A servo control device for a machine tool generates a command for an actuator in order that the position of a driven object such as a tool or a table provided in the machine tool follows the command value. There are various types of actuators for the servo control device such as a rotary motor, a linear motor, and a piezoelectric element.
A servo control, as executed by a servo control device for a machine tool, which drives a mechanical system in a mechanical device such that the position of a tool relative to a workpiece precisely follows an instructed trajectory in order to machine the workpiece into a designed shape, is referred to as “trajectory control” or “contouring-motion control”. These controls are executed accurately by using a numerical control device or a servo control device provided in a numerical control device. A mechanical system of a control-target mechanical device is provided with a plurality of shafts, each of which is controlled by a motor. Driving of the motor is controlled by using the servo control device.
In the servo control device, a response error is caused due to a disturbance such as a friction that occurs in the mechanical system, or vibrations of the mechanical structure. As a typical example, in a case where two servo control devices set as perpendicular to each other are used to instruct an arc trajectory, when the movement direction of a feed shaft is reversed at a point where the quadrant of the arc is changed across to another quadrant, a response error occurs. When the amount of this error is scaled-up in the radial direction and is plotted, then the trajectory has an outwardly protruding shape. Therefore, this error is referred to as “quadrant projection”. When a response error on the trajectories such as a quadrant projection is caused, this forms undesirably a blemish or scratch on the machined product. As another example, a response error is caused by mechanical vibrations generated when a motor is accelerated/decelerated. The mechanical vibrations are generated when a mechanical structure is vibrated by a driving force or a driving reaction force generated when a motor is accelerated/decelerated. When the mechanical vibrations are generated, a blemish is formed on the machined surface, or an uneven machined surface is undesirably formed thereby.
In correction of a response error due to a friction or vibrations, there is a frequently used error reducing method that estimates a generated error by using a model so as to input a necessary correction command for cancelling out the error. In order to perform this method, it is necessary to determine a model parameter by which the amount of generated response error is made equal to or smaller than the allowable value.
The linear approximation of an error model or the model order reduction is performed so as to implement the error model in a practical controller of the servo control device. Therefore, even when a parameter value is identified on the basis of measurement results, the amount of generated response error is not minimized in some cases. Accordingly, a control parameter adjustment device is needed which has a control parameter adjustment function of searching for or finely adjusting a model parameter such that the amount of response error caused in a servo control is made equal to or smaller than an allowable value.
For example, in Patent Literature 1, a servo control device has a model to input a correction torque in a step shape, when the motion direction is reversed, in order to correct a quadrant projection. The servo control device corrects a torque command and updates the correction torque repeatedly up until the amount of quadrant projection generated during arc motion becomes equal to or smaller than a threshold. A method for deciding an optimal parameter through this operation has been disclosed.
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