Abstract:
A wireless field device that performs a wireless communication with other field devices in a wireless network through a gateway. The device includes: a battery incorporated in the field device to activate the field device; and a wireless communication module configured to exchange radio signals with the gateway to perform the wireless communication with said other field devices. When the field device starts procedure for joining in the wireless network, firstly, the wireless communication module continuously accesses the gateway in a first period to receive an advertisement signal from the gateway, and when the wireless communication module fails to receive the advertisement signal in the first period, the wireless communication module intermittently accesses the gateway until receiving the advertisement signal from the gateway.

Description:
This application claims priority from Japanese Patent Application No. 2010-149710, filed on Jun. 30, 2010, the entire contents of which are herein incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     Embodiments described herein relate to a wireless field device. 
     2. Related Art 
     Conventionally, at places where a cable cannot be wired, for example, in water supply or sewer facilities such as manholes, a flowmeter that has a wireless communication function and serves as a wireless field device is attached to a pipe through which water flows. 
     In collecting data or inspecting a facility, a worker receives data from a flowmeter in a manhole, for example, by performing a wireless communication with the flowmeter using a hand-held terminal which serves as a wireless communication device. 
     Field devices include various devices having a communication function such as a pressure gauge, a differential pressure gauge, a thermometer, a level meter, and a flowmeter and devices such as a valve positioner that have a communication function and perform a measurement or control (valve positioner) that directly relates to a process. Field devices other than a valve positioner are called (industrial) transmitters. 
     Devices which receive a signal from such field devices and perform indication, recording, control, etc., such as an indicator, a recorder, an adjuster, a distributed control system device, an alarm device, etc. are system-side devices and are called receivers as opposed to the above-described concept of transmitters. 
     Assuming, as mentioned above, the use at places where ordinary wiring cannot be made, wireless field devices are of a battery-driven type. To enable the wireless field devices to operate for a long time, the wireless field devices are provided with a lithium battery as a power source. 
       FIG. 8  shows the system configuration of an example wireless network which uses such wireless field devices. As shown in  FIG. 8 , a wireless network topology  1  includes I/O devices  2 - 6 , routing devices  7  and  8 , and a gateway  9  and has a star-mesh configuration. 
     Each of the I/O devices  2 - 6  is a sensor such as a differential pressure gauge, a pressure gauge, or a thermometer or a manipulation end such as a valve positioner and has a wireless communication function as supported by IEEE 802.15.4. 
     The routing devices  7  and  8  have an advertisement function of issuing an advertisement to nearby devices on a regular basis and a routing function of sending route information and messages. Alternatively, the I/O devices  2 - 6  may be provided with the routing function. 
     The gateway  9  has a function of connecting the wireless network topology  1  and a plant network  10  and thus realizes connection between a control system  11  and the I/O devices  2 - 6 . 
     In the example of  FIG. 8 , the I/O devices  2  and  3  can perform a wireless communication with the gateway  9  via the routing device  8  and the I/O devices  4 - 6  can perform a wireless communication with the gateway  9  via the routing device  7 . 
     An I/O device  12  does not receive advertisements from the routing device  7  or  8  and does not belong to the wireless network topology  1 . For example, such a situation occurs immediately after a time point when the I/O device  12  is powered on after power-on of the other I/O devices  2 - 6 . 
       FIG. 9  is a sequence diagram which allows the I/O device  12  to join the wireless network topology  1 . The sequence diagram of  FIG. 9  is directed to a case that the I/O device  12  receives an advertisement from the routing device  8  and joins the wireless network topology  1  which complies with, for example, the wireless communication standard ISA 100.11a which utilizes IEEE 802.15.4. 
     Referring to  FIG. 9 , at step SQ 1 , the routing device  8  periodically sends, to nearby devices, an advertisement for urging them to join the wireless network topology  1 . The I/O device  12 , which has not joined the wireless network topology  1  yet, receives the advertisement sent from the routing device  8 . 
     After receiving the advertisement, at step SQ 2  the I/O device  12  sends, to the routing device  8 , join requests that are directed to a system manager  91  and a security manager  92  provided in the gateway  9 , respectively. 
     After receiving the join requests, at step SQ 3  the routing device  8  transfers, to the system manager  91 , the join request that is directed to the security manager  92 . 
     After receiving the join request, at step SQ 4  the system manager  91  transfers the join request to the security manager  92 . The system manager  91  and the security manager  9  may be provided outside the gateway  9 . 
     At step SQ 5 , the routing device  8  transfers, to the system manager  91 , the join request that is directed to the system manager  91 . 
     After the security manager  92  receives the join request, at steps SQ 6 -SQ 8  a security join permission is finally communicated to the I/O device  12 . 
     Furthermore, when the system manager  91  has received, from the I/O device  12 , the join request directed to itself, at steps SQ 9  and SQ 10  a system manager join permission is finally communicated to the I/O device  12 . 
     At step SQ 11 , the I/O device  12  sends a security confirm request to the system manager  91 . 
     At step SQ 12 , the system manager  91  communicates the security confirm request to the security manager  92 . 
     At step SQ 13 , the security manager  92  sends a security confirm response to the I/O device  12 . 
     After completion of the procedure of  FIG. 9 , the I/O device  12  can send process data such as a differential pressure, a pressure, a temperature, or the like to the gateway  9  at a constant cycle. The constant cycle is a time interval (e.g., about 1 second to 1 hour) that can be set arbitrarily by the user. 
     Process data that has been transmitted to the gateway  9  is processed by the control system  11 , whereby monitoring by the user is performed or a process control is performed by sending a wireless signal via the gateway  9  to an I/O device that is an operation terminal such as a control valve. 
     Each of the I/O devices  2 - 6  and  12 , after joining in the wireless network topology  1 , usually suspends the operation of the internal circuits to save energy consumption of the built-in battery. Each of the I/O devices  2 - 6  and  12  is activated at the constant cycle, that is, only when it is necessary to send process data. After sending of process data, the operation of each of the I/O devices  2 - 6  and  12  is suspended until the next calculation and sending of process data. 
       FIG. 10  is a graph showing an example current consumption characteristic of the I/O device  12  before and after joining in the wireless network topology  1 . The horizontal axis represents time and the vertical axis represents the current consumption. The sequence process of  FIG. 9  is executed in a period Ta that is from power-on of the I/O device  12  to completion of joining (indicated by a broken line). In the period Ta, the I/O device  12  is always kept operational to receive signals successively and hence the current consumption always has a large value Ia. 
     In a period Tb that starts from the completion of joining, the I/O device  12  is activated at a constant cycle T, that is, only when it is necessary to send process data. The I/O device  12  repeats a cycle of calculating and sending process data and then being kept inactive until the next calculation and sending of process data. In a period T 1  of each cycle T, the I/O device  12  is kept inactive and the current consumption has a small value Ib (&lt;Ia). In the other period T 2 , the I/O device  12  is operational to send process data. Each arrow denoted by symbol tp indicates that the I/O device  12  is sending process data to the gateway  9  by a wireless communication. 
     As can be seen from  FIG. 10 , after the completion of joining, the combination of the suspension period T 1  in which the current consumption is Ib and the active period T 2  in which the current consumption is Ia is repeated at the constant cycle T, and the suspension period T 1  accounts for a large part of the cycle T. Therefore, the energy consumption of the battery can be reduced by elongating the constant cycle T. 
     JP-A-2003-134030 discloses a technique for adding a wireless communication function to a field device, and JP-A-2003-134261 discloses a communication system which uses field devices having a wireless communication function. 
     Incidentally, in constructing the wireless network shown in  FIG. 8 , the gateway  9  needs to be installed and activated first. However, because of delayed purchase of equipment, configuration of the plant network  10  or the control system  11 , or some other reason, the gateway  9  may be installed after installation of the I/O devices  2 - 6  and  12  or the routing devices  7  and  8 . 
     On the other hand, in general, field devices used as the I/O devices  2 - 6  and  12  and the routing devices  7  and  8  do not have a power-on/off switch outside the cabinet. This is because such field devices used in a dangerous atmosphere are required to have a pressure-resistant, explosion-proof structure. In general, such field devices are powered on/off by attachment/detachment of a battery. 
     Although it is conceivable to provide a switch inside the cabinet of a field device, opening the lid of the cabinet to turn on or off the switch is a burden to the user. It is not permitted to open the cabinet at a place where pressure resistance and explosion proof are required. Therefore, it is necessary to turn on the power switch or insert a battery immediately before installing a field device at an intended site. 
     As a result, in a state that the gateway  9  has not been installed yet, field devices used as the I/O devices  2 - 6  and  12  and the routing devices  7  and  8  have to wait for an advertisement (step SQ 1  in  FIG. 9 ) in the power-on state for a long time. That is, the state that the period Ta in which the current consumption has the large value Ia (see  FIG. 10 ) continues until the gateway  9  is installed and activated. 
     In actual measurement examples, the current consumption in the period Ta is several tens of times the average current consumption in the period Tb (see  FIG. 10 ) and one-day delay of installation of the gateway  9  results in consumption of energy that would be consumed in several months if the gateway  9  were installed. In battery-driven wireless field devices, the battery life is an important issue and the fact that the battery energy may be consumed in the state of the period Ta shown in  FIG. 10  for a long time is a serious problem. 
     The same situation occurs when the gateway  9  has failed: A large amount of battery energy is consumed in field devices used as the I/O devices  2 - 6  and  12  and the routing devices  7  and  8  until completion of replacement of the gateway  9 . 
     Powering on field devices after completion of installation or replacement of the gateway  9  is not practical for the following reasons. First, as mentioned above, the cabinet cannot be opened at a place where pressure resistance and explosion proof are required. 
     In a large plant, several hundreds or several thousands of field devices may be distributed. Powering on all the field devices after completion of installation or replacement of the gateway  9  not only increases personnel expenses but also increases, for example, the probability of occurrence of trouble due to scuffing of the lid of a cabinet and danger in a pressure-resistant area due to incomplete closure of a lid. 
     Even if a battery is inserted into place and the lid of its cabinet is closed in a safe area such as a user work bench area, the battery energy is consumed uselessly until a start of regular communications. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention address the above disadvantages and other disadvantages not described above. However, the present invention is not required to overcome the disadvantages described above, and thus, an exemplary embodiment of the present invention may not overcome any disadvantages. 
     Accordingly, it is an illustrative aspect of the present invention to provide a wireless field device whose battery life is elongated by reducing the current consumption before joining in a wireless network. 
     According to one or more illustrative aspects of the present invention, there is provided a wireless field device that performs a wireless communication with other field devices in a wireless network through a gateway. The device includes: a battery incorporated in the field device to activate the field device; and a wireless communication module configured to exchange radio signals with the gateway to perform the wireless communication with said other field devices. When the field device starts procedure for joining in the wireless network, firstly, the wireless communication module continuously accesses the gateway in a first period to receive an advertisement signal from the gateway, and when the wireless communication module fails to receive the advertisement signal in the first period, the wireless communication module intermittently accesses the gateway until receiving the advertisement signal from the gateway. 
     Other aspects and advantages of the present invention will be apparent from the following description, the drawings and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a wireless field device  20  according to an embodiment of the present invention; 
         FIG. 2  is a graph showing an example current consumption characteristic of the field device  20  of  FIG. 1  before and after joining in a wireless network; 
         FIG. 3  is a state transition diagram of the field device  20 ; 
         FIG. 4  explains the details of the states shown in  FIG. 3 ; 
         FIG. 5  explains the details of the state transitions shown in  FIG. 3 ; 
         FIG. 6  is a block diagram of a wireless field device  20 A according to another embodiment of the invention; 
         FIG. 7  is a graph showing an example current consumption characteristic of the field device  20 A of  FIG. 6  before and after joining in a wireless network; and 
         FIG. 8  shows the system configuration of an example wireless network using wireless field devices; 
         FIG. 9  is a sequence diagram which allows an I/O device  12  to join a wireless network topology  1  in the wireless network of  FIG. 8 ; and 
         FIG. 10  is a graph showing an example current consumption characteristic of the I/O device  12  before and after joining in the wireless network topology  1 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Exemplary embodiments of the present invention will be hereinafter described with reference to the drawings.  FIG. 1  is a block diagram of a wireless field device  20  according to an embodiment of the invention. As shown in  FIG. 1 , the wireless field device  20  includes an antenna  21 , a wireless communication module  22 , an MPU  23 , a sensor  24 , a power management unit  25 , a battery  26 , an infrared communication unit  27 , a display unit  28 . 
     For example, an infrared transceiver  30  is a field device setting tool incorporating an infrared transmitter and receiver, and performs an infrared communication with the infrared communication unit  27 . 
     The wireless communication module  22  is configured to exchange radio signals with the gateway  9  (see  FIG. 8 ) via the antenna  21 . A reception result of the wireless communication module  22  can be communicated to the MPU  23 . In accordance with the instruction from the MPU  23 , the wireless communication module  22  can send failure information, a process value PV, etc. to the gateway  9 . 
     The MPU  23 , which is a central processing unit, has a computing unit such as a microprocessor and storage unit such as a RAM and an EEPROM. The MPU  23  converts, corrects, or converts into a user-specified scaling value such as a percentage value a process quantity PV such as a process pressure, temperature, or a flow rate of fluid in a pipe (not shown) detected by the sensor  24 , and supplies a calculation result to the wireless communication module  22 . 
     The MPU  23  receives a setting change request, for example, from the user by radio via the wireless communication module  22 . The MPU  23  controls and diagnoses the individual internal units of the field device  20  and, if a problematic diagnosis result is obtained, informs the user of failure information etc. via the wireless communication module  22 . Furthermore, the MPU  23  supplies diagnosis results and a calculation result of a process value PV to the display unit  28  and causes them to be displayed on the display unit  28 . 
     The power management unit  25  regulates a power voltage that is supplied from the battery  26  and supplies a necessary voltage to the MPU  23 . 
     The power management unit  25  supplies power to the individual internal units of the field device  20  every prescribed cycle by switching the power voltage by performing on it an on/off control that is determined by the CPU  23  according to an intermittent operation cycle time specified by the user. 
     Furthermore, the power management unit  25  also on/off-controls the supply of power to the individual internal units of the field device  20  according to a reception signal that is transmitted from the infrared communication unit  27 . 
     The management functions of the power management unit  25  may be provided in the central processing unit of the MPU  23 . As for the power management portion of the power management unit  25 , waiting mode such as a power saving mode can be set. The power management unit can cause a transition to a standby state by itself after it has operated for a prescribed time. The standby state can be canceled when an external interrupt signal, for example, is received from the infrared communication unit  27 , for example. 
     The battery  26 , which is a lithium battery, for example, can be inserted into and removed from a prescribed portion inside the field device  20  by opening a lid (not shown). 
     The infrared communication unit  27  receives a signal from the infrared transceiver  30  through a glass cover (not shown), and supplies a reception result to the power management unit  25 . 
     Capable of sending a signal to the infrared transceiver  30 , the infrared communication unit  27  can send out a calculation result of a process value PV and a failure diagnosis result produced by the MPU  23  in place of the wireless communication module  22 . The infrared communication unit  27  may be provided on the display unit  28 . 
     The display unit  28  is an LCD, for example. 
     The user operates the infrared transceiver  30  to send, to the infrared communication unit  27 , a setting signal for powering on or off the above-configured field device  20 . The infrared communication unit  27  supplies the received power setting signal to the power management unit  25 . 
     If being in a standby state, the power management unit  25  is activated by a setting signal supplied from the infrared communication unit  27 , an external interrupt signal, or the like. If being in operation, the power management unit  25  receives a setting signal. According to a signal received from the infrared communication unit  27 , the power management unit  25  stops the supply of power to the wireless communication module  22 , the MPU  23 , the sensor  24 , and the display unit  28 . The power management unit  25  turns on and off the supply of power to the infrared communication unit  27  at regular intervals (e.g., every 1 second) to allow it to detect infrared light. 
     When the supply of power to the wireless communication module  22  from the power management unit  25  is stopped, the wireless communication module  22  stops sending and receiving radio waves completely. 
     As mentioned above, when the battery  26  is removed from the field device  20 , the supply of power to the power management unit  25  is shut off and the supply of power to the wireless communication module  22 , the MPU  23 , the sensor  24 , and the display unit  28  is stopped. The field device  20  may be configured so that when the battery  26  is inserted, the power management unit  25  restarts supply of power to the wireless communication module  22 , the MPU  23 , the sensor  24 , and the display unit  28  irrespective of reception of an on/off signal from the infrared communication unit  27 . 
       FIG. 2  is a graph showing an example current consumption characteristic of the field device  20  of  FIG. 1  before and after joining in a wireless network. The horizontal axis represents time and the vertical axis represents the current consumption. The sequence process of  FIG. 9  is executed in a period Tc that is from power-on of the field device  20  to completion of joining (indicated by a broken line). In the period Tc, as long as the field device  20  is operational, the field device  20  receives signals successively and hence the current consumption has a large value Ic. 
     As in the period Tb shown in  FIG. 10 , in a period Td that starts from the completion of joining, the field device  20  is activated at a constant cycle T, that is, only when it is necessary to send process data. The field device  20  repeats a cycle of calculating and sending process data and then being kept inactive until the next calculation and sending of process data. In a period T 6  of each cycle T, the I/O device  12  is kept inactive and the current consumption has a small value Id (&lt;Ic). In the other period T 7 , the field device  20  is operational to send process data. Each arrow denoted by symbol tp indicates that the field device  20  is sending process data to the gateway  9  by a wireless communication. 
     If the field device  20  has not joined the wireless network even after a lapse of a prescribed time T 3  from the start of the period Tc, the MPU  23  renders the wireless communication module  22  inactive. The prescribed time T 3  can be changed by the user using the infrared transceiver  30 , for example, in a range of one week to one month, for example, according to an installation plan of the gateway  9 . If necessary, the prescribed time T 3  may be set at about 8 to 24 hours. 
     In a period T 4 , the infrared communication unit  27  is kept inactive and the current consumption is reduced to Ic′ because no receiving operation is performed. However, the field device  20  cannot join the wireless network because it cannot receive an advertisement from the routing device  7  or  8  or the gateway  9 . The length of the period T 4  is about 1 hour and can be changed by the user. 
     A period T 5  is a period which starts after the period T 4  and in which the wireless communication module  22  is made operational again by the MPU  23 . In the period T 5 , the wireless communication module  22  waits for reception of an advertisement. In the period T 5 , the current consumption has the large value Ic as in the period T 3 . The length of the period T 5  is about 5 to 10 minutes and can be changed by the user. If the wireless communication module  22  cannot receive an advertisement in the period T 5 , the MPU  23  again renders the wireless communication module  22  inactive. 
     If the gateway  9  is installed and starts to operate sometime in the period Tc, the field device  20  repeats the cycle consisting of the periods T 4  and T 5  until the wireless communication module  22  receives an advertisement. When the wireless communication module  22  has received an advertisement, the period Tc is finished and the period Td is started which is the same as the period Tb of the conventional example shown in  FIG. 10 . 
       FIG. 3  is a state transition diagram of the field device  20 .  FIG. 4  explains the details of the states shown in  FIG. 3 .  FIG. 5  explains the details of the state transitions shown in  FIG. 3 . 
     In  FIGS. 3-5 , “deep sleep” indicates a state that the entire field device  20  is inactive. The MPU  23  is in a standby state and can be activated by an external interrupt signal that is supplied from the power management unit  25 . 
     “Start” is a state that the field device  20  is performing activation processing. “Long wait” corresponds to the advertisement waiting period T 3  (see  FIG. 2 ) during which to join the wireless network. 
     “Sleep” is a state that the wireless communication module  22  is inactive because the field device  20  could not join the wireless network while it was in the “long wait” state. This state corresponds to the period T 4  of the period Tc (see  FIG. 2 ). 
     “Short wait” is a state that the wireless communication module  22  is active and hence can receive an advertisement. This state lasts only the short time T 5  after the “sleep” state. 
     “Comm” is a state that the field device  20  is making a communication after joining the wireless network. This state corresponds to the period Td (see  FIG. 2 ). 
     Symbol S 1  denotes a transition from “deep sleep” to “start.” The field device  20  is activated by insertion of the battery  26  or an event that the infrared communication unit  27  receives an infrared signal from the infrared transceiver  30  and supplies it to the power management unit  25 . 
     Symbol S 2  denotes a transition from “start” to “long wait.” A timer for management of the period T 3  is started upon completion of the field device activation processing. 
     Symbol S 3  denotes a transition from “long wait” to “comm.” The wireless communication module  22  receives an advertisement, whereupon it performs processing for joining the wireless network. 
     Symbol S 4  denotes a transition from “long wait” to “sleep.” The wireless communication module  22  does not receive an advertisement within the setting time of the timer for management of the period T 3 . The wireless communication module  22  is rendered inactive and a timer for management of the period T 4  is started. 
     Symbol S 5  denotes a transition from “sleep” to “short wait.” The setting time of the timer for management of the period T 4  expires, whereupon a timer for management of the period T 5  is started and the wireless communication module  22  is activated. 
     Symbol S 6  denotes a transition from “short wait” to “comm.” The wireless communication module  22  receives an advertisement, whereupon it performs processing for joining the wireless network. 
     Symbol S 7  denotes a transition from “short wait” to “sleep.” The wireless communication module  22  does not receive an advertisement within the setting time of the timer for management of the period T 5 . The wireless communication module  22  is rendered inactive and the timer for management of the period T 4  is started. 
     Symbol S 8  denotes a transition from “comm” to “long wait.” The wireless communication module  22  determines that the field device  20  has left the wireless network, whereupon a timer for management of the period T 3  is started. 
     Symbol S 10  denotes continuation of “deep sleep.” The battery  26  is kept removed and the infrared communication unit  27  receives no infrared signal from the infrared transceiver  30 . 
     Symbol S 11  denotes continuation of “long wait.” The wireless communication module  22  does not receive an advertisement and the timer for management of the period T 3  is doing a counting operation. 
     Symbol S 12  denotes continuation of “sleep.” The timer for management of the period T 4  is doing a counting operation. 
     Symbol S 13  denotes continuation of “short wait.” The wireless communication module  22  does not receive an advertisement and the timer for management of the period T 5  is doing a counting operation. 
     Symbol S 14  denotes continuation of the state “comm” in which the field device  20  belongs to the wireless network. The MPU  23  performs prescribed calculation processing and the wireless communication module  22  sends process data to the gateway  9  at transmission timing. 
     Symbol S 20  denotes a transition from a certain indefinite state to “deep sleep.” The battery  26  is removed in a certain indefinite state, the entire field device  20  is rendered inactive. 
     Symbol S 21  denotes a transition from “sleep” or “short wait” to “long wait.” The infrared communication unit  27  receives an infrared signal from the infrared transceiver  30  in the state “sleep” or “short wait,” whereupon the timer for management of the period T 3  is started. 
     As described above, if the wireless communication module  22  has not received an advertisement in the prescribed period T 3  which is part of the period Tc, the cycle that the wireless communication module  22  is kept in active for the prescribed time T 4  and then kept operational for the prescribed time T 5  is repeated. As a result, the energy consumption of the battery  26  can be saved and the field device  20  can be added to the wireless network automatically after installation of the gateway  9  without the need for turning on the power switch by opening the lid of the field device  20  installed at a certain site. 
     Incidentally, in actual plants, a wireless network is in many cases constructed by installing many field devices. Assume a case that the gateway  9  was disconnected from the wireless network topology  1  temporarily for inspection or replacement work and then connected to the wireless network topology  1  again. In this case, if the wireless communication modules provided in the respective field devices are activated at approximately the same time points, accesses from a large number of field devices to the gateway  9  are concentrated. This may result in a situation that the gateway  9  cannot deal with those accesses it its processing ability is insufficient and a considerable number of field devices are rendered in a sleep state. A long time may be taken until complete recovery of the wireless network. 
     One countermeasure is to deviate (distribute) the activation times of the wireless communication modules provided in the respective field devices from each other by a very small time and thereby prevent access concentration to the gateway  9 . 
       FIG. 6  is a block diagram of a wireless field device  20 A according to another embodiment of the invention, which takes care of such a countermeasure. The units having the same or corresponding ones in  FIG. 1  are given the same reference symbols as the latter. As shown in  FIG. 6 , a time width controller  29  is connected to the power management unit  25 . The time width controller  29  sets unique, prescribed delays to the respective field device  20 A as for operations that the power management units  25  suspend the supply of power to the wireless communication modules  22  according to signals received from the MPU  23 . Although in  FIG. 6  the time width controller  29  is an independent function block, it may be incorporated in the MPU  23  or the power management unit  25 . 
       FIG. 7  is a graph showing an example current consumption characteristic of the field device  20 A of  FIG. 6  before and after joining in a wireless network. Periods and current consumption values having corresponding ones in  FIG. 2  are given the same reference symbols as the latter. The characteristic of  FIG. 7  is different from that of  FIG. 2  in that a prescribed time width T 4 ′ which is set by the time width controller  29  so as to be unique to each field device  20 A is inserted between the period T 4  in which the wireless communication module  22  is rendered inactive and the period T 5  in which the wireless communication module  22  is made operational again. 
     The time width T 4 ′ is about 10% of the time width T 4  and is calculated by the time width controller  29  according to the following Equation (1): 
     
       
         
           
             
               
                 
                   
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                   = 
                   
                     
                       
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                         T 
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                         4 
                       
                       63 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           A 
                           
                             EUI 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             64 
                           
                         
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                         MOD 
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                         63 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where MOD represents a remainder and A EUI64  has a value that is determined according to an EUI-64 address of the field device  20 A. A EUI64  may be the EUI-64 address itself. Alternatively, to increase the processing speed, A EUI64  may be the sum of the lowest 4-byte words of the EUI-64 address that are given by the following Equation (2). 
     The I/O devices  2 - 6  and  12  shown in  FIG. 8  are given respective IEEE EUI-64 bit addresses. The EUI-64 bit address is a 64-bit address which was standardized by IEEE and serves to uniquely identify a device. The upper 24 bits represent a number that is uniquely assigned to a manufacturer. As for the remaining, lower 40 bits, different numbers are assigned to respective devices by the manufacturer.
 
 A   EUI 64   =B   3   +B   2   +B   1   +B   0   (2)
 
     where B 0 , B 1 , B 2 , and B 3  are the values of the lowest byte, the second lowest byte, the third lowest byte, and the fourth lowest byte, respectively, of the EUI-64 address of the field device  20 A. 
     Since as described above different time widths T 4 ′ that are set by the time width controller  29  for respective field devices  20 A are added to the time width T 4 , different suspension times are set for the respective field devices  20 A. As a result, the I/O devices  2 - 6  and  12  shown in  FIG. 8  are activated at different time points. 
     Concentration of accesses from the I/O devices  2 - 6  and  12  to the gateway  9  can be prevented, whereby the load of the gateway  9  can be reduced (distributed). 
     Alternatively, a time width T 3 ′ which is given by the following Equation (3) may be added to the time width T 3 . This provides the same advantage as in the case that the time width T 4 ′ is added to the time width T 4 . 
     
       
         
           
             
               
                 
                   
                     T 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       3 
                       ′ 
                     
                   
                   = 
                   
                     
                       
                         0.1 
                         ⁢ 
                         T 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         4 
                       
                       63 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           A 
                           
                             EUI 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             64 
                           
                         
                         ⁢ 
                         MOD 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         63 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     While the present invention has been shown and described with reference to certain exemplary embodiments thereof, other implementations are within the scope of the claims. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.