Abstract:
An apparatus for marking predetermined known overhead positional locations within a coordinate space, for viewing by an image acquisition system. The apparatus comprises a plurality of marker tags, grouped in one or more rows, each row having an axis, each row being supported by a row support. Each marker tag comprises an optically opaque, dark colored corrugated substrate, substantially rectangular in shape. A label having a unique machine-readable barcode symbology printed thereon is positioned centrally on the substrate so that a dark colored border of the substrate surrounds the label. Each row support comprises a first support cord and a second support cord. The first support cord supports a first lateral edge of the marker tags in a row group in a fixed, spaced-apart positional arrangement. The second support cord supports the second lateral edge of the marker tags in the row in a slidable support arrangement.

Description:
Benefit of U.S. Provisional Application 61/011,084, filed Jan. 14, 2008 is claimed. 
    
    
     RELATED APPLICATION 
     PCT/US2005/043755, published as WO2006/065563, Method and Apparatus for Determining Position and Rotational Orientation of an Object. 
     BACKGROUND OF THE INVENTION 
     Determining the position and rotational orientation of an object within a defined space is a practical problem that has brought many solutions. For example, Global Positioning Systems (GPS) is a widely recognized position determination technology, but it lacks rotational orientation determination capability for stationary objects. GPS operability suffers indoors from signal attenuation and reflections, so it is not a good choice for indoor applications. Ultrasonic methods that operate well indoors have been designed to replicate GPS capability, but they, too, lack rotational orientation determination. In prior art optical position location systems various markers are used. 
     PRIOR ART 
     U.S. Pat. No. 6,556,722 discloses a method, wherein circular barcodes are utilized to indicate reference positions within a television studio. In this optically based method, a television studio camera is equipped with a secondary camera which views position markers set onto the studio ceiling in known locations. The markers are constructed of concentric ring barcodes which are developed specifically for that purpose. Camera position is determined by capturing an image of at least three markers and performing geometric analysis in a digital computer to determine accurate location within the three-dimensional studio space. The invention discloses proprietary circular ring barcodes, which cannot be read by commercial machine vision systems, and requires a multiplicity of markers to be within view. 
     U.S. Pat. No. 5,832,139 discloses a method and apparatus for determining up to six degrees of freedom of a camera relative to a reference frame which comprises an optically modulated target with a camera and processing the camera&#39;s output video signal with a digital computer. The target may have a single pattern, multiple patterns, or patterns of varying size, and multiple targets may be used. The invention analyzes the parallax, or “warping” of square target patterns into non-square quadrilaterals within the field of view in order to determine six degrees of freedom of the camera. It does not present common barcode symbols as a choice for passively modulated targets, and does not use the inherent identity of barcode symbols for both automated means and non-automated position determination means. 
     A number of machine vision-based systems exist, especially for vehicle and robot guidance, however, most analyze physical surroundings by viewing downward toward floor markings, or horizontally toward local scenery or reflective markers. For example, U.S. Pat. No. 6,728,582 provides a system and method for estimating the position of an object in three dimensions using two machine vision cameras interconnected with a machine vision search tool. A nominal position for each camera&#39;s acquired image of the object is determined and a set of uncertainty vectors along each of the degrees of freedom is generated. This method requires viewing multiple objects with multiple cameras in order to make the weighted estimation of the position of the object. 
     In view of the foregoing, there is a need for a simple, easy to install, optical position marker apparatus useful with image acquisition systems such as machine vision and navigation systems. 
     SUMMARY OF THE INVENTION 
     An apparatus for marking predetermined known overhead positional locations within a coordinate space, for viewing by an image acquisition system, is disclosed. The apparatus comprises a plurality of marker tags, the marker tags being grouped in one or more rows, each row having an axis, the marker tags in a row being supported by a row support. Each marker tag comprises an optically opaque, dark colored corrugated substrate, substantially rectangular in shape. An adhesive-backed label having a unique machine-readable barcode symbology printed thereon is positioned centrally on the substrate so that a dark colored border of the substrate surrounds the label. Each row support comprises a first support cord and a second support cord. The first support cord supports a first lateral edge of the marker tags in a row group in a fixed, spaced-apart positional arrangement. The second support cord supports the second lateral edge of the marker tags in the row in a slidable support arrangement. The first support cord is attached to an overhead support structure at each end with a tensioning device and the first support cord is drawn to a predetermined tension, thus establishing a substantially straight first lateral row edge. The tensioning devices on the first support cord permit precise positioning of the marker tag group along the row axis. The second support cord is also attached to the support structure at each end with a tensioning device and the second support cord is drawn to substantially the same tension of the first cord, so that the marker tags are supported in a substantially horizontal plane. The slidable support of the second edge allows the marker tags of a row group to align along the first lateral edge and eliminates any skewing of the marker tags due to unequal tensions in the support cords. A spreader bar is provided at each end of the support cords to establish a fixed spacing of the support cords corresponding to the spacing of the first and second lateral edges of the marker tags, thus preventing the application of lateral forces to the substrates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an arrangement of a position marker apparatus having multiple row groups of overhead optical position marker tags in accordance with the present invention; 
         FIG. 2  is a plan view of a row group of marker tags showing the support cords supporting the row group of marker tags, the ends of the support cords having loops through which cable ties pass to attach the row group to a support structure and to adjust the support cord tension; 
         FIG. 3  is an enlarged plan view of a marker tag; 
         FIG. 4A  is a perspective view showing a first attachment arrangement of a marker tag to two support cords, also illustrating the support cords being fed through holes in a spreader bar; and 
         FIG. 4B  shows a second attachment arrangement of a marker tag to two support cords. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is an improved optical position marker apparatus usable with the system of the related application, PCT/US2005/043755 WO2006/065563, incorporated herein by reference, for the tracking of vehicles and stored goods within a warehouse or factory setting which requires a plurality of individually unique position markers, arranged at predetermined known positional locations. As described in the related application, in a factory or warehouse setting, the object to be located is typically on the floor, and the position markers are placed overhead. The overhead support structure, such as a roof truss support, is sufficiently high above the working area so as not to interfere with operations. 
       FIG. 1  illustrates the apparatus  10  of the present invention in use in the system of the related application. The optical position marker apparatus  10  described herein allows light to pass from overhead light fixtures to the work area, and air to flow freely for heating and ventilation. Suspension of the optical position marker apparatus  10  is provided by mechanical supports, such as overhead beams or trusses of a building structure. The apparatus  10  comprises one or more row groups  20 , illustrated as row groups  20 - 1 ,  20 - 2 ,  20 - 3  in  FIG. 1 . 
     As described in the related application, an image acquisition system is mounted on an object, such as an industrial materials handling vehicle, typically a forklift. The field of view FOV of the image acquisition system is shown in dashed lines as an inverted pyramid. The image acquisition system acquires an image of one or more position markers within view and the image is then decoded by commercially available machine vision equipment to determine the identity of the one or more position markers. The location of a position marker with the acquired image is then used to determine the position and rotational orientation of the object. Each position marker  30  (best seen in  FIG. 3 ) bears a unique barcode symbol  40 B that contains a rotational orientation feature  40 R, thereby identifying both the position coordinate and rotational orientation of the position marker within the predefined coordinate space. The position and rotational orientation in “real” space are them computed from machine vision data using a series of programmed instructions which translate the relative positional and rotational orientation data to coordinates expressed in usable units such as feet or meters relative to a know datum. Results are stored, displayed, and/or transmitted to another system, such as an inventory control system, where data may be used to record object location, to direct vehicle guidance, or for other navigation or control purposes. 
     Although only one position marker must be within view of the object, if more than one position marker is within view (as seen in  FIG. 1 ) the position and rotational orientation of the object may be calculated from each position marker within view to verify and improve the precision of the position and rotational determination. 
     A marker tag  30  is created by affixing a label  40  ( FIG. 3 ) to a substantially stiff substrate  50  to add mechanical strength. A corrugated plastic substrate, such as black polypropylene corrugated sheeting, four millimeters thick, available under the trade name Coroplast, from Coroplast Corporation of Dallas, Tex., is preferred since it provides for easy attachment of the support system. The corrugated plastic substrate is sized larger than the barcode label to provide a dark border  50 D ( FIG. 3 ), termed an “optical quiet zone”, around each barcode symbol  40 B. For clarity of illustration, in  FIG. 3  the substrate  50  is cross hatched, so that the individual elements, i.e. the label  40 , the barcode symbol  40 B and the rotational orientation feature  40 R may best be seen. 
     The dimensions of a marker tag substrate  50  and the barcode symbol  40 B are selected according to the desired field of view for the camera of an image acquisition system and the distance between the camera and the elevation of the position marker apparatus  10 . Barcode symbols ranging from about three centimeters square to about twenty-four centimeters square have been used. Barcode symbols, each containing a unique identification encoded in two-dimensional barcode symbology are printed on label stock. Retro reflective barcode labels are preferred to improve the contrast of the image and thus the signal-to-noise ratio and the quality of the signal output from the machine vision camera. A retro reflective material sold under the trade name Scotchlite™ from 3M Corporation of St. Paul, Minn. is preferred, although other label stocks available from many suppliers, such as Duratran II thermal transfer label stock, Part No. E06175 available from Intermec Corporation of Seattle, Wash., are suitable. Barcode labels may be printed using a common barcode label printer such as Intermec model 3800 thermal transfer barcode label printer or a commercial inkjet printer such as Roland XC-540 model printer. 
     As may be seen in  FIG. 2  the marker tags  30  are attached to a first and second support cords  60 A,  60 B of a supporting cord or cable assembly  60 , known as a row support, in a fixed, spaced-apart positional arrangement to create row groups  20  of marker tags. The support cords  60 A,  60 B of the support cord assembly  60  should have substantially no stretch. A cord material, such as one eighth inch diameter antenna cable, having a diamond braided polyester outer jacket with a Kevlar® core used for radio antenna support, available from Erin Rope Corporation of Blue Island, Ill., has been found suitable. 
     In a preferred embodiment as seen in  FIG. 4A  (also  FIG. 3 ) a corrugated substrate  50  is used and each support cord  60 A,  60 B is threaded through a corresponding corrugation channel  50 C,  50 D adjacent to each lateral edge  50 L 1 ,  50 L 2  of the substrate  50 . The first support cord  60 B is threaded through a corrugation channel  50 C adjacent to the first lateral edge  50 L 1  of the substrate  50 . Two fasteners  62 ,  63  such as a nylon cable ties, also known as a zip ties, available from NELCO Products Incorporated of Pembroke, Mass., are cinched down tightly on the first support cord  60 A, one at each side of the marker tag  30  to hold the marker tag in place on the cord  60 A, thus establishing a fixed, spaced-apart positional arrangement (best seen in  FIG. 2 ) for the marker tags  30  in a row group  20 . The second support cord  60 B is threaded through a corrugation channel  50 D adjacent to the second lateral edge  50 L 2  of the substrate  50  to support the edge in a slidable manner. 
     Alternatively, as shown in  FIG. 4B , the first lateral edge  50 L 1  of each marker tag  30  may be fastened tightly to the first support cord  60 A using two or more fasteners  64 ,  65 , such as nylon cable ties, to establish the fixed, spaced-apart positional arrangement for the marker tags  30  in a row group  20 . The second lateral edge  50 L 2  is attached loosely to the second support cord  60 B with two additional fasteners  66 ,  67 , such as the nylon cable ties shown, to establish support of the marker tag  30  in a slidable manner, so that the marker tag  30  can move freely on the second support cord  60 B. 
     In both embodiments of  FIGS. 4A and 4B , the second support cord  60 B supports the second lateral edge  50 L 2  of each marker tag  30  in a slidable manner. This slidable support of the second lateral edge insures that the marker tags  30  will be aligned along the first lateral edge  50 L 1 , and thus along the row group axis  20 A ( FIGS. 1 and 2 ). This slidable support of the second lateral edge thus prevents any skewing tension on the marker tags  30 . 
     As seen in  FIGS. 2 and 4A  a stiff bar, termed a spreader bar  70 , is used at each end  20 E 1 ,  20 E 2  of each row group  20  of marker tags to maintain the support cords  60 A,  60 B at the proper separation, thus preventing the application of lateral forces to the substrates  50 . The spreader bar  70  can be made of any suitable material such as fiberglass composite, metal or plastic. Holes  70 H 1 ,  70 H 2  formed adjacent to each end of the spreader bar  70  receive the corresponding support cords  60 A,  60 B to establish the support cord spacing. 
     As best seen in  FIGS. 2 and 4A , a loop  60 L is created at each end of each support cord  60 A,  60 B such as by tying a bowline knot, so that the loop will not slip or close up when tension is applied. Tensioning fasteners  72 ,  74 , such as heavy duty cable ties, are then inserted through each loop  60 L at the end of each support cord  60 A,  60 B to serve as tensioning devices. A twenty centimeter (fourteen inch) long heavy duty nylon cable tie available from NELCO Products Incorporated is the preferred tensioning device to attach the support cords to an overhead support structure S. It may be appreciated that by adjusting each fastener  72 ,  74 , the positions of the marker tags  30  of a row group  20  may be precisely established. 
     As may be appreciated from  FIG. 2 , the barcode labels  40 B of all marker tags  30  in a row group  20  are typically oriented in the same direction relative to the row axis  20 A and the substrates  50  are supported with the barcode labels facing downward ( FIG. 1 ) so they may be viewed by an image acquisition system adjacent the floor below. Each row group  20 - 1 ,  20 - 2 ,  20 - 3 , etc. of marker tags is supported in a straight line along the row axis  20 A and the row axes, such as  20 A- 1  and  20 A- 2  ( FIG. 1 ), of adjacent rows  20 - 1 ,  20 - 2  are typically parallel to each other. 
     Those skilled in the art, having benefit of the teachings of the present invention asset forth herein, may effect modifications thereto. Such modifications are to be construed as lying within the contemplation of the present invention, as defined by the appended claims.