THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE
NATURE OF THE INVENTION

GROUND FAULT PROCESSING CIRCUIT
A) TECHNICAL FIELD
[0001] The present invention generally relates to ground fault processing system used to switch-off starter / generator in case of generation of ground fault. The present invention is used in any product comprising of starter/generator or differential inputs.
B) BACKGROUND OF THE INVENTION
[0002] At present the ground fault processing device has a ground fault sensor. The output of the sensor is differential in nature and is fed to a ground fault processing circuit. The output of the ground fault processing circuit is compared with a threshold to switch off the starter/ generator. Moreover the setting of the thresholds with respect to the different modes of operation of the starter / generator is very difficult.
[0003] The US Patent number 5945802 discloses a ground fault detection device and protection method for a variable speed alternating current (AC) electric motor. The device used for detecting the presence of a ground leakage path in an electric motor system includes an AC has a pair of serially connected switching devices coupled between positive and negative direct current (DC) voltage buses for each phase of the motor. The motor receives power from an inverter provided with a current detector for sensing the current in the negative voltage bus. A gating pulse is applied to each switching device during the energization of the motor so that a ground leak path is

detected when a current is produced at the negative voltage bus due to the applied gating pulse.
[0004] The US patent number 5386183 discloses a method and device for sensing a ground fault in a motor control system. A motor control unit provided for monitoring the currents in each phase of the motor to output values representative of each phase current, has a contactor for opening and closing each phase. The motor control unit regulates the contactor based on the values representing each phase currents. The contactor is opened when a ground fault is detected at the motor.
[0005] The US patent number 5363047 discloses a ground fault detection device provided with a chare transfer variable oscillator. A pair of microprocessor based counters calculates difference between the current level measured with and without grounding center tap to compute a fault current dynamically.
[0006] Thus the currently available devices could not calculate threshold level for different modes of operation dynamically and easily. None of the currendy available ground fault processing devices has an analog to digital converter (ADC) to change the differential output and to modify the gain of the operational elements in the fault processing circuit to output unipolar values.
[0007] Hence there is a need to develop a ground fault processing circuit provided with an ADC and a simple circuit to convert differential signal into single ended signals. Also there is a need to develop a ground fault processing circuit to take care of both the positive and the negative imbalances in the input voltages, to modify the gain setting dynamically and to change the threshold levels easily.

C) OBJECTS OF THE INVENTION
[0008] The primary object of the present invention is to develop a ground fault processing circuit to process the ground fault signals and feed the processed signals to ADC for digitization.
[0009] Another object of the present invention is to develop a ground fault processing circuit to ensure zero output under normal operating conditions.
[0010] Yet another object of the present invention is to develop a ground fault processing circuit to ensure positive output even when the imbalance in the input voltages is negative.
[0011] Yet another object of the present invention is to develop a ground fault processing circuit to provide a linear positive output proportional to the input difference.
[0012] Yet another object of the present invention is to develop a ground fault processing circuit to ensure protection to starter/generator in the event of a ground fault.
[0013] Yet another object of the present invention is to develop a ground fault processing circuit to feed the output to ADC so that threshold can be set and changed in software.

[0014] Yet another object of the present invention is to develop a ground fault processing circuit to ensure a positive output to ADC as most of the ADCs used in processor are unipolar.
[0015] Yet another object of the present invention is to develop a ground fault processing circuit to set gains so that the circuit can be used for any differential input level.
[0016] Yet another object of the present invention is to develop a ground fault processing circuit to make use of the most commonly used components (COTS).
[0017] These and other objects and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.
D) SUMMARY OF THE INVENTION
[0018] The above mentioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and studying the following specification
[0019] The preferred embodiments of the present invention provides a ground fault processing circuit with an analog to digital converter (ADC) to change the differential output into unipolar output and to modify the gain of the functional elements in the processing circuit to set a threshold value. The output of the ADC is compared with the threshold set in software to trip a starter/generator.

[0020] According to one embodiment of the present invention, the ground fault processing circuit has a ground fault sensor to detect the imbalance in the input voltage levels and an analogue to digital converter (ADC). The differential inputs from the ground fault sensor are processed to obtain a unipolar output and is connected to the ADC. The output of the ADC is compared with a preset threshold value to detect and take a countermeasure against a ground fault.
[0021] The inputs from the ground fault sensor are summed using an adder. The output of the adder is passed through an inverter and a non-inverter that are connected back to back. The out put of the inverter and the non-inverter are connected using diodes and input in to the ADC. The output of the ADC is compared with a preset threshold to detect and to take corrective measure against a ground fault.
[0022] The outputs of the inverter and the non-inverter are two are diode-ORed, thereby ensuring a unipolar output to the ADC. The gain of the inverter and non-inverter can be adjusted to suit the different input voltage levels. The ground fault inputs are equal in magnitude but opposite in sign to begin with, thereby producing a zero output at the ADC. When a ground fault is present, the output of the processing circuit is changed based on the magnimde of the ground fault. The ADC converts the input into a digital value. A threshold value for detecting a ground fault is set using a software program. The output of the ADC is compared with the preset threshold value to detect and correct a ground fault. For example the different thresholds like IV or 3.7V difference at the input can be treated as faults. When the ADC output value exceeds the preset threshold, a preventive action is taken by the processor.

[0023] The Ground fault processing circuit is used to switch-off the starter/generator in the event of a ground fault. The output of the ground fault sensor is differential in nature and is fed to the ground fault processing circuit. The processing circuit consists of an inverter and a buffer that are connected back-to-back and isolated using diodes. The output of the processing circuit is fed to the ADC. The output of the processing circuit is always positive, since the single supply ADC (unipolar) embedded in a processor requires positive voltage inputs. The processing circuit inverts the input, when the input is negative. The processing circuit passes the input through a buffer, when the input is positive. The gain of the inverter and the buffer are adjusted to suit the different input voltage levels so that the gain is varied to match the input signal magnitudes. Under normal operating conditions, the differential inputs to the circuit are equal in magnitude but opposite in sign so that the input to the ADC will be 0 Volts. In the event of a ground fault, there is an imbalance in the input voltage levels. The difference in the inputs generates a non-zero input at the ADC. The ADC converts the input voltage to a corresponding digital value and estimates a threshold value to switch-off the generator. The thresholds are different in different modes of operation of the starter/generator and may be easily set using a software program. Thus the ground fault processing circuit of the present invention detects and analyzes faults of any signs and magnitudes.
[0024] Thus the various embodiments of the present invention provides a ground fault processing circuit with a simple circuit to convert the input differential signals into a single ended signal, and may be used in a variety of embedded systems employing an embedded ADC. The circuit protects an electrical system in the event of ground fault. The ground fault processing circuit of the present invention has a few components (COTS), thereby reducing the cost and size of the ground fault processing device.

[0025] These and other objects and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.
E) BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which:
[0027] FIGURE. 1 illustrates a schematic block circuit diagram of a ground fault processing circuit according to one embodiment of the present invention.
[0028] FIGURE. 2 illustrates a functional block diagram of a ground fault processing circuit according to one embodiment of the present invention.
[0029] FIGURE. 3 illustrates a block circuit diagram of a ground fault processing circuit according to one embodiment of the present invention.
[0030] Although specific features of the present invention are shown in some drawings and not in others. This is done for convenience only as each feature may be combined with any or all of the other features in accordance with the present invention.

F) DETAILED DESCRIPTION OF THE INVENTION
[0031] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which the specific embodiments which may be practiced is shown by way of illustration. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that the logical, mechanical and other changes may be made without departing from the scope of the embodiments. The following detailed description is therefore not to be taken in a limiting sense,
[0032] The present invention provides a ground fault processing circuit provided with an analog to digital converter to provide a unipolar output. According to one embodiment of the present invention, the ground fault processing circuit has a ground fault sensor to detect the imbalance in the input voltage levels and an analogue to digital converter (ADC). The differential inputs from the ground fault sensor are output to the ADC to obtain a unipolar output. The output of the ADC is compared with a preset threshold value to detect and take a countermeasure against a ground fault.
[0033] The differential inputs from the ground fault sensor are summed through an adder circuit. The inputs are summed through a resistor and are passed to a processing circuit including an inverter and a non-inverter/buffer that are connected back to back. The outputs of the inverter and the non-inverter/buffer are diode-ORed, thereby generating a unipolar output to the ADC. The gain of the inverter and non-inverter/buffer may be adjusted to suit the different input voltage levels. The imbalances in the input voltages are equal in magnitude but opposite in sign thereby producing a

zero output at the ADC. During the generation of the ground fauh, the output of the processing circuit is varied based on the magnitude of the ground fault. The ADC converts the input into a digital value. A threshold value for detecting the ground fault is estimated using a software application. The output of the ADC is compared with the preset threshold value to detect and correct the ground fault. For example different thresholds like IV or 3.7V difference at the input can be treated as faults. When the output digital value from the ADC exceeds a preset threshold level, a preventive action is taken by the processor.
[0034] The Ground fault processing circuit is used to switch-off the starter/generator in the event of a ground fault. The output of the ground fault sensor is differential in nature and is fed to the ground fault processing circuit. The processing circuit includes an inverter and a buffer that are connected back-to-back and isolated using diodes. The output of the processing circuit is fed to the ADC. The output of the circuit is always positive, since a single supply ADC (unipolar) embedded in a processor requires positive voltage inputs. The processing circuit inverts the input when the input is negative. The processing circuit passes the input through a buffer/ non-inverter, when the input is positive. The gain of the inverter and the buffer are varied to match the input signal magnitudes. Under normal operating conditions, the differential inputs to the processing circuit are equal in magnitude and opposite in sign. Under these conditions the input to the ADC will be 0 Volts. When a ground fault is generated, there is an imbalance in the input voltage levels and a difference exists between the inputs so that a non-zero input is passed to the ADC. The ADC converts the input voltage into a corresponding digital value. A preset threshold for switching -off the generator/ starter is estimated using a software application. The thresholds are different for different modes of operation of the

starter/generator and may be easily set using the software application. The ground fault processing circuit is able to detect and correct ground faults of any signs and magnitudes.
[0035] The FIG. 1 indicates a schematic block circuit diagram of the ground fault processing circuit. A ground fault sensor detects and measures the differential inputs. The inputs from the ground fault sensor are output to a processing circuit. The processing circuit has an inverter and a buffer/ non-inverter that are connected back to back. The inputs are summed through a resistor and are passed through an inverter and a non-inverter connected back to back. The outputs of the two are diode-ORed, thereby ensuring a unipolar output to the ADC. The gain of the inverter and non-inverter can be adjusted to suit the different input voltage levels. The inputs are equal in magnitude but opposite in sign to begin with, thereby producing a zero output at the ADC. During the generation of a ground fault, the output of the processing circuit is changed based on the magnitude of the generated ground fault. The ADC converts the input into a digital value. A preset threshold for switching -off the generator is set using a software application. For example, the different threshold values like IV or 3.7V difference at the input may be treated as ground faults. Whenever the output digital value from the ADC exceeds the preset threshold value, a preventive action is taken by the processor.
[0036] The FIG, 2 shows a functional block diagram of a ground fault processing circuit according to one embodiment of the present invention. A ground fault sensor detects and measures the differential inputs. The inputs from the ground fault sensor are output to a processing circuit. The processing circuit has an inverter and a buffer/ non-inverter that are connected back to back. The inputs are summed through a resistor and are passed through an inverter and a non-inverter connected back to back. The outputs of the two are diode-ORed, thereby ensuring a unipolar output to the ADC. The gain of the

inverter and non-inverter can be adjusted to suit the different input voltage levels. The inputs are equal in magnitude but opposite in sign to begin with, thereby producing a zero output at the ADC. During the generation of a ground fault, the output of the processing circuit is changed based on the magnitude of the generated ground fault. The ADC converts the input into a digital value. A preset threshold for switching -off the generator is set using a software application. For example, the different threshold values like IV or 3.7V difference at the input may be treated as ground faults. Whenever the output digital value from the ADC exceeds the preset threshold value, a preventive action is taken by the processor.
[0037] The FIG. 3 indicates a schematic block circuit diagram of the ground fault processing circuit according to one embodiment of the present invention. With reference to FIG. 3 the differential inputs from the ground fault sensor are summed using an adder. The ground fault processing circuit includes an adder formed by resistors Rl and R2. One end of the resistors Rl and R2 are connected to the inputs and the other end of the resistors are connected together. The adder circuit converts the differential inputs into a single ended output. The output of the adder circuit is fed to a processing circuit including the non-inverter/ buffer and an inverter that are connected back-to-back. The non-inverter is formed by OP-AMP UlA and the resistors R3, R4, and R5. Any input to the processing circuit (at the output of the adder circuit) will be applied with an appropriate gain. The gain of the circuit is controlled by the resistors R4 and R5. The inverter is formed by an OP-AMP UIB and the resistors R6 and R7. Any input to this circuit (at the output of the adder circuit) will be applied with appropriate gain and will appear negated at the output. The gain of the circuit is controlled by resistors R6 and R7. The out put of the inverter is amplified and inverted. The outputs of both the inverter and the non-inverter circuits are ORed using diodes Dl and D2, which allow only the

positive outputs to reach the ADC and block the negative outputs. The diodes are Schottky diodes with a low diode drop value. The diodes D3 and D4 provide protection to ADC by preventing the ADC input from exceeding the supply voltage.
[0038] The operation of the processing circuit will become more clear, when we consider the different input magnitudes. If the input VP > VN, then the output of adder circuit will be positive, the output of non-inverter will be positive, and that of inverter will be negative. The OR circuit allows the output of the non-inverter to pass through the ADC, while blocking the output of inverter. When the input VP < VN, then the output of adder circuit will be negative, output of non-inverter will be negative, and that of inverter will be positive. The OR circuit allows the output of the inverter to pass through ADC, while blocking the output of non-inverter. The input at ADC will be proportional to the difference in the input voltages (VP-VN) with appropriate gain and sign. Thus the circuit provides unipolar output irrespective of the sign of the input. The circuit works well for any common mode signal present on the differential inputs, as the common mode signal gets cancelled at the summer stage.
[0039] Thus the processing circuit provides a unipolar output irrespective of the sign of the input. The circuit works well for any common mode signal present on the differential inputs, as the common mode signal gets cancelled at the adder stage.
G) ADVANTAGES OF THE PRESENT INVENTION
[0040] Thus the various embodiments of the present invention provide a ground fault processing circuit to have balanced inputs under normal operating conditions. The ground fault processing circuit ensures protection to a starter/ generator during the

generation of a ground fault reliably and safely. The threshold level used for switching off starter/ generator and for taking a corrective action during the generation of the ground fault is set easily based on the mode of operation of a motor using a software application. The circuit provides a positive output to the ADC always as the ADC as most of the ADCs used in the processor are unipolar. Thus the circuit provides a unipolar output to the ADC always. The processing circuit detects voltages of both positive negative signs during the ground fault detection period. The gain setting of the inverter and the buffer may be set individually using the software application. The processing circuit is formed with the most commonly used components (COTS) such as resistors, diodes and amplifiers so that the cost and size of the processing circuit is reduced drastically. The circuit converts the differential signal into single ended signals and may be used in a variety of embedded systems using an embedded ADC.
[0041] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the present invention described herein and all the statements of the scope of the invention which as a matter of language might be said to fall there between.

CLAIMS
What is claimed is:
1. A ground fault processing circuit comprising:
A ground fault processing sensor to detect and measure difference in input voltage levels;
A processing circuit connected to said sensor to receive differential input signal levels and to output unified signal;
An analog to digital converter connected to said processing circuit to receive unified signal to output a digital value corresponding to input differential signal;
Wherein the output digital value from said analog to digital converter is compared with a preset threshold value to detect ground fault to switch off an electrical device.
2. The ground fault processing circuit according to claim 1, wherein the preset threshold value is calculated using a software application.
3. The ground fault processing circuit according to claim 1, wherein the preset threshold value is calculated corresponding to different modes of operation of said electrical device.
4. The ground fault processing circuit according to claim 1, wherein the electrical device is a starter.

5. The ground fault processing circuit according to claim 1, wherein the electrical de vice is a generator.
6. The ground fault processing circuit according to claim 1, wherein the analog to digital converter is an embedded type analog to digital converter.
7. The ground fault processing circuit according to claim 1, wherein the analog to digital converter outputs a digital value which is proportional to the difference in input voltage levels.
8. The ground fault processing circuit according to claim 1, wherein said processing circuit has an inverter and a non inverter connected back to back to back to provide a unipolar signal regardless of the sign of the input differential signal.
9. The ground fault processing circuit according to claim 1, further comprises an adder circuit to sum the input signals from said sensor before being output to said processing circuit,
10. The ground fault processing circuit according to claim 9, wherein the adder
circuit adds the differential input signals from said sensor to output a single
ended input signal to said processing circuit.
11. The ground fault processing circuit according to claim 1, further comprises a OR circuit to convert the output of the processor circuit to output a unipolar signal to ADC.
12. The ground fault processing circuit according to claim 11, wherein the OR circuit outputs only a positive signal to the ADC regardless of the magnitude and sign of the input differential signal.
13. The ground fault processing circuit according to claim 1, wherein the gain of the inverter and the non inverter are varied based on the input differential signals.


Rakesh Prabhu,
Patent Agent, ALMT Legal,
No.2, Lavelle Road,
Bangalore-560001,
INDIA
To
The Controller of Patents, The Patent Office, At Chennai