Location system adapted for use in multipath environments

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TECHNICAL FIELD OF THE INVENTION

The invention relates generally to position location systems, and more particularly relates to a radiolocation system adapted for use in an environment subject to multipath effects. In even greater particularity, the invention relates to such a system that accomplishes position location using either (a) time-of-arrival differentiation for radiolocation transmissions received at multiple receivers (high resolution), or (b) area-detection using receivers that receive radiolocation transmissions from assigned areas (low resolution).

RELATED PATENT

This patent incorporates by reference the subject matter of U.S. Pat. No. 4,864,588, titled Remote Control System, Components and Methods, which is assigned to Hillier Technologies Limited Partnership.

BACKGROUND OF THE INVENTION

Position or object location systems are finding increasing application in manufacturing and materials handling environments. For example, such systems have utility for factory automation, including such applications as tool automation, process control, robotics, autonomous guided vehicles, computer-integrated-manufacturing (CIM), and just-in-time (JIT) inventory control.

One approach to position location systems uses transmitters, or tags, attached to objects to be tracked, and an array of receivers for receiving tag transmissions throughout a tracking area. Tag transmissions can be effected by radio, ultrasonic or optical communications, using various techniques for identifying object movement or location in the near range of a receiver.

Radio communication provides a high degree of accuracy and performance superior to ultrasonics and optics in terms of: (a) range per watt of power; and (b) penetrability through opaque structures. However, a problem with radio communications in the typical business environment--which includes walls, silvered windows and other fixed structures--is that, for the frequencies of interest (i.e., above 100 MHz), random reflections introduce multipath distortions in tag transmissions arriving at a given receiver. Moreover, in such an environment, the unpredictable attenuation of transmissions passing through walls and other structures makes signal strength only marginally useful for communicating distance/location information.

Accordingly, a need exists for a position location system capable of being used to locate objects in an environment subject to the effects of multipath reflections.

SUMMARY OF THE INVENTION

The invention is a location system adapted for use in environments subject to multipath effects, implementing object location by (a) time-of-arrival differentiation using tag transmissions received at multiple receivers (high resolution embodiment), or (b) area-detection using receivers that receive tag transmissions from an assigned area (low resolution embodiment). In either the high resolution or low resolution embodiment, a radiolocation system can be implemented with spread spectrum communications for unlicensed operations.

In one aspect of the invention, the location system includes, for each object to be located within a tracking area, a TAG transmitter that transmits, at selected intervals, TAG transmissions including at least a unique TAG ID. An array of receivers is distributed within the tracking area.

For a high resolution embodiment, the array of receivers is distributed such that TAG transmissions from a given TAG transmitter located anywhere in the tracking area are received by at least three receivers (for two dimensional tracking).

Each receiver includes a time-of-arrival circuit and a data communications controller. The time-of-arrival circuit triggers in response to the arrival of a direct-path TAG transmission, providing a time-of-arrival TOA-COUNT synchronized to a system synchronization clock available at each receiver. The data communications controller is responsive to the triggering of the time-of-arrival circuit for providing a corresponding TOA-DETECTION packet including at least the TAG ID from the TAG transmission and the TOA COUNT.

A location processor receives TOA-DETECTION packets communicated from each receiver, and determines the location of a TAG (and its associated object) from at least three TOA-DETECTION packets corresponding to the TAG transmissions for that TAG received by different receivers.

For a low resolution embodiment, each receiver of the array is assigned a specific location-area, such that it receives TAG transmissions almost exclusively from TAGs located in that area. Implementing a radiolocation system based on receiver-assigned areas can be accomplished in various ways, such as by using directional antennas at the receivers, or by cooperatively selecting receiver spacing and TAG transmitter power so that TAG transmissions are received by the most proximate receiver.

Each receiver includes a data communications controller. The data communications controller in each receiver is responsive to the receipt of a TAG transmission for providing a corresponding AREA-DETECTION packet including at least the TAG ID from the TAG transmission.

A location processor receives AREA-DETECTION packets from each receiver, and determines the location of each object based on the respective receiver that received the TAG transmissions to which it is most proximate.

The location system can be implemented using spread spectrum radio communications, which allows unlicensed operations. In this aspect of the invention, each TAG transmitter includes a spread spectrum transmitter that outputs TX-packets, including at least the TAG ID, according to a spread spectrum data communications protocol. The TAG transmitters operate at a predetermined power level. Each radiolocation receiver includes a spread spectrum receiver that receives the spread spectrum TAG transmission, recovers the TAG ID, and outputs an RX-packet that includes the TAG ID.

A data communications controller at each receiver is responsive to the RX-packet to provide a DETECTION-packet, including at least the TAG ID, for communication to a location processor. The location processor receives DATA-packets from each receiver, and determines object location.

In more specific aspects of the invention, the exemplary high-resolution embodiment of a radiolocation system is used to locate objects such as wafer boxes in a semiconductor manufacturing facility. An array of radiolocation receivers is coupled to a radiolocation system processor over a LAN (local area network).

Each TAG transmitter includes, in addition to a spread spectrum transmitter, a motion detect circuit and a periodicity control circuit. The TAG transmitter is enabled for transmission only while object motion is detected by the motion detector. While the object is in motion, the TAG transmitter transmits at regular intervals determined by the periodicity control. Each TAG transmission includes a motion status (Initiated, Continuing, Stopped) in addition to TAG ID. In addition, the TAG can include means for entering other information (by an operator or otherwise) for communication to the system processor.

Each radiolocation receiver includes, in addition to a spread spectrum receiver, a TOA trigger circuit, a time base latching circuit and a programmable controller. The TOA trigger circuit triggers within the early cycles of the arrival of a TAG transmission, providing a TOA DETECT trigger. The time base latching circuit is responsive to the TOA DETECT trigger to latch the time base TOA COUNT from an 800 MHz time base counter, which is synchronized to a 200 MHz system synchronization clock provided by the system processor over the LAN. The programmable controller receives the TAG ID and motion status recovered by the spread spectrum receiver and the TOA COUNT from the time base latching circuit, and provides a TOA-DETECTION packet communicated over the LAN to the system processor.

The time-of-arrival detection circuitry in the receiver provides adjustable noise sensitivity for differentiating between TAG transmissions and random pulsed noise. The TOA trigger circuit provides the TOA-DETECT trigger when the input signal level exceeds an adjustable signal-level threshold, while the time base latching circuit signals that a valid TAG transmission has been received when the duration of the TOA-DETECT trigger exceeds a programmable signal duration threshold.

The technical advantages of the invention include the following. The location system is adaptable to use in a multipath environment, such as found in manufacturing and other business facilities, where the reception of direct-path transmissions is affected by the presence of multipath noise. Object location can be accomplished by either a high resolution approach using time-of-arrival differentiation, or a low resolution (low cost) approach using area detection by receivers configured to detect TAG transmissions from an assigned area. Using unlicensed, commercially available spread spectrum equipment facilitates discrimination between the direct-path transmissions of interest and multipath noise.

To conserve power, each TAG transmitter can include a motion detector, with TAG transmission being limited to (or concentrated in) intervals when an object is being moved. TAG transmissions can include, in addition to TAG ID, motion status and other information input by an operator or otherwise.

For the high resolution embodiment, TOA-DETECTION triggering and time base TOA COUNT latching can be separated from the spread spectrum communications function to permit the use of commercially available spread spectrum equipment. A high-speed TOA triggering circuit provides TOA-DETECTION triggering within the early cycles of the arrival of a TAG transmission. A time base latching circuit using a synchronized time base counter operating in the range of 800 MHz provides resolution on the order of 10 feet. Noise filtering optimizes time-of-arrival detection for TAG transmissions, providing adjustable signal-level and signal-duration thresholds to minimize the effects of random pulsed noise.

For a more complete understanding of the invention, and for further features and advantages, reference is now made to the following Detailed Description of an exemplary embodiment of the invention, taken in conjunction with the accompanying Drawings. Although the Detailed Description, and the Drawings, are with respect to a specific, exemplary embodiment of the invention, various changes and modifications may be suggested to one skilled in the art, and it is intended that the invention encompass such changes and modifications as fall within the scope of the appended Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a illustrates a semiconductor fabrication facility as an environment containing walls and other fixed structures that cause multipath reflections.

FIG. 1b illustrates an exemplary array of receivers for the radiolocation system of the invention, together with a fixed array of calibration transmitters.

FIGS. 2a-2c are functional block diagrams of a TAG transmitter, a radiolocation receiver (with time-of-arrival detection) and the LAN interface for the radiolocation system;

FIG. 3 diagrammatically illustrates the position location processing operation for the high resolution embodiment of the radiolocation system using time-of-arrival differentiation for object location;

FIGS. 4a-4c functionally illustrates a spread spectrum communication system (transmitter and receiver) for use in the radiolocation system; and

FIGS. 5a-5b schematically illustrate the time-of-arrival circuitry for the high resolution embodiment, with FIG. 5a illustrating the receiver front and the TOA detect circuit, and FIG. 5b illustrating the time base latching circuit.

DETAILED DESCRIPTION OF THE INVENTION

The Detailed Description of exemplary embodiments of the radiolocation system of the invention, adapted for use in multipath environments, is organized as follows:

1. Radiolocation System--TOA Detection

1.1. TAG Transmission

1.2. Reception and TOA Detection

1.3. LAN Communications

1.4. Position Location Processing

1.5. Calibration

2. Radiolocation System--Area Detection

3. Spread Spectrum Communication

4. TAG Transmitter

5. Radiolocation Receiver

5.1. TOA Trigger Circuit

5.2. Time-Base Latch Circuit

5.2. Programmable Controller

APPENDIX A

APPENDIX B

This Detailed Description incorporates by reference the subject matter of U.S. Pat. No. 4,864,588, titled Remote Control System, Components and Methods, and assigned to Hillier Technologies Limited Partnership, together with any divisionals of that patent.

1. Radiolocation System--TOA Detection. In the exemplary embodiment, the radiolocation system is used to track and locate objects (such as wafer boxes) in an automated semiconductor fabrication facility. The radiolocation system is configured for high-resolution object location (on the order of 10 feet or less) using time-of-arrival differentiation and an 800 MHz synchronized time base clock.

FIG. 1a illustrates a semiconductor fabrication facility 10 with numerous segregated areas, such as GaAs Lithography Photolithography 12, GaAs Etch 13, Metalization 14 and Ion Implant 15, each enclosed by partitions or walls, including walls 16, 17 and 18.

FIG. 1b illustrates the fabrication facility 10 showing only the walls 16, 17 and 18. Located within the facility (in or adjacent to the ceiling) is an array of radiolocation receivers 20, including individual receivers 22, 24 and 26.

Numerous objects move within the facility, such as wafer boxes transported on conveyor systems. These objects must be tracked, and their location identified, to implement efficient automated fabrication operations.

Attached to each object to be tracked is a TAG transmitter. Each TAG transmitter associated with an object transmits TAG transmissions that are received by the receiver array. For example, the TAG transmissions from a TAG transmitter located at 30 are received by, at least, radiolocation receivers 22, 24 and 26. Each TAG transmission is a TX-packet that includes a TAG ID uniquely identifying each TAG (i.e., each object).

In addition to TAG transmitters on each object, a number of fixed-position TAG transmitters 35 are located around the facility. These TAG transmitters, which have a known position with respect to each receiver, are used for system calibration.

Each radiolocation receiver in the receiver array 20 receives TAG transmissions, and accurately detects time-of-arrival using an 800 MHz time base counter. For each TX-packet in a TAG transmission, the receiver generates corresponding TOA-DETECTION packets, which are communicated over a LAN (local area network) to a radiolocation system processor 40.

System processor 40 performs all object-location computations. In addition, the system processor 40 generates a 200 MHz system synchronization clock 42 from which the 800 MHz time base count in each receiver is derived. System processor 40 is coupled through a LAN interface 44 to the network, which is used for data communications between the system processor and the receiver array, and for providing the 200 MHz system synchronization clock.

The system processor includes object-tracking database storage, with user access to the object location information being provided by a graphics workstation through a graphical user interface.

1.1. TAG Transmission. TAG transmissions between TAGs (objects) and the receiver array are implemented using spread spectrum communications in the 902-928 MHz band. In the environment illustrated in FIG. 1, radio transmissions in that frequency band are subject to multipath reflections. Using spread spectrum communications for the TAG transmissions is advantageous in separating direct-path transmissions from multipath reflections (see, Section 2).

FIG. 2a is a functional block diagram of a TAG transmitter 50, which includes:

(a) a spread spectrum transmitter 52 for transmitting spread spectrum TAG transmissions (TX-packets);

(b) a battery saving circuit 54 for enabling the spread spectrum transmitter when the TAG (object) is being moved; and

(c) a motion detection circuit 56 for detecting TAG (object) motion; and

(d) a periodicity control circuit 58 for controlling the re-transmission interval of the spread spectrum transmitter.

For the exemplary embodiment, the TX-packet in each periodic TAG transmission includes not only the appropriate TAG ID, but also one of three motion status indications: Motion Initiated, Motion Continuing and Motion Stopped.

To conserve power and to increase the available population of TAG transmitters, each spread spectrum transmitter 52 is normally in a power-saver mode, being enabled for transmission by battery saving circuit 54 only while its associated object is being moved to a new location. Object motion is detected by motion detector 56, which provides an appropriate indication to the battery saving circuit.

In response to a motion indication, battery saving circuit 54 initiates a transmit mode by enabling spread spectrum transmitter 52 for an initial TAG transmission. The TX-packet in this initial TAG transmission includes, in addition to the TAG ID, a Motion Initiated status.

While the object remains in motion (as detected by motion detector 56), periodicity control 58 causes spread spectrum transmitter 52 to re-transmit TAG transmissions at selected intervals (such as every 15 seconds). The TX-packets in these periodic TAG re-transmissions include, in addition to the TAG ID, a Motion Continuing status.

When the object arrives at its new location and becomes stationary, motion detector 56 stops providing an object motion indication to battery saving circuit 54. After a predetermined period in which the object is stationary (such as 30 seconds), the battery saving circuit disables periodicity control 58, and causes the spread spectrum transmitter to transmit a final TAG transmission with a TX-packet including a Motion Stopped status.

The TAG transmitter remains in the non-transmitting power-saver mode until the next movement of the object. As an alternative to completely disabling TAG transmissions while an object is stationary, during such stationary times, the TAG transmitters could be programmed to transmit a low duty cycle TAG transmission that provides a No Motion status indication.

The TAG transmissions propagate through the facility, and are received by the receiver array. Because these transmissions must propagate through partitions, walls and other obstructions that introduce unpredictable levels of attenuation, signal strength at the receivers does not provide any useful information from which object location can be deduced. Moreover, these obstructions introduce multipath reflections that are also received by the receivers, albeit after the arrival of the direct-path transmission.

1.2. Reception and TOA Detection. TAG transmissions arrive at the various receivers with a time-of-arrival differential that depends upon the corresponding time-of-arrival (or path-length) differential between multiple receivers and the TAG transmitter (object), and is substantially unaffected by signal-attenuating obstructions in the path of the TAG transmission.

To implement a high resolution embodiment of the radiolocation system, this time-of-arrival differential can be used to determine object location with a high level of resolution if each receiver provides reliable and accurate time-of-arrival detection for a received TAG transmission. TOA detection requires: (a) reliable triggering on the time-of-arrival for the direct-path TAG transmission; and (b) a stable synchronized time base.

Failure to consistently and accurately trigger on arrival of the early cycles of the direct-path TAG transmission (which will arrive before any associated multipath reflections)--despite random changes in temperature, humidity and/or circuit performance--causes reliability problems that translate into errors in TOA detection, and therefore, location computation. However, even if TOA triggering is accurate, failure to achieve a stable synchronized time base (or knowledge of relative time differences) reduces the accuracy of time-of-arrival detection based on TOA triggering.

In addition, TOA triggering must be independent of the strength of the TAG transmission signal (which is subject to attenuation in the path between the object and a given receiver). Failure to trigger independent of signal strength, also known as dispersion delay, introduces time-of-arrival triggering disparities depending upon direct-path attenuation.

For the exemplary embodiment, the radiolocation system processor 40 provides a 200 MHz system synchronization clock over the LAN to each of the receivers in the array 20. At each receiver, the 200 MHz system clock is converted by conventional phase coherent frequency multiplication to an 800 MHz TOA time base clock that is synchronized with all other receivers. This approach to providing a time base for time-of-arrival detection enables the receivers to be synchronized to within about 1.25 nanoseconds, thereby allowing location resolution through TOA differentiation to within about two feet.

The selection of a 200 MHz system synchronization clock with up-conversion at each receiver to the desired 800 MHz time base clock is a design choice resulting from the selection of a specific LAN data communication system for providing the system synchronization clock (see, Section 1.3). The radiolocation system of the invention is readily adaptable to other schemes for providing a system synchronization clock for deriving an appropriate receiver time base for the desired location resolution.

FIG. 2b is a functional block diagram of a radiolocation receiver 60, which includes:

(a) a receiver front end 62 for amplifying and conditioning the received TAG transmission (TX-packet);

(b) a TOA detect trigger 64 for detecting the arrival of the direct path TAG transmission and providing a TOA DETECT indication;

(c) a time base latching circuit 65 for latching, in response to a TOA DETECT indication, the associated time base TOA COUNT of the synchronized 800 MHz time base counter;

(d) a spread spectrum receiver 66 for receiving the TX-packet from each TAG transmission, and generating an RX-packet including the TAG ID and the motion status;

(e) a programmable controller 68 for assembling the latched TOA COUNT from the time base latching circuit, along with the recovered TAG ID and motion status, into a TOA-DETECTION packet; and

(f) a network interface 69 for interfacing the communication of TOA-DETECTION packets over the LAN.

In addition, a power supply provides both TTL, ECL and radio circuitry power.

Receiver front end 62 receives each TAG transmission, and performs conventional amplification and filtering.

A received TAG transmission is applied to TOA trigger 64 for time-of-arrival triggering--the TOA trigger provides a TOA DETECT indication within the early cycles of the TAG transmission. The rapid detection of a triggering event is achievable with a high speed comparator using conventional peak energy detection in the TOA trigger.

TOA DETECT is provided to time basellatching circuit 65 as an indication of the arrival of a TAG transmission wavefront. The time base latching circuit latches the associated time base count of the 800 MHz time base clock (up-converted from the 200 MHz system synchronization clock). In addition, the time base latching circuit performs digital noise filtering to attempt to ensure that a TOA DETECT indication from TOA trigger 64 is associated with a spread spectrum TAG transmission rather than random pulsed noise.

When time base latching circuit 65 indicates the arrival of a TAG transmission, the associated TX-packet is applied to spread spectrum receiver 66. The spread spectrum receiver extracts the TAG ID and motion status from the TX-packet, and outputs an RX-packet that includes TAG ID and motion status.

For each TAG transmission, programmed controller 68 retrieves the latched time base count from time base latching circuit 65, along with the RX-packet from spread spectrum receiver 66. The programmed controller assembles this time-of-arrival information (TAG ID, motion status and time base TOA COUNT) into a TOA-DETECTION packet for communication over the LAN to the radiolocation system processor.

As an object moves from one location to another, farther from some receivers, closer to others, each radiolocation receiver detects changing time-of-arrival measurements for the associated TAG transmissions. For a given TAG transmission, the time-of-arrival detection operation at each receiver differentiates between the arrival of the direct-path TAG transmission and the subsequently-arriving multipath reflection signals, triggering on the arrival of the early cycles of the direct-path TAG transmission prior to the confluence of the multipath components.

The ability to receive a valid TAG ID despite multipath effects is enhanced by the space diversity inherent in spread spectrum communications (see, Section 2). Effectively, each receiver can be considered an element of a space diverse antenna, facilitating the rejection of multipath noise.

After TOA detection of a received direct-path TAG transmission, the TOA-DETECTION packet assembled by the programmed controller is communicated to the radiolocation system processor over the LAN.

1.3. LAN Communications. Referring to FIG. 1b, each receiver in the radiolocation receiver array is coupled over a LAN to radiolocation system processor 40 (the LAN cabling is not shown). System processor 40 continuously receives TOA-DETECTION packets (TAG ID, motion status and time base TOA COUNT) from each of the receivers as they detect TAG transmissions.

The receivers are coupled to the system processor for two independent communications operations: (a) data communication; and (b) receiver time base synchronization (providing repeatability to within a few hundreds of picoseconds). The exemplary embodiment implements these two communications operations using a single coaxial-cable based ARCNET local area network.

The ARCNET LAN uses a token passing protocol and a data transmission rate of 2.5 Mbits per second. Communication is over standard RG62 coaxial cable that will accommodate signal frequencies of up to 200 MHz without significant attenuation problems. Thus, the 200 MHz system synchronization clock can be multiplexed onto the normal ARCNET data communications traffic without any significant degradation.

FIG. 2c is a functional block diagram illustrating the LAN interface at the radiolocation system processor and the receivers. At the system processor, a LAN interface 81 includes an ARCNET interface (RIM) card 82 and a 200 MHz clock interface. A diplex filter 84 multiplexes the 200 MHz system synchronization clock 83 onto the 2.5 MHz ARCNET signal, and outputs the resulting LAN signal onto the network as normal ARCNET packet traffic.

The LAN communications from the system processor are received by the receivers of the radiolocation array. At each receiver, a LAN interface 86 includes a diplex filter 87 that demultiplexes the LAN signal to recover the 200 MHz system synchronization clock. The ARCNET packet is provided to an ARCNET interface (RIM) card 88, while the 200 MHz clock is provided through a clock interface 89 to the time base latching circuit (not shown).

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