Communications – electrical: acoustic wave systems and devices – Distance or direction finding – With time interval measuring means
Reexamination Certificate
1999-12-30
2001-11-13
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Distance or direction finding
With time interval measuring means
C367S129000
Reexamination Certificate
active
06317386
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention concerns ultrasonic location systems.
FIELD OF INVENTION
An ultrasonic location system has been proposed for indoor environments. The system is able to determine the positions and orientations of objects with a high degree of accuracy by measuring the times-of-flight of ultrasonic pulses emitted by transmitters placed on the objects to be tracked, and detected by receivers placed at known points around the room in which they are located. Since the speed of sound in air can be found from measurement of the air temperature in the room, distances from a transmitter to the receivers can be determined, and a process called trilateration is used to calculate the 3D position of the transmitter (and hence the object) from the receiver positions and the computed distances.
The transmitter and receiver must by synchronised, so that pulse times-of-flight can be measured accurately. To achieve this synchronisation, each ultrasonic transmitter unit also has a unique address and a bidirectional radio link to a central controller. The central controller periodically polls each ultrasonic transmitter by sending an addressing message across the radio link, causing the addressed transmitter to emit an ultrasonic pulse. Simultaneously, the central controller resets the time-of-flight counters on the receiver units via a wired network. Since the time-of-flight of the radio signal is insignificant, the ultrasonic pulse will be emitted at practically the same moment that the time-of-flight counters are reset.
In a preferred system the ultrasonic signal currently used for distance measurements is a single, short (<1 ms) pulse, and identification of the source of each ultrasonic pulse is achieved by ensuring that only one transmitter will be triggered to emit an ultrasonic signal within a given period of time, called a timeslot. Where a system includes a central controller whose radio transmissions extend over several rooms, each with its own receiver matrix and containing some of the transmitters, ultrasonic signals can be correctly ascribed to transmitters (which can move between the rooms) by triggering only one of the transmitters known to be located in each of the rooms in each timeslot.
The length of time selected for each timeslot, is the time required for the reverberation associated with an ultrasonic pulse to die down within a room within which the measurements are to be taken. Typically this period is of the order of 20 ms. Each timeslot starts when a transmitter is triggered. During each timeslot, only the pulse from that transmitter can reach receivers in a room, and since the central controller knows which transmitter was triggered, the pulse information gathered by receivers can be ascribed to the correct transmitter. However with a timeslot length of 20 ms, such a system is limited to a maximum of 50 transmitter addresses, and therefore position updates per second.
It is an object of the present invention to provide an improved location system in which information transmitted in the ultrasonic channel can be used to identify the transmitter which has emitted an ultrasound pulse received by one or more receivers.
It is a further object to provide an enhanced transmitter identification system in which a greater number of transmitters in an environment can be triggered and monitored per second, than in systems proposed to date.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, the ultrasound signal emitted by each transmitter when triggered, is encoded with a unique identity and the receivers are provided with decoding means to determine the identity of the transmitter from which each ultrasound pulse has been received.
In one embodiment of this aspect of the invention each ultrasound signal emitted by a transmitter has added thereto identification data which can be decoded by the receivers.
However, efficient ultrasonic transducers have limited bandwidth, and data can therefore only be sent from the transmitters at a relatively low rate. Additionally the large number of reflections caused by flat surfaces in indoor environments (such as walls, furniture, etc.) can result in frequent inter-symbol interference in any ultrasonic data stream received by the receivers.
Furthermore in a system involving a large number of transmitters, the identification address space would have to be large (perhaps 48 bits or more), and the time taken to transmit each address over the ultrasonic channel could therefore be significant. Thus while fill identification is possible in such a system, the rate at which distance-measurement pulses can be emitted by the transmitters where a large number of transmitters is involved, can become reduced, capping the rate at which they can be located and monitored by such a system.
By reducing the identification data to be transmitted so as to identify the source of an ultrasonic transmission, using information within the transmitted signal, the smaller amount of transmitter address information can be transmitted via the ultrasonic channel without so significantly affecting the rate at which the ultrasonic transmitters can be triggered. However by reducing the identification data, it may not reliably identify the source.
According to another aspect of the present invention and so as to overcome the disadvantage linked to the reduced ID data approach, in an ultrasonic location system in which there is an ultrasonic transmission channel, address information to be transmitted using ultrasound is encoded so as to enable a plurality of transmitters to be permitted to emit ultrasound pulses during the same timeslot to permit a corresponding plurality of different transmitter locations to be determined within that timeslot.
In accordance with a preferred feature of the invention therefore, at the beginning of each new timeslot, N different transmitter addresses are selected by the controller and transmitted so as to enable each of the N transmitters to ascertain which of N different encoding techniques is to be used by it when transmitting its ultrasound signal in the timeslot.
The encoding may for example be achieved by pulse shaping or pulse position modulation. However other encoding schemes may be employed.
In one embodiment up to four bits of addressing information may be encoded within an ultrasonic pulse. In such a system each timeslot can be used by 16 transmitters, each of which can emit a differently encoded ultrasonic signal.
In a location system as aforesaid which includes a central controller, the latter may be provided with information as to which transmitter is using a particular encoding and receivers can uniquely ascribe incoming pulses to different transmitters accordingly.
The location rate of such a system can therefore be increased sixteen fold to a system in which unencoded ultrasound pulses are used. Thus in the example given, where the previous maximum number of updates was 50, the system could now address 16×50=800 transmitters per second, and likewise 800 updates per second.
If encoding using pulse shaping is employed, each receiver may be adapted to identify pulse shape to enable each transmitted pulse to be identified as to the originating transmitter.
According to a preferred arrangement, in a system in which there are more than 16 transmitters to be triggered, and which incorporates a central controller, the latter can be adapted to command different groups of transmitters to use different particular pulse encoding techniques during each timeslot. This is most simply achieved by incorporating pulse encoding information in an addressing message sent to all transmitters, so that each transmitter is allocated a particular pulse encoding technique by the central controller at the beginning of each timeslot.
In such a system in which there are 16 different pulse encoding possibilities, the radio message sent at the start of each timeslot will typically contain the addresses of 16 of the transmitters, and in the transmission each address is allocated one of the 16 pulse e
AT&T Laboratories-Cambridge Limited
Lee, Mann, Smith McWilliams, Sweeney and Ohlson
Lobo Ian J.
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