Radio-frequency badge for location measurement

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BACKGROUND

1. Technical Field

The invention is related to location tracking systems, and more particularly to a system and process for determining the location of persons and objects carrying radio frequency (RF) transmitters that transmit data messages to at least one RF receiver connected to a computer in a computer network. The receivers forward data received from the transmitters to the network, along with radio signal strength indicator (RSSI) data, for computation of the location of the person or object carrying each transmitter.

2. Background Art

Knowledge of the location of people and objects is essential to the viability of many current mobile and ubiquitous computing schemes. For example, in a mobile computing environment, a user of a mobile computing device (e.g., notebook computer, handheld PC, palm-size PC, Personal Digital Assistant (PDA) or mobile phone) may wish the device to provide directions to a particular location in a building, such as the nearest printer, snack room, restroom, etc., or perhaps directions to a particular conference room or office within the building. This type of information is dependent on knowing the current location of the user. Mobile computing device users also typically expect messages and other notification information to be provided to them wherever they happen to be. However, some notifications can be dependent upon the user's location. For instance, a user might be notified that he or she is near a printer where a user-submitted document has been printed. Again the user's current location is needed to make such a notification. A mobile computing device user might also want to know the location of other people in the building, in order to find them or obtain information about them. For example, a user might want to get a list of the names of people attending the same meeting. To obtain this information, it is necessary to know what people are at the location of the meeting. The foregoing are just a few examples of the need to know the location of people. It is easy to imagine many other situations where knowledge of the location of people would be useful to a mobile computing device user.

Location information is equally critical in so-called ubiquitous computing. Ubiquitous computing revolves around extending computational activities beyond the current desktop model and into the environment. In future homes and offices, access to computing should be as natural as access to lighting. Users should not be required to go to a special place (i.e., the desktop) to interact with the computer. Rather, the computer should be available to interface with the user anywhere in the home or office (or more generally anywhere in an arbitrarily large environment), through whatever set of devices is available, be they fixed or carried by the user.

It is noted that the term computer is used loosely here in that the user actually would have access to a wide variety of computing and information services, which will likely employ many computers and "smart" devices such as the aforementioned PDA's, mobile phones, etc. For example, computing services such as web browsing, document editing, or video conferencing are envisioned. Thus, it should be understood that when the term computer is used in connection with the concept of ubiquitous computing, in actuality many computers may be involved non-exclusively in a single interactive session.

The usefulness of an ubiquitous computing system hinges on the ability to maintain an awareness of the users, particularly their locations. One goal of such a system would then be to understand the physical and functional relationship between the users and various I/O devices. This knowledge could be employed to allow a user to move from room to room while still maintaining an interactive session with the computer. In addition, knowledge about who and what is in the vicinity of a person can be used to tailor a person's environment or computing session to behave in a context-sensitive manner. For example, knowing the location of a person in a building can be used to infer what activity that person is engaged in and then the environment or computing session can be adjusted appropriately.

There are several current technologies for automatically determining the location of people and objects. For example, one of the first of such location systems uses diffuse infrared technology to determine the location of people and objects in an indoor environment. A small infrared emitting badge (sometimes referred to as a button or tag) is worn by each person, or attached to each object, whose location is to be tracked. The badge automatically emits an infrared signal containing a unique identifier every 10 seconds, or upon request of a central server. These requests are transmitted to the badges via a series of fixed infrared sensors placed throughout the indoor environment--typically mounted to the ceiling. The sensors also receive the infrared emissions from badges within their line-of-sight. The central server, which is hardwired to each sensor, collects the data received by the sensors from the badges and provides it to a location program for processing. These types of systems do not provide the actual 3D location of the person or object carrying the badge. Rather, the person's or object's location is deemed to be within the room or area containing the infrared sensor that received the emission from the badge attached to the person or object. In addition, these systems, being infrared-based, are susceptible to interference from spurious infrared emissions from such sources as fluorescent lighting or direct sunlight. Further, diffuse infrared-based systems have a limited range, typically only several meters. Thus, except in small rooms, multiple sensors are required to cover the area. In addition, since the sensors must be within the line-of-sight of the badges, a sensor must be placed in every space within a room that cannot be seen from other parts of the room. As a result, a considerable number of sensors have to be installed and hardwired to the central server. This infrastructure can be quite expensive and in some cases cost prohibitive.

Other existing indoor location systems attempt to improve the accuracy of the location process using a combination of radio frequency and ultrasonic emission. In these systems, a central controller sends a request for location data via a short range radio transmission to each badge worn by the people, or attached to the objects, whose location is being tracked. In response, the badges emit an ultrasonic pulse to a grid of fixed receivers, which are typically mounted to the ceiling. Each receiver that "hears" the ultrasonic pulse emitted from a badge reports its distance from the badge to the central controller via hardwired connections. Specifically, a synchronized reset signal is sent to each receiver at the same time the location request is transmitted to the badges. This reset signal starts a timing procedure that measures the time between the reset signal and the receipt of a ultrasonic pulse for a badge within range of the receiver. The receiver then computes its distance from the badge emitting the pulse and reports this to the central controller. An ultrasound time-of-flight lateration technique is then used by the controller to accurately determine the locations of the badges. While these types of systems do provide very accurate location information, they again require an expensive infrastructure in form of multiple receivers mounted throughout the environment which must be hardwired to the central controller. In addition, the accuracy of these systems has been found to be adversely affected if the placement of the receivers is less than optimal. Further, there is a concern associated with animals being sensitive to ultrasonic emissions.

A variation of the combined radio frequency and ultrasonic location system requires the badges to determine their own location, presumably to compute directions, and the like, and to provide the information to a person carrying the badge. In this case there is no centralized controller that determines locations of all the badges. Specifically, ultrasonic emitters are mounted in various locations around an indoor space. The badges include a radio frequency transceiver. Whenever location information is desired, the badge transmits a radio frequency signal. The emitters pick up the signal from the badges and respond with an ultrasonic pulse. The badge unit measures the time it takes to receive each ultrasonic pulse emitted by an emitter within range of the badge. In addition to the ultrasonic pulse, the emitters also transmit a radio frequency signal that identifies the emitter and its location. From the timing and emitter location information, the badge triangulates its own position. The infrastructure is not as problematic in this latter system since there can be fewer emitters and they are not hardwired into any kind of centralized controller. However, only the badge unit knows its location. Thus, there is no centralized database to provide location information to help locate persons in the building. In addition, the badges are relatively complex in that they must include both a radio frequency transceiver and an ultrasonic receiver, as well as the processing capability (and so power burden) to compute their location.

In yet another indoor location system, radio frequency LAN wirelesss networking technology is used to determine the position of people, or more specifically a computing device employing the wireless LAN technology (such as a notebook computer). In this system, base stations are deployed within the indoor environment to measure the signal strength and signal to noise ratio of signals transmitted by the wireless LAN devices. A centralized program takes the signal information from the base stations and employs a lateration process to estimate the location of the transmitting unit. This system has the advantages of requiring only a few base stations and using the same infrastructure that provides the building's general purpose wireless networking. However, person or object being tracked must have a device capable of supporting a wireless LAN, which may be impractical on small or power constrained devices.

Other current systems also employ radio frequency technology to locate people and objects in an indoor environment. One such system uses a centralized base station and a series of antennas spread throughout the environment that each emit a RF request signal which is received by badges within range of the antenna. These badges, which are attached to people and objects whose location is being tracked, transmit a RF signal in reply with an identifying code embedded therein. The location of the badge relative each antenna is computed using a measurement of the time it takes for the base station to receive the reply via the various antennas after the request is transmitted. However, the antennas have a narrow cone of influence, which can make ubiquitous deployment prohibitively expensive.

Electromagnetic sensing is also employed for position tracking. These types of systems generate axial DC magnetic field pulses from a fixed antenna.

The system then computes the position of the receiving antennas by measuring the response in three orthogonal axes to the transmitted field pulse. However, the infrastructure needed for these systems is expensive and the tracked object must be tethered to a control unit.

Finally, position tracking has been accomplished using computer vision techniques. In these systems, cameras are employed to determine where persons or objects of interest are located in an indoor environment. While these types of position tracking systems can be quite accurate, the processing required to analyze each camera frame is substantial, especially when complex scenes are involved. Thus, the infrastructure costs for these systems can be very high.

SUMMARY

The present invention is directed toward a system and process for determining the location of persons and objects in an environment that overcomes the limitations of existing location systems by utilizing existing infrastructure to minimize overhead costs and by employing a compact, simple radio frequency (RF) transmitter as a badge.

The system includes a plurality of battery-powered, radio frequency (RF), transmitters (TXs) that are carried by the person or object being tracked. One, or typically more, RF receivers (RXs) are used to receive location messages transmitted by TXs within signal range of the RX. Each RX is connected to a computer, which receives data messages from the RX generated using a location message received from a TX. A centralized computer is in communication with each of the computers associated with a RX via a conventional network. The centralized computing device tracks the location of each person or object carrying a TX, using data derived from data messages forwarded to it from the RX-connected computers.

The location messages transmitted by each TX include at least a transmitter identifier which uniquely identifies the particular TX transmitting the location message. Each RX receiving the location message from a TX measures the strength of the signal carrying the message to produce a radio signal strength indicator (RSSI). The RX then generates a data message that is forwarded to the centralized computer. This data message includes the transmitter identifier from the location message, the RSSI associated with the location message and a receiver identifier that uniquely identifies the particular RX sending the data message. The centralized computing device is preprogrammed to know the person or object associated with each TX and the physical location of each RX, thereby allowing the centralized computer to determine the location of each person or object carrying a TX using the RSSI, transmitter identifier and receiver identifier provided in a data message.

The location messages transmitted by the TXs can also include error detection data, preferably in the form of a message count and conventional checksum value. The message count is simply a number which is incremented each time a TX transmits a location message. The error detection data can be used in different ways. For example, the RX can determine whether a location message received from a TX has a message count increment one unit above the last, previously received, location message transmitted from that TX. If the message count is too high, then it is deemed that an interim location message was lost and the RX foregoes providing a data message corresponding to the out-of-sequence location message to the computer network. The same policy can be followed if the RX finds a received location message is incomplete or corrupted should the checksum not match the data received. Alternately, the RX can include the message count and checksum values received from a TX in the data message it forwards to the centralized computer. The centralized computer then decides whether or not to use the other data in the data message for locating purposes.

The battery-powered TX of the foregoing location tracking system has unique conservation features for extending the life of the battery. In general, the TX is constructed using a microcontroller, an accelerometer which is connected to the microcontroller and which provides a signal indicative of the severity of motion to which the TX is being subjected, one or more manually-operated function selection switches which are connected to the microcontroller and which activate and deactivate particular functions of the TX, a RF transmitter unit which is also connected to the microcontroller and which transmits the location message supplied to it by the microcontroller, and finally a power supply that includes a battery for powering the electronic components of the TX. The power saving features essentially involve using the accelerometer signal to curtail transmission of location messages during periods when there is no movement of the TX about an environment, such as an office building. By foregoing the transmission of location messages when the person or object has not moved, the power required to send the transmission is saved. This task can be accomplished by counting the number of times the accelerometer signal exceeds an accelerometer signal level threshold in a prescribed period of time (e.g., 1 second). If the count does not exceed a prescribed number (e.g., 2), the location message is not transmitted. If, however, the count does exceed the prescribed number a transmission is initiated. The accelerometer threshold represents a signal level over which it is likely the person or object carrying the TX is actually moving about the environment.

Further power can be saved by powering down the TX if no substantial movement has occurred for a period of time. Specifically, whenever the transmission of the location message is not made owing to a lack of movement, the microcontroller of the TX waits a prescribed timeout period (e.g., 2 seconds) and then counts the number of times the accelerometer signal exceeds the accelerometer signal level threshold in the aforementioned prescribed period of time. It is then determined if the count exceeds the aforementioned prescribed number. If the count does not exceed the prescribed number, it is next determined if a prescribed shutdown time limit (e.g., 1 minute) has been passed since the last transmission of a location message by the TX. Whenever it is determined that the shutdown time limit has not been exceeded, the process of waiting and sampling the accelerometer signal is repeated. If at any time during this process it is discovered the count exceeds the prescribed number, then the transmission of a location message is initiated. If, on the other hand, the count is not found to exceed the prescribed number during any iteration up to the time the shutdown time limit is exceeded, the TX is powered down to extend the life of the battery.

The TX stays in the foregoing powered down condition until one of the following occurs. First, preferably the aforementioned manually-operated function selection switches includes a "power on" switch. If a user activates this switch, the TX is powered up, regardless of whether it was in the power saving shutdown mode or not. In addition, the TX can be equipped with a motion-activated tilt switch. This switch remains open when the TX is at rest, but when the TX is moved it closes. If the TX is in the powered down condition when the tilt switch closes, a signal is sent to the microcontroller that caused the TX to be powered back up. Thus, if a powered down TX is moved, it reactivates. Finally, the microcontroller can be programmed to "wake up" periodically during the shutdown mode (e.g., once every hour) and to initiate the transmission of a location message. This last feature is useful in finding lost badges.

The manually-operated function selection switches can also optionally include a continuous transmission mode switch. This switch when activated causes a location message to be transmitted at prescribed intervals (e.g., every 1 second). This continuous transmitting mode of operation would override the power saving features and would remain in force until a user manually deactivates the switch. Another switch that can be included in a send-once switch, which when activated causes the microcontroller to transmit a location message regardless of when the location message would have been transmitted had the send-once switch not been activated. This is a one-time event, however, and the power saving mode of operation would be reestablished once the transmission is complete.

It is noted that the accelerometer data can also be included in the location message. This data can be used by the centralized computer to perform motion studies and the like. Specifically, an accelerometer signal history in the form of a count of the number of times the accelerometer signal exceeded the accelerometer signal level threshold in the aforementioned prescribed period of time is included in the location message. Preferably, a separate count is included for each consecutive prescribed period of time occurring since the last transmission of a location message.

Another useful feature that can be incorporated into the TX in the present location tracking system is a personal identification number (PIN) scheme. In this scheme, a user enters a PIN into an input apparatus, such as a number keypad on the TX. The number is then stored by the microcontroller. Whenever a location message is transmitted by the TX, the microcontroller includes the PIN in the message. The PIN is used by the location tracking system to identify the person or object carrying the TX. However, there is an issue of what to do about an activated TX with a stored PIN number that somehow becomes separated from the person or object with which it is associated. This can be handled using the accelerometer signal. In one version, the microcontroller monitors the accelerometer signal to determine if the TX is moving through the environment, and whenever it is determined that the TX has not moved for a period of time, the PIN is erased and no location message is transmitted until a replacement PIN number is entered into the TX. In another version, the accelerometer signal is made up of separate x-axis and y-axis signals. The microcontroller monitors the accelerometer signal to determine if the TX has been placed in an orientation other than an expected orientation, and whenever it is determined that the TX has been place in an unexpected orientation for a prescribed period of time, the PIN is erased and no location message is transmitted until replacement PIN number is entered.

In addition to the just described benefits, other advantages of the present invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.

DESCRIPTION OF THE DRAWINGS

The specific features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

FIG. 1 is a diagram depicting a location tracking system according to the present invention.

FIG. 2 is an image depicting the exterior of one version of the RF transmitter, or badge, employed in the location tracking system of FIG. 1.

FIG. 3 is a block diagram illustrating the internal components included in the RF transmitter employed in the location tracking system of FIG. 1.

FIG. 4 is a flow chart diagramming a process for conserving the battery life of a RF transmitter employed in the location tracking system of FIG. 1.

FIG. 5 is an image depicting the exterior of one version of the RF receiver employed in the location tracking system of FIG. 1.

FIG. 6 is a block diagram illustrating the internal components included in the RF receiver employed in the location tracking system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of the preferred embodiments of the present invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

Referring to FIG. 1, the location tracking system according to the present invention determines the location of persons and objects carrying radio frequency (RF) transmitters 100 that transmit messages to at least one RF receiver 102. Each receiver 102 is connected to a computing device 104, such as a personal computer (PC) that is in turn part of an existing network 106 of such computing devices. The receivers 102 forward data received from the transmitters 100, along with radio signal strength indicator (RSSI) data, to a centralized computer 108 via the network 106. The centralized computer computes the location of persons and objects associated with each transmitter based on the forwarded data and RSSI, using conventional methods. The location tracking system can be used in any environment, indoors or out. For instance, the receivers could be placed in every office in a building to determine which office a person or object is closest to.

One of the major advantages of the foregoing location tracking system is that it employs an existing computer network, thereby avoiding the considerable infrastructure cost associated with many of the previously described location systems. In addition, unlike existing systems, the transmitters used in the present location system are compact, simple RF transmitters only. There is no need for the transmitters to receive timing signals or any other data from the rest of the system to operate.

Specifically, in one version of the transmitter shown in FIG. 2, the transmitter (TX), or badge as it is often called, is a small palm-sized unit resembling an automobile key fob with buttons used to remotely control the door/trunk locks and car alarm. In this case, the buttons are respectively used to turn the TX on and off, transmit a message, and engage a continuous transmit mode. However, the TX can take on other forms, particularly ones that are much thinner and smaller than the depicted prototype. For example, other prototypes no bigger than a large coin have been constructed. It is also envisioned that TXs having the size and thickness of a credit card are possible. This latter version could be carried in a wallet or worn like a security badge. It is further envisioned that the TX could be configured to fit inside articles commonly carried with a person, such as a pen.

Referring now to the block diagram of FIG. 3, the general construction of the TX will be described. The heart of the TX is a PIC microcontroller 300 (e.g., a PIC 16C620 8 bit micro, FLASH ROM 512 Bytes, 128 byte RAM, 1 uA standby@3 v, 15 uA @32 Khz flush-mount Flash PIC microcontroller), which is connected to several other components. For example, the signal output of an accelerometer 302 is connected to the microcontroller 300. In the prototype TXs, the accelerometer 302 produced separate x-axis and y-axis signals that were combined to form the signal output sent to the microcontroller 300. The accelerometer's output signal can be used for a variety of purposes as will be described later. In versions of the TX where battery power is conserved by shutting down the unit if no appreciable movement is detected (i.e., the TX is operating in a "battery saving" mode that will be described in detail later), a tilt switch 304 is connected to the microcontroller. The tilt switch 304 closes and an interrupt signal to the microcontroller to "wake" it up, whenever the TX is moved after having gone into its shut down mode. There are also one or more manually-operated switches connected to the microcontroller 300. In the prototype TXs, four switches where included, although more or fewer switches could be used depending on what functions it is desired to make available for manual activation or deactivation. In the prototype TXs, push-button switches were used, however any type of switch could be employed. The first two of the switches 306, 312, are "power-on" and "power-off" switches, respectively. Whenever the power-on switch 306 is activated, the TX is powered up and begins operations. Conversely, when the power-off switch 312 is activated, the TX is off regardless of the position of the tilt switch 304. Thus, this switch 312 acts as a hard shutdown and is unaffected by movement of the TX. This hard shutdown mode not only saves battery life when the TX is not needed for providing location information, but affords a degree of privacy to a person carrying the TX since he or she can shut the unit off when they do not want their location known. The power-on and power-off switches are mutually exclusive in that whenever on is activated, the other is deactivated. The third switch 308 is a "continuous transmission mode" switch. This switch 308 is also normally off, and when it is off, the TX acts in the aforementioned "battery saving" mode.

However, when the continuous transmission mode switch is activated, the TX transmits a location message at prescribed intervals (e.g., every 1 second), regardless of whether the accelerator signal indicates the TX is moving or not. While battery life is not c