ELECTRO-MAGNETIC COMPATIBILITY

Introduction• Advancements in technology and tighter federal regulations translate into

more electrical systems in vehicles.• Need to reduce the electromagnetic interference (EMI) between these

systems, and guarantee immunity to RI.

Introduction• When electrically charged particles, such as electrons, are put into

motion, they create a magnetic field. When these particles are made to oscillate, they create electromagnetic radiation.

• This can include radio waves, visible light, or x-rays, depending on the frequency of the oscillation.

• In a vehicle, these EM rays could prove problematic.

• In an automotive electronic system, EMI can adversely affect the performance of an integrated circuit internally, as well as that of other electronic components in close proximity.

• Sources include: Electronic Engine Controls, High Current Systems in EV/HEVs, Optional Convenience and Entertainment systems .

• Since wires are the longest circuit element in a vehicle, they provide the largest surface area to radiate and receive EM radiations.

Inductive coupling

• Inductive coupling and Common Impedance are two common ways, EMI will cause EMC failure in the automotive environment.

Introduction

Introduction• Twisted pairs of wires are essential to reduce crosstalk and EMI.• This invention is old and dates back to the late 19th century.

Background

• Conductive wires placed in a magnetic field generate current. The strength of the magnetic field determine the value of the current as proved by Ampere’s law:

The seemingly simple twist of a pair, or pairs, of wires can reduce crosstalk, RFI and EMI.

Background

• The most common type of electrical and data conductors is the Unshielded Twisted Pair (UTP).

• In a vehicle, the Control Units talk on local vehicle networks, which are similar to a common computer LAN, but based on different protocols.

• By crossing the wires in predetermined segments at equal lengths, the EMI problem can be minimized.

In a CAN bus all the communicating devices are connected to the same two wires, labeled CAN-High and CAN-Low. All the devices must use the bus at the same speed. At each end, the two wires are connected with a 120 ohm termination resistor.

Cross Talk• As the frequency of the

magnetic field increases, its effect becomes stronger.

• When wires are not tightly twisted, the result is Near End Crosstalk (NEXT).

• Noise is picked up from an adjacent pair and cause signal fault and/or attenuation.

Background

• A twisted pair reduces coupling from magnetic fields up to 100 KHz.• For pairs to work effectively, uniform twists should exist along the total length of the wire.• Twisted pairs generate the best results in lower frequency and audio applications.

Ford requirements for twisted pairs: A minimum twist rate of 4 turns per 50 mm. One turn equals a 180 degree rotation. The twisting must be uniform throughout the

length of the wire.

• Vehicle wire harnesses vary in quality according the supplier and place of build.

• Loosely twisted wires will cause EMI interference and failure of EMC testing.

Twisted WiresThere exists a chronic issue with the wire harnesses that are put together for Ford prototype vehicles:• Lack of quality control from the supplier is causing a decline in adherence to FORD’s

requirements for twisted wires, and is a major cause of EMC test failure.• This is costing the company money and man hours.

Conclusions• Incorrectly twisted pairs is rapidly becoming a common cause of

failure for Electro-Magnetic Compatibility testing. • Netcom is undergoing a rewrite of the FORD requirement for

twisted pairs to make adequate twisting a critical characteristic, thus forcing suppliers to adequately check for quality.

• Research must be conducted on the BUS’ robustness when it comes to radiated interference.