Crack detector for heated glass panel

Abstract: An improvement is provided for an electrically heated glass panel (10) of the type where a resistive film (18) is disposed on the glass panel and is connected between and along the length of a pair of spaced bus bars (20,22) each of which is terminated in a respective voltage input terminal (24,26) and where one of the bus bars (20) includes a connector section (28) to which the resistive film is not connected extending from the voltage input terminal (24) of the bus bar to a junction point (30) on the bus bar beyond which the resistive film is connected to the bus bar. A sensor conductor (34) is disposed on the glass panel and connected from at or beyond the junction point on the one bus bar to a sensor terminal (36) and extends generally adjacent to the connector section of the bus bar. The power supply (32) is disabled in response to the presence of a predetermined voltage difference between the sensor terminal and the input terminal of the one bus bar indicating the presence of a break (33) in the connector section of the bus bar so as to prevent the development of an arc across the break.

Description: This invention relates to a crack detector for an electrically heated glass panel such as the windshield of an automotive vehicle. More particularly, the invention relates to a heated glass panel of the type where a resistive film is disposed on the glass panel and is connected between and along the length of a pair of spaced bus bars each of which is terminated in a respective voltage input terminal. It is common in such heated glass panels that at least one of the bus bars includes a connector section (to which the resistive film is not connected) extending from the voltage input terminal of the bus bar to a junction point on the bus bar beyond which the resistive film is connected to the bus bar. A high voltage is applied across the voltage input terminals of the bus bars from a power supply to energize the resistive film when it is desired to heat the glass panel. Such an arrangement is shown in US-A-4565919. US-A-3941975 discloses an arrangement for disconnecting the power supply to a window heater when a sensor conductor breaks. A crack detector for an electrically heated glass panel in accordance with the present invention is characterised over US-A-4565919 by the features specified in the characterising portion of Claim 1. In the event of a break or crack in the glass panel extending through the connector section of the one bus bar, undesired arcing could result when the high voltage is applied across the voltage input terminals. To prevent such arcing, the invention provides a sensor conductor disposed on the glass panel and connected from the junction point on the bus bar to a sensor terminal. The sensor conductor extends generally adjacent to the connector section of the one bus bar. Further, in accordance with the invention, means are provided for disabling the power supply to prevent the application of the high voltage across the voltage input terminals of the bus bars in response to the presence of a predetermined voltage difference between the sensor terminal and the voltage input terminal of the one bus bar indicating the presence of a break in the connector section of the bus bar. In addition, pursuant to the invention, low voltage (preferably battery voltage) may be applied to the voltage input terminal of the one bus bar prior to turning on the high voltage power supply. If there is a break in the connector section of the bus bar, the predetermined voltage difference will appear between the sensor terminal and the voltage input terminal of the one bus bar and the high voltage power supply will not be turned on. If the low voltage was not first applied to the voltage input terminal of the one bus bar, the high voltage power supply would be turned on until the presence of the break was detected and then the power supply would be shut off. During this turn on/shut off process, an undesired incipient arc condition could develop across the break. The foregoing "continuity test" feature of the invention eliminates the possibility of such an incipient arc condition. The low voltage is too low to produce any arc across the bus bar break. The invention is now described, by way of example, with reference to the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings, in which:- Figure 1 is a schematic diagram of one embodiment of the invention; Figure 2 is a cross-section of a portion of the (multilayered) glass panel shown in Figure 1; Figure 3 is a more detailed schematic diagram of a specific (second) embodiment of the invention; and Figure 4 is a flow diagram useful in explaining the operation of the Figure 3 embodiment. Referring to Figures 1 and 2 of the drawing, a (multilayered) glass panel 10, which may be the windshield of an automotive vehicle, comprises an inner glass layer 12, an outer glass layer 14, and an intermediate layer 16 of clear insulator material. Disposed over the inner surface of the outer glass layer 14 is a resistive film 18 which is transparent. The resistive film 18 is connected between and along the length of a pair of spaced, electrically conductive, bus bars 20 and 22 which are also disposed on the inner surface of the outer glass layer 14. The bus bars 20 and 22 are terminated in voltage input terminals 24 and 26, respectively. Further, the bus bar 20 includes a (generally vertical) connector section 28 (to which the resistive film 18 is not connected) extending from the voltage input terminal 24 to a junction point 30 beyond which the resistive film 18 is connected to the bus bar 20. A power supply 32 is connected across the voltage input terminals 24 and 26 of the bus bars 20 and 22 for applying a high voltage, e.g., 70-90 volts DC, therebetween to energize the resistive film 18 when it is desired to heat the (multilayered) glass panel 10. In the event of a crack or break in the outer glass layer 14 extending through the connector section 28 of the bus bar 20 (as shown for example by the line 33) undesired arcing across the break can result when the high voltage is applied between the voltage input terminals 24 and 26. To prevent such arcing, the invention provides a sensor conductor 34 also disposed on the inner surface of the outer glass layer 14 of the glass panel 10. The sensor conductor 34 is connected to the bus bar 20 at the junction point 30 and extends, generally adjacent the connector section 28 of the bus bar 20, to a sensor terminal 36. Further, in accordance with the invention, a voltage comparator 38 is responsive to the presence of a predetermined voltage difference between the sensor terminal 36 and the voltage input terminal 24 of the bus bar 20, indicative of a crack extending through the connector section 28 of the bus bar 20, to disable the power supply 32 to prevent the application of the high DC voltage across the voltage input terminals 24 and 26 of the bus bars 20 and 22, respectively. The power supply 32 and voltage comparator 38 may be implemented within the arrangement shown in our published European patent applications nos. 0256690 and 0256689 which are hereby incorporated as if set forth fully herein. Referring to the above mentioned publications, the windshield crack detector arrangement shown and described therein is replaced by the embodiment of this invention illustrated in Figure 3 hereof. Elements shown and described in Figure 3 hereof that are also shown and described in the above mentioned publications are designated in Figure 3 hereof by the same respective numerals used to designate such elements in the above mentioned publications except that the numerals are primed in Figure 3 hereof. Elements shown and described in Figure 3 hereof that are not shown and described in the above mentioned publications are designated in Figure 3 hereof by unprimed numerals. Parts which are similar to those in Figure 1 have been given the same reference numerals. Referring to Figure 3 hereof, the digital computer 126′ (which may be a Motorola Model No. MC 6805R2 microcomputer) includes pins 21 and 23 which are analogue voltage inputs to an analogue to digital converter unit of the digital computer 126′. Specifically, pin 21 is connected to the junction between a pair of voltage divider resistors 40 and 42 which are connected in series between the sensor terminal 36 of sensor conductor 34 and ground. Similarly, pin 23 is connected to the junction between a pair of voltage divider resistors 44 and 46 which are connected in series between the voltage input terminal 24 of bus bar 20 and ground. Preferably, the total resistance of the voltage divider networks 40-42 and 44-46 is sufficiently high that the current drain from voltage input terminal 24 and sensor terminal 36 is negligible. Filter elements may be employed in conjunction with the voltage divider networks 40-42 and 44-46 as desirable. In operation, a crack in the glass panel 10 of the windshield (such as indicated by line 33) passing through connector section 28 of bus bar 20 (or through sensor conductor 34) will cause a voltage difference between the voltage input terminal 24 of bus bar 20 and the sensor terminal 36 of sensor conductor 34. This voltage difference will be reflected between pins 2l and 23 of the digital computer 126′ via voltage divider resistors 40-46. Provided that the voltage difference between voltage input terminal 24 and sensor terminal 36 is greater than a given threshold difference (e.g., 10 volt difference), the digital computer 126′ will interpret the corresponding voltage difference between pins 21 and 23 as indicating the presence of a crack 33 in the glass panel 10 having the potential to produce an undesired arc condition unless the power supply 32 is disabled or deenergized. As related to the present crack detector invention, the digital computer 126' functions in accordance with a program embodied within a faults routine flow chart as shown in Figure 4 which is the same as the faults routine flow chart shown in Figure 8 of the above mentioned publications except for changes occasioned by the present invention. Elements shown and described in Figure 4 hereof that are also shown and described in Figure 8 of the above mentioned publications are designated in Figure 4 hereof by the same respective numerals used to designate such elements in Figure 8 of the above mentioned publications except that such numerals are primed in Figure 4 hereof. Elements shown and described in Figure 4 hereof that are not shown and described in Figure 8 of the above mentioned publications are designated in Figure 4 hereof by unprimed numerals. Referring to Figure 4 hereof, the faults routine checks the heated glass panel circuitry for any problems or faults. The faults routine begins at point 294′ and proceeds as described in the above mentioned publications with the following exception: At decision step 48, the digital computer 126' determines whether the voltage difference between pins 21 and 23 indicates that the glass panel 10 is cracked through the connector section 28 of the bus bar 20 (or sensor conductor 34). If the decision at decision step 48 is yes, the program proceeds to set the fault flag true at step 298′ and then proceeds to execute an off routine as described in Figure 12 of the above mentioned publications which disables or deenergizes the power supply 32. A crack (such as that indicated by line 33) may occur in the glass panel 10 during a period when the power supply 32 is turned on or during a period when the power supply 32 is shut off. In the first case, with the power supply 32 energized and applying the high DC voltage across the voltage input terminals 24 and 26, the crack detector arrangement of the invention is effective to sense the cracked condition and shut off the power supply 32 as previously described. The operation is the same in the second case when the power supply 32 is next energized to apply the high DC voltage between the voltage input terminals 24 and 26. In either case, however, an incipient arc may develop across the crack 33 in the connector section 28 of the bus bar 20 before the power supply 32 can be shut off. This incipient arc condition is short-lived, and while undesirable, it is tolerable. Fortunately, pursuant to an added feature of the invention, the foregoing undesired incipient arc condition can be eliminated altogether for the case where the crack 33 occurs during a period when the power supply 32 is shut off. Specifically, this may be accomplished by applying battery voltage (or a similar low voltage which is too low to produce any arc across the crack 33) from line 14′ to the voltage input terminal 24 of the bus bar 20 via a diode 50 and a current limiting resistor 52 (Figure 3). With this arrangement, the digital computer 126′ will detect the presence of a cracked windshield condition immediately upon operation of the vehicle ignition switch (not shown) which applies the low battery voltage to line 14′ and will disable the high voltage power supply 32 before it can be turned on or energized. Note that for this arrangement to be effective, there must be no current path from the voltage input terminal 24 to ground through the power supply 32. It should be recognized that the foregoing embodiments of the invention are disclosed for purposes of illustration only and are not intended to unduly limit the invention. As will be appreciated by those skilled in the art, other modifications to the illustrated embodiments are possible within the spirit and scope of the invention. For example, one of the voltage input terminals 24 or 26 may be grounded and the power supply 32 altered accordingly. In some applications, the high voltage applied between the voltage input terminals 24 and 26 may be AC rather than DC. Further, the voltage comparator 38 may be implemented by means other than the digital computer 126′, for example, by an analogue operational amplifier such as a Motorola Model No. LM 2902 together with appropriate companion elements.

Claims: A crack detector for an electrically heated glass panel (10) of the type where a resistive film (18) is disposed on the glass panel (10) and is connected between and along the length of a pair of spaced bus bars (20,22) each of which is terminated in a respective voltage input terminal (24,26) and where one of the bus bars (20) includes a connector section (28) to which the resistive film (18) is not connected extending from the voltage input terminal (24) of the bus bar (20) to a junction point (30) on the bus bar (20) beyond which the resistive film (18) is connected to the bus bar (20), and including a power supply (32) for applying a high voltage across the voltage input terminals (24,26) of the bus bars (20,22) to energize the resistive film (18) when it is desired to heat the glass panel (10); the crack detector preventing in the event of a break in the glass panel (10) extending through the connector section (28) of the one bus bar (20) undue arcing across such break when the high voltage is applied to the voltage input terminals (24,26) and being characterised by, a sensor conductor (34) disposed on the glass panel (10) and connected from at or beyond the junction point (30) on the one bus bar (20) to a sensor terminal (36) and extending generally adjacent to the connector section (28) of the bus bar (20); and disabling means (38,126′) for disabling the power supply (32) to prevent the application of the high voltage across the voltage input terminals (24,26) of the bus bars (20,22) in response to the presence of a predetermined voltage difference between the sensor terminal (36) and the voltage input terminal (24) of the one bus bar (20) indicating the presence of a break in the connector section (28) of such bus bar (20). A crack detector as claimed in Claim 1, comprising means (50,52) for applying a low voltage to the voltage input terminal (24) of the one bus bar (20) incapable of producing an arc across any break in the connector section (28) of such bus bar (20) prior to applying the high voltage across the voltage input terminals (24,26) of the bus bars (20,22), for producing the predetermined voltage difference in the presence of a break. A crack detector as claimed in Claim 1 or Claim 2 wherein the glass panel (10) is a windshield of a motor vehicle. An electrically heated glass panel having a crack detector as claimed in any one of Claims 1 to 3.

Inventors: RAUSCH RICHARD A, RUFF DONALD O, RAUSCH, RICHARD A., RUFF, DONALD O.
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1989
Main classification: H05B3
IPC codes: B60J1, B60S1, G01N27, H05B3, H02H5, B32B17