Abstract:
A method for positioning a mobile platform at a station that includes moving the mobile platform near a bumper by applying torque to a drive shaft. The mobile platform is maneuvered until a contact stop of the platform is touching the bumper. A locking mechanism is engaged to the mobile platform before the torque to the drive shaft is released to assure position retention at the station. A system for precise positioning of a mobile platform at a station is provided that includes a stationary bumper at said station. A mobile platform has stop contacts, a drive shaft, and sensors for measuring contact with said stationary bumper. The mobile platform having a locking mechanism for preventing movement of the drive shaft in response to input from at least one of the sensors in response to contact with the stationary bumper to provide the precise positioning of said mobile platform.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a non-provisional application that claims priority benefit of U.S. Provisional Application Ser. No. 61/873,683 filed 4 Sep. 2013; the contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention in general relates to mobile platforms, and in particular to a positioning system that facilitates the movement and parking of a mobile platform (vehicle) into a location with a high degree of precision, while also keeping the platform in place. 
       BACKGROUND OF THE INVENTION 
       [0003]    The growth of the use of robots in manufacturing, as well as flexible manufacturing has necessitated the need to move robots to different stations along an assembly line. The required robotic movement has necessitated the mounting of robots on mobile platforms, where the platforms must be positioned into a known location at a repeatable high accuracy in the order of less than 1 mm. Examples applications of robots mounted to mobile platforms include the maneuvering of a robotic assembly arm to a location where the robotic arm can execute assembly tasks, or the maneuver of a robotic surgery system to the right place to conduct surgery on a patient. 
         [0004]    In general, in order to achieve a high level of accuracy and precision when moving a mobile platform two technologies will have to operate in concert, a location tracking system, to measure at high accuracy where the vehicle is, and at what orientation, and a vehicle platform control system that can move the vehicle at high precision in the desired position. While vehicle control systems with high accuracy are readily available, these control systems are typically slow in order to maintain precision control. Furthermore, prior art systems that attain a higher speed and maintain accuracy tend to be overly complex for most applications. Thus, there has conventionally been a tradeoff between the complexity of a control system and the level of accuracy attained by a control system. This trade-off between the complexity and level of accuracy typically also applies to the tracking systems. 
         [0005]    Thus, there exists a need for a mobile platform that can be repositioned at a high level of precision and accuracy that is also cost effective. 
       SUMMARY OF THE INVENTION 
       [0006]    A method for positioning a mobile platform at a station that includes moving the mobile platform near a bumper by applying torque to a drive shaft. The mobile platform is maneuvered until a contact stop of the platform is touching the bumper. A locking mechanism is engaged to the mobile platform before the torque to the drive shaft is released to assure position retention at the station. 
         [0007]    A system for precise positioning of a mobile platform at a station is provided that includes a stationary bumper at said station. A mobile platform has a plurality of stop contacts, a drive shaft, and sensors for measuring contact with said stationary bumper. The mobile platform having a locking mechanism for preventing movement of the drive shaft in response to input from at least one of the sensors in response to contact with the stationary bumper to provide the precise positioning of said mobile platform. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIGS. 1A-1D  illustrate a method for positioning a mobile platform with contact stops according to embodiments of the invention; 
           [0009]      FIGS. 2A-2E  illustrate a method for positioning a mobile platform configured with short range proximity sensors in addition to contact stops according to embodiments of the invention; 
           [0010]      FIG. 3  illustrates the parking of the mobile platform of  FIG. 2  according to embodiments of the invention 
           [0011]      FIG. 4  illustrates a flowchart for a method for parking a platform or autonomous vehicle using only bumpers B and stop contacts as shown in  FIGS. 1A-1D ; 
           [0012]      FIG. 5  illustrates a flowchart for a method for parking a platform or autonomous vehicle using stop contacts and pressure sensors or tactical sensors mounted to a platform or vehicle according to embodiments of the invention; 
           [0013]      FIG. 6  illustrates a flowchart for a method for parking a platform or autonomous vehicle using proximity sensors to align the vehicle close to the walls ( 12 ,  14 ) of a bumper according to embodiments of the invention; and 
           [0014]      FIG. 7  illustrates a flowchart for a method for a mobile platform or vehicle to follow a course or path by following with an absolute position reference system according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    An inventive positioning system is provided that facilitates the movement and parking of a mobile platform (vehicle) into a location or station with a high degree of precision, while also keeping the platform in place. The present invention has utility in a variety of fields including robotic manufacturing, remote inspection and testing, and surgery. In contrast to the prior art, an inventive system can locomote at a comparatively high speed of tens or hundreds or even thousands of rotations per minute and only slow in proximity to a barrier. As used herein. A “barrier” and “bumper” are used synonymously to refer the fixed component of controlled contact between an mobile platform and fixed structure defining a known position. The high degree of precision is achieved by driving the platform slowly into the barrier. In some embodiments , at least three stops on the vehicle make contact with the barrier to determine both position and orientation. Once the position and orientation of the platform is determined, and before the torque on the platform drive system is released, a locking mechanism is engaged to fix the platform position. In a particular embodiment, the locking mechanism is a brake is applied at the drive shaft of the platform wheels so as to maintain a sheer force on the tires to keep the vehicle in place. It is appreciated that other locking mechanisms are operative herein an illustratively include a locking pin engaging a transmission gear; a solenoid lock, and an electro-mechanical locking device like an Electro-Magnet. It is noted that the sheer force on the wheels should exceed any anticipated forces on the vehicle while in a parked position. It is appreciated that a mobile platform according the present invention is readily translated on a continuous track (caterpillar tread), or an inch worm-type propulsion system. The application of the locking mechanism to the drive train also allows for a mechanical drive system with backlash or low resolution motion feedback to be positioned accurately. Embodiments of mobile platform may be an omni-directional vehicle that has the ability to drive sideways, or may have a four-wheel steer system. 
         [0016]    The platform is equipped in some embodiments with a contact point stop adapted to engage the barrier, while in other embodiment in which lateral movement along the barrier after contact is desired, the platform is equipped with a caster wheel. In still other embodiments, the caster wheel is mounted with a sensor that measures the force on the caster wheel; with the applied force exceeding a preselected threshold is used to engage the locking mechanism. 
         [0017]    The use of a platform stop-bumper to drive up against positions, a mobile platform in known designated positions at a high level of accuracy, while eliminating the complexity of a highly accurate position tracking system, and for a highly precise vehicle control system. In certain embodiments of the inventive mobile platform low cost proximity sensors are combined with the use of the bumper to improve the speed of positioning of the platform. The mobile platform or vehicle controller used in embodiments of the invention has the ability to command the vehicle. The controller includes the control algorithms for dead-reckoning and algorithms to perform the bumper move. In still other embodiments, the controller includes the input capture for the proximity sensors to line-up the platform before executing the contact barrier-bumper. 
         [0018]    Embodiments of the inventive method provide additional benefits for autonomous guidance of vehicle platforms. Typical autonomous navigation and control systems use a combination of dead-reckoning (information from wheel speeds and inertial sensors) with external absolute position reference information (e.g., laser based, radio frequency (RF) based, based on magnetic strips, or on simultaneous localization and mapping (SLAM). External absolute reference frame systems tend to be costly, often require extensive installation efforts, and are typically sensitive to environmental conditions. Embodiments of the inventive method described herein can be used as a low cost highly accurate external positioning reference system for this purpose. For applications that do require the vehicle platform to travel longer distances between stations, without the opportunity to park at a stop-bumper, there will still be a need for an external position reference and tracking system. These could be any laser-based, or RF-based as for example detailed in any of U.S. Pat. No. 8,417,444; U.S. Pat. No. 8,010,133; U.S. Pat. No. 7,983,694; U.S. Pat. No. 7,403,783; or U.S. Pat. No. 8,214,147. 
         [0019]    Besides the benefit of eliminating the need for complex locating, vehicle control, and high performance mechanical drive trains with high speed and no backlash, certain embodiments of the present invention are used to position the mobile platform also improve the robustness and durability of a system using embodiments of the inventive method. Particularly when the inventive system is installed in a harsh machining and manufacturing environment, where conditions may prohibit some of the complex technologies that otherwise would be required. 
         [0020]    The infinite operating scenario of certain inventive embodiments is as follows: 
         [0000]    1) Manually drive the platform into the proximity of a bumper.
 
2) Activate the automatic bumper platform alignment task that will park the vehicle in the accurately known position with the stops against the bumper.
 
3) Activate a path following maneuver that will autonomously navigate the vehicle into the proximity of another bumper station.
 
4) Activate the automatic bumper platform alignment task that will park the vehicle in the accurately known position with the stops against the bumper.
 
5) Use the robotic arm or allow other platform payload to execute its task.
 
6) Continue to the next task from step 3)
 
Each time the vehicle lines up with the barrier, the system regains its accuracy to drive to the next station based on dead-reckoning.
 
         [0021]    Path following for embodiments of the mobile platform, as mentioned in the preceding paragraphs, can be conducted either by command following, “dead-reckoning”, or by autonomous guidance with position tracking. 
         [0022]    Command following means that the platform or vehicle motion commands are recorded in time, and that the vehicle will be provided with exactly the same drive commands over time, during the path following process. 
         [0023]    Dead-reckoning is a method that uses wheel speeds and inertial sensors to estimate the platform or vehicle motion in space over time. The estimation of the vehicle position and orientation is then compared with the vehicle motion during the recording, or is compared to a pre-programmed reference profile. The vehicle is commanded to follow the desired trajectory in time. 
         [0024]    Path following with absolute position references means that the platform or vehicle constantly compares its estimated position from dead-reckoning, to an absolute reference position from an external geo-location system. During path following, the vehicle is constantly correcting its motion to follow the desired trajectory accurately. 
         [0025]    Besides the simplicity benefit of eliminating the need for expensive positioning technologies, embodiments of the mobile platform also improves the robustness and durability of a system using this method. Particularly when the system is installed in a dirty machining and manufacturing environment, where conditions may prohibit some of the expensive technologies needed otherwise. While the present invention is further illustrated with respect to an orthogonal barrier structure, it is appreciated that a barrier operative in the present invention can have any number of shapes including a linear barrier, as well as posts of various cross sections including circular, rectilinear, and polygonal cross-sections. 
         [0026]    Referring now to the figures, where like elements are identified with identical numerical designators between figures,  FIGS. 1A-1D  illustrate an embodiment of a first inventive method for positioning a mobile platform  10  with a high degree of precision, when the platform  10  is in an unknown position and orientation, but is relatively close to a bumper B. As shown in  FIG. 1A , the platform  10  has a set of wheels  18  that are powered by a drive train  16 . The platform  10  is configured with three or more stops contacts (C 1 , C 2 , C 3 ) for positioning the platform  10  with respect to a landing right angle bumper B with sides  12  and  14  that may define or correspond to a X-Y plane. In  FIG. 1B  the platform is maneuvered until a first contact is made between bumper side  12  and stop contact C 1  (it is noted that C 2  may also be used). As shown in  FIG. 1C , the platform  10  continues to move toward bumper side  12  until stop contact C 2  hits the bumper side  12  (alternatively C 1  will contact side  12  if C 2  was first to contact side  12 ). In  FIG. 1D  with both stop contacts C 1  and C 2  touching bumper side  12 , the platform  10  rides along bumper side  12  until stop contact C 3  touches. It is noted that the bumper B may also have markings to facilitate for variation in the Z axis or vertical plane to adjust for non-planar conditions experienced by the platform  10 . A platform  20  in certain embodiments having a vertical adjustment mechanism to position a platform supported payload such a robotic arm to a controlled vertical position. A vertical adjustment mechanism operative herein includes those conventional to the art such as a worm gear, stepper motors, and the like. 
         [0027]      FIGS. 2A-2E  show a platform  20  configured with short range proximity sensors (S 1 , S 2 , S 3 ) in addition to contact stops (C 1 , C 2 , C 3 ). Examples of proximity sensors may include, but are not limited to, infra-red, sonar, magnetic, etc. The use of proximity sensors allows the platform to move into position faster since contact with the landing right angle bumper B is only done once an initial alignment of the platform is determined with the proximity sensors (S 1 , S 2 , S 3 ). For example, the platform  20  may initially be moved to within an inch of the bumper B as shown in  FIGS. 2B-2D , and then moved into final position with the stop contacts (C 1 , C 2 , C 3 ) touching the sides  12  and  14  of the bumper B as shown in  FIG. 2E . 
         [0028]    Furthermore, in certain embodiments an absolute tracking reference system, for example, based on RF, laser, or magnetic strips may be used to drive the platform to a first landing position, and then apply then use the stop contacts (C 1 , C 2 , C 3 ) and proximity sensors (S 1 , S 2 , S 3 ) for the final positioning of the platforms  10  or  20 . 
         [0029]      FIG. 3  illustrates the parking of the mobile platform  20  of  FIG. 2  where an operator manually maneuvers the platform  20  near the bumper B, and the operator then activates the proximity sensors (S 1 , S 2 , S 3 ) to automatically have the platform position itself with the bumper B with an auto park program. 
         [0030]      FIG. 4  illustrates a flowchart for a method  30  for parking a platform or autonomous vehicle using only bumpers B and stop contacts as shown in  FIGS. 1A-1D . The platform  10  follows a pre-recorded or pre-programmed path (step  32 ), either from recorded motion commands, or following a series of position waypoints from a trajectory, until the path is complete (step  34 ). When the path is complete, the vehicle should be in the vicinity of the park location. With bumpers only, the vehicle will be commanded to slowly drive into the bumpers (step  36 ) on two corner sides or the vehicle, until the torque on the wheel shaft or motor drive exceeds a certain limit (step  38 ), indicating the vehicle is actually pushing into the wall. This is when the parking maneuver is complete. 
         [0031]      FIG. 5  illustrates a flowchart for a method  40  for parking a platform or autonomous vehicle using stop contacts and pressure sensors or tactical sensors mounted to the platform or vehicle. The platform  20  follows a pre-recorded or pre-programmed path (step  42 ), either from recorded motion commands, or following a series of position waypoints from a trajectory, until the path is complete (step  44 ). After the path is complete, the vehicle will drive forward first (step  46 ), until the forward contact stop C 3  touches the wall  14  of the bumper B (step  48 ). Then the vehicle will be commanded to drive sideways (step  50 ) until one of the side contact stops (C 1 , C 2 ) touches the wall  12  of the bumper B (step  52 ). A determination (step  54 ) is made whether the front contact C 3  is still contacting the bumper B, where if the contact C 3  is not touching the bumper B, the platform is again driven forward (step  46 ) based on the determination made at step  54 . The vehicle is subsequently commanded to rotate (step  58 ), based on a determination of whether both side contact stops (C 1 , C 3 ) are touching the wall  12  of the bumper B. The process  40  stop when all three stop contacts (C 1 , C 2 , C 3 ) are touching the walls ( 12 ,  14 ) of the bumper B. 
         [0032]      FIG. 6  illustrates a flowchart for a method  60  for parking a platform or autonomous vehicle using proximity sensors to align the vehicle close to the walls ( 12 ,  14 ) of the bumper B, after the pre-recorded path or pre-programmed path following has completed. The platform  20  follows a pre-recorded or pre-programmed path (step  62 ), either from recorded motion commands, or following a series of position waypoints from a trajectory, until the path is complete (step  64 ). A determination is made at step  66  if the proximity sensors (S 1 , S 2 , S 3 ) detect the walls ( 12 ,  14 ) of the bumper B. At step  68 , the platform  20  is controlled so that the proximity sensors (S 1 , S 2 , S 3 ) read a predefined distance at the alignment points to the walls ( 12 ,  14 ) of the bumper B. The vehicle motion commands are aimed to control the error between the measurement from the proximity sensors and a pre-defined distance from the wall to zero. Following a positive confirmation (step  70 ) that all the proximity sensors (S 1 , S 2 , S 3 ) are at the predefined distances to the walls ( 12 ,  14 ) of the bumper B and the platform  20  is aligned, the platform  20  slowly creeps or moves towards the corner of the bumper B (step  72 ). A determination is made at step  74  whether the torque exceeds normal driving conditions for free movement. If the torque exceeds normal driving conditions, this indicates that the platform is now in contact with the walls ( 12 ,  14 ) of the bumper B and the process concludes with a brake applied to the drive train. 
         [0033]      FIG. 7  illustrates a flowchart for a method  80  for a mobile platform or vehicle to follow a course or path by following (step  82 ) with an absolute position reference system, where the vehicle constantly compares (step  86 ) its estimated position from dead-reckoning, to an absolute reference position from an external geo-location system (step  84 ) to determine if the platform or vehicle is on path. During path following, the vehicle is constantly correcting its motion to follow the desired trajectory accurately by following commands to move towards the current waypoint in the path (step  88 ). A determination is made at step  90  of whether the platform or vehicle has completed the path or course. If the path or course has been completed, the parking maneuver is executed for the platform or vehicle as described in the preceding methods. 
         [0034]    The foregoing description is illustrative of particular embodiments of the invention, but is not meant to be a limitation upon the practice thereof. The following claims, including all equivalents thereof, are intended to define the scope of the invention.