User Requirements, Modelling e Identification. Lezione 1 prj Mesa (Prof. Ing N. Muto) 1.1 Introduction: A customer has requested the establishment of a system for the automatic orientation of a string of solar panels in order to optimize the energy generation. Afterwards the data provided through an interview with the client himself has raised some informal requests that then have to be analyzed, formalized and subject to verification and approval of the customer. After this step we will have a document that we will call "USER REQUIREMENTS" (from now on UR) and that will serve as a guide for the development of the system. The UR are key to proceed safely and without the danger of misinterpreting the instructions of the customer. Also it set aside the developer from "program changes" by the customer himself. In fact while admitting changes in the process, because they have a very high cost often enough, it must be recorded that the initial indications provided were different. Customer requirements: The informal requests of the customer are that the system must be able to run on both a scale model than on the real system,the motors must also be managed manually, it must be present a man-machine interface with graphic. It's still required that once identified the optimal position of the string of solar panels toward the light source, it must be maintained, pursuing its in time. The handling system includes n. 2 "stepper" motors, one for the "ascension" movement and one for "declination" of the star to point and chase. Nothing has been defined on the modality to find the best position and maintain it, so we can decide it freely. Even the delivery times must be clearly defined to avoid surprises at work. In our case, the work must be tested on the scale model within April 30, 2012. 1.3 Knowledge and skills In order to formalize the specification we can use the skills acquired during the study of system theory, in particular, should be drawn to the topics: definition and classification of a system, modelling and identification of a system.
1.3.1 Definition and classification of systems A system is a set of parts that interact by exchanging energy or information with a commute purpose. Many things that surrounds us, from the simplest to the most complex can be considered SYSTEMS, such as a ballpoint pen, a computer, a bicycle, a cell phone. Usually, we think that a system there are the causes of the EFFECTS. The causes can be considered INPUT to the system and the effects can be thought as OUTPUT of the system. Then we can think of a first simple GRAPHICAL REPRESENTATION of a system: There are various ways to classify the systems, based on what we want to observe. If we want to refer man-made systems or existing in nature, such as splitting we can all systems in NATURAL or ARTIFICIAL. A classifications makes more sense from a technical standpoint instead considers the BEHAVIOUR OF A SYSTEM with regard to the following aspects: RELATIONSHIP WITH THE ENVIRONMENT A system can be CLOSED, i.e. which does not interact with an external environment, from which it is perfectly and definitely isolated! Obviously there in no system actually CLOSED! If a system interacts with the environment in which it is located, it is called OPEN system. The exchange may relate to energy, matter and information. The mechanical, electrical, computer are definitely open. ABILITY TO REMEMBER This aspect relates to the ability of a system to store energy, information or material that alters the behaviour even in the presence of the same stresses. If a system has this capability, then you define SEQUENTIAL and it is possible to define a CONDITION, that the system has the ability to store so that its output depends not only on inputs but on its history. A system that does not have this ability is called COMBINATORIAL, for which its think of a pocket calculator, we observe that the result of a certain algebraic operation is always the same. If you observe a lift, we can understand that the effect is obtained by pressing same key internal or external, depending on the level ta which the elevator is located.
DETERMINISTIC or ALEATORY REPORT between CAUSES and EFFECTS If it is possible to provide in a secure and repetitive effects of certain stresses, even in the presence of states, then the system can be defined deterministic. For example, the elevator is a deterministic system because it certainly does not invent his behaviour in an unpredictable way (unless it is the protagonist of an horror film!). Instead a sphere from which extract the numbers in a lottery is a system in which deliberately NOT be possible to predict the output. Even a simple coin that is launched can be classified as a not predictable system, i.e. ALEATORY. STABILITY 'THE PHYSICAL STRUCTURE OF THE SYSTEM Once built or devised a system we can ask whether its physical structure is or is not stable. For example a missile used to put satellites into orbit changes its structure during its operation the same, namely that consumes fuel up to the instant of the launch was an integral part of its structure. A kitchen oven instead maintains, in theory, preserving its structure and thus also an elevator. We say theoretically because in reality all things wear out and therefore the parts of a system. CHANGE INPUT and / or OUTPUT IN TIME If a system has no evolution as we observe it then it can be defined STATIC but in reality this depends on the time that we look at it. The mountains seem as static but we know that the whole earth's crust moves over the millennia. A system which has instead of the stresses and effects that vary in time of observation is defined DYNAMIC, as most of the systems in which we are interested in the technology. TYPE OF PHYSICAL PRESENT IN THE SYSTEM In a system may be present with magnitudes that vary with continuity, type the temperature of a human body, or there may be stresses and effects which nevertheless have a finite number of values that can assume. Returning to the example of the lift we can understand that plans can go up on possible are over, and so is the kind of movement that can be done. We then define "DISCRETE" systems that have to do with variables that have a finite number of values that can take. We define "CONTINUOUS" systems that have variables with an infinite number of values. We will learn later that a computer can NOT handle continuous magnitudes as it has an OVER memory!
1.3.2 Modelling of a SYSTEM: In technology we consider a system because you want to study, monitor, edit or otherwise but often a real system is too complex to deal with as it is physically and then you have recourse to the creation of a MODEL, namely the SIMPLIFIED representation of the system. The simplification of course will cover the aspects that we don't care treat while we will maintain the significant characteristics from our viewpoint, that is the CONTEXT in which we want to work with the system. In a formal way we can say that: "The model is a simplified representation of a physical system, in order to simplify the study and analysis of its most significant characteristics, given the context in which we place" The models can be classified according to criteria. In particular we considering the purpose of the model, we can mention: DESCRIPTIVE model It 'a model that DESCRIBES the system, "photographing" it completely, without, however, interested in what he does and how he does it. PREDICTIVE model In this case the model it deals with describing what the system do, without interest how it is done the system itself or how he can do what he does. PRESCRIPTIVE model In the prescriptive model instead considering HOW it is realized what the system does, without interest in its structure. Or, if we consider the nature of the model, we can talk about SYMBOLIC model. If you are unable to represent the model in an ABSTRACT, mathematical and formal way, then the model is defined symbolic. Obviously you can not always have a symbolic model, given the great complexity of many systems, both natural and artificial. ANALOG model In this model there is based on the characteristic of having a behavior similar to the real system under study.
1.3.3 SYSTEM IDENTIFICATION As a visual model to realize a model of a system, you can use a block diagram, in which a rectangle interacts with the arrows entering and leaving. This tool, however, prefers the functional aspect of the system rather than a structural one, that allows us to understand what he's doing but little about how the system is "constructed" the system itself. If we introduce also the NOISE of a system, defined as unintended stresses are otherwise of the effects, and the PARAMETERS, which are structural characteristics and invariant, at least during the time of observation, we can represent a system as follows: INPUTS NOISE OUTPUTS PARAMETERS In a rigorous way we can define: INPUTS INDEPENDENT variable magnitudes, we can change voluntarily. OUTPUTS Sizes EMPLOYEES variables that change as a result of the values of INPUT. NOISE UNCONTROLLABLE variables that make one feel their effect. PARAMETERS Entity or quantities that remain CONSTANT throughout the period of observation. The task of devising a clear, complete, accurate and entities named in the table of system identification and is an essential step in order to model and study a system.