Control system for hybrid construction machine

1. A control system for a hybrid construction machine, comprising:

a variable-capacity main pump;

an engine that drives the main pump;

an engine rotation speed controller that controls the rotation of the engine;

a generator;

a battery that stores power generated by the generator;

a variable-capacity sub-pump connected to a discharge side of the main pump and adapted to assist the main pump;

an assist control mechanism that controls the sub-pump to output a commanded assist output;

a storage that stores a coefficient table for an assist correction coefficient used to reduce an assist output of the sub-pump by controlling the assist control mechanism when the storage amount of the battery is below a threshold value, a coefficient table for an engine rotation speed correction coefficient used to increase the rotation speed of the engine when the storage amount of the battery is below the threshold value, and the threshold value for the storage amount of the battery; and

a controller that determines whether or not the storage amount of the battery is below the threshold value, and when the storage amount of the battery is below the threshold value, reduces the assist output of the sub-pump by controlling the assist control mechanism based on the assist correction coefficient, increases the discharge amount of the main pump by controlling the engine rotation speed controller based on the engine rotation speed correction coefficient and increasing the rotation speed of the engine, and increases an output of the main pump by increasing the rotation speed of the engine by as much as a reduction in the assist output of the sub-pump.

2. The control system according to claim 1 , wherein:

the storage stores a first threshold value and a second threshold value smaller than the first threshold value for the storage amount of the battery; and

the controller reduces the assist output of the sub-pump based on the assist correction coefficient when the storage amount of the battery is below the first threshold value and zeroes the assist output of the sub-pump based on the assist correction coefficient when the storage amount of the battery is reduced to the second threshold value.

3. The control system according to claim 1 , further comprising:

a circuit system that is connected to the main pump and includes a plurality of control valves;

a hydraulic motor connected to the main pump and adapted to rotate the generator; and

a neutral flow path in which oil discharged from the main pump flows when all the control valves in the circuit system are maintained at neutral positions;

wherein the discharge amount of the main pump is maintained at a standby flow rate by the action of a pilot pressure generated in the neutral flow path and the hydraulic motor causes generation of a standby regenerative power by the action of the standby flow rate when all the control valves of the circuit system are maintained at the neutral positions;

wherein the storage stores a table of a standby regeneration correction coefficient used to increase the standby regenerative power when the storage amount of the battery is below the threshold value; and

wherein the controller increases the rotation speed of the engine and the standby regenerative power by controlling the engine rotation speed controller based on the standby regeneration correction coefficient when the storage amount of the battery is below the threshold value and all the control valves of the circuit system are at the neutral positions.