Method and apparatus for detecting leakage resistance in an electric vehicle

Abstract: A method and a circuit are described for measuring the leakage path resistance in an electric vehicle having an isolated high voltage traction battery and an auxiliary battery grounded to the vehicle. The circuit operates by periodically applying a selected excitation signal and comparing the voltage induced on an energy storage element by the excitation signal to a selected reference voltage during a selected time period. An alternate embodiment of the present invention provides a circuit which operates by applying a periodic excitation signal and comparing the phase shift of the voltage induced on an energy storage element to the phase shift of a signal derived from the excitation signal.

Description: The present invention relates to leakage path detection circuits, and more particularly to a method and apparatus for detecting leakage current through a leakage path in an electric vehicle having an isolated high voltage traction battery and an auxiliary battery grounded to the vehicle. Conventional electric vehicles are known to those skilled in the art to be equipped with a high voltage traction battery for use as a primary source of power and an auxiliary battery for use in powering various electrical components. For obvious design reasons, the high voltage traction battery is typically isolated from the vehicle chassis to prevent the formation of a current path when a person concurrently contacts the vehicle and one of the traction battery terminals. Even in this arrangement, however, it is still possible to cause a short circuit between the traction battery and the body of the vehicle due to the potential leakage of electrolyte from the battery or for other reasons. If such leakage does occur, a resistive path is developed through which leakage current may flow. Previous work in detecting resistive paths of the type referenced above have involved deliberately providing a return path for current to flow through the resistive path into the vehicle chassis and back to the traction battery. Unfortunately, these designs create the very resistance path that the test is intended to detect. Thus, if a person concurrently touches the vehicle and the traction battery during the test, a resistive path may develop, and result in the unwanted flow of current. The other disadvantage known to those skilled in the art is that the mechanical contacts historically used in prior art leakage path detection devices become dirty and require regular servicing. See, for example, U.S. Patent No. 3,696,367 issued to Inamdar on October 3, 1972. Inamdar discloses a ground fault detection circuit for use with dual voltage battery systems, comprising first and second ground connected circuits which are alternatively energised depending on the location of a ground fault in any one of three lines connected to the system. Those skilled in the art will recognise that Inamdar physically connects the high voltage and low voltage batteries used by the electric vehicle. See, also, U.S. Patent No. 3,786,466 issued to Naito, et al., on January 15, 1974. Naito discloses a leakage detecting circuit for detecting leakage from a source of power, such as a battery, to a source of reference potential. The circuit comprises a pair of voltage detecting circuits respectively connected to the positive and negative terminals of the battery to ground through first and second switching means. As disclosed by Naito, the battery is insulated from ground so that if a leakage path exists between the battery and ground, an electrical circuit is closed which includes a voltage detection device and one of the switching means. Conventional electrical leakage detecting devices such as those referenced above have proven inoperable due to their inability to detect leakage current without creating the very leakage situation for which they are designed to detect. WO 91/19205 A, from which the respective preamble of Claims 1, 2, 8 and 9 is derived, is concerned with an apparatus for detecting a resistance between a vehicle battery terminal and, more particularly, to an apparatus for detecting the resistance between both the negative and positive battery terminals and the vehicle frame, and producing a command signal responsive to the resistance reaching a predetermined level. The apparatus includes a microprocessor for producing a clock signal. A charging circuit receives the clock signal, responsively charges a first capacitor to a voltage level determined by the frame resistance and produces a responsive capacitor voltage signal. The magnitude of this is compared with a reference frame voltage signal by means of a comparator. If the frame resistance is less than a predetermined level, then the capacitor voltage signal is greater than the frame voltage signal and the comparator produces a command signal indicating a fault condition. The apparatus of WO 91/19205 A does not disclose a number of features in the invention to be described. It is an object of the present invention to overcome the problems of the prior art leakage detection devices by providing a method and apparatus for detecting leakage current through a leakage path in an electric vehicle by relying on the known operating characteristics of an RC circuit. As disclosed herein, the circuit arrangement of the present invention relies on attempting to store energy in a plurality of energy storage devices which are designed to resist breakdown under high differential voltage. According to a first aspect of the present invention, there is provided a leakage path detection circuit for detecting leakage current through a leakage path in an electric vehicle, the circuit comprising: first voltage storage means having a first terminal and a second, grounded terminal, excitation means for periodically applying a selected excitation signal to the first terminal of the first voltage storage means to induce a voltage to be stored on the first voltage storage means; comparator means having a first input in electrical contact with the first terminal of said first voltage storing means and a second input for a selected DC reference voltage, the comparator means being effective to compare said voltage stored by said first voltage storage means to the selected DC reference voltage during a selected time period and to generate an output signal indicating whether said stored voltage exceeds said selected DC reference voltage; second voltage storage means in electrical contact with the comparator means and protect means in electrical contact with said comparator means and the respective first terminal of said first voltage storage means for protecting said comparator means from voltage spikes; characterised in that the circuit, which is adapted for detecting leakage current in an electric vehicle having an isolated high voltage traction battery and an auxiliary battery grounded at one battery terminal to the vehicle, has the protect means in electrical contact with the excitation means; the input terminal of the second voltage storage means is adapted for receiving the excitation signal from the excitation means and is in electrical contact with the first input of the comparator means, the second voltage storage means has an output terminal in electrical contact with the traction battery, and the output signal from the comparator means is determinative of the leakage current of the leakage path. The disclosed method and apparatus for detecting leakage current are also applicable for use in co-operation with an AC power source. The invention thus additionally provides in accordance with a second aspect, a leakage path detection circuit for generating an output signal to detect leakage current through a leakage path in an electric vehicle, the circuit comprising: first voltage storage means having a first terminal and a second, grounded terminal; excitation means for periodically applying a selected excitation signal to the first terminal of the first voltage storage means to induce a voltage to be stored on the first voltage storage means; comparator means having a first input in electrical contact with the first terminal of said first voltage storage means and a second input for a selected reference signal; second voltage storage means in electrical contact with the comparator means; and protect means in electrical contact with said comparator means and the respective first terminal of said first voltage storage means for protecting said comparator means from voltage spikes; characterised in that the circuit, which is adapted for detecting leakage current in an electric vehicle having an isolated high voltage traction battery and an auxiliary battery grounded at one battery terminal to the vehicle, has the protect means in electrical contact with the excitation means; the said reference signal received by the second input of the comparator means is derived from the excitation signal; the input terminal of the second voltage storage means is adapted for receiving the excitation signal from the excitation means and is in electrical contact with the first input of the comparator means; the second voltage storage means has an output terminal in electrical contact with the traction battery; and the comparator means is adapted to generate the output signal to correspond to the phase shift between said voltage induced on said first voltage storage means and said reference signal during a selected time period. Further aspects of the invention are set forth in independent Claims 8 and 9 of the appended claims. In summary, the invention provides a method and apparatus for detecting leakage current by relying on the known operating characteristics of an RC circuit in combination with comparator means to compare the induced voltage to a signal derived from an excitation signal and generate an input signal indicating the leakage path resistance during a selected time period. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram of a prior art leakage detection system; Figure 2 is a schematic circuit diagram of one of the leakage path current detection circuit of the present invention; Figure 3 is a schematic circuit diagram of an alternate embodiment of the present invention; Figure 4 is a block diagram of the method steps of one embodiment of the present invention; and Figure 5 is a block diagram of the method steps of an alternative embodiment of the present invention; Figure 6 is a graphical schematic of the voltage induced on the first energy storage means of the present invention through application of a stepped voltage over time. With reference to Figure 1 of the drawings, there is shown a prior art leakage path detection circuit indicated generally by reference number 10. Prior art circuit 10 includes an isolated high-voltage traction battery 12 and an auxiliary battery 14. As shown in Figure 1, a leakage path is depicted by reference numeral 16 through use of a mechanical contact 18. Typically, auxiliary battery 14 is grounded along with the AC line 20 to the vehicle chassis 22. As referenced above, prior art system 10, by design, creates the very resistance path it is intended to detect. Thus, a person coming in contact with traction battery 12 concurrently with the vehicle chassis 22 may suffer a shock. Referring now to Figure 2, the leakage path detection circuit of the present invention is indicated generally by reference numeral 24. Detection circuit 24 includes a pair of first and second energy storage devices such as capacitors 26 and 28. Capacitor 26 is provided with an input terminal and a grounded output terminal. Similarly, capacitor 28 is provided with an input terminal and an output terminal in electrical contact with traction battery 12. There is further provided buffered excitation means 30 for periodically applying a selected signal 32 to the detection circuit 24. Comparator means such as operational amplifier comparator 34 is also provided in electrical contact with capacitors 26 and 28 for comparing the voltage induced on capacitor 26 to a selected DC reference voltage during a selected time period. Comparator 34 thereafter generates an output signal indicating whether the induced voltage exceeds the selected voltage. Finally, protect means generally designated by reference numeral 36 are provided in electrical contact with excitation means 30, comparator means 34 and the respective input terminals of capacitors 26 and 28 for protecting comparator 34 and excitation means 30 from voltage spikes. In the circuit design depicted in Figure 2, the output signal provided by comparator 34 is used to determine the resistance of the leakage path. Still referring to Figure 2, protect means 36 are shown to include, for example, pairs of resistors 38 and 40 and pairs of clamping diodes 42 and 44 for dissipating unwanted voltage spikes. Significantly, the leakage path detection circuit referenced above requires no moving mechanical contacts. Instead, the circuit relies on attempting to store energy in capacitor 26 which is designed to resist breakdown under high differential voltages. In operation, if a resistive path 46 exists anywhere between traction battery 12 and the auxiliary battery 14, capacitor 28 will be capable of storing energy according to the known equation Q = CV. Those skilled in the art will recognize that the energy stored in capacitor 28 will depend on the voltage across capacitor 26 and the amount of resistance in the resistive path 46 between traction battery 12 and the auxiliary battery 14 or vehicle chassis 22. The circuit of Figure 2 operates by periodically applying a selected excitation signal such as pulse 32 to the circuit. If there is no resistive path, the voltage across capacitor 26 will change according to the equation: for time t o < t < t 1 , where t o is the instant of excitation, t 1 is the time of the trailing edge of pulse 32, a is the amplitude of the excitation, R 1 is the sum of the values of resistors 38 and C 1 is the value of capacitor 26. If a resistive path does exist, the voltage across capacitor 26 will be changed more slowly due to the energy absorbing capacity of capacitor 28. In a DC design, comparator 34 thus compares the voltage induced on capacitor 26 to a selected DC reference voltage during a selected time period and generates an output signal indicating whether the induced voltage exceeds the selected voltage. In contrast, in an AC design, a comparator compares the voltage induced on a capacitor to a signal derived from a periodic excitation signal and generates an output signal indicating the phase shift between the induced voltage and the derived signal during a selected time period. See, for example, the alternative embodiment of a leakage path detection circuit shown in Figure 3. A periodic signal such as sine wave 52 is generated by buffered excitation means 54. Diode pairs 56 and 58, resistor pairs 60 and 62, and capacitors 64 and 66 are configured as in the embodiment of the invention presented in Figure 2. The buffered excitation signal is fed to the non-inverting input of comparator 68 and the voltage across capacitor 64 is coupled to the non-inverting input of comparator 70. As the leakage path resistance 72 varies, the phase difference between these two signals will vary in a predictable manner at the frequency of interest. Considering the angular frequency of excitation to be ω and considering the phase to be zero, the phase shift between the excitation signal and the voltage across capacitor 64 can be expressed as: where, and where R 1 represents the sum of the values of resistors 60, R 2 represents the resistance of the resistive path, C 1 represents the value of the capacitor 64 and C 2 represents the value of capacitor 66. Comparators 70 and 68 compare each of these signals to ground. The compared results are fed to exclusive OR (EXOR) gate 74 which yields a train of pulses whose width is proportional to the phase difference between the two signals. Thus, the actual phase difference can be ascertained by measuring the width of these pulses directly by analog or digital circuitry or alternatively, by a circuit which low pass filters the pulses to generate a D.C. level proportional to phase shift. The deviation of this phase shift from a phase indicative of no leakage or acceptable leakage can thus be detected, and can be used to determine the leakage resistance. With reference to Figures 2-5, the method steps of the present invention will be described in further detail. These steps include the provision of first and second energy storage means. The first energy storage means includes an input terminal 47 and a grounded output terminal 48. The second energy storage means includes an input terminal 49 and an output terminal 50 in electrical contact with the traction battery 12. By periodically applying a selected excitation signal to the first and second energy storage means such as capacitors 26 and 28, a voltage is induced on capacitor 26. This induced voltage is thereafter compared (in a DC design) to a selected DC voltage and an output signal is thereafter generated indicating whether the induced voltage exceeds the selected voltage during a selected time period. This output signal is used to determine the resistance of the leakage path. In an AC design, the voltage induced on capacitor 64 is compared to a signal derived from the excitation signal and an output signal is thereafter generated indicating the phase shift between the induced voltage and the derived signal during a selected time period. Figure 6 shows four time plots 82, 84, 86 and 88 of the circuit of Figure 2 where the leakage path resistance is set to infinity, 380kΩ, 38kΩ, and 3.8kΩ, respectively. R 1 is set to 10kΩ, C 1 is set to 1µF and C 2 is set to 10µF. The input voltage is stepped from zero volts to five volts at t = 1 msec as shown in plot 80. For the electric vehicle, ten milliamps has been generally recognized by those skilled in the art to be the maximum amount of leakage current. This current correlates to a resistive path of approximately 38k ohms. As can be seen 20 milliseconds after the step voltage is applied, the capacitor 28 has approximately 4.32 volts if no resistive path exists and approximately 3.6 volts in the 38k path exists. This difference can be detected and used for the leakage detection as referenced above. It should be noted that in the particular application described herein, 3.6 volts has been selected as the threshold voltage for comparison. Those skilled in the art, however, will recognize that any voltage may be selected in accordance with the amplitude of the excitation voltage, the objectives of the leakage path to be detected and the circuit used therein.

Claims: A leakage path detection circuit for detecting leakage current through a leakage path in an electric vehicle, the circuit comprising: first voltage storage means (26) having a first terminal (47) and a second, grounded terminal (48), excitation means (30) for periodically applying a selected excitation signal to the first terminal (47) of the first voltage storage means (26) to induce a voltage to be stored on the first voltage storage means (26); comparator means (34) having a first input in electrical contact with the first terminal (47) of said first voltage storing means (26) and a second input for a selected DC reference voltage, the comparator means being effective to compare said voltage stored by said first voltage storage means (26) to the selected DC reference voltage during a selected time period and to generate an output signal indicating whether said stored voltage exceeds said selected DC reference voltage; second voltage storage means (28) in electrical contact with the comparator means (34) and protect means (36) in electrical contact with said comparator means (34) and the respective first terminal (47) of said first voltage storage means (26) for protecting said comparator means (34) from voltage spikes; characterised in that the circuit, which is adapted for detecting leakage current in an electric vehicle having an isolated high voltage traction battery and an auxiliary battery grounded at one battery terminal to the vehicle, has the protect means (36) in electrical contact with the excitation means (30); the input terminal (49) of the second voltage storage means (28) is adapted for receiving the excitation signal from the excitation means (30) and is in electrical contact with the first input of the comparator means (34), the second voltage storage means (28) has an output terminal (50) in electrical contact with the traction battery, and the output signal from the comparator means (34) is determinative of the leakage current of the leakage path. A leakage path detection circuit for generating an output signal to detect leakage current through a leakage path in an electric vehicle, the circuit comprising: first voltage storage means (64) having a first terminal and a second, grounded terminal; excitation means (54) for periodically applying a selected excitation signal to the first terminal of the first voltage storage means (64) to induce a voltage to be stored on the first voltage storage means (64); comparator means (68,70) having a first input in electrical contact with the first terminal of said first voltage storage means (64) and a second input for a selected reference signal; second voltage storage means (66) in electrical contact with the comparator means (68,70); and protect means (56,58,60,62) in electrical contact with said comparator means (68,70) and the respective first terminal of said first voltage storage means (64) for protecting said comparator means (68,70) from voltage spikes; characterised in that the circuit, which is adapted for detecting leakage current in an electric vehicle having an isolated high voltage traction battery and an auxiliary battery grounded at one battery terminal to the vehicle, has the protect means (56,60) in electrical contact with the excitation means (54); the said reference signal received by the second input of the comparator means (68,70) is derived from the excitation signal; the input terminal of the second voltage storage means (66) is adapted for receiving the excitation signal from the excitation means (54) and is in electrical contact with the first input of the comparator means (68,70); the second voltage storage means (66) has an output terminal in electrical contact with the traction battery; and the comparator means (68,70) is adapted to generate the output signal to correspond to the phase shift between said voltage induced on said first voltage storage means (64) and said reference signal during a selected time period. A circuit as claimed in claim 1 or 2, wherein said first voltage storage means (26, 64) comprises a capacitor. A circuit as claimed in claim 1 or 2, wherein said second voltage storage means (28,66) comprises a capacitor. A circuit as claimed in claim 1 or 2, wherein said selected time period is 20ms. A circuit as claimed in claim 1, wherein said selected reference voltage is a reference voltage derived from the level of said excitation signal. A circuit as claimed in claim 1 or 2, wherein said excitation means (30,54) is a switch capable of alternately applying the excitation signal and grounding the circuit. A method of detecting leakage current through a leakage path in an electric vehicle, the method comprising: employing excitation means (30) for periodically applying a selected excitation signal which induces a voltage to be stored on first voltage storage means (26) having a first terminal (47) and a second, grounded, terminal (48); supplying a selected DC reference voltage; employing comparator means (34) having a first input in electrical contact with the first terminal (47) of said first voltage storing means (26) and a second input for the selected DC reference voltage to compare said voltage stored by said first voltage storage means (26) to the selected DC reference voltage during a selected time period and to generate an output signal indicating whether said stored voltage exceeds said selected DC reference voltage; providing protect means (36) to protect said comparator means (34) from voltage spikes; and providing second voltage storage means (28) in electrical contact with the comparator means (34); characterised by ; applying the method to a vehicle having an isolated high voltage traction battery and an auxiliary battery, the auxiliary battery being grounded at one battery terminal to the vehicle; putting the protect means (36) in electrical contact with the excitation means (30); putting the input terminal of second voltage storage means (28) in electrical contact with the first input of the comparator means (34), said input terminal of said second voltage storage means (28) being adapted for receiving a voltage applied by said excitation means (30); providing the second voltage storage means (28) with an output terminal (50) in electrical contact with the traction battery; and using the output signal from the comparator means (34) to determine the leakage current of the leakage path. A method for generating an output signal to detect leakage current through a leakage path in an electric vehicle, the method comprising: employing excitation means (54) for periodically applying a selected excitation signal which induces a voltage to be stored on the first voltage storage means (64) having a first terminal and a second, grounded terminal; supplying a reference signal; employing comparator means (68,70) having a first input in electrical contact with the first terminal of said first voltage storage means (64) and a second input for the reference signal to compare said voltage induced on said first voltage storage means (64) to the reference signal; providing protect means (56,58,60,62) to protect said comparator means (68,70) from voltage spikes; and providing second voltage storage means (66) in electrical contact with the comparator means (68,70); characterised by ; applying the method to a vehicle having an isolated high voltage traction battery and an auxiliary battery, the auxiliary battery being grounded at one battery terminal to the vehicle; putting the protect means (56,60) in electrical contact with the excitation means (54); deriving the reference signal provided to the second input of the comparator means (68,70) from the excitation signal; putting the input terminal of the second voltage storage means (66) in electrical contact with the first input of the comparator means (68,70), said input terminal of the second voltage storage means (66) being adapted for receiving the excitation signal from the excitation means (54); providing the second voltage storage means (66) with an output terminal in electrical contact with the traction battery; and generating the output signal from the comparator means (68,70) to correspond to the phase shift between said voltage induced on the first voltage storage means (64) and said reference signal during a selected time period.

Inventors: GALE ALLAN ROY, GALE, ALLAN ROY
Applicants: FORD MOTOR CO, FORD MOTOR COMPANY
Application year: 1993
Main classification: G01R27
IPC codes: G01R31