Crack detector for electrically conductive windshield

Abstract

A circuit and method for interrupting power applied to an electrically conductive panel such as a vehicle windshield or window upon cracking of the conductive coating or element thereon. The circuit includes structure for (1) monitoring the resistance of the conductive element and (2) interrupting power applied thereto when the resistance of the conductive element assumes a value indicative of element cracking.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. An electrically heated panel assembly comprising: a panel including an electrically conductive coating;   cable means for conducting direct current through said coating;   controllable switch means coupled in series with said cable means for opening and closing a circuit through said cable means, said controllable switch means including a control terminal;   a ground resistor coupled in series with said cable means between said conductive coating and ground;   comparator means for comparing the voltage across said coating and the voltage across said ground resistor; and   control means coupled to said control terminal and responsive to said comparator means for opening said controllable switch means when the relation of the coating voltage to the ground resistor voltage is unacceptable.   
     
     
       2. A panel assembly as defined in claim 1 further comprising means for sensing the temperature of said panel, and wherein said control means is further responsive to said temperature sensing means and includes means for opening said controllable switch when the temperature of said panel exceeds a preselected value. 
     
     
       3. A panel assembly as defined in claim 1 wherein said comparator means comprises a summing amplifier including a balance pot. 
     
     
       4. A panel assembly as defined in claim 1 wherein said panel is a window panel. 
     
     
       5. An improved electrically conductive panel assembly including a panel having an electrically conductive element, circuit means for conducting direct current through the element, and switch means for opening and closing said circuit means, wherein the improvement comprises said switch means comprising: a controllable switch coupled in series with said circuit means, said controllable switch including a control terminal;   current measuring means for measuring the current passing through the element when said controllable switch is closed;   voltage measuring means for measuring the voltage applied across the element;   calculator means responsive to said current measuring means and said voltage measuring means for producing an output indicative of the resistance of the element; and   control means responsive to said calculator means and coupled to said control terminal for opening said controllable switch when the resistance of the element is unacceptable.   
     
     
       6. An improved panel assembly as defined in claim 5 wherein said current measuring means comprises a ground resistor coupled in series with the circuit means, said ground resistor having a fixed known resistance, whereby the voltage across said ground resistor is directly proportional to the current through the resistor and element. 
     
     
       7. An improved panel assembly as defined in claim 6 wherein said calculator means comprises a summing amp having inputs connected across the electrically conductive element and across said ground resistor. 
     
     
       8. An improved panel assembly as defined in claim 7 wherein said summing amp includes a balance pot enabling said summing amp to be factory balanced. 
     
     
       9. An improved panel assembly as defined in claim 5 further comprising temperature sensing means for sensing the temperature of the panel, and wherein said control means is further responsive to said temperature sensing means and includes means for opening said controllable switch when the panel temperature exceeds a preselected temperature. 
     
     
       10. An improved panel assembly as defined in claim 5 wherein said panel is a window panel. 
     
     
       11. A method of interrupting electrical D.C. power to a resistively heated panel upon cracking of the conductive element therein, said method comprising the steps of: monitoring the resistance of the conductive element by monitoring the voltage applied to the conductive element and the current passing through the conductive element and calculating the resistance using Ohm's law; and   interrupting power to the conductive element when the resistance of the element assumes an undesired value.   
     
     
       12. A method as defined in claim 11 further comprising monitoring the temperature of the panel, and wherein said interrupting step includes interrupting the power when the panel temperature assumes an undesired value. .Iadd. 
     
     
       13.  An improved electrically conductive panel assembly including a panel having an electrically conductive element, circuit means for conducting direct current through the element, and switch means for controlling the current passing through said circuit means, wherein the improvement comprises said switch means comprising: a controllable switch coupled in series with said circuit means, said controllable switch being actuable between first and second states and including a control terminal;   current signal means for providing a signal indicative of the current passing through the element;   voltage signal means for providing a signal indicative of the voltage across the element;   calculator means responsive to said current signal means and said voltage signal means for producing an output indicative of the resistance of the element; and   control means responsive to said calculator means and coupled to said control terminal for actuating said controllable switch from one of the states to the other of the states when the resistance of the element is unacceptable. .Iaddend. .Iadd.14. An improved panel assembly as defined in claim 13 wherein said current signal means comprises a ground resistor coupled in series with the circuit means, said ground resistor having a fixed known resistance, whereby the voltage across said ground resistor is directly proportional to the current through the resistor and element. .Iaddend. .Iadd.15. An improved panel assembly as defined in claim 14 wherein said calculator means comprises a summing amp having inputs connected across the electrically conductive element and across said ground resistor. .Iaddend. .Iadd.16. An improved panel assembly as defined in claim 15 wherein said summing amp includes a balance pot enabling said summing amp to be factory balanced. .Iaddend. .Iadd.17. An improved panel assembly as defined in claim 13 further comprising temperature signal means for providing a signal indicative of the temperature of the panel, and wherein said control means is further responsive to said temperature signal means and includes means for actuating said controllable switch when the panel temperature exceeds a preselected temperature. .Iaddend. .Iadd.18. An improved panel assembly as defined in claim 13 wherein said panel is a window panel. .Iaddend.