Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DISPLAY DEVICE CONTROLLED BY A DIODE ~ATRIX NETWORK
Background of the invention
The present invention relates to display devices
comprising electrically controlled display elements
which are energised selectively by means of a diode matrix
network ; the unitary display elements may for example
comprise flaps mounted to pivot between a display position
and a retracted position, their pivoting being controlled
by a relay, which is preferably a double_acting relay ;
these display elements may also consist of luminous devices.
The invention is more particularly applicable to
display panels or devices which are disposed in places
where access is difficult or in places which are only
accessible during short or infrequent periods. This is the
case in particular for the indicator panels used on motor-
ways and especially on high density traffic urban motor-
ways or roads for which an interruption of traffic for
purposes of maintenance risks causing problems and must
therefore be limite~ in dura~n and frequency.
Description of the Prior Art
In a display device having electro-mechanical display
elements of known design, each relay controlling the tilting
of a flap is actuated in both directions to move the flap
to its display position and to return it to its retracted
position by means of a diode matrix network comprising a
respective diode connected to each relay for each direction
of the control current. The failures which can affect
each display element are of two kinds :
- in the first kind, the electrical connection is
interrupted , for example by the breakage of a wire of a
coil or when a control diode open-circuits; such a failure
only affects the defective display element and the message
displayed remains legible.
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- on the other hand, when one of the two control
diodes of a display element short-circuits, the derivative
currents from the short-circuit diode may create a pertur-
bation over a large part of the display panel, whose
message may become illegible.
Objects of the invention
An object of the invention is to provide a dis-
play device of the kind referred to above in which
a failure concerning one display element does not cause
perturbation in the rest of the display device and, more
particularly, in which a short-circuit in a control diode
does not create parasitic currents on other display
.elements.
Description of the invention
According to the present invention, there is
provided an electromechanical display apparatus comprising
discrete movable display elements which can adopt two po-
sitions, and several relays by which each of said elementscan be individually activated, said relays being energized
in a selective manner by means of a diode matrix network,
each relay being energized by a pair of diodes, each diode
of a pair of diodes having one contact connected to one
contact of its respective relay, the two diodes of one
pair of diodes being connected in an opposite conductive
direction, each diode of a pair of diodes hav:ing its second
contact connected to an actuat:ing e:Lectri.ca:l. suppLy :L:ine,
display apparatus where.irl an additional diode is connected
in series with each diode of said pairs oE diodes in the
same conductive direction; and for maintenance purpose,
each discrete display element is energized with a prede-
termined voltage which does not activa-te said discrete
display elements, said predetermined voltage being e~ual
. . .
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to at least s~e times the forward poten-tial drop across one diode.
The presence of the second control diodes in
series with the first diode prevents a short-circuit
failure in one diode producing parasitic currents in a
control circuit of other display elements; such parasitic
currents will only appear in the unlikely event
of simultaneous short-circuits in both diodes of a same
control circuit.
Moreover, since test voltages are insufficient
to actuate the display elements, a test operation may
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be performed without displaying erroneous or illegible
messages on the array during maintenance.
When one diode short-circuits, the potential drop
across it becomes very small, and the current flowing
in the control circuit under a potential difference
which is somewhat higher than the sum of the normal
forward potential drops across the series connection
of the diodes is increased by an amount which is suf-
ficiently large to be dete~ted readily by measurement
of the current flowing.
In a preferred embodiment of the invention,
test means operable in automatic mode during periods
of non-utilisation of said array are provided, said
test means including means for recording the test results.
This enables the number of repair operations on the
display array and matrix network to be reduced, since
it is sufficient to replace the components of a display
element recorded as defective.
Advantageously, said test means comprises
micrO-prOcessOr means, for detecting short-circuit
diodes, said micro-processor means also controlling
the selective actuation of said supply means.
Preferably, the apparatus includes a plurality
of said arrays and of said matrix networks, a central
station, and telecommunication means for transmitting
the test results from said matrix networks to said
central station. The defective display arrays and net-
works being known in advance, it is sufficient to inter-
vene on the failed devices only, which iurther reduccs
the interventionsneceC~sary
Description of the drawings
Other features and advantages of the invention
will appear from the following description, given by way
of example, with reference to the accompanying drawings
in which :
- Fig. 1 is a schematic electric diagram of
part of a display device of known design ;
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- Fig. 2 is a similar schematic diagram showing
a display device in accordance with the present invention.
Description of the Preferred embodiment
Fig. 1 is a diagram of part of the electrical
circuit of a prior art display device having electromechanical
display elements such as hinged flaps (not shown) which can
adopt two positions : a first display position in which they
are visible on the display board, and a second rest position in
which they are retracted and invisible on the display board.
The movement of the display flaps is actuated in each direction,
towards the display position and towards the retracted posi-
tion, by means of a coil 1 supplied by a diode matrix network
so as to enable current to pass in one direction or
the other between first and second terminals of the coil.
The network comprises "column" conductors
shown vertically in the drawing, to each of which are
connected one of the terminals of the coils 1 of a respec-
tiie column,and ~line" conductors 3 and 4 shown horizontally
in the drawing r the line conductors 3 are each connec-
ted to the second terminals of the coils of a respective
line through a first control diode 5, each of the diodes
5 being connected to the conductor 3 by the same electrode
(for example the anode, as in the example shown).
Similarly, each of the conductors 4 is connected
to the second terminal of the coils 1 of a respective
line through a diode 6 connected the opposite way from the
diodes 5, that is to say with its anode connected to the
coil in the example shown.
The actuation of a display element to move it
to its retracted position or to its display position is
obtained by applying a suitable voltage between the con-
ductor 2 of the relevant column and the conductor 3 or 4
of the relevant line for the element considered.
When a diode 5 short-circuits, derivative
currents are c~ed and part of the display device
malfunctions so that the displayed message may beco~e
incomprehensible.
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In accordance with the embodiment of the
present invention, as shown in Fig. 2, which is a similar
diagram to Fig. 1, a second diode is connected in series
with each of the control diodes, the two diodes in series
ha~ing the same forward conductive direction.
Thus, each actuation of the display element,
that is to say each movement of the display flap in the
embodiment shown, is controlled by means of the pair of
diodes in series, that is to say the diodes 5 and 5'
for one movement, and diodes 6 and 6' for the opposite
movement.
Because the drop in potential across the diodes
in the forward direction is small relative to the supply
potential for the coils, the in~roduction of a second diode
in series has a negligeable influence on the normal ope-
ration of the display device. Thus, for a supply potential of
30 ~ , giving a control current of 3A, the reduction of
control current is only 2 %. However, in order to detect
a short-circuit diode, the network may be energised at
a potential which is sufficiently low that the display
elements are not actuated, but which is at least as high as
several times the forward potential drop across one diode,
so t~at the increase in current produced by the short-
circuit of a diode is sufficiently large in relative
value to be measured and detected readily. For example ,
in the case indicated above, if the 3~Y supply potential
is replaced by a 5V potential, which is 7 times the
forward potential drop across a diode, the increase in
current resulting from the short-circuit of one diode and
from the consequent elimination of the forwarcl potential drop
across this diode is of the order of 20 % which is readi-
ly detected.
Advantageously, the operation of detecting
short-circuit diodes is performed while the display device
or board is inoperative ; this operation can be performed
automatically by means of a control device testing all
the display devices in succession, the result of these
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tests being recorded so that when access to the display
device is obtainable, it will only be necessary to
replace the display elements detected as defective.
In the case of control of the display device
by a micro-processor integrated into the display device,
the operation of detecting short-circuit diodes may be per-
formed by this micro-processor.
It is also possible for a certain number of
display devices to form part of a group controlled by a
central station to which they are connected by a telecommu-
nication system ; this is the case for example in the case
of a series of display panels along a motorway or express
road, In this case, the result of the operations of
detecting short-circuit diodes is adva~tageous~y transmitted
to the central station so as to know in advance which display
panels have defective elements, which avoids intervening
on display devices which have no defective elements ; this
further reduces malntenance times.
