Radio-frequency badge for location measurement

• US 6,720,922 B2
• Filed: 03/29/2002
• Issued: 04/13/2004
• Est. Priority Date: 03/29/2002
• Status: Expired due to Term

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Microsoft Corporation
• Inventors: Needham, Roger M., Williams, Lyndsay
• Primary Examiner(s): PHAN, DAO LINDA
• Application Number:
  US10/112,217
• Publication Number:
  US 20030184475A1
• Time in Patent Office: 746 Days
• Field of Search: 342/463, 342/465, 342/457, 342/464, 455/456.1, 455/456.3, 455/457
• US Class Current: 342/465
• CPC Class Codes: G01C 21/20 Instruments for performing navigational calculations G01C21/24, G01C21/26 take precedence
• CPC Class Codes: G01S 2205/09 for tracking people
• CPC Class Codes: G01S 5/017 Detecting state or type of motion
• CPC Class Codes: G01S 5/019 Energy consumption
• CPC Class Codes: G01S 5/0244 Accuracy or reliability of position solution or of measurements contributing thereto
• CPC Class Codes: G01S 5/14 Determining absolute distances from a plurality of spaced points of known location

Abstract

A system and process for determining the location of persons and objects using transmitters (TXs) that transmit location messages to at least one receiver connected to a computer, in a network of computers. The TXs are small, battery-powered, radio frequency transmitters carried by the person or object whose location is being tracked. The location message signals are picked up by one or more receivers (RXs) scattered throughout an environment. Each TX transmits a code with a unique identifier, and each RX also has a unique identifier. The RXs measure the signal strength of each location message transmission. Each RX is connected to a computer, which is in turn connected to a network. The RX computers send the data messages including the TX identifier, radio signal strength, and RX identifier to a central computer via the network. The central computer uses this information to determine the location of the TXs.

Claims

1. A location tracking system for determining the location of persons and objects using transmitters that transmit location messages to at least one receiver connected to a computing device in a network of computing devices, comprising:

a plurality of battery-powered, radio frequency (RF), transmission-capable only transmitters (TXs), each of which is carried by the person, or attached to the object, whose location is to be determined;

at least one RF receiver (RX), each of which receives location messages transmitted by TXs within signal range of the RX;

at least one computing device, each of which is in communication with a different one of the RX and receives data messages from the RX each of which is generated using a location message received from a TX;

a centralized computing device which is in communication with each of the computing devices associated with a RX via said network, wherein said centralized computing device determines the location each of person carrying a TX or object having an attached TX, using data derived from data messages forwarded to it from a computing device associated with a RX.

2. The system of claim 1, wherein the location messages transmitted by each TX comprise a transmitter identifier which uniquely identifies the particular TX transmitting the location message.

3. The system of claim 2, wherein each RX measures the strength of the signal carrying a location message received from a TX to produce a radio signal strength indicator (RSSI), and wherein the data message sent to the computing device associated with each receiver and forwarded to the centralized computing device comprises the transmitter identifier contained in the location message received from the TX, the RSSI associated with the location message and a receiver identifier that uniquely identifies the particular RX sending the data message.

4. The system of claim 3, wherein 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 carrying a TX or object having an attached TX using the RSSI, transmitter identifier and receiver identifier provided in a data message generated by a RX in response to the receipt of a location message from that TX.

5. The system of claim 1, wherein the location messages transmitted by each TX comprise error detection data.

6. The system of claim 5, wherein the error detection data comprises a message count which is incremented each time a TX transmits a location message.

7. The system of claim 5, wherein the error detection data comprises a checksum value.

8. The system of claim 7, wherein the error detection data further comprises a message count which is incremented each time a TX transmits a location message, and wherein the RX determines whether a location message received from a TX has a message count one increment above the last previously received location message transmitted from that TX, and whenever it is not foregoes providing a data message corresponding to the out-of-sequence location message to the computing device associated with the RX.

9. The system of claim 7, wherein the RX determines whether a location message received from a TX in incomplete based on the location message'"'"'s checksum message, and whenever it is determined that the location is incomplete forego providing a data message corresponding to the incomplete location message to the computing device associated with the RX.

10. The system of claim 7, wherein the error detection data further comprises a message count which is incremented each time a TX transmits a location message, and wherein the message count is included in the data message generated by a RX that received the location message, and wherein the centralized computing device receiving the data message generated by the RX that received the location message determines whether the message count is one increment above the last previously transmitted location message from that TX.

11. The system of claim 7, wherein the checksum is included in the data message generated by a RX that received the location message, and wherein the centralized computing device receiving the data message generated by the RX that received the location message determines whether the location message was complete using the checksum.

12. The system of claim 1, wherein each TX comprises:

a microcontroller;

an accelerometer which is connected to the microcontroller and provides a signal to the microcontroller 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 connected to the microcontroller and which transmits the location message whenever supplied to it by the microcontroller; and

a power supply for powering the electronic components of the TX.

13. The system of claim 12, wherein the TX microcontroller includes data representing the accelerometer signal history in the location message.

14. The system of claim 13, wherein the accelerometer signal history is in the form of a count of the number of times the accelerometer signal exceeded an accelerometer signal level threshold in a prescribed period of time.

15. The system of claim 14, wherein a separate count is included in the location message for each consecutive prescribed period of time occurring since the last previous transmission of the location message.

16. The system of claim 12, wherein the one or more manually-operated function selection switches of the TX comprises a continuous transmission mode switch which is normally in the open, deactivated position, and which when activated by manually closing the switch causes the location message to be transmitted at prescribed intervals.

17. The system of claim 16, wherein said prescribed interval is approximately every 1 second.

18. The system of claim 16, wherein the TX microcontroller monitors the accelerometer signal to determine if the TX is moving through an environment, and precludes the transmission of the location message when the TX is not moving, so as to conserve power and minimize collisions at a receiver with location messages transmitted by other TXs in the environment, unless the continuous transmission mode switch is activated.

19. The system of claim 16, wherein the TX microcontroller monitors the accelerometer signal to determine if the TX is moving through an environment, and powers down the TX location message if the TX has not moved through said environment since said last previous transmission, thereby conserving power and minimizing collisions at a receiver with location messages transmitted by other TXs in the environment.

20. The system of claim 19, wherein the one or more manually-operated function selection switches of the TX comprise:

a power-on switch which when activated by manually closing the switch causes the TX to be powered up; and

a power-off switch which when activated by manually closing the switch causes the TX to be powered down.

21. The system of claim 20, wherein the one or more manually-operated function selection switches of the TX further comprise a motion-activated tilt switch which is connected to the microcontroller and which is normally open during times when the TX is substantially at rest, but closes at least momentarily when the TX is moved, and wherein whenever the TX has been powered down for lack of motion and subsequently moved the tilt switch closes and causes an interrupt signal to be sent to the microcontroller which in turn causes the microcontroller to power up the TX, if the power-on switch is activated.

22. The system of claim 20, wherein the one or more manually-operated function selection switches comprise a send-once switch which is normally in the open, disengaged position, and which when activated by manually closing the switch causes the microcontroller to transmit the location message regardless of when the location message would have been transmitted had the send-once switch not been activated, if the power-on switch is activated.

23. The system of claim 12, wherein the RF transmitter unit of the TX comprises:

an antenna extending therefrom; and

a 433 MHz FM 14400 KBaud low power transmitter having a maximum range of approximately 50 feet.

24. The system of claim 12, wherein the power supply of the TX comprises a battery.

25. The system of claim 24, wherein the location messages transmitted by the TX comprise a battery voltage level which is used to determine if the battery is failing and need of replacement.

26. The system of claim 12, wherein each TX further comprises a personal identification number (PIN) input apparatus with which a user enters a PIN into the TX which is stored by the TX microcontroller.

27. The system of claim 26, wherein the TX microcontroller includes the PIN in the location message, and wherein the PIN is used by the location tracking system to identify the person carrying the TX or the object to which the TX is attached.

28. The system of claim 26, wherein the TX microcontroller monitors the accelerometer signal to determine if the TX is moving through an environment, and whenever it is determined by the microcontroller that the TX has not moving through the environment during a prescribed period of time, the PIN stored by the microprocessor is erased and no location message is transmitted until replacement PIN number is entered into the TX.

29. The system of claim 26, wherein the accelerometer signal comprises separate x-axis and y-axis signals, and wherein the TX 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 by the TX microcontroller that the TX has been place in an unexpected orientation for a prescribed period of time, the PIN stored by the TX microprocessor is erased and no location message is transmitted until replacement PIN number is entered into the TX.

30. The system of claim 12, wherein the TX microcontroller monitors the accelerometer signal to determine if the TX is moving through an environment, and curtails the transmission of location messages by the TX during periods wherein the accelerometer signal indicates that the severity of motion is below a motion threshold that is attributable to movement of the person or object associated with the TX about an environment.

31. The system of claim 30, wherein curtailing the transmission of location messages by the TX, comprises the TX microcontroller:

counting the number of times the accelerometer signal exceeds an accelerometer signal level threshold in a prescribed period of time;

whenever the count does not exceed a prescribed number, deeming that the accelerometer signal indicates the severity of motion is below the motion threshold and foregoing transmission of the location message; and

whenever the count does exceed the prescribed number, deeming that the accelerometer signal indicates the severity of motion exceeds the motion threshold and initiating a transmission of the location message.

32. The system of claim 31, wherein the prescribed period of time is approximately 1 second.

33. The system of claim 31, wherein the prescribed number is two.

34. The system of claim 31, wherein curtailing the transmission of location messages by the TX further comprises, whenever the transmission of the location message is not made because the accelerometer signal indicates the severity of motion is below the motion threshold, the TX microcontroller:

(a) waiting a prescribed timeout period;

(b) counting the number of times the accelerometer signal exceeds the accelerometer signal level threshold in the prescribed period of time;

(c) determining if the count exceeds the prescribed number;

(d) whenever the count does not exceed the prescribed number, determining whether a prescribed shutdown time limit has been exceeded since the last transmission of a location message by the TX;

(e) whenever it is determined that the shutdown time limit has not been exceeded, repeating (a) through (d);

(f) whenever it is determined that the shutdown time limit has been exceeded, powering down the TX; and

(g) whenever the count exceeds the prescribed number, initiating a transmission of the location message.

35. The system of claim 34, wherein the prescribed timeout period is approximately 2 second.

36. The system of claim 34, wherein the shutdown time limit is approximately 1 minute.

37. The system of claim 34, wherein each TX further comprises a motion-activated tilt switch that is open when the TX is substantially at rest, and which whenever the TX is powered down because it was determined that the shutdown time limit had been exceeded, powers on the TX if activated due to motion of the TX.

38. The system of claim 34, wherein the TX microcontroller periodically powers up a TX which was powered down because it was determined that a shutdown time limit had been exceeded, and transmits a location message.

39. The system of claim 38, wherein TX microcontroller periodically powers up a TX, which was powered down because it was determined that a shutdown time limit had been exceeded, once every hour.