In a method for detaching a frozen charge from an inner wall of a grinding tube of a tube mill, a motor of a drive system is operated in a first mode to rotate the grinding tube for grinding. In a second mode, the motor of the drive system is operated to remove the frozen charge from the inner wall of the grinding tube such that operation of the motor to effect a rotational movement of the grinding tube is repeatedly disturbed by a disturbance signal to cause a repeated disturbance within a variable time interval, with the time interval of the repeated disturbance being shortened.

In a method for detaching a frozen charge from an inner wall of a grinding tube of a tube mill, a motor of a drive system is operated in a first mode to rotate the grinding tube for grinding. In a second mode, the motor of the drive system is operated to remove the frozen charge from the inner wall of the grinding tube such that operation of the motor to effect a rotational movement of the grinding tube is repeatedly disturbed by a disturbance signal to cause a repeated disturbance within a variable time interval, with the time interval of the repeated disturbance being shortened.

A method of designing a control device that controls a controlled variable of a plant to a set point, the control device including a feedback control system for outputting a manipulated variable of the plant based on an output of a feedback controller and a disturbance estimation value, including a step of selecting one of a first order transfer function, a second order transfer function, a third order transfer function, a first order and time delay transfer function, a second order and time delay transfer function, and a third order and time delay transfer function as a transfer function of the nominal plant in accordance with characteristics of the plant; and a step of determining a transfer function of the feedback controller based on a gain and time constant of the nominal plant.

Disclosed is a system comprising: a plant (P) to be controlled; a controller (C); a first weight function (W1); a second weight function (W2); and a disturbance observer, wherein the first weight function (W1) receives a difference value between a disturbance (w) and an output (u.sub.DO) of the disturbance observer as an input value, the second weight function (W2) receives a difference value between the output value of the controller (C) and the output (u.sub.DO) of the disturbance observer as an input value, the plant (P) to be controlled receives, as an input value, the sum value of the disturbance (w) and the difference value between the output value of the controller (C) and the output (u.sub.DO) of the disturbance observer, and the controller (C) receives an output value of the plant (P) to be controlled as an input value.

A target generating unit outputs a target value signal of a control target, and a position encoding processing unit outputs a position detection signal of the control target. A PID compensator calculates a control amount for causing the control target to follow a target position based on a difference signal between the target value signal and the position detection signal, and outputs it to an adding unit. A disturbance estimation observer outputs the result of estimation of disturbance to the adding unit. The adding unit adds the control amount output of the PID compensator to the output of the disturbance estimation observer to calculate a total control amount. A storage unit stores a control amount during position control to a fixing position as an attitude difference correction amount, and the disturbance estimation observer performs disturbance estimation using a control amount obtained by subtracting a value stored in the storage unit during position control to a moving position from the total control amount.