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A hybrid construction machine including an internal combustion engine, a motor generator, a hydraulically driven working element, and a rotation mechanism rotationally driven by a rotation electric motor includes a charge storage part provided between the motor generator and the rotation electric mo

A hybrid construction machine including an internal combustion engine, a motor generator, a hydraulically driven working element, and a rotation mechanism rotationally driven by a rotation electric motor includes a charge storage part provided between the motor generator and the rotation electric motor, wherein the charge storage part includes a constant voltage charge storage part configured to maintain its voltage value within a certain range; and a variable voltage charge storage part configured to tolerate a variation in the voltage value of the charge storage part due to the electric motor and the generator operation of the motor generator or the power running and the regenerative operation of the rotation electric motor.

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1. A hybrid construction machine, comprising: an internal combustion engine;a hydraulic pump connected to the internal combustion engine;a motor generator connected to the hydraulic pump;a working element driven by the hydraulic pump;a rotation mechanism rotationally driven by a rotation electric mo

1. A hybrid construction machine, comprising: an internal combustion engine;a hydraulic pump connected to the internal combustion engine;a motor generator connected to the hydraulic pump;a working element driven by the hydraulic pump;a rotation mechanism rotationally driven by a rotation electric motor; anda charge storage part connected to an inverter of the motor generator and an inverter of the rotation electric motor,wherein the charge storage part includes a constant voltage charge storage part configured to maintain a voltage value thereof within a certain range, the constant voltage charge storage part including a capacitor provided between the inverter of the motor generator and the inverter of the rotation electric motor;a step-up and step-down converter electrically connected to the constant voltage charge storage part; anda variable voltage charge storage part whose charging and discharging are controlled by the step-up and step-down converter. 2. The hybrid construction machine as claimed in claim 1, further comprising: a step-up and step-down drive control part configured to control switching of a voltage raising operation and a voltage lowering operation of the step-up and step-down converter,wherein the step-up and step-down converter has a first side thereof connected to the constant voltage charge storage part and has a second side thereof connected to the variable voltage charge storage part, the step-up and step-down converter being configured to raise or lower the voltage value of the constant voltage charge storage part by varying a voltage value of the variable voltage charge storage part. 3. The hybrid construction machine as claimed in claim 2, further comprising: a voltage value detecting part configured to detect the voltage value of the constant voltage charge storage part,wherein the step-up and step-down drive control part is configured to control the switching of the voltage raising operation and the voltage lowering operation based on the voltage value detected by the voltage value detecting part. 4. The hybrid construction machine as claimed in claim 2, wherein the step-up and step-down drive control part is configured to control the switching of the voltage raising operation and the voltage lowering operation in accordance with an operating state of the rotation electric motor so that the voltage value of the constant voltage charge storage part falls within the certain range. 5. The hybrid construction machine as claimed in claim 2, wherein the step-up and step-down drive control part is configured to provide a time of no operation in switching one to another of the voltage raising operation and the voltage lowering operation of the step-up and step-down converter. 6. The hybrid construction machine as claimed in claim 3, wherein: the step-up and step-down converter includes a step-up switching device for controlling a supply of electric power from the variable voltage charge storage part to the constant voltage charge storage part;a step-down switching device for controlling a supply of electric power from the constant voltage charge storage part to the variable voltage charge storage part; anda reactor connected to the step-up switching device and the step-down switching device, andthe step-up and step-down drive control part includes a voltage control part configured to control driving of the step-up switching device or the step-down switching device so that the voltage value of the voltage value detecting part becomes a voltage target value;a current control part configured to control driving of the step-up switching device or the step-down switching device so that a value of a current flowing through the reactor becomes a predetermined current threshold; anda control switching part configured to perform switching selectively to one of the voltage control part and the current control part so that a load on the step-up and step-down converter becomes less than or equal to a predetermined load. 7. The hybrid construction machine as claimed in claim 6, wherein the control switching part is configured to perform switching to a drive control by the current control part in response to an absolute value of the current flowing through the reactor becoming greater than the current threshold during performance of a drive control by the voltage control part. 8. The hybrid construction machine as claimed in claim 6, wherein the control switching part is configured to perform switching to a drive control by the voltage control part in response to the voltage value of the voltage value detecting part returning to the voltage target value during performance of a drive control by the current control part. 9. The hybrid construction machine as claimed in claim 6, wherein in switching one to another of the voltage control part and the current control part, the step-up and step-down drive control part is configured to correct an initial value of a control target value after the switching. 10. The hybrid construction machine as claimed in claim 2, wherein the step-up and step-down drive control part is configured to perform such control as to increase a charging or discharge current in a dead zone region of the step-up and step-down converter. 11. The hybrid construction machine as claimed in claim 2, wherein the step-up and step-down drive control part comprises: a main control part configured to calculate a PWM duty value for driving the step-up and step-down converter so that the voltage value of the constant voltage charge storage part follows a target voltage value;a compensation duty value calculating part configured to calculate a compensation duty value for compensating for the PWM duty value in a predetermined low current region in a current value characteristic of the step-up and step-down converter relative to the PWM duty value; anda combining part configured to perform a combining operation of combining the compensation duty value with the PWM duty value calculated by the main control part. 12. The hybrid construction machine as claimed in claim 11, wherein the main control part is configured to calculate the PWM duty value by PI control based on a deviation between the voltage value of the constant voltage charge storage part and the target voltage value, further comprises a replacing part configured to replace an integral component value included in the PWM duty value with an opposite of a proportional component value at a time of starting an activation of the combining operation. 13. The hybrid construction machine as claimed in claim 11, wherein the compensation duty value calculating part is configured to calculate, as the compensation duty value, a duty value corresponding to the PWM duty value at an inflection point on a step-up side or a step-down side in the current value characteristic of the step-up and step-down converter relative to the PWM duty value. 14. A method of controlling a hybrid construction machine including an internal combustion engine, a hydraulic pump connected to the internal combustion engine, a motor generator connected to the hydraulic pump, a working element driven by the hydraulic pump, a rotation mechanism rotationally driven by a rotation electric motor, and a charge storage part connected to an inverter of the motor generator and an inverter of the rotation electric motor and including a constant voltage charge storage part and a variable voltage charge storage part, the method comprising: maintaining a voltage value of the constant voltage charge storage part within a certain range, wherein the constant voltage charge storage part includes a capacitor provided between the inverter of the rotation electric motor and the inverter of the motor generator connected to the hydraulic pump connected to the internal combustion engine; andcontrolling charging and discharging of the variable voltage charge storage part by a step-up and step-down converter electrically connected to the constant voltage charge storage part.