Note: Descriptions are shown in the official language in which they were submitted.
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Title: MONITORING APPAR~TUS
Back~round of the invention:
This invention relates to monitoring apparatus,
particularly, but not exclusively for monitoring traffic
flow in a tunnel.
Monitoring apparatus for intruder detection i8 known,
for example that manufactured by the Vision Research
Company Limited under the trade name Pixstore 256. Such
apparatus generally comprises a video camera connected
to a monitor screen via a control unit which is further
connected to an intruder detection device eg an infra
red, micro wave, ultrasonic or perimeter type device,
the control unit actuating an alarm and freezing the
image from the video camera at the instant of detection
thus allowing a 'snap shotl picture of the intrus$on.
It i8 a d$sadvantage of such a system that it i~ only
capable of responding to and recording an incident at
the time the incident occurs and is thus unsuitable for
providing information as to how the $ncident arose. The
system is thus unsu$table for mon$toring traffic related
incidents.
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SummarY of the invention
According to the invention in a first aspect, there is
provided monitoring apparatus comprising a camera, an
image memory for storing images from the camera, an
incident detector for generating an incident detection
signal when an incident occurs, the image memory being
responsive to the detection signal whereby a plurality
of said images prior to the incident are identified; and
means for reviewing said prior images
Preferably, the monitoring apparatus is used for
detecting a traffic incident, preferably in a tunnel,
the camera being a video camera providing images to a ~ -
camera controller which stores the images periodically
in an image memory The incident detector preferably `
comprises a road sensor, for sensing when a vehicle
passes in the field of view of the camera and a
proces~or for determining the occupancy of the road and,
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; from this information, if an incident has occurred, the
inol~d-nt d-t-ctor then gen-rating the incident detection
ignal
';Other preferred features of the invention are mentioned
in dependent Claims 2 to 20
~` It is a further disadvantage of the prior art apparatus
that the control unie is disposed at a central
onitoring location, ~ervicing a plurality of video
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cameras so that malfunction of the unit cau~es the whole
monitoring system to be disabled.
According to the invention in a second aspect there is
provided monitoring apparatus comprising a plurality of
monitoring stations each having a camera and a local
camera controller, the camera controller having an image
memory for storing images from the camera; and a remote
central controller connected to the camera controllers
for selectively receiving said stored images.
According to the invention in a third aspect, there is
provided a method of determining the existence of a
traffic incident comprising the ~teps of measuring the
average speed of vehicles passing a sensor in a first
time period, measuring the average speed of vehicles
passing the sensor in second time period shorter than
the first time period, calculating the difference
between the average speeds for the two time periods and
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generating an incident detectlon signal when the
~ - difference is above a predetermined threshold.
i: Brief descri~tion of the drawings:
~;~ An embodlment of the lnvention will now be described, by
way of example, with reference to the accompanying
drawings, in which:
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Fi gure 1 is a 8 chematic diagram 8 howing the di 8 pO8 i tion
of camera6 and incident detectors of an embodiment of
the invention in one carriage way of a road tunnel
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Figure 2 i8 a schematic diagram showing an embodiment of
the inventlon
Figure 3 i8 a schematic diagram of a camera controller ;~
of the embodlment of Figure 2
Figure 4 is a schematic diagram of the data conversion
unit of the embodiment of Figure 2
Figure 5 i~ a schematic diagram of a 6ite controller of `~
the embodiment of Figure 2
Figure 6 is a schematic diagram of a vehicle detection
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module~ of the embodiment of Figure 2
p-~orl~tlon of the ~referred -mbodiment
; Ref-rring to the flgures, an mbodiment of monitoring
apparatus according to the invention is shown
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The monitoring apparatus i8 axranged for use in traffic
monltoring, to monitor and detect a traffic incident on
a hlghw~y and, specifically, in a road tunnel
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As shown in Figure 1, a plurality of video cameras 10,
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12, 14 are attached at spaced intervals to the wall of a
tunnel, each camera having a field of view 22, 24, 26
which overlaps with the fiela of view of the preceaing
camera. The cameras monitor one carriage way 30 havlng
two lanes 32, 34 in the tunnel 20 and each camera 10,
12, 14 has associated therewith a plurality of vehicle
sensors 40 - 53 .... of which sensors 40 - 45 are
as60ciated with camera 10 and sensors 46 - 51 are
associated wlth camera 12. As de~crlbed below, the
sensing of an incldent by one or more of the sensors
a8~0ciated with the camera will cause the retention of
images showing both the incident and the circum6tances
leading up to the incident. The sensors in each lane
are separated by a distance D whlch is chosen according
to condltions and is preferably 60m.
A block diagram of an embodiment of monitoring apparatu6
of the invontion as a whole is shown in Figure 2.
Bach camera 10, 12, is connected to a local camera
controller 60, 62, 64 each controller processing image6
received fro~ its respective camera. All camera
controllers àre connected to a central controllor 66
remote from the camera sites vla a data bus 70, the
central controller having a supervising computer 72,
with associated hard disc storage and hard copy
facilities, a monitor 74 and a data conversion unit 76
shown in more detall ln Flgure 4 and formed of 8tandard
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electronic circuits providing D/A and A/D conversion,
modulatlon and demodulation and flltering of video
signals and instruotions. The unit 76 act~ as an I/0
and processing interface between the computer 72,
monitor 74 and camera controllers 60 - 64.
Camera controller 60 is shown in more detail in Figure 3
and includes an A/D converter 80 for converting the
analogue video image signal from the video camera 10 to
a digital equivalent and providing an output digital
signal to a central processing unit 82. The CPU 82
stores frames of the digital video image signal at
interval~ in a memory stack 84 typically having capacity
for 32 frames with a resolution 192 x 300 pixels x 64
grey levels, implying a memory requirement of
approximately 24 K bytes/frame. The interval period may
be either constant or variable and is preferably of at
least one second duration, 80 that the memory stack
provides a record of past events of at least 32 seconds
duration.
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The CPU 82 i~ further connected to a data Interface for
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transmitting/receiving signals to/from central ~ ~-
controller 66~via bus 70. Each camera controller has a
unique address and upon an instruction signal from the
central computer 72 tagged w1th the address, can
transmit real time video images direct from the video
oamera 10 or can transmit all or part of frame store
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84 The camera controllers only transmits on
instruction from central computer 66 The CPU 82 is
also responsive to an inoident detection signal, from
central control 66 on bus 70 or from an incident
detector (described below) on data line 88 to freeze the
contents of frame memory 84 in the event of an incident
being detected, 90 that the contents of the frame store,
which records past images prior to the detected incident
are retained for transmi 8 9 ion to the central controller
66, thus providing a record of the circumstances leading
up to the detected incident Optionally, the CPU can
move a frame pointer 80 that some of the frames labelled
F are stored and future, post in¢ident images are stored
higher up the stack, thus allowing retention of past
events and at the same time continued monitoring of
current events
The frame memory 84 may optionally be formed as a robust
detatohable cartridge eg of the type disclosed in
co-pending European patent application No 89303333 2,
thl- allowing r-moval and retrieval of image data held
in the cartridge in the event of ~ystem failure The
cartridge may be form-d from physical and thermal shock
resistant materials, for example a polycarbonate case
having epoxy re~in potting, ~o that retrieval of the
cartridge when the camera controller ha~ suffered all or
partial de3truction will still be possible Use of a
d-tatchable cartridge also allow~ for corroboration,
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after an incident, between the content of the frame
memory and the image data tran~mltted to the central
controller 66 ~ -
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The central controller 66 is further oonnected, via a
data bus 90, to a plurality of incident detector6 92,
94, 96 Each incident detector comprises a site
controller 100, 102 104 Each site controller has
connected thereto three vehiole detection modules (VDM~)
120, 122, 124; 126; 128; 130; each of which
contains signal processlng aircuitry for a pair of the
vehicle sensors 40, 41 , the pair being connected to -~
the respective VDM Each site controller is further
aonnected to an associated one of the camera controllers
via signal lines 140, 142, 144
The vehicle detection module 120 is shown in more detail
in Figure 6 The road seneor 40 comprises a square four
turn inductive loop 200 which is connected to the VDM
120~via an i~olating transformer 210 ~he VDM inaludes
an alternating current source 220 having a frequency of
approximately 60 kHz and a frequency measuring circuit
comprising a programmabl- down counter 230, an elapse ;
counter 240 connected to counter 230 by line 235 and `;
driven by higb pe-d~clock (10 MHz) 250 for measuring
the duration of the o~clllation count and a CPU 260
Each sensor 40, 41 ha~ its own alternating current `~
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source 220, the fr-quency m-asuring circuit being
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connected to one or other of the sensors by means of
switch 270 under control of CPU 260 in a time division
manner. Preferably, the CPU 260 switahes between
sensors 40, 41 at one millisecond intervals.
In use, the sensor 40 i8 placed in the road lane either
attached on the road surface or 3unk into the road
surface. The inductance of the loop 200 will drop when
a metal bodied vehicle passes over. This in turn will
affect the oscillation frequency of the circuit
comprising the loop 200 and source 220. The frequency
measuring circuit measures this frequency by counting
down a selected number of oscillations of the
alternating voltage signal; when the counting operation
is being performed, a signal on line 235 changes level
thus providing a start/stop sign~l to counter 240 which
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measures the time duration of the count, this giving the
period of the alternating signal and thus its
frequency. The count value is passed to CPU 260 via bus
255.
Preferably, the CPU controls the counter to output a
start/stop signal on line 235 after a plurality of
osclllations (eg 8, 16 or 32) selected by user
configurable ~witches 237, to improve resolution
accuracy.
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The CPU 260 processe~ the counter information and
compares the derived frequency measurement with a
threshold, producing a true/false slgnal indicating
presence adjacent the loop 40 of a vehicle. The signal
is sent to the site controller 100 on bus 265.
The site controller 120 i 8 shown in more detail in
Figure S and i8 of stand alone construction, based on
the MARRSMAN 600 traffic management controller
manufactured by the applicants. The site controller 120
has a I/O circuit 300 which received frequency
information signals on buses 265 - 267 from respective
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VDMs 120, 122, 124. The I/O circuit also provides an
- output on bus 140 to camera controller 60.
The I/O circuit and all other functions of the site
controller are controlled by a CPU 310 to which is
further connected a keyboard/display 315, to allow on
1te initiali~atlon and input/output of data from the
site controllerj a data cartridge 320 and interface 325 : :~
preferab}y of a type dlsolosed ln European patent
application No 89303333.2, for storlng incldent and
oooupancy data, a ROM/RAM 355 for storing CPU programmes
and operational data and a network interface 330 whlch
provldes a oommunlcations link with data bus 90.
The network interface 330 includes a processor 335 and
two port universal asyncronous receiver transmitter
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(UART) 340 for data transfer. A switched bypass bus 350
having a plurality of electromechanical relays i8
further provided, the relays being blased closed but
held open by the site controller CPU 310. The bus 350
acts to 'short circuit~ the network lnterface 330 in the
event of power failure of the site controller 120 or
when the site controller cannot make sense of signals
being transmitted through UART 340, 80 that failure of
one site controller w1ll not affect the operation of
others connected to data bus 90 downstream of the failed
site controller.
The CPU has random access and read only memories 355 for
internal data storage and for storing contrsl and
incident identification programmes. The site controller
120 is connected to a power supply and also has a local
;~ backup supply in the form of a rechargable battery (not
shown) for use ln the event of a general power fallure.
~In u~e, the site controller receives the vehlcle
presenae lnformatlon from the VDMs 120, 122, 124 and
from thls calculates the degree of ~occupancy~ and
''density' of any one sensor by vehicles. Occupancy is
defined as the number of consecutlve seconds that a
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vehicle has been sensed as present by the sensor.
Density is defined as the percentage time in a given
time interval that the loop has sensed the presence of a
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vehicle or vehicles This information is then used to
oalculate if a traffic incident has ocaurred in
accordance with an algorithm Such algorithms are ~nown
to those skilled in the art, as exemplified by ths high
occupancy (HIOCC) algorithm developed by the Transport
and Road Research Laboratory (T~RL) as disclosed in TRRL
supplementary reports Nos 775 (Automatic incident
detection, experience with two TRRL algorithm HIOCC; J F
Collins 1983) and 526 (Automatic inoident deteotion -
TRRL algorithms HIOCC and PATREG; J F Collins, C M
Hopkins and J A Martin 1979)
The HIOCC algorithm as disclosed in the above documents,
the contents of which are incorporated herein by
reference, operates by detecting stationary or slow
moving vehicles to indicate a traffic queue caused by an
incident or by congestion It looks for several ~ -
oonsecutive seconds of high detector occupancy to detect
queues and ino1dents in high traffio flows A programme ~
in acoordance with the flow diagram of Figure 3 of ~ ;
r-port 526 is stored in ROM in site oontroller 120 and -~ `
CPU 310 processes the oooupancy data from VDMs 120 - 124
in acoordance with the programmed algorithm The
resultant occupancy, denslty and incident data is stored
locally in data cartridge 320 and is also sent to the
central oontroller 66 via data bus 90
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When the algorlthm detects a trafflc lncident, an
incident detection signal i8 sent both to the central
controller and to the camera controller 60 associated
with the site controller 100. The incident detection
signal causes the oamera controller to freeze a
predetermined number of image~ in the frame store 84 as
previously descrlbed, thus provlding a stored record of
the circumstances leading up to the incldent as detected
by the incident detector.
In addition to the HIOCC algorithm, the site controller
uses a speed (as opposed to occupancy) based algorithm
using two ad~acent loop sensors eg 41, 43 in any one
lane. Such an algorlthm provides additional information
concerning slow moving vehicles - whi¢h are, in
themselves, a traffic hazard. Furthermore, by basing
analysi~ on speed, speeding violations may also be
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detected.
An example of a suitable algorithm is as follows:
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Two measurement intervals Tl and T2 (between one minute
i ' and twenty fqur hours depending on occupancy) are
chosen, interval Tl representing a relatively longer
perlod than T2. Uslng ~djacent loop detectors 41, 43
the average speed in each interval Tl, T2 is calculated
and updated as each vehicle passes, givlng average
speeds S1, S2. The difference between these speeds (Sl
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-S2) glves an indication of short term speed variation
away from the long term average and if over a
predetermined threshold SD gives an indication of an
incident. Furthermore, if S1 or S2 or the instantaneous
vehicle speed fall outside predetermined high or low
speed thresholds THH, THL, this also gives an indication
of a probable incident. Examples of suitable parameters
are T1 = 1 hour, T2 = 6 minutes, SD = 5 kph, THH = 200
~ kph, THL ~ 20 kph.
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The site controllers 100, 102, 104 form nodes of a local
area network (LAN) having a standard format and
operating protocols, each node passing messages along ~ -
the data bus 90. Each controller has a unique network
identifier, with identifiers being reserved for ~all
stat~ons~ called and the central control 66.
h~- topology of the networX is a daisy chain with out
go1ng~message~ being passed to the end of the line and
naomlng~messages belng passed back to the central
controller 66. Each site oontroller is responsible for
passing messagès along the line when a character arrives
i~ it is bufered until the whole message is complcte. It
is then retransmitted.
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A1l s1t- controllers examine me6~ages, discard corrupt ~;
m-ssage~, accept tho~e w1th matching addresses and pass
on others.
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When a site controller has a message of its own for the
central controller 66, it tests the status of the
incoming line from the previous site controller. If the
line is busy, the site controller will continue to
re-tranmsit data from the previous site controller until
the line is clean, at whlch point the site controller
will aommence transmission of its own message. During
thi~ tlme any incoming me~ages from the outlying site
controllers will be buffered in the 6ite controller UART
for re-transmission at the earliest opportunity.
Site controllers are assigned unique addresses and all
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messages from site controllers to the central controller
66 are tagged with this address. Messages from the
central controller to slte controllers are either 'all
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-~ ~ stations' to all site controllers or 'addressed' to
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individual site controller8.
Me-8ageo are transmitted along the LAN as the data field
of a network packet, packets having the following format:
i~` ' 1. Packet header
2. Destination address
3. Source address
4. Control flag
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A: Acknowledgement
B: Text mes~age
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C: Incident detection/alarm mes~age
D: Status message
5. Field check sum
6. Data box size
7. Data block
The packet receiving protocol for each site controller
is as follows:
1. Each complete message is re-transmitted along the
line.
2. Incoming characters are also placed into the
incoming message buffer.
3. When a full packet is received the unit compares the
destination address with its own node id. -~ -~
4. If the id and address do not match no further action
is taken.
5.~ If the id and address match then the packet iB
interpreted, involving the following:
A. The oh-cks ar- miscalculated ~ -
` B. If the cheok sum is incorrect a bad packet error
is flagged
C. Th- response to a bad packet error is to do
nothing and allow the souroe to time out and resend
D. If the checks are incorrect, the data block is
passed to the CPU for action and an acknowledgement
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~ (ACR) packet is sent to the central controller.
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The packet sending protocol of the site controllers is
as follows:
1. The messages are formatted by the site controller
CPU into a p~cket having the form noted above.
2. The packet is placed in an outgoing packet queue and
the count of re-tries set to zero.
3. The packet is transmitted.
4. If an ACX packet i8 received from the target
node/central controller with the correct packet ID then
the packet has been successfully transmitted and is
removed from the queue.
If no reply is received before a predetermined time out
then the packet is re-sent and the count of retries for
this packet is incremented.
If the count of time out re-tries reaches the user
opecified maximum then a message time out error is
flagged.
During normal operation when no incident detections are
present the central controller 66 will poll the status
of each site controller by sending a request for a
status packet. In reply to the status request the units
will respond with a data packet giving the following
parameters:
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1. Occupancy (the number of consecutive seconds for
which a sensor has been found to be occupied).
2. Density: the percentage of tlme at a given lnterval
for which the loop has been occupied.
3. Incident detection status.
4. Loop status.
S. Mislcellaneous.
When an inaident detection signal is generated, the
sensing unit sends an incident detection packet to the
central controller 66. The central controller 66 ~: ;
acknow~edges receipt of the pa¢ket (otherwise the packet
is re-sent). The central controller then acts to freeze
the memory store for the relevant camera/camera
controller. In parallel, an incident detection signal
i8 sent direct from the site controller to its
associated ¢amera controller.
While the embodiment of the invention as described above
ha~be-n appli-d to a road traffic sensing sy8tem, this
not~to be~construed as limltative. For example, the ~ ~:
invention may be used in a driver's cab of a train, the
incident detector being respon8ive to an automatic
warnIng system (AW6) 'line ocoupied' signal showing, for
xample, wh-n a train has gone through a danger signal.
Alternatively, the lnoident detector could be a sensor
aonnected to the front buffers 0f the train 80 that data
18 stored on impact w1th another vehicle or ob~ect on
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the track. In suah cir¢um~tances, the data cartrldge
could be made to 'black box' ~tandards.
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