Electricity: motive power systems – Constant motor current – load and/or torque control
Reexamination Certificate
1999-05-19
2001-11-13
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Constant motor current, load and/or torque control
C318S254100, C318S434000, C318S430000, C318S609000, C318S610000, C388S815000
Reexamination Certificate
active
06316893
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of adaptively compensating for load changes, particularly in connection with an electric motor drive which comprises a motor, the shaft of the motor being connected to a load having a known moment of inertia; a device, such as an inverter, to control the motor, a torque controller being arranged in connection with the device; and a speed controller arranged to control the speed of the motor.
The present electric motor drives enable a load to be controlled with great precision. For instance, the angular speed or the torque that is acting on the shaft of the motor can be made to keep the desired value quite accurately. Particularly in speed-controlled groups provided with several motors that are interconnected in one way or another, it is extremely important to keep the angular speed of each motor of the group under strict control. Such a speed-controlled group can be a cylinder group of a paper machine, for example. The speed of the rotating rolls of the cylinder group should be controlled such that the speed of the paper web running between the rolls remains the same at each roll.
When a speed-controlled group is subjected to a load change, the speed of the rotating rolls changes from its set value, in which case the speed controller of the motor drive reacts and, eventually, corrects the speed by returning it to the set value. In connection with paper machines, load changes affecting the web speed are caused for instance by cleaning doctor blades of cylinders pressing against the rolls, blades of coating stations pressing against backing rolls, and various closing nips, i.e. rolls that are pressed against each other. When the load of a motor drive increases, for instance when a cleaning doctor is pressed against a roll, the speed of the drive tends to drop. In the opposite situation, in other words when, for example, the cleaning doctor departs from the surface of the roll, the speed of the drive tends to increase. In connection with paper machines, when the paper web runs between the rolls, abrupt uncompensated changes in speed also affect the web tension of the paper in the machine direction.
Previously known attempts to compensate for predictable load changes employ a method of adding an experimentally determined additional command to the torque commands of an electric motor drive at a suitable moment. Such a procedure for compensating for load changes is, however, rather inefficient since it is difficult to determine a suitable additional torque command. Furthermore, load changes vary in magnitude and time of occurrence owing to mechanical wear and changing delays.
BRIEF DESCRIPTION OF THE INVENTION
An object of the present invention is to provide a method which avoids the disadvantages described above and enables load changes to be compensated in a more reliable manner. This object is achieved by the method of the invention, the method being characterized by
determining, during the load change, an estimate (M
h
) for an external torque disturbance which causes the load change, and
employing the torque disturbance estimate (M
h
) determined during the previous load change for compensating for the external torque disturbance in controlling the torque of the motor.
The method of the invention is based on the idea that during a load change, it is possible to determine a torque compensation which can be utilized in connection with the next load change to compensate for a corresponding torque disturbance. The advantages of the invention are fast and reliable adaptation to predictable load changes, and the reliable load change compensation provided by the method.
The invention further relates to an arrangement for adaptively compensating for load changes, particularly in connection with an electric motor drive which comprises a motor, the shaft of the motor being connected to a load having a known moment of inertia; a device, such as an inverter, to control the motor, a torque controller being arranged in connection with the device; and a speed controller arranged to control the speed of the motor, the speed controller receiving as its input a difference of a speed command and an actual value of the speed of the motor from an adder unit. The arrangement is characterized by comprising a compensator arranged to determine a torque compensation for compensating for an external torque disturbance on the basis of a torque disturbance to which the load is subjected, an output torque of the device controlling the motor, and a moment of inertia and an angular speed of the load.
By means of such an arrangement, the advantages of the method of the invention can be achieved by a simple and reliable structure.
REFERENCES:
patent: 5263113 (1993-11-01), Naitoh et al.
patent: 5467004 (1995-11-01), Matsuo et al.
patent: 5545957 (1996-08-01), Kudo et al.
patent: 5650700 (1997-07-01), Mutoh et al.
patent: 5666034 (1997-09-01), Seoung et al.
patent: 5994868 (1999-11-01), Takeuchi et al.
ABB Industry OY
Dykema Gossett PLLC
Nappi Robert E.
Smith Tyrone
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