University of Michigan, Ann Arbor
June 26-27, 2000
Sponsored by NSF and ERC/RMS
The purpose of this presentation is to introduce the GMPT future control software requirements. Having technologies and products that satisfy these requirements will accelerate the realization of the OMAC concept. An overview of GMPT and other end user's manufacturing requirements is presented first. These requirements are then translated into control system requirements in general and control software requirements in specific. Technical elements that are necessary to meet the control software requirements are presented next. Finally, technical topics that require further research are suggested.
The IEC 61499 project is expected to produce at least two Publicly Available Specifications (PASs) this year for a two-year trial use period. This specification extends the application of IEC 61131-3 concepts to the domain of distributed industrial process automation and control. Key to this extension is the replacement of the concept of a "program" by the concept of a "distributed application" represented by the flows of data and events in an extended function block diagram. Additional key elements include the introduction of event-driven state machines for control of algorithm execution, and service interface function blocks which can provide interfaces to any operating system-provided services defined as specified in the Open System Interconnection (OSI) service model. Software portability is supported by XML DTDs (Document Type Definitions) for reusable software library elements.
If time permits, a demonstration will be given of prototype IEC 61499-based software tools for the modeling and simulation of manufacturing process control and automation.
For more information, see http://www.holobloc.com/papers/opendist/
When comparing software development methods and development requirements for coordination applications, we realize that a productivity crisis is about to happen, which cannot be managed alone with traditional validation techniques (i.e. reviews, simulation and testing), but urgently requires validation technology innovation. Coordination applications are typically component-based, event-driven, and characterized by a clear separation between the tasks to be coordinated and their coordination. These particular characteristics, not encountered in general software applications, can be effectively exploited for validation purposes, and constitute - together with expressive, clear and verifiable application descriptions, and powerful verification algorithms - the basis of the validation technology innovation proposed by the VALID tool set.
The talk will first focus on current trends and issues in the development of validated coordination applications and the related customer requirements. Subsequently, the VALID tool set will be introduced. This tool set supports the design of software applications by means of a well-defined methodology and guidelines, it can be used stand-alone or in conjunction with other standard development environments and frameworks, and integrates modeling, verification, simulation, as well as the generation of executable code, test cases and documentation. The main benefits to software developers are:
Finally, application examples from medical engineering, robotics, automotive body electronics and GUI applications will be presented.
An FMC is defined herein as a system comprising automatic processing machines, typically serviced by robotic material-handling devices, working under the control of a supervisor. An FMC is, thus, supposed to have two levels of flexibility: At the device level, flexibility for part production or part transfer is achieved via re-configuration and re-programming. At the system level, a supervisory (workcell) controller is synthesized to monitor and control the workcell using commercial controllers (such as, industrial PLCs).
In the academic literature, FMCs have been modeled as Discrete-Event Systems (DESs) utilizing a variety of control theories, most notably by Petri Nets (PNs) and Ramadge-Wonham Automata theory. The literature has also advocated the use of PCs for the direct control of FMC, utilizing formal control theories and, thus, providing an open hardware architecture. In our work, however, we advocate the use of a hybrid PC/PLC supervisory-control system that is based on the strengths of each device: namely, PCs provide users with computational and human/machine-interface flexibility in order to efficiently communicate with PLCs that in turn allow efficient communication with cell devices and their control.
Our talk will present a novel and generic PC/PLC-based software/hardware architecture for the control of FMCs. The proposed implementation methodology is based on the utilization of any one of the available formal DES control theories in conjunction with state-of-the-art industrial programmable-logic controllers (PLCs).
The methodology has been verified to be a viable technique through its actual implementation in our laboratory using a PLC-controlled industrial robotic-workcell testbed. The specific control theory used is a combination of Extended Moore Automata and Ramadge-Wonham Automata, which has been developed by our research group.
The modular control software architecture has been developed for MS-Windows environments (running on one PC that is interfaced to the PLCs) and allows the use of different formal control theories as well as different commercial PLC hardware. The effective graphical user interface provides a transparent programming environment, where users are not expected to have a full knowledge of the formal control theory used.
In this talk we will discuss the automated generation of supervisors or logic controllers for flexible manufacturing systems. In general, the problem rapidly becomes intractable. We will discuss, conditions and typical manufacturing system topologies for which the problem (of generating and running a maximally-permissive supervisors) remains tractable and easy to implement. Schemes under which such controllers can be hierarchically decomposed (into cell, system and plant controllers) while maintaining their non deadlocking properties will also be discussed. Finally, some our current research in shared (human-computer) supervisory control will be discussed.
The modeling problem is characterized by concurrent and asynchronous events. Petri nets are well suited for modeling manufacturing systems because they capture the precedence relations and interactions among these events. In addition, a strong mathematical foundation exists for the analysis of system properties such as deadlock, conflict, and boundedness.
The Petri net model can also be used as a real-time controller for a manufacturing system. The flow of tokens through the net establishes the sequence of events to carry out a specific manufacturing task.
Petri nets are also a very valuable performance analysis tool. When time is added to the firing of the transitions, it becomes possible to calculate such measures as throughput, production rates, average machine utilization, and the probability that a machine will be blocked or starved.
Applications to a machining workstation will be presented along with numerous examples from transfer lines and production networks.
In this talk, we describe a new formal representation for a logic controller of a high-volume transfer line using Petri nets. The starting point for the control design is a timing bar chart, created by the mechanical designers of the machining system. The timing bar chart can be converted to a Petri net representing the logic control for the normal operation cycle. This formal representation allows the logic controller to be verified as live, safe, and reversible before it is implemented on the hardware. The incorporation of other control modes (manual mode, failure recovery, etc.) into the logic control framework will also be discussed. The Petri net structure is easily converted to Sequential Function Charts, a standard PLC programming language, for ease of implementation. The logic controller is constructed in a modular fashion, following the modularity of the transfer line hardware, enabling the transfer line control system to be more easily modified or reconfigured. This talk represents joint work with Pramod Khargonekar and Euisu Park.