Electricity: motive power systems – Positional servo systems
Patent
1990-07-25
1991-12-17
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
318561, 318603, 318616, G05B 501
Patent
active
050737465
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of controlling the speed of a servomotor, which method is capable of preventing the servomotor from generating pulsations at the time of rotating at low speeds.
2. Description of the Related Art
In various machines (such as an NC machine tool) using a servomotor as a drive source, it is known to control the rotation speed of the servomotor so as to drive an operating section of a machine (e.g., a tool table movable along the feed axis of a machine tool) at a predetermined speed. This type of machine generally comprises a servo system, which is arranged to detect an actual rotation speed of the servomotor based on a generation frequency of a feedback pulse supplied from an encoder each time the servomotor rotates through a predetermined rotation angle, and is arranged to supply a driving current to the servomotor in accordance with the deviation between the actual speed and a desired speed, to thereby rotate the servomotor at the desired speed.
Recently employed are software servo systems accommodating therein processors installed with speed control programs. A typical software servo system comprises a speed control loop section (FIG. 1), which is operable to subtract the product of an actual motor speed fb and a proportional gain k2 from an integrated value of a deviation between a speed command value VCMD and the value fb, to thereby derive a torque command Tqc. In FIG. 1, symbol 20 denotes an integration term whose transfer function is represented by k1/S (k1 indicates the integration gain). The transfer functions of the servomotor, represented by Kt and Jm, for a torque constant and inertia, respectively, are depicted in blocks 22 and 23.
In the conventional software servo system, feedback pulses supplied from the encoder are detected at intervals of a period equal to a speed control processing period, and a speed parameter (hereinafter referred to as estimated speed) indicative of the actual motor speed is derived by dividing the number of the pulses detected in one detection period by the same period. According to this detection method, however, the actual motor speed sometimes cannot be correctly represented by the estimated speed derived on the basis of the number of detected pulses.
For example, in the case that feedback pulses from an encoder, arranged to generate 10,000 pulses for each rotation of the servomotor, are detected at intervals of a period equal to a speed control processing period TS (e.g., 2 msec), 20,000 pulses are generated for each minute, i.e., two-thirds of a pulse are generated in each detection period when the servomotor is rotated at a low speed of twice per minute. In other words, one feedback pulse is detected in each of two detection periods among three successive detection periods and hence the calculated value of the estimated speed in these two detection periods is 1/TS, while no feedback pulse is detected in the remaining one period and the calculated value of the estimated speed in this period is "0" (FIG. 2). As a result, the estimated speed indicative of the actual speed fb significantly fluctuates between the value of "0" and the value of "1/TS". Therefore, according to the conventional speed control method, an error is likely to occur in the actual speed detection at the time of the low speed servomotor rotation during which a contribution of one feed back pulse to the detected number of pulses is large, and thus pulsations of the motor rotation are likely to occur. As a result, for instance, irregularities occur in the rotation of the feed axis of the machine tool driven by the servomotor, thereby causing vibration.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a speed control method for a servomotor, which method is capable of smoothly rotating the servomotor, without causing pulsations of the motor rotation even when the servomotor rotates at low speeds.
To achieve the above object, according to one aspect of the present invention, the
REFERENCES:
patent: 4266171 (1981-05-01), Mashimo
patent: 4503374 (1985-03-01), Sakano
patent: 4528491 (1985-07-01), Takeuchi et al.
patent: 4617635 (1986-10-01), Shimizu
Iwashita Yasusuke
Sakamoto Keiji
Fanuc Ltd.
Moskowitz Larry
Shoop Jr. William M.
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