The relationship between automotive functional safety and reliability

At present, with the rapid development of the automotive industry, cars are also being redefined. When designing automotive electronic and electrical systems, it is inseparable from the consideration of functional safety and reliability design. Correctly understanding the relationship between the two will help to better analyze and solve problems.

What is car reliability?

Automobile reliability refers to the ability of an automobile to complete specified functions within a specified time and under specified conditions of use. It is a relatively complex comprehensive performance, which broadly includes automobile trouble-freeness (durability), maintainability and preservation. Trouble-freeness is an important aspect. Its main evaluation indicators are trouble-free probability (reliability), cumulative failure probability and failure rate. Automobile durability is evaluated by a series of life indicators, which mainly include average life (or average trouble-free working mileage or time), rated life, characteristic life, reliable life, effective life, etc. In addition to the indicators of maintainability, maintenance rate, average maintenance time, etc., the evaluation of maintainability can also use a series of indicators that can show that the automobile is easy to maintain and easy to inspect, diagnose, disassemble, and repair after a failure occurs, such as maintenance cycle, maintenance workload, and maintenance cost (yuan/1000km).

What is automotive functional safety?

With the rapid popularization of automobile intelligence and electrification technology, there are more and more in-vehicle controllers and various electronic components, and all kinds of electronic components are subject to the risk of systematic failure and random hardware failure, so the corresponding automotive functional safety is becoming more and more important. In the automotive electronics industry, the international functional safety standard ISO26262 (a standard based on IEC61508 (Generic standard for Functional Safety of electrical/electronic systems) and applicable to the automotive industry) and the corresponding national standard GB/T34590 define functional safety as: avoiding unreasonable risks caused by failures in electronic and electrical systems. That is, random hardware failures and systematic failures will not lead to incorrect functions of the safety system, thereby causing human injury and death. ISO 26262 is the first functional safety standard in history applicable to road vehicles.

The relationship between functional safety and reliability

1. The difference between functional safety and reliability

Functional safety and reliability are two completely different concepts. The differences are as follows:

Different focus

Functional safety emphasizes the control of risks, while reliability focuses on increasing the service life of products.

Different Purposes

The purpose of reliability design is to maintain the function of the system and ensure that the product can work for a long time without failure; the purpose of functional safety design is to prevent accidents and avoid casualties and property losses.

Different design approaches

Commonly used design methods for reliability include derating design, redundant design, tolerance design, simplified design, thermal design, EMC (electromagnetic compatibility) design, etc.; the core of functional safety is the design of safety mechanisms, including redundant design, diagnostic design, alarm design, and design for transition to a safe state.

Reliability design focuses on coping with the system status before and when a fault occurs, and the response strategy is to try to reduce the probability of a fault or functional failure. Functional safety design focuses on coping with the system status after a fault occurs to prevent personal injury after a fault occurs, and the response strategy is fault diagnosis and fault response (transition to a safe state or enter an emergency operation state).

Different analysis methods

Although both reliability analysis and functional safety analysis use fault tree analysis (FTA), the focus of the analysis is different. Functional safety focuses on the analysis of risk hazards, while reliability focuses on the analysis of functional failures.

Different verification methods

The verification methods used for reliability design are also applicable to functional safety design verification, but some design verification methods for functional safety, such as fault injection design, are not applicable to reliability design verification.

Different management methods

Functional safety design has higher management requirements and uses more management methods. Among them, management processes such as safety requirements management, safety analysis, and safety design are not applicable to reliability design, but the management methods of reliability design are applicable to functional safety design.

Different metrics and evaluation methods

Commonly used metrics for functional safety include single point fault measure (SPFM), latent fault measure (LFM), and probability of random hardware failure that violates safety goals (PMHF).

Common reliability measurement indicators include: reliability R(t), failure rate (failure rate) λ(t), mean time between failures MTBF, reliability life, etc.

The two have different measurement indicators, which leads to different evaluation methods. In short, the better the functional safety, the higher the reliability, and the higher the reliability, the better the functional safety. Products with good functional safety may have lower reliability. Conversely, if the safety mechanism is not considered in the product design, the functional safety of a product with high reliability will not be very good.

2. The relationship between functional safety and reliability

Functional safety and reliability are closely linked. The relationship between the two is as follows:

Reliability is the foundation of functional safety

It only makes sense to consider product functional safety when the product is reliable. There is no way to talk about functional safety for unreliable products.

"Failure" is the common root of reliability and functional safety. In terms of preventing failures, reliability and functional safety have the same goal. When the safety mechanism is determined, the higher the reliability, the better the functional safety. In this case, measures to improve system reliability are also conducive to improving functional safety. Generally speaking, unreliable systems will lead to unsafe systems. For example, when the electronic power steering system EPS fails, it will not only affect the normal driving steering operation, but may also cause the vehicle to deviate from the normal track, causing personal injury or death.

• Have a common theoretical basis

The basic theories of reliability engineering, such as probability theory and mathematical statistics, common distribution functions, failure rate and failure distribution, bathtub curve, common reliability models, failure rate prediction and other theories, are also applicable to functional safety design.

• Shared design approach

Because reliability is the basis of functional safety design, the common methods of reliability design are also applicable to functional safety design. For example, redundant design is not only a common design method for reliability, but also a common safety mechanism design for functional safety.

• Shared analytical approach

Analysis methods and tools such as FMEA and FTA are applicable to both reliability design analysis and functional safety design analysis.

• Have a common verification method

Test methods related to reliability, such as climate environment testing, electromagnetic environment testing, mechanical environment testing, etc., are also applicable to functional safety testing. Verification methods such as review and simulation are applicable to both functional safety design and reliability design. Of course, some methods used for functional safety testing, such as fault injection testing, are not applicable to reliability testing.

• Have common development tools

Many popular reliability design tools on the market, such as Isograph, Medimianalyze, etc., are also suitable for functional safety development.

• Contains similar management requirements

Whether it is functional safety or reliability, there are not only technical requirements, but also management requirements, and the management processes are also very similar. The management requirements of reliability design are also applicable to functional safety design. The reason why there are many connections between functional safety and reliability is that "failure" is their common basis. All theories about fault analysis and fault handling are applicable to both functional safety and reliability. The difference between the two is due to their different research focuses. Functional safety focuses on risk control, while reliability focuses on extending life.