Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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AMENDED SHEETS
Method for testing the operability of speed measuring devices
fGr traffic control
Te hnical Field
The in~ention relates to a method of testing the operability
of stationarily installed speed measuring devices for traffic
control, with which devices sensors are ~uried in the road
surface. the sensors responding to vehicles passing thereover,
signal processing means being provided, to which the sensor
means can be connected and which provide speed measuring
value~, taking ~ picture of a vehicle by the camera being
triggered, if the~ passing vehicle exceeds a permissible
; maximum speed, the signal processing means containing error
detection~ means which` prevent the taking into account of the
2~ speed m~asuring values, lf certain errors occur.
Bac~round Art
S~:at~ionar~ speed measuring devices are known far detecting the
~Z5 ~exceeding of permissible maximum speeds by vehicles~ These
spe~ measuring devices contain sensors, which are buried in
the road surface~at predetermined distance and which respond
to;~vehicles passing thereover. Such sensors may be
piezoelectric cables ~r cables otherwise sensitive to pressure
(FR-~-Z,471,066; GB-A-?,084,774; EP-A-0,267,032). The sensors
provide electric sensor signals, when the wheels of a vehicle
pas -t~ereover. The speed can be d~termined by means of the
; signal processing means from the time interval between the
sensor signals. If the vehicle speed thus measured exceeds a
~35 predetermined threshold, ~hich is equal to a permissible
maximum speed, then the signal processing means cause the
releasing of a photographic camera. The camera takes a picture
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of the speeding vehicle. With this picture, also the license
number and, if appropriate, also the driver are photoyraphed.
The neasured speed and other data (location, date and local
time) are reflected into the photographic exposure. It is also
5 known to represent tis information by matrices of LEDs and to
reflect the data during film feeding character-by-character
onto the edge of the film (DE-C-3,034,161). Furthermore, it is
known to place three sensors one behind the other (EP-B-
0,042,546; "Radio Mentor Electronic" 44 (1978), 10-20, EP-A-
0,387~0g3)~ The three sensors determine a total of three
measuring distances: A first measuring distance extends
between the first and second sensors, as viewed in the
direction of motion of the traffic. A second measuring
distance extends between the second and third sensors.
Eventually, a third measuring distance is obtained betwqen thefir~t and second sensors. It is known to derive speed
measuring values from the time intervals betwe~n the sensor
signals for all three measuring distances.
The speed measuring values may ~orm the basis of a proceeding
for violation of traffic regulations. Therefore the speed
; ~measuring values have to stand court verification. Therefore,
all~peed measuring values are discarded which could give rise
to any doubts. Therefore, the signal processing means contain
2~ error recognition means which rate speed measuring values as
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aulty in accordance with certain criteria. Normally the
releasing of the camera is prevented with such speed measuring
values rated as faulty. Such faulty speed measuring values are
in no way recorded.
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The criteria according to which speed measuring values can be
rated as faulty can be the following: A pulse from a sensor is
~- missing. The deviations of the speed measuring values obtained
with the three measuring distances are too large. The first
measuring pulse is not provided by the first sensor. Two
sensors are triggered at approximately the sam~ time~
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Furthermore, it is necessary to calibrate the installation
accurately. This is a rather expensive procedure, which is
carried ou~ by a calibration authority. The conditions at the
installation site may, however, change in the course of time,
for example due to dislocation of the road surface, changes of
the subsoil etc. For this reason, it is necessary to check the
operability of the device at comparatively large intervals.
Conventionally this is done by re-calibration. Such re-
calibration~ again, is an expensive measure.
Disclosure of the Invention
It is the object of the invention, with a device of the type
defined in the beginning, to check the operability of the
device in a simple way and to avoid expensive re-calibration
at least in the majority of cases.
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~ A~cording to the invention, this object is achieved in that
:
~ (a) a number of different error criteria are observed,
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(b) the frequency of the occurrence of these different
error criteria prior or shortly after the deployment
of the installation is recorded as an error
distribution typical for the installation site on the
:
basis of ~ number of measurements sufficiently large
or statistical purposes,
(c) the instantaneous error distribution during operation
is determined at predetermined intervals and is
compared with the recorded, initial error
distribution, significant differences of this error
distributions pointing at changes at the speed
measuring device or its environment.
The invention is based on the realisation that the various
error criteria, in the statistical mean of a rather large
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number of measurements, provides a typical frequency
distribution of the errors for each particular installation.
This fre~uency distribution represents a kind of "fingQrprint"
of the installation. Such a frequency distribution is
determined immediately following the first calibration or
prior to this first calibration. As long as this frequency
distribution remains substantially unchanged in the course of
time, i.e. for months, it can be assumed that also the
installation itself has remained substantially u~changed. If,
however, the frequency distribution of the errors changes
significantly, this will point to variations in the function
or structure of the installation. Then a check or, if
necessary, re-calibration of the installation has to take
place.
While, with the prior art installations, the errors recognized
;in ~accordance with various criteria merely cause the
assoc~iated speed measuring value to be discarded, according to
the invention the operation of the installation is monitored
on a~ long time basi~s by~means of error statistics. The errors
ccùr ~also with perfectly operating and freshly calibrated
inst~llations. But de~iations from a frequency distribution of
hese~errors typical o~ the particular installation are used
as~a criterion of changes in the installation.
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Modifications of the ~invention are subject matter of the
depen~ent claims.
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An~embodiment of the invention is described in greater detail
hereinbelow with reference to the accompanying drawings.
Brlef;Description of the Drawinqs
; Fig.1 ~shows a block diagram of a speed measuring device with
a device for statistically testing its operability.
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Fig.2 shows typical signal waveforms as obtained with
sensors of ~he speed measuring devi~e sf Fig.1.
Fig.3 shows a diagram of the error distribution, which
diagram is characteristic of a particular speed
measuring device at a particular measuring site.
Pre~erred Embodiments of the Invention
The Speed measuring device contains three sensor~ 10, 12 and
14 buried in the road surface with fixed spacings one behind
the other. The sensors 10, 12 and 14 are piezoelectric
cables, which extend transversely across the road or one
half of the road. The sensors lo, 12 and 14 generate
signals, when a vehicle passes thereover. Fig.2 shows
typical siynals as *hey~are obtained in normal operat;ion in
two: neighboring ~ensors, for example the sensors 10 and
12. ~ Each signal exhibits a first peak 16 or 18,
respectively, and a~ second peak 20 or 22, respectively. The
first~peaks are caused ~by the front wheels of the vehicle.
The~ second: peaks~ are caused by the rear wheel~ of the
vehicle. The signals represent dying-out os~illations. The
:respeotive front:edges~ of the signa~s serve as ~easuring
points. For ~the speed measurement, the~ time intervals
between~signals provided by the various sensors lO, 12 and
14, ~or example 16 and 18, are measuredO This is achieved
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by~means of three ~ounters 24, 26 and 28. The coun~er 24 is
started by the signal 16 of sensor 10 and counts counting
: pul~es, which are provide a~ high frequency, by a quartz-
controlled oscillator. The counting is stopped ~y the
signal 18 from the neighboring counter. The
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counter 26 is started by the signal 18 of the second sensor
12O Then the counter 26 also counts counting pulses, which are
provided at a high frequency by a quartz-controlled
oscillator. The counter 26 is stopped by the first signal of
the third sensor 14. Eventually, the counter 28 is started by
the signal of the first sensor 10 and also counts counting
pulse~ at a high frequency from a quartz controlled
oscillator. The counter 28 is stopped by the first signal of
the third sensor. Thereby, the counts provide measures of the
time which the vehicle required to cover the measuring
distan~es between the first and second sensors 10 and 12,
respectively, between the second and the third sensors 12 and
14, respectively, and between the first and the second sensors
10 and 14, respectively. A computer determines the speed from
these times and the known lengths of the measuring distances
in accordance with the relation speed equals distance divided
by time. This value is displayed by means of a display device
32~ If the o~tained speed measuring value wxceeds a
predetermined threshold corresponding to the permissible
~maximum speed, a camera 36 is released through an output 34.
The~threshold, at which, if exceeded, a photographic exposure
is~tri~geredr is set by an input device 3~.
Th~speed measuring device includes error recognition means
~25 ~ 40.~ The error recogni~tion means 42 include error analysing
means,~ which are~ illustrated by a correspondingly marked
blo~k. The signals of the three sensors 10, 12 and 14 are
applied~ to the error analysing means 42. The error analysing
means~ recognize errors of an error type "1" and error os an
3~ error type "2". An error of error type "1" is present, !if two
; or all sensors provide a signal simultaneously. Then a count
; of one of the counters 24, ?6 or 28 becomes zero.Such signals
An then be caused by cross-talk between the sensors. An error
of the error type "2" is present, if the siynals appear in the
wrong sequence. Also in this case, the counters do not count
any counting pulses. Then the signals may be caused by a
vehicle driving in the opposite direction. In both cases, the
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measuring value is suppressed by the error analysing means 42.
The camera is not released.
The errors of type "l" detected by the error analysing means
42 are counted in a counter 44. The errors of type "2"
detected by the error analysing means 42 are counted in a
counter 48.
The counts which have not been discarded as faulty by the
error analysing means 42 are supplied, as shown by arrows 50,
to comparison means 52~ which are represented by a
correspondingly marked block. The ~omparison means 52 compare
the times determined by means of the counters 24, 26 and 28
~or the speeds resulting therefrom). An error of type "3".
Such an error is present if the obtained speed measuring
values deviate from each other by more than a certain amount,
for example 3 km/h or 3 percent with speeds above lO0
km~h.Al~o in such a case, the speed measuring value is rated
uncertain and is not taken into account. The camera is not
triggered. Such deviations can, in particular, occur, if the
vehi¢le decelerates while passing the measuring distances.
Also the number of detected errors of type "3" is counted by a
counter 54. If none of the errors of types "l", 1'2" or 11311 has
occurred, the time or speed measuring value is appli~d to the
comput~:r 30.
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The~computer, eventually, detects errors of a type "4". These
rrors consist in that npt all sensors lO, 12 and 14 have
provided a signal. Also these errors are counted in a counter
56.
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also no error of type "4" is prèsent, the speed measuring
value from the computer 30 is applied to an output register
58. If the speed measuring value exceeds the permissible
maximum speed, the the computer 30 triggers the camera 36
through an output 60. The camera is a motor-driven recording
camera.
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Furthermore, the computer controls a frame counter 62 which
counts the number of exposures made. Furthermore, a local time
and date display 64 is controlled by the computer.
Furthermore, the total number of the measured vehicles
(whether exceeding the permissible maximum speed or not~ is
counted by a counter 66. An ouput register 68 contains a code
which identifies the respective installation. Eventually, the
permissible maximum speed valid at the site of the
installation is stored in an output register 70. Such data can
be inputted into the computer through a keyhoard.
During normal operationj the camera 36 is released, when an
excessive speed is detected. During the subsequent film
feeding, information is transferred to the edge of the film
through a LED-matrix controlled by the output registers. This
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is dvne in the way described in DE-A-3,034,161. When an
excessive speed is detected in the normal way, the pattern
shown in the right strip in Fig.1 is obtained. The displayed
data are: the speed measuring value at the location 72, the
current number of the frame from frame counter 62 at the
location 74, local time and date from output register 64 at
locatlon~76, the code from output register 68 at location 78,
and~the maxomum speed from output register 70 at location 80.
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A similar picture (calibration photograph) i5 taken, when the
installation is calibrated for the first time.
Coun~ers 44, 48, 54 and 56 are provided for monitoring the
operability of the installation. In addition, the counters 62
and 66 are used for this purpose. If, after a sufficiently
large~number of observed vehicles, which permits statisticai
evaluation, the total num~er of the observed vehicles, the
` number of the detected violations and the frequencies of the
35 various types of errors are plotted in a diagram of the kind
o~ Fig.3, this will result in a partic~lar "waveform" 82. It
has been found, that this waveform 82 is characteristic of a
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particular installation and a particular installation site. At
one location, the probability that the permissible maximum
speed is exceeded is higher, for example due to the road
situation, at some other location the probability is lower.
Accordingly, a certain ratio of the total number of vehicles
and of the number of detected violations will ensue.
Similarly, there will be a certain probability for the
occurrence of the various types of errors for each
installation site and each installation. At one location, it
is more likely that a driver decelerates or accelerates than
at some other location. Correspondingly the fre~uency of an
error of the type 113-- will vary. A similar situation exists
with location-dependent probabilities that the sensors are
actuated by passing vehicles driving in the opposite direction
or that vehicles leaving or entering the lane actuate only one
or two sensors. The ~requency of the occurrence of errors can
also be due to the characteristics of the speed measuring
device or, for example, to the road, in the road surface of
which the sensors a~re buried. For example, ~ibrations can
2û ~ o~:cur in the road which falsify individual measured values. As
long as this s$atistically detected "error profile" remains
substantially unchanged, it can be assumed that there are no
substantial changes~ of the installation. If~ however,
deviations ~f this error profile occur, this will point to
;25 ~changes o~ the ins~allation or of its environment, which may
necessitate checking or re-calibration.
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; In ~order to obtain the "error profiles", the counts of the
counters 44, 48, 54~and 56 and of the counters 62 and 66 are
photographed in certain time intervals together with date and
local time and the code by "statistical photographs". These
statistical photographs are triggered either by the counter
200 after each 200 exposures, or by the clock after each 4
hoursO This is illustrated in Fig.l by the connections 84 and
86.
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The statistical photographs have a pattern as shown in Fig.1
by the left strip on the right side. The information is also
transferred to the edge of the film by a LED-matrix during the
fi~m feeding. The count of the counter 44 appears at location
8~. The count of the counter 48 appears at the location 90.
The count of the counter 54 appears at the location 92. The
count of the counter 56 appears at the location 94. There is
no recording of a speed measuring value or of a permissible
maximum speed. However the number of the pictures taken from
counter 62, date and lncal time from output register 64, the
total number of the observed vehicles from counter 66 and the
code from output register are recorded at the locations 74,
76, 96 and 78, respectively.
By means of a manually operated switch 98, a calibration
photograph can be triggered manually. ~n the same way, a
statistical photograph can be triggered by a manually operated
switch 100.
The procedure of monitoriny the operability with the device
described is as follows:
At ~irst, a first caIi~ration of the installed installation is
made by~the competent calibration authority, for example the
25;~ Physi~alisch-Technische Bundesanstalt. The calibration is
d~Gumented, amon~ other ways, by calibration photographs.
Thereafter, the ~requencies of the errors of the various types
and~ the total number of observed vehicles and the number of
detected violations are determined~ during a statistical
period of 200 e~posures or 4 hours, and are documPnted by a
statistical photograph of the type illustrated in Fig.1~ Such
; a statistical photograph can also be triggered manually.
~ Therefrom, an error profile of the kind shown in Fig.3 can be
; obtained. The data thus obtained are placed in the records. At
certain intervals, the newly obtained statistical photographs
and error profiles are compared with the data in the records.
In case that this reveals significant deviations, re-
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calibration can be effected. Such re-calibration may, however,
be unnecessary. as long as the statistical photographs provide
substantially the same error profiles as those obtained
immediately after the first calibration. The statistical
photographs are triggered automatically.
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