Slewing-type working machine

1. A slewing-type working machine comprising:

a lower travel body;

an upper slewing body mounted on the lower travel body so as to be able to be slewed;

a hydraulic motor which is a drive source for slewing the upper slewing body;

a slewing electric motor connected to an output shaft of the hydraulic motor to generate regenerative power by utilization of the slewing of the upper slewing body;

a hydraulic pump which is a supply source for supplying to the hydraulic motor hydraulic oil for operating the hydraulic motor;

a slewing operation device to which an operation is applied to command slew driving and slew braking of the upper slewing body;

a control valve which is operated to control supply of hydraulic oil to the hydraulic motor and discharge of hydraulic oil from the hydraulic motor on the basis of the operation applied to the slewing operation device;

a brake valve which is connected to motor both-side lines connected to both sides of the hydraulic motor respectively to make a hydraulic brake action;

a communication valve configured to be switched between an open position in which the communication valve brings a line which is one of the motor both-side lines and is connected to an outlet side of the hydraulic motor into direct communication with a tank or a line which is the other of the motor both-side lines and is connected to an inlet side of the hydraulic motor, so as to bypass the control valve, and a close position in which the control valve blocks the communication;

a controller which outputs a communication-valve-switching command for switching the position of the communication valve, and a torque command for specifying a torque of the slewing electric motor and inputs the torque command into the slewing electric motor, wherein the controller includes an abnormal-switching detection section which detects occurrence of an abnormal switching in the communication valve and the controller performs:

(i) determining, as a target value, a pressure which would be generated in the hydraulic motor in case the communication valve was absent, or a torque determined based on the pressure, based on an operation state of the slewing operation device and a slewing state of the upper slewing body;

(ii) determining, as an actual value, a pressure actually generated in the hydraulic motor or a torque determined based on the pressure; and

(iii) outputting the torque command on the basis of a value obtained by subtracting the actual value from the target value.

2. The slewing-type working machine according to claim 1 , wherein the controller determines a motor outlet-side pressure which would be generated on an outlet side of the hydraulic motor if the communication valve was absent, based on a meter-out opening area of the control valve determined based on an amount of the operation applied to the slewing operation device and a flow rate in the hydraulic motor;

setting a target torque as the target value based on the motor outlet-side pressure;

calculating, as the actual value, an actual torque actually generated in the hydraulic motor; and

outputting a torque command for the slewing electric motor, on the basis of a torque obtained by subtracting the actual torque from the target torque.

3. The slewing-type working machine according to claim 1 , wherein the controller performs, at a time that a slewing direction commanded by the slewing operation device differs from an actual slewing direction, setting a target torque which is the target value based on a motor inlet-side pressure which would be generated on an inlet side of the hydraulic motor in case the communication valve was absent, calculating, as the actual value, an actual torque actually generated in the hydraulic motor from the motor inlet-side pressure and a motor outlet-side pressure, and outputting, as a torque command for the slewing electric motor, a torque obtained by subtracting the actual torque from the target torque.