EMC漫话（17）
数百辆汽车美国家高速交通安全机关（ National Highway Traffic SafetyAdministration (NHTSA)）以及全世界其他地方类似的机构召回。理由是“安全气囊的意外弹出”。包括几乎所有知名汽车厂商的产品，包括 BMW, Chrysler,Ford, General Motors, Hyundai, Land Rover, Mazda, Mercedes-Benz, Saab, Toyota,Volvo and Volkswagen。下面是一个美国司机的报告。
“我用左手拿着手机，向前伸出大约一臂长度，以便拨号，这时手机几乎碰到靠近方向盘的中心位置。突然安全气囊像个炸弹一样弹出，我的左手被剧烈打弯几乎碰到上臂，头被向左后方向扭转好像要被从脖子上扭下来。气囊打在手上的疼痛十分剧烈，像火烧一样，好像永远也好不了了。”
这个司机做了一些调查，结论是“我认为，在某个环境下，手机天线会在汽车的内部电路上耦合出电流，当手机靠近气囊的点火电路时，电流可能大到足以引爆气囊。气囊弹出时，我的手机距离方向盘中心位置的距离大约1个半英寸，一个美国从事EMc试验的机构说，在这个距离上，手机产生的电场大约为70V/m。”
The above driverdid some investigation, and concludes that: “The thinking is that, in certaincircumstances, the electric current coupled into the vehicle wiring from thecell phone antenna when it is close to an air bag igniter can be enough tocause deployment of the air bag. The antenna of my cell phone was, at most, aninch-and-a-half from the centre of the steering wheel when the air bag wentoff. A US organization involved in EMC testing said that the field at such asmall distance from a mobile phone is likely to be in the region of 70V/m.”
It is impossibleto say with absolute certainty that the cell phone set off the air bag. Thereare too many unknowns: the exact strength of the 1.9GHz current required totrigger the air bag; the exact distance of the cell phone antenna from theigniter; and the exact strength of RF field emitted from the cell phone’santenna and its coupling factors into the vehicle’s wiring. (Editor’s note: But it seems very unlikelythat the airbag should operate spuriously at the exact time that the cellphonewas close to its igniter.)
The AutomotiveEMC Directive requires whole cars sold in Europe to be tested for immunity at aminimum of 30V/m up to 1GHz, in Europe. Since the above cellphone operated at1.9GHz it is outside the range of this testing and the susceptibility of thecar’s systems at this frequency is unknown. Also the testing is done withcontinuous wave (CW) and amplitude modulation (AM), not with the pulsedmodulated (PM) signals typical of a mobile phone.
There are nolegal immunity requirements for the USA – but all the reputable motorcarmanufacturers apply immunity tests anyway to help reduce their risks ofliability lawsuits. The EMC immunity specification employed by the manufacturerof the vehicle involved in the above requires electronic ‘components’(subassemblies) to pass tests at 200V/m from 1-400MHz in a stripline or TEMcell, and 80V/m from 0-1000MHz in an anechoic chamber. Plus the whole vehicleis tested with radiated external fields at 200V/m from 6-30MHz, 140V/m from30MHz-1.3GHz, and 70V/m from 1.3-3GHz – but these are the external field strengths: the fields inside the vehicle duringthese tests are not controlled so are unknown (the same comment applies toAutomotive EMC Directive immunity testing).
The cell phone concerned operated at 1.9GHz, hence it was outsideof the frequency range for the ‘component’ testing range – and the wholevehicle testing might not have created 1.9GHz fields at the steering wheel withfield strengths comparable with those created by the close proximity of a cellphone. So neither this particular manufacturer’s tests, nor tests under theAutomotive EMC Directive, could be sure to reveal the susceptibility of theairbag igniter to very close proximity of a cell phone transmitting
at 1.9GHz. Notethat about half of the cell phones in Europe operate at 1.8GHz, using the GSMsystem, so this brief analysis also applies to them.
(Adapted from information sent in by Martin O’Hara of theAutomotive EMC Network www.autoemc.net by email in April 2004.)
306) Cell phones interfere with Lexus sensors
After incidents where cell phone calls apparently interfered witha sensor in some 1998 Lexus GS300
