NBSIR 77-1241

Seismic Detection of Motor

Vehicles

James M. Kenney

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

March 1977

Final

U.S. Department of Transportation

Federal Highway Administration

Offices of Research and Development

Washington, D.C. 20234

NBSIR 77-1241

SEISMIC DETECTION OF MOTOR

VEHICLES

James M. Kenney

Institute for Materials Research

National Bureau of Standards

Washington, D C. 20234

March 1977

Final

U.S. Department of Transportation

Federal Highway Administration

Offices of Research and Development

Washington, D.C. 20590

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

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
'
PREFACE

Highway measurements were made by permission of the Maryland State

Highway Administration. The vibration sensors used were fabricated by

S. Roth, P. Freeze, and S. Edelman. The work was supervised and assisted

by S. Edelman.

ii

TABLE OF CONTENTS

INTRODUCTION 1

MEASUREMENT PROCEDURES AND RESULTS 2

CONSTRUCTION OF A SEISMIC SWITCH (FEASIBILITY MODEL) 18

RESULTS 21

CONCLUSIONS 23

RECOMMENDATIONS 24

APPENDIX - A SIMPLE DESCRIPTION OF PIEZOELECTRIC SENSORS 27

iii
LIST OF ILLUSTRATIONS

FIGURE 1. Set-up for single-vehicle (time-domain) road vibration

measurements

.

4

FIGURE 2. Typical time-domain recording of single-vehicle passby

undivided asphaltic concrete secondary road.

on 5

FIGURE 3. Average spectra, 0-50 Hz. Noise match between channels 7

FIGURE 4. Set-up for multiple-vehicle (frequency-domain) highway

vibration measurements.

9

FIGURE 5. Average spectra, 0-50 Hz, from shielded ceramic sensor

divided segmented portland cement concrete highway.

on 10

FIGURE 6. Average spectra, 0-50 Hz, from shielded ceramic sensor

divided unsegmented portland cement concrete highway.

on 11

FIGURE 7. Average spectra, 0-50 Hz, from shielded ceramic sensor

divided asphaltic concrete highway.

on 12

FIGURE 8. Average spectra, 0-50 Hz, from piezoelectric polymer sensor

on segmented portland cement concrete highway turnoff.

13

FIGURE 9. Average spectra, 0-50 Hz, from shielded ceramic sensor

asphaltic concrete secondary road.

on 14

FIGURE 10. Average spectra, 0-50 Hz, from shielded ceramic sensor

divided segmented portland cement concrete highway.

on 15

FIGURE 11. Schematic diagram of seismic switch. 19

FIGURE 12. Set-up for testing seismic switch. 22

FIGURE A— 1

.

Construction of piezoelectric sensors 28
INTRODUCTION

The use of raised reflective markers on the pavement to supplement the

painted lines separating lanes on highways has proven quite effective in

southern areas. In northern areas, however, their use has not been feasible

due to their vulnerability to destruction by snowplows. While efforts are underway to modify the design of the markers to protect them from snowplows,

the Federal Highway Administration of the Department of Transportation is

also investigating the feasibility of using flush-mounted electric lights as active-source substitutes for the (passive) reflectors. In order to conserve

power, particularly if batteries were used, it would be desirable to have these

lane-marker lights lit only when vehicles were near. The National Bureau of

Standards was asked to determine if it were technically feasible to trigger

these lights by seismic means, i.e., by the detection of road vibration induced by an oncoming vehicle.

1
MEASUREMENT PROCEDURES AND RESULTS

A search of the literature revealed no previous experimental work in

the detection of pavement vibration induced by motor vehicles. The preliminary

phases of this study were therefore necessarily engaged in the establishment

of suitable techniques for the detection and analysis of such vibration. The

first objective of this effort was to determine the presence of vibration at

distances suitable for the illumination of lane marker lights. The DOT-established minimum distance for this purpose is 25 metres (82 feet), with a design

goal of 50 metres (164 feet), for a vehicle of 900 kilograms (2000 lbs) travelling 50 kilometres per hour (30 miles per hour).

The earliest measurements were made using battery-powered ac meters and

preamplifiers. These proved of little value due to the long meter response

time relative to the passby time of even a slow vehicle. Useful experience was obtained during these measurements, however, in the establishment of reliable

sensor bonding techniques and in suitable sensor design. The most satisfactory

type of sensor for this purpose was a ceramic (lead zirconate titanate) piezoelectric double disc, 2 1/2 cm (1 in) in diameter, 6 mm (1/4 in) thick, and

with a mass of about 29 g (1 oz). A description of piezoelectric sensors is

given in the Appendix. A quick-setting gypsum plaster was used to bond this

ceramic disc to the road, and also to bond a metal "seismic" (reaction) mass of

about the same diameter to the opposite (top) side of the disc. This mass, of

about 150 g (1/3 lb) and 24 mm thick, together with the mass of the ceramic,

served to convert acceleration to force. A heavy metal shield covering the

sensor was also bonded to the road with plaster, to provide isolation from

airborne noise and electromagnetic pickup (ignition noise, CB transmission, etc.),

reduce wind noise, and protect against precipitation. Wind noise, in particular, was found to produce large spurious signals from any type of unshielded sensor,

and appeared to be the limiting factor in amplifying signals from even a well-shielded

one

.

A battery-powered oscilloscope provided the first indication that vehicles

could be detected at reasonable distances, but human reaction time made distance

evaluation difficult.

2
Acquisition of a battery-powered instrumentation recorder enabled the

first reliable (and permanent) evidence of detection distance to be presented.

After some experimentation, the most satisfactory time-domain data was obtained

(as shown in figure 1) by use of a 30 metre (100 foot) cable between the recorder

and one preamplifier ("channel 2"); the input of the latter was connected to a piezoelectric polymer strip cemented to the pavement 30 metres "up-traffic" from

the ceramic vibration sensor. This strip had an active area length of about

53 cm (21 in). A description of piezoelectric polymers is given in the Appendix.

The pavement was asphaltic concrete (AC) , with portland cement concrete (PCC)

curbs retaining the soil of the surrounding broad lawns. (Some readers may be

more familiar with the less technical terms "asphalt" or "blacktop" for this

type of pavement.)

The right front and right rear wheels of a vehicle passing over the piezoelectric strip generated two very sharp pulses. (Diode limiters and cascaded

attenuators were used to protect the preamplifier.) As may be seen from the

lower traces in figure 2, these pulses are quite distinctive. The important

thing to note in these oscillograms is that the road vibrations, shown in the

top traces, exhibit an amplitude distinctly above the noise levels (approximated

at the far left of the top trace) in the region where the wheel-spikes occur,

thus demonstrating that the vehicle was detectable at the 30 metre distance.

From an analysis of the oscillograms of 13 single- vehicle passbys,

it was estimated that in 9 of these the signal was percentibly

above the noise level just before the front wheels of the vehicle passed over

the piezoelectric polymer strip. Average vehicle speeds over the 30 metre distance were estimated from the separation between the leading (front wheel)

voltage spike and the apparent peak of the vibration transient envelope detected

by the ceramic sensor. (A second piezoelectric polymer strip could have been

used to more precisely mark the moment the vehicle passed opposite the ceramic

sensor.) Average vehicle speeds ranged from an estimated 16 to 18 km/h (10 to

11 mph) (which produced a poorly defined transient envelope) to 73 km/h (45 mph)

for two vehicles. Average speed for the 13 vehicles was 44 km/h (27 mph). Four

vehicles had an average speed over 50 km/h (30 mph), all of which were judged

easily detectable at 30 metres or more. Of the four vehicles judged not detectable

(or questionable), two were the slowest. Of the seven vehicles with speeds of

33 km/h (21 mph) or over but less than 50 km/h (30 mph), five were judged detectable

3
CURBS §E8X ^ S - ! 4

FIGURE

1.

Set-up

for

single-vehicle (

timedomain)

road

vibration

measurements.

Right

wheels

of

vehicle

(going

from

left

to

right

in

figure)

passing

over

piezoelectric

polymer

strip

generated a

distinctive

pair

of

narrow

pulses ,

thereby

recording

when

vehicle

was

30m

(100')

from

sensor,

and

also

permitting

an

estimation

of

average

vehicle

speed

(see

figure

2).
tv tOOmS

(a) Low gain for ceramic sensor output (top trace) shows passby transient at its

highest amplitude.

200mV tOO*S

(b) High gain for ceramic sensor output shows that transient is above noise level (leftmost part of top trace) when vehicle is more than 30 metres from ceramic sensor, indicated

by pair of narrow wheel spikes in the output of the piezoelectric polymer strip at this

distance (bottom trace).

FIGURE 2. Typical time domain recording of single vehicle passby on undivided AC*

secondary road (on NBS grounds). Top trace in each photo is output of shielded ceramic

sensor; bottom trace is output of piezoelectric polymer strip 30 metres from ceramic sensor

passed over by right wheels of vehicle. Vertical scaling is arbitrary; horizontal scale

is 0-10 seconds (1 second per major horizontal division) due to 10:1 increase in tape

speed on playback. Estimated speed of vehicle is 33 km/h (21 mph).

* Asphaltic concrete (no Portland cement concrete underlayer)

5
at 30 metres. The fastest in this speed range, however, was not, possibly

being an unusually light vehicle. (Vehicle types were not correlated with

transients: most, however, were ordinary passenger cars.) From these measurements it was concluded that the probability of detection with even a relatively

unsophisticated trigger circuit would be quite high.

The next stage was to attempt a frequency analysis of the signal in order

to determine the passband requirements for a seismic switch. After considerable

effort had been expended in capturing passby transients in the spectrum analyzer

and matching their spectra with time-domain oscillograms, it was found that the

observed spectral structure was poorly defined, and was not reproducible from

one passby to another. Most importantly, it was not feasible to obtain a spectrum

from only that part of the transient when the vehicle was at a distance from the sensor. It was found, however, that distinctive and reproducible vehicle vibration spectra could be built up to significant amplitude by spectrum averaging —

a valuable feature of the analyzer used. To separate useful signal (sensor output)

from instrumentation noise, channel 2 of the recorder was used as a noise reference,

matching that of channel 1, which was used for the sensor signal, as shown in

figure 3 where the small separation between traces is due to slightly greater noise

from the reference channel. Of four available preamplifiers, two were chosen

which by adjustment of channel gain matching pots (provided in the preamplif ierto-recorder adaptors to compensate for microphone sensitivity differences) would

allow the closest match of instrument noise between channels at the record-gain

setting commonly used. Spectral analysis of the channel 2 "reference noise" was

made from each length of tape from which a channel 1 signal analysis had been made.

A dual averager memory in the spectrum analyzer allowed both spectra to be stored.

They could then be photographed together by double exposure while being alternately displayed on an oscilloscope (the preferable method) or by a single long

exposure with the analyzer alternating the two spectra in rapid sequence.

6
FIGURE 3. Average spectra, 0-50 Hz (5 Hz per major horizontal division). Noise

match between channels. With no sensor at the input of either channel, the channel

noise levels were matched to the degree shown. The sensor impedance is assumed to

be sufficiently high at these frequencies as to have negligible effect. On this and

all following spectrograms, the vertical deflection is linear with voltage; vertical

scaling is arbitrary, but signal processing is identical for both channels. Horizontal

scaling is linear with frequency. All spectra shown are average, covering very many

individual spectra and many vehicle passbys.

7
The procedure used for most road vibration recordings was to connect

the sensor and its preamplifier to the recorder with the 30 m cable previously mentioned, the sensor being down-traffic from the recorder, as shown in figure 4. The recorder was manned inside a vehicle parked just

off the shoulder pavement of the road, from which position the tape could be

rapidly stopped as vehicles passed opposite the car (initially) or (later) as they passed a safety cone set on the outer edge stripe of the nearest lane

50 m from the sensor. The tape was not restarted until the vehicles were at

least those distances past the sensor, and with no others within + 50 m of the sensor. Recordings were also made with the recorder in continuous operation.

The sensor was bonded to the pavement directly on the outer edge stripe of the

nearest lane, which is the nominal "edge" of the pavement of the road proper.

(While usually paved, the shoulders of the highways used were at a somewhat

lower level or were paved with somewhat different appearing material.)

To obtain an overall (record- reproduce cycle) frequency response down to about

2 1/2 Hz, most recordings were made at a tape speed of 3.8 cm/s (1.5 in/s) and

reproduced at 38 cm/s (15 in/s), thus raising all frequencies by a factor of 10.

The spectrum analyzer frequency ranges to be cited are one-tenth of the actual

instrument range settings, thus referring to the original signal frequencies

at the recorder input.

Examples of the average frequency spectra obtained from highway pavements

in various locations by the above technique are shown in figures 5 through 10.

It may be seen from these spectra that the signal (represented by the gap

between the upper and lower traces in figures 5 through 9) peaks in the vicinity

of 15 Hz (12 to 17 Hz). Although differing in fine detail, the general shapes

of the spectra from pavements of different material or construction are similar.

The discrete spectral-lines in figure 9 demonstrate the possibility of false

alarms due to seismic sources other than traffic or wind. Figure 10 indicates a high probability that the seismic switch would be activated by opposite- lane

vehicles unless the threshold level for triggering was so high that the sensitivity

to oncoming near-lane vehicles would be poor.

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(a) Vehicles > 50 metres from sensor in either direction.

(b) Vehicles at all distances from sensor.

FIGURE 5. Average spectra, 0-50 Hz, from shielded ceramic sensor on divided segmented

PCC* highway (MD-29). On these spectrograms, the bottom trace is the channel 2 reference

noise

.

*Portland cement concrete.

10
(a) Vehicles > 50 metres from sensor in either direction.

(b) Vehicles at all distances from sensor.

FIGURE 6. Average spectra, 0-50 Hz, from shielded ceramic sensor on divided unsegmented

PCC highway (1-95). On these spectrograms, the bottom trace is the channel 2 reference

noise

.

* Portland cement concrete.

11
(a) Vehicles > 30 metres from sensor in either direction.

(b) Vehicles at all distances from sensor.

j!* FIGURE 7. Average spectra, 0-50 Hz, from shielded ceramic sensor on divided AC

highway (1-270). On these spectrograms, the bottom trace is the channel 2 reference noise.

* Asphaltic concrete (no portland cement concrete underlayer).

12
FIGURE 8. Average spectra, 0-50 Hz, from piezoelectric polymer sensor on segmented

PCC highway turnoff (unfinished extension of 1-95). Vehicles > 30 metres from

sensor in either direction. Rubber cement bond. On this spectrogram, the bottom

trace is the channel 2 reference noise.

k

Portland cement concrete.

13
(a) No near vehicles in either direction, including opposite lane (on same pavement).

(b) Vehicles > 30 metres from sensor in either direction, including opposite lane.

FIGURE 9. Average spectra, 0-50 Hz, from shielded ceramic sensor on AC secondary

road (on NBS grounds). On these spectrograms, the bottom trace is the channel 2 reference

noise

.

*Asphaltic concrete (no portland cement concrete underlayer).

14
FIGURE 10. Average spectra, 0-50 Hz, from shielded ceramic sensor on divided segmented

PCC* highway (MD-29). Top trace: vehicles > 50 metres from sensor in either direction;

bottom trace: no close vehicles in either direction (except those in opposite lanes, in

both cases, and for all other highway spectra).

*Portland cement concrete.

15
An effort was made to fabricate suitable piezoelectric sensors from

polymer sheets (poly viny lidene fluoride). Designs included simple doublesheet sandwiches of various sizes and shapes (see Appendix) similar to the

ceramic disc assembly with and without an added seismic mass, where the con- tacting electrodes from each sheet are connected to a cable center-conductor

and are shielded by the outer electrodes which are connected to the cable

ground. Also tried were multi-layer sandwiches with the layers connected

electrically in series, and resonant sensors where the spring element in the

mass-spring system was either an elastic foam pad or the elasticity of the

piezoelectric polymer film itself. Most of these did not have a sufficiently

large output relative to the amplifier or wind noise, or were found to be highly

susceptible to pickup of 60 Hz power-line current and harmonics. (One exception

is shown by figure 8). High resistance in the ground plane connections following

repeated flexing (which cracked the conductive rubber used in the ground connection) was believed partly responsible for this pickup; poor insulation from the

road surface was another factor (particularly during wet weather). The brass

ground clip shown in the appendix drawing was added at a later date in response

to this problem, but no measurements were made in this application using sensors having it. The wet pavement present during most of the polymer sensor measurements

caused repeated bonding failures, making evaluation of these devices difficult.

These same problems also interfered with attempts to record traffic vibration in the soil alongside the road. A strong similarity was noted between the

vibration spectra obtained with vehicles at 50 m and beyond, and those obtained

with no near-lane traffic (i.e., those recorded with no near- lane vehicles in

sight or with near-lane vehicles in sight but only at very much greater distances)

as shown in figure 10. This suggested that soil transmission may be more significant than pavement transmission, since it was believed that the lower trace in

figure 10 was due largely to opposite lane traffic, which was separated from the

near lane traffic by a wide (unpaved) median strip. It was thought that a frequency selective soil transmission would also account for the similarity between

average spectra obtained from highway pavements of greatly different construction,

as shown by figures 5 through 9.

Attempts to measure soil vibration alongside a highway were made using

various designs of piezoelectric polymer sensors. These were largely unsuccessful for the same reasons as noted above for pavement measurements. Even thorough

wrapping with tape before burial generally failed to eliminate the strong 60 Hz

pickup, and what appeared to be intermodulation products were observed about the

60 Hz spectral line in many oscillograms.

16
A ceramic sensor was buried alongside the road, after being wrapped in

tape and plastic sheeting. To ascertain its acoustical effects, the soft

insulation was cut away over the front face (opposite the reaction mass) of

the ceramic disc and replaced with a rigid epoxy. This change had little

effect, however. Except for amplitude differences, the corresponding spectra

were essentially identical. An inexplicable shifting with time toward higher

frequencies was exhibited by this sensor, following burial for three weeks.

The stronger high frequencies could be due to better packing of the soil

around the sensor, but this should not have reduced the low frequencies.

In all of the buried-sensor oscillograms, it was apparent that the detertpd

spectra were quite different from those obtained from the pavement-mounted

sensors, so that the shapes of the observed pavement-vibration spectra could

not be attributed to the soil transmission properties. Lack of good reproducibility, however, limits the value of these observations.

Although the detection of vibration in the soil could not be accomplished

with any degree of reproducibility, and though the mode of transmission of pavement vibration was not determined, there was confidence in the seismic detectability of motor vehicles based upon the time-domain oscillograms and upon the pavement vibration spectra. Accordingly, it was decided to proceed with the construction

of a seismic switch to demonstrate the validity of this conclusion.

17
CONSTRUCTION OF A SEISMIC SWITCH (FEASIBILITY MODEL)

The sole purpose of the seismic switch (vibration- level detector) built

at NBS was to prove that vehicles could be reliably detected at the established

minimum or goal distances. No attempt was made to limit size, cost, or power

consumption, or to make it rugged and reliable — all factors necessary for

ultimate field development. The circuit developed for this feasibility study

is given in figure 11, but it cannot serve as a prototype for a production

model, even for a very limited production run, since some of the components

were expensive, bulky, and had a high current drain.

For successful use with the ceramic sensor used earlier for most of the

vibration-spectra study, a broadly- peaked amplifier passband centered at 14 Hz,

(measured, nominally 15 Hz) and down 10 dB at 5 Hz and 40 Hz (measured), with

a measured center frequency gain of about 100 dB, proved satisfactory. Three

op-amp stages were used: a non-inverting voltage follower followed by two

ac-coupled stages of nominally 54 dB gain each. About 8 dB of overall gain at

the 14 Hz center frequency was lost due to overlap of high and low frequency

roll-off. The total amplifier noise was (very roughly) 60 mV at the output

with the input shorted and 80 mV with the input open or, respectively, 0.6 yV

and 0.8 yV referred to the input. With the ceramic sensor at the input, the

best acoustic isolation of the sensor resulted in an output of about 150 mV, or 1.5 yV referred to the input. The noise was well below typical triggering levels

and did not appear to be a significant factor in the detection- range vs. falsealarm-rate trade-off using this type of sensor.

18
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The tuned amplifier stages were followed by a zener-diode limiter and by

a circuit module which combined the functions of rectifier, filter, dc comparator,

and output transistor. This module was used to drive a series combination of a light-emitting diode, a 3 kHz tone generator (alarm) and the coil of a relay

which was used in tests to drive a second tone generator at the end of a 50 m

cable to indicate when triggering had occurred to observers at the design goal

distance. This relay was also suited to either power or trigger a lane-marker

light. The threshold level at which triggering occurred could be adjusted by

a 10-turn potentiometer (with counter-type lockable dial) which set the reference

voltage for the dc comparator; this control was adjusted to obtain the desired

detection range with minimum false alarm rate.

To increase the gain (for use with piezoelectric polymer sensors), trimpots were added to adjust the gain of both gain stages, and also as offsettrimmers for these stages. Since unity dc gain was not used due to the very

low passband frequencies, of f set-trimming was needed to prevent saturation due

to the larger output offset at these higher gains. The performance figures

given above are for the minimum gain settings of the gain trimpots.

A major problem in the development of the seismic switch was the suppression of turn-off transients which tended to re- trigger the circuit. A combination of power supply decoupling and the shielding of internal input and output

leads was found effective.

20
RESULTS

Using the combination of a ceramic sensor (bonded to the asphalt pavement

and shielded as it was during the vibration studies) and the electronic switch

with the characteristics described above, set up as shown in figure 12, triggering

could be made to reliably occur for single vehicles at or beyond the 50 m distance,

a large percentage of which had speeds obviously below 50 km/h (30 mph). After a proper threshold level was established, there were almost no vehicle passbys that

failed to trigger at this distance, none that failed to trigger at close to this

distance, and very few false alarms. These tests were all made on the NBS grounds,

and therefore did not indicate what the false alarm rate would be due to oppositelane vehicles separated from the sensor by a median strip; that such false alarms

can be expected to be quite high, however, is indicated by figure 10, but time did

not permit verification of this.

Time also did not permit a statistical study of trigger success and false-alarm

rate. The variables of threshold level and road, vehicle, and switch characteristics,

combined with the need for single-vehicle passbys, would have necessitated an ex- tremely lengthy study to produce meaningful data, one quite beyond the scope of the

investigation, and probably of little value from a practical viewpoint considering

the latitude in triggering distance (between minimum and goal). The almost complete success obtained at this one location, however, indicates that acceptable results

can almost certainly be obtained at most highway locations, exclusive of falsealarms due to opposite-lane traffic or to unusual seismic background noise.

While the effect of water on the road was not investigated, there is little

likelihood that seismic vibrations at such low frequencies (as opposed to airborne

noise at higher frequencies) could be altered by even a measurable amount by surface

water, let alone by an amount significant in comparison with the effect of small

changes in other variables (such as vehicle weight or tire balance).

21
22

Set-up

for

testing

seismic

switch.

Test

utilized

single-vehicle

passbys

(going

from

left

to

right

in

figure)

.
Attempts at using the switch with piezoelectric polymer sensors were

largely unsuccessful, even with the increased gain previously mentioned. One

sensor, however, a 7 1/2 cm (5 in) diameter simple sandwich, gave very promising

results: triggering occurred with fair reliability at about 25-30 m (25 m is

the minimum distance objective). However, a second polymer sensor essentially

identical to this one could not be made to work. It is speculated that the

variability exhibited by the polymer sensors was due largely to slight but

significant differences in their bonding. The successful attempts are believed

to have resulted from obtaining a "strain-gauge" action, where the pavement

surface waves (Rayleigh waves, having a lateral as well as vertical motion)

stretch the polymer film, rather than compress it, resulting in a far larger

signal output due to the stress being applied through the cross-sectional

area of the film, which is small compared with its face area. This mode of

operation requires a taut film, however, which is difficult to reproducibly

obtain by simple bonding techniques.

CONCLUSIONS

It has been conclusively demonstrated that seismic detection at a distance

of 50 m (164 ft) of motor vehicles travelling 50 km/h (30 mph) is technically

possible, and that it can be done using fairly simple equipment. Before

practical use can be made of this concept, however, considerable additional

work will be required.

23
RECOMMENDATIONS

While there are unanswered questions concerning such details as the dependence

of seismic vehicle detection upon a number of variables, some of which are difficult

if not impossible to quantify, the most important questions affecting the advisability

of continued efforts in the detection area would now seem to be in other areas. Three

of the more obvious of these questions are: (1) Can a lane-marker light be developed

which is compatible with snowplows and yet providing, with reasonable power consumption

sufficient light to be visible in competition with reflections from a wet pavement

of headlights of oncoming vehicles (the situation in which lane markers are most

desperately needed and possibly the only one that can really justify their cost)?

(2) Can a battery supply (if power cables in the pavement are unacceptable) be

developed for this light capable of lasting an economically acceptable length of

time in the brutal environment of a highway pavement? (3) Is the whole idea

economically feasible? Compared with these fundamental questions, the remaining

doubts concerning seismic detectability are of minor importance. Keeping this

perspective in mind, the following investigations would be desirable if the overall

feasibility of the marker-light concept is deemed sufficient to warrant the

necessary costs:

1. To obtain minimum-cost sensors, piezoelectric polymers should be developed

for this application. That this is not an easy task is demonstrated by the substantial but largely unsuccessful efforts made under this program. That it is a realistic task, however, is demonstrated by the occasional (though unrep roducible)

success

.

2. The minimum requirements for wind (etc.) shielding should be determined.

This is also a difficult task, because of its ill-defined requirements and hardto-measure variables.

24
3. If greater confidence in detectability on different roads is desired, a methodical study should be made of the amplitude and shape of pavement vibration

spectra vs. distance, preferably using a recorder with a large number of channels,

with good matching between sensors and between channels. This should be done at

many locations on every type of road.

4. A similar quantitative investigation of pavement vibration spectra from

opposite-lane traffic should be made at the same locations as in (3). In addition,

the false-alarm rate at these locations could possibly be determined using the

feasibility-model switch.

5. A low-cost low-power switch circuit should be developed suitable for

(at least) a limited production run, in order to be able to demonstrate the

electrically-illuminated lane marker concept using a sufficient number of lights

for a realistic evaluation. While the 15 Hz tuning has proven successful, additional

experimentation with passband characteristics is highly desirable prior to quantity

production. Minimization of the false alarm rate at some locations may require

different characteristics.

6. The most likely cause of false alarms — opposite lane traffic vibrations —

could possibly be discriminated against by development of a directional sensor. The

lateral component of the surface (Rayleigh) waves could be utilized for directionality

by (a) laterally mounting an accelerometer-type sensor (such as the ceramic type

used in this investigation), i.e., with the active direction (perpendicular to the

broad face of the disc) parallel to the pavement and to the direction of traffic, or

(b) developing a strain-gauge type sensor (such as a piezoelectric polymer) in the

form of a thin strip parallel to the direction of traffic and bonded to the pavement

at both ends with provision for taking up slack and pre-stressing.

7. The effects of recessing sensors in the pavement should be determined.

While desirable from the point of view of environmental protection and ease of

packaging, different switch frequencies may be required for best operation.

25
In addition to its possible use in lane marking, the seismic switch has

other possible uses, such as triggering the illumination of warning signs along

the roadway at particularly dangerous locations. The flashing of an illuminated

warning would obviously be much less likely to be overlooked than the present

passive caution signs. The presence of slow-moving vehicles hidden from sight to

following vehicles by hills or curves could be warned of in this way by the use of appropriate time delays in turning the sign on and off, and by the use of large

switch hysteresis, i.e., a much lower threshold for turn-off than for turn-on.

While there are other conceivable methods for detecting vehicles for such purposes,

the seismic method has the advantages of eliminating or minimizing the use of

cables, and of being inherently well-protected (especially if buried) against

environmental deterioration of either the device itself or its performance (and

also against vandalism). The development of sufficiently directional sensors may

make attractive the seismic operation of demand-type traffic-control signals.

An obvious non-highway use would be for intrusion detection, since extreme

sensitivity of the feasiblity model to footsteps has been observed; this is

believed to have been investigated for the military. (Nearby pedestrian traffic

would obviously pose a difficult f alse-alarm-rate problem for seismically switched

lane-marker lights or other signals.)

26
APPENDIX - A SIMPLE DESCRIPTION OF PIEZOELECTRIC SENSORS

Piezoelectric materials are dielectrics in which there has been a net

internal displacement of charge between opposing surfaces of the material. The

charge displacement may be due to the alignment of polar molecules or to the

injection of free charges into the material. This is accomplished by exposing

the material to a strong electric field while it is heated sufficiently to allow

polar molecules to rotate into alignment with the electric field or to allow

charge to penetrate the surface. The molecules and charges are then immobilized

by cooling the material before the electric field is removed. If electrodes on

the (poled) surfaces are connected together, then charge will flow between the

electrodes into surface layers at the boundaries between the electrodes and the

poled material sufficient to terminate the surface field of the latter (since no electric field can penetrate the conductors). (With or without electrodes, ions

in the air would be attracted to the electrodes or the poled surfaces, neutralizing

the electric field even without electrode shorting).

If stress is applied to the poled material, the resultant strain will alter

the induced surface charge. A voltage will then appear between the electrodes

equal to the change in surface charge divided by the capacitance between the

electrodes. If the electrodes are momentarily short circuited while a steady

stress is applied, removal of the stress will alter the surface charge by an equal amount in the opposite direction, reversing the polarity of the resulting

voltage. An alternating stress, as from a sound field, will therefore generate

a corresponding alternating voltage across a high impedance electrical load, such

as a voltage amplifier, attached to the electrodes, or will drive an alternating

current through a low impedance load, such as a charge amplifier.

The piezoelectrics used in this investigation were lead zirconate titanate,

a crystalline ceramic, and poly (viny lidene fluoride), a flexible plastic. The

piezoelectric ceramic was obtained commercially as electroded and poled discs,

so that it was only necessary to assemble a pair of discs together and attach a shielded cable, as shown in figure A-l. The electrically paralleled discs formed

a fully shielded transducer since the grounded electrodes of the discs were on the outside and connected together at their edges by a layer of silver-filled

conducting epoxy.

27
POLYMER

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28
The piezoelectric polymers were electroded and poled at NBS using

commercially available sheets of poly (vinylidene fluoride). They were

assembled as self-shielding pairs, similar to the ceramic transducer.

Details of a typical piezoelectric polymer fabrication procedure used

at NBS have been given in a previous NBS report*.

The mode of operation for the ceramic sensor, and the usual mode for

the polymer sensor, was as an accelerometer. Forces were developed through the

sensor due to vertical acceleration by the vibrations transmitted to it from

the road surface. Since the conversion of acceleration to force requires mass,

the mass of the sensor was generally augmented by an added mass cemented to the

top surface of the sensor. Because of the finite acoustic impedance presented

by the pavement, and the finite acoustic impedance of the sensor, the force

developed, and hence the output voltage, was not proportional to the added mass:

a limit on the amount of usable mass was reached beyond which no further significant increase in output could be obtained.

An absolute measurement of road surface vibration was considered impractical

for this project due to the calibration difficulties imposed by the combination

of sensor mass and source (pavement) impedance.

*DeReggi, A. S., Piezoelectric Polymer Transducer for Impact Pressure Measurement

NBSIR 75-740, Final Report Covering Period December 15, 1973 to December 31, 1974

Section 2.2 "Transducer Construction", pp. 7-11. Available through NTIS.

29
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 77-1241

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

SEISMIC DETECTION OF MOTOR VEHICLES

5. Publication Date May 1977

6. Performing Organization ('ode

7. AUTHOR(S)

James M. Kenney

8 . Performing Organ. Report No. FHWA-RD-76-161

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON. D.C. 20234

10. Project/Task/Work Unit No. FCP-3111-112

11. Contract/Grant No. P.o. 4-1-0192

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

U.S. Department of Transportation

Federal Highway Administration

Offices of Research and Development

Washington, D.C. 20590

13. Type of Report & Period

Ma^vef§^4 - May 1976

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The technical feasibility of detecting the anproach of motor vehicles by

seismic means was determined by the recording and analysis of road vibration,

and verified by the construction of a seismic switch which was reliably triggered

at a distance of 50 metres (164 ft) along the pavement by vehicles travelling

at 50 km/h (30 mph).

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Ceramic transducers; motor vehicle sensing;

piezoelectric polymers; road lane markers; road vibration; seismic detection;

traffic noise

18. AVAILABILITY Unlimited

For Official Distribution. Do Not Release to NT1S

Order From Sup. of Doc., U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C 1 3 y Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIEIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

34

22. Price

$ 4.50

USCOMM-DC 29042-P74