Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Automatic route guidance vehicle
Reexamination Certificate
2000-03-31
2002-02-05
Zanelli, Michael J. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Automatic route guidance vehicle
C318S587000
Reexamination Certificate
active
06345217
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to automated-guided vehicle (AGV) systems and, in particular, to navigation and control systems for guiding an automated-guided vehicle along a system guide path. The invention finds application in material handling, such as movement of material within a factory, as well as on-road and off-road vehicles.
Automated-guided vehicles have become extremely effective at movement of materials between processes in a manufacturing plant. Each of a plurality of AGVs automatically carries a load from a pickup point to a discharge point along a system guide path. Navigation of the AGV is typically either by reference to fixed guides, such as guide wires positioned in the floor along the guide path, or by dead-reckoning. Dead-reckoning systems utilize sensors within the AGV in order to monitor the heading, rate-of-change of heading and distance traveled by the AGV along its longitudinal axis which is controlled to coincide with the guide path. The dead-reckoning systems are advantageous because they avoid the enormous expense of placing guide wires in the floor along the entire guide path. Additionally, such dead-reckoning systems are flexible because the guide path layout may be altered by programming changes in the controls rather than requiring tearing and repositioning of the guide wires.
Dead-reckoning systems rely upon an integration of the rate of turn of the vehicle and the distance traveled to maintain position information of the vehicle. Because such measurements tend to drift with time, it is known to supplement the dead-reckoning navigation system with a location verification system, such as markers positioned at known locations along the system guide path. These markers are sensed by a sensor assembly on the AGV as the AGV moves along the guide path in order to verify and compensate, if necessary, the position of the AGV.
One type of marker is a cylindrical magnet positioned in the floor which is sensed by a sensor assembly made up of a series of magnetic sensors, such as Hall-effect sensors, laterally spaced along the bottom of the AGV body. Every time the vehicle passes over a magnet, a location of the body with respect to the magnet is determined from the outputs of the magnetic sensors and used to update the position information of the vehicle. An example of such a system is disclosed in U.S. Pat. No. 4,772,832 to Okazaki et al. Such a system is also utilized in automated-guided vehicle systems marketed by Applicants' assignee, Rapistan Systems of Grand Rapids, Mich. The Rapistan Systems AGV is marketed under various model numbers, such as Model No. DT-100,and is embodied in various forms. These include tuggers and unit load carriers, to name a few. The sensor assembly employed in the Rapistan Systems AGV includes a series of magnetic sensors in the form of Hall-effect sensors spaced approximately 1 inch apart. The position of the vehicle with respect to a magnet is determined by identifying the three sensors having the highest output and interpolating the values of the outputs to identify the maximum magnetic field intensity sensed by the sensor assembly. While the theoretical accuracy of such system is within ¼ inch, there is a tendency for the existing algorithm solution to group, or settle, at the integer distance values, i.e., every inch, which corresponds to the locations of the magnetic sensors. This tendency to settle on the integers reduces the accuracy of the detection of the magnet.
Another difficulty with the known magnet-based position update system is that the magnet is positioned with one of its opposite poles directed upwardly so that the magnetic field sensed by the sensor assembly is always unipolar and of the same polarity. While this makes identification of the maximum magnetic field strength easier, the system is unable to determine the direction that the vehicle body passes over each magnet. Information on the direction that the vehicle passes each magnet could be helpful to the system.
For example, each vehicle must be initialized into the system at an initialization station. An initialization station consists of two spaced apart magnets at a unique distance. The initialization station provides initial position and bearing information to the vehicle dead-reckoning system to allow the vehicle to travel along the guide path. Because it is necessary that each initialization station be uniquely identified to the vehicle, various schemes have been proposed to provide that information to the vehicle. One scheme is to vary the spacing between the magnets at the initialization station. For example, the two magnets may be spaced apart at unique separation distances which, for example, may be between 4 feet and 15 feet. In order to accommodate tolerances, the unique separation distances are provided in steps, such as 6-inch steps, in order to ensure that one initialization station is not mistaken for another. The requirement that the separation of the magnets be uniquely assigned in no finer than 6-inch increments and between fixed limits, such as 4 feet and 15 feet, severely limits the number of initialization stations in the system. In this example, no more than 22 stations are possible. This, in turn, limits the flexibility in constructing very large systems.
Furthermore, with the known initialization station, it is possible to erroneously initialize the vehicle traveling in the wrong direction across the two magnets. This may occur even though an arrow may be inscribed on the floor adjacent the magnets. When a vehicle is initialized in this manner, operation of the vehicle is erroneous.
Proposals have been made for magnetic position update systems that present magnetic fields of opposite polarity, generated by the north and south poles of the magnet at the surface of the pathway, for sensing by the sensor assembly. One such system is disclosed in U.S. Pat. No. 4,908,557 issued to Sudare et al. In Sudare et al., a position of the vehicle with respect to each magnet is determined by repeatedly scanning the magnetic sensors as the vehicle passes over the magnet in order to determine a region pair which includes two regions of equal level of magnetic magnitude. A shortest distance is obtained with respect to the two regions and a center position of the two regions is selected on the basis of the shortest distance. This is supposed to represent a null point between the opposite polarity fields. While the Sudare et al. system provides the ability to determine the direction that the vehicle passes over each magnet, it is not without its difficulties. The Sudare et al. system has precision of measurement on the order of magnitude of center-to-center spacing of the Hall-effect sensors.
One of the difficulties in sensing a position of an AGV body with respect to a magnet assembly producing a bipolar magnetic field is that the maximum strength of the associated polarity of the field does not tend to correspond with any physical place on the magnet, such as the north face or south face. In order to be able to determine a relative position between an AGV body and a magnet assembly, a predetermined position on the magnet assembly must be selected for sensing. Because the maximum field strength for a polarity does not necessarily correspond with a location on the magnet generating the bipolar magnetic field with respect to the surface the vehicle travels, it becomes difficult to locate the magnet assembly in the coordinates of the factory floor. Another difficulty with sensing the bipolar magnetic field is that, unlike a unipolar magnetic field, the axis passing through the north and south poles can become skewed with respect to the guide path unless expensive placement techniques are used. Such skewing could significantly affect the ability to establish a unique position of the magnet assembly with respect to the vehicle body.
SUMMARY OF THE INVENTION
The present invention provides an automated-guided vehicle system in which position updates are obtained from magnets generating bipolar magnetic si
Black Andrew R.
Ko Clyde Miin-Arng
Zeitler David W.
Gibson Eric M
Rapistan Systems Advertising Corp.
Van Dyke Gardner, Linn & Burkhart, LLP
Zanelli Michael J.
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