Abstract:
A wireless mesh network that makes use of strategically placed nodes with radios having superior communication range through the use of high gain antennas, MIMO technology, high power transmitters, high sensitivity receivers, a combination thereof, etc. Besides the obvious effect of extending the physical coverage area of a network, the effect of the longer distance links formed by the nodes with superior range is a flattening of the wireless mesh network by reducing hop count, thereby improving performance, latency, reliability, cost, and power consumption among other factors.

Description:
BACKGROUND 
     The present invention relates to wireless mesh network systems. In particular, the invention relates to wireless mesh network systems having an improved performance through the use of selected nodes with different communication characteristics. 
     In many industrial settings, control systems are used to monitor and control inventories, processes, and the like. Often, such control systems have a centralized control room with a host computer that communicates with field devices that are separated or geographically removed from the control room. 
     Generally, each field device includes a transducer, which may generate an output signal based on a physical input or generate a physical output based on an input signal. Types of transducers used in field devices include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow sensors, positioners, actuators, solenoids, indicators, and the like. Traditionally, analog field devices have been connected to the process subsystem and the control room by two-wire twisted-pair current loops, with each device connected to the control room by a single two-wire twisted pair loop. Typically, a voltage differential is maintained between the two wires of approximately 20 to 25 volts, and a current between 4 and 20 milliamps (mA) runs through the loop. An analog field device transmits a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. An analog field device that performs an action under the control of the control room is controlled by the magnitude of the current through the loop, which is modulated by the ports of the process subsystem under the control of the controller. 
     While historically field devices were capable of performing only one function, more recently hybrid systems that superimpose digital data on the current loop have been used in distributed control systems. The Highway Addressable Remote Transducer (HART) protocol superimposes a digital carrier signal on the current loop signal. The digital carrier signal can be used to send secondary and diagnostic information. Examples of information provided over the carrier signal include primary and secondary process variables, diagnostic information (such as sensor diagnostics, device diagnostics, wiring diagnostics, process diagnostics, and the like), operating temperatures, sensor temperature, calibration data, device ID numbers, configuration information, and so on. Accordingly, a single field device may have a variety of input and output variables and may implement a variety of functions. 
     Another approach uses a digital communication bus to connect multiple field devices to the host in the control room. Examples of digital communication protocols used with field devices connected to a digital bus include Foundation Fieldbus, Profibus, Modbus, and DeviceNet. Two way digital communication of messages between a host computer and multiple field devices can be provided over the same two-wire path that supplies power to the field devices. 
     Typically, remote applications have been added to a control system by running very long homerun cables from the control room to the remote application. If the remote application is, for example, a half of a mile away, the costs involved in running such a long cable can be high. If multiple homerun cables have to be run to the remote application, the costs become even higher. Wireless communication offers a desirable alternative, and wireless mesh networks have been proposed for use in industrial process control systems. However, to minimize costs, it is also desirable to maintain existing control systems and communication protocols, to reduce the costs associated with changing existing systems to accommodate the wireless communication. 
     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 not typically located nearby or may not be permitted into the hazardous locations where the instrumentation (sensors and actuators) must be located without incurring significant installation expense. The economic 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 properly 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, each device must be capable of routing messages for itself as well as other devices in the mesh network (so called mesh to the edge). The concept of messages hopping from radio to radio through the network is beneficial because lower power 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 as in a point-to-point system which employs remote devices talking directly to a distant centralized base station. 
     A mesh network 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 messages enhances data reliability by ensuring there is at least one path for messages to use even if another path becomes blocked or degraded due to environmental influences or interference. 
     Some mesh network protocols are deterministically routed such that every node 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 node relays the messages for their descendants through the network to some final destination such as a gateway. The parenting nodes 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. 
     Normally, all nodes in the mesh network employ radios (RF transceivers) of the same quality each having the same link budget (Tx power-Rx sensitivity). To bridge long distances, many nodes are sometimes required to relay messages. In an industrial setting, it may be physically impossible or environmentally prohibited to install radios in certain locations to make these links. An improved method for using wireless mesh networks in an industrial setting is needed. 
     SUMMARY 
     A wireless mesh network includes a first plurality of nodes including radios having a first communication range and a second plurality of nodes with including radios having a second, different communication range. The first and second plurality of nodes are capable of communicating with any other node within their communication range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a conventional mesh network with two clusters of nodes. 
         FIG. 2  is a diagram showing a wireless mesh network with two clusters of nodes interlinked by nodes with radios utilizing higher gain Yagi antennas. 
         FIG. 3  is a diagram showing a wireless mesh network with two clusters of nodes interlinked by nodes with radios that have higher gain Yagi antennas. 
         FIG. 3A  is a diagram of a wireless mesh network, similar to the network of  FIG. 3 , that further includes a node intermediate the two clusters. 
         FIG. 4  is a diagram showing a wireless mesh network with three clusters of nodes redundantly connected together with nodes that have radios with higher gain Yagi antennas. 
         FIG. 5  is a diagram showing a wireless mesh network including nodes with higher range radios. 
         FIG. 6  is a diagram showing a wireless mesh network including a longer distance link formed by nodes including a pair of transceivers at one end and a single transceiver at the other end. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides a method for constructing mesh networks for field devices where conventional techniques would require additional nodes or would not exhibit the required connectivity for a mesh network to operate. A node includes at least one radio and one field device, though it may have several of each. Many nodes are installed in a fixed location where its corresponding field device is designed to operate. When there is a control room adapted to operate with the field devices, the control room is capable of communicating with at least one node such that it is operable over the mesh network. 
     For a mesh network to function properly, full connectivity to all the nodes is needed so that data to/from all of the field devices is available. It is not always possible to install additional nodes to relay communications from remote nodes in the industrial setting because it may be impossible or environmentally prohibited to place them there. By introducing superior nodes with increased communication range or specialized antennas, obstacles which would prevent connectivity are overcome. 
     Previous designs with high power radio nodes were used in a hierarchy where a local node would transmit to a high power radio node which would then communicate with another high power radio node that would finally deliver the message to another local node. The high power radio nodes would typically use a different protocol or even a different communications medium. This multi-tiered network requires more hops to transmit a message as it must be transmitted from the lower level to a higher level, across the higher level and then back down from the higher level to the lower level. In a flat network, there are no tiers and no required hierarchy. Each node can communicate directly with any other node within the range of its radio. Performance is better because there are fewer hops resulting in lower latency, higher reliability, and lower power consumption. 
     Traditionally, so called flat networks employed only nodes with the same link budget. To span a distance, multiple nodes would be positioned to relay the message. The other alternative was to employ selected higher power radio nodes on a multi-tiered hierarchical system. The present invention allows a high power radio node to communicate with any other node in range. The resulting network is flat and includes the superior radio nodes needed to provide full network connectivity in an industrial setting. 
       FIG. 1  illustrates nodes as they may be installed in an industrial setting clustered around a piece of equipment or a location where environmental readings are taken. Cluster  10  has omni-directional radio nodes  12 ,  14 ,  16 , and  18  with ranges R 12 , R 14 , R 16 , and R 18  respectively. Similarly, cluster  20  has omni-directional radio nodes  22 ,  24 ,  26 , and  28  with ranges R 22 , R 24 , R 26 , and R 28 . A node can complete a transmission to any other node within its range. Node  12  can send transmissions to nodes  16 , and  18 ; node  14  can send to nodes  16  and  18 , etc. No direct link exists between nodes  12  and  14 . Messages sent between these nodes are routed through other nodes. For example, a message from node  12  to node  14  could first go from node  12  to node  16  then to node  14  or it could go from node  12  to node  18  to node  14 . A message hops each time it is relayed through another node. Latency is directly related to the number of hops required to complete a transmission. This network suffers from the problem that there is no way to send messages between clusters  10  and  20  as there are no nodes between the networks to relay messages. In an industrial setting, it may not be possible to install nodes in the intermediate locations. There are a variety of reasons why this may be the case. It may be physically impossible to locate a functioning radio there, it may be a sensitive/hazardous area where installing a permissible node is cost prohibitive, or the owner of the system may not have the land rights between locations, etc. 
       FIG. 2  shows a wireless mesh network containing cluster  30  of onmi-directional radio nodes  32 ,  34 ,  36 , and  38  and cluster  50  of omni-directional radio nodes  52 ,  54 ,  56 , and  58  where clusters  30  and  50  are arranged similarly to the network of  FIG. 1 . This mesh network additionally includes node  42  in cluster  30  and node  62  in cluster  50  utilizing radios with Yagi antennas which have coverage areas R 42  and R 62  respectively. A small 2.4 GHz Yagi antenna measuring 4.5″ long and 3.5″ in diameter can provide  10  dBi forward gain and 55 degree beam width versus only 2.1 dBi gain for a standard dipole (360 degree omni-directional) antenna. 
       FIG. 3  shows this network with all ranges illustrated. The antenna selection and placement of the nodes allows at least one path for every necessary transmission. For example, a message can be sent from node  72  to node  98  by hopping from node  72  to node  82  to node  102  to node  98 . A return path for a reply message can follow this same path in reverse. If two nodes can communicate in one direction they can communicate in the opposite direction by the principle of reciprocity. 
     The network is also functional if a different return path is used to enable bi-directional communication. For example, the message can be sent from node  98  to node  72  by hopping from node  98  to node  94  to node  102  to node  82  to node  72 . 
     The strategic placement and use of these high gain antennas increases the link budget for those nodes and allows them to achieve a longer range connection without having to place several additional nodes between cluster  70  and cluster  90 . A variety of different types of antennas can be used based on the physical environment where the mesh network is installed. This arrangement is particularly useful when the location prevents the installation of additional nodes. 
     In the example shown in  FIG. 3 , there are three ranges to consider. The Yagi-Yagi range between two nodes with Yagi antennas is greater than the Yagi-Omni range between a node with a Yagi antenna and one with a 2 dBi standard dipole omni-directional antenna. In turn, the Yagi-Omni range is greater than the omni-omni range between two nodes with 2 dBi standard dipole omni-directional antennas. 
       FIG. 3A  illustrates these three different ranges. The wireless mesh network shown in  FIG. 3A  is similar to the network shown in  FIG. 3 , except for the addition of node  101  positioned intermediate clusters  70  and  90 . Node  101  is an omni-directional radio node which, in this example, is positioned far enough from both clusters  70  and  90  that the Omni-Omni antenna range is not large enough to permit direct communication between node  101  and any of the omni-directional nodes  72 ,  74 ,  76 ,  78  of cluster  70  or  92 ,  94 ,  96 ,  98  of cluster  90 . 
     Yagi-Omni range R 82/101  of the combination of nodes  82  and  101  is sufficient to provide a link between cluster  70  and node  101 . Similarly, Yagi-Omni range R 102/101 , of the combination of nodes  102  and  101  is sufficient to provide a link between cluster  90  and  101 . The Yagi-Yagi range of the combination of nodes  82  and  102  is greater than the Yagi-omni ranges, and can provide a direct link between clusters  70  and  90 . 
       FIG. 4  shows a wireless mesh network with node clusters  110 ,  130 , and  150  which are redundantly connected together with Yagi antenna equipped nodes  118 ,  122 ,  138 ,  142 ,  158 , and  162 . The range lines for omni-directional radio nodes  112 ,  114 ,  116 ,  132 ,  134 ,  136 ,  152 ,  154 , and  156  have been omitted. Each node has a sufficient gain to transmit to the other nodes within its cluster but not between clusters. If interference or noise were to break one of the links connecting clusters  110  and  130 , for example between nodes  118  and  142 , full connectivity of the network would remain. When something such as interference prevents a direct link between nodes  118  and  142 , a message can be sent by hopping from node  118  to node  112  to node  116  to node  122  to node  162  to node  156  to node  154  to node  158  to node  138  to node  132  to node  134  to node  142 . 
     Yagi antennas are very directional, so they must be pointed to communicate in the desired direction. Node  122 , is not pointed at node  112  so they cannot communicate directly. Nodes  122  and  156  have an intermediate range since they form a Yagi-Omni link. They may or may not be in direct communication with each other. Node  158  is not pointed at node  156 , so they cannot communicate directly. Node  142  is not pointed at node  132 , so they cannot communicate directly. 
       FIG. 5  shows a wireless mesh network with high range radio node  182  with coverage area R 182  in cluster  170  and high range radio node  202  with coverage area R 202  in cluster  190 . The range of nodes  182  and  202  can be increased in a number of ways. One method is to install a radio with a higher power transmitter and a more sensitive receiver to achieve a higher link budget. Another method is to use technology such as MIMO to increase the range. MIMO technology creates a higher link budget by providing gain through spatial diversity. The same signal is transmitted using two or more transmitters at the same time from one radio and correlating the signals received on two or more separate receivers in another radio. True MIMO technology has multiple receivers and multiple transmitters including multiple antennas at both ends of the link. However, not all nodes need to be upgraded to MIMO technology to obtain a benefit. 
     A simpler method shown in  FIG. 6  uses multiple transceivers in node  210  (antennas  222  and  224 ) and a single transceiver in other nodes  212  (antenna  226 ) and  214  (antenna  228 ). This arrangement provides significant benefits to signal strength. When multiple transceiver node  210  receives a signal, it does so using separate receivers on separate antennas. Because the antennas are separated from each other, the signal transmitted from the sending node has a slightly different path to each antenna. Due to variation in path loss, node  210  can pick which receiver has received the signal with the best quality. When sending a signal, node  210  can then choose the best antenna to use which then has the best path to transmit a signal back to the original sending node. In effect, the improved reception increases the range of node  212  from R 212-1  to R 212-2  and from R 214-1  to R 214-2  for radio  214 . The improvements also greater range R 210  for node  210  due to the ability to select the best path at any time. This technique achieves gain through signal and path selectivity. 
     Conventional mesh networks consist of nodes that are all of the same quality with the same link budget. The present invention describes mesh networks where some nodes have superior radios or superior antennas. Latency is reduced because messages can be transmitted using fewer hops. This improves the performance of the mesh network. Higher link budgets can be achieved by using varying antenna designs, for examples 10 dBi YAGI vs. 2 dBi dipole, using transmitters of higher power, for example +15 dBm vs. 0 dBm, using receivers of higher sensitivity, for example −110 dBm vs. −90 dBm, or a combination of the above. Directional antennas other than Yagi antennas, such as sector antennas or parbolic antennas, may also be used. 
     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.