Architecture Modeling - SPES 2020

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Architecture Modeling the functional perspective would specify the desired control function. We expect at a minimum abstraction levels to be used at interfaces between subprocesses along the supply chain hierarchy. Additional abstraction levels can be chosen depending on the tradeoff between design time and optimization needs – as discussed in the next section. 5.1.3 Example In the following, the application of the primitive steps in component-based design presented in section 4 is shown with the aid of a common example used in aeronautics case studies. First, in section 5.1.3.1 the example of a wheel braking system is presented with a general description of its functionality and a schematic overview. To demonstrate the different concepts we walk exemplarily through the design process starting in section 5.1.3.2 with snapshots of combinations of perspectives and abstraction levels. After that, the primitive design steps decomposition, allocation and realization are shown exemplarily at selected parts of the previously presented model snapshots. In section 5.1.3.3, we elaborate on the question of how an allocation-link of components of the logical and technical perspective could be proved using a virtual integration test by means of realtime contracts. Finally, we show in section 5.1.3.4 how the fourth primitive step implementation could be proven by a satisfaction-check against a safety contract using fault-tree analysis. Here, the main purpose is not to give an overview on safety analysis but to show how such methods can be integrated in the SPES methodology. Finally, in section 5.1.3.5 we introduce a fourth level of abstraction, the Unit Level, to show exemplarily how realtime contracts may be checked using scheduling analysis. 5.1.3.1 Aeronautic Example: Wheel Braking System The Wheel Braking System (WBS) described in the ARP 4761 [5, p. 168 to 280] is a common example used in aeronautics case studies. The WBS is supposed to decelerate the aircraft on the ground by controlling the hydraulic pressure for the wheel brake actuators. The central control unit of this system is called the brake system control unit (BSCU). In Figure 5.1 a schematic overview of the WBS introduced in the ARP 4761 is depicted containing control units as well as hydraulic and mechanical elements. Therefore, in the SPES context this figure would be part of the technical perspective while the BSCU is modeled at a very high abstraction level. The system operates two independent hydraulic circuits connected to each wheel of the aircraft and in three different modes Normal, Alternate and Emergency which will be supplied by either the green or the blue hydraulic lines or by the reserve offered by an accumulator. The Selector Valve located in both the green (Normal) and blue (Alternate) line guarantees that only one of the lines can propagate hydraulic pressure. If both lines are operational, the green one is passed through. Switch over to the blue line occurs if either the green pressure fails, the BSCU becomes in-operational (green pressure is cut off at Shut-Off Selector Valve) or if it is explicitly selected by the pilot (not shown in the figure). The default mode of operation is Normal mode. In this mode the aircraft can decelerate either automatically or manually by the pilot. In both cases the BSCU generates the necessary control and anti-skid commands for the Meter Valve in the green hydraulic line that operates individual hydraulic pistons located on every wheel. If a switch-over to Alternate mode occurs (see above) automatic braking is not possible. The rate of deceleration is controlled by the pedals and transmitted mechanically to the respective meter valve. The hydraulic pressure is transmitted to an independent set of brake pistons operating on sets of wheels. If the BSCU is still working, it will provide anti-skid commands via the Anti-Skid Shut-Off Valve. When the system is in Alternate mode and pressure to the blue line 72/ 156
Pwr1 Pedal_Pos1 Pwr2 Pedal_Pos2 CMD_AS1 BSCU Monitor 1 BSCU1 AS1 Valid1 Valid2 Valid_ Switch Monitor 2 BSCU2 Command 1 Command 2 CMD_AS2 AS2 Select_Switch Architecture Modeling Valid AS CMD_AS Shut Off Selector Valve Green Pump Normal Meter Valve Isolation Valve Selector Valve Anti Skid Shut Off Valve Meter Valve Blue Pump Figure 5.1: Schematic Overview of the Wheel Braking System Alternate Reserve Accumulator Mech Pedal Position Wheel fails it switches to Emergency mode. In this mode the pressure is supplied via an accumulator. Due to the limited capacity it is not possible to apply and release the brakes multiple times so that it must be ensured that Anti-Skid Shut-Off Valve is disabled. Please note, that the models we are presenting in the following sections might slightly differ in some details from the ARP 4761 and that we do not show each perspective on each abstraction level for the sake of brevity. 5.1.3.2 Abstraction Levels, Perspectives and Design Steps In this section, we present how the wheel braking system is modeled on three abstraction levels (Aircraft Level, System Level, Device Level) using the SPES architecture modeling concepts. A fourth abstraction level named Unit Level is described in section 5.1.3.5. The example currently covers four different perspectives starting with the operational perspective on Aircraft Level where user-activities and actors that operate the system are identified. This is usually the first design-step we consider in this framework and builds the reference for succeeding phases. When all needs of operational actors are acquired, in the functional perspective the functions are modeled that realize the operational activities. Furthermore, these functions may be decomposed into subfunctions. In the logical perspective logical components are defined that realize the functions identified in the previous step. A technical perspective is not part of the Aircraft Level. On the System Level, the logical architecture is refined by decomposition and an initial technical architecture is proposed that is refined at the Device Level by adding redundant components due to safety reasons. 73/ 156
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Architecture Modeling ∗ Andreas B
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Contents 1 Introduction 1 2 Quick-T
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Bibliography [1] ARINC 653 - Avioni
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Architecture Modeling [26] Holger G
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Architecture Modeling - SPES 2020

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