Standardized Assembly Process (Skill) Descriptions

The description of an assembly process is in line with the new IEC 61499 Standard which provides an object oriented control structure and reuse of program logic for PLC’s. The standard is applicable to the system control, which consist in the operation of the MAS. This is the step that occurs after the process of configuration. The use of this function block concept in the configuration methodology allows direct mapping between the configuration of the system and the control of the system. If the same function block descriptions are used both for configuration and for control, then the setting up of the system control will be shortened. Figure 4.3 provides an overview of the conceptual assembly process block, the main

62 characteristics of the Assembly Process description for the purpose of configuration are:

· The Assembly Process type – which has to be extracted from the existing list of assembly process types within the Assembly Process Library, so it is included in the assembly process template.

· The Control Ports – which provide control variables, both inputs and outputs, for operating the assembly process (namely: Start, Interrupt, Finished, etc.). These also provide the means for plugging together two different assembly processes. This will enable the establishment of the control sequence.

· The Parameter Ports – which exist for assembly processes interactions, providing complex data when present. A typical use of these is a force feedback loop, where the value of the force would be passed on to other processes via a parameter port. In other word this provides the information flow of a given assembly process configuration.

.. . .. . .. . .. . Control Ports (Inputs) Control Ports (Outputs) Parameter Ports (Inputs) Parameter Ports (Outputs) Assembly Process Block

Figure 4.3 - Overview of conceptual assembly process block

The Assembly process needs to be able to encapsulate all of the characteristics, but also to be structured in a way that can be enhanced in the future to add other characteristics. The proposed assembly process description establishes a main node for the assembly process, which contains a set of attributes (Name, AssemblyProcessID and Description). The node will contain five child nodes to provide the additional information that can be common across other assembly processes, namely Assembly Process type, Control Ports, Parameter Ports,

63 Configuration Characteristics and Composed. These can be seen in the overview of the XSD description provided in Figure 4.4.

Figure 4.4 - Overview of Assembly Process Skeleton (XSD)

The Assembly Process type is linked with the Assembly Process Library, therefore the template will contain a list of possibilities, which restricts the choice of assembly process type to the types that exist in the Assembly Process Library. This enforces the use of the same terminology to define the same type of skill.

The Control Ports node contains several nodes of the type “Control Port”. This enables the possibility of an infinite number of ports. However, at least four are always required, namely two inputs, Start and Interrupt and two output, Finished and

64 Error. Each port will contain the interface information which provides the information for the other control ports which can be plugged into the current one. The Parameter Ports is an optional node that contains several nodes of the type “Parameter Port”. In simple assembly processes it is expected that this node will not exist. However, in more complex assembly processes this will be required. When this node exists it will also require interface information for establishing to what other ports it can be plugged into.

4.2.4.1. Assembly Process Configuration Assessment Characteristics

The assembly process description needs to contain configuration relevant information. It has been identified in the literature that the most important parameters for the configuration methodology are Time, Cost, Quality and Flexibility (Chryssolouris [14]). All these aspects could be related to the assembly process. However, it is considered that only characteristics that are intrinsic to the assembly process should be associated with it. For the purposes of this work, flexibility has

been defined as the spare capabilities within a given system. Therefore, it is not

specific to an assembly process but rather to the non used assembly processes present in the given system.

The Cost of the process could be viewed as a relevant characteristic, but the process cost would have a marginal impact on the system cost. However, a system running cost can be inferred if its cost is defined for each assembly process. This information should be an average value by a unit of time. So, the first required characteristic of an assembly process is “Running Cost” and is defined as:

“The average cost per unit of time to activate and run a given assembly process” The execution of an assembly process always has a time constraint, as “Time” is required for performing any assembly process. Therefore, “Time” is another important characteristic that needs to be established for each process in order to determine the cycle time of a given system. Consequently, “Time” is the second required characteristic of an assembly process, and is defined as:

“The average time required to perform an given assembly process”

Quality is a concept intrinsically related to the product. However, accuracy and repeatability are assembly process characteristics which have an impact on the

65 quality of the product. Furthermore, these are characteristics that are related to the defined flexibility of a system. Therefore these need to be included in the assembly process description. To do so it is required to clarify what is the difference between accuracy and repeatability, which can be summarized by Figure 4.5, where an overview of the two concepts can be seen.

Bad Accuracy, Good Repeatability Bad Accuracy, Bad Repeatability

Good Accuracy, Good Repeatability Good Accuracy, Bad Repeatability

Target

Accuracy

Repeatability

Figure 4.5 - Overview of Distinction between Accuracy and Repeatability

Accuracy tells us how close a measurement is to achieve its intended target. The difference between the target and the achieved result is the accuracy of the assembly process. Thus, “Accuracy” is the third required characteristic of an assembly process and is defined as:

66 “A numeric value that establishes the average difference between the achieved assembly process result and its intended target”

The repeatability of an assembly process is its ability to achieve the same target in a repeatable manner. The perfect precision is achieved if an assembly process is capable of obtaining the same result, regardless of the number of times it is executed. So the repeatability is the deviation obtained when performing repeatedly the same assembly process under the same conditions. So, “Repeatability” is the fourth required characteristic of the assembly process and it is defined as:

“The deviation of results obtain from running the assembly process under the

same conditions several times”

The configurations characteristics described should appear in the format of a statistical distribution. This will provide better insight into the assembly processes, allowing a more realistic description of the assembly process. This will also allow more information for system optimization issues. The normal distribution will be used in this work, which can be seen in Equation 1. This distribution has two variables that enable its definition the mean value (µ) and the variance of that value (). Thus all values for the configuration characteristics will have the following form:

Equation 1 - Normal Distribution (Snedecor and Cochran [129])

4.2.4.2. Composition of Assembly Processes

The Composed node provides the means to define composite assembly processes. These composite assembly processes are composed of more elementary assembly processes. This allows for the high level definition of assembly processes and provides the basis for combining assembly processes into new assembly processes.

67 Assembly Process Block Assembly Process Block Assembly Process Block

Assembly Process Block

Figure 4.6 - Composite Assembly Process Block Overview

The composite assembly process also need to cater for the connectivity issues that arise from being composed of elementary assembly processes. Therefore, there is a need for establishing how to connect the control and parameter ports. Another important aspect to consider is the possibility of a composite assembly process having alternative realizations. This should be an element that the description should also cater for. The Composed node will contain several composition nodes to deal with the alternatives possible. These will in turn contain assembly process nodes, and a connection node that establishes which port nodes connect to one another. Figure

4.7 presents that XSD structure that incorporates all the description and enables the

definition of composite assembly process blocks.

This provides the model for the definition of templates that can be stored in the Assembly Process library. Once stored these can be retrieved and used by the actors involved in the configuration methodology.

68 Figure 4.7 - XSD Structure that Enables the Definition of Complex Assembly Processes