1. A servo control device (1a, 1b, 1c) that generates, based on an operation detection signal representing a detected operating position or operating speed of a controlled object (2, 20) and an operation command given from a upper-level apparatus as a target value for the operating position or the operating speed of the controlled object, a driving force command, which is a command for generating a driving force given to a motor that drives a mechanical system of the controlled object, such that the operating position or the operating speed of the controlled object follows the operation command, the servo control device comprising:
a reference model unit (3, 21) that calculates, based on the operation command, a model operation signal representing a model operating position or model operating speed, with which an assumed model of the controlled object follows the operation command according to a predetermined characteristic, and calculates a model driving force, which is a driving force necessary for the assumed model of the controlled object to operate to coincide with the calculated model operation signal;
a variable-compensation calculating unit (4, 10, 22) that adds one or both of a value obtained by multiplying a first-order differential value of a control deviation, which is a deviation between the model operation signal and the operation detection signal, by a first-order control gain, which is a second control gain, and a value obtained by multiplying a second-order differential value of the control deviation by a second-order control gain, which is a third control gain, to a value obtained by multiplying the control deviation by a zeroth-order control gain, which is a first control gain, to calculate a variable compensation signal and outputs the variable compensation signal;
a gain changing unit (7, 14, 25) that changes, during an operation of the controlled object, a value of one or both of the first-order control gain and the second-order control gain used for calculation of the variable compensation signal by the variable-compensation calculating unit based on at least one of the model operation signal, the operation detection signal, and the driving force command and reflects the value on calculation of the variable compensation signal;
an integral compensator (5, 11, 23) that integrates the variable compensation signal; and
an adder (6, 12, 24) that adds an output of the integral compensator to the model driving force and outputs it as the driving force command.

2. The servo control device (1a, 1b, 1c) according to claim 1, wherein the gain changing unit (7, 14, 25) changes a value of the control gain based on the control deviation, which is a deviation between the model operation signal and the operation detection signal.
3. The servo control device (1a, 1b, 1c) according to claim 1, wherein, when the model operation signal and the operation detection signal are respectively signals of the operating position and the control deviation is a position deviation,
the gain changing unit (7, 14, 25) changes a value of the second-order control gain, and the variable-compensation calculating unit (4, 10, 22) uses, when the second-order
control gain is used for calculation of the variable compensation signal, the second-order
control gain, a value of which is changed by the gain changing unit.
4. The servo control device (1a, 1b, 1c) according to claim 1, wherein, when the model
operation signal and the operation detection signal are respectively signals of the operating
position and the control deviation is a position deviation,
the variable-compensation calculating unit (4, 10, 22) adds a value obtained by multiplying the first-order differential value of the control deviation by the first-order control gain and a value obtained by multiplying the second-order differential value of the control deviation by the second-order control gain to a value obtained by multiplying the control deviation by the zeroth-order control gain to generate the variable compensation signal, and
the gain changing unit (7, 14, 25) changes, based on a value obtained by dividing the second-order control gain by an inertial value of the controlled object (2, 20), a value of at least one control gain among the zeroth-order, first-order, and second-order control gains used for calculation of the variable compensation signal by the variable-compensation calculating unit.