Position-tracking system

[0001] The present application is being contemporaneously filed with a commonly-owned application Ser. No. 11/054,905 by the same inventors entitled “Position-Tracking Device for Position-Tracking System.”

[0002] The present invention is generally directed to systems and methods for tracking the position of objects.

[0003] Position-tracking systems seek to identify the location of mobile objects in real-time and are used in a wide variety of applications, including transportation, logistics management, healthcare, security, etc. Position-tracking systems that can provide continuous location information are desirable for applications that require non-interrupted visibility of the mobile object through a journey. For objects that are moved with a vehicle (such as inter-modal shipping containers or trailers), being able to locate the position of the objects in real-time can help improve the efficiency of supply-chain and inventory management while reducing the loss in productivity of assets. As businesses increasingly depend on a global supply chain and distribution networks, more goods and assets are being moved around for longer distances and with more parties involved in the process. In order to facilitate an automated, error-free data exchange for logistics management, the availability of continuous information about these assets in real-time is more valuable than “discrete” information that is only available for certain regions, albeit with perhaps higher position resolution.

[0004] Existing position-tracking systems typically employ satellite, radio wave or other wireless technologies. However, such systems typically suffer from limitations caused by line-of-sight requirements for location signals and/or interference caused by the environment in which they operate (e.g., multi-path fading or radio waves in an indoor setting). In addition, although much effort has gone into extending the operating range for Global Positioning System (GPS) and GPS-like technologies with more and more sophisticated software implementations, GPS systems are still generally ineffective and unreliable for location tracking indoors or in other environments where obstructions to the satellite signals present challenges. Further, other non-GPS based position technologies that have been proposed to date are largely based on a “local GPS” system using an assembly of locally deployed wireless antennas to mimic the function of the GPS satellites. Due to the high cost of designing and implementing such systems, however, they remain a costly proposition.

[0005] Other object-tracking technologies, such as RF ID tags or bar codes, may be acceptable for tracking objects where the objects generally travel only within a small, controlled environment, such as a warehouse, where their travel paths are static and well-defined. These technologies, however, cannot suitably track the position of the object once it leaves the controlled environment.

[0006] In one general aspect, the present invention is directed to a position-tracking system for tracking the position of an object. According to various embodiments, the tracking system includes a tracking device that is connected to or otherwise affixed to the object to be tracked. In other embodiments, the tracking device may be connected to equipment that is used to move the objects to be tracked. The tracking device may include, among other things, an inertial sensor assembly, radio transceivers and a processor. The position tracking system may also include a host processing system that is in communication with the tracking device.

[0007] The position tracking system may provide variable-resolution position information based on the environment in which the object is moving. For example, while in a “wide resolution” area where very high position resolution is not required, the system may compute a general position for the object based on a wireless telephone network Cell-ID/map correlation architecture. That is, the system may utilize Cell-ID information obtained from a wireless telephone network in conjunction with a database of area maps to determine a general position of the object (e.g., within the geographic area covered by a certain Cell-ID). In this case, the position tracking system is said to operate in the “wide area” mode.

[0008] When the object enters an area (such as a facility, warehouse, campus, etc.) where greater position resolution is required (a so-called “high resolution area”), the position tracking system may transition to a corresponding high-resolution mode. The mode transition may be activated, for example, by a wireless activation signal provided at the ingress of the high-resolution area. The greater position resolution may be realized using other wireless signals, from a wireless aiding system within the high-resolution environment, combined with the inputs from the inertial sensors. The tracking system (e.g., at either the tracking device, the host processing system, or distributed between these components) may compute the amount of displacement in the longitudinal, latitudinal and altitudinal directions based on the measurements from the inertial sensors. When aided by the wireless reference position signals from the wireless aiding system, the combined implementation can provide enhanced accuracy (e.g., on the order of meters) without ever losing coverage of the object, regardless of the layout of the high-resolution environment. When the tracked object exits the high-resolution area, the wireless activation signal at the point of egress may deactivate the high-resolution mode and the system may automatically revert back to the wide area mode.

[0009] For object tracking in the high-resolution mode, the system may exploit distinct patterns of motion that can be identified as motion “signatures” that are characteristic of certain types of motion. The position tracking system may estimate the state of the object (e.g., the heading, position, velocity of the object) based on kinematic models for the current mode of the object. Adaptive and cascaded Kalman filtering or similar techniques, for example, may be employed in the analysis to more accurately estimate the position and velocity of the object based on the motion pattern identified.

[0010] The state history of the object may be stored and used in the state estimate computation to make continuous adjustments to the state estimates in the past. For instance, the inertial sensor measurements and a priori estimations for a configurable time period at a configurable frequency may be stored in a data storage device. As new information becomes available, the a priori estimations may be adjusted to provide more accurate final estimations (with a time delay). For example, when the tracking system determines that the object has become “stationary” based on an identified match to a motion signature, any residual estimated velocity may be corrected (or, smoothed) backwards in time to provide an improved velocity and, hence, position estimations from the original calculation.

[0011] The host computer system may maintain a centralized, real-time record of the locations of the tracked objects. The record can be stored in a database and/or it may be a direct input to a logistic or other IT management system so that the location information can be further processed or utilized by other applications

[0012] In another general aspect, the present invention is directed to an electronic device comprising an inertial sensor assembly and a processor in communication with the inertial sensor assembly. The processor is programmed to determine an appropriate power level for the processor based on input from then inertial sensor assembly. For example, the processor may be programmed to enter a sleep mode when the processor determines from the input of the inertial sensor assembly that the electronic device had limited movement for a certain period of time. The electronic device may be a position tracking device for use in a position tracking system, or any other electronic device including an on-board processor.

[0013] These and other aspects of the present invention will be apparent from the description to follow.

[0020] FIG. 1 is a diagram of a position-tracking system 10 according to various embodiments of the present invention for tracking the position of one or more mobile objects in real-time (i.e., within a small response time) as the objects travel about. The tracked objects may be any tangible object that is capable of moving, whether by its own mobility system or because it is capable of being transported by other means, such as conveyor belts, vehicles, lifts, persons, etc. Accordingly, the tracked objects may be goods, containers or supports for goods, vehicles or equipment for moving goods, etc. Also, the tracked objects or goods may or may not have RFID tags. That is, the position tracking system described herein may be used in conjunction with RFID technology or in lieu of it.

[0021] The tracking system 10 tracks the position of the objects as they travel through areas, such as area 12, where very high position location resolution is not needed, and areas, such as area 14, where high position resolution is needed. “Wide resolution” areas may include areas where the objects are in transit between locations such as, for example, intermodal transportation hubs, shipyards, rail yard, etc. In such areas, position resolution on the order of hundreds or thousands of meters may be acceptable. On the other hand, “high resolution” areas, whether indoor or outdoor environments, are areas that require more precise location information for logistical purposes, such as on the order of a few meters or less. Examples of high-resolution areas include manufacturing facilities, campuses, warehousing facilities, etc.

[0022] The position-tracking system 10 may include, according to various embodiments, one or more tracking devices 16 and a host computer system 18 that communicates with the tracking devices 16. The tracking devices 16 may be attached or otherwise connected to the objects to be tracked, for example. In FIG. 1, three tracking devices 16_1-3 are shown, one attached to a truck 22 traveling in the wide-resolution area 12, a second connected to goods on a first forklift 23₁ in the high-resolution area 14, and a third connected to a second forklift itself 23₂. It should be recognized that the tracking devices 16 could be attached or otherwise connected to other types of vehicles, goods, containers for the goods, equipment, etc. For example, tracking devices could be connected to mobile assets (e.g., goods), such as the tracking device 16₂ in FIG. 1, or to equipment (e.g., forklifts) that are used to move mobile assets around a facility, such as the tracking device 16₃ in FIG. 1. By tracking the location of equipment, for example, the location of object/goods that are not capable of moving by themselves can be effectively tracked in certain applications, as long as information on which object(s) is associated with the tracked equipment at any given time is available. Also, the tracking system 10 may track a greater number of objects than the two shown in FIG. 1.

[0023] The tracking devices 16 preferably include processing capabilities that allow them to estimate their real-time position based on, for example, inertial sensor inputs and wireless signals. The tracking devices 16 may operate in one of two modes—a wide area mode and a high-resolution mode—and compute their estimated position differently depending on its mode of operation. For example, when a tracking device 16 is in the high-resolution area 14 (such as connected to goods on the forklift 23₁ or connected to the forklift 23₂ itself), the tracking device 16 may correspondingly be in its high-resolution mode and compute its estimated position with an accuracy on the order of a few meters or less based on the input from inertial sensors and wireless signals from a wireless aiding system. The wireless aiding system may include, as shown in FIG. 1, one or more reference point transmitters 28 for providing a reference location to the tracking device 16. When the tracking device 16 is in the wide resolution area 12 (such as connected to goods or containers on the truck 22), it may correspondingly be in the wide area mode and its estimated position in this mode may have less resolution, such as on the order of hundreds or thousands of meters. Such a dual resolution mode approach may be satisfactory in position tracking applications where highly accurate position information is not needed in the wide resolution areas but is needed in other environments. Indeed, the dual mode approach may be preferable because it does not require the wireless location aiding system for the high-resolution area to be built out in the wide resolution areas.

[0024] The tracking device 16 may transmit its estimated position to the host computer system 18. This may be done continuously, periodically, randomly, pseudo-randomly, and/or upon a request from the host computer system 18. The host computer system 18 may maintain a centralized, real-time record of the locations of the tracked objects. The record can be stored in a database 24 and/or it may be a direct input to a logistic or other IT management system so that the location information can be further processed or utilized by other applications.

[0025] The host computer 18 may be implemented as one or a number of networked computers, such as servers, PCs, workstations, etc. In various embodiments, as described above, the tracking device 16 may compute its estimated position and transmit the position to the host 18, although according to other embodiments, the position determination processing may be distributed between the processing capabilities of the tracking device 16 and the host 18. Also, although only three tracking devices 16 is shown in FIG. 1, it should be recognized that at any one time the host computer system 18 may be in communication with a fewer or greater number of tracking devices. Further, the host computer system 18 may control and monitor the tracking devices 16 via control and monitoring signals sent to the tracking devices 16.

[0026] The wireless aiding system used in the high-resolution area 14 may include a number of reference point transmitters 28 positioned throughout the high-resolution area 14. Each reference point transmitter 28 may wirelessly transmit a high accurately reference location position to the tracking devices 16, such as with RF, acoustic, optical, IR or other suitable signals, such that the tracking devices 16 can compute their position based on the reference as well as with input from inertial sensors, as described in more detail below. The high-resolution areas 14 may also include a number of radio access points 30. The radio access points 30 may provide a wireless gateway (e.g., via the IEEE 802.11 or IEEE 802.15.4 protocols) for communications between the position tracking devices 16 and the host computer system 18. The radio access points 30 may be in communication with the host 18 via a data transfer network 20 such as, for example, a LAN, a corporate intranet, a WAN, a MAN, a TCP/IP network, a broadband computer network, a wireless communication network, or a combination of one or more of these data transfer networks.

[0027] The reference point transmitters 28 and the radio access points 30 may be strategically placed throughout the high-resolution area 14 so as to avoid interference caused by obstructions in the environment and/or co-channel interference, yet reduce the number of each that is needed to provide adequate position resolution and communications with the host 18. For example, in various applications it may be advantageous to position the reference point transmitters 28 points along or near common travel paths for the objects in the environment.

[0028] FIG. 2 is a block diagram of a tracking device 16 according to various embodiments of the present invention. In the high-resolution mode of operation, the tracking device 16 may compute its estimated position based on (1) inputs from a number of inertial sensors in an inertial sensor assembly 40 and (2) received signals from the reference position transmitters 28. The inertial sensor assembly 40 may detect movement by the tracking device 16, such as lateral, vertical or rotational movement, and provide corresponding data signal inputs to a processor 42. The inertial sensor assembly 40 may include a number of accelerometers (not shown) and/or gyroscopes (not shown) for detecting motion by the tracking device 16. For example, according to various embodiments, three accelerometers and three gyroscopes may be used, one for each or multiple degree(s) of freedom (x, y, z, roll, pitch, yaw) for the tracking device 16. According to other embodiments, a lesser or greater number of accelerometers and/or gyroscopes may be used. The accelerometers/gyroscopes may be, for example, micro-devices, such as MEMS-based devices. According to other embodiments, different types of inertial sensors may be used, such as optical-based inertial sensors. The processor 42 may include one or more microprocessors. For an embodiment where multiple processors are used, the tracking device 16 may employ parallel processing.

[0029] In addition to the processor 42 and the inertial sensor assembly 40, the tracking device 16 may include a reference position receiver 44, a number of RF transceivers 46A-C, and a wireless telephone network transceiver 48. The reference position receiver 44 may receive signals from the reference point transmitters 28 and forward the received reference position information to the processor 42 to aid in the position determination process. The RF transceiver 46A may communicate with the radio access points 30 and/or with other tracking devices 16. As such, the RF transceiver 46A may report object location information (as computed by the processor 42) back to the host 18 via the radio access points 30, as well as receive control and monitoring signals from the host 18 and send responses thereto.

[0030] The wireless telephone network transceiver 48 may communicate with a wireless telephone network that includes base stations 50 (see FIG. 1). Information from such communications may be used by the processor 42, as described below, in calculating the location information of the object in the wide area mode of operation. While in the wide resolution area 12, the tracking device 16 may communicate with the host 18 via, for example, satellite radio signals or other long-range radio communication signals sent from the RF transceiver 46B and received by a transceiver 21 associated with the host 18. According to other embodiments, the tracking device 16 may communicate with the host via a telephone network (not shown), such as the wireless telephone network associated with the base stations 50.

[0031] The tracking device 16 may also include a memory device 52 and a power source, such as battery 54. The memory device 52 may be in communication with the processor 42, and store instructions and data for the processor 42. The memory device 52 may be implemented, for example, as one or more RAM units or flash memory devices, or any other suitable memory device(s). The battery 54 supplies power to the various power-consuming components of the tracking device 16.

[0032] As shown in FIG. 2, the tracking device 16 may include a high-resolution module 60 and a wide area resolution module 62 for computing the position of the tracking device in the corresponding modes of operation. The modules 60, 62 may be implemented as software code to be executed by the processor 42 using any suitable computer instruction type such as, for example, Java, C, C++, Visual Basic, etc., using, for example, conventional or object-oriented techniques. The software code may be stored as a series of instructions or commands on a computer readable medium, such as a memory device 52.

[0033] A wireless signal transmitted from points of egress/ingress between the wide resolution areas 12 and the high-resolution areas 14, such as from a transmitter 66 at gate 68 in FIG. 1, may be used by the tracking device 16 to switch between the wide area mode and the high-resolution mode. That is, when the tracking device 16 receives the egress/ingress activation signal (via, e.g., the RF transceiver 46C), the tracking device 16 may switch its mode of operation, for example, from the high resolution mode to the wide area mode, or vice versa, depending on whether the tracking device is leaving or entering the high resolution area 14. A number of such transmitters 66, such as one for each point of egress/ingress, may be used. Also, according to other embodiments, the modes of the tracking device 16 may be manually toggled, such as with a bistable switch at the point of egress/ingress.

[0034] In the wide area mode, the tracking device 16 may determine its general position, according to various embodiments, based on cell-ID information received from the wireless telephone network base stations 50. That is, the tracking device 16 may be in communication with one of the base stations 50 of the wireless telephone network according to conventional call hand-off technology employed by the wireless telephone network and obtain the Cell-ID from the base station 50 in the hand-shake/registration communications with the base station 50. Each base station 50 may serve a certain geographic area (a “cell site”). The wide area resolution module 62 of the tracking device 16 may correlate the received Cell-ID information to map information stored in the memory device 52 to determine the general location of the tracking device 16 (e.g., within the geographic area covered by the base station 50). Thus, as the object travels between cell sites as it travels throughout the wide resolution area 12, the wide area resolution module 62 may update its position as it enters each new cell site based on the new Cell-ID information that it receives. The geographic area of a cell in a wireless telephone network typically ranges from hundreds to a few thousand meters. In such a scheme, the positioning resolution varies as the object moves from areas with higher base station densities to areas with lower base station densities.

[0035] Further, if there is pre-planned route information for the object and that information is available to the tracking system, that information can be used to further refine the location estimation. For example, if it is known that the object is to be transported on a truck on particular highway in a particular region, when the wide area resolution module 62 determines the object is in the region based on the received cell-ID, it may further refine its location estimation to be on or near the highway in the geographic region covered by the relevant base station 50. The pre-planned route information may be stored in the memory device 52 of the tracking device 16, or it may be stored in a memory unit associated with the host 18 (e.g., the database 24). In the later case, the host 18 may compute the refined location estimation based on the pre-planned route information.

[0036] According to yet other embodiments, the wide area resolution module 62 may reside with the host 18. In such an embodiment, the cell-site map information may be stored in the database 24 and the tracking device 16 may report back to the host 18 the Cell-ID information. The host 18 may then perform the correlation between the Cell-ID information and the cell-site map information to compute the general location of the tracking device 16.

[0037] According to other embodiments, instead of or in addition to using the Cell-ID information to compute the location of the object in the wide resolution areas 12, the tracking device 16 may include a GPS receiver (not shown) to aid in the location determination process. For an embodiment where both the Cell-ID information and the GPS information are used to track the object, the wide area resolution module 62 may use both inputs to verify the location of the object.

[0038] The high-resolution module 60 of the tracking device 16 may compute the location and orientation of the object based on the inertial sensor inputs and the inputs from the wireless aiding system. The inertial sensors may provide outputs that measure motions of the object, such as linear accelerations and rates of rotation. In the description to follow, the vector â is used to identify the measured x, y and z accelerations, and the vector ŵ is used identify the x, y, and z rotation rates as measured by the inertial sensor assembly 40. The measurements of the sensors may be assumed to contain noises that can be represented by stochastic models such as:
(Measured Data)=(Sensor Bias)+(Scale Factor)×(Actual Data)+(Drift Rate)×(Time)
where “Sensor Bias”, “Scale Factor”, and “Drift Rate” are random variables with possible time-varying characteristics.

[0039] FIG. 3 is a diagram of a state estimation algorithm employed by the high-resolution module 60 according to various embodiments of the present invention to estimate the state of the tracked object in a high resolution area. For terminology purposes, three different frames of orientation are considered in describing the process: (1) the Inertial Frame, which is the coordinate frame of the tracking device 16, (2) the Body Frame, which is the coordinate frame of the object to which the tracking device 16 is affixed or connected (in both cases x is in the body forward direction), and (3) the Navigation Frame, which is the coordinate frame of the environment in which the object is moving. The “state” of the object computed by the high-resolution module 60 may include values for the following parameters (where vectors are denoted in lower case with an “^” and matrices are denoted in upper case with an “→”):

• • [0040] the position, {circumflex over (p)}, a vector representing the position of the object in the Body Frame;
  • [0041] the bias estimate, {circumflex over (b)}, a vector representing the bias in â and ŵ (denoted as {circumflex over (b)}^â and {circumflex over (b)}^ŵ respectively);
  • [0042] the alignment, {right arrow over (I)}, a rotation matrix that maps from the Body Frame to the Inertial Frame;
  • [0043] the attitude, {right arrow over (R)}, the rotation matrix that maps from the Navigation Frame to the Inertial Frame;
  • [0044] the active mode, m, a scalar representing the current mode of the object (e.g., stopped, moving in x, y, and/or z, etc.);
  • [0045] the velocity, {circumflex over (v)}, a vector representing the velocity of the object in the Navigation Frame; and
  • [0046] the corrective state, {right arrow over (E)}, a matrix of correction factors.
    In other embodiments, the state estimate may comprise a subset of these parameters.

[0047] Referring to FIG. 3, at block 80 the inputs from the inertial sensor assembly 40 (â and ŵ) at time t=k are stored in a memory location (a buffer S1). At step 82, a current mode detector algorithm detects the current mode of the object, i.e., the mode at t=k−p, where p is a delay value, based on the inertial sensor inputs and the estimated state of the object at t=k−p−1 (stored in a buffer S2 at block 84). The current mode of the object may be a motion mode that is characterized by certain motion patterns or signatures indicative of the mode. For example, in the case of a car, one mode may be “moving forward,” which may be characterized by a large acceleration in the x direction and lasting until a large deceleration. Another mode may be “stopped with the ignition off,” which may be characterized by extremely small or no acceleration in the x, y or z direction, as well as extremely small or no rotation. The mode “stopped with the ignition on” may be characterized by slightly greater acceleration or rotation than the “stopped with the ignition off” mode, but still with relatively small acceleration and rotation measurements. Other modes, such as “turning,” “moving backward,” etc. may be defined, and such modes may be defined for different transportation modes (forklift, crane, carried by human, conveyor belt, etc.) The current mode of the object may be detected by matching the current motion signatures to the patterns of the defined modes. The current mode of the object may be used, as described below, in determining the location or state of an object because kinematic (or object movement) models may be constructed and used to define and/or limit the possible movements of the object when in a particular mode.

[0048] When the current mode detector 82 determines that there is a mode transition at time t=k, it stores the time point of the transition t_mⁱ in a buffer S3 at block 94. The current mode detector 82 may also reference the buffer S3 to recall when the last mode transition occurred.

[0049] At step 86, a current mode state estimator algorithm computes the current state estimate of the object (i.e., state estimate at t=k−p) and the state estimate is written into the buffer S2 at block 84. As mentioned above, the “state” estimate of the object may include values for the parameters {circumflex over (p)}, {circumflex over (b)}, {right arrow over (I)}, {right arrow over (R)}, m, {circumflex over (v)}, and {right arrow over (E)}, or a subset of these parameters. As described in more detail below in conjunction with FIG. 5, the state estimate determination may be calculated based on the inertial sensor values (e.g., â and ŵ), the mode of the object at t=k−p (as determined at step 82) and the state estimate at t=k−p−1 (stored in the buffer S2 at block 84). If there was a mode change, the correction information may associated with the mode change may be written to a buffer S4 at block 90. Also, if there is a correction resulting from a mode transition, the current mode state estimator 86 may write into the buffer S3 at block 94 that a correction was available at time t=k, which is stored as time t_gⁱ.

[0050] Available correction inputs 88 at time t=k, such as external correction inputs available from, for example, the reference point transmitters 28, may be stored in the buffer S4 at block 90. Notationally, the correction inputs may be represented by the correction information matrix {right arrow over (C)}, which may include several vectors, including {circumflex over (p)}, {circumflex over (v)} and û (where û is a vector of the yaw, pitch and roll Euler angles of the attitude {right arrow over (R)}). Also, as mentioned about, mode transition correction information from the current state estimator algorithm 86 may be stored in the buffer S4.

[0051] At step 92, a state estimate corrector algorithm computes a corrected state estimate of the object at time t=k−p−q based on the historical state information stored in buffer S2, inputs regarding when the last mode transition occurred (time t_gⁱ) from the buffer S3, and the correction information stored in buffer S4, where q is an additional delay representing the amount of time that passes between the arrival of correction information. The correction algorithm 92 may check to see whether any corrections have arrived in the buffer S4 and, if so, apply those corrections to the state estimates in the buffer S2 to generate corrected state estimates. For example, assume that at time t_gⁱ correction information has arrived either (a) from the correction inputs 88 or (b) as a result of a mode transition detected by the current mode detector 82 and its resulting correction computed by the current mode state estimator 86 or (c) both (a) and (b). The corrector algorithm 92 may then compute and apply the corrections as it computes to the state estimates in the buffer S2 for the time period t_gⁱ⁻¹ to t_gⁱ. The state estimation at time t=k−p−q, where q is an additional delay to account for the corrected information, may then be supplied as the output of the high resolution state estimation algorithm. In this approach, t_gⁱ−t_gⁱ⁻¹<g, such that the corrector algorithm 92 can work on all past states. Thereafter, the current mode state estimator 86 may use the new corrected state estimate as it computes the ongoing states estimates for the object.

[0052] A vector ĝ of the correction history, e.g., a list of the last i time points where a state correction was provided (e.g., ĝ=[t_g¹,t_g² . . . ,t_gⁱ] may be stored in the buffer S3 at block 94. The buffer S3 may also store a vector ĥ of the mode history, e.g., a list of the last j time points where the mode of the object was switched (e.g., ĥ=[t_m¹,t_m² . . . ,t_m^j]) That is, when a change in the current mode is detected at step 82, the vector ĥ stored in the buffer S3 at block 94 is updated.

[0053] In certain embodiments, the buffer S2 may store data at a slower rate than the other buffers. For example, the buffer S2 may store data at a rate of 1 Hz, whereas the other buffers may store data at a rate of 96 Hz. In that case, the buffer S2 stores every 96^th value. As the buffer S2 outputs state estimation information, its time stamp may indicate the necessary fractional time stamp adjustment.

[0054] The process of the detecting the current mode of the object (step 82 of FIG. 3) may be a function of the various modes that the body (object) supports and the ease with which transitions between the modes can be detected. Typically, a conclusive decision that a mode transition detection is valid takes a period of time, p, to confirm the transition. For a body/object that has multiple modes, a generic process for detecting the current mode of the body/object according to various embodiments of the present invention is shown in FIG. 4. At step 100, a straight integration is performed on the inertial sensor inputs (e.g., â and ŵ) at time t=k and the state estimate at time t=k−p−1 (stored in the buffer S2, see FIG. 3) to arrive at a first state estimate for the time period t=[k−p, k]. The state estimate for the straight integration for this time period may be stored in a buffer (buffer SS) at block 101. Also, a number (N) of motion models 102 may be used to provide additional state estimate for the time period t=[k−p, k] based on the inertial sensor inputs at time t=k and the state estimate at time t=k−p−1, as described in more detail below. The state estimate for the various motion models may be stored in buffers (buffers SM1 to SMN) at blocks 104. At step 106, the various state estimates for the time period t=[k−p, k] produced by the straight integration and the various motion models are analyzed and, based on the analysis, a final decision as to the mode at time t=k−p is made. According to various embodiments, a curve-fitting algorithm may be used to select the current mode.

[0055] A disadvantage of using a curve fitting algorithm is that to generate one time-increment of state estimation for each of the N motion models and the straight integration, (N+1)*p calculations must be performed, which may be too computationally expensive for some applications. To compensate, according to various embodiments, the current mode estimation can be performed at a lower frequency than the actual state estimation. In addition, if the probability of a mode transition is high, the resolution can be increased to better identify the transition time.

[0056] For bodies/objects with few motion modes or very distinct acceleration patterns, statistical analysis can also or alternatively be performed on the inertial sensor data to detect the current mode. For example, if the body is a car, which is either stationary or moving, statistical analysis of acceleration in the x direction may be sufficient to detect motion. For instance, the statistical analysis algorithm may analyze the standard deviation of a moving time window of the acceleration values. Changes in the standard deviation may be used to indicate mode transitions. The amount of time spent in the mode may then be used to ensure that the mode change detection was valid before a final decision is made that a mode transition has occurred.

[0057] Also, according to various embodiments, right after a transition it may be assumed that there are no other transitions for a time period, such as a fraction of p. During this time period, it may not be necessary to perform additional mode transition detection.

[0058] FIG. 5 is a diagram of the process of the current mode state estimator algorithm 86 for determining the current state estimate (the state estimate at time t=k−p) of the object according to various embodiments of the present invention. At step 110, a straight integration is performed on the inertial sensor inputs (e.g., â and ŵ) at time t=k and the state estimate at time t=k−p−1 (stored in the buffer S2, see FIG. 3) to arrive at a state estimate for the time period t=[k−p, k]. Alternatively, since the current mode detector algorithm 82 may perform this operation, the state estimate may be read from the buffer SS (see block 101, FIG. 4).

[0059] A current mode motion model 112 has a model for the allowed motion patterns for the object given the current motion mode, as determined by the current mode detector 82. For example, if the object is traveling in a car or other similar vehicle type in terms of allowed motions, the current mode motion model 112 may assume that the object cannot move in the body y direction. On the other hand, for example, if the object is being lifted by a forklift, the current mode motion model 112 may assume that the motion has to be in the z direction, and no motion may be expected in the x-y plane. Based on the allowed motion patterns for the given mode, the current mode motion model 112 computes a state estimate for the object at time t=k−p that is consistent with the motion model. A number of such motion models may be used for different motion types, with different assumptions for different modes as applicable. Because the actual state of the object is probably somewhere between the state estimates computed by the straight integration 110 and the current mode motion model 1112, a Kalman filter 114, or some other suitable filtering technique, may take the two different state estimates and corresponding confidence factors (based on, e.g., known sensor noise and known model shortcomings) to compute a state estimate that is a weighted sum of the two state estimates. The Kalman filter 114 may be a standard Kalman filter or an adaptive one. According to various embodiments, a number of adaptive, cascaded Kalman filters may be used. The parameters of the filter(s) may be derived from noise characteristics of the inertial sensors, the accuracy of the current mode motion model 112 and other tuning.

[0060] The state estimate corrector algorithm 92 may correct the state estimates stored in the buffer S2 based on the corrective information stored in the buffer S4. The corrector algorithm 92 may differentiate between different types of corrective data in making corrections. For example, in various embodiments, the corrector algorithm 92 may differentiate between frequent and infrequent correction types. For infrequent corrections, such as when a position update is provided (such as from one of the reference position transmitters 28), the appropriate heading and/or velocity corrections that would shift the estimated position of the object to the updated position are computed. In making this determination, the initial heading information may be assumed to be incorrect and that the velocity error accumulated linearly. Once the heading and velocity corrections (and sometimes the bias corrections) are calculated, the results may be applied to correct the entire state trajectory in the buffer S2. In various embodiments, the corrector algorithm 92 may use a closed form solution to determine the heading and velocity corrections based on the estimated object location (without corrective inputs) and the updated object location.

[0061] The following describes one embodiment of how the state estimate corrector algorithm 92 may operate. Assume that a tracked object traverses a trajectory from an initial position {circumflex over (p)}₀ to a final position {circumflex over (p)}_n over a period of T seconds. Let v₀ define the initial speed and let ĥ₀ define the initial speed heading in a navigation frame, i.e., a vector with unit magnitude, where v₀*ĥ₀ would be the velocity in the navigation frame. Let the time interval T be divided into n sub increments, each of length dt (i.e., T=n*dt). Let {circumflex over (p)}_i, v_i and ĥ_i denote the position, speed, and speed heading, respectively, at time i*dt, and let {circumflex over (p)}_n, v_n and ĥ_n be the final position, speed, and speed heading. Further, assume that the final position is calculated as:
{circumflex over (p)}_n={circumflex over (p)}₀+Σ((ĥ_i*v)*dt).
For simplicity, let {circumflex over (p)}_n=0 in the following analysis.

[0062] Let {circumflex over (f)}_i be a unit heading vector in the navigation frame at time i. Let {right arrow over (R)}_i be the rotation matrix that maps from the inertial frame to the navigation frame. Assume that at time n, some correction information is given and its impact on the current state is to be calculated. Three types of error may be considered: (1) velocity estimation errors due to accelerometer noise and incorrect initial conditions, (2) speed heading errors due to gyroscope noise and incorrect initial conditions, and (3) accumulating velocity error due to both.

[0063] For velocity estimation errors (Case 1), the following types of velocity estimation errors should be accommodated while performing dead reckoning from the initial to the final position of an object:

• • [0064] The v₀ value may be wrong, and all v_i need to be incremented by dv in some heading {circumflex over (f)}_i in the navigation frame.
  • [0065] The estimate from v₀ to v_n may be accumulating bias linearly, i.e., v_i need to be incremented by i*dv in some heading {circumflex over (f)}_i.
  • [0066] The calculation from v₀ to v_n may be accumulating bias with an arbitrary weight sequence and v_i need to be incremented by w_i*dv in some heading {circumflex over (f)}_i.
    It should be noted that first and second points are special cases of the third point. In the first situation w_i=1, and in the second situation w_i=1.

[0067] For cars or other similar vehicles, {circumflex over (f)}_i is typically equal to ĥ_i since the speed heading is typically in the body-x direction and there is minimal body-y or body-z speed to correct. As such each {circumflex over (f)}_i is known and given at time i*dt. If one wants to accommodate error accumulation in all axes in the body frame, representing the three sensor biases, then {circumflex over (f)}_i can be rewritten as {circumflex over (f)}_i={right arrow over (R)}_i*{circumflex over (f)}, where {circumflex over (f)} is a constant heading in the body frame, representing the error accumulation ratio in the body frame, and {right arrow over (R)}_i is a rotation matrix, rotating the body frame to the navigation frame. Therefore, the position that accounts for the errors may be given as:

[0068] $\begin{array}{c}{\hat{p}}_c=\sum \left(\left({\hat{h}}_i*v_i+{\hat{f}}_i*w_i*\mathrm{dv}\right)*\mathrm{dt}\right)\\ =\mathrm{dv}*\mathrm{dt}*\sum \left(w_i*{\hat{f}}_i\right)+{\hat{p}}_n\\ =\mathrm{dv}*\mathrm{dt}*\sum \left(w_i*{\stackrel{->}{R}}_i\right)*\hat{f}+{\hat{p}}_n\end{array}$

[0069] Assume at time n a correction is obtained. The above equation can be used if the provided correction information is the correct position {circumflex over (p)}_c,_in which case the correct values for dv and {circumflex over (f)} can be selected to make sure that the trajectory is shifted from {circumflex over (p)}_n to {circumflex over (p)}_c, and the updated velocity may be computed at time n.

[0070] Alternatively the correction information may be a new velocity (i.e. dv and {circumflex over (f)}) and the new position may be calculated.

[0071] Let

[0072] ${\hat{h}}_a=\frac{\sum w_i*{\hat{f}}_i}{\sum w_i}$ ${\stackrel{->}{H}}_b=\frac{\sum w_i*{\stackrel{->}{R}}_i}{\sum w_i}.$
The heading ĥ_a and the rotation matrix {right arrow over (H)}_b can be updated at each time increment and remembered as part of the system state. As such when at time n, a correction arrives, the correction can be applied without having to recalculate the state trajectory for t=[0,n]. Therefore, at any time n, one can either solve for {circumflex over (f)} and dv, or apply their impact.

[0073] Where the heading ĥ_i values are wrong (Case 2 above), they may be corrected by rotating them by an amount described by the rotation matrix {right arrow over (EH_i)}. If {right arrow over (EH_i)}={right arrow over (EH₀)}, the error model may assume the initial heading was wrong. If it is desired to model some heading error accrual, it may be represented by a series of varying {right arrow over (EH_i)}. The new point may be given as:
Σ(({right arrow over (EH)}_i*ĥ_i*v_i+{right arrow over (EH)}_i*{circumflex over (f)}_i*w_i*dv)*dt).
Here, the vectors {circumflex over (f)}_i have been rotated as well, since the heading error typically represents the misalignment between body frame and navigation frame. But this term can be dropped depending on the needs of the model.

[0074] Because, for the general {right arrow over (EH_i)}, or even for small angle {right arrow over (EH_i)}, there is no immediate simplification that enables the heading error accumulation to be modeled and represented, it may be assumed that {right arrow over (EH_i)} are all equal to {right arrow over (EH)}, which is equivalent to saying the initial heading was wrong. In this case the problem is simplified to:

[0075] $\stackrel{⟶}{\mathrm{EH}}*\sum \left(\left({\hat{h}}_i*v_i+{\hat{f}}_i*w_i*\mathrm{dv}\right)*\mathrm{dt}\right)=\stackrel{⟶}{\mathrm{EH}}*\left({\hat{p}}_n+\mathrm{dv}*\mathrm{dt}*\sum \left(w_i*{\hat{f}}_i\right)\right)=\stackrel{⟶}{\mathrm{EH}}*\left({\hat{p}}_n+\mathrm{dv}*\mathrm{dt}*\left(\sum w_i\right)*{\hat{h}}_a\right)$
This equation can then be used at time n to accommodate the correction information obtained.

[0076] The last equation above applies to the case for a vehicle where the initial heading assumption was wrong and velocity error is accumulating (Case 3 above). In this case, if a position update is provided and given a new point {circumflex over (p)}_c, this correction can be accommodated by selecting an {right arrow over (EH)} to rotate {circumflex over (p)}_n to a point {circumflex over (p)}_r such that a translation from {circumflex over (p)}_r in the direction {right arrow over (EH)}*ĥ_a leads to {circumflex over (p)}_c. The amount of translation to {circumflex over (p)}_c is adjust of the dv value. Alternatively, one could first translate {circumflex over (p)}_n to a point {circumflex over (p)}_t along ĥ_a such that {circumflex over (p)}_t can then be rotated to {circumflex over (p)}_c by picking {right arrow over (EH)}. If given a velocity correction, the other unknowns can be solved for.

[0077] FIGS. 6 and 7 provide a simplified example of how the reference position transmitters 28 may be used to correct/update the position of the object in the high-resolution mode. FIG. 6 is a simplified diagram of a high-resolution area 14 with walls/obstacles 150 situated throughout the environment. FIG. 7 shows the x- and y-position error in the calculated position of the object/tracking device 16 as a function of time as it moves throughout the environment. As can be seen in this example, when the object/tracking device 16 reaches the first reference position transmitter 28₁ at time t1, the position of the object/tracking device 16 is updated/corrected such that the x and y errors are zero at time t1. As the object/tracking device 16 continues to move throughout the environment, errors accumulate in both the x and y directions until the object/tracking device 16 reaches the second reference position transmitter 28₂ at time t2, at which time the position of the object/tracking device 16 is updated/corrected, and so on as the object/tracking device 16 moves throughout the environment.

[0078] The host processing system 18 may have knowledge (stored in the database 24, for example) about physical constraints of the environment and make additional position refinements based on that information. For example, the host processing system 18 may know the location of certain walls or other permanently located obstacles in the environment and, based thereon, refine the reported position of an object if the reported position coincides with such a wall or obstacle. Also, for certain object types, the host processing system 18 may assume that no two objects are in the same position at the same time.

[0079] FIG. 8 is a diagram of the tracking device 16 according to another embodiment of the present invention. As shown in FIG. 8, in this embodiment the tracking device 16 additionally includes a power management module 160. The power management module 160 may be implemented as software code to be executed by the processor 42 using any suitable computer instruction type such as, for example, Java, C, C++, Visual Basic, etc., using, for example, conventional or object-oriented techniques. The software code may be stored as a series of instructions or commands on a computer readable medium, such as a memory device 52.

[0080] According to various embodiments, the power management module 160 may receive the inputs from the inertial sensor assembly 40 and, based thereon, determine an appropriate power level for the tracking device 16 (such as, e.g., full power, reduced duty cycle, hibernation or sleep mode) that is needed for proper operation of the tracking device 16 at that particular moment. For example, according to one embodiment, for a position tracking system 10 where continuous position sensing and computation of the tracking device 16 is only needed when the object under surveillance is in motion, the power management module 160 can, for example, cause the processor 42 to enter a sleep mode if no (or minimal) movement is detected for a certain time period. Also, for example, the power management module 160 may select a reduced duty cycle mode when the tracking device 16 is in the wide area resolution mode and select a full power mode when the tracking device 16 is in the high resolution mode (except when the processor 42 enters the sleep mode due to lack of movement, for example, regardless of operational mode). Although this description of the power management module 160 is in the context of a tracking device with an on-board processor, such a power management module that is responsive to inertial sensor inputs in selecting the power level can be used for any device with an on-board processor.