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Electronics in the The complexity of the electronic systems used in a modern car means that extensive testing is required to ensure that these systems do not suffer from electromagnetic interfer­ence. Here’s how Ford tested the systems used in its EF Falcon. One major criterion that a vehicle’s electronics systems must satisfy is electromagnetic compatibility. The last thing a driver needs is to have an airbag trigger unexpectedly or to have the engine stall because of interference with the engine manage­ment module from a nearby transmitter. For this reason, all electronic systems must be thoroughly tested to ensure that they can not be disrupted by electromagnet­ic interference (EMI). Nor should the systems themselves generate EMI to a degree which is either illegal or which interferes with 8  Silicon Chip the operation of other systems. Furthermore, the car’s electronics should be able to with­stand a diversity of abuses, ranging from a suddenly disconnected battery to electrostatic discharges generated by people sliding in and out of the seats. Electromagnetic interference A car presents a very hostile environment for electronic circuitry. Not only are there physical factors involved, such as vibration and heat, but there may also be high-level EMI from 50Hz to over 1GHz in areas where the vehicle operates. Strong emitters typically include power lines; radio navigation systems; AM, FM and TV transmitters; amateur and mobile radios; cellular phones and radar. The potential effect of this EMI on a vehicle can vary from a flashing clock display to engine stalls during the use of a mobile phone. The possible effects of EMI on vehicle electronic systems can have legal and safety implications. An airbag trigger or anti-lock braking system adversely affected by EMI has serious ramifications, while the digital odo­m­ eter incorporated in all EF Falcons is required by Australian Design Rules (ADRs) to operate without data corruption. In-car EMI EMI generated by electronic and electrical components within the car is not produced at the same high levels as by radio transmitters or power e new EF Falcon By JULIAN EDGAR Pt.3: Avoiding Electromagnetic Interference lines. However, because of its close proximity and the use of common wiring harnesses, it can still cause significant problems. The EMI generated by a car can be divided into two types: narrowband and broadband. Narrowband EMI is generated mainly by microprocessor modules and consists of discrete inLeft: the development of a new car now involves extensive elec­ tromagnetic compatibility testing to ensure that the numerous electronic systems will operate reliably in all environments. Testing of the EF Falcon was carried out in a special facility located in the United States. terference harmonics related to the microprocessor clock frequency. Two main problems are associated with this type of EMI. First, fringe area reception of FM radio can be degraded when one of the interference harmonics falls within the broadcast band­width of the station frequency. And second, there can be a prob­lem with mobile radios where continuous scanning of a number of frequencies is carried out. If the interference harmonic falls on or close to a scanned frequency, the radio can lock onto the interference signal and be effectively disabled. Broadband EMI, on the other hand, is generated by switching transients Fig.1: the Ford electromagnetic compatibility test facility, Michigan, USA. May 1995  9 wiring located on the front and the sides of the car are directly exposed to the field. The transverse electromagnetic cell is also calibrated to produce free space fields but, because the field distribution is more uniform, a turntable is not used. Remote control Fig.2: the transverse electromagnetic (TEM) cell is used for testing at frequencies below 20MHz. The dynamometer allows the vehicle to be ‘driven’ while being tested. Fig.3: the anechoic chamber is used for vehicle testing with frequencies from 20MHz to 1GHz. Note the turntable which allows the car to be rotated during testing. from ignition coils, motor commutators, solenoids and relays. This can cause radio interference and corruption of engine management sensor inputs. EMI compatibility testing Testing of the EF Falcon for electromagnetic compatibility (EMC) was carried out at Ford’s state-of-the-art facility at Michigan, USA. Fig.1 shows the layout of this facility. High-level narrowband EMI testing is carried out in two specially constructed test cells. These comprise a transverse electromagnetic cell designed for testing below 20MHz (Fig.2) and a shielded anechoic chamber for testing above 20MHz (Fig.3). Note that both test cells have chassis dyna­mom­ eters installed, allow­ ing the test­ ed vehicle to be run under load while remaining stationary. 10  Silicon Chip Although test procedures in the anechoic chamber vary de­pending on standards, the process essentially involves irradiat­ing the vehicle with RF signals at a set field strength. Because the field within the chamber varies with frequency and position, the Ford method initially establishes the free space values of the electric fields; ie, the calibrated field strengths are first measured in the empty chamber without the vehicle distorting the field. In addition, to improve field distribution around the car, the antennas are set as far back as possible and so very high powers are used for testing. During susceptibility testing of the vehicle, the amplifier can be set to produce the test field strength at varying frequen­cies. Because the antenna is fixed, a turntable is used to rotate the car after each frequency sweep. This ensures that all compon­ents and It’s worth noting here that dangerously high field levels are present in the EMI chambers during testing. As a result, all vehicle functions are activated and monitored remotely. Two video cameras are used to view the instrument cluster and the centre console, while an intake manifold vacuum gauge (installed within view of one of the cameras) monitors engine performance. To prevent RF from leaking into the control room, all video, audio and other test signals are routed using optical fibres. In addition, all actuators and switches on the car are pneumatically controlled to eliminate a potential source of unwanted RF which would affect the accuracy of the test proce­ dure. The switches controlled in this manner are used to inter­rupt fuse lines to enable emergency shutdown. They are also used to reset electronic modules, so that start-up routines can be monitored, and for the extraction of fault codes. Actuators are also installed to activate switches for cruise control operation and to depress the brake pedal to test ABS operation. Test procedure The actual testing is performed both when the car is at idle and also at 70km/h. The test starts by sweeping each fre­quency band at the highest test field strength and during this process the frequencies where susceptibility affects appear are noted. At the conclusion of the sweep test, the frequencies where potential problems existed are pinpointed, with the RF level increased until failure is observed. The field strength at which this occurs is noted and then checks are made against standards criteria. The specific fault criteria for which each system is exam­ined are listed in Table 1. Internally-generated EMI The production of EMI by the car is regulated in the US by Federal TABLE 1: EMC TEST FUNCTIONS System Functions Monitored Engine management Vacuum gauge monitoring for engine stumbles or stalls; production of fault codes Anti-lock braking Brake fluid pressure reduction as appropriate at each wheel Automatic transmission Shift of transmission to limp-home constant third gear mode; production of fault codes Cruise control Constant speed cruising ability measured by dynamometer roller speed; sudden throttle opening as indicated by the vacuum gauge Airbag Warning of light illumination, indicating the presence of fault codes Body electronics module Timing of intermittent wiper period; production of fault codes Instrument cluster Errata gauge, LCD, odometer or warning light operation Communications Commission (FCC) standards and these have been adopted by Ford as the corporate standard for EMI generation. In Australia, a voluntary Australian Standard applies and both radi­ated EMI tests are performed. Narrowband EMI testing is performed in the transverse elec­ tromagnetic cell, with special emphasis placed on any interactive problems caused by vehicle wiring and components. Ignition test­ing is carried out in the open air with the engine running, with a field plot of the EMI carried out around the car. These values are then checked against the AS 2557 standard, which is designed to control vehicle EMI on TV and radio broadcasts and on communi­cations services. Other testing Other tests involve electrostatic discharges, load dumping and the effects of low battery voltage. Electrostatic discharges (ESD) occur when a charged body comes in contact with parts of the car. It can cause damage in two ways: (1) by direct charge injection into sensitive semicon­ductors via the housing or wiring; and (2) by indirect RF radia­tion generated by the discharge. The test for ESD susceptibility is conducted with a commer­ cially produced simulator. A high voltage power supply is used to charge a capacitor to the required voltage, with the simulator then brought close to the instrument panel to cause the dis­ charge. The test voltage starts at 4kV and is increased to 15kV. At locations where it is conceivable that a person standing outside the vehicle could contact the interior, the test voltage is increased to no less than 25kV. All components are checked for damage after each discharge. Load dump testing is necessary to evaluate the effect of a heavy current being switched off in a car in which the battery is disconnected. Such a situation could occur if the battery termi­nals are corroded or loose and can result in a surge of up to 150V being generated on the 12V supply line. The test procedure involves running the engine at 2000 rpm with the battery disconnected. A resistive load drawing 80% of the alternator’s rating is then suddenly disconnected, after which all accessory items are checked to ensure that they have not been destroyed by the load dump. The effect of low battery voltage (eg, due to a broken alterna­tor belt) is also explored. In this case, testing is carried out with the battery and alternator disconnected, and the car run from a high-current voltage source. The supply voltage is then gradually reduced while various functions in the car are monitored. Typically, the alternator, ABS and airbag warning lights glow first to warn of abnormal operating conditions. At lower volt­ ages, the cruise control, instrument cluster and other components shut down until, finally, SC the engine stalls at about 6V. Acknowledgement Thanks to Ford Australia and the Society of Automotive Engineers for permission to use material from the “SAE Australa­sia” journal of October/November 1994. May 1995  11