NBSIR 78-872 ®

ELECTROMAGNETIC COMPATIBILITY EVALUATION OF

SEVEN ELECTRONIC BRAKE SYSTEMS

J.W. Adams

H.E. Taggart

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80303

March 1978

Final Report

Prepared for:

National Highway Traffic Safety Administration

Department of Transportation

Washington, DC. 20590

NBSIR 78-872

ELECTROMAGNETIC COMPATIBILITY EVALUATION OF

SEVEN ELECTRONIC BRAKE SYSTEMS

J.W. Adams

H.E. Taggart

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80303

March 1978

Final Report

Prepared for:

National Highway Traffic Safety Administration

Department of Transportation

Washington, D.C. 20590

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

1. Background -'-

2. Electromagnetic Field Strength Criteria 1

3. Test Procedures 2

4. Results 3

5. References 3

iii

Electromagnetic Compatibility Evaluation of Seven Elect ronic Brake Systems

J. W. Adams and H. E. Taggart

Seven electronic brake systems were subjected to electromagnetic fields

inside a TEM cell to determine their susceptibility to these fields. Worstcase criteria for field strength levels were established based on previous

near-field measurements. The frequency range covered in these tests was from

10 kHz to 1 GHz. The results on each brake system are subject to many factors.

These include measurement accuracy of criteria levels and susceptibility

levels, cable lengths, geometric arrangement of components and cables, reliability of antilock-failure warning display, and time variation of fields.

The seven brake systems are referred to as units A through G. No unit was perfect in all respects. Unit D showed no failure under any test condition,

but did have an indicator failure under three conditions. Unit B had 16 failure

indications and 8 valve-clicks without failure indication. Units A, C, E, F, and G each indicated numerous failures at levels an order of magnitude below

the criteria level.

Key Words: Electromagnetic compatibility; electronic brakes; susceptibility.

1. Background

In order to comply with the Federal Motor Vehicle Safety Standard 121 (FMVSS-121),

truck manufacturers have elected to use electronic antilock devices in their braking

systems. The work reported here was performed at the request of the National Highway

Traffic Safety Administration (NHTSA) to determine whether those systems are susceptible

to electromagnetic interference (EMI) and to attempt to detect failure modes under

worst-case conditions. Failures due to EMI are difficult to either predict or determine

after the fact due to the extreme mobility of vehicles and sources and their complex

interactions.

2. Electromagnetic Field Strength Criteria

The same criteria curve is shown on each of seven figures. Figures 1 through 7. The curve is based on worst-case field strength conditions in the sense that worst case occurs

in the immediate proximity of a radio transmitter where both electric (E) and magnetic (H)

fields are orders of magnitude higher than at distances several wavelengths away from the

transmitter. These worst-case levels have been measured previously by NBS with support

from NHTSA and reported elsewhere [1].

In summary of this previous work, electric field strength levels in and around vehicles

with on-board transmitters range mostly between 10 and 300 volts per meter, while field

strength levels in and around vehicles adjacent to vehicles with transmitters range from 5 to 100 volts per meter. Field strength levels in and around vehicles adjacent to fixed,

high-power transmitters are generally less than those reported above with the exception of some AM broadcast stations. These levels are sometimes higher than those reported above.

In this referenced work, no near-field measurements have been made above 500 MHz, but

the extrapolation used above 500 MHz is predicted from theory [2]. There are plans to make

these measurements subsequently.

The criteria curve is actually somewhat lower than suggested in this previous work.

This is subject to interpretation and judgment. The basis for using a lower curve is

primarily based on the following factors. First, the ANSI C-95. 1-1974 Standard and

Occupational Safety and Health Standard, 29CFR1910 . 97 , for personnel hazards requires

levels for individuals to be less than 200 volts per meter when averaged over any sixminute period. This averaging (of power density or E^) allows much higher field strength

levels for time periods short compared to six minutes. If people are exposed to fields of more than 200 volts per meter when around vehicles, then legal power levels
need be reduced In the future so that neither people nor vehicles are exposed

to fields greater than 200 volts per meter. Nevertheless, present legal power limits

produce fields considerably in excess of 200 volts per meter around vehicles. Finally, if

filtering and shielding are adequate to protect a unit against fields of 200 volts per

meter, this is normally sufficient to protect it against even higher fields. If there is

insufficient filtering and shielding, as is the case In several of these units, failures

show up at levels far below the criteria level.

At present, the portion of the spectrum of greatest concern is between 10 MHz and

500 MHz, since there are more mobile transmitters in this range than any other, and

electromagnetic energy is coupled more efficiently in this frequency range than any other

for objects of vehicle-size dimensions.

Each unit was tested in at least two different TEM cells [3]. This was necessary in

order to achieve the desired field strength levels over the 10 kHz to 1 GHz frequency

range. Failure detection depended principally on reaction of the failure-indication

system. A stethoscope was used to detect audible clicks by the air valves when the unit

was inside the large TEM cell. The factors that must be considered are as follows.

a. The power available and the spacing between the center septum and the bottom of

the TEM cell determine the unperturbed field strength level E inside the cell. This may

be calculated from

where P is net power into the cell, is the characteristic impedance of the cell, and d is the spacing between the septum and the floor of the cell.

b. The unit under test perturbs the fields at all frequencies. This is a spatial

variation around the unit, and maximum to minimum may vary by a factor of 2 or 3 to 1.

c. At resonant frequencies of the unit under test, the field is greatly changed,

usually by a factor of 10 to 1 over the unperturbed levels.

d. Each system was tested in at least two different geometrical arrangements. For

rigid systems, orientation of the unit under test causes variations since the field is a vector quantity. For "flexible" systems in which several components are connected by

flexible metallic cables, there seems to be no favored orientation for maximum coupling

of energy. What is important at frequencies up to 100 MHz, is the total length of the

maximum dimension of the system under test. The larger this dimension, the larger is the

ratio of physical length to electrical wavelength, and the amount of energy coupled is

correspondingly greater. Thus, lower susceptibility levels would be expected when a large

TEM cell (d = 1.5 m) is used than when a small TEM cell (d = 0.8 m) is used. Above 100 MHz (sometimes 200 MHz), the susceptibility levels are about the same for both size cells,

since the physical dimension of the cables is a multiple of half wavelengths, just as it

is in the larger TEM cell.

e. The field strength level (E^) shown on the plots of data are those calculated from

net power and separation distance of septum to floor of the TEM cell. The levels measured

with an isotropic probe are normally within 0.5E^ to 3E^, usually higher. This means that even though the field strength level was some particular value without the unit being tested

present , the fields are perturbed by the unit, and vary from .5E to 3E at various

locations around the unit. If the unit is at a resonant frequency, fields may be an order

of magnitude or more higher.

Both electric and magnetic fields were present in the TEM cell during tests since the

cell was resistively terminated.

3. Test Procedures

o o

2
4. Results

In the figures, one for each unit, "X" represents indication of a failure as indicated

by activation of the light in the failure indication system, regardless of any other system

response. An indicates an audible response from the air valve (from one to multiple

clicks) that was not signalled by the failure indication light. An "n" indicates a false

indication of failure due to a glow of the failure indication light caused by direct rf

heating of the light. Failures that were recorded at field strength levels above the

criteria level may be discussed and evaluated at a later time, but will not be discussed

further in this report. All units performed satisfactorily before and after the EMI tests.

Unit A had sis failures at levels an order of magnitude below the criteria level,

and a multiplicity of failures below the criteria level. It had two possible failures

(370 MHz and 480 MHz) not indicated by the visual warning system.

Unit B had about two dozen failures at levels below the criteria curve. Several of

these were potential failures indicated by audible response of the air valve and not

indicated by the visual failure indicator.

Unit C had innumerable failures at levels an order of magnitude below the criteria

level. This unit had a unique potential failure mode. The air valve s^idtched several

(2 to 8) times over 2 to 10 seconds when an applied field was abruptly discontinued. This

simulates release of a microphone key on a mobile transmitter. The failure warning light

indicated a failure at the end of the clicks.

The only problem ^^ith unit D was with the antilock failure indication subsystem. Tne

failure indication subsystem gave false indications at three frequencies. These were due

to direct rf heating of the warning light and a light-switching module was burned out at about 120 volts per meter at 48 MHz.

Units E, F, and G had a multiplicity of failures below the criterial level; each imit

had several failures an order of magnitude below the criteria level. There is inherent

shielding provided by metal cases, but there is apparently insufficient filtering of the

cables

5. References

[1] Adams, J. W. , Kanda, M. , Shafer, J., and Wu, Y., Xear-field electric field strength

levels of EM environments applicable to automotive systems, IEEE E-IC Symposium,

Aug. 2-4, 1977, Seattle, Washington.

[2] Reference data for radio engineers, ITT, International Standard Book Nxmber:

0-672-21218-8, Library of Congress Catalog Card Number: 75-28960, 1975.

[3] Crawford, M. L., Generation of standard EM fields using TEM transmission cells, IEEE Trans, on Electromagnetic Compatibility, Vol. EMC-16, No. 4, November 1974.

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NBS-I14A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. P\'\> A VION OK RHI'OHT NO.

NBSIR 78-87?

2. Gov't Acccs

No.

3. Recipient's Acce.s.sion Nc

4. Ti ri.!-: AND siiivi rn.H 5. Publication Date March 1978

Electromagnetic Compatibility Evaluation of Seven Electronic

Brake Systems

6. Performing Organization Code

276.03

7. AUTHOR(S)

John Adams

8. Performing Organ. Report No.

9. PERI-ORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Boulder, Colorado 80303

10. Project/Task/Wori< Unit No. 2763 487

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address {Street, City, State, ZIP)

National Highx\7ay Traffic Safety Administration

Department of Transportation

Washington, D. C. 20590

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or /ess factual summary of most sigfiificanl information. If document includes a significant

bibliography or literature survey, mention it here.)

Seven electronic brake systems were subjected to electromagnetic fields

inside a TEM cell to determine their susceptibility to these fields. Worstcase criteria for field strength levels were established based on previous

near-field measurements. The frequency range covered in these tests was from

10 kHz to 1 GHz. The results on each brake system are subject to many factors.

These include measurement accuracy of criteria levels and susceptibility

levels, cable lengths, geometric arrangement of components and cables, reliability of antilock-fallure warning display, and time variation of fields.

The seven brake systems are referred to as units A through G. No unit

was perfect in all respects. Unit D showed no failure under any test condition,

but did have an indicator failure under three conditions. Unit B had 16 failure

indications and 8 valve-clicks without failure indication. Units A, C, E, F,

and G each indicated numerous failures at levels an order of magnitude below

the criteria level.

17. KEY WORDS (six to twelve entries; ^Ifdxabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Electromagnetic compatibility; electronic brakes; susceptibility.

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