Electricity: motive power systems – Positional servo systems – 'reset' systems
Patent
1997-07-28
1999-08-10
Ip, Paul
Electricity: motive power systems
Positional servo systems
'reset' systems
318610, G05D 300, G05D 312
Patent
active
059363690
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of estimating a disturbance load externally exerted on a servomotor for driving a feed shaft of a machine tool, a robot arm, etc.
BACKGROUND ART
In a control system of a servomotor for driving a feed shaft of a machine tool, a robot arm, etc., there is known a method of preventing damage to a mechanical system by estimating a force externally exerted on the servomotor by a disturbance estimation observer to monitor the estimated value. FIGS. 5a and 5b (PRIOR ART) are block diagrams illustrating a principle of the conventional disturbance load estimation method. In FIG. 5a, a servomotor 10 receives a torque command (current command) Tc outputted from a control system which performs a proportional control with respect to position and a proportional-plus-integral control with respect to speed, and outputs an actual torque T. A movement of a movable member such as a feed shaft of a machine tool, a robot arm, etc, which is driven by the servomotor, is sometimes hindered for some reason such that the driven member collides with some other object and the tool is excessively pressed against a workpiece. Such a condition is equivalent to an occurrence of an external disturbance load torque Td exerted on the motor 10. Conventionally, a difference between an acceleration estimated based on the torque command Tc and an actual acceleration detected by a speed detector mounted on the motor 10 is obtained at a term 11, and this acceleration difference is converted into a torque value as an estimated disturbance load torque Td* at a torque conversion term 12.
FIG. 6A (PRIOR ART) is a block diagram of a conventional control system for obtaining an estimated disturbance load torque, and illustrates the case where the disturbance load is estimated by the acceleration comparison. In FIG. 6A, the servomotor control system is expressed by a term 8 for speed loop processing, a term 1 for current loop processing, and terms 2 and 3 for the transfer function of the servomotor 10, and an observer 40 for obtaining the estimated disturbance load torque Td* is expressed by terms 5, 6 and 7. The term 5 is a term for multiplying an actual current Ic to be supplied to the servomotor by a parameter .alpha. (quotient obtained by dividing a torque constant Kt of the motor by inertia Jm), the term 6 is a differential term for differentiating an actual speed v, and the term 7 is a term to be multiplied by a parameter 1/.alpha..
FIG. 6B (PRIOR ART) is a block diagram of a conventional system which embodies components of a control system for a machine tool. As illustrated, control apparatus 30 represents a control apparatus such as a numerical control apparatus (CNC) to control a machine tool. Control instructions are transferred between common memory 32 and then transferred between digital servo circuit 34. Digital servo circuit 34 includes a CPU for executing position, speed, and current control processing and outputs a torque command, i.e. current command. The torque command is amplified by servo amplifier 36 and then transferred to motor 38. Pulse coder 40 generates a predetermined number of feedback pulses per revolution of motor 38 to thereby feedback position and speed information to digital servo circuit 34.
In the servomotor control system shown in FIG. 6A, the torque command (current command) Tc is obtained at the term 8 for speed loop processing. At the term 1 for current loop processing, a voltage command is obtained based on the torque command Tc, and an actual current Ic corresponding to the voltage command is supplied to the servomotor, to thereby drive the servomotor. The observer 40 for estimating the disturbance load torque Tc obtains a difference between the value (Ic.multidot..alpha.) which is obtained at the term 5 by multiplying the actual current Ic by the parameter .alpha., and the value dv/dt which is obtained at the term 6 by differentiating the actual speed v of the motor, and this difference {(Ic.multidot..alpha.)-dv/dt} is multiplied by the parameter 1/.al
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Iwashita Yasusuke
Kawamura Hiroyuki
Fanuc Ltd
Ip Paul
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