FR2792076A1 - Testing apparatus for verifying integrity of electrodes in plasma screen comprises supply rails polarising electrodes, and antenna measuring electric field from electrodes - Google Patents

Abstract

Testing apparatus for network of electrodes (21) is placed on slab (20), comprises supply rails (22, 23) and antenna (24). Supply rails allows polarisation of electrodes so that they can produce electric field. Antenna has guiding rails (31, 32) and gate (33) to measure electric field from electrodes. An Independent claim is also included for the testing method.

Description

A 2792076

  The invention relates to a device for testing a plasma panel and a method for testing said panel. More particularly, the test checks the integrity of the electrodes of a flat screen, and in particular of a plasma panel. Plasma panels, hereinafter called PAP, are flat type display screens. There are two main families of PAP, namely PAP whose operation is of the continuous type and those whose operation is of the alternative type. The PAP generally comprise two insulating slabs (or substrate), each carrying one or more networks of parallel electrodes and delimiting between them a space filled with gas. The slabs are assembled to each other so as to define intersections between the electrodes of said arrays. Each intersection of electrodes defines an elementary cell to which corresponds a gas space partially delimited by barriers and in which an electric discharge occurs when the cell is activated. The electric discharge causes ionization of the gas in the elementary cell, accompanied

  of a light emission from the cell.

  If an electrode of a PAP is cut, a black line appears on the screen which damages the image and makes the PAP unusable. During manufacture, the electrodes can be damaged at any time due to a handling error. On the one hand, the most critical moments are

  of the etching of the electrodes and on the other hand during the assembly of the tiles.

  A visual test on a lit screen makes it possible to very quickly detect an electrode rupture but only allows sorting at the end of production. It is preferable to test the slabs during the etching of the electrodes in order to be able to repair them immediately and also just after assembly so as not to carry out the pumping and filling of the PAP on the screens.

defective.

  Conventionally, the electrode continuity test is carried out as shown in FIG. 1. Each electrode 2 of a panel 1 is tested by a continuity controller 3, for example an ohmmeter, by means of two probes 4 and 5 placed at each end of the electrode to be tested. Such a device requires that all the electrodes 2 lead to the two opposite sides of the slab which support them. The width of the electrodes is of the order of a hundred tim, which requires precise positioning for each measurement. Repeating these measurements results in a significant test time. Such a device also does not make it possible to detect short circuits between neighboring electrodes. The invention proposes to test the electrodes by capacitive coupling with an antenna. The slab (or PAP) is positioned on a frame which

  polarizes the electrodes of a slab. An antenna then moves over

  above the slab (or PAP) to measure the electric field produced by the electrodes. The deviation (s) in the measured electric field allow

  determine if the electrodes are cut or not.

  The subject of the invention is a test device for an array of electrodes placed on at least one support slab, the device comprising supply means for polarizing the electrodes so that the latter produce an electric field; at least one antenna provided with guide means so that it can be moved parallel to the support slab at an almost constant distance from the electrodes in order to measure a

  electric field from said electrodes.

  In order to improve the readability of the measurement, the supply means comprise a first means for polarizing half of the electrodes at a polarization voltage and a second means for polarizing the other half of the electrodes with the opposite of the voltage polarization, and in that said first and second means are arranged

  to polarize the electrodes alternately.

  The invention also relates to a method for testing an array of electrodes placed on at least one support slab, characterized in that the electrodes are polarized in order to emit an electric field, and in that at least an antenna is moved parallel to the support slab at an almost constant distance from the electrodes in order to measure a field

  electric from said electrodes.

  In order to improve the readability of the measurement, half of the electrodes are biased with a bias voltage, and the other half of the electrodes are biased with the opposite of the bias voltage, and in that, the electrodes are biased alternately the invention will be better understood and other features and

  advantages will appear on reading the description which follows, the

  description referring to the appended drawings among which:

  Figure 1 shows schematically a test device according to the prior art, Figure 2 shows schematically a test device according to the invention, Figures 3 and 4 show the invention according to a first embodiment, Figures 5 and 6 represent the measurement principle implemented in the invention, FIG. 7 represents a measurement circuit used in the invention, FIGS. 8 and 9 represent the signals measured by the antenna in the absence of an electrode cut-off, FIG. 10 represents the internal signals of the measurement circuit of FIG. 7 when an error is detected, FIG. 11 represents a device according to the invention with two antennas, FIG. 12 represents an antenna end used in the detection device Figure 11, Figures 13 and 14 show two modes of operation

of the device of FIG. 11.

  FIG. 2 represents a test device for a plasma panel slab 20, the electrodes 21 of which open out alternately on either side of the slab 20. Those skilled in the art can notice that such a

  configuration makes it possible to widen the electrodes 21 on their contact end.

  The slab 20 is placed on two rails 22 and 23 so that half of the electrodes are in contact with each rail 22 or 23 alternately. The rail 22 is supplied electrically at a bias potential, and the rail 23 is supplied with a voltage equal to the reverse of the bias voltage. The two rails 22 and 23 behave as supply means for biasing half of the electrodes with a bias voltage and the other half of the electrodes with the opposite of the bias voltage alternately. In the preferred example, the bias voltage is sinusoidal and is for example equal to V * sin (o) t), the inverse of the voltage being equal to -sin (ot). The antenna 24 moves

parallel to the tested slab.

  Any form of bias voltage can be used, including direct voltages, but it becomes difficult to filter out low frequency noise. Also, it is possible to use a single bias voltage, but this then amounts to measuring a variation in capacitance very precisely, which in the case of a plasma panel can be difficult to interpret. The alternation of sinusoidal signals on the electrodes allows a simplicity of setting

  implemented coupled with very good readability of the measurements.

  Figures 3 and 4 show an embodiment of a table for testing plasma screen tiles according to the invention. The table comprises a plate 30 which supports the two supply rails 22 and 23 and two guide rails 31 and 32. The supply rails 22 and 23 also serve as support for the slab 20 to be tested. A gantry 33 which supports the antenna 24 is connected to the guide rails in order to be able to translate parallel to the slab to be tested. Moving the gantry 33

  takes place for example using a stepping motor (not shown).

  Preferably, the antenna 24 is a conductive wire connected to the gantry 33 by adjustment elements 34 and 35 which make it possible to adjust the height of the antenna 24 relative to the slab 20. The adjustment elements 34 and 35 can be omitted if the table is only used to test slabs whose

  The thickness is defined as being fixed.

  FIG. 5 illustrates the test principle implemented in the invention. The antenna 24 moves parallel to the slab 20 and therefore parallel to the electrodes 21 supported by the slab. Obviously, a parallelism error is always possible, it is not a problem if the movement of the antenna 24 remains substantially parallel to the plane of the screen 20. A modeling of the electric field produced by the electrodes 21 and picked up by the antenna 24 amounts to treating the interaction of each electrode 21 with the antenna 24 as a capacitor 50 to 54, the dielectric of which consists of an insulating layer of the slab 20, the insulator 55 surrounding the antenna 24 and l between the antenna 24 and the slab 20. Such a

  modeling is shown in Figure 6.

  In the example described, only five capacitors 50 to 54 are taken into account. In reality, with an antenna produced using a wire, 50 to 100 capacitors must be taken into account, the number depending on the size of the antenna 24, on the size and the spacing of the electrodes 21 and the distance between the electrodes 21 and the antenna 24. The electrical model of FIG. 6 represents two voltage sources 60 and 61 each connected to an armature of half of the capacitors 50 to 54. The voltage source 60 and 61 provide two voltages of opposite sign. The other armature of the capacitors 50 to 54 is connected to the antenna 24. To detect a possible short circuit between two neighboring electrodes 21, it suffices to measure the current using an ammeter 62 placed in series with a voltage sources 60. The antenna 24 is connected to the input of an amplifier 63 whose output is connected to a control circuit 64 of which

  an exemplary embodiment is detailed in FIG. 7.

  The control circuit 64 has an input which receives the signal from the antenna 24 after amplification in the amplifier 63. A bandpass filter 70, one input of which corresponds to the input of the control circuit 64 eliminates noise picked up by the antenna 24, providing a filtered signal on an output. Those skilled in the art can notice that the parasitic continuous and low frequency components (sector) are easily rejected since the polarization signal is sinusoidal (the frequency being chosen to allow simple filtering). A mixer 71 having two inputs and one output receives on one of the inputs a voltage proportional to one of the bias voltages, for example sin (oet). The other of the inputs of the mixer 71 is connected to the output of the bandpass filter 70. The signal supplied on the output of the mixer corresponds to

  produces signals present on its inputs.

  A low-pass filter 72 has an input connected to the output of the mixer 71 and an output. The cutoff frequency corresponds to a frequency less than the frequency of the bias voltage but greater than the envelope frequency which modulates the signal picked up by the antenna 24. The output of the low-pass filter 72 provides a signal which is proportional to the envelope of the electric field picked up by the antenna 24. A comparator 73 compares the signal leaving the low-pass filter 72 with zero volts in order to provide a signal representative of the sign of the envelope on an output. A pulse generator 74 has an input connected to the output of the comparator 73 and an output which supplies a pulse each time the envelope changes sign. A 75 meter has an input

  reset, clock input and counting output.

  The reset input is connected to the output of the pulse generator 74 so that the counter is reset each time the envelope changes sign. A clock signal H is supplied on the clock input. A high clock signal frequency will preferably be chosen to have a good counting resolution. The 75 meter

  measures the time between two sign changes of the envelope signal.

  A comparator 76 has a data input connected to the counting output of counter 75 and an output which provides a binary signal

  error active if the count is greater than a predefined value Dmax.

  The binary signal can be processed in different ways. A simple and effective way of using the binary error signal consists in controlling a marking device integral with the gantry 33 each time the said error signal is active. The result thus obtained makes it possible to pinpoint faulty electrodes. Obviously, others

  processing of this error signal can be considered.

  In FIGS. 8 and 9 are shown pieces of curves of the voltage present at the output of the bandpass filter 70 when the electrodes are not cut. Figure 8 corresponds to the end of the slab. The envelope signal 80 corresponds to a damped sinusoid because when one is located on one end of the panel, the antenna is subjected to electrodes distributed unilaterally. The amortization comes from the successive taking into account of the electrodes bilaterally. For practical reasons, depreciation is shown over three periods while, in practice, depreciation lasts ten to twenty periods. A period corresponds to the passage of the antenna 24 at constant speed above

  two successive polarized electrodes with voltages of opposite sign.

  FIG. 9 corresponds to the curve measured in the middle of the slab, the envelope signal 80 is then of constant amplitude. The envelope signal 80 can be written in the form f (x) * sin (t / I), with x the position of the antenna 24 relative to one end of the screen 20, f (x) the function damping and 6 the duration for the antenna to travel (at constant speed) the distance between

  two electrodes of the same polarity.

  Using FIG. 10, we will now explain how an electrode cutout is detected. FIG. 10A represents the measured signal which leaves the filter 70 when there is no cut and when one is located in the middle of the screen 20. The measured signal 100 is a signal of the form K * sin ( ce) * sin (t / b). The measured signal 100 is assigned to the envelope 80 as explained above. But if an electrode is ever cut, this amounts to more or less greatly reducing the capacity of a

  capacitor 52 corresponding to the position where the cut is located.

  The influence of the capacitor 52 of reduced capacity is maximum when the antenna 24 is located just above it. Thus we can measure a signal whose envelope is deformed as indicated on curves 101 and 102. Curve 101 corresponds to an electrode cut near the edge o it is supplied, and curve 102 corresponds to an electrode cut far from the edge o it is powered. At the cut electrode, the envelope has

plus a constant duty cycle.

  FIG. O10B represents the signal leaving the filter 72 in the cases corresponding to curves 101 and 102. Subsequently the curves in solid lines will relate to curve 101 and the curves in dotted lines will relate to curve 102. FIG. 10C represents the curve which

  corresponds to the signal leaving comparator 73 when it receives signal 101.

  The pulse generator 74 then supplies pulses as shown in Figure 10D. A person skilled in the art will note that the cut of the electrode is here detected by the absence of pulses. FIG. 10E represents on the one hand a counting curve 103 which is reset to zero during each pulse and which is representative of the output of the counter 75, and on the other hand the counting threshold Dmax. The curve in FIG. 10F represents the binary error signal which is active when the count is greater than Dmax. The curves shown in FIGS. 1OG to 10J correspond to the case of the envelope 102. Here, it can be seen that the error is detected by the displacement of the zero crossings of the envelope 102. The

  error signal is here active twice for a single cut electrode.

  A person skilled in the art can notice that if the cut is very far from the supply end, it is hardly detectable with such a system because it is necessary to provide a margin of error on the threshold Dmax to compensate for a possible phase jitter in particular due to the influence of an electrode cut off at a certain distance. A defect in the table 30 is that it requires manipulation of the plasma panel when it is desired to test the electrodes of the two slabs after assembly. In addition, it is necessary to use movable rails and have a square table whose side corresponds to the largest

length of a plasma panel.

  FIG. 11 shows a measurement table 110 having four supply rails 111 to 114 parallel two by two in order to be able to supply the electrodes situated in two perpendicular orientations. The table also includes two guide rails 115 and 116 which serve to guide two gantries 117 and 118 which each support an antenna carrier 119 and 120. Each of the antenna carriers 119 and 120 is guided in translation along the gantry 117 and 118 who supports it. Each antenna holder 119 and 120 supports a reduced size antenna, each antenna having its own processing circuit. FIG. 12 represents an example of an antenna end of reduced size in the shape of a point. The end of the antenna consists of a metallic core 121 surrounded by an insulator 122, the whole surrounded by a metallic shield 123 connected to ground. Such an antenna is only sensitive to what is in a small area. A small antenna will take about ten electrodes into account. Furthermore, the antennas are moved at the ends of the slab which are used to feed the electrodes, as shown in Figures 13 and 14, so that the cuts always appear as capacitor suppressions on at least one of the antennas which

  whatever the location of the cut.

  Obviously, many variants are possible without departing from the scope of the invention. The analysis of the measured signals can be done in a completely different way. One can for example digitize the signal and carry out a digital processing which calls for a precise modeling of the phenomenon. Those skilled in the art will also understand that it is possible to use more than two antennas, the shapes of which can vary considerably. The supply means are not necessarily rails and can be replaced by any other type of contact. The guide means can be completely different from those shown, from the moment when these allow a parallel movement of the antenna (s) relative to the electrodes to be tested. Also, the invention has been described for testing plasma screen tiles, it goes without saying that the invention can be used to test any type of screen comprising an array of electrodes, for example a crystal screen screen liquids.

Claims (13)

  1. Test device for an array of electrodes (21) placed on at least one support slab (20), characterized in that the device comprises: supply means (22, 23) for polarizing the electrodes so that these produce an electric field; - at least one antenna (24) provided with guide means (31 to 33) to be able to be moved parallel to the support slab (20) at an almost constant distance from the electrodes (21) in order to measure a field   electric from said electrodes (21).   2. Device according to claim 1, characterized in that the supply means comprises a first means (22) for biasing half of the electrodes at a bias voltage (Vsin (cot)) and a second means (23) for biasing the other half of the electrodes with the opposite (-Vsin (ot)) of the bias voltage, and in that said first and second means are arranged to bias the electrodes of alternately.   3. Device according to one of claims 1 or 2,   characterized in that it comprises means (34, 35) for adjusting the distance   between the at least one antenna (24) and the screen (20).   4. Device according to one of claims 1 to 3,   characterized in that the antenna (24) is a wire parallel to the panel (20).   5. Device according to one of claims 1 to 3,   characterized in that it comprises four conductive rails (111 to 114)   parallel two by two and at least two antennas.   6. Device according to claim 5, characterized in that the antennas are points whose one end moves in a plane   parallel to the electrode array.   7. Device according to one of claims 1 to 6,   characterized in that the supply voltage is a sinusoidal voltage.   8. Device according to one of claims 1 to 7,   characterized in that the supply means comprise means (62)   to measure the current flowing through the electrodes.   9. Method for testing an array of electrodes (21) placed on at least one support slab (20), characterized in that the electrodes (21) are polarized in order to emit an electric field, and in that '' at least one antenna (24) is moved parallel to the support slab (20) at an almost constant distance from the electrodes (21) in order to measure a field   electric from said electrodes (21).   10. Method according to claim 9, characterized in that half of the electrodes is biased with a bias voltage   (Vsin (cot)), and the other half of the electrodes is polarized with the opposite (-   Vsin (cot)) of the bias voltage, and in that, the electrodes are alternately biased   1il. Method according to one of claims 9 or 10,   characterized in that the supply voltage is a sinusoidal voltage.   12. Method according to one of claims 9 to 1 1,   characterized in that the current flowing through the electrodes is measured.   13. Method according to one of claims 9 to 12,   characterized in that if an electrode is cut then the electrode cut is localized.   14. Method according to one of claims 9 to 13,   characterized in that two antennas are moved near the ends   of the slab which are used to supply the electrodes.