Abstract:
A commodity delivery system. The system includes a plurality of commodity delivery devices. The plurality of commodity delivery devices are configured to at least one of transport the commodity, modify the commodity, and monitor the commodity. A subset of the plurality of commodity delivery devices is further configured to receive a beacon from a network device, designate a plurality of transmission time slots based on the beacon, the transmission time slots synchronized for all of the plurality of commodity delivery devices, detect an error condition, select a transmission time slot following the detection of the error condition, and transmit a last gasp message during the selected transmission time slot.

Description:
BACKGROUND 
       [0001]    The invention relates to power outage reporting (last gasp) by devices on a commodity delivery network. Specifically, the invention relates to slotting of last gasp communication messages to reduce collisions between communications from multiple devices. 
         [0002]    Many modern commodity delivery networks (e.g., electric, gas, etc.) include smart devices along the delivery pathway. These smart devices monitor various aspects of the commodity (e.g., usage) and include communication capabilities to report back to the commodity provider. 
       SUMMARY 
       [0003]    In one embodiment, the invention provides a commodity delivery system. The system includes a plurality of commodity delivery devices. The plurality of commodity delivery devices are configured to at least one of transport the commodity, modify the commodity, and monitor the commodity. A subset of the plurality of commodity delivery devices is further configured to receive a beacon from a network device, designate a plurality of transmission time slots based on the beacon, the transmission time slots synchronized for all of the plurality of commodity delivery devices, detect an error condition, select a transmission time slot following the detection of the error condition, and transmit a last gasp message during the selected transmission time slot. 
         [0004]    In another embodiment the invention provides a method of informing a commodity provider of an interruption in the delivery of the commodity. The method includes receiving a beacon by a first commodity delivery device, designating, by the first commodity delivery device, a plurality of transmission time slots based on the beacon, detecting an interruption in delivery of the commodity, selecting a transmission time slot following the detection of the interruption in the delivery of the commodity, and transmitting a last gasp message during the selected transmission time slot. 
         [0005]    In another embodiment the invention provides a commodity delivery device. The device includes a communication circuit, a commodity monitoring circuit, and a controller. The communication circuit is configured to transmit and receive messages. The commodity monitoring circuit is configured to detect an interruption in delivery of the commodity. The controller is configured to receive an indication of the interruption in delivery of the commodity from the commodity monitoring circuit, detect a beacon received by the communication circuit, designate a plurality of transmission time slots based on the beacon, randomly select one of the plurality of transmission time slots, and transmit via the communication circuit a last gasp message in the selected one of the plurality of transmission time slots when the indication of the interruption in delivery of the commodity is received 
         [0006]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a diagram of a commodity delivery network. 
           [0008]      FIG. 2  is a diagram of a communication network overlaid on the commodity delivery network of  FIG. 1 . 
           [0009]      FIG. 3  is a time line of a prior art last gasp communication scheme. 
           [0010]      FIG. 4  is flow chart of an exemplary process for communicating last gasp messages that reduces the probability of a collision between communications from multiple commodity delivery devices. 
           [0011]      FIG. 5  is a time line of last gasp communication messages transmitted using the process of  FIG. 4 . 
           [0012]      FIG. 6  is a block diagram of a commodity delivery device for executing the process of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0014]      FIG. 1  shows an exemplary commodity delivery network  100 . The commodity delivery network  100  in this example is for delivering electricity to end-users (e.g., homes, businesses, etc.). The delivery network  100  includes a power generation plant  105 , a plurality of substations  110  which receive high-voltage power from the power generation plant  105 , a plurality of transformers  115 , a plurality of feeders or laterals  120 , and a plurality of end-devices  125  (e.g., a smart electric meter). The power generation plant  105  generates electricity and provides high-voltage power to the substations  110 . The substations  110  step down the voltage they receive from the power generation plant  105 , and supply the stepped-down voltage to the transformers  115 . The transformers  115  again step down the voltage and supply this voltage, via the feeders/laterals  120 , to end-users (e.g., residential buildings) through the end-devices  125 . The end-devices  125  monitor, and in some cases control, usage of the commodity by the end-users. 
         [0015]      FIG. 2  shows an exemplary construction of the communication network  200  which overlays the delivery network  100 . The communication network  200  enables communication between the utility provider (e.g., the power generation plant  105 ) and the components (e.g., the plurality of substations  110 , the plurality of transformers  115 , the plurality of feeders or laterals  120 , and the plurality of end-devices  125 ) of the delivery network  100 . The communication network  200  includes a wide-area-network (WAN)  205  via which a back office  210  of the utility provider communicates with a local-area-network (LAN). The LAN includes a plurality of gateways  215 , a plurality of relays  220 , and a plurality of end-devices  125 ′. The components of the delivery network  100  can perform one or more of the communication network  200  functions (e.g., the gateway  215 , the relay  220 , and the end-device  125 ′ functions), or they can perform none of the communication network  200  functions. In addition, one or more of the gateways  215  and/or relays  220  can be communication network only devices. That is, they perform communication network functions but do not perform any commodity delivery functions. 
         [0016]    U.S. Pat. No. 7,853,417 filed on May 17, 2007, and entitled “Methods and System for Utility Network Outage Detection,” the entire content of which is hereby incorporated by reference, describes the operation of the communication network  200  in greater detail. In addition, the invention can be practiced using other communication network constructions. For example, U.S. patent application Ser. No. 12/510,168 filed on Jul. 27, 2009, and entitled “Meshed Networking of Access Points in a Utility Network,” and U.S. patent application Ser. No. 12/167,592 filed on Jul. 3, 2008, and entitled “Network Utilities in Wireless Mesh Communications Networks,” the entire content of each is hereby incorporated by reference, disclose mesh communication networks for use in utility distribution systems. 
         [0017]    When power to an end-device  125  (or other distribution device) is interrupted (e.g., due to a power outage or a downed power line supplying the end-device  125 ), the end-device  125  quickly detects the interruption. In order to assist the utility provider in determining an extent of a power outage, the end-device  125  sends out a “last gasp” message indicating that it has lost power. Some end-devices  125  have an internal power source (e.g., a battery backup); however, the majority of end-devices  125  do not have an internal power source. The end-devices  125  without an internal power source maintain (e.g., via a capacitor) operational power for only a short period of time (e.g., several hundred milliseconds) following the power interruption. Accordingly, the end-devices  125  have only a short window of opportunity to transmit the last gasp message before their operational power is too low to function. 
         [0018]    In the event of a power outage affecting more than a single end-device  125  (e.g., a downed power line, a blown transformer  115 , a substation  110  failure, etc.), hundreds or thousands of end-devices  125  (or transmission devices such as transformers  115  and laterals  120 ) detect the outage nearly simultaneously. The end-devices  125  attempt to send last gasp messages to inform the utility provider of the outage. Last gasp messages are very short (e.g., only containing information identifying the sending device  125  and that the message is a last gasp), taking only a couple of milliseconds to transmit. However, when a large number of devices  125  detect the outage at virtually the same time, and send their last gasp messages at virtually the same time, messages from multiple devices “collide” causing the messages to be lost. Therefore, prior art networks have used a slotting method of communication to improve the number of messages that do not collide with other messages and get through to a self-powered relay  210  or gateway  215 . 
         [0019]      FIG. 3  shows a time line  300  demonstrating a prior art slotting method for an end-device  125 ″. At time  305  the end-device  125 ″ detects a power interruption. The device  125 ″ then designates a plurality of time slots  310  beginning at time  315  with the last time slot  320  occurring prior to the time at which the device  125 ″ will no longer maintain power to transmit its last gasp message. Each time slot  310  has a duration T s  which is essentially equivalent, but slightly longer, than the time needed to transmit the last gasp message. The device  125 ″ then randomly chooses one of the time slots  322 , and transmits its last gasp message during the time slot  322 . 
         [0020]      FIG. 3  also shows a time line  300 ′ demonstrating the prior art slotting method for a second end-device  125 ′″. At time  325  the end-device  125 ′″ detects a power interruption. The device  125 ′ then designates a plurality of time slots  330  beginning at time  335  with the last time slot  340  occurring prior to the time at which the device  125 ′″ will no longer maintain power to transmit its last gasp message. Each time slot  330  has a duration T s  which is essentially equivalent, but slightly longer, than the time needed to transmit the last gasp message. The device  125 ′″ then randomly chooses one of the time slots  345 , and transmits its last gasp message during the time slot  345 . 
         [0021]    By slotting the last gasp transmissions, the number of collisions and lost messages are greatly reduced. However, as shown in  FIG. 3 , because the end-devices  125 ″ and  125 ′″ detect the outage at different points in time,  305  vs.  325 , the plurality of slots  310  designated by device  125 ″ overlap  350  the plurality of slots  330  designated by the other device  125 ′″. Therefore, even though two devices  125 ″ and  125 ′″ choose different slots  322  and  345 , the overlap  350  causes both of their last gasp transmissions to be lost. The chance (probability) of a collision using this slotting method has been shown to be equal to e −2G  (i.e., ALOHA protocol). Where G equals the load on a given channel. 
         [0022]      FIG. 4  shows a process by which the probability of a collision is significantly reduced through synchronization of time slots using a beacon signal. On power up, the end-device  125  waits to receive a beacon signal (step  400 ). The gateway  215  transmits the beacon signal which is received by a plurality of end-devices geographically near the gateway  215 . In some embodiments, the beacon signal is cyclically transmitted following a predetermined time period (e.g., once every five seconds). In other embodiments, the beacon signal is transmitted during periods when there is no other communication taking place. In some embodiments, the beacon signal is generated by a device external to the communication network  200  (e.g., an atomic clock). 
         [0023]    Once the end-device  125  receives the beacon signal, the end-device  125  generates a plurality of time slots starting at a predetermined time following receipt of the beacon signal (step  405 ). Because each end-device  125  receives the beacon signal at the same time, all of the end-devices  125  generate essentially the same time slots. Should an error (e.g., a power outage) occur (step  410 ), the end-device  125  randomly selects a time slot (step  415 ) and transmits a last gasp message (step  420 ) before losing power (step  425 ). If an error did not occur (step  410 ), the end-device  125  checks for another beacon signal (step  430 ). If a new beacon signal is received, the end-device continues by generating a new plurality of time slots (step  405 ). If a new beacon signal is not received, the end-device again checks for an error (step  410 ). 
         [0024]      FIG. 5  shows time lines  500  and  500 ′ (for end-devices  125 ″ and  125 ′″ respectively) illustrating the process of  FIG. 4 . Each end-device  125 ″ and  125 ′″ receives a beacon signal at time  505 . The end-devices  125 ″ and  125 ′″ generate a plurality of time slots  510  starting at time  512 . The time slots  510  are synchronized (i.e., start and end at the same time) for both devices  125 ″ and  125 ′″. Device  125 ″ detects a power interruption at time  515 , and randomly selects to transmit its last gasp message in the fourth time slot  520 . Device  125 ′″ detects a power interruption at time  525  which is slightly later than the interruption is detected by device  125 ″. Device  125 ′″ randomly selects to transmit its last gasp message in the third time slot  530 . Because the time slots are synchronized, based on receipt of the beacon signal, the last gasp transmission from device  125 ″ ends at time  535  and the last gasp transmission from device  125 ′″ starts at time  535 . Because the time slots  510  are synchronized, they do not overlap and the messages do not collide with one another. 
         [0025]    Synchronizing the time slots for all devices in an area improves the probability of a transmission not colliding with a transmission from another device. The probability can be determined using the formula e −G  (i.e., slotted ALOHA protocol). 
         [0026]      FIG. 6  shows an embodiment of a commodity delivery device  600  for performing the processes described above. The device  600  includes a monitoring circuit  605 , a controller  610 , a communication circuit  615 , and an antenna  620 . The monitoring circuit  605  is coupled to a commodity delivery conduit  625 , and monitors one or more parameters of the commodity. In some embodiments, the monitoring circuit  605  passes the commodity to an output conduit  630 . In other embodiments, the commodity delivery conduit  625  and the output conduit  630  are the same conduit. The monitoring circuit  605  provides an indication of the one or more parameters monitored to the controller  610 . 
         [0027]    The controller  610  can provide information on the one or more parameters monitored to the communication circuit  615  for transmission to another device (e.g., the back office system  210 ). The controller  610  can also receive communications (e.g., polling requests, beacon signals, etc.) from other devices (e.g., a gateway  215 ) via the communication circuit  615 . 
         [0028]    Various features and advantages of the invention are set forth in the following claims.