Abstract:
A field device includes a power control module, a network interface module that communicates over a wireless network, and a device interface module for operating transducers, such as a sensor or an actuator. The power control module controls distribution of electrical power so that the network interface module receives electrical power while it is attempting to join the wireless network. Once the network interface module has joined the wireless network, the power control module allows the network interface module and the device interface module to share electrical power.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from provisional application Ser. No. 60/848,262 filed Sep. 29, 2006, which is incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to devices that communicate over a wireless mesh network. In particular, the present invention relates to power management in devices operating on wireless mesh networks. 
     Wireless data communication and control will be a dominant player in future sensor automation, process control, security, and safety regulation. One of the important requirements for wireless data communication and control is that the devices communicating over the network minimize their power consumption. 
     In wireless mesh network systems designed for low power, sensor/actuator-based applications, many devices in the network must be powered by long-life batteries or by low power energy-scavenging power sources. Power outlets, such as 120 VAC utilities, are typically not located nearby or may not be allowed into the hazardous areas where the instrumentation (sensors) and actuators must be located without incurring great installation expense. The need for low installation cost drives the need for battery-powered devices communicating as part of a wireless mesh network. Effective utilization of a limited power source, such as a primary cell battery which cannot be recharged, is vital for a well functioning wireless device. Batteries are expected to last more than 5 years and preferably as long as the life of the product. 
     In a true wireless mesh network, which may also be referred to as a self-organizing multi-hop network, each device must be capable of routing messages for itself as well as other devices in the network. The concept of messages hopping from node to node through the network is beneficial because lower power RF radios can be used, and yet the mesh network can span a significant physical area delivering messages from one end to the other. High power radios are not needed in a mesh network, in contrast a point-to-point system which employs remote devices talking directly to a centralized base-station. 
     A mesh network protocol allows for the formation of alternate paths for messaging between devices and between devices and a data collector, or a bridge or gateway to some higher level higher-speed data bus. Having alternate, redundant paths for wireless messages enhances data reliability by ensuring there is at least one alternate path for messages to flow even if another path gets blocked or degrades due to environmental influences or due to interference. 
     Some mesh network protocols are deterministically routed such that every device has an assigned parent and at least one alternate parent. In the hierarchy of the mesh network, much as in a human family, parents have children, children have grandchildren, and so on. Each device (or “node”) relays the messages for their descendants through the network to some final destination such as a gateway. The parenting devices may be battery-powered or limited-energy powered devices. The more descendants a node has, the more traffic it must route, which in turn directly increases its own power consumption and diminishes its battery life. 
     In order to save power, some protocols limit the amount of traffic any node can handle during any period of time by only turning on the radios of the nodes for limited amounts of time to listen for messages. Thus, to reduce average power, the protocol may allow duty-cycling of the radios between On and Off states. Some protocols use a global duty cycle to save power such that the entire network is On and Off at the same time. Other protocols (e.g. TDMA-based) use a local duty cycle where only the communicating pair of nodes that are linked together are scheduled to turn On and Off in a synchronized fashion at predetermined times. Typically, the link is pre-determined by assigning the pair of nodes a specific time slot for communications, an RF frequency channel to be used by the radios, who is to be receiving (Rx), and who is to be transmitting (Tx) at that moment in time. 
     Mesh networks use a process known as “joining” to incorporate new devices into the secured network. During the joining process, a number of information exchanges and configurations take place. 
     The new device may scan through all available network channels or may use a predetermined channel or subset of channels to discover similar devices within radio range. The new device searches for the existing network nodes the new device has available to it in order to gain membership into the network. The presence of each device within earshot is recorded. The new device sends a message to establish a handshake protocol with a neighbor device, asks to join the network, and provides a device number and network ID. The neighbor communicates the request to a network manager, which for example may be a software program running on a network gateway or a server connected to the gateway. The new device will provide its “neighbor” list to the network manager so that the network manager can determine the links that must be established to allow the new device to participate in the network. 
     The new preferably device uses its pre-configured security information to decode a joining message from the network manager and sends back the expected security response along with other information necessary for the network manager to establish links from the new device to other devices in the network. 
     The new device and its new parents and children receive and implement configuration information from the network manager to establish the required links. The new device is then fully joined and participating in the network. 
     In most networks, the joining process described above happens only when new devices join the network. The process may take 15 to 20 minutes depending on the network activity in the neighborhood of the new device. 
     BRIEF SUMMARY 
     A field device capable of wireless data communication includes a network interface module for communicating over a wireless network and a device interface module for operating a transducer such as a sensor or an actuator. Distribution of electrical power to the network interface module and the device interface module is controlled by a power control module. The power control module allocates power so that the network interface module receives electrical power while it is attempting to join the wireless network. Once the network interface module has joined the wireless network, the power control module allocates power so that the network interface module and the device interface module share electrical power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a wireless mesh network that includes multiple field devices that define nodes of the network. 
         FIG. 2  is a block diagram showing a wireless field device representative of one of the nodes of the wireless mesh network of  FIG. 1 . 
         FIG. 3  is a flow diagram illustrating the control of power distribution to a network interface module and a device interface module of the wireless field device of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows self-organizing mesh network  10 , which includes network manager  12  and individual devices or nodes  14 A- 14 I. Self-organizing mesh network  10  is a wireless communications network in which individual devices  14 A- 14 I pass data through multiple paths. 
     Network manager  12  may comprise, for example, a software application running on a network gateway or on a host computer. Network manager  12  can communicate directly (a single hop) with some of the devices (in this case devices  14 A,  14 B,  14 C, and  14 F) and can communicate indirectly (multiple hops) with the remaining devices. 
     According to one embodiment, when each of the devices  14 A- 14 I joined network  10 , network manager  12  preferably provided that device with a schedule to use in talking to other devices within network  10 . Each device is provided with slots representing specific times and radio frequencies which they use to pass data to and from nearby devices that are either children or parents to that device. 
     In one embodiment, devices  14 A- 14 I are field devices in a distributed industrial process system. The field devices may be transmitters having a sensor (or sensors) to monitor a process parameter such as pressure, temperature, flow rate, or fluid level. Alternatively, the field device may include an actuator for providing the control function in response to a control command signal received over network  10 . 
       FIG. 2  shows a block diagram of node or field device  14 , which may be representative of each of the individual devices  14 A- 14 I. Device  14  includes power source  20 , power control module (PCM)  22 , transceiver  24 , network interface module (NIM)  26 , device interface module (DIM)  28 , and transducer  30 . 
     Power source  20  may be a battery, or a renewable energy source such as a solar cell, thermoelectric cell, atomic battery, or energy scavenger in conjunction with a storage capacitor. The effective lifetime of field device  14  depends upon the capacity of power source  20 , and whether it is renewable or not. 
     Power control module  22  controls the distribution or allocation of energy from power source  20  to the other components of field device  14 . In particular, power control module (PCM)  22  allocates power between the network communication functions performed by transceiver  24  and network interface module (NIM)  26 , and the device functions performed by transducer  30  and device interface module (DIM)  28 . 
     Transceiver  24  provides the wireless communication between device  14 , and other similar devices within mesh network  10 . In particular, transceiver  24  will receive control messages routed to it over network  10 , will transmit responses to control messages based upon the actions of transducer  30  and DIM  28 , and will relay messages to parent and children nodes as required by the mesh network communication protocol. 
     Network interface module (NIM)  26  controls the operation of transceiver  24 . It processes the incoming messages received by transceiver  24 , and it formats the outgoing messages to be transmitted by transceiver  24  through the mesh network. NIM  26  is also responsible for the joining process, in which device  14  joins and becomes a part of mesh network  10 . 
     Transducer  30  may be a sensor or sensors for sensing a process parameter (such as pressure, temperature, flow, or fluid level). Transducer  30  may include additional sensors for sensing secondary parameters or variables that can also be reported over the wireless mesh network, or may be used by field device  14  in processing the measurement of the primary process parameter. For example, when the primary process parameter is pressure or flow, a temperature sensor may be used to provide a sensed temperature signal for correcting temperature dependence of the sensed primary parameter signal. 
     In other embodiments, transducer  30  may be an actuator that performs a mechanical function based upon a control input received over the mesh network. For example, transducer  30  may be a valve actuator used to control flow of a fluid in a process that is being controlled. 
     Device interface module (DIM)  28  provides the power to transducer  30  to perform the sensing or actuating function. When transducer  30  is a sensor, DIM  28  processes the sensor signal, and produces a sensor output that is provided to NIM  26  for transmission in a message over wireless mesh network  10 . When transducer  30  is an actuator, DIM  28  provides the control or command input to the actuator based upon a message that has been received by transceiver  24  and provided to DIM  28  by NIM  26 . 
     During normal operation, power control module (PCM)  22  allocates power from power source  20  between NIM  26  and DIM  28 , so that both operation of transceiver  24  and operation of transducer  30  can occur. This sharing of power by NIM  26  and DIM  28  occurs so long as device  14  is a part of network  10 . 
     When device  14  first joins network  10 , as well as during times when communication has been lost and device  14  must reestablish contact with and rejoin network  10 , consumption of power by DIM  28  and transducer  30  is not necessary. Power consumption by NIM  26  is at a maximum during a joining sequence, and then is lower during normal operation when network connection has been established. During the joining sequence, therefore, PCM  22  allocates the power only to NIM  26 . 
     During a joining sequence, NIM  26  causes transceiver  24  to listen for transmissions from nearby devices that are part of network  10 . Upon detecting the presence of one or more neighboring devices, NIM  26  causes transceiver  24  to send a handshake protocol message to the neighbors. Upon establishing communication with neighbors, NIM  26  sends a message to one of the neighbors asking to join the network. This message includes the device number and network ID of device  14 . The neighbor then forwards the message asking to join the network to network manager  12 , which performs a join authorization process and configures device  14  to network  10 . Network manager  12  determines which neighboring devices will be parents and children of device  14 , and establishes the network schedule for when transceiver  24  is to listen for messages directed to device  14 , when it is to send messages, and on what channels transmission and reception should take place. This configuration of device  14  is performed through a series of configuration messages sent by network manager  12  to device  14 . 
     The initial power requirement for NIM  26  to join network  10  is high and the join process may take a significant amount of time. As a result, the power needs of NIM  26  are high during the join process. All other power consumption is curtailed by PCM  22  during the join process, and all available power is allocated to NIM  26 . 
     Once the joining process has taken place, NIM  26  will turn transceiver  24  on and off according to a network schedule provided by network manager  12 . The duty cycle can be very low, with transceiver turned on for short periods when it is its turn to receive or transmit messages. In this way, transceiver  24  is not consuming power during time periods when no messages will be sent to or from device  14 . 
     According to one embodiment, once NIM  26  has joined network  10 , PCM  22  makes power available to DIM  28  as well as NIM  26 . Each time that DIM  28  wakes up (e.g. in accordance with an internal time schedule of device  14 , or in response to a message received by transceiver  24  and supplied by NIM  26  to DIM  28 ), DIM  28  will initiate a check to see whether NIM  26  is presently connected to network  10 . If NIM  26  has not established a connection to network  10  through a join processes, or if communication with network  10  has been lost, NIM  26  will indicate that it is not connected to network  10 . In that case, DIM  28  will be placed in a sleep mode, thereby substantially reducing its power consumption, and all available power will be allocated by PCM  22  to NIM  26  until NIM  26  has been successful in joining or rejoining network  10 . 
     If NIM  26  indicates that it is connected to network  10 , then DIM  28  is permitted to initiate operation of transducer  30 . This may involve a sensor measurement, and signal processing of that sensor measurement by DIM  28 . The processed sensor signal is then provided by DIM  28  to NIM  26  where it is stored until the sensor output can be provided in a message transmitted over the network. 
     When a message has been sent over the network containing a sensor output, an acknowledge signal may be sent back to device  14  indicating that the message containing the sensor output reached its destination. The acknowledge signal provides an indication to NIM  26  that NIM  26  is still active within the network. Similarly, control messages directed over network  10  to device  14  may request data such as the sensor output or operation of an actuator. Receipt of a control message also indicates to NIM  26  that it is still a part of network  10 . 
       FIG. 3  shows a flow diagram illustrating one embodiment of the power allocation within device  14 . As shown in  FIG. 3 , operation begins with startup of device  14  (step  50 ). Upon startup, PCM  22  initially allocates power from power source  20  to NIM  26  (step  52 ). NIM  26  then initiates a joining process (step  54 ). Until the joining process has been successful and NIM  26  indicates that it is now in the network, power continues to be allocated substantially to NIM  26  (step  56 ) by PCM  22 . 
     Once NIM  26  has completed the joining process and indicates that it is in network  10 , PCM  22  then allocates power between NIM  26  and DIM  28  (step  58 ). DIM  28 , whenever it becomes active, initiates a check of the status of NIM  26  (step  60 ). DIM  28  may be activated periodically according to an internal schedule of device  14 , or may be activated in response to a message received over network  10 .by transceiver  24  and NIM  26  (step  62 ). 
     Upon initiation of a check by DIM  28 , PCM  22  determines whether or not NIM  26  is currently in network  10  (step  64 ). If NIM  26  is not in network  10 , PCM  22  causes DIM  28  to enter a sleep mode (step  66 ). With DIM  28  in a sleep mode, power is once again allocated substantially to NIM  26  (step  52 ). 
     If NIM  26  indicates that it is connected to network  10 , then DIM  28  activates transducer  30  (step  68 ). DIM  28  provides power to transducer  30  and either receives a sensor signal or signals, or causes an actuator to operate, depending on the type of transducer. DIM  28  then provides data to NIM  26  reporting the results of the transducer activity (step  70 ). This may be a sensor output derived by DIM  28 , or may be feedback on the operation of an actuator. 
     NIM  26  stores the data from DIM  28  until the next time slot for transmission of data over network  10 . NIM  26  formats the data received from DIM  28  into a message, and causes the message to be transmitted by transceiver  24  over the network at the appropriate time (step  72 ). 
     The power management of device  14  by PCM  22  separates power requirements for NIM  26  and DIM  28 , and allows those power requirements to be satisfied in a sequence. This reduces the total power requirements whenever device  14  is joining wireless network  10 . NIM  26  is provided with the power it needs to perform the joining process in order to establish connection to a wireless network. DIM  28  is inactive and not powered unless NIM  26  has joined the network. 
     The power management takes advantage of the reduced power requirement for NIM  26  after the joining process has taken place. Once NIM  26  is in normal network operation, DIM  28  can be powered up and used to operate transducer  30  as needed. 
     The power management process can be performed internally by device  14  using internal hardware, software or firmware. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.