alternative means of balancing at low forward velocity is to provide a means of oscillating the vehicle in the {circumflex over (z)} direction close to the ground contact. This situation is a variant of the inverted pendulum problem and has been solved for the case where the oscillation is a simple harmonic motion (z(t)=Aeiωt).
JGC{umlaut over (θ)}−Mgl sin θ={umlaut over (z)}sin θ=−MlAω2eiωt sin θ
For sufficiently fast oscillations, although the vehicle roll, θ(t), deviates from zero, it recovers in a type of limit cycle solution and balance is maintained. Note that such a scheme doesn't require feedback or control to achieve this effect. It does, however, require that the oscillation be largely and consistently in the {circumflex over (z)} direction.
Note that all of these strategies for balance in the roll direction are distinct from the notions of balance in the Dean Kamen patents related to the Segway™ (U.S. Pat. Nos. 5,975,225, 6,357,544, and 6,367,817). In the vehicles described in those patents, the vehicles are statically stable in the roll direction by virtue of having widely separated wheels. However, they are unstable in the pitch direction and apply the known solution to the inverted pendulum control problem to “balance” in the pitch direction. Fundamentally, such balance couples the position of the vehicle and its pitch. Put simply, the vehicle must move forward or backward somewhat to correct for a deviation in vehicle pitch. This can be intuitively understood by appreciating that balancing an inverted broom on one's palm is a classic example of the inverted pendulum control problem. In contrast, the monowheel vehicle that is the subject of this invention cannot simply move sideways to correct for vehicle lean in the roll direction due to the no-slip or limited slip condition presented by the wheels at the ground contact.
Dynamic Balance (YZ plane) at Non-Zero Forward Velocity:
Although the nominal desired lean angle (tilt) at zero forward velocity will typically be zero because it represents an equilibrium point, it will be nonzero at higher speeds. In a turn ({dot over (x)}≠0 and {dot over (γ)}≠0), although the body may be experiencing substantial lean (θ>>0), inertial forces due to centripetal acceleration generates a torque about the CG. As a result, the equilibrium point is moved to a nonzero position wherein the inertial torque balances the weight-induced torque. Assuming the center of mass is elevated above and separated from the GC by a distance 1, a torque is generated outwards from the circle of radius, r, with a magnitude:
Then
τinertial=τweight
Ml{dot over (x)}{dot over (γ)}=Mglsin θ,
resulting in a new equilibrium lean angle of
As a sanity check, this dynamic result should still be valid in the static case of {dot over (x)}=0 and {dot over (γ)}=0, indeed θ=0. Thus, this value represents the nominal equilibrium and therefore, the typical desired lean angle as a function of both forward speed and yaw rate.
Righting the Vehicle from a Fallen Pose:
There are numerous motivations to consider the case of the vehicle in a prone position with its wheels not in contact with the ground. Beyond the obvious that it should be inevitable that the vehicle would eventually end up in this position unintentionally, intentional motivations might be to conserve power in this minimum potential energy state or simply to make it easier to hide.
In order to right the fallen tandem monowheel vehicle, a means must be found to exert a torque about the body. External forces can be created by use of actuators or thrusters. Internal forces can be created by reacting against a reaction inertia (quasi-static case) or precessing a gyroscope by leveraging Newton's third law roughly stated that for every action there is an equal and opposite reaction.
For the large torques required to right the fallen vehicle, then a different dynamic case must be employed. Consider
l=Rw−RCOM cos φ.
In order to right the vehicle, a torque about the GC needs to be created. For the general case, this torque can only be manifested as an external torque to the monowheel structure. However, application of this torque between a reaction inertia, Jf, and the monowheel structure, Js, allows this torque to be applied as an internal torque resulting in acceleration of both inertias albeit in opposite directions with a resultant force, Fr, by the ground on the GC and a component, Fy, in the ŷ direction resisting slip.
Summing torques around the GC yields:
where JGC is the net effective rotational inertia about the ground contact. From the above, we see that if the dynamics are reduced to zero, then the external torque, τGC, is simply equal to the weight-induced torque which matches intuition. Rearranging results in:
τ=JGC{umlaut over (θ)}−Mglsin θ
Based on the above result, it can be determined that if the acceleration of the body about θ is ignored, then the torque generated by the time rate of change of the flywheel inertia would reduce to that necessary to counter only the weight-induced torque which is the static value. Thus, it's no surprise that {tilde over (θ)}=0. A larger rate than this is necessary to produce acceleration of the body in the negative direction ({tilde over (θ)}<0). So then, what is that acceleration? We seek the acceleration that would restore θ=0 with {dot over (θ)}=0. From energy conservation between states:
Then, if the body could be instantaneously accelerated to a rotational velocity, {dot over (θ)}, and all of that energy was translated into gravitational potential energy and all other stored energy such as in the flywheel in the system remained constant, comparing the final desired state (θ=0 and {dot over (θ)}=0) to an initial state (θ≠0 and {dot over (θ)}≠0) evaluated at t=0+ yields:
Note that although this is mathematically valid for all θ, it is likely impractical much beyond θ=π/2 as it is imperative that the no lateral slip condition be maintained. Solving yields:
Note that although calculated instantaneously, since the trajectory of θ is continuous to the end condition, this expression is also a valid description of {dot over (θ)}(t) as a function of θ(t) although it is not a unique trajectory satisfying the beginning and ending conditions. Differentiating both sides with respect to time gives:
Inserting this result into the equation above and dropping the “initial” subscript provides an expression for the necessary torque:
τ=2Mgl sin θ.
It is interesting to note that this expression simplifies so much. Based on the equations and assuming no losses, the necessary input is to simply provide double the weight-induced torque. It may not be intuitive that the result doesn't depend on JGC, but if one considers that although the input should be linearly larger for a larger body rotational inertia, the stored kinetic energy also increases linearly with this parameter and cancel.
If the torque, τ, comes from a reaction inertia, then:
Therefore,
meaning that the flywheel must decelerate or accelerate in the opposite direction to θ. Due to energetic limitations, it is typically more practical to brake the flywheel rapidly than to accelerate rapidly. For the tandem monowheel, reaction inertias exist in the form of the two flywheels when oriented such that their spin axes are parallel to {circumflex over (x)} or |ψ−γ|=π/2. Or, in other words, for establishing an optimal orientation of the flywheels prior to righting the vehicle (prior to braking the flywheels), the flywheels' spin axes are perpendicular to the diameters of the wheels (and parallel to the roll axis). In this configuration, the angular momentum of the flywheels is parallel to the roll axis. The two flywheel pivots can be subsequently reoriented and their respective flywheels spun up to speed.
Using the dynamic method, there are three candidate trajectories for righting the system:
1. A simple rotation about GC to the upright position.
2. A dynamic rotation about GC beyond the upright position such that the GC or “foot”, or current contact point with the ground (as it is a locus of moving contact), leaves the ground, resulting in an airborne or aerial phase. This “hop” is recovered by appropriate foot placement to absorb the lateral translational kinetic energy.
3. A simple rotation about the GC to a somewhat elevated position and then accelerating forward along a circular or spiral trajectory.
For trajectory following with the simple rotation method #1, a controller such as shown in
τf=KPθ(θd−θ)+KDθ({dot over (θ)}d−{dot over (θ)})=KPθe+KDθė
For θd={dot over (θ)}d=0 and evaluating at θ=π/2, this suggests:
In the context of the control system diagram of
For the dynamic method #2, an open loop torque can be supplied initially as long as a closed loop method can be employed during ballistic flight. Although energetically suboptimal, since there is precedence for “foot placement” during the ballistic phase in the literature (Raibert, M. H., Brown, H. B., Jr. 1984. Experiments in balance with a 2D one-legged hopping machine. ASME J. Dynamic Systems, Measurement, and Control 106:75-81), this seemingly more complicated method may actually be easier from a control perspective. For θ≦π/2, such an open loop torque can be simply:
τf=2KOMgl sin θ,
for KO>1.
During the ballistic phase, the GC is placed such that it is laterally farther away from the COM as the lateral velocity increases. Unlike the robots in the Raibert papers which were specifically designed such that the rotational inertia of the legs was much smaller than that of the reaction inertia “body,” for the tandem monowheel, the rotational inertia in the roll direction is much larger than the flywheel. As such, conservation of angular momentum in the ballistic phase must be considered in order to accurately place the GC. Therefore, it is desirable to incorporate two flywheels: the first as described previously, and a second rotating in the opposite direction, that operates at constant angular velocity during the “leap” and rapidly decelerates during the ballistic phase to cause a rotation of the structure in the opposite direction for GC placement.
For the dynamic method #3, if ground space permits, one way to use smaller torques and still right the vehicle is to separate the righting process into the following sequence combining a static step with a dynamic step. Firstly, apply an initial torque to tilt the vehicle up enough to allow the body to clear the ground and the wheel to contact the ground with sufficient contact pressure so as to not slip laterally. Subsequently, negotiate a circle, leveraging inertial forces due to centripetal acceleration to cause a rotation of the body (assuming the center of mass is elevated above and separated from the GC by a distance l) outwards from the circle:
where r is the radius of the circle, {dot over (γ)} is the angular frequency, and {dot over (x)} is the speed. Additionally, the vehicle can accelerate and negotiate a spiral trajectory of ever decreasing radius to hasten the effect:
Note that since the first action preferentially exerts a torque on a vehicle of low rotational inertia, it would be preferential for the vehicle to possess the ability to move its center of mass such that it is close to the GC in the first action and farther from it in the second action. If so, then
From this differential description of the torque, we can see that the inertial torque increases with acceleration ({dot over (x)}>0), a decreasing radius ({umlaut over (γ)}>0), and increasing COM-GC separation distance (i>0). Similarly, based on this equation, there is a means of reducing the torque as the vehicle is righted, namely by deceleration and by increasing the trajectory radius. Once the body is sufficiently upright, the balance control system can take over and the circular or spiral trajectory can be aborted.
Vehicle Acceleration:
In conventional monowheel systems, excessive acceleration or deceleration results in the structure of the vehicle spinning in the opposite direction due to coupling of the wheel actuation and pitch attitude control. This condition is colloquially known as “gerbiling.” An analysis of the dynamics of the structure in rotation yields:
If the wheel torque, τw, exceeds the weight-induced torque, Mgr sin φ, then the rotational inertia of the structure accelerates. If it exceeds the maximum value of Mgr, then the structure spins. This result also illustrates how the maximum wheel torque used to accelerate a conventional monowheel vehicle cannot exceed the critical value of
without causing the “gerbiling” condition. Note that even if this condition is satisfied, the pitch may still be nonzero and have a maximum magnitude of |φ|≦x/2.
Fundamentally, the problems of coupled pitch to wheel acceleration and the possibility of “gerbiling” occur because the vehicle structure is the only reaction inertia available. Alternatively, a design solution to this problem can be found by providing an alternative reaction inertia preferably with an axis in the ŷ direction that is free to accelerate or decelerate in the opposite direction. If this reaction inertia has its center of mass at its center of rotation such as in a flywheel, then no gravitational potential energy is stored and the dynamics of the system become:
where the flywheel kinematics have been simplified for the nominal case of θ≈0 and φ≈0.
This expression clearly demonstrates that if the pitch is in the vicinity of its nominal value of φ=0 and |ψ−γ|=0, then if
τf=−τw,
then there will be no net acceleration on the structure ({umlaut over (φ)}=0). Thus, if
Jw+MR2=KJf,
then the desired flywheel accelerations should be:
{dot over (ω)}f=−K{dot over (ω)}w
Moreover, if the flywheel is gimbaled so that it can pivot, then an additional, gyroscopic term can be added for an additional degree of control:
Js{tilde over (φ)}=−Mgr sin φ−((Jw+MR2){dot over (ω)}w+Jf{dot over (ω)}f cos(ψ−γ)+Jfωf{dot over (ψ)}cos(ψ−γ)).
Thus, the pitch of the structure can be modulated both by accelerating or decelerating the flywheels as long a sufficient component of the momentum is along the ŷ axis or by precessing the gyroscopes about the {circumflex over (ψ)} axis, or by a combination of both control inputs. For example, in order to maintain the current pitch, then {umlaut over (φ)}=0. Thus:
As a result, the present pitch attitude, φ, can be maintained in the presence of wheel acceleration, {dot over (ω)}w, given the present orientation of the flywheel in inertial coordinates, ψ−γ, by a combination of flywheel speed change, {dot over (ω)}f, and orientation change, {dot over (ψ)}.
A single actuator can be used to torque both inertias to reduce the number of actuators although double the torque will be required. Alternatively, if the torques are supplied by separate actuators, or if the flywheel or the wheel or both is/are coupled to a single actuator with a variable transmission, then the net torque, i.e., the difference in applied torques can be used to servo a somewhat arbitrary pitch angle.
τw−τf=KPφ(φd−φ)−KDφ{dot over (φ)}
Alternatively, a gyroscope with a pivoting axis oriented vertically and a spin axis oriented along the {circumflex over (x)} axis can be twisted along the pivot axis to produce a reaction torque to resist the vehicle structure from gerbiling. Gyroscope arrangements with alternative axis orientations are possible that will achieve the same effect as long as the precession axis is along the ŷ axis. In another embodiment, the flywheel axes are mounted on a gimbal that can changes the angles of the axes in multiple dimensions, rather than the axes only being pivoted generally about a vertical axis.
Raising the Center of Mass (COM):
In conventional monowheeled vehicles, it is undesirable for the pitch to exceed a value even close to π/2 to avoid upsetting the rider. However, in an unmanned monowheel vehicle, it may be desirable to do so since there are two solutions to the weight-induced torque for |φ|≦π. The solution with the larger angle might be desired in order to increase the distance between the COM and the GC, for example, or to decrease the tipping frequency.
Generalized Vehicle Control Strategy:
Up to now, we have discussed how gyroscopic actuators (flywheels) could be used to correct tilt about the {circumflex over (x)} axis and control pitch including resisting vehicle structure acceleration about the ŷ axis. Also, in a simplistic case it was mentioned that the two actuators should generally always act opposite each other. However, that isn't necessarily optimal. For example, with a pair of gyroscopic actuators, there are four input variables (front flywheel spin speed change and precession speed, and rear flywheel spin speed change and precession speed) to contribute to servoing the net desired torque vector which is a sum of the desired roll, pitch, and yaw torques:
{right arrow over (τ)}tot={right arrow over (τ)}{circumflex over (x)}+{right arrow over (τ)}ŷ+{right arrow over (τ)}{circumflex over (z)}
For the purpose of illustration, the desired yaw torque can be extracted to allow focus on the desired roll and pitch torques. As a result, solutions can be chosen such that the sum of the gyroscopic torques introduced by the precession speeds equals the desired roll torque and the difference equals the pitch torque:
One can see that if the desired pitch torque is zero, then indeed, the amount of necessary precession speed of both gyroscopic actuators is the same. One skilled in the art can appreciate that other combinations of the four gyroscopic actuator control variables are conceivable.
A generalized vehicle control strategy that seeks to simplify control by largely decoupling these variables might proceed according to the following logic:
At low speeds when both wheels maintain traction, control
At higher speeds, lock the differential by preventing the differential bevel gear 72 from rotating by servoing the current position (zero angular velocity) of the steering motor.
Preferential Dynamics and Additional Analysis:
Once righted, in the simplest case with no momentum components spinning about the ŷ axis, summing torques around the GC yields the following dynamic relationship:
Then
JGC{tilde over (θ)}+Mglθ=J{tilde over (θ)}+Kθθ=0.
This yields an expression for the “tipping frequency” exhibited by the vehicle:
where M is the total vehicle mass, l is the distance from the ground contact (GC) to the vehicle's center of mass and JGC is the total mass moment of inertia about the GC. Thus, for improved roll control to resist tipping, it is preferable to have the vehicle rotational inertia as large as possible.
Speed Control:
The wheel torque should be controlled depending on the mode desired. If in a position control mode with a constant destination position, xd, then a candidate control law may take the form of a regulator:
τw=KPx(xd−x)−KDx{dot over (x)}+KIx∫(xd−x)dt
where {dot over (x)}=ωwR.
If in a velocity control mode with a constant desired velocity, {dot over (x)}d, then a candidate control law may be:
τw=KPv({dot over (x)}d−{dot over (x)})−KDv{tilde over (x)}
Alternatively, a controller that takes both speed and position into account can be considered as independent state variables.
Differential Transmission Assembly:
The vehicle employs a differential transmission assembly 60 to supply power from the internal combustion engine and transmission assembly 30 to the two wheels 2 and 4, much like in a rear wheel drive automobile.
The drive sprocket 31 (
The differential transmission assembly also includes an active differential gear assembly that is used for steering the vehicle at zero or low forward velocities (rather than tilting the vehicle when moving). A differential pinion gear 73 extends from the differential bevel gear 72 and has differential pinion gear teeth 75 that engages with steering crown gear teeth 81 of a steering ring gear 80. A steering motor 40 (
When the vehicle is moving in a forward direction, the differential pinion gear 73 rotates around the wheel axes as a result of being engaged with the rotating bevel gears 70 and 71. Since the steering motor 40 is typically disengaged from the differential pinion gear 73 when the vehicle is moving in a forward direction, there is little drag on the propulsion system due to the steering assembly.
The various gears use the following bearings: differential ring gear/carrier inner bearing 54, differential ring gear/carrier outer bearing 55, steering ring gear bearing 56, and pinion gear bearing 57.
Thus, the drive torque, τD, to the left and right wheels 2 and 4 (for propulsion) is the scaled sum of the torques to the left and right wheels, τL and τR, respectively. Also, the steering torque, τS, is the scaled difference of the torques to the two wheels:
Thus, the wheel torques can be expressed as:
This entire differential assembly may be sealed and contain a viscous fluid and appropriate features to operate as a viscous coupling unit such that slow, relative motions of the two outputs are permitted, yet suppress rapid rotations as might occur during the case of a single wheel spinning. Although all gears in these figures are shown with straight cut teeth, one skilled in the art can appreciate that they may be cut other ways as well such as helical and double helical.
The various electronic controls and attitude sensors (e.g., inclinometers, gyroscopes including both spinning type and vibrating structure type, accelerometers, GPS triangulation receivers, etc.) are provided on an upper platform 21 (
In one embodiment, the vehicle is programmed to transport itself to a particular destination using GPS coordinates. In another embodiment, a camera mounted on the vehicle, such as supported by the frame 20, is used to provide feedback to a remote supervisory control facility, where an operator then controls the speed and direction of the vehicle using a joystick or other control. The communications may be tethered or by various telemetry methods including ultrasonic, radio frequency, optical, or any of various methods enabling physical separation of the vehicle from its supervisory control location. The balancing of the vehicle is performed automatically, as previously discussed. The vehicle may be used for surveillance in hostile environments or may be used to deliver payloads.
The speed of the vehicle is controlled by the control system controlling the amount of fuel and air injected into the combustion engine cylinders. In
While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention. The term “invention,” as used herein, frequently refers to the overall vehicle, although individual components within the vehicle may themselves be inventive when not in combination with all other described features of the vehicle. Therefore, the claims themselves represent the true inventions.
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| 8906117 | Jul 1989 | WO |
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| Number | Date | Country | |
|---|---|---|---|
| 20110191013 A1 | Aug 2011 | US |
