EM Simulation Plays Major Role In Automotive Design

The automobiles of the next century will, very likely, be built largely from electrical–not mechanical–components. It’s possible that only the wheels and pistons will remain purely mechanical. Even engine cams could be replaced by electromagnetic (EM) sensors and actuators.

Examples of the “electrical takeover” already are prevalent. Antilock braking systems that replace traditional hydraulic components with EM sensors and actuators are common in vehicles ranging from one-ton trucks to two-seat sports cars. Research projects promise to replace mechanical power-assisted steering mechanisms with variable-speed magnetic actuator systems, and traditional passive-spring and shock-absorber suspension systems with active systems consisting of EM sensors and actuators.

And the future will see many more electronic sensors in cars, including global positioning sensors to plot the course of the car on detailed maps, and proximity sensors to warn drivers of cars entering their blind spot or within a danger zone ahead. Trip computers and telepay systems for automatically paying tolls also will become more popular.

With so many electrical and electronic components being built into automobiles, the need for EM field simulation is paramount. At the same time electronic systems in cars become more complex, their operation becomes more critical to vehicle safety.

Advances in electronics mean that automobile components become smaller and operate at higher frequencies and faster rise times. This accentuates the potential for undesirable and sometimes catastrophic coupling between otherwise unrelated components and the corresponding need to model electromagnetic compatibility (EMC) effects. As a result, issues of EM reliability of automobile components and electromagnetic interference (EMI) between different components become essential.

Unfortunately, prototyping costs escalate as new and untried systems rush through the design cycle. Often the components show a different behavior in the car than during an isolated certification.

Although testing is essential, to reduce costs, computer tools have emerged to simulate electrical and EM components. The sensors, actuators and other EM components in automobiles are EM in nature; the excitation and logic functions are circuit related.

Design programs must combine EM field simulation with electronic circuit simulation. The goal of these programs is to create a virtual car in the computer, at least for the electrical components, with the capability to simulate all of the car’s electrical characteristics. While this goal is not yet fully realized, using EM computer-aided design (ECAD) tools has proven effective in many automotive product areas.

The finite element method (FEM) has emerged as the technique of choice for EM field simulation. In the FEM, the complex geometry of an automobile and of its internal parts is represented in the computer by thousands of tiny subcomponents called finite elements. The EM field in each element is approximated in the computer and the entire system is solved for local field values.

Coupled with circuit simulation for the electronic components, the finite-element field solution provides an accurate, reliable and complete model of the electrical components in a car. To illustrate, we will examine two examples using ECAD simulation: signal-integrity analysis and EMC analysis.

Signal-Integrity Analysis

The integrity of signals originating in the many electronic components and control systems in an automobile presents an essential and formidable challenge to reliable operation. The connection of numerous components must be done over relative long distances in complex cable trees with many closely packed wires carrying a variety of signal types and current levels.

In many applications, such as motor management and ignition control, the cables and wires often terminate in complex connectors with hundreds of pins. Different bus systems use different clock frequencies. Even the main battery cable does not operate at DC anymore, but carries base frequencies of 10 to 20 kHz.

Computer programs couple EM field analysis with time-domain simulation to model signal-integrity effects on closely spaced conductors. This simulation system handles arbitrary geometries and material distributions. Such tools are particularly effective in modeling these types of design problems:

(o) Signal integrity of electronic circuits, components and PCB layout.

(o) Simulation of reflections, ringing and ground bounce in connectors and PCBs.

(o) Effects of cable and wiring structures and bus systems on each other.

(o) Construction of equivalent circuits of such structures for system-level simulation.

Figure 1 shows a 3-D model of a car body with the simulation of a generator cable tree laid next to another cable tree. The two cable trees contain the generator-cable-battery connection, the CPU-bus-control electronics connection, and additional wiring within both cable trees.

The effects of induced eddy currents in the car body as a possible source of noises can be calculated by the field simulator. Circuit equivalents of the transmission lines and connectors for the system-level time-domain signal analysis are also produced automatically.

EMC Analysis

Some of the most pressing areas of automotive engineering today concern EMC. Topics of interest include:

(o) The effect of incident EM radiation on component reliability.

(o) The placement of electrical components to reduce EM susceptibility.

(o) The placement of antennas, including the integration of antennas, into the car structure.

(o) Radiated emissions from PCBs and cables.

Again, computer simulation provides a cost-effective way to avoid time-consuming build-and-test methods. This is particularly true in EMC, where it is difficult to separate the individual effects by analyzing measured data from a complete system. Computer simulation provides information and insights that are very helpful in improving product designs.

Due to the nature of radiated fields and the frequencies involved, EMC simulation tools must solve the complete set of Maxwell’s equations. This type of simulation is called a full wave solution. Design parameters computed are far- and near-field radiation patterns, scattering parameters and equivalent circuit values.

To compute the radiated fields produced by a mobile telephone in a BMW automobile, the geometry of the car was entered into the computer along with a model of the driver. The driver model was quite detailed and included the eye sockets, nasal passages and other aspects of the human anatomy.

Figure 2 shows the results obtained from the Maxwell SI Eminence computer program. It is a full-wave EM field solver that combines the FEM with absorbing boundary conditions to model open-region radiation and susceptibility problems accurately.

Computer simulation has the capability to use the same model in other situations. For example, little additional work was required to obtain the effect of an incident plane wave for susceptibility studies of the BMW.

Conclusion

The pace of change in the industry is quickening as automobiles evolve toward a nearly all-electric future. ECAD accelerates this change by allowing many prototyping steps to be done inexpensively on the computer, creating “virtual prototypes” of a variety of automobile components. ECAD is also used to model interference effects in wires and cables as signal lines from a variety of electrical and electronic components are routed and packed in close proximity.

ECAD tools are also well-suited to study the EM susceptibility and EM compatibility of automobile components. These simulations make automobiles safer and more reliable, and have become an indispensable tool in automobile design.

About the Authors

Davor Gospodaric is the founder and Technical Leader of Research and Development at Trimerics GmbH, Germany. He holds a Ph.D. in electrical engineering from the University of Stuttgart.

John W. Silvestro is a Senior Staff Engineer at Ansoft Corp. He earned B.S.E.E., M.S.E.E. and Ph.D. degrees in electrical engineering from Case Western Reserve University.

Zoltan J. Cendes is a Professor of Electrical and Computer Engineering at Carnegie Mellon University. In 1982, he founded Ansoft, and today is the Technical Leader of Research and Development at the company. Previously, Dr. Cendes was affiliated with McGill University, Union College and General Electric Corp. He graduated from McGill with M.S.E.E. and Ph.D. degrees in electrical engineering.

Ansoft Corp., Four Station Square, Suite 660, Pittsburgh, PA 15219, (412) 261-3200.

Copyright 1995 Nelson Publishing Inc.

April 1995


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