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DESIGN & PRODUCTS AUTOMOTIVE ELECTRONICS perpendicular to it - see figure 1. Centered over the IC, the magnetic field is strongest in the same plane as the Ic, whereas the field perpendicular to the IC is almost zero. At an off-center position, the situation reverses: the field in the IC’s plane is near to zero, whereas the perpendicular field reaches its maximum. Hence, moving the brake pedal and magnet from left to right yields a cosine for the in-plane field and a sine for the perpendicular field. These signals can be used to calculate the position using an arcTan function. Since all the fields diminish to zero in the extreme off-center position, linearization is required for the outer positions to be measured precisely. Linearization data is stored in an on-chip EEPROM - see figure 2. Once the architecture of such an Ic, the core technology and its function are clear, and the Fig. 2: Simplified block diagram of a brake pedal position functional blocks are defined, work on the FMEDA can begin. asIL B requires a failure rate of <100fIT sensor ASIC from ams. and a single-point fault metric of 90%. fIT – failure capabilities of the host system carefully. for example, in crash in Time – is a unit for measuring reliability. 1fIT = 1 failure in 1 avoidance systems that use a laser to detect other cars, a cam- billion hours of operation. The fIT rate is a function of the wafer era can be used in parallel to check the plausibility of the output fabrication process technology used, the Ic area and operat- of the system. This would enable the asIL level for the system’s ing temperature, and can thus be calculated for the operating main Ic to be lowered, which would in turn reduce its cost. conditions of any given application. Generally speaking, in fact, car makers will not pay compo- Each block of the IC is analyzed in turn in terms of its fail- nent suppliers more for increasing the safety level of features in ure modes and the effect of failures. Consider for example a their cars. This means that suppliers must make an intelligent common functional block in analog and mixed-signal Ics: an and lean implementation of their part’s safety requirements. amplifier. The engineer must consider those parameters which determine the amplifier’s performance, such as gain, offset Example of application of ISO26262 and stability. Gain could fall below its specified value, causing So how in practice does the implementation of ISO26262 affect the amplifier to fail completely, or it could rise above its speci- the design and operation of an automotive Ic? fied value, leading to measurement errors. A similar analysis of This is best illustrated by reference to an application example. the ways in which stability, offset and other parameters could electronic technology has become more prevalent in braking be compromised must also be carried out, and the probability systems with the proliferation of hybrid cars that recover electri- of failure (based on the FIT rate calculation described above) cal energy in braking. Precise measurement of the brake pedal’s determined. position within its travel enables an ecu to decide whether all of the failure modes will then be assessed for their impact to use the electric motor to brake the car, or the conventional on the specified safety level (whether the failure mode in itself mechanical brakes. a typical safety requirement for this applica- is safety-critical or not). For any of the failure modes, a check tion is asIL B. since there is no parallel, redundant method for is performed to determine whether one or more of the existing sensing brake position, the Ic therefore has to meet the require- diagnosis features can detect the failure and bring the system ments of asIL B. to a safe state, for instance by switching the output to a ‘signal The latest generation of hybrid vehicles use a magnetic Hall- failure’ band. standard diagnosis features in a magnetic posi- effect position sensor ASIC to measure the position of a ring tion sensor will include: magnet affixed to the brake pedal mechanism. - Measurement of the external and internally-generated advanced position sensors use a 3d Hall sensor array which supply voltages, providing a warning flag and eventually a reset can measure magnetic fields both in the plane of the IC and if preset thresholds are breached. Table 1: Simplified FMEDA example for one block. The total FIT rate of the block is 1,54FIT according to its share of total die area. The dangerous and undetected FIT rate is 0,062+0,006 FIT (column: residual faults). Thus the metric for this isolated block would be 1-(0,006+0,062)/1,54 = 95%. 18 Electronic Engineering Times Europe November 2012 www.electronics-eetimes.com


EETE NOVEMBER 2012
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