Design Patterns for Distributed Dependable Control Systems
Design patterns are used in design guidance in the case of the design of a new control architecture or at least in the case of the design of a new control application. Design patterns are currently used in design aid emerging from building architecture but made more widespread by information sciences.
The description and application of design patterns is based on the principle of describing a solution to a problem within a special context. Therefore, a design pattern includes the description of the basic problem, the context of the problem (the field where the problem has occurred), drivers important for the solution, the solution of the problem itself, and the background of the origin, and, finally, the possible application fields of the solution, Requirements for Field Control Systems.
The design of field control systems is driven by a set of requirements emerging from technology and economics. Generally, in the control architecture the Field Control Level aims to control the physical execution of manufacturing processes and, therefore, to drive them via actuators and measure them via sensors. Hence, the core individual entities of this level to be considered are the manufacturing process, sensors, actuators, and control devices
The design of field level control systems forces special requirements on the design process, on the control system architecture, and on the integrated devices. Few are listed below.
- Low building and maintenance costs
- Robustness with respect to operability under malfunctions
- Flexibility with product types, product volume, and equipment
- Application-dependent control system development
- Component-based development approach
- Human integration and friendliness
- Compliancy with existing system and standards
- Integration with existing control devices and legacy systems.
Rationale of Design Patterns
The basic problem of control is the necessity of controlling the physical values in- fluencing the controlled system based on the current state of the controlled system.
One best known general solutions is that of the closed-loop, feedback-based control system. In general, we can say that the usability of this solution for the general problem is not limited to a particular case; hence the feedback control design pattern can be applied - in principle - to any automation system.
Nevertheless, the description of the solution itself is not sufficient.
It has to be accompanied by:
- A description of the problem the solution is targeting on
- The surrounding and environmental conditions the design pattern is valid within, and
- The drivers and ancillary conditions that imposed this solution.
If design patterns are described in this way, they can be easily used to catalog and document design knowledge. In this way, knowledge exchange among scientists and practitioners, system integrators and end users, and people of different manufacturing sciences can be improved by the common knowledge base and the common description framework that pattern languages provide. This communication problem is very relevant within the automation world. Specialists from information sciences, mechanical engineering, process industry, electrical engineering, and other fields have to work together towards a common goal.
Design Pattern for Distributed Field Control Systems
Design Pattern - Distributed Control Applications
Distributed control applications are based on the cooperative solution of a control problem which are in a set of independent but cooperating intelligent and non-intelligent control devices which usually given in the case of interacting mechatronical units within a manufacturing system.
The distribution problem requires the allocation of different parts of the control application to control devices.
The main drivers for a solution to the above-mentioned problem are the reusability of control application parts as well as the maintainability and flexibility of the control system itself.
The solution has to ensure that the necessary knowledge about products that have to be produced, including knowledge about necessary manufacturing processes that have to be executed and the necessary knowledge about manufacturing resources that have to be applied (production system knowledge) is integrated within the control design process and, ultimately, within the control application.
The solution of the mentioned problem is based on the definition of control functions containing control system building blocks and their unique allocation to control devices. The combination and interaction of these control functions will establish the control application.
The control functions associated to the control devices can be classified within two main function classes.
The first class consists of process functions. These functions are used to control the manufacturing process and its progress. They provide a complex system of manufacturing process steps that largely enable the manufacture of the desired products. The process functions are sequenced in a so-called half-order relation. Each process function may have predecessor steps, successor steps, and parallel or concurrent steps. This half-order, together with the individual process functions, represents the overall product knowledge contained in the control application.
The second class of functions are safety functions. These functions contain all control activities necessary to safeguard machines, material, environment, and human beings against dangerous events. Safety functions interact with other safety functions as well as with process functions. They act as observers for process functions to prevent dangerous situations caused by the activities of process functions. These interactions, together with the safety functions and the process functions, will represent the production system knowledge contained in the control application.
Dependable control system design and evaluation has to focus in the following:
1. Fault tolerance based on non - redundancy.
2. Avoid stovepipe systems.
Design Pattern - Reusability of Control Software Building Blocks
Distributed control applications have to reflect the modular design of manufacturing systems based on mechatronical units.
The main driver for the solution of the reuse problem is the possibility of different viewpoints for the decomposition of manufacturing systems and manufacturing system control applications. The decomposition into control application building blocks can be based on the following decomposition rules:
- Assignment of different production functions and safety functions to different building blocks
- Assignment of different mechatronical units to different building blocks
- Assignment of different technologies to different building blocks
- Assignment of different technological parts of a device to different building blocks
- Assignment of different devices to different building blocks.
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