Patent Application
20070133598
1. Field of the Invention
The present invention relates to a wireless sensor network and mesh computer having a plurality of elements which communicate with each other. More particularly, it relates to the design and operation of computing nodes within the mesh for determining distances and positions between said nodes.
2. Discussion of Related Art
A wireless sensor mesh network is formed from a densely deployed collection of unreliable distributed sensors and nodes with limited bandwidth, memory and computational capabilities connected in a mesh network layout. A characteristic of a mesh network is that it is constrained by its energy resources, is self-organizing and fault tolerant. The sensor and compute nodes are usually battery powered and are operated in an unattended and inaccessible environment. Over time, as the sensors and compute nodes fail the neighboring nodes will modify their behavior so that the overall system continues to function. New sensor and compute nodes are deployed on top of the partially functional system thereby extending the performance and longevity of the system indefinitely.
To optimize the use of the network resources and to coordinate functionalty between devices, it is desirable to have the mesh nodes understand their spatial relation to each other. Specifically, the distance between nodes must be determined with some degree of accuracy. It is not practicable to physically measure the distances between nodes. Furthermore, as nodes fail and are replaced, the position of a given node with respect to their neighbors will change from time to time. Thus, a process is necessary to automatically determine the distance between nodes.
There are a number of methods that enable the mesh nodes to determine their relative position. One of these methods is to use a Received Signal Strength Indicator (RSSI) which measures the received signal from a known transmitter then, using a computational algorithm and a model of the channel loss versus the distance, compute an estimate of the spacing between the transmitter and receiver. Another method is to use a pulse of very short duration, sometimes referred to as ultrawideband (UWB) signaling, and measure the time of arrival (TOA) of the signal. Given the propagation speed of the signal and the time of arrival, the distance can be computed. Yet another method is to use an out-of-band signaling technique with two or more transmitted signals such as a low frequency radio or acoustic beacon along with a high frequency carrier. There are deficiencies with each of these various methods. In particular, they require complex structures for transmitting, receiving, and processing the necessary signals. Furthermore, such systems have significant limitations in determining distances with accuracy.
Other methods of position detection and location include, the global positioning system (GPS) is a system which uses transmissions from multiple satellites to determine the position of a receiver and LORAN. Additionally, several systems exist for determining position of cellular telephones based upon signals from cell towers. However, each of these systems requires significant computational and processing capability and consume a significant amount of energy during the computation of their location. They are not easily adapted to the desired small size and low power consumption of a mobile sensor in a mesh computer system. They also cannot be adapted to utilize existing antenna and transmissions mechanisms within the sensor. A completely separate receiver system is required for determining the position.
The present invention overcomes many of the deficiencies of the prior art through a system and method for determining distances between two communicating devices. According to one aspect of the invention, a method for determining distance utilizes a transmitter transmitting a plurality of synchronized periodic signals, each signal having a frequency which is a multiple of a base frequency. According to an aspect of the invention, the periodic signals are sinusoidal. A receiver determines a phase of each of the signals at a synchronized time, such as the zero crossing time of sinusoidal waves at the transmitter. The receiver utilizes the phases of the signals to determine a distance from the transmitter. According to another aspect of the invention, the plurality of periodic signals are sent as a time multiplexed signal. According to another aspect of the invention, the plurality of periodic signals are superimposed on an unrelated transmission.
According to another aspect of the invention, the periodic signals are provided through rotation of the components of quadrature signals. The receiver determines the rotational phase of the signal for use in determining the distance. According to another aspect of the invention, the quadrature signal is processed to create the rotation. According to another aspect of the invention, a repeater is used to create the transmission signals. A first device transmits the periodic signals. A second device, the repeater, receives the signals and re-transmits them to the first device. A phase locked loop is used to synchronize the signals in the repeater. The first device processes the signals from the second device to determine the distance between the devices. Since the signals are synchronized in the second device, the first device can determine the zero crossing point to the signal and its phase at the time of zero crossing.
According to another aspect of the invention, a mesh system utilizes the method of the invention to determine the position of all of the devices on the mesh. Two devices (a device and its repeater) can determine absolute distance along a line of transmission. Three devices (two devices and a repeater) can determine distance in a plane. Four devices in the mesh utilize another process for determining exact position in three dimensions. The remaining devices determine their distances from the known positions of the four devices using the method of the present invention. With the known distances, the positions of all devices in the mesh are determined in three dimensional space.
The present invention provides a method for determining a distance between nodes in a network or mesh. The method utilizes sinusoidal radio frequency waves transmitted from one of the nodes to the other node. A single node can transmit the necessary signals as discussed below. Each node in the network, upon receiving the signals can determine its distance from the transmitting node.
The theory of operation of the method is illustrated in
The potential error factor is reduced through the use of additional sinusoidal waves. The succeeding waves 12, 13, 14 are synchronized with the base wave and have frequencies which are multiples of the base frequency. According to a preferred embodiment, the frequency of each succeeding wave is twice that of the preceding wave. The receiving node 21 determines the phase of each of the succeeding waves 12, 13, 14 at the same zero-crossing point 16 for the transmitting node 20. Each phase is determined with a degree of error. As the frequency increases, the error in the phase determination represents a smaller error in the distance determination. By determining the relative phases of the waves at each of the frequencies, the distance between the transmitting node and receiving node can be accurately determined. For example, a base frequency of 100 MHz has a wavelength of approximately 3 meters. With only eight waves, the distance between nodes can be determined within approximately 0.125 meters. Additionally, since the use of successive waves reduces the error in the phase determination, depending upon the number of waves used and the desired precision of determining distances, the phase determination with respect to any particular wave can be imprecise without limiting the invention. Additional precision of the measurement can be obtained by time averaging the rotational phase and interpolating the signal. It may be sufficient to merely determine the zero-crossing points of any wave and determine whether the phase is in the first or second half of the wave.
The present invention is not limited to sinusoidal signals. The method can be used with any set of periodic signals having the frequency relationship described above. A phase of a signal can be determined for any periodic signal. Additionally, since the method of the present invention can accurately determine distances based upon simply the zero crossing points of a sinusoidal wave, it can determine the distance from periodic signals having a single pulse per period.
The method for determining the distance between nodes as set forth above does not depend upon the devices used to transmit and receive the sinusoidal waves. Furthermore, any device can be used within the receiver for determining the phase of the sinusoidal waves. Nevertheless,
The receiver 130 similarly includes a quadrature antenna system 131 and a signal processor 131. At the receiver 130, the signal processor 131 operates to determine the rotational phase of the received signal. First, the signal processor sums the inputs from the in-phase and quadrature antennas. Summing the two inputs eliminates any amplitude modulation in the transmission signal and provides a stronger signal irrespective of the position of the antennas. Using known methods and circuitry, the signal processor 131 determines the rotational phase of the signal. The rotational phase represents the sinusoidal signals 10 used in determining the distance between the devices as discussed above.
The devices illustrated in
As discussed above, the receiver 21 determines the distance from the transmitter 20 based upon the phase of the sinusoidal waves 10 when the transmitter is at a zero-crossing point 16.
The method of the present invention can be used in conjunction with other methods for determining distanced. For example, a Received Signal Strength Indicator (RSSI) method, using known structures, can be used to estimate a distance between a transmitter and receiver. However, an RSSI method decreases in accuracy as the distance between the nodes increase. The method of the present invention can be used for greater accuracy once the distance has been estimated using the RSSI method. Such a combination would permit distances longer than the wavelength of the base frequency to be determined.
Similarly, the method of the present invention can be combined with Ultrawideband (UWB) signaling methods. The present invention utilizes periodic signals transmitted from one device to another. The nature of the signals are irrelevant to the method of the present invention. Thus, UWB signals can be used for transmitting the periodic signals.
Having disclosed at least one embodiment of the present invention, various adaptations, modifications, additions, and improvements will be readily apparent to those of ordinary skill in the art. Such adaptations, modifications, additions and improvements are considered part of the invention which is only limited by the several claims attached hereto.
