A lower ASIL may lead to an easier implementation for designers, but it may not always be the right approach. If we do a cost-benefit analysis for a system design with and without ASIL decomposition, we often find that the decomposed design is probably unnecessary and more expensive to implement. Then there is also Freedom from Interference. This is required by ISO to provide evidence showing sufficient independence between the decomposed functions and the elements they are allocated to.
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There are cars that can park themselves or sense traffic ahead and adjust their rate of speed accordingly, and even experimental vehicles that can operate without driver input. The control of the transmission is now electronic. There are multiple LCD screens displaying a variety of information, one of them being a touch screen that controls the audio system and the interaction with cell phones. While there are many advantages to the recent improvements, there is also additional risk.
It is more important than ever to recognize any potential risks of hardware or software related failures. The results of such failures could result in severe injury or possible loss of life. The International Organization for Standardization ISO recognized this need and published the ISO standard to help ensure functional safety of electrical and electronic systems in road vehicles.
In addition, it includes guidelines and regulations for assigning a risk level to an electronic system, software or component, evaluate the risk and document the testing to ensure safety of road vehicle electronic systems. It is important that we cover what the standard is and is not applicable to. The standard also defines methods for using ASILs to specify needed safety requirements to achieve an acceptable level of residual risk. ISO also includes requirements for proper validation and verification methods to make sure that a satisfactory level of safety has been achieved.
These new systems introduce a new risk of possible failure modes. Some of the possible system failures could result in injury or possible fatalities.
Safety in manufacturing is serious business. They could result in recalls and possible litigation. Companies need to make every effort to assure safe operation or use of their product. They need to be diligent in identification and evaluation of risk in their designs and follow through with effective measures to reduce or eliminate that risk. The ISO standard was published to help companies ensure functional safety of their electrical and electronic systems.
Organizations looking to implement ISO should understand the goal is to identify and analyze risk early in the product development process. In addition, they must establish safety goals and achieve these goals through a comprehensive validation plan. How to Implement ISO In many cases, a new standard is introduced during a new product introduction or pilot project. Adopting and implementing a new standard is often met with many challenges, but studies have shown that the ISO standard integrates well with existing safety concepts in the automotive industry.
Many companies are already realizing the benefits of identifying and evaluating risk related to electronic systems and applying appropriate testing throughout the product life cycle. The fundamental deliverables for ISO include development of a Safety Plan, creating Safety Goals, building and documenting your Safety Case, identifying the Safety Lifecycle and validation and verification of hardware and software systems, components and units.
The ISO standard consists of 11 sections and hundreds of pages. It would be impossible to cover all the information in a brief review. Therefore, this page will focus on some key terms and definitions, along with the risk identification and analysis methods contained within the standard.
Many people focus on the ASIL score, the method of ranking the possible hazards. Nevertheless, what we must remember is that the ISO standard is a goal-based standard. It is based on establishing safety goals, identifying risk and developing a plan to meet those goals. Safety goals are the top-level safety requirements of an item or element. They precede development of the functional safety requirements for elimination or avoidance of unreasonable risk for a potential hazardous event.
Safety goals should be expressed as functional objectives and not technical solutions. However, ASILs are only one piece of the process in determining and verifying the required dependability of an item, element or component based on the risks and possible consequences resulting from a failure.
Key Terms Before we go any further we need to look at some key terms used within the standard. In order to understand the standard, you must first learn the key terms and definitions used in application of the standard. Some of the key terms and definitions used include, but are not limited to, the following list: Item: Refers to a specific system or collection of systems that perform a particular function of the vehicle to which the ISO safety life cycle applies. The Item is the highest level identified in a process or system and is usually the initial point for development and analysis of the systems.
Component: One or more software units or hardware parts. Automotive Safety Integrity ASIL : This helps identify the ISO requirements and safety measures to apply for avoiding unreasonable risk within the design and function of an item or element.
Software Unit: The lowest level of the software that may be used for standalone testing. Hazard Analysis and Risk Assessment HARA : Methodology used to identify and categorize possible hazardous events relating to items, develop safety goals and ASILs for development of prevention or mitigation of the potential hazards to avoid unreasonable risk. It should be determined at the beginning of the development process. The planned system functions should be analyzed with respect to possible hazards.
The Exposure factor consists of five different classifications, Severity has four and Controllability has four. The definitions allow much discretion on the part of the evaluator, designer, builder, and supplier of each component, element or item and the automaker as well. The purpose of this document is to provide guidelines for classifying the three factors used to develop an ASIL. This document should help reduce the number of assumptions made regarding the severity, probability of exposure, and controllability factors.
However, the new guidelines may not eliminate the necessity to make some assumptions when determining ASILs. The ASIL is part of the safety goal and is innate to each successive safety requirement. The functional and technical safety requirements are assigned to all the design elements, beginning with the preliminary design concepts all the way down to the software and hardware elements.
Through decomposition during the development phase, the ASIL rating can be customized to the next level of the system design. To further clarify; an element that addresses a particular safety goal, assigned a specific ASIL rating, can be broken down into two independent elements, each with a possible lower ASIL rating.
The benefit is that the cost of development to a lower ASIL is generally lower. The stipulation is that each of the decomposed elements must address the same safety goal and take on the same safe state.
Another thing to remember is that decomposition of the software element requires thorough investigation of the software and hardware independence. However, the hardware metrics are not impacted by the decomposition of the software. Validation Testing and Qualification Within the ISO section 4, the standard covers software, hardware and even testing tool qualifications. The section contains several requirements and tables that indicate analysis and testing requirements based on the ASIL rating.
This clause is applicable to components or systems having previously been in use in other applications without incident. Proven reliable systems that remain unchanged from previous vehicles are certifiable under the ISO standard.
Therefore, by combining certifiable components from similar applications and from applications used extensively throughout the industry, prior to the standard, the system complexity can be minimized and the certification requirements reduced.
Software Qualification The qualification process for software and its component or units requires the following actions: Defining the software functional requirements Determination of resource usage Predicting software behavior during various fault situations Software errors are analyzed and resolved throughout the design process. Software testing is performed under normal operating conditions and during the insertion of various types of faults to determine how it reacts to abnormal inputs.
Software development and testing requirements are addressed in part 6 of the standard. The standard covers requirements for: Initiating Software Development Software Unit Design Software Implementation The analysis and testing process can be reduced through utilization of existing qualified software during the development process. Examples of proven and qualified software components that could be utilized include but are not limited to driver software, libraries, databases and operating systems.
Hardware Qualification The qualification process for hardware components generally consists of two purposes: Specify how the component fits into the overall system design Assess all probable failure modes Hardware components are validated through comprehensive testing under various operational and environmental conditions.
Basic hardware components may be qualified through standard qualification processes. However, more complex hardware components require ASIL evaluation, decomposition and validation testing. The Safety Case A Safety Case must be developed to validate that our item or element will achieve our dependability goals, using all the applicable methods and evidence consisting of quality management, formal design verification, software code analysis, system testing, or proven-in-use data.
The Safety Case should validate that our system meets the safety goals we determined previously and confirm dependability is acceptable for the assigned ASIL. Quality-One provides Knowledge, Guidance and Direction in Quality and Reliability activities, tailored to your unique wants, needs and desires.
Tag Archives: ASIL decomposition
There are cars that can park themselves or sense traffic ahead and adjust their rate of speed accordingly, and even experimental vehicles that can operate without driver input. The control of the transmission is now electronic. There are multiple LCD screens displaying a variety of information, one of them being a touch screen that controls the audio system and the interaction with cell phones. While there are many advantages to the recent improvements, there is also additional risk. It is more important than ever to recognize any potential risks of hardware or software related failures. The results of such failures could result in severe injury or possible loss of life. The International Organization for Standardization ISO recognized this need and published the ISO standard to help ensure functional safety of electrical and electronic systems in road vehicles.
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