Patent Publication Number: US-2023164087-A1

Title: Control system and control method

Description:
TECHNICAL FIELD 
     The present invention relates to a control system and a control method. 
     BACKGROUND ART 
     A time delay in communication processing in a communication network may adversely affect the performance of real-time control of a terminal of the Internet of Things (IoT) (hereinafter referred to as “IoT terminal”). For example, as the time delay in communication processing increases, the time required to stabilize the control system (hereinafter referred to as “settling time”) may increase, deteriorating the performance of real-time control. Further, for example, as the time delay in communication processing increases, the settling time of the control system in the communication network may increase, causing the IoT terminal to be out of control. 
     In general, the longer the distance between a device under control as defined (hereinafter referred to as “control target device”) and a control device that controls the control target device is, the longer the time delay in communication processing becomes. Further, the larger the number of communication devices that are passed through in a communication between the control target device and the control device, the longer the time delay in communication processing becomes. 
     In recent years, techniques related to edge computing have been proposed. In edge computing, a control device is installed in the vicinity of a control target device to control the control target device, so that the time delay in communication processing can be shortened. 
     However, not only the length of the time delay but also the jitter (variation) of the time delay adversely affects the performance of the real-time control of the control target device. For example, the time delay in communication processing in the communication device on the station building side changes according to the changes in the communication band, the number of communication devices, and the communication protocol in the communication network. This causes jitter in the time delay in communication processing between the control target device and the control target device. Therefore, in real-time control of the control target device, it is necessary to reduce the jitter of the time delay. 
     Further, PTL 1 discloses a network management device that aims at setting a buffer size so as to shorten the time delay. Further, PTL 2 discloses a bandwidth allocation device that aims at setting a cycle time for dynamic bandwidth allocation (DBA) so as to achieve both a time delay and a bandwidth utilization efficiency. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] Japanese Patent Application Publication No. 2014-138307 
         [PTL 2] Japanese Patent Application Publication No. 2016-111582 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     However, in the conventional control systems, it may not be possible to reduce the time delay in communication processing in a communication network and the jitter of the time delay. For example, the network management device according to PTL 1 does not cover any optical access network, so that it is not possible to reduce the time delay in communication processing and the jitter of the time delay in an optical access network. Further, for example, the bandwidth allocation device according to PTL 2 is not directed to the real-time control of an IoT terminal and does also not consider the jitter due to the change in the state of a communication network, so that it is not possible to reduce the jitter of the time delay in communication processing in the communication network. 
     In view of the above circumstances, an object of the present invention is to provide a control system and a control method which are capable of reducing at least one of the time delay in communication processing and the jitter of the time delay in a communication network. 
     Means for Solving the Problem 
     One aspect of the present invention is a control system including a control device that controls a control target device; and one or more communication devices that execute communication processing between the control target device and the control device. Each of the communication devices includes a division determination unit that determines whether data used for controlling the control target device per a certain cycle has been divided into a plurality of cycles each being the certain cycle; and a control unit that, when it is determined that the data used for control per the cycle has been divided, sets a size of a packet to be increased so that data required for control per the cycle is contained in the packet, or sets a length of a burst frame to be increased so that data required for control per the cycle is contained in the burst frame. 
     One aspect of the present invention is a control method performed by a control system including a control device that controls the above-mentioned control target device; and one or more communication devices that execute communication processing between the control target device and the control device. The control method includes a division determination step of, by each of the communication devices, determining whether data used for controlling the control target device per a certain cycle has been divided into a plurality of cycles each being the certain cycle; and a control step of, by each of the communication devices, when it is determined that the data used for control per the cycle has been divided, setting a size of a packet to be increased so that data required for control per the cycle is contained in the packet, or setting a length of a burst frame to be increased so that data required for control per the cycle is contained in the burst frame. 
     Effects of the Invention 
     According to the present invention, it is possible to reduce at least one of the time delay in communication processing and the jitter of the time delay in a communication network. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating a configuration example of a control system according to a first embodiment. 
         FIG.  2    is a diagram illustrating a configuration example of a first communication device according to the first embodiment. 
         FIG.  3    illustrates an example of a relationship between packet size and settling time in the first embodiment. 
         FIG.  4    illustrates an example of a relationship between data size and packet size in the first embodiment. 
         FIG.  5    is a flowchart illustrating an operation example of the control system according to the first embodiment. 
         FIG.  6    is a diagram illustrating a configuration example of a control system according to a second embodiment. 
         FIG.  7    is a diagram illustrating a configuration example of a second communication device according to the second embodiment. 
         FIG.  8    illustrates an example of a relationship between burst frame size and settling time in the second embodiment. 
         FIG.  9    illustrates an example of a relationship between data size and burst frame size in the second embodiment. 
         FIG.  10    is a flowchart illustrating an operation example of the control system according to the second embodiment. 
         FIG.  11    is a diagram illustrating a configuration example of a control system according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described in detail with reference to the drawings. 
     First Embodiment 
       FIG.  1    is a diagram illustrating a configuration example of a control system  1   a  according to a first embodiment. The control system  1   a  is a system that executes real-time control of a control target device such as an IoT terminal. The control system  1   a  executes motion control of the control target device in real time. 
     The control system  1   a  includes one or more control devices  2 , one or more first communication devices  3 , and one or more control target devices  4 . In the first embodiment, a control device  2 , one or more first communication devices  3 , and a control target device  4  form a single star (SS) network as an example. 
     Hereinafter, the direction from the control device  2  toward the control target device  4  is referred to as “downlink”. The direction from the control target device  4  to the control device  2  is referred to as “uplink”. Hereinafter, uplink data used for control and downlink data used for control are collectively referred to as “data used for control”. 
     The control device  2  is a device that controls the control target device  4 . The control device  2  acquires from the first communication device  3  certain data transmitted in uplink from the control target device  4  as uplink data used for controlling the control target device  4 . The certain data may be, for example, sensor data generated by the control target device  4  or image data generated by the control target device  4 . This image data may be still image data or moving image data having a predetermined frame rate. 
     In order to control the control target device  4 , the control device  2  generates instruction data based on the certain data as uplink data used for controlling the control target device  4 . The instruction data includes, for example, a code number indicating the type of instruction. The instruction data may include control data such as a setting value used for controlling the control target device  4 . The setting value may be, for example, a value representing the operating amount of the IoT terminal, or a value representing the allocation of the bandwidth for communication. The control device  2  transmits instruction data as downlink data used for controlling the control target device  4  to a following first communication device  3 . Specifically, the control device  2  transmits the instruction data in downlink to the control target device  4  so that the instruction data passes through the first communication devices  3  (the following first communication devices  3 ) disposed downstream of the control device  2 . 
     The first communication devices  3  are each a switch. A first communication device  3 - 0  acquires the instruction data from the control device  2 . The first communication device  3 - 0  transmits the instruction data in downlink to a first communication device  3 - 1 . The first communication device  3 - 1  acquires the instruction data from the first communication device  3 - 0 . The first communication device  3 - 1  transmits the instruction data in downlink to the control target device  4  so that the instruction data passes through the first communication devices  3  disposed downstream of the first communication device  3 - 1 . Note that a plurality of control target devices  4  may be disposed following the first communication device  3  (end node) disposed at the most downstream. 
     The first communication device  3  disposed at the most downstream acquires from the control target device  4  the certain data transmitted in uplink as uplink data used for controlling the control target device  4 . A first communication device  3 -( n +1) (n is an integer of 0 or more) acquires from a first communication device  3 -( n +2) the certain data transmitted in uplink from the control target device  4 . The first communication device  3 -( n +1) transmits to a first communication device  3 - n  the certain data transmitted in uplink from the control target device  4 . The first communication device  3 - 0  transmits to the control device  2  the certain data transmitted in uplink from the control target device  4 . 
     The control target device  4  is a device under control as defined. The control target device  4  is, for example, an IoT terminal or an industrial robot. The control target device  4  acquires from the first communication device  3  disposed at the most downstream the instruction data transmitted in downlink. The control target device  4  generates the certain data in accordance with the instruction data. For example, the control target device  4  generates sensor data as uplink data used for controlling the control target device  4 . For example, the control target device  4  may generate moving image data having a predetermined frame rate as uplink data used for controlling the control target device  4 . 
     The control target device  4  transmits uplink data used for controlling the control target device  4  to the first communication device  3  disposed at the most downstream. Specifically, the control target device  4  transmits the certain data such as sensor data or moving image data in uplink to the control device  2  as uplink data used for controlling the control target device  4 . 
     Next, the details of the first communication device  3  will be described. 
       FIG.  2    is a diagram illustrating a configuration example of the first communication device  3  according to the first embodiment. The first communication device  3  includes a first communication unit  30  (transfer interface), a first storage unit  31 , a transfer unit  32 , a second storage unit  33  (buffer), a second communication unit  34  (transfer interface), a division determination unit  35 , a delay information acquisition unit  36 , a delay determination unit  37 , and a control unit  38 . 
     The first communication unit  30  of a first communication unit  30 - 0  acquires from the control device  2  the packet containing the instruction data transmitted in downlink. The first communication unit  30  of the first communication device  3 -( n +1) acquires from the first communication device  3 - n  the packet containing the instruction data transmitted in downlink. The first communication unit  30  records in the first storage unit  31  the packet containing the instruction data transmitted in downlink. 
     The first communication unit  30  acquires from the first storage unit  31  the packet containing the certain data transmitted in uplink. The first communication unit  30  of the first communication device  3 -( n +1) transmits to the first communication device  3 - n  the packet containing the certain data transmitted in uplink. The first communication unit  30  of the first communication unit  30 - 0  transmits to the control device  2  the packet containing the certain data transmitted in uplink. 
     The first storage unit  31  stores the packet containing the instruction data transmitted in downlink from the first communication unit  30 . The first storage unit  31  stores the packet containing the certain data transmitted in uplink from the transfer unit  32 . 
     The transfer unit  32  acquires from the control unit  38  information on a packet size determined by the control unit  38 . The transfer unit  32  generates a packet having the size determined by the control unit  38 . 
     The transfer unit  32  acquires from the first storage unit  31  the packet containing the instruction data transmitted in downlink. The transfer unit  32  includes in the generated packet the instruction data transmitted in downlink. The transfer unit  32  records in the second storage unit  33  the generated packet containing the instruction data transmitted in downlink. 
     The transfer unit  32  acquires from the second storage unit  33  the packet containing the certain data transmitted in uplink. The transfer unit  32  includes in the generated packet the certain data transmitted in uplink. The transfer unit  32  records in the first storage unit  31  the generated packet containing the certain data transmitted in uplink. 
     The second storage unit  33  stores the packet containing the instruction data transmitted in downlink from the transfer unit  32 . The second storage unit  33  stores the packet containing the certain data transmitted in uplink from the second communication unit  34 . 
     The second communication unit  34  acquires from the second storage unit  33  the packet containing the instruction data transmitted in downlink. The second communication unit  34  of the first communication device  3 - n  transmits to the first communication device  3 -( n +1) the packet containing the instruction data transmitted in downlink. 
     The second communication unit  34  of the first communication device  3 - n  acquires from the first communication device  3 -( n +1) the packet containing the certain data transmitted in uplink. The second communication unit  34  records in the second storage unit  33  the packet containing the certain data transmitted in uplink. The second communication unit  34  transmits to the delay information acquisition unit  36  the packet containing the certain data transmitted in uplink. 
     The division determination unit  35  acquires from the second storage unit  33  the packet containing the instruction data transmitted in downlink. The division determination unit  35  determines whether or not the instruction data used for controlling the control target device  4  per a certain cycle (e.g., packet transmission cycle) has been divided into a plurality of cycles. The division determination unit  35  transmits to the control unit  38  a determination result indicating whether or not the instruction data has been divided into a plurality of cycles. 
     The division determination unit  35  acquires from the second storage unit  33  the packet containing the certain data transmitted in uplink. The division determination unit  35  determines whether or not the size of the data used for control is within the packet size. Specifically, the division determination unit  35  determines whether or not the certain data used for controlling the control target device  4  per a certain cycle (e.g., packet transmission cycle) has been divided into a plurality of cycles. The division determination unit  35  transmits to the control unit  38  a determination result indicating whether or not the certain data has been divided into a plurality of cycles. 
     The delay information acquisition unit  36  and the delay determination unit  37  (time delay monitoring unit) measure the time delay in the entire control system  1   a . The method for measuring the time delay in the entire control system  1   a  is not limited to a specific method. For example, the delay information acquisition unit  36  and the delay determination unit  37  measure the time delay in the communication processing of the entire control system  1   a  by using the control method disclosed in a reference (Japanese Patent Application Publication No. 2020-21410). 
     For example, the delay information acquisition unit  36  of the first communication device  3 - n  measures the time delay in the communication processing of the entire control system  1   a  based on the total of processing times in communication processing of one or more first communication devices  3  (the following first communication devices  3 ) disposed downstream of the first communication device  3 - n . The delay information acquisition unit  36  acquires delay information indicating the time delay in communication processing from the one or more first communication devices  3  disposed downstream of the first communication device  3 - n . Here, the delay information acquisition unit  36  acquires from the second communication unit  34  the packet containing the certain data transmitted in uplink. The delay information acquisition unit  36  sequentially measures the processing time in communication processing based on time information included in the header of the packet of the certain data transmitted in uplink and the reception time of the packet of the certain data transmitted in uplink. The delay information acquisition unit  36  adds up the processing times in communication processing for the first communication devices  3  disposed downstream. The delay determination unit  37  acquires delay information indicating the total of time delays in communication processing from the delay information acquisition unit  36 . The delay determination unit  37  determines whether or not the total of time delays indicated by the delay information exceeds a first threshold value. The first threshold is determined in advance. The delay determination unit  37  transmits to the control unit  38  a determination result indicating whether or not the total of time delays indicated by the delay information exceeds the first threshold value. 
     For example, the delay information acquisition unit  36  may sequentially measure the time delay in the communication processing of the entire control system  1   a  based on a round trip time (RTT) of a signal transmitted to the first communication device  3  disposed at the most downstream. The delay information acquisition unit  36  sequentially measures the round trip time based on time information included in the header of the packet of the certain data transmitted in uplink and the reception time of the packet of the certain data transmitted in uplink. The delay determination unit  37  acquires from the delay information acquisition unit  36  the round trip time as the delay information indicating the time delay in communication processing. The delay determination unit  37  determines whether or not the time delay (round trip time) indicated by the delay information exceeds the first threshold value. The delay determination unit  37  transmits to the control unit  38  a determination result indicating whether or not the time delay indicated by the delay information exceeds the first threshold value. 
     The control unit  38  acquires from the division determination unit  35  a determination result indicating whether or not the certain data has been divided into a plurality of cycles. When it is determined that the certain data has been divided into a plurality of cycles, the control unit  38  sets the packet size to be increased so that at least one of the time delay and the jitter of the time delay is reduced. The control unit  38  transmits information on the packet size to the transfer unit  32 . 
     The control unit  38  may acquire from the delay determination unit  37  a determination result indicating whether or not the time delay indicated by the delay information exceeds the first threshold value. When it is determined that the time delay exceeds the first threshold value, the control unit  38  sets the packet size to be increased so that at least one of the time delay and the jitter of the time delay is reduced. The control unit  38  transmits information on the packet size to the transfer unit  32 . 
       FIG.  3    illustrates an example of a relationship between packet size and settling time in the first embodiment. The horizontal axis represents packet size (network parameter). The vertical axis represents settling time of a control system in a communication network. In real-time control of IoT terminals, an increase in the time delay in communication processing leads to an increase in the settling time of a control system in a communication network. Therefore, the vertical axis may represent time delay. 
     If the size of the certain data transmitted in uplink is less than a threshold value (small data size), the smaller the size of the packet containing the certain data is, the shorter the settling time (time delay) in the entire communication network is. 
     If the size of the instruction data transmitted in downlink is less than a threshold value (small data size), the smaller the size of the packet containing the instruction data is, the shorter the settling time (time delay) in the entire communication network is. 
     If the size of the certain data transmitted in uplink is greater than or equal to the threshold value (large data size) and if the size of the packet containing the certain data is less than or equal to a predetermined size (too small packet size), the settling time (time delay) in the entire communication network is longer. This is because the certain data is divided into a plurality of packets, so that the time (time delay) required to receive all the certain data transmitted in uplink increases. 
     If the size of the instruction data transmitted in downlink is greater than or equal to the threshold value (large data size) and if the size of the packet containing the instruction data is less than or equal to a predetermined size (too small packet size), the settling time (time delay) in the entire communication network is longer. This is because the instruction data is divided into a plurality of packets, so that the time (time delay) required to receive all the instruction data transmitted in downlink increases. 
       FIG.  4    illustrates an example of a relationship between data size and packet size in the first embodiment. The horizontal axis represents time. As illustrated in the first row from the top of  FIG.  4   , if the size of data used for control per a cycle (e.g., packet transmission cycle) is greater than the packet size, the data required for control per a cycle is divided into a plurality of packets. The data required for control per a cycle cannot be acquired by one packet received, so that the time (time delay) required to receive all the data required for control increases. 
     When it is determined that the time delay exceeds the first threshold value, the control unit  38  sets the packet size to be increased to be greater than the data size, as illustrated in the second row from the top of  FIG.  4   . The control unit  38  transmits information on the packet size to the transfer unit  32 . As a result, the data required for control per a cycle can be acquired by one packet received, so that at least one of the time delay and the jitter of the time delay is reduced. 
     The control unit  38  may periodically reduce the packet size by a predetermined size. The control unit  38  determines whether or not at least one of the time delay and the jitter of the time delay increases when the packet size is periodically reduced. When it is determined that at least one of the time delay and the jitter of the time delay increases, the control unit  38  returns the packet size to the original size. 
     When it is determined that the time delay exceeds the first threshold value, the control unit  38  derives an appropriate size for the packet (data size) by using, for example, the gradient method, as illustrated in the third row from the top of  FIG.  4   , so that at least one of the time delay and the jitter of the time delay is reduced. The control unit  38  transmits information of the appropriate size for the packet to the transfer unit  32 . This makes it possible to dynamically respond to changes in the status of the communication network. 
     When jitter occurs for the size of the data used for control per a cycle, the control unit  38  may increase the packet size to give a margin to the packet size, as illustrated in the fourth row from the top of  FIG.  4   . When jitter exceeding a predetermined threshold value “p” occurs with a certain probability “a” or more, the control unit  38  sets the packet size length “1” to be increased to give a margin to the packet size length. This makes it possible to reduce the time delay even when jitter occurs for the size of the data used for control per a cycle. 
     Next, an operation example of the control system  1   a  will be described. 
       FIG.  5    is a flowchart illustrating the operation example of the control system  1   a  according to the first embodiment. The division determination unit  35  determines whether or not the data used for controlling the control target device  4  per a certain cycle has been divided into a plurality of cycles (step S 101 ). The certain cycle is, for example, a packet transmission cycle. 
     When it is determined that the data has not been divided into a plurality of cycles (step S 101 : NO), the division determination unit  35  executes the operation of step S 101  again after a predetermined time shorter than the packet transmission cycle elapses. When it is determined that the data has been divided into a plurality of cycles (step S 101 : YES), the control unit  38  sets the packet size to be increased so that the data required for control per cycle is contained in the packet (step S 102 ). The control unit  38  returns the processing to step S 101 . 
     As described above, the division determination unit  35  determines whether or not the data (at least one of the instruction data and the certain data) used for controlling the control target device  4  per a certain cycle (e.g., packet transmission cycle) has been divided into a plurality of cycles. When it is determined that the data used for control per cycle has been divided, the control unit  38  sets the packet size to be increased so that the data required for control per cycle is contained in the packet. 
     In this way, the control unit  38  changes the packet size so that the data required for control per cycle is contained in the packet. As a result, the data required for control per cycle can be received at one time, so that it is possible to reduce at least one of the time delay in communication processing and the jitter of the time delay in a communication network such as a single star network. It is also possible to improve the accuracy of the control of the IoT terminal so that the IoT terminal is not out of control. 
     Second Embodiment 
     A second embodiment is mainly different from the first embodiment in that the control system includes a second communication device and a third communication device. For the second embodiment, the differences from the first embodiment will be mainly described. 
       FIG.  6    is a diagram illustrating a configuration example of a control system  1   b  according to the second embodiment. The control system  1   b  is a system that executes real-time control of a control target device such as an IoT terminal. The control system  1   b  executes motion control of the control target device in real time. 
     The control system  1   b  includes one or more control devices  2 , a second communication device  5 , one or more third communication devices  6 , and one or more control target devices  4 . In the second embodiment, the control device  2 , the second communication device  5 , the one or more third communication devices  6 , and the one or more control target devices  4  form an optical access network such as PON (Passive Optical Network). 
     The control device  2  is a device that controls the control target devices  4 . The control device  2  acquires from the second communication device  5  certain data transmitted in uplink from the control target devices  4  as uplink data used for controlling the control target devices  4 . 
     The control device  2  transmits instruction data as downlink data used for controlling the control target device  4  to the following second communication device  5 . Specifically, the control device  2  transmits the instruction data to the control target devices  4  in downlink so that the instruction data passes through the second communication device  5  and the third communication devices  6 , which are disposed downstream of the control device  2 . 
     The second communication device  5  is an optical line terminal (OLT). The second communication device  5  is installed in, for example, a station building. The second communication device  5  acquires the instruction data from the control device  2 . The second communication device  5  transmits the instruction data in downlink to the third communication devices  6 . Specifically, the second communication device  5  transmits the instruction data in downlink to the control target devices  4  so that the instruction data passes through an optical fiber and the third communication devices  6  disposed downstream of the second communication device  5 . 
     The second communication device  5  acquires, from each of the third communication devices  6  (the following each third communication device  6 ) disposed downstream of the second communication device  5 , the certain data transmitted in uplink as uplink data used for controlling the control target device  4 . The second communication device  5  transmits to the control device  2  the certain data transmitted in uplink from the control target device  4 . 
     The third communication devices  6  are each an optical network unit (ONU). Each third communication device  6  is installed in, for example, each subscriber&#39;s house (customer&#39;s house). The third communication device  6  acquires the instruction data addressed to a following control target device  4  from the second communication device  5 . A third communication device  6 - n  (n≥1) transmits the instruction data in downlink to a control target device  4 - n  (n≥1). Specifically, the third communication device  6  transmits the instruction data in downlink to the control target device  4  (the following control target device  4 ) disposed downstream of the third communication device  6 . Note that, for each third communication device  6 , a plurality of control target devices  4  may follow the third communication device  6 . 
     The third communication device  6  acquires, from the control target device  4  disposed downstream of the third communication device  6 , the certain data transmitted in uplink as uplink data used for controlling the control target device  4 . The third communication device  6  transmits to the second communication device  5  the certain data transmitted in uplink from the control target device  4 . 
     The control target devices  4  are each a device under control as defined. A control target device  4 - m  (m is an integer of 1 or more) acquires from a third communication device  6 - m  the instruction data transmitted in downlink. The control target device  4  generates the certain data in accordance with the instruction data. 
     The control target device  4 - m  transmits to the third communication device  6 - m  uplink data used for controlling the control target device  4 - m . Specifically, the control target device  4  transmits the certain data such as sensor data or moving image data in uplink to the control device  2  as uplink data used for controlling the control target device  4 . 
     Next, the details of the second communication device  5  will be described. 
       FIG.  7    is a diagram illustrating a configuration example of the second communication device  5  according to the second embodiment. The first communication device  3  includes a third communication unit  50  (optical communication unit), a signal processing unit  51 , a third storage unit  52 , a transfer unit  53 , a fourth storage unit  54  (buffer), a fourth communication unit  55  (optical communication unit), a division determination unit  56 , a delay information acquisition unit  57 , a delay determination unit  58 , and a control unit  59 . 
     The third communication unit  50  acquires from the control device  2  a burst frame (continuous frame) including the instruction data transmitted in downlink. The third communication unit  50  transmits to the signal processing unit  51  the burst frame including the instruction data transmitted in downlink. 
     The third communication unit  50  acquires from the signal processing unit  51  a burst frame including the certain data transmitted in uplink. The third communication unit  50  transmits to the control device  2  the burst frame including the certain data transmitted in uplink. 
     The signal processing unit  51  executes predetermined signal processing. The predetermined signal processing is, for example, dynamic bandwidth allocation (DBA) processing for each third communication device  6 . The signal processing unit  51  includes in the instruction data the execution result of the predetermined signal processing. The signal processing unit  51  acquires from the third communication unit  50  the burst frame including the instruction data transmitted in downlink. The signal processing unit  51  records in the third storage unit  52  the burst frame including the instruction data transmitted in downlink. 
     The signal processing unit  51  acquires from the third storage unit  52  the burst frame including the certain data transmitted in uplink. The signal processing unit  51  transmits to the third communication unit  50  the burst frame including the certain data transmitted in uplink. 
     The third storage unit  52  stores the burst frame including the instruction data transmitted in downlink from the signal processing unit  51 . The third storage unit  52  stores the burst frame including the certain data transmitted in uplink from the transfer unit  53 . 
     The transfer unit  53  acquires information on a burst frame length determined by the control unit  59  from the control unit  59 . The transfer unit  53  generates a burst frame having the length determined by the control unit  59 . 
     The transfer unit  53  acquires from the fourth storage unit  54  the burst frame including the certain data transmitted in uplink. The transfer unit  53  includes in the generated burst frame the certain data transmitted in uplink. The transfer unit  53  records in the third storage unit  52  the generated burst frame including the certain data transmitted in uplink. 
     The fourth storage unit  54  stores the burst frame including instruction data transmitted in downlink from the transfer unit  53 . The fourth storage unit  54  stores the burst frame including the certain data transmitted in uplink from the fourth communication unit  55 . 
     The fourth communication unit  55  acquires from the fourth storage unit  54  the burst frame including the instruction data transmitted in downlink. The fourth communication unit  55  transmits to each third communication device  6  the burst frame including the instruction data transmitted in downlink. In the header of the burst frame including the instruction data, identification information of the control target device  4  which is the destination of the instruction data is described. 
     The fourth communication unit  55  acquires from the third communication device  6  the burst frame including the certain data transmitted in uplink. The fourth communication unit  55  records in the fourth storage unit  54  the burst frame including the certain data transmitted in uplink. The fourth communication unit  55  transmits to the delay information acquisition unit  57  the burst frame including the certain data transmitted in uplink. 
     The division determination unit  56  acquires from the fourth communication unit  55  the burst frame including the instruction data transmitted in downlink. The division determination unit  56  determines whether or not the instruction data used for controlling the control target device  4  per a certain cycle (e.g., burst frame transmission cycle) has been divided into a plurality of cycles. The division determination unit  56  transmits to the control unit  59  a determination result indicating whether or not the instruction data has been divided into a plurality of cycles. 
     The division determination unit  56  acquires from the fourth communication unit  55  the burst frame including the certain data transmitted in uplink. The division determination unit  35  determines whether or not the size of the data used for control is within the burst frame length. Specifically, the division determination unit  35  determines whether or not the certain data used for controlling the control target device  4  per a certain cycle (e.g., burst frame transmission cycle) has been divided into a plurality of cycles. The division determination unit  56  transmits to the control unit  59  a determination result indicating whether or not the certain data has been divided into a plurality of cycles. 
     The delay information acquisition unit  57  and the delay determination unit  58  (time delay monitoring unit) measure the time delay in the entire control system  1   b . The method for measuring the time delay in the entire control system  1   b  is not limited to a specific method. For example, the delay information acquisition unit  57  and the delay determination unit  58  measure the time delay in the communication processing of the entire control system  1   b  by using the control method disclosed in a reference (Japanese Patent Application Publication No. 2020-21410). 
     For example, the delay information acquisition unit  57  measures the time delay in the communication processing of the entire control system  1   b  based on the processing time in the communication processing of the following third communication device(s)  6 . The delay information acquisition unit  57  acquires delay information indicating the time delay in communication processing from the third communication device  6 . Here, the delay information acquisition unit  57  acquires from the fourth communication unit  55  the burst frame including the certain data transmitted in uplink. The delay information acquisition unit  57  sequentially measures the processing time in communication processing based on time information included in the header of the burst frame of the certain data transmitted in uplink and the reception time of the burst frame of the certain data transmitted in uplink. The delay determination unit  58  acquires delay information indicating the time delay in communication processing from the delay information acquisition unit  57 . The delay determination unit  58  determines whether or not the time delay indicated by the delay information exceeds a first threshold value. The delay determination unit  58  transmits to the control unit  59  a determination result indicating whether or not the time delay indicated by the delay information exceeds the first threshold value. 
     For example, the delay information acquisition unit  57  may sequentially measure the time delay in the communication processing of the entire control system  1   b  based on a round trip time of an optical signal transmitted to the following third communication device(s)  6 . The delay information acquisition unit  57  sequentially measures the round trip time based on time information included in the header of the burst frame of the certain data transmitted in uplink and the reception time of the burst frame of the certain data transmitted in uplink. The delay determination unit  58  acquires from the delay information acquisition unit  57  the round trip time as the delay information indicating the time delay in communication processing. The delay determination unit  58  determines whether or not the time delay (round trip time) indicated by the delay information exceeds the first threshold value. The delay determination unit  58  transmits to the control unit  59  a determination result indicating whether or not the time delay indicated by the delay information exceeds the first threshold value. 
     The control unit  59  acquires from the division determination unit  56  a determination result indicating whether or not the certain data has been divided into a plurality of cycles. When it is determined that the certain data has been divided into a plurality of cycles, the control unit  59  sets the burst frame length to be increased so that at least one of the time delay and the jitter of the time delay is reduced. The control unit  59  transmits information on the burst frame length to the transfer unit  53 . 
     The control unit  59  may acquire from the delay determination unit  58  a determination result indicating whether or not the time delay indicated by the delay information exceeds the first threshold value. When it is determined that the time delay exceeds the first threshold value, the control unit  59  sets the burst frame length to be increased so that at least one of the time delay and the jitter of the time delay is reduced. The control unit  38  transmits information on the burst frame length to the transfer unit  53 . 
       FIG.  8    illustrates an example of a relationship between burst frame size and settling time in the second embodiment. The horizontal axis represents burst frame size (network parameter). The vertical axis represents settling time of a control system in a communication network. The vertical axis may represent time delay. 
     If the size of the certain data transmitted in uplink is greater than or equal to the threshold value (large data size) and if the size of the burst frame containing the certain data is less than or equal to a predetermined size (too small burst frame length), the time delay in the communication processing of the entire communication network is longer. This is because, due to the certain data being divided into a plurality of burst frames, it takes time to receive the certain data transmitted in uplink. 
     If the size of the instruction data transmitted in downlink is greater than or equal to the threshold value (large data size) and if the size of the burst frame containing the instruction data is less than or equal to a predetermined size (too small burst frame length), the time delay in the communication processing of the entire communication network is longer. This is because, due to the instruction data being divided into a plurality of burst frames, it takes time to receive the instruction data transmitted in downlink. 
       FIG.  9    illustrates an example of a relationship between data size and burst frame size in the second embodiment. The horizontal axis represents time. As illustrated in the first row from the top of  FIG.  9   , if the size (length) of the data used for control per a cycle (e.g., burst frame transmission cycle) is longer than the length of the burst frame, the data required for control per a cycle is divided into a plurality of burst frames. The data required for control per a cycle cannot be acquired by one burst frame received, so that the time (time delay) required to receive all the data required for control increases. 
     When it is determined that the time delay exceeds the first threshold value, the control unit  59  increases the burst frame length to be greater than the data size, as illustrated in the second row from the top of  FIG.  9   . The control unit  59  transmits information on the burst frame length to the transfer unit  53 . As a result, the data required for control per a cycle can be acquired by one burst frame received, so that at least one of the time delay and the jitter of the time delay is reduced. 
     The control unit  59  may periodically reduce the burst frame length by a predetermined length. The control unit  59  determines whether or not at least one of the time delay and the jitter of the time delay increases when the burst frame length is periodically reduced. When it is determined that at least one of the time delay and the jitter of the time delay decreases, the control unit  59  returns the burst frame length to the original length. 
     When it is determined that the time delay exceeds the first threshold value, the control unit  59  derives an appropriate length for the burst frame (data size) by using, for example, the gradient method, as illustrated in the third row from the top of  FIG.  9   , so that at least one of the time delay and the jitter of the time delay is reduced. The control unit  59  transmits information on the appropriate length for the burst frame to the transfer unit  53 . This makes it possible to dynamically respond to changes in the status of the communication network. 
     When jitter occurs for the size of the data used for control per a cycle, the control unit  59  may increase the burst frame length to give a margin to the burst frame length, as illustrated in the fourth row from the top of  FIG.  9   . When jitter exceeding a predetermined threshold value “p” occurs with a certain probability “a” or more, the control unit  59  sets the burst frame length “1” to be increased to give a margin to the burst frame length. This makes it possible to reduce the time delay even when jitter occurs for the size of the data used for control per a cycle. 
     Next, an operation example of the control system  1   b  will be described. 
       FIG.  10    is a flowchart illustrating the operation example of the control system  1   b  according to the second embodiment. The division determination unit  56  determines whether or not the data used for controlling the control target device  4  per a certain cycle has been divided into a plurality of cycles (step S 201 ). The certain cycle is, for example, a burst frame transmission cycle. 
     When it is determined that the data has not been divided into a plurality of cycles (step S 201 : NO), the division determination unit  56  executes the operation of step S 201  again after a predetermined time shorter than the burst frame transmission cycle elapses. When it is determined that the data has been divided into a plurality of cycles (step S 201 : YES), the control unit  59  sets the burst frame length to be increased so that the data required for control per cycle is contained in the packet (step S 202 ). The control unit  59  returns the processing to step S 201 . 
     As described above, the division determination unit  35  determines whether or not the data (at least one of the instruction data and the certain data) used for controlling the control target device  4  per a certain cycle (e.g., burst frame transmission cycle) has been divided into a plurality of cycles. When it is determined that the data used for control per cycle has been divided, the control unit  38  sets the burst frame length to be increased so that the data required for control per cycle is contained in the burst frame. 
     In this way, the control unit  38  changes the burst frame length so that the data required for control per cycle is contained in the burst frame. As a result, the data required for control per cycle can be received at one time, so that it is possible to reduce at least one of the time delay in communication processing and the jitter of the time delay in an optical access network such as PON. It is also possible to improve the accuracy of the control of the IoT terminal so that the IoT terminal is not out of control. 
     Third Embodiment 
     A third embodiment is mainly different from the first embodiment and the second embodiment in that the control system includes a first communication device, a second communication device, and a third communication device. For the third embodiment, the differences from the first embodiment and the second embodiment will be mainly described. 
       FIG.  11    is a diagram illustrating a configuration example of a control system  1   c  according to the third embodiment. The control system  1   c  is a system that executes real-time control of a control target device such as an IoT terminal. The control system  1   c  executes motion control of the control target device in real time. 
     The control system  1   c  includes one or more control devices  2 , a plurality of first communication devices  3 , a second communication device  5 , one or more third communication devices  6 , and one or more control target devices  4 . In the third embodiment, the control device  2 , the plurality of first communication devices  3 , the second communication device  5 , the one or more third communication devices  6 , and the one or more control target devices  4  form an optical access network such as PON. 
     The control device  2  is a device that controls the control target devices  4 . The control device  2  acquires from a first communication device  3 - 0  certain data transmitted in uplink from the control target devices  4  as uplink data used for controlling the control target devices  4 . 
     The control device  2  transmits instruction data as downlink data used for controlling the control target device  4  to the following first communication device  3 - 0 . Specifically, the control device  2  transmits the instruction data in downlink to the control target devices  4  so that the instruction data passes through the first communication devices  3 , the second communication device  5 , and the third communication devices  6 , which are disposed downstream of the control device  2 . 
     The first communication devices  3  are each a switch. The first communication device  3 - 0  acquires the instruction data from the control device  2 . The first communication device  3 - 0  transmits the instruction data in downlink to the second communication device  5 . 
     The first communication device  3 - 0  acquires from the second communication device  5  the certain data transmitted in uplink from the control target device  4 . The first communication device  3 - 0  transmits to the control device  2  the certain data transmitted in uplink from the control target device  4 . 
     The second communication device  5  is an optical line terminal. The second communication device  5  acquires the instruction data from the first communication device  3 - 0 . The second communication device  5  transmits the instruction data in downlink to the third communication devices  6 . Specifically, the second communication device  5  transmits the instruction data in downlink to the control target devices  4  so that the instruction data passes through an optical fiber, the first communication devices  3 , and the third communication devices  6 , which are disposed downstream of the second communication device  5 . 
     The second communication device  5  acquires, from each of the third communication devices  6  (the following each third communication device  6 ) disposed downstream of the second communication device  5 , the certain data transmitted in uplink as uplink data used for controlling the control target device  4 . The second communication device  5  transmits to the first communication device  3 - 0  the certain data transmitted in uplink from the control target device  4 . 
     The third communication devices  6  are each an optical network unit. The third communication device  6  acquires the instruction data addressed to the following control target device  4  from the second communication device  5 . A third communication device  6 - n  (n≥1) transmits the instruction data in downlink to a first communication device  3 - n  (n≥1). Specifically, the third communication device  6  transmits the instruction data in downlink to the control target device  4  (the following control target device  4 ) disposed downstream of the third communication device  6 . 
     The third communication device  6 - n  (n≥1) acquires, from the first communication device  3 - n  (n≥1) disposed downstream of the third communication device  6 - n  (n≥1), the certain data transmitted in uplink as uplink data used for controlling a control target device  4 - n  (n≥1). The third communication device  6  transmits to the second communication device  5  the certain data transmitted in uplink from the control target device  4 . 
     The first communication device  3 - n  (n≥1) acquires the instruction data from the third communication device  6 - n . The first communication device  3 - n  (n≥1) transmits the instruction data in downlink to the control target device  4 - n . Note that, for each first communication device  3 - n  (n≥1), a plurality of control target devices  4  may follow the first communication device  3 - n  (n≥1). 
     The first communication device  3 - n  (n≥1) acquires from the control target device  4 - n  the certain data transmitted in uplink as uplink data used for controlling the control target device  4 . The first communication device  3 - n  (n≥1) transmits to the third communication device  6 - n  the certain data transmitted in uplink from the control target device  4 - n.    
     The control target devices  4  are each a device under control as defined. A control target device  4 - m  acquires from a first communication device  3 - m  the instruction data transmitted in downlink. The control target device  4  generates the certain data in accordance with the instruction data. 
     The control target device  4 - m  transmits to the first communication device  3 - m  uplink data used for controlling the control target device  4 - m . Specifically, the control target device  4  transmits the certain data such as sensor data or moving image data in uplink to the control device  2  as uplink data used for controlling the control target device  4 . 
     As described above, the division determination unit  35  determines whether or not the data (at least one of the instruction data and the certain data) used for controlling the control target device  4  per a certain cycle has been divided into a plurality of cycles. When it is determined that the data used for control per cycle has been divided, the control unit  38  sets the packet size to be increased so that the data required for control per cycle is contained in the packet. When it is determined that the data used for control per cycle has been divided, the control unit  38  sets the burst frame length to be increased so that the data required for control per cycle is contained in the burst frame. 
     In this way, the control unit  38  changes at least one of the packet size and the burst frame length. As a result, the data required for control per cycle can be received at one time, so that it is possible to reduce at least one of the time delay in communication processing and the jitter of the time delay in a single star network or an optical access network. It is also possible to improve the accuracy of the control of the IoT terminal so that the IoT terminal is not out of control. 
     Part or all of the devices of the control system are realized as software by a processor, such as a CPU (Central Processing Unit), executing a program stored in a storage unit including a non-volatile recording medium (non-transitory recording medium). The program may be recorded on a computer-readable recording medium. The computer-readable recording medium is a non-transitory recording medium, for example, a portable medium such as a flexible disk, an optical magnetic disk, a portable medium, an ROM (Read Only Memory) or a CD-ROM (Compact Disc Read Only Memory), or a storage device such as a hard disk built in computer system. 
     Part or all of the devices of the control system may be implemented using hardware including an electronic circuit or circuitry using, for example, LSI (Large Scale Integration circuit), ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), FPGA (Field Programmable Gate Array), or the like. 
     Although the embodiments of the present invention have been described in detail above with reference to the drawings, the specific configuration is not limited to such embodiments, and includes any designs and the like without departing from the spirit and scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to control systems such as IoT terminals and industrial robots. 
     REFERENCE SIGNS LIST 
     
         
           1   a ,  1   b ,  1   c  Control system 
           2  Control device 
           3  First communication device 
           4  Control target device 
           5  Second communication device 
           6  Third communication device 
           30  First communication unit 
           31  First storage unit 
           32  Transfer unit 
           33  Second storage unit 
           34  Second communication unit 
           35  Division determination unit 
           36  Delay information acquisition unit 
           37  Delay determination unit 
           38  Control unit 
           50  Third communication unit 
           51  Signal processing unit 
           52  Third storage unit 
           53  Transfer unit 
           54  Fourth storage unit 
           55  Fourth communication unit 
           56  Division determination unit 
           57  Delay information acquisition unit 
           58  Delay determination unit 
           59  Control unit