Distributed Manufacturing Architecture

Distribute Manufacturing Architecture is an implementation of ADS for factory automation which has been developed and maintained by a technical committee at MSTC/JOP in Japan. Its broadcast-type protocol enhances ease of construction, fault tolerance and maintainability of systems. MDA protocol is understood as observer pattern in the design-pattern point of view. The committee released second version of MDA protocol to enhance flexibility of message syntax. 1. DMA technical committee at MSTC/JOP Manufacturing Science and Technology Center (MSTC) has supported Japan FA Open Systems Promotion Group (JOP) since 1996. Objectives of the group is to promote utilization of de-facto standards that proliferate in PC and Internet industry and to develop or help developing technologies to apply such PC/Internet standards to manufacturing industries. The technologies JOP develops or helps developing should not only be open to public but also supported by as many vendors as possible. To this end, JOP organizes technical committees to define and maintain technologies as specifications and project groups to implement and verify the technologies. The Distributed Manufacturing Architecture Technical Committee is one of those committees. It utilizes Autonomous Decentralized Systems technology to manufacturing fields. While the ADS technology has been implemented with mainframes, mini-computers and proprietary networks for decades, it seems to best fit to implement distributed automation systems using open PC and Internet technologies. 2. A broadcast-type protocol The DMA committee started with a communication protocol specification proposed by Hitachi Ltd., original developer of ADS. The protocol is using UDP/IP/Ethernet as networking standards and implemented on Windows as well as vendor specific controllers. Differing from conventional two-way data communication, an MDA node broadcasts messages to all other nodes in the system. The receiving nodes check Transaction Code Data (TCD) that is attached every message and know whether the message makes sense to the node. This architecture reduces dependencies among nodes greatly. The committee categorized the TCD towards its standardization for manufacturing industry, while it adopted the protocol specification as proposed for the first release of the committee specification [1]. 3. DMA as observer pattern The Distribute Manufacturing Architecture can be recognized as an implementation of Observer Pattern, a term that recent object-oriented design studies defines as one of the behavioral pattern [2]. The Observer Pattern, also mentioned as Publisher-Subscriber Pattern, is a design technique to implement event notification subsystem while minimizing dependency among distributed nods. In almost all factory automation systems some process must do its work when other process changes its state. In this situation the latter, called Subject, shall inform the former, Observer, with its new status when the change occurs. To do so, the Subject must know a-priori what Observer waits for the information. That may add a huge burden to its system designer. The Observer Pattern reduces the potential burden with provision for Observer to register itself to the Subject at its initialization. The Subject then notifies any Observer according to the registered request. The data or status the Observer should know may be given as part of the notification message from the Subject (Push Model), or the Observer may access the Subject to obtain the information when notified (Pull Model). The MDA protocol is a supreme implementation of the Push Model. Broadcasting virtually removes requirement of Observer registration because in the MDA systems senders do not care about receivers. Message with TCD Subject broadcasts can convey enough information required for the Pull Model. 4. DMA protocol release 2 The DMA technical committee released its second version as a result of its continued efforts to improve MDA [3]. The release 2 keeps the basic architecture the same as the first version mentioned above and focuses on enhancement of data portion of the messages. The previous version assumes that every TCD has its specific data format for its main body; sender and receiver must know the format. This may be effective in light of bandwidth utilization, but surely is a burden for design and maintenance of the system. The release 2 defines associative arrays for message data structure: all information following TCD consists of name, data type and value. Several of the data set can be combined in a single message. The name and data type describes itself sufficient for receiver to handle them without specific knowledge of message format. Furthermore, ASN.1 (ISO/IEC 8824/8825) is adopted to describe the associative arrays and data type definition is derived from the PLC programming standard (IEC11313). Adoption of these standards is expected to make implementation easy while maximizing data flexibility though message length and processing time will increase comparing with those in the previous version. Considering recent rapid progress of network bandwidth and microprocessor speed, those will not be obstacles even for controllers in very near future.