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Accommodating the needs of ISO26262 in MEMS Gyro based automotive applications

Accommodating the needs of ISO26262 in MEMS Gyro based automotive applications

Technology News |
By eeNews Europe



Not so many years ago these were limited to the high-end vehicle models but today, driven mainly by the reducing cost of such sensors, they are finding their way into most models. Although for some applications there has been a viewpoint that MEMS gyros and accelerators have not been well balanced in terms of costs and performance. For example, active suspension applications require four extremely accurate and stable sensors mounted on the wheels in order to meet the necessary input signals to achieve a smooth reliable chassis dynamics. The cost of such highly stable and accurate sensors has traditionally been high.

Another application that is now mandatory in the US is preventing front passengers being ejected from the vehicle during an accident. This is particularly the case for vehicles with a high centre of gravity such as SUVs and pick-up trucks where cases of passengers being flung from the vehicle in a roll-over situation is very high. In most automotive applications using a MEMS gyro it is only the yaw rate that is detected. However for roll-over detection you also need to measure the roll rate. This requires another gyro be employed to detect the X-axis of the vehicle. Similar movement detection is also used for adjusting the high light angle when a vehicle is heavily loaded and is mandatory for Xenon headlights. MEMS-based sensors are already part of our everyday life, but compared to those commonly found in popular applications, such as smartphones, but the sensors used in such applications are nowhere near as stable and accurate as those used for automotive applications and their characteristics vary widely due to temperature, vibration and other environmental factors.

The MEMS device is also just the core component of a system that in most cases requires software filtering of noise, adaptive learning algorithms and being able to zero any offset of effects of changing temperature and vibration, not to mention changes that take place during the automotive production process; all in all, active chassis and electronic stability control applications are really very complex. As with so many automotive applications, including the MEMS sensor device itself, embedded software is omnipresent.

Figure 1 – Example of a combined gyro and accelerometer sensor device used in automotive applications – the SCC2000 from Murata. For higher resolution, click here

All software within a vehicle falls within the ISO26262 standard. Titled “Road vehicles – Functional Safety”, the standard is part of the broader ISO61508 Functional Safety of Electrical/Electronic/Programmable Electronic Safety-related Systems. The standard applies to any aspect of safety-critical features that takes control away from the driver and takes control of steering, speed and braking functions.

For engineers developing safety functions using MEMS they will need to ensure their design will comply with the ISO26262 standard. The consequences of failing to comply could result in litigation should an accident occur, so this matter needs to be taken seriously. The most important aspect of the development process is to identify all the application requirements and highlight those that have the potential to impact safety. Based on such safety analysis, a mapping exercise is performed that looks at both the software and hardware platform, and assigns a safety risk classification according to the automotive safety integrity levels (ASIL) QM, A, B, C, or D. ASIL D denotes that in the event of a malfunction, the potential for a severe life-threatening or fatal injury requires the highest level of safety assurance. Diagnostics are a fundamental part of any ISO26262 certification and are used to reduce failure rate. The overall system needs to achieve certification but part of that process will rely on the fact that the sensing components themselves, upon which the application takes its inputs, can also indicate their own operational status.

For example, Murata’s SCC2000 is already compliant with the ISO26262 standard and a fail-safe specification is provided for engineers wishing to incorporate the sensor into their device, as well as design support by Murata. The approach taken is called ‘Safety Element out of Context’ (SEooC). The sensor is designed with a generic application in mind and a self-testing framework is designed accordingly. The combined sensor is also meets the AEC-Q100 stress test qualification requirements for electronic components used in automotive applications. It is an ideal platform for use in both active and passive safety designs in automotive applications such as hill start assistance (HSA), active steering, and adaptive cruise control for advanced driver assistance systems (ADAS).

Figure 2 – Table of safety goals and safety requirements for ESC application (excerpt, not all requirements shown)

Figure 2 illustrates the safety goals of the sensor so that should the sensor start to exhibit erroneous data it will fail-safe and communicate this via the failure flags to the host application. In addition, the SCC2000 runs a self-test diagnostic during power-up to check critical sensor functions. There is also a continuous checking of about 20 parameters that the ASIC device in the sensor monitors without any impact on the sensor detection process itself.

For example, Murata’s SCC2000 series sensor has five status registers that indicate; status overall, gyro sensor status 1, gyro sensor status 2, accelerometer status and the status of the common blocks. The ASIC device, see Figure 3, includes interfaces both for the angular rate sensor element and the multi-axis accelerometer element. Both interfaces are having mixed-signal architecture including analog signal preprocessing, analog-to-digital conversion (ADC) and digital post-processing. Interfaces are sharing some common blocks in the analog (power supply regulators, voltage references, oscillator, temperature sensor) and in the digital part (SPI, register map, non-volatile memory interface).

Figure 3 – Functional ASIC block diagram Murata SCC2000

Figure 4 indicates the sensor status block and how errors flags are set. If, for example, the internal fail-safe diagnostics detects over-range or saturation conditions then the corresponding status bit registers are set to indicate a failure.

Figure 4 – Handling of failure flags – Murata SCC2000

Murata’s SCC2000 offers best in class temperature dependency, shock sensitivity and bias stability characteristics and consists of a low-g 3-axis accelerometer with two angular rate sensor options of either X or Z-axis detection, together with a 32-bit digital SPI interface. The sensor has a software selectable 10 Hz or 60 Hz low pass filter that can be configured via SPI. Gyro range is ±125 degrees per second with sensitivity of 50 LSB per degree per second – additional measurement ranges are available upon request. Typical accelerometer offset temperature drift is ±6 mg for the 2 g sensor and ±12 mg for the 6 g version. Gyroscope offset temperature drift is typically in the range ±0.5 °/s for the 125 °/s X & Z -axis product versions. The Gyroscope has a typical offset short-term bias stability of 1 °/h for the 125 °/s X-axis device and 2 °/h for the 125 °/s Z-axis product.

The use of MEM sensors has enabled an array of safety functions to make our car journeys much safer. For such complex systems the availability of key components that themselves have been pre-qualified against safety standards such as ISO26262 greatly assists the design engineer.

About the author:

Jan Pekkola is Senior Product Engineer, Sensor Products at Murata Europe.

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