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As modern technology is more advancing, the demand from customers and vehicle manufacturers to install various new electrical and electronic systems into automobiles has increased dramatically. The rising demand lead to installation of onboard sophisticated electronic control systems in automobiles to enhance vehicle safety and driver comfort. The system includes Control Area Networks (CAN), Engine Management Systems (EMS), Anti-Lock Braking Systems (ABS), safety systems, communications, mobile, wireless headsets, entertainment systems, variety of DC motors and controllers. The physical size of different vehicle control systems used are reduced dramatically due to demanding light weight automobiles for better performance.
As electronic systems go smaller and lighter it becomes more complex with software embedded on it. As more systems present large number of wires need to be installed to connect them, thus increasing the cost of manufacturing. To reduce cost and weight all modern vehicles manufacturers are planning to go wireless or use CAN bus multiplex wiring system on board. Placing many of electrical and electronic system on vehicle in small confined space is still a problem with Electromagnetic Interference (EMI) of these onboard systems from interfering among each other causing cross talk (radiated and conducted emissions). If these systems are not controlled properly they may fail or cause severe problems, because as an unfortunate all electronics suffer any sort of interference.
Fig.1 EMI to Vehicle Electronics
Familiar Situation of EMI
As youths get more attracted towards new technology they tend to install costlier and better music systems or other gadgets into their personal cars to get more entertainment and pleasure. These systems include high watt speakers, amplifiers, sub-woofers and LCD displays on their car. These remain threat always as they were not recommended by car manufacturer or neither it is been tested for Electromagnetic Compatibility (EMC) after installation. Whenever these systems are operated during vehicle movement they can cause problems to normal vehicle behavior. Some issues were unintended slowing down vehicle speed even though enough throttle pedal pressed, sometimes increase in speed than required throttle given, engine cut off or cruise control accelerates the car beyond driver control. Amplifiers and sub-woofers used are of high ratings of around 1000W and require high current for operation. These systems can easily interfere with vehicle electronics or they may in turn get disturbed by other vehicle operations or by radiations present in the environment around. EMI problem with music systems will not be a major issue as very few people add them but excluding them there are many other EMI problems through environment or onboard vehicle electronics (Electronic Control Units).
Developing technology in modern automobile industry, computers got involved with automobiles and their engines. Modern engine's operation is now controlled by computers ECU (Engine Control Unit) which are also called as EMS (Engine management System). Embedded microcontrollers are used in variety of vehicle systems. Using a variety of sensors, the EMS controls the functions of the engine and allows aÂ proper management of the engine's operations. Acceleration, braking (ABS), fuel injection and many other functions of car are controlled electronically for better comfort, fuel efficiency and safety. Modern Engine Control Units were launched due to the technological advancement to embedded microprocessors (CPU) that was fast enough to provide a real-time operations.
Due to EMI engine operations are more adversely affected. Engine management Systems are mostly influenced by several factors including harsh environments onboard and external. The automotive environment contains severe threats including many power transients, radio frequency interference (external and onboard radio transmitters and receivers) electrostatic discharge, power line electric and magnetic fields. The major threat for EMC designers is to focus on making automobile environment less susceptible to EMI during operations. These problems can only be overcome through good EMI design technique.
This assessment is focused on EMC issues such as different EMI threats to vehicle electronics, mechanisms that producing such problems mainly on automobile engine management system and Electronic Control Units. There are different methods also included which could improve performance of the desired system. Different tests are also being conducted on vehicle in simulation chamber and also on some systems at design level to check whether system is working fine under different conditions for worst cases as well.
EMC Issues Associated with vehicle Electronics:
As technology is quickly advancing, automobile manufacturers are trying to make more safe cars and practically modern vehicles are safer than before but still EMI problems can cause some severe accidents. As cars can go anywhere interference caused to its onboard electronics is sometimes unpredictable. The main problems of EMI to automobiles are electrostatic discharge from humans, power line fields(transformers), radio frequency immunity, roadside broadcast, cellular telephone, airfield radar systems, cars own systems(main cause of power transient) and many more. There is also major problem of EMI which can cause to vehicles such as, after an accident vehicle is repaired, when owners modify them by adding additional entertainment systems onboard or by external body modifications. If these modifications are done by using devices or systems which are not of EMC standards this can cause additional problems of EMI by defeating the interference control measures placed by original vehicle manufacturer.
The main mechanisms that produce EMI are:-
Conductive Emissions: It is generated by switching of solenoids in gearbox, relays and by commutation of electric motors. These are transient in nature. The emissions are conducted along the wiring harness and spread into the power supply terminals of onboard electronic systems. These transients get coupled inductively or capacitively into signal leads of various systems.
Conductive Susceptibility: These high voltage transients get superimposed into vehicle power supply of 12v or 24v and can significantly harm the electronic systems of the vehicle.
Radiated Emissions: Radiated emissions arise from two sources in the vehicle.
Conducted transients which are generated by electrical systems which starts emitting radiation since the wiring harness acts as an aerial.
Emissions from electronic systems which involve high speed logic such as microprocessor circuitry. Harmonics generated from clock pulses of 1MHZ or greater which extend over 100MHz. These are either radiated directly from the system (microprocessor) or from the wiring harness. Radiated emissions from ignition system also interfere with other vehicles or with domestic receivers.
Radiated Susceptibility: Since vehicle in an inherent manner is likely to be a good conducting body as it is subjected to harsh electromagnetic environment onboard and mobile transceivers. Highest rate of problems at 20-200MHz band is due to wiring harness and vehicle body itself. The fixed onboard transmitters around the vehicle produce great amount of power but relatively less field strength, but mobile transmitters are opposite by emitting less power with greater field strengths affecting more on host and adjacent vehicle.
Thus also illustrated by referencing the following equation
E = (âˆš30PG)/r v/m
Here 'E' is the field strength in (v/m) produced by an antenna with radiated power as 'P'(W) and 'r'(m) is the distance between the source and victim. 'G' is antenna gain. Consider the case were neglecting antenna gain were on-board transmitter emits radiations on ECU. Assuming distance between the source and victim is 1m or less in case between circuit components. The power dissipated is assumed as 10W. The field strength experienced by the vehicle is calculated as
E = [âˆš (30*10)]/1 = 17.32 v/m
From the above equation its clearly visible that the field strength is directly proportional to radiated power and inversely proportional to distance between coupling components. If either distance is reduced and power radiated is more the field intensity will be high.
Fig.2 Electromagnetic Spectrum
The Electromagnetic Spectrum in fig.2 shows the range all frequencies and their associated wavelengths. Not all radiations of the spectrum have greater effect on vehicle electronics but the above diagram clearly explain the basic idea that frequency-"f" is inversely proportional to wavelength-"Î»". Hence f=c/ Î» were "c=3*10^8m/s. Therefore, energy-"E=hc / Î» J. As wavelength decreases effect of emissions on components also decreases with inverse effect on frequency.
Methods to improve system performance:
It is seen that vehicle systems are definitely prone to EMI problems. If EMC issues have to be improved in modern electronics of automobile, problems occurring in the design process of such systems should be controlled and if it is not done it becomes difficult and more expensive to handle it later. Although it is impossible to control emissions such as RF interference, electrostatic discharge, magnetic fields and many other EMI from external sources. Only proper shielding, filtering and grounding of such onboard systems could reduce the risk of interference on them.
Few EMI reduction methods on circuit level design of electronic systems are:
Try moving most of the components to PCBA (Printed Circuit Board Assembly) and try good shielding techniques on them.
Use less noisy components in circuit so that it least interfere with each other.
Reducing coupling between circuits by good physical separation to avoid EMI.
Minimizing the ground loop from the circuit back to the power supply and by reducing the ground impedance by proper grounding model.
Using ferrite products as it acts as an absorber of EMI energy by emitting less amount of heat.
Reducing capacitive and inductive coupling of microprocessor and other IC's (integrated microcontrollers) by reducing the rise time of pulses as this process decreases the higher frequency components of the signal by reducing harmonics.
Improvements should be done to improve vehicle efficiency, stability and reliability. As many series and parallel wires run all around the car, interference caused to wires and noise from them creates a radioactive loop. To control noise, the noisy leads should be twisted together as this method will decrease the area of loop by decreasing magnetic coupling. If shielding could be done on these twisted leads a better solution for the problem could be achieved. The leads which are leaving the shielded area should be filtered as well. In cars chassis is used as common ground, if most of the cables are placed near chassis it will minimize inductive coupling of wires with other components. The length of cable used in vehicles should always be kept as short as possible because a long cable will act as an efficient antenna. The leads which are not shielded should be kept as short as possible to avoid capacitive coupling. Proper shielding of all cables running around the car with proper installation should be maintained all round vehicle life and also these cables should meet EMC international and government standards.
As shielding helps to avoid interference from entering the components circuit, the conducted harmonics within shielded area cannot be fully removed. It can be only eliminated using a filter which will remove harmonics without affecting the actual signal. The different types of shielding techniques are BLS (board-level shields) on PCB's this approach involves proper shielding selection and optimal circuit designing including proper division, board stack-up, as well as high-frequency grounding of the board and filtering techniques. As mentioned before if these techniques are designed at the beginning stage, very less impact to schedule and cost can be attained. The proper designed and installed PCB shield can actually remove the entire loop area because the affected circuit will be inside the shield. Components with greater noise should be moved away from the sensitive area.
For instance, board layout affects EMI control greatly RF and wire mesh gaskets and many more. Electrically Conductive Elastomers are widely used in cars as it shields from both environment and EMI. These can be custom made or cut into any shape required for variety of applications. These provide shielding effectiveness up to 120db at 10GHz. Nickel-zinc ferrite products are used extensively in signal line and EMI filtering as cable cores, chip beads, CAN-Bus chokes and connector plates.
Form in Place (FIP) the EMI gaskets can be used in any conductive plated, painted or metallic surfaces of electronics cover that requires environmental sealing has complex or rounded surfaces and it has minute devices requiring a good gasket thus protecting the cover against internal including external radiated interference and environmental elements. FIP gaskets allow more critical packaging space for PCB level than any components. The room temperature solving gasket materials actually eliminate need of costly heat curing systems which cause single component mixture eliminate ingredient mixing. This shortens production cycles. These have shielding effectiveness of excess 70--100 dB up to 18 GHz.
The factors that must be taken into account to arrive at a correct solution is the equation for emissions from a basic circuit, it is given by
E = 1.316 AIF2/ (dS) -------------equation (1)
Where E = field strength in (Î¼V/m)
A = loop area in square centimeter (sqm)
I = drive current in amperes(A)
F = frequency in MHz(Hz)
d = separation distance in meters(m)
S = shielding ratio between source and point of measurement.
Analyzing equation-(1), it is realized that frequency is the biggest factor because of the emissions increasing as the square of the frequency (F) that also increases. For current (I), emissions increase linearly, which is also true for loop area (A). The distance (d) is set by the test specification and 1.316 is a standard constant. The system designer has no control over these last two parameters as then must not be considered.
The equation for susceptibility is given by
Vi= 2Î AEFB/(300S) ---------------equation (2)
Where Vi = volts induced into loop (V)
B = bandwidth factor (B=1 in band and B=circuit attenuation when out of band)
Equation-(2) indicates that the susceptibility is directly proportional to loop area (A), frequency (F) and the bandwidth factor (B). Frequency (F) is specified by the operating environment as is the EMS trength (E).
Testing of vehicle to meet EMC standards:
Firstly testing the vehicles electronic immunity to radiated fields can be tested by placing the vehicle in a large non-resonant chamber of 10m*10m*5m height in a RF (Radio Frequency) absorber being 1.5m long and expose it to a frequency range of 1- 10GHz with radiating field strength of up to 60 v/m. The vehicle is being tested without any driver seated. Inside the test car very little as possible testing equipments are being placed to avoid any change in response of electronic systems caused by testing equipments. The vehicle is monitored to check whether ECU or any electronic component behavior is altered at high frequency range with large field strength. In this test ECU is connected to spectrum analyzer using optic fiber cable to find out the maximum immunity level that the ECU can withstand. Spectrum analyzer is also used in testing of EMS and many other electronic microcontrollers.
The current absorbed by wiring harness during this test is more due to less distance from the radiating source. Considering vehicle placed at a distance of 20m, the current induced on wiring harness is being scaled up using current injection technique to check the behavior of the system at higher effective field strength.
Three different positions of antenna are used as transmitter by placing one in front of the test vehicle and other two on sides. As lot of wiring harness is accumulated at sides and edges of the vehicle the field strength will be significantly greater at these corners. Testing at these corners will be done at large intensity for about 100v/m for worst case conditions.
Testing vehicle is the accurate way of checking the immunity of vehicle electronics to radiated field. However more convenient way is to check each and every component of electronics (EG: on PCB's) during the design and manufacturing stage as supplier of these systems doesn't require vehicle to test each electronic components.
As most EMI occurs through wiring harness due to inherent coupling. The best method to test systems immunity is by BCI (Bulk Current Injection) technique. The next testing step involves measuring the current absorbed by wiring harness while vehicle is being subjected to radiations of relatively low field strength of 1v/m. This test is done at open test site at a distance of 20m radiating antenna to prevent distortion from near field radiation effects and to subject the vehicle to uniformly distributed field.
Different tests for far field strengths are conducted from 1-100v/m over a wide frequency range using different harnesses and field polarizations. Using BCI as a diagnostic technique in a EMC chamber on a whole vehicle testing levels at which susceptibility occurs at each system can be determined. This test data will be helpful to increase the immunity of the system by doing suitable modification to the required systems. Various standard testing methods such as ISO can be used for both whole vehicle testing and system (or component) level testing.
Testing method of component level testing will more often eliminate problems occurring at design states by making whole vehicle testing more cost effective. The major onboard components such as CAN Bus network, ECU, EMS, other electronic control systems which control operation of engine will be tested twice on circuit level and on whole vehicle after its implemented. This would better prove that system will meet the required standards. Thus time involved in whole vehicle testing will be reduced making it more immune to harsh environments.
By skipping the important step of simulation the problems which present underneath will never be known. Transient emissions from the different electrical components can be measured and noticed using spectrum analyzer. Line Impedance Stabilizing Network (LISN) is also widely used to test/simulate impendence of vehicle wiring harness.