Patent Application
20130038358
Wireless sensor nodes may used in a variety of applications. One such application is seismic monitoring for use in various endeavors such as oil exploration, study of plate tectonics, and so on. In these and other uses, a large number of such sensor nodes disposed at different locations over a multiple-kilometer area may measure and collect data on the same event.
In such applications, the sensor nodes are typically positioned where they do not have access to the electrical grid, so their electrical power is self-contained. In addition, it is frequently desired that the sensors collect data at certain intervals over an extended period of time, and store this data in a record for each interval. As such, it is desirable to reduce or minimize the electrical power usage of a node, in order to allow the sensors to operate for a longer time without replacement or replenishment of their power source.
Analyzing the data collected from multiple sensors includes correlating the data collected at a given time by one sensor with the data collected at the same given time by other sensors. To determine the time associated with the collected data, a sensor may include an on-board time resource, such as a global positioning system (GPS) receiver, which is operated as needed to provide a sufficiently accurate time value for all of the data records. The on-board time resource, unfortunately, typically consumes a significant amount of electrical power during operation. This results in increased electrical power usage by the node, which in turn reduces the length of time in which the sensor can operate without replacement or replenishment of its power source.
Analyzing the data collected from multiple sensors includes correlating the data collected at a given time by one sensor with the data collected at the same given time by other sensors. To determine the time associated with the collected data, a sensor may include an on-board time resource, such as a global positioning system (GPS) receiver, which is operated as needed to provide a sufficiently accurate time value for all of the data records. The on-board time resource, unfortunately, typically consumes a significant amount of electrical power during operation. This results in increased electrical power usage by the node, which in turn reduces the length of time in which the sensor can operate without replacement or replenishment of its power source.
Referring now to the drawings, there is illustrated an example of a wireless sensor node in a mesh network. A wireless transceiver in the sensor node receives, from an upstream node in the network, a time value (referred to as “beacon time”, or “hopped time”) which corresponds to global time as sent by the transmitter of the upstream node's wireless transceiver. An undetermined latency delay in communication of the time value through the upstream node's transmitter and the sensor node's receiver causes the beacon time as received at the sensor to be offset from global time by the latency. The sensor node uses a power-consuming resource to determine the global time, and computes the latency using the hopped time and the global time. For values of beacon time subsequently received by the node, the node uses the latency to compute the global time corresponding to the beacon time, without using the power-consuming resource. For each beacon time, a timestamp including the global time and the corresponding local time provided by an on-board clock is recorded. Power is conserved at the sensor node by determining global time for the subsequent receptions of beacon times without using the power-consuming resource. The resulting reduction or minimization in power usage advantageously allows the sensor node to operate for a longer period of time without replacement or replenishment of its power source.
Considering now a schematic view of a system that is illustrative and non-limiting with respect to the present teachings, a system 100 includes a plurality of wireless seismic sensor nodes 102, such as sensor nodes 102A through 102D and 102N. The respective nodes 102 are distributed over a land surface 104 such that a sensor array 106 is defined. Each seismic sensor node 102 is configured to sense or detect incident seismic energy 108 and to digitally quantify corresponding seismic data. In one example, each node 102 samples the incident seismic energy 108 at a rate of five-hundred times per second. Other digital sampling rates can also be used. Each node 102 includes an onboard local clock configured to provide a local clock time value. Each node 102 is further configured to store the seismic data and respective timestamps on an ongoing basis.
The system 100 may also include a source of artificial seismic energy 110. In some examples, the source 110 may be embodied as a truck or and vehicle having electro-mechanical resources configured to produce an outgoing seismic stimulus 112. Generally and without limitation, the outgoing seismic stimulus 112 is reflected and refracted by way of various subterranean strata and features 114, 116 and 118. The reflected and/or refracted energy results in seismic energy 108 incident to the respective nodes 102. The precise definition or constituency of such features 114-118 not germane to an understanding of the present teachings, and further elaboration is not needed herein. The seismic data associated with the seismic energy 108 sensed by the set of nodes in the sensor array 106 in response to the seismic stimulus 112 is typically post-processed at a data center. The timestamps are used during post-processing to correlate or cross-correlate the seismic data among the various nodes 102. The seismic data may also be correlated to the global time. The accuracy with which the seismic data can be correlated is dependent in part upon the accuracy of the time values contained in the timestamps.
The system 100 further includes a global positioning system (GPS) satellite 120. The satellite 120 is illustrative of any of a plurality of such GPS satellites in Earth orbit. The satellite 120 provides global time values by way of wireless signaling 122. As defined herein and in the appended claims, the term “global time” shall be broadly understood to mean a time value provided by, or traceable to, a national or international standards entity such as an atomic clock or other resource. Some or all of the seismic sensor nodes 102 may be configured to directly receive global time by way of the wireless signaling 122. The corresponding nodes 102 perform such time value reception on an intermittent basis, such as periodically, in response to a predefined event, or in accordance with another suitable scheme. Reception of global time at a node 102 consumes a significant amount of power, so in order to conserve power each seismic sensor node 102 is configured to acquire global time by way of the signaling 122 sparingly, a few times per day for example.
The sensor array 106 may, in some examples, take the form of a wireless mesh network of the sensor nodes 102. As defined herein and in the appended claims, the term “mesh network” shall be broadly understood to mean a network topology in which nodes serve to relay (or “hop”) data to other nodes along defined paths. The paths may be defined as the network is brought up, and may be re-defined as individual nodes join the network (e.g. by powering up), or leave the network (e.g. by powering down).
A particular node 102 in a wireless mesh network may be configured to receive a time value from an upstream node 102 in its defined path via a wireless signaling mechanism 126. For example, each node 102 may include a wireless transceiver that operates according to the IEEE 802.11 communication standard for implementing wireless local area networks in order to implement the wireless signaling mechanism 126. The time value transmitted from one node to another via the wireless signaling mechanism 126 is referred to herein as beacon time. As defined herein and in the appended claims, the term “beacon time” shall be broadly understood to mean a time value received via the wireless signaling mechanism 126 at a node in the mesh network from an upstream node in the mesh network. The beacon time may be transmitted from one node to another via the wireless signaling mechanism 126 in the same manner as any other data, or through a special-purpose time data mechanism.
Consider an example path in the mesh network in which data, such as a time value, is hopped from node 102A to node 102B, then from node 102B to node 102C, and finally from node 102C to node 102D. The wireless transceiver of node 102A has a wireless communications range 130 that allows it to communicate with node 102B and node 102N. However, the range of node 102A is insufficient to wirelessly communicate directly with nodes 102C and 102D. Similarly, node 102B has the range to wirelessly communicate with nodes 102A, 102C, and node 102C has the range to wirelessly communicate with nodes 102B, 102D. This arrangement allows data to be relayed from node 102A to node 102D in a total of three hops. Moreover, it advantageously allows the use in each node 102 of a wireless transceiver that consumes less power, thus conserving the power source of the node 102.
In preparation for transmitting, the upstream node typically sets the beacon time to a value that the upstream node considers to be global time. However, a time latency or delay occurs during the transmission process that in turn causes the beacon time received at the receiving node to be different from global time. The majority of the latency is due to the time it takes to send the data through the transmitter of the upstream node and to receive the data through the receiver of the receiving node. For a given pair of nodes 102, the latency typically is relatively constant from transmission to transmission. However, a change in the topology of the network, and particularly a change in the identity of the upstream node, can change the latency.
Consider now, and with reference to
Consider now, with reference to
The node 300 includes power handling circuitry 326 configured to receive electrical energy from a power source 324, such as for example a storage battery, and to provide conditioned or regulated power to the various resources and circuits of the node 300. Non-limiting examples of operations performed by the power handling 326 include voltage regulation, current limiting, and so on.
The node 300 includes a controller 302. The controller 302 is configured to control various operations of the node 300. The controller 302 can be defined by or include any suitable resources such as, without limitation, a microprocessor, a microcontroller, a state machine, an application specific integrated circuit (ASIC), digital or analog or hybrid circuitry, and so on. In one example, the controller 302 is configured to operate in accordance with a computer-readable program code.
The node 300 also includes a seismic sensor 304 configured to provide an electronic signal 348 corresponding to seismic energy 306 incident thereto. The electronic signal 348 is coupled from the sensor 304 to the controller 302. In turn, the controller 302 is configured to digitally quantify the signals 348 and to store the resulting seismic data 332 within a data storage media 308. The storage media 308 can include any suitable computer-accessible data storage such as non-volatile memory, solid-state memory, magnetic storage media, optical storage media, and so on. Other types of storage media 308 can also be used.
The seismic sensor node 300 also includes a wireless receiver, typically a global positioning system (GPS) receiver 314, configured to receive global time values by way of wireless signals 316. Wireless signals 316 may be substantially equivalent to wireless signals 122. The GPS receiver 314 consumes a significant amount of electrical power during operation. The seismic sensor node 300 also includes a wireless transceiver 318 configured to perform bidirectional communication, over the mesh network, between the node 300 and various external entities by way of wireless signals 320. Wireless signals 320 may be substantially equivalent to wireless signals 126. Over a given time period, acquiring time information via the wireless transceiver 318 typically consumes significantly less power than acquiring time information via the GPS receiver 314. The GPS receiver 314 typically takes between several seconds and almost one minute to power up and acquire time information, while the wireless transceiver 318 can power up, receive a timing beacon, and retransmit the beacon much more quickly. Thus, even in configurations where the wireless transceiver 318 is used to acquire time beacons more frequently than the GPS receiver 314 would be used to acquire time information, the total amount of power consumed by the wireless transceiver 318 would be significantly less than that consumed by the GPS receiver 314.
The node 300 further includes a local clock 310. The local clock is configured to provide a local time value to the controller 302. The local clock 310 can be specifically defined by any suitable electronic circuitry, a dedicated purpose integrated circuit, and so on. The local clock 310 is also configured to be reset or resynchronized from time to time by way of the controller 302. Other suitable clocks can also be used. The local clock 310 of one node 300 is typically not synchronized to the local clock 310 of another node 300. The clock is typically driven by an oscillator 312. In one example, the oscillator 312 may be a low jitter temperature-compensated crystal oscillator. However, the clock 310 is typically not synchronized to global time, and may exhibit rate error relative to global time. As defined herein and in the appended claims, the term “rate error” shall be broadly understood to mean how much faster or slower the local clock is running compared to global time, often represented in parts per million. The output frequency of oscillator 312 typically varies with temperature and other factors, and is therefore not constant.
To compensate for this deviation in the rate error of the local clock 310 within a node 300, the node 300 intermittently receives global time (or a signal from which global time can be derived) from an external source, and then generates a timestamp that includes the global time and the corresponding local time. In one example, this may occur approximately every 10 to 20 seconds. The controller 302 is configured to receive or acquire GPS time values 342 by way of the GPS receiver 314. However, the GPS receiver 314 consumes a substantial amount of power, and operating the GPS receiver 314 to receive global time this frequently would undesirably shorten the time to replenishment or replacement of the power source 324. In order to conserve power by not operating the GPS receiver 314, the controller 302 acquires beacon time values 344 by way of the wireless transceiver 318 more frequently than via the GPS receiver 314. The controller 302 determines the latency 336 of beacon time transmission through the transmitter of the upstream node, the air, and the receiver of the receiving node. Then, using the determined latency 336, each beacon time 344 is converted to the corresponding global time.
The controller 302 is further configured to generate, for each of the beacon times 344, a timestamp 334 that includes the computed global time corresponding to the beacon time 344 and a local time 346 provided by the local clock 310. Such timestamps 334 are stored to the storage media 308 by the controller 302.
A set of the wireless sensor node 300 can be distributed over a and surface area such that a sensing array is defined. Each sensor node (e.g., node 300) operates autonomously and without hardwired connection to a central data acquisition hub. Each sensor node senses incident seismic energy 306 and stores, on the storage media 308, seismic data 332 that includes the corresponding local time. Seismic data measurement and storage is typically performed at regular intervals. The storage of the seismic data 332 and the storage of the timestamps 334 is performed asynchronously. That is, the storage of each seismic data 332 is not necessarily accompanied by, or contemporaneous with, the storage of a corresponding timestamp 334.
The stored seismic data 332 and timestamps 334 may later be retrieved from the storage media 308 and communicated to an entity distinct from the seismic sensor node 300 by wired, wireless, or other signaling. The onboard storage media 308 within the node 300 can then be erased, written over, or otherwise reused. In this way, a vast array of wireless seismic sensor nodes 300 can be deployed for field operation without having interconnecting wiring for electrical power, data acquisition or time clock synchronization.
Considering beacon time in greater detail, and with reference to
In one example, the beacon time 344 associated with each timing beacon 410 is implemented via the time synchronization function (TSF) of an 802.11 wireless transceiver, such as wireless transceiver 318 of node 300. In the sending node, the TSF counter may be set to the global time of the sending node, and thus the TSF counter becomes the beacon time. Alternatively, a global time offset value may be calculated that relates the TSF counter to the global time of the sending node, and thus the beacon time is equivalent to the TSF counter plus the global time offset value. In other examples, the beacon time 344 may be implemented using a function or a type of data frame other than TSF.
After each beacon 410A-D is received, the controller 302 of node 300 uses the previously determined value of the latency 336 to convert the received beacon time 344 of each timing beacon 410 to global time, and records a new timestamp 334 of the global time and the local time 346.
The controller 302 of node 300 may, on occasion, decide to recompute the latency 336. The decision may be based on the length of time since the last latency recomputation, or the occurrence of some event in the node 300 or in the mesh network. In the example beacon transmission sequence of
Consider now, and with reference to
Latency=Global Time−Beacon Time+Delta Time
At 516, the global time corresponding to at least some of the beacon times is computed using the latency. However, the power-consuming resource is not used. At 518, for each beacon time, the global time and a local time corresponding to that beacon time are recorded. In some examples, at 520, the global time is transmitted from the sensor node to a downstream node in the mesh. In some examples, at 522, seismic data is measured using a sensor in the node, and the seismic data is recorded together with the local clock value that corresponds to the seismic data measurement.
In some examples, determining the global time using the power consuming resource may be repeated, at 506, after multiple beacon times have been wirelessly received. In other examples, determining the global time using the power-consuming resource may be repeated, at 508, after the upstream node in the mesh is replaced by a different upstream node.
It is noted that time information is transmitted through the mesh in one direction: from an upstream node to a receiving node. No time information is transmitted from the receiving node back to the upstream node. Furthermore, the local clocks of the upstream node and the receiving node are not time-synchronized to each other.
Some examples of the wireless sensor node 300 of
Considering now an example of the operation of the method 500 in a mesh network, and with reference to
Consider now, and with reference to
Method 700 begins at 702 by determining whether the latency of time beacon transmission from an upstream node to the receiving node is to be computed or recomputed. If so (“Yes” branch of 702), then at 704 a GPS receiver in the receiving node is enabled. As has been discussed heretofore, the GPS receiver consumes a considerable amount of power during operation, and it is advantageous to minimize the amount of time it is in operation. At 706, it is determined whether a GPS pulse per second (PPS) signal generated by the GPS receiver has been received. This signal is typically generated once every second by the GPS receiver. If not, (“No” branch of 706), operation continues at 712. If the PPS signal has been received, then at 708 the GPS time corresponding to the PPS signal, referred to as TGPS0, is obtained from the GPS receiver, and the current time of the node's local clock is stored as T0. At 712, it is determined whether a timing beacon has been received from the upstream node by the wireless transceiver in the node. If not, (“No” branch of 712), then execution returns to 706. In environments where the timing beacon is received much less frequently than the PPS signal, such as once every 10 to 20 seconds, returning to 706 will obtain the GPS time corresponding to the next PPS such that, when the timing beacon is received, it will be within one second of the PPS signal.
If the timing beacon has been received (“Yes” branch of 712), then at 714 the beacon time corresponding to the timing beacon, referred to as TBEACON, is obtained from the wireless transceiver, and the current time of the node's local clock is stored as T1. At 722, since both the GPS time and the beacon time have been obtained, the GPS receiver is disabled to conserve node power.
At 744, TDELTA is calculated according to the formula:
TDELTA=T1−T0
At 752, the latency of time beacon transmission from an upstream node to the receiving node is computed according to the formula:
LATENCY=TGPS0−TBEACON−TDELTA
At 754, the global time corresponding to the beacon time is calculated according to the formula:
TGLOBAL=TBEACON+LATENCY
At 756, a timestamp corresponding to the timing beacon is recorded. The timestamp includes the global time, TGLOBAL, and the local clock at the time the timing beacon was received, T1. The timestamp is typically stored as a record in a timestamp database in the node such as, for example, timestamps 334 of node 300.
At 758, it is determined whether the node that has received the timing beacon from the upstream node has been configured to transmit global time to a downstream node in the mesh network. If so, and if it is to do so at the present time (“Yes” branch of 758), then at 760 a timing beacon is configured to transmit a timing beacon to the downstream node, using TGLOBAL as the time value for TBEACON. Following this, or if the node does not transmit global time to a downstream node at the present time (“No” branch of 758), the method returns to 702.
If, at 702, the latency is not to be computed or recomputed (“No” branch of 702), then at 732 it is determined whether a timing beacon has been received from the upstream node by the wireless transceiver in the node. If not (“No” branch of 732), the method returns to 702. If the timing beacon has been received (“Yes” branch of 732), then at 736 the beacon time corresponding to the timing beacon, referred to as TBEACON, is obtained from the wireless transceiver, the current time of the node's local clock is stored as T1, and the method branches to 754 to compute TGLOBAL using the previously-determined value of latency.
Considering now the operation of the method 700, and with reference to
Event 910B indicates reception of a timing beacon by the wireless transceiver of the node as detected at step 712. The value of the beacon time obtained at step 714 is 1:00:03.00. The value of the node's local clock at time T1 (TLOCAL1), obtained at step 714, is 7:30:55.674.
These example values for TGPS0, TLOCAL0, TBEACON, and TLOCAL1 are used in subsequent calculations. TDELTA, computed using the formula of 744, is 0.674. LATENCY, computed using the formula of 752, is 2.674. Finally, the global time TGLOBAL corresponding to timing beacon 910B, computed using the formula of 754, is 1:00:05.674.
Consider now, and with reference to
Relative to the method of
Method 800 begins at 802 by determining whether the latency of time beacon transmission from an upstream node to the receiving node is to be computed or recomputed. If so (“Yes” branch of 802), then at 804 a GPS receiver in the receiving node is enabled. As has been discussed heretofore, the GPS receiver consumes a considerable amount of power during operation, and it is advantageous to minimize the amount of time it is in operation. At 806, it is determined whether a GPS pulse per second (PPS) signal generated by the GPS receiver has been received. This signal is typically generated once every second by the GPS receiver. If not, (“No” branch of 806), operation continues at 812. If the PPS signal has been received, then at 808 the GPS time corresponding to the PPS signal, referred to as TGPS0, is obtained from the GPS receiver, and the current time of the node's local clock is stored as T0. At 812, it is determined whether a timing beacon has been received from the upstream node by the wireless transceiver in the node. If not, (“No” branch of 812), then execution returns to 806. In environments where the timing beacon is received much less frequently than the PPS signal, such as once every 10 to 20 seconds, returning to 806 will obtain the GPS time corresponding to the next PPS such that, when the timing beacon is received, it will be within one second of the PPS signal.
If the timing beacon has been received (“Yes” branch of 812), then at 814 the beacon time corresponding to the timing beacon, referred to as TBEACON, is obtained from the wireless transceiver, and the current time of the node's local clock is stored as T1. At 818, the method waits until another PPS signal is received. At 820, once the PPS signal has been received, the GPS time corresponding to this additional PPS signal, referred to as TGPS2, is obtained from the GPS receiver, and the current time of the node's local clock is stored as T2. At 822, since the two GPS times and the beacon time have all been obtained, the GPS receiver is disabled to conserve node power.
At 842, the correction factor R for the rate error of the local dock is calculated according to the formula:
R=(TGSP2−TGSP0)/(T2−T0)
At 844, TDELTA is calculated according to the formula:
TDELTA=(T1−T0)*R
At 852, the latency of time beacon transmission from an upstream node to the receiving node is computed according to the formula:
LATENCY=TGPS0−TBEACON+TDELTA
At 854, the global time corresponding to the beacon time is calculated according to the formula:
TGLOBAL=TBEACON+LATENCY
At 856, a timestamp corresponding to the timing beacon is recorded. The timestamp includes the global time, TGLOBAL, and the local dock at the time the timing beacon was received, T1. The timestamp is typically stored as a record in a timestamp database in the node such as, for example, timestamps 334 of node 300.
At 858, it is determined whether the node that has received the timing beacon from the upstream node has been configured to transmit global time to a downstream node in the mesh network. If so, and if it is to do so at the present time (“Yes” branch of 858), then at 860 a timing beacon is configured to transmit a timing beacon to the downstream node, using TGLOBAL as the time value for TBEACON. Following this, or if the node does not transmit global time to a downstream node at the present time (“No” branch of 858), the method returns to 802.
If, at 802, the latency is not to be computed or recomputed (“No” branch of 802), then at 832 it is determined whether a timing beacon has been received from the upstream node by the wireless transceiver in the node. If not (“No” branch of 832), the method returns to 802. If the timing beacon has been received (“Yes” branch of 832), then at 836 the beacon time corresponding to the timing beacon, referred to as TBEACON, is obtained from the wireless transceiver, the current time of the node's local clock is stored as T1, and the method branches to 854 to compute TGLOBAL using the previously-determined value of latency.
Considering now the operation of the method 800, and with reference to
Event 924 indicates generation of the second, additional PPS signal by the GPS receiver of the node as detected at step 820. The value of the GPS time corresponding to the second PPS signal obtained at step 820 (TGPS2) is 1:00:06.000; the PPS signals are one second apart and highly accurate. The value of the node's local clock at time T2 (TLOCAL2), obtained at step 820, is 7:30:56.010. Thus it can be observed that the local clock of the node measures the time interval between the two PPS signals 922, 924, as 1.010 seconds, slightly greater than the correct time interval of 1.000 seconds. In other words, the local clock is running slightly fast.
These example values for TGPS0, TLOCAL0, TBEACON, TLOCAL1, TGPS2, and TLOCAL2 are used in subsequent calculations. The correction factor R for the rate error of the local clock, computed using the formula of 842, is 0.990. TDELTA, computed using the formula of 844, is 0.667. LATENCY, computed using the formula of 852, is 2.667. Finally, the global time TGLOBAL corresponding to timing beacon 910B, computed using the formula of 854, is 1:00:05.667.
From the foregoing it will be appreciated that the wireless sensor node and methods provided by the present disclosure represent a significant advance in the art. Although several specific examples have been described and illustrated, the disclosure is not limited to the specific methods, forms, or arrangements of parts so described and illustrated. This description should be understood to include all novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. The foregoing examples are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application. Unless otherwise specified, steps of a method claim need not be performed in the order specified. The disclosure is not limited to the above-described implementations, but instead is defined by the appended claims in light of their full scope of equivalents. Where the claims recite “a” or “a first” element of the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements.
