IL100035A

IL100035A - Arrangement for controlling liquid crystal display

Info

Publication number: IL100035A
Application number: IL10003591A
Authority: IL
Original assignee: Mannensmann Kienzle Gmbh
Priority date: 1990-11-16
Filing date: 1991-11-11
Publication date: 1994-11-11
Also published as: EP0486922A2; DE4036521A1; DE4036521C2; ATE142348T1; IL100035A0; JPH0675089B2; JPH04268463A; DE59108139D1; EP0486922B1; EP0486922A3

Abstract

A device is described for visually checking standard liquid crystal displays (LCDs) for correct operation. For an LCD to be used in devices which must be calibrated, there are increased requirements with respect to the operational reliability. In accordance with the principle of influence due to coupling a test voltage onto defective points, interruptions in feed lines or segment electrodes interrupted on the glass supports, a case of a defect is made visible by a controllable flashing. The solution consists of coupling a test voltage (11, 27, 27, 51) onto the segments electrodes (1a, 21, 32, 41) by means of an electrode (10, 24, 34, 46) which is conductive throughout and which covers the rear of an LCD (6, 23, 33, 43). Application: display in taximeters, fuel pumps. <IMAGE>

Description

ARRANGEMENT FOR CONTROLLING LIQUID CRYSTAL DISPLAY 'ί>τυ Κ ΠΛ midri mpD ΪΊΤΟ Device for checking a liquid crystal display (LCD) The invention relates to a device for visually checking perfect functioning of standard liquid crystal displays (LCDs = liquid crystal displays ) , in particular in equipment which must be precisely calibrated, where the checking according to the principle of electrostatic induction by coupling a test voltage to defective segment electrodes isolated owing to interruptions in an incoming line or on a glass carrier makes an error optically visible by a flashing which may be checked for.

Liquid crystal displays (LCDs) are increasingly used in many fields of measuring technology for the digital display of physical measuring results. Particularly high demands are made on the functional reliability of the LC displays when liquid crystal displays are used in devices which must be precisely calibrated, for example in a taxi meter, fuel pumps, consumption meters, cash registers and similar devices.

With the use in such devices measures have also become known which are suitable for reducing or making it possible to detect erroneous displays. By means of a simple redundancy, with a double segment construction of the display elements an increased reliability in the representation of the characters or numbers to be displayed is achieved. In the event of a failure of only one part of the doubly represented segment, a legibility of a controlled digit is retained with the simultaneous detection of a partially defective functioning of the display. By means of this device, admittedly, some of the erroneous displays are detected, but there is also the possibility of a legible digit still being produced on the simultaneous failure of a double segment bar and a wrong display produced in this manner remaining undetected. Moreover, in order to introduce this measure a particular liquid crystal display of an appropriate construction and an adapted control are required.

A further improvement in the reliability of the display is achieved by constructing the display elements as a matrix display. An increased Hamming distance which is achievable therewith increases the representation reliability, again in dependence on the matrix format. Complete reliability, however, is not guaranteed even with this arrangement, since a failure of a segment bar is not signalled. Depending on the multiplexing rate, with this arrangement the contrast is reduced when the display is viewed from the side.

Another known device for detecting erroneous displays uses the process of current loop technology, in which a test current is routed through the segment electrodes from one terminal to another independent terminal. Here, a check as to the presence or absence of a test current gives information on the state of each individual segment or of the display. For this function check, a particular, application-specific liquid crystal display and a special control are necessary. Owing to the inevitable doubling of the LCD terminals, on the other hand, a correspondingly higher probability of failure of the contact points is again to be reckoned with.

By means of a high-resistance overlay of a test voltage across the LCD terminals, an increased detectability of signal interruptions to the liquid crystal display is achieved in that a signal interruption activates the test voltage and is displayed by flashing of the segment concerned. If an application of this device is not considered from a structural point of view, the test process can only be retrofitted with great difficulty. The check of the signal path which may be achieved therewith extends only as far as the terminals of the liquid crystal display; the contrast-forming point, the segment electrode, is not monitored as to its functioning. Even with segment groupings, in particular in multiplexing systems, it is admittedly achieved that, in the event of a failure of segment groups, a segment combination is produced which is not interpretable as a number, but a special form of the liquid crystal display and a particular control are required therefor. Nor is this test process any more adequate with, for example, 14-segment displays, since as a result of group failures of segments numbers or characters which are interpretable can be produced again.

The object of the invention is to provide a device for the visual check of the functioning of liquid crystal displays (LCDs) in equipment, which device may be used on LC displays in the standard version and with a standard control and may be retrofitted as a checking device without complicated structural measures in equipment with LC displays and which also may equally be used in 14-segment, 16-segment or dot matrix displays.

This object is achieved in that the coupling of a test voltage to the segment electrodes is effected by means of an electrode which is conductive throughout and covers the rear surface of an LCD.

Advantageous embodiments of the subject of the invention are specified in the sub-claims.

Example embodiments of the invention are described and illustrated in the description which follows and in the drawings, in which: Fig. 1 shows a diagrammatical section diagram through an LCD with function check and terminal connections by way of conductive rubber contact strips, Fig. 2 shows an equivalent circuit diagram for the LCD function check with additional coupling of the test voltage on the viewing side of the LCD, Fig. 3 shows an equivalent circuit diagram for the LCD function check with line interruption in the cover electrode of a segment, Fig. 4 shows an equivalent circuit diagram for the LCD function check with line interruption in the rear electrode of a segment/ Fig. 5 shows a diagrammatical illustration of an LC display with function check and interruption in the incoming line to a segment.

Fig. 1 illustrates a diagrammatical section diagram through a liquid crystal display (also designated LCD) with a function check according to the invention. As is known, in terms of construction, a liquid crystal display comprises a cover glass plate 1 on the viewing side and a rear glass plate 2 which is arranged parallel thereto, which are connected to one another at a certain spacing in the outer zone by means of a glass solder or an adhesive layer 3 and thus form a cell compartment 6 which is filled with a nematic liquid crystal and is hermetically closed. The inner surfaces of the glass plates 1 and 2 are coated with transparent electrode layers of a cover electrode la and a rear electrode 2a in the correspondingly desired display configuration. Stuck with adhesive onto the outer surfaces of the field effect displays, in particular onto the cover glass plate 1, is a cover polarizer 4 on the viewing side, and a rear polarizer 5 with an integrated reflector onto the rear side. For supplying a control voltage, contact strips 7 or conductive rubber connectors for supplying a signal from a circuit board 8 to the LCD are provided. In the same manner, contact strips are provided as incoming lines 9 which effect the transmission of a test voltage from a test voltage signal source 11 by way of signal lines on the circuit board 8 and the incoming lines 9 to an electrode 10 for coupling the test voltage to the LCD. Arranged on the other end of the electrode 10 is an outgoing line 14, also constructed as a contact strip, for tapping the test voltage and incoming line to a control loop 12, 13. The control loop 12, 13 comprises a loop resistor 12 for checking the test voltage and a tap line 13 for transferring a loop signal.

Fig. 2 shows an equivalent circuit diagram for the LCD function check with an additional coupling of the test voltage to the viewing side of the LCD. In simplified illustration, there is illustrated a cover electrode 21 of a segment and, parallel thereto, a rear electrode 22 of a segment, between which liquid crystal 23 is enclosed. Arranged behind the liquid crystal cell is an electrode 24 for coupling the test voltage. An interruption 25 in the rear electrode 22 simulates the error state. A segment control signal source 26 is connected conductively to the cover electrode 21 and the rear electrode 22. A test voltage signal source 27 is connected to the coupling electrode 24 and can at the same time supply an additional coupling electrode 29 on the viewing side of the LCD with a test signal voltage. Both voltage sources 26 and 27 are applied across a common reference potential (earth) 28 of the circuit according to Fig. 2. With applied voltage sources 26 and 27 an electrical field El is produced in the region between the cover electrode 21 and a part of the rear electrode 22 of a segment which is cut off and isolated by the interruption 25. A further electrical field E2 is produced between the isolated part of the rear electrode 22 and the coupling electrode 24. A third electrical field E3 is created between the part of the intact rear electrode 22 and the coupling electrode 24. Finally, a fourth electrical field E4 exists in an intact region between the electrodes 21 and 22 of the segment concerned.

Another example embodiment is specified in Fig. 3, in which an equivalent circuit diagram for the LCD function check is shown with a line interruption 35 in a cover electrode 32 of a segment. The symbolically illustrated cell compartment between the cover electrode 32 and a rear electrode 31 arranged parallel thereto is once again filled with a liquid crystal 33. An electrode 34 for coupling the test voltage is connected to a test voltage signal source 37, the other side of which is applied across a reference potential (earth) 38. The segment control signal source 36 is linked to the cover (32) and the rear electrode 31 and is connected to the reference potential 38 by means of the rear electrode 31. In the operating state, an electrical field El is once again produced between the rear electrode 31 and an interrupted part of the cover electrode 32. An electrical field E2 is produced between the coupling electrode 34 and the disturbed part of the segment with the isolated cover electrode 32. An electrical field E3 is produced between the coupling electrode 34 and an intact region of the segment, and finally an electrical field E4 is produced in the intact part of the segment .

The illustration according to Fig. 4 specifies an equivalent circuit in which a rear electrode 42 of the segment has an interruption 44. The segment is illustrated symbolically by a cover electrode 41, a rear electrode 42 (with interruption 44) and liquid crystal 43 enclosed therein, and is acted upon by a segment control signal source 48. Serving for coupling a test voltage is an electroluminescent illumination foil 45 to 47 which substantially comprises a transparent cover electrode 45, a reflective rear electrode 46 and a light-emitting layer 47 inserted therebetween. A control voltage source 49 for the illumination foil 45 to 47 is linked to the input side of the transformer 50 for converting the control voltage from the voltage source 49 to operating values for the illumination foil 45 to 47. On the secondary side of the transformer 50 there is provided a central tap for coupling a test voltage from a test voltage signal source 51. The control signal source 48 and the test voltage signal source 51 are applied across a common reference potential (earth) 52.

Fig. 5 specifies a 7-segment display 60 with an arrangement of seven cover electrodes la corresponding to the segment bars 61 to 67 for the representation of digits from "0" to "9", the incoming lines 71 to 77 of which are guided out of the liquid crystal cell to a terminal strip 69. Arranged in congruence behind the segments 61 to 67 is a common rear electrode 68 to the segments which is guided to the outside by way of a rear electrode terminal 70. Finally, as Fig. 1 also specifies, there is provided on the rear side of the 7-segment display an electrode 10 (corresponding to the electrode 10 in Fig. 1) which is conductive throughout and covers the rear surface of the entire LCD and serves for coupling the test voltage from the test voltage signal source 11. For the simulation of an error case, according to the illustration according to Fig. 5, an interruption 59 in the incoming line 72, for example to the segment bar 62, is shown. An electrical field going out from the coupling electrode 10 acts on the segment bar 62 cut off by itself from the control such that the segment bar 62 in operation is distinguished from the other segment bars 61 and 63 to 67 by "flashing" and optically displays the error case. This procedure is explained below.

In the widely available field effect liquid crystal displays (LCDs) - to be regarded as standard - the contrast formation in a segment is effected by an electrical field E4 which forms between the corresponding electrodes la and 2a of a segment in the liquid crystal 6, 23, 33 , 43 located therebetween on applying a corresponding control voltage 26, 36, 48. Here, the light vector in the liquid crystal layer 6, 23, 33, 43 is rotated such that light penetrating through the polarizers 4 and 5 on the cell-delimiting glass plates 1 and 2 is either "let through" or "restrained" (absorbed).

If an interruption 59 occurs in the incoming line 72 of an electrode of a segment 62 or of a segment group (for example in multiplexing displays), then an electrical field E4 can of course no longer be created by the control voltage 26, 36, 48, i.e. the segment 62 fails. The same occurs if, for example, depending on very narrow conductor paths on the glass of the LCD, interruptions 25, 35, 44 occur which from the outside are "not detectable" in operation.

The electrode 22, 32, 42, 62 isolated thus by the error case "interruption" 25, 35, 44, 59 is electrically neutral with respect to the corresponding segment electrode 2a, 21, 31, 41. This neutrality may be influenced by electrostatic induction, i.e. by means of a "powerful" external electrical field, so that by capacitive coupling the "isolated" electrode 22, 32, 42, 62 can be brought dynamically to a potential deviating from the corresponding segment electrode 2a, 21, 31, 41. The electrical field El produced here between the segment electrodes then results again in a contrast formation in the manner described above.

Since the remaining, final transition resistor of an "isolated" electrode 22, 32, 42, 62 leads to removal of the field produced by electrostatic induction to the corresponding segment electrode 2a, 21, 31, 41, the "isolated" electrode 22, 32, 42, 62 must be "charged" again, for example by changing the polarity of the "powerful" external field.

On doubling the frequency of changing of the external field E2, the electrical field El between the isolated electrode 22, 32, 42, 62 and the corresponding segment electrode 2a, 21, 31, 41 is created and removed. A segment defective in this way appears to be "flashing" and may be easily visually detected and thus marked as defective.

The potential (amplitude of the test voltage) necessary for coupling the test voltage is dependent on: a) the minimum value of the electrical field between the corresponding segment electrodes (for example 21 to 22 and the inner sides of pos. 1 to pos. 2) for influencing the liquid crystal 23 and 6 for the purpose of rotating the light vector, b) the spacing ratios of the segment electrodes in the centre (for example pos. 21 and 22, and 1 and 2 on the inner side) for the test voltage coupling electrode 24, 29 and 10, c) the ratios of the effective, i.e. the electricity constants of the materials between the electrodes with e le: value of the liquid crystal cell (segment) and ε 2e: value of the materials between the liquid crystal cell and the test voltage coupling electrode .

Conventional LCDs require a cell voltage of /U21 22/~ 3 V for contrast formation.

In the case of an interruption 25, the field produced by the test voltage divides into El, the portion of the cell, and E2, the portion outside the cell.

The voltage division is therefore approximately dependent on the relative spacings d21j 22 to d22t 2, the active effective dielectric constants ele, e2e and the active surface ratios F21 22 to F22.23* /u21 22/ 2e d21.22 * F24 /U22 24/ ε le . d22i 24 . F22 where d22 24 2. 1·5 mm: Spacing of the test voltage coupling electrode 24 from a segment electrode 22 d21 22 .8 μια : Spacing of the segment electrodes 21, 22 from one another /U21 . 3 V : Amount of the contrasting voltage across the segment electrodes 21, 22 /U22 2 / 1 Amount of the test voltage to be applied, from 27 e le = 8 : Effective, relative dielectric constant of the materials between the segment electrodes 21 and 22 e 2e = 10 : Effective, relative dielectric constant of the materials between the segment electrode 22 and the test voltage coupling electrode 24 F22 : Electrode surface of the "isolated" electrode 22 F24 : Electrode surface as the average value of the surface of the "isolated" electrode 22 and the test voltage coupling electrode 24 F24/F22 ~ 5 ... 15 : Estimated active ratio of the electrode surfaces of a disrupted segment .

In the case of a highly simplified observation, an amount of the test voltage of /U22t 23/ ~ 40 V is found which is necessary to bring about a contrast in the case of interruptions of the type indicated above.

If a further test electrode 29 (transparent) is mounted on the cover side of the LCD (viewing side) and is acted upon by means of the same test voltage from the test voltage signal source 27, then the amount of the test voltage with the same contrast is reduced to approximately half the value.

If an interruption 35 occurs in the cover electrode 32 according to Fig. 3, then here too in the interrupted, isolated part of this electrode 32 a charge shift is caused by the test voltage coupling electrode 34 and a test voltage from the test voltage signal source 37.

The segment rear electrode 31 located between the test voltage electrode 34 and the isolated segment electrode 32 has a substantially smaller surface by comparison with the test voltage coupling electrode 34, so that its screening action is minimal .

The test voltage across the coupling electrode 34 also acts on the segment electrodes still connected in conventional manner to the segment control signal source 36. The charge shift provoked by the test voltage from the electrode 34 causes, however, only a negligible, overlaid current in the segment control source 36 and correspondingly a negligible voltage stroke which does not have a contrasting effect.

A cyclic charging of the test voltage coupling electrode 34 by means of for example a "free running" choke which is charged cyclically by means of a charging current to generate the charging pulse is sufficient to generate the test voltage for the coupling electrode 34 from a conventional 5-volt supply (not illustrated in detail). The charging/discharging time then determines the "flashing frequency" of a defective segment. Other circuits, such as for example capacitive charging pumps, may also be used.

For checking the test voltage coupling, as indicated in Fig. 1, a checking device 12, 13 is conceivable which ensures the presence of the test voltage on the electrode 10 by means of a tap line 13 mounted separately from the connection point of the test voltage coupling electrode 10. If for example an interruption occurs in the incoming line of the test voltage signal source 11 to the electrode 10 or at the test tap point 13, then this can be established by the checking device 12, 13 and correspondingly evaluated.

Abstrac : Device for checking a liquid crystal display (LCD) A device is described for visually checking perfect functioning of standard liquid crystal displays (LCDs). For the use of an LCD in equipment which must be precisely calibrated, there are increased requirements with respect to functional reliability. According to the principle of electrostatic induction by coupling a test voltage to defective points, interruptions in incoming lines or segment electrodes interrupted on the glass carriers an error case is made visible by a flashing which may be checked for. The solution consists in coupling a test voltage (11, 27, 27, 51) to the segment electrodes (la, 21, 32, 41) by means of an electrode (10, 24, 34, 46) which is conductive throughout and covers the rear surface of an LCD (6, 23, 33, 43).

Use: Display in taxi meters, petrol pumps.

Fig. 1

Claims

CLAIMS :

1. A device for visually checking perfect functioning of segment shaped liquid crystal displays (L.C.D.) which by coupling with an electrical alternating field created by a test voltage signal source, due to interruption of its feeding conduits delivers a faulty display, does make optically recognizable whatever fault by means of flashing determined and controllable by frequency of the alternating field, characterised thereby that the coupling of the test voltage (11,27,37,51) with the segment electrodes ( la, 21 ,32,41) is effected capacitatively, thus wholly contactless, by means of a fully conductive surface electrode (10,24,34,46) which overlaps the rear surface (2) of the liquid crystal display.

2. The device according to claim 1 characterised by formation of a control loop (12,13) for the test voltage such that the coupling of the test voltage signal source (11) is effected via an in-conduit (9) with electrode (10)" over a resistance (12) of the loop (12,13) whereat the presence of a correct test voltage is ascertainable.

3. Device according to claim 1 characterised thereby that coupling of a test voltage with the segment electrode (21,22) is effected by means of a transparent electrode (29) which overlaps the cover surface of the segment electrode (21) of an L.C.D. at the viewing side,

4. A device according to claim 1 characterised thereby that coupl ng of a test voltage is effected via an illumination foil located behind L.C.D. in accordance with the electro-luminescense principle, such that the surface conductive illumination foil (47) is subjected also to a test voltage (51) in addition to approach voltage (49) .

5. Devices according to claim 1 whenever part of equipment which subject to calibration surveyance,

6. Devices for visibly checking perfect functioning of L.C.D, displays, substantially as hereinbefore described with reference to the appended diagrammatic drawings. COHEN ZEDEK & RAPAPQRT P,0, BOX 33116, TEL AVI ATTORNEYS FOR APPLICANT