Holonic Self-Organization of MetaMorph via Dynamic Virtual Clustering

3.5.1

Holonic MetaMorphic Architecture

Within the HMS consortium, part of our research has focused on how to dynamically reconfigure a multiagent system, according to need, so that it develops or retains holonic structures (Zhang and Norrie 1999). For this, we have developed a mathematical framework (see Sections 3.6 and 3.7) that enables automatic holonic clustering within a generic (nonholonic) multiagent system (MAS). The method is based on uncertainty minimization via fuzzy modeling of the MAS. This method appears to have promise

for reconfiguring distributed manufacturing systems as holonic structures, as well as for investigating the potential for a nonholonic manufacturing system to migrate toward a holonic one.

In this section, using metamorphic mechanisms for distributed decision-making in an agent-based manufacturing system, the concept of dynamic virtual clustering is extended to manufacturing process control at the lower levels (Zhang and Norrie 1999). Event-driven dynamic clustering of resource control services and cooperative autonomous activities are emphasized in this approach.

As mentioned in Section 3.3, virtual clustering in MetaMorph is a dynamic mechanism for organiza- tional reconfiguration of the manufacturing system during run-time. An organization based on virtual clusters of entities can continually be reconfigured in response to changing task requirements. These tasks can include orders, production requests, as well as planning, scheduling, and control. A cluster exists for the duration of the task or subtask it was created for and is destroyed when the task is completed. Mediators play key roles in the process and manage the clusters. Instead of having preestablished and rigid layers of hierarchically organized mechanisms, a mediator-based metamorphic system can use reconfiguration mechanisms to dynamically organize its manufacturing devices. The necessary structures of control are then progressively created during the planning and execution of any production task. In this dynamically changing virtual organization, the partial control hierarchies are dynamic and transient and the number of control layers for any specific order task are task-oriented and time-dependent. It will be seen that holonic characteristics such as “clusters-within-clusters” groupings exist at different organizational levels.

3.5.2

Holon Types in MetaMorph’s Holarchy

A basic HMS architecture can be based on four holon types: product holon (PH), product model holon (PMH), resource holon (RH), and mediator holon (MH). A product holon holds information about the process status of product components during manufacturing, time constraint variables, quality status, and decision knowledge relating to the order request. A product holon is a dual of a physical “component” and information “component.” The physical component of the product holon develops from its initial state (raw materials or unfinished product) to an intermediate product, and then to the finished one, i.e., the end product. A product model holon holds up-to-date engineering information relating to the product life cycle (configuration, design, process plans, bills of materials, quality assurance procedures, etc.). A resource holon contains physical and information components. The physical part contains a production resource of the manufacturing system (machine, conveyor, pallet, tool, raw material, and end product, or accessories for assembling, etc.), together with controller components. The information part contains planning and scheduling components.

In the following development of a reconfigurable HMS architecture using the four basic holon types, a mediator holon serves as an intelligent logical interconnection to link and manage orders, product data, and specific manufacturing resources dynamically. The mediator holon can collaborate with other holons to search for and coordinate resource, product data, and related production tasks. A mediator holon is itself a holarchy. A mediator holon can create a dynamic mediator holon (DMH) for a new task such as a new order request or suborder task request. The dynamic mediator holon then has the responsibility for the assigned task. When the task is completed, the DMH is destroyed or terminates for reuse. DMHs identify order-related resource clusters (i.e., machine group) and manage task decomposition associated with their clusters.

3.5.3

Holonic Self-Organization

The following example will illustrate holonic clustering within this architecture. Figure 3.4 shows the initial activity sequence following the release to production of an order for 100 of a particular product. This product is composed of three identical parts (to be machined) and two identical subassemblies (each to be assembled). As shown in Figure 3.4, following the creation of the appropriate product holon, there are created the relevant part and subassembly holons. The requests for manufacturing made by these

latter holons to appropriate production holons (which function as high-level Production Managers for a manufacturing shop-floor plan or part dispatch) result in the creation of dynamic mediators for the machining and assembly tasks. Subsequently, each production holon coordinates inspection or assembly of the parts or subassemblies according to the production sequence prescribed by the production model holon (from its stored information). More complex situations will occur, when products having many components requiring different types of production processes are involved.

After physical and logical machine groups are derived (for example, via group-technology approaches), the necessary control structures are created and configured using control components cloned from template libraries by a DMH. The machine groups, their associated and configured controllers, then form a temporary manufacturing community, termed a virtual cluster holon (VCH), as shown in Figure 3.5. The VCH exists for the duration of the relevant job processing and is destroyed when these production processes are completed. The physical component of a VCH is composed of order-related parts, raw materials or subproducts for assembly, manufacturing machines and tools, and associated controller hardware. Within these manufacturing environments, parts develop from their initial state to an inter- mediate product and then to the finished one. The information component of a VCH is composed of cluster controller software-components, the associated DMH, and intermediate information on the order and the related product. Each cluster controller is further composed of multilayer control functions that execute job collaboration, control application generation and controller dynamic reconfiguration, process execution, and process monitoring, etc.

3.5.4

Holonic Clustering

The life cycle of a dynamic virtual cluster holon has four stages: resource grouping; control components creation; execution processing; and termination/destruction. The dynamic mediator holon is involved in the stages 1 and 2. The first cluster that is created is the schedule-control cluster shown in Figure 3.5. A cluster can be also considered to be a holonic grouping. The controller cluster next created is composed of three holonic parts: collaboration controller (CC), execution controller (EC), and control execution (CE) holon. One CE holon can be associated with more than one physical controller (execution platform such as real-time operation system and its hardware support devices) and functions as a distributed- node transparent-resource platform for execution of cluster control tasks at the resource level. In the prototype system under development, the CC, EC, and CE holons collaborate to control and execute the

FIGURE 3.4 Holonic clustering mechanism.

Order Release Holon Part Holon Batch Size=300 Request: 300 Part - X Production Holon Production Holon Machining Creates Creates Creates Creates Creates Dynamic Mediator Dynamic Mediator Production Task: P-6329 Production Task: P-6895 Assembling Request: 200 Sub_Assy-Y Sub_Assy Holon Batch Size = 200 Product Holon Batch Size = 100 Product Model Holon Request: Create Product Holon (100)

distributed tasks or applications on a new type of distributed real-time operating system recently imple- mented (Zhang et al. 1999). The distributed tasks or applications are represented using the Function Block (FB)-1499 specification, which is a draft standard described by the IEC for distributed industrial- process measurement and control systems.

As shown in Figure 3.5, the dynamic mediator holon records and traces local dynamic information of the individual holons in its associated virtual cluster community. It is important to note that during the life cycle of the DMH, this mediator may pass instantaneous information of the partial resource holons to some new virtual cluster communities while the assigned tasks on these resource holons are being completed.

The dynamic characteristics of the event-driven holon community become more complicated as the population grows. In the next section, we present an approach for automatic grouping into holonic clusters depending on the assigned task. This approach, due to its strong mathematical foundation, should be applicable to large multiagent systems.