MECHATRONIC DESIGN AND SUPERVISORY CONTROL THEORY FOR FLUID POWER APPLICATIONS Eduardo A. P. Santos*, José Eduardo R. Cury** and Victor J. De Negri*** *LAS/PUC-PR – Po. Box 16210, 80215 901, Curitiba – PR, Brazil,
[email protected] **LCMI/DAS/UFSC – Po. Box 476, 88 040 900, Florianópolis – SC, Brazil,
[email protected] ***LASHIP/EMC/UFSC – Po. Box 476, 88 040 900, Florianópolis – SC, Brazil,
[email protected]
ABSTRACT The aim of this work is to integrate the use of formal tools for analysis, control and synthesis of systems in mechatronic design. Besides, the work presents an integrated design approach for fluid power systems, in which the control system is conceived during the design process of the physical plant, making the global design more efficient and reliable.
KEYWORDS Mechatronics, automation, control, design methodology, supervisory control, fluid power systems design, integrated design.
INTRODUCTION The large-scale introduction of microelectronics and computer systems in mechanic devices as a whole, has significantly grown in pneumatic control designs along the eighties. Microprocessors, micro-controllers and programmable controllers, at more and more competitive prices, have gradually replaced the pneumatic signal processing elements and the Electropneumatic relays, causing an increasing use of electric and electronic sensors as signal elements [1]. The pneumatic industrial automation, mainly that associated to microelectronics and computer resources, fits more complex automation processes, better each day. The use of traditional design methods (for example, intuitive and cascade) has not complied with the modern demands for high design quality, such as quick simple execution, good supervision and maintenance and standardization. Besides, they cover solutions for sequence specifications only, not covering other type of specifications, such as inter-blocking, communication among controllers, fairness, liveliness, and safety. It’s observed that these types of specifications are handled through the experience and inspiration of the control system designer, resulting in design with lowreliability and hard maintenance. Being so, the design
process must be carefully planned and systematically executed to lead to an efficient development of a fluid power system. Crucial is the use of systematic procedures, which are able to integrate and optimize the different aspects involved in a design fitting the many technologies and making an effective team interaction possible. In this sense, special attention is dedicated to the product integrated development concept [2], related to product development, multidisciplinarity, team integration and simultaneity of activities. The present work aims to contribute to the systematization of fluid power system designs. By using the concept of product integrated development, the aim is to integrate the physical system design and the control system design, in order to obtain a general methodology for conceiving fluid power systems. In this sense, the work makes use of concepts from mechatronics design and supervisory control theory, in order to turn the plant design into the controller design.
MECHATRONIC DESIGN Increasingly, machines and industrial designs have used electronics and computer for control and information processing, resulting in better performance flexibility and reliability for the whole system. Still considering electric, mechanic, hydraulic and pneumatic devices, normally found in industrial equipment, we face the union of components with various technological principles, demanding a cooperative work of different specialists in its analysis, design and maintenance. A universally accepted definition of the term mechatronics is the integration of a number of disciplines such as mechanics, electronics, electrical, computer, control, and software engineering using microelectronics to control mechanical devices. In addition to product design, mechatronics as a design philosophy penetrates and is applied to production design, monitoring, and control with the objective of achieving high-quality products at optimal running conditions [2]. Independently on the application domain and its complexity, mechatronic products and processes can be
decomposed into an information sub-system and energetical/material sub-system, shown in Figure 1. The information system involves equipment that processes signals and information data, such as computers, programmable controllers, analogue and digital controllers, among others. The energetic/material system applies to machines, devices, physical and chemical processes that transform energy and/or matter.
Energy Material Information
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Energy Material Information
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for mechatronic systems design derive from those and, consequently, apply the same fundamental concepts. In this sense, functional analysis has the same importance in mechatronic design.
Information System
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Figure 1 – Display of a mechatronic system [3]. As Figure 1 illustrates, the information system must be able to extract information from the energetical/material part, process them, and later, use them to modify its running process. Besides the exchange of information between these two sub-systems, there is also the input and output of energy (ene), matter (mat) and information (inf), in relation to the external environment. For the product and processes design, the traditional engineering design community has widely accepted the conceptual design methodology according to Pahl and Beitz [4]. First, a designer determines the entire function by analyzing the specifications of the product to be designed and built. He or she then divides the function recursively into subfunctions, a process that produces a functional structure. Second, for each divided subfunction, the designer uses a catalog to look up the most appropriate functional element – a component or a set of components that perform a function. Finally, the designer composes a design solution from those selected elements. As figure 2 illustrates, this methodology defines function as the transformation between input and output of energy, matter and information. The development and application of design methods for mechanical systems and software, procedural or object oriented, precedes mechatronics as a research area. Therefore, it is natural that all proposed methodologies
Figure 2 – Functional hierarchy in the traditional design methodology. It is known that the fundamental objective of design procedure is to transform functional specification into an object that accomplishes it. Therefore, the authors understand that the designers must use models that progressively lead to the final physical structure of the system in development. This representation form needs to start in the initial phases of the design. Therefore, the diagrammatic representation of the function structure created in conceptual design should have the same meaning of an object interconnection, which is not shown in the traditional methodology according to Pahl and Beitz [4]. In this sense, a core model of mechatronic systems, as Figure 1 illustrates, uses the notation of Petri net Channel/Instance (C/I net), which has two basic elements: the round shaped channels, which indicate the passive elements (where energy, material and information resources flow by); the instances, represented by rectangles, which indicate the active elements (processes), that consume and transform the resources. These elements are linked by directed arcs that determine the way of the resource flow. A more meaningful characteristic of the C/I net is that it clears out the physical connection among machines or devices and also channels where matter flows by. This characteristic is fundamental for the control system design, during the early levels of the physical subsystem conception, representing an integrated design of the fluid power system. The next step of the design is using
the supervisory control theory on the synthesis of fluid power systems controllers.
SUPERVISORY CONTROL THEORY Discrete event systems (DESs) are useful modeling abstraction for certain, mainly man made systems, such as manufacturing systems and communication networks. The main characteristic of such systems is that at each time instant they occupy a discrete symbolic-valued state, and perform state-changes on the occurrence of events. Events occur asynchronously and instantaneously at discrete intervals of time. Thus, a DES behavior is described by the sequences of events that occur, and the corresponding sequences of visited states. The Supervisory Control Theory introduced by Ramadge and Wonham [5] is a general approach to the synthesis of control systems for DESs. Given a DES describing the uncontrolled behavior, the plant, and a specification for the controlled behavior, a supervisor can be automatically synthesized to control the plant to stay within the specification. Ramadge and Wonham´s theory of supervisory control [5] uses formal languages and automata to model both the uncontrolled DES and the specification for the controlled behaviour. In their approach, a DES execution is modeled as a sequence of events. The set of all such events forms a language and represents all the possible executions of the system. The basic problem of supervisory control is to modify the open-loop behavior of a DES by eliminating sequences of events from the system behavior. The objective is to restrict the behavior of the system so that is contained in a desired behavior, called specification. This is achieved by constraining the discrete event generator to execute events only in strict synchronization with another system, called supervisor. Ramadge and Wonham postulate that the plant spontaneously generates all events, and that the events are divided into two classes: controllable and uncontrollable events. The controllable events Σc can be prevented from occurring by synchronization with the supervisor while the uncontrollable events Σu over which a supervisor has no authority. In the model of Ramadge and Wonham, the supervisor acts as a passive device, tracking events produced by the plant and restricting the behavior of the plant by dynamically disabling the controllable events (see figure 3).
Plant List of forbidden events
Generated events
Supervisor
Figure 3 – Supervisory control schema [5].
One of the matters in the model of Ramadge and Wonham is that, when a large number of tasks must be executed by the control system, the approach may have a very unfavorable computer performance once the number of states that represent the system increases exponentially with the number of component elements of the system. This restricting factor has been considered by several authors that attempt to overcome these computational difficulties by exploiting different aspects of the system [6]. A way to diminish the complexity in the synthesis of supervisors is by dividing the control task into various sub-tasks, which are solved by using the classical model. This approach is called modular synthesis [5]. A better approach, with relation to the classical modular synthesis is the one proposed by De Queiroz and Cury [6][7], called local modular synthesis. This approach takes advantage of the modular structure of the plant in the design. The results presented by De Queiroz e Cury [6][7] show that the locally modular synthesis induces a natural noncentralized structure for supervisors, accordingly figure 4. Disabled events Sub-plant 1
Event
Sub-plant 2
Supervisor 1 Event
Supervisor 2
Disabled events
Figure 4 – Local modular control schema [6][7]. In fact, each local supervisor only needs to exchange information with its corresponding local plant. In the case of changes in the plant or in the specifications, respected the non-blocking condition, the control modules can be redesigned, based only on local information. A distributed control system with more flexibility and higher computer simplicity is obtained. Those results will be used in this work for the system control design, for all the inner advantages of this approach.
RESULTS AND DISCUSSION The methodology which is proposed on this work has been worked out on the design of a part-handling system (PHS), that aims to assemble, pack and stock pencil sharpeners. The design at LASHIP – Hydraulics and Pneumatics Systems Laboratory of Federal University of Santa Catarina. First, the mechatronic system that will be built is represented, with the use of the C/I net. Figure 5 illustrates the global functional
model of the assembling and packing module of the PHS. By following the functional analysis, the model from Figure 5 is gradually refined, with the goal of finding lower complexity functions, where conceptions can be associated. Figure 5 shows the existence of two designs running simultaneously: the physical plant design (energetical/material) and the control system design (information processing).
The design of the packing module control system is started from the functional model in Figure 6. The basic operational specification of PHS is to avoid the occurrence of overflow and underflow in the buffers, so that the part-flow happens orderly. Function F3 - To open package
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Figure 7 – Principles solution for functions F2, F3 and F4.
PHS - Assembly/Package
PrE x = Select product package x, with x = 1,2 ... 9 QPrEx = Select quantity of PrEx Pey = Part y, with y = 1 ... 5 Embz = Package z, with z = 1 ... 3 PrEi = Produced package product i, with i = 1 ... QPrE1, QPrE1+1 ... QPrE1+QPrE2,..., QPrE8+1 ... QprE8+QPrE9
Figure 5 – Functional model of assembly and package modules. By following the functional decomposition of the packing module, one can get to the configuration which is shown in Figure 6. Simultaneously, from this functional structure, a local modular synthesis is used to obtain the controllers of the various local plants.
By applying the results presented in [6][7], local plants are built from the composition of subsystems under specification. Figure 6 shows the functional model with the local plant LP. The basic model for each function and for the specification of the non-occurrence of overflow and underflow in the channel (C/I notation) or buffer of local plant LP are shown in Figure 8. αi
βi Automata Fi, i = 2,4.
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F1 – To capture package F2 – To transport package F3 – To open package F4 – To transport package F5 – To pack up
B1, B2, B3, B4 – Package buffers B5 – Sharpeners buffer LP – Local Plant
Figure 6 – Functional model of packing module. From this functional structure, it is possible to select principles solution for each of the subfunctions. Figure 7 shows solution principles for some of the subfunctions of the packing module.
β2
α4 Automata Ei , specification to buffer B3.
Figure 8 – Function and specification models. (α) events represent the beginning of the functional operations (passive or being disabled, so controllable), and (β) events represent the end-of-functional operations events (can not be disabled, so, not controllable). After the controllers for each local plant are synthesized, it is necessary to check the existence of conflict in the simultaneous operation of such controllers. This property is called local modularity, and if checked, means that the operation of the supervisors is non-conflicting and optimum [7]. Local supervisors form a hierarchy of the global control system, once they act as operation coordinators of the several mechanisms that form the plant. These supervisors must then, be translated into a PLC programming language. Sequential Function Chart
(SFC) [8] has been chosen for the work, considering its inner advantages in relation to others (such as the ladder diagram and instruction list). The translation is illustrated in Figure 9. α1 β1
β2
α2
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α1
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1 β2
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Figure 9 – Translated local supervisor in a SFC. It is now necessary to synthesize the low-level controllers, those that control the operation sequences of the mechanisms. Figure 10 illustrates the control system for the local plant LP (see figure 6) on the packing module, having the notations from Figure 7 as correspondents.
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REFERENCES
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As opposed to traditional methods, this approach shows how it is possible to structure control implementation and follow rules to achieve better productivity and quality in design. By proceeding in this fashion, this approach helps the implementation of designs and further assists in troubleshooting. In the long run, systems implemented according to this method are easy to maintain and can be modified easily. In most cases, fluid power control systems are implemented empirically, using experience and standard combinational logical techniques. In turn, this leads to circuits that cause problems at start-up, and end-up being very difficult to maintain or to accommodate changing requirements. Besides, the aim of the integrated design of the physical plant with the system aggregates quickness and reliability to the global design, saving time and resources. The use of C/I net as a core analysis model, has been quite satisfactory, once it gradually shows physical interconnections among machines and devices. Finally, the assembling and packing modules are now in late construction phases. The target now is to implement controllers by using formal synthesis procedures, as proposed by Ramadge and Wonham [5] and De Queiroz e Cury [6][7].
End of sequence 7-13
13
=1
Figure 10 – Control system of a local plant LP from the packing module. The first level corresponds to the communication among controllers, so that the parallel operation in the system does not cause conflict. The second level corresponds to the sequential control of the devices and mechanisms. They handle information as signal sensors and commands to the electrovalves. One advantage of this architecture is the conception of a distributed control system. A local controller is more easily built, modified, updated and corrected.
[1] BOLLMANN A. Fundamentals of Pneutronic Industrial Automation. ABHP, Brazil, 1998. 278. (in Portuguese). [2] POPOVIC D., VLACIC L. Mechatronic in Engineering Design and Product Development. Marcel Dekker, Inc. New York, USA, 1999. [3] DE NEGRI V. J. Model structuring of automatic systems and its application for a test bench of hydraulic systems. Doctorate thesis, Federal University of Santa Catarina, Brazil, 1996. (in Portuguese) [4] PAHL G., BEITZ W. Engineering design – a systematic approach. Springer – Verlag UK, 1988.544. [5] RAMADGE P. J., WONHAM W. M. The control of discrete event systems. Proceeding of the IEEE, 77 (1): 81-98, January 1989. [6] DE QUEIROZ M. H., CURY J. E. R. Modular Control of Composed Systems. In: Proceedings of the American Control Conference. Chicago, June 2000. [7] DE QUEIROZ M. H., CURY J. E. R. Modular Supervisory Control of Large Scale Discrete-Event Systems. In: Discrete Event Systems: Analysis and Control. Kluwer Academic Publishers, USA. 103-110. [8] IEC, International Electrotechnic Commission, Preparation of Function Charts for Control Systems, publication 848, 1988.
