Abstract:
In one possible implementation, a method is provided for determining contactor health including measuring a differential voltage between a first utility line voltage and a second utility line voltage on a primary side of a contactor and on a secondary side of the contactor. The measuring is performed with both an unloaded current and with a load current. The unloaded and loaded measurements are performed at the primary side and the secondary side, and are made with the contactor closed. It includes determining a difference between a secondary unloaded voltage and a secondary loaded voltage and subtracting a difference between a primary unloaded voltage and a primary loaded voltage to provide a contactor voltage drop. The contactor resistance is determined by dividing the contactor voltage drop by the loaded current.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of PCT/US2011/033134 by Flack, entitled CONTACTOR HEALTH MONITOR CIRCUIT AND METHOD, filed on 19 Apr. 2011, herein incorporated by reference in its entirety, which claims the benefit of the following U.S. Provisional Patent Application, which is herein incorporated by reference in its entirety: 
     U.S. Provisional Application 61/325,791, by Albert Flack, filed 19 Apr. 2010, entitled CONTACTOR HEALTH MONITOR CIRCUIT. 
    
    
     BACKGROUND 
     Electric vehicles may now be charged with utility power and with high current. As a result, even a small resistance in the line can be inefficient and can generate significant heat, which could pose a serious hazard. What is needed is a means to monitor the charging circuit to reduce the potential of overheating, and improve the efficiency of the circuit. 
     SUMMARY 
     In one implementation a method is provided for determining contactor health including measuring a differential voltage between a first utility line voltage and a second utility line voltage on a primary side of a contactor and on a secondary side of the contactor. The measuring is performed with both an unloaded current and with a load current. The unloaded and loaded measurements are performed at the primary side and the secondary side, and are made with the contactor closed. The method further includes determining a difference between a secondary unloaded voltage and a secondary loaded voltage and subtracting a difference between a primary unloaded voltage and a primary loaded voltage to provide a contactor voltage drop. The contactor resistance is determined by dividing the contactor voltage drop by the loaded current. 
     In one possible embodiment, a contactor health monitor circuit is provided which includes a first diode connected at a terminal end to a line one primary side terminal of a contactor and a second diode connected at a terminal end to a line two primary side terminal of the contactor. A third diode connected at a terminal end to a line one secondary side terminal of the contactor and a fourth diode connected at a terminal end to a line two secondary side terminal of the contactor. The circuit further includes an operational amplifier circuit. The first diode and the second diode are connected together so as to provide a summed output of the line one primary side contactor terminal and the line two primary side contactor terminal to a first input of the operational amplifier circuit. The circuit also has the third diode and the fourth diode being connected together so as to provide a summed output of the line one secondary side contactor terminal and the line two secondary side contactor terminal to a second input of the operational amplifier circuit. 
     In yet another embodiment, a contactor health monitor circuit is provided having a first operational amplifier connected at a first input to a line one primary side terminal of a contactor and connected at a second input to a line two primary side terminal of the contactor. The circuit also includes a second operational amplifier connected at a first input to a line one secondary side terminal of the contactor and connected at a second input to a line two secondary side terminal of the contactor. A summing amplifier is connected at a first input to an output of the first operational amplifier summed with an output of the second operation amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention will be better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  shows a schematic view of a cable to connect utility power to an electric vehicle (not shown) and some associated circuitry. 
         FIG. 2  shows a simplified schematic view of an embodiment of a contactor health voltage monitor circuit. 
         FIG. 3  shows a simplified schematic view of an alternate embodiment of a contactor health voltage monitor circuit. 
     
    
    
     DESCRIPTION 
       FIG. 1  shows a schematic view of a cable  100  to connect utility power to an electric vehicle (not shown) along with some associated circuitry. In the embodiment of  FIG. 1 , the cable  100  contains L 1  and L 2  and ground G lines. The cable  100  connects to utility power at one end  100   u  and to an electric vehicle (not shown) at the other end  100   c . The electric vehicle (not show) could have an onboard charger, or, the electric vehicle end  100   c  of the cable  100  could be connected to a separate, optionally free standing, charger (not shown). The separate charger (not shown) would in turn be connected to the electric vehicle for charging onboard batteries, or other charge storage devices. In other embodiments not shown, a charger could be integrated into the cable  100 , if desired. 
     The contactor  140  mechanically disconnects/connects the utility power L 1  and L 2  from/to the vehicle connector  100   c . Over time, the impedance of the contactor  140  increases. As such, the health of the contactor  140  must be monitored to insure that the impedance does not get too high. Thus, the terminals  11   p ,  12   p ,  13   s , and  14   s  of the contactor  140  are monitored by a contactor health monitor circuit  180 . As used herein, the terminals  11   p  and  12   p  are referred to as being on the “primary side” of the contactor  140  and the terminals  13   s  and  14   s  are referred to as being on the “secondary side” of the contactor  140 . The voltages on the terminals  11   p ,  12   p ,  13   s , and  14   s  are labeled as signals AC_ 1 , AC_ 2 , AC_ 3 , and AC_ 4 , respectively. 
     In general, a contactor health voltage monitor circuit is subject to large absolute errors at the amplifier stages due to tolerance errors in the high voltage buffer components. These errors can make absolute channel-to-channel difference comparisons useless for monitoring the very small voltage changes across the contactor. A better method is to use the two high voltage buffer stages as relative change indicators rather than absolute voltage values. This means that, for a given voltage channel, the difference in voltage readings (zero current and loaded current) from one voltage level to another level is only the given linearity error for the circuit. This is a very small error and is not affected by the precision of the resistive elements. The accuracy of the A/D converter is the governing precision determinator. For a 10 bit converter this is about 2 bits, 0.4%. 
       FIG. 2  shows a simplified schematic of one possible embodiment of a contactor health monitor circuit  200 . The signal AC_ 1  is supplied via resistors R 7 , R 11 , R 16 , and R 93 , to an inverting input of the operational amplifier  210  into operational amplifier  210 . The signal AC_ 2  is supplied via resistors R 4 , R 8 , R 13 , and R 95 , to a non-inverting input of the operational amplifier  210 . A middle reference voltage signal ADC VREF MID, such as 1.5 volts, is combined via a resistor R 100  with the signal AC_ 2  to bias the output of the operational amplifier  210  above zero so that the output does not go below zero. The output of the operational amplifier  210  is supplied via resistor R 50  as signal A/D 1  to an A/D converter. 
     The signal AC_ 4  is supplied via resistors R 96 , R 99 , R 103 , and R 104  to an inverting input of the operational amplifier  220  into operational amplifier  220 . The signal AC_ 3  is supplied via resistors R 20 , R 26 , R 35 , and R 106 , to a non-inverting input of the operational amplifier  220 . A middle reference voltage signal ADC VREF MID, such as 1.5 volts, is combined via a resistor R 107  with the signal AC_ 3  to bias the output of the operational amplifier  210  above zero so that the output does not go below zero. The output of the operational amplifier  220  is supplied via resistor R 59  to an A/D converter as signal A/D 3 . 
     The output of the operational amplifier  210  and the operational amplifier  220  are summed and supplied to the inverting input of the summing amplifier  230 . The summing amplifier amplifies the difference between the primary differential (L 1  to L 2 ) voltage and the secondary differential (L 1  to L 2 ) voltage. The output of the summing amplifier  230  is supplied via resistor R 55  to an A/D converter as signal A/D 2 . 
     Diodes D 3  and D 2  provide overvoltage protection for the operational amplifier  210 . Diodes D 35  and D 7  provide overvoltage protection for the operational amplifier  220 . 
     In the embodiment of  FIG. 2 , the output signals A/D 1  and A/D 3  may be stored prior to summing for calculation to determine the resistance R c  of the contactor  140 . Alternately, the output signal A/D 2  may be stored for calculation to determine the resistance R c  of the contactor  140 . 
       FIG. 3  shows a simplified schematic of an alternative embodiment of a contactor health monitor circuit  300 . In the contactor health monitor circuit  300 , the AC_ 3  and AC_ 4  signals are combined after passing through diodes D 10  and D 9 , respectively. The combined signal is supplied via resistors R 4  and R 8  to the inverting input of operational amplifier  310 . The diodes D 10  and D 9  allow only the positive voltages to combine so the signals AC_ 3  and AC_ 4  do not cancel. 
     In the contactor health monitor circuit  300 , the AC_ 1  and AC_ 2  signals are combined after passing through diodes D 2  and D 1 , respectively. The combined signal is supplied via resistors R 9  and R 10  to the non-inverting input of operational amplifier  310 . The diodes D 2  and D 1  allow only the positive voltages to combine so the signals AC_ 3  and AC_ 4  do not cancel. 
     In the contactor health monitor circuit  300 , the output of the operational amplifier  310  is provided to an A/D converter for storage and use in determining the resistance R c  of the contactor  140 . 
     The embodiment of  FIG. 3  provided improved accuracy because it has fewer resistors and because there is no common mode issues because the diodes D 1 , D 2 , D 9 , and D 10  prevent swings of negative-to-positive voltage into/out of the operational amplifier  310 . The diodes D 1 , D 2 , D 9 , and D 10  allow the input to be entirely positive and within the operating range of the amplifier  310 , which reduces the error and gives a better gain because it does not have to split the output between a positive and negative value midpoint. As such, by using diodes D 1 , D 2 , D 9 , and D 10  to rectify, it doubles the range of accuracy. 
     Further, the embodiment of  FIG. 3 , contains fewer components, less resistors and operational amplifiers. Referring to  FIG. 3 , in some embodiments, R 3 , R 4 , R 5 , R 6 , R 8 , R 9 , R 10 , and R 12  have resistance values of 55K, 3 M, 100, 600 k, 10K, 3 M, 10K, and 55K in ohms, respectively, and C 1 , and C 3  have capacitance of 0.01 uFarad and 0.0001 microFarad, respectively. 
     Referring to  FIG. 2 , however, in some embodiments, R 7 , R 11 , and R 16  have a combined resistance of 3.00 M ohms; R 4 , R 8 , and R 13  have a combined resistance of 3.00 M ohms; R 96 , R 99 , and R 103  have a combined resistance of 3.00 M ohms; R 20 , R 26 , and R 35  have a combined resistance of 3.00 M ohms. In various embodiments, R 93  is 20.0K, R 94  is 10.0K, R 95  is 20.0K, R 100  is 10.0K, R 50  is 100, R 104  is 20K, R 105  is 10.0K, R 106  is 20K, R 107  is 10.0K and R 59  is 100 ohms. Further, in various embodiments, R 39  is 1.00 M, R 40  is 10.0K, R 42  is 10.0K, R 59  is 100, R 39  is 1.00 M, and R 55  is 100 ohms. Additionally, capacitor C 13  and C 28  are each 0.1 uFarad, C 18  is 0.001 uFarad, C 20  is 0.001 uFarad, C 30  is 0.01 uFarad, C 55  is 0.01 uFarad, C 57  is 0.01 uFarad, C 58  is 0.01 uFarad, C 65  is 0.01 uFarad. Moreover, in some embodiments, most of the resistors are +/−0.1%. The embodiment of  FIG. 3 , however, contains fewer components, less resistors and operational amplifiers so can be more cost efficient. 
     Referring to  FIGS. 1-3 , in operation, both embodiments of the contactor health monitor circuit  200  and  300  measure the differential voltage between L 1  and L 2  (nominally 240 volts) on the primary side of the contactor  140  at  11   p  and  12   p  and on the secondary side of the contactor  140  at  13   s  and  14   s . These measurements are made in both the unloaded (zero current) and the loaded (with current). Both the unloaded and loaded measurements at the primary side and the secondary side are made with the contactor  140  closed. The unloaded measurements are made before current is applied to a load (battery) within the vehicle (not shown). 
     The difference between the secondary unloaded and the secondary loaded voltages is subtracted from the difference between the primary unloaded and the primary loaded voltages. This number is divided by the load current to produce the resistance R c  of the contactor  140 . Note that the resistance R c  of the contactor  140  is the total resistance for both L 1  and L 2  paths through the contactor  140 . 
     Based on this approach, the method for determining contactor impedance R c  may be implemented as follows. Record the contactor  140  no load AC voltages on the primary and secondary, Vp_NL and Vs_NL, respectively. Monitor the AC current and wait for a maximum current value to be obtained. Record the contactor  140  loaded AC voltages on the primary and secondary, Vp_L and Vs_L, respectively, and the loaded AC current value A_L. Use the calculation below to determine the contactor impedance, where
         PD=Primary Difference   SD=Secondary Difference   CD=Contactor Drop   R c =Contactor Resistance
 
 PD=Vp   —   NL−Vp   —   L  volts
 
 SD=Vs   —   NL−Vs   —   L  volts
 
 CD=PD−SD  volts
 
 R   c   =CD/A   —   L ohms  
       

     These steps may be performed several times and the results averaged for many readings. 
     It is worthy to note that any reference to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment may be included in an embodiment, if desired. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
     The illustrations and examples provided herein are for explanatory purposes and are not intended to limit the scope of the appended claims. This disclosure is to be considered an exemplification of the principles of the invention and is not intended to limit the spirit and scope of the invention and/or claims of the embodiment illustrated. 
     Those skilled in the art will make modifications to the invention for particular applications of the invention. 
     The discussion included in this patent is intended to serve as a basic description. The reader should be aware that the specific discussion may not explicitly describe all embodiments possible and alternatives are implicit. Also, this discussion may not fully explain the generic nature of the invention and may not explicitly show how each feature or element can actually be representative or equivalent elements. Again, these are implicitly included in this disclosure. Where the invention is described in device-oriented terminology, each element of the device implicitly performs a function. It should also be understood that a variety of changes may be made without departing from the essence of the invention. Such changes are also implicitly included in the description. These changes still fall within the scope of this invention. 
     Further, each of the various elements of the invention and claims may also be achieved in a variety of manners. This disclosure should be understood to encompass each such variation, be it a variation of any apparatus embodiment, a method embodiment, or even merely a variation of any element of these. Particularly, it should be understood that as the disclosure relates to elements of the invention, the words for each element may be expressed by equivalent apparatus terms even if only the function or result is the same. Such equivalent, broader, or even more generic terms should be considered to be encompassed in the description of each element or action. Such terms can be substituted where desired to make explicit the implicitly broad coverage to which this invention is entitled. It should be understood that all actions may be expressed as a means for taking that action or as an element which causes that action. Similarly, each physical element disclosed should be understood to encompass a disclosure of the action which that physical element facilitates. Such changes and alternative terms are to be understood to be explicitly included in the description. 
     Having described this invention in connection with a number of embodiments, modification will now certainly suggest itself to those skilled in the art. The example embodiments herein are not intended to be limiting, various configurations and combinations of features are possible. As such, the invention is not limited to the disclosed embodiments, except as required by the appended claims.