Electricity: motive power systems – Positional servo systems – Antibacklash systems
Patent
1988-08-04
1990-04-10
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
Antibacklash systems
318677, 318600, 318567, 318603, G05B 1101
Patent
active
049163752
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a servomotor control apparatus for controlling the positioning of a movable element of an industrial robot or NC machine tool.
DESCRIPTION OF RELATED ART
A fully-closed loop-type control circuit is illustrated in FIG. 3 as an example of a CNC servo-control circuit. In the Figure, an information processing circuit X includes a CPU and a memory and forms a command signal for a servomechanism Y. The servomechanism Y is constituted by a comparator circuit a for comparing the command signal and a fed back position signal, a servo drive circuit b to which are inputted an error signal obtained by the comparator circuit a and a fed back velocity signal, a servomotor c, a velocity detector d for detecting the velocity of the servomotor c, and a linear scale e for position detection mounted on a mechanical load f such as a table.
The characterizing feature of a fully-closed loop servo system is that an arrangement in which the machine body is incorporated in the positioning servo loop is realized by mounting the position detector on the mechanical load f of the servomotor.
FIG. 4 is a block diagram of a servo system of this type, in which K denotes position gain, k.sub.1 and k.sub.2 gains, T.sub.L disturbance, K.sub.T a torque constant, J.sub.m rotor inertia, and J.sub.L load intertia.
The relationship between the position command and torque command of the servo system shown in FIG. 4 can be expressed by representing an integration term (Z.sup.-1) by a transfer element subjected to a Z conversion (a pulse transfer function conversion), as shown in FIG. 5. In FIG. 5, MC(i) indicates a move command formed by an NC apparatus, VEL(i) indicates a servomotor velocity signal, and POS(i) represents an amount of movement of the machine in a sampling period T.
Besides using this fully-closed loop system, it is also possible to carry out positioning control by a semi-closed loop-type servo-control system which extracts a position signal from a servomotor output shaft in front of the mechanical load or from a ball screw shaft coupled to the servomotor output shaft. With this system, a comparatively inexpensive resolver, pulse encoder or the like is used as a position detecting element and it is possible to realize dynamic precision. However, since a mechanism-related error such as feed screw pitch error has a direct influence upon control precision, static precision with regard to position is poor in comparison to dynamic precision. The type of, servo-control system that is used is determined by the user depending upon the type of position detecting element employed and to what degree of accuracy the mechanical load is to be controlled. However, there are many cases in which an NC apparatus will make common use of both systems.
When the rotating direction of a servomotor changes in response to a command for changing the direction of movement of a table or arm, a response delay regarding the position command occurs in accordance with the servomotor velocity-torque characteristic shown in FIG. 2. Response delays occurs because each axis of the mechanical load possesses a control error element ascribable to backlash or friction. In other words, the rotation of the servomotor of a predetermined axis approaches a stopped state when there is a reversal in the direction of rotation, and torque is no longer proportional to velocity before and after the operating quadrant reverses.
With the semi-closed loop servo-control system, a backlash command from the NC apparatus side is inputted to the servomechanism together with the position command, thereby correcting the servomotor position signal to improve upon the response delay when the direction of rotation reverses. However, as described in connection with FIG. 3, the fully-closed loop servo-control system is arranged so that the mechanical load is included in the positioning servo loop, and a backlash component is contained in the position signal obtained from the linear scale e. As a result, the servo-control system position signal
REFERENCES:
patent: 4196380 (1980-04-01), Gustavsson et al.
patent: 4743822 (1988-05-01), Futami et al.
patent: 4743823 (1988-05-01), Fujita
patent: 4792738 (1988-12-01), Yamazaki et al.
Kurakake Mitsuo
Sakamoto Keiji
Bergmann Saul M.
Fanuc Ltd.
Shoop Jr. William M.
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