Servo control device with function to reduce lowering during a braking process

Abstract

Servo control device (20) for controlling a servo motor (24) which drives a center-of-gravity axis (4) of a machine tool or an industrial machine (22), the servo control device comprising: a brake signal output section (26) configured to output a brake activation signal and a brake release signal to a mechanical brake (2) with a backstop (12) for holding the center-of-gravity axis; a torque command generation section (28) configured to generate a torque command value for controlling a position of the servo motor;and a torque limit generation section (30) configured to generate a torque limit for limiting the torque command value generated by the torque command generation section, wherein the torque limit generation section is configured to continuously decrease the torque limit from a first value greater than a torque corresponding to gravity to a second value less than the torque corresponding to gravity after the brake signal output section outputs the brake activation signal, wherein the center of gravity axis (4) begins to descend at the moment at which the torque limit falls below the torque corresponding to gravity, and the amount of descent of the center of gravity axis (4) is limited within a distance (L1) corresponding to a width of the dead travel (12).

Description

BACKGROUND OF THE INVENTIONThe present invention relates to a servo control device having a function of reducing an amount of lowering of a center-of-gravity axis when the center-of-gravity axis is decelerated.In a configuration in which a feed axis, etc., of a machine tool or an industrial machine is driven, in the case where an external force, in particular, gravity (hereinafter, such an axis is referred to as a "center-of-gravity axis") acts on an axis driven by a servomotor, the center-of-gravity axis may lower by gravity when the power to the servomotor is cut off and the center-of-gravity axis is in an uncontrolled state.As a technique of lowering a center-of-gravity axis, for example, JP 2003-131 701 A discloses a control device for a servo motor. The control device is configured to output a command for lifting a center-of-gravity axis of a machine driven by a servomotor by a predetermined distance and a command for braking the center-of-gravity axis by a braking device when the machine is in an emergency stop state or a power failure state.Further, JP 2010-215 369 A discloses a servo control system used in a turning operation machine such as a crane truck, wherein a load axle is driven by an electric motor through a plurality of transmission wheels. In order to reduce an influence due to backlash between the transmission wheels, the servo control system is configured to limit an output torque of the motor during passage of a predetermined period of time corresponding to the time required for moving a single transmission wheel by a backlash width when the running direction of the motor is reversed or decelerated.It is described in JP 2003-131 701 A that the object of the invention in this document is to prevent lowering or lowering of the center of gravity axis when the center of gravity axis is decelerated. Further, in JP 2003-131 701 A, when a means for stopping the center of gravity axis during the braking operation is to be switched from the motor controller to a mechanical brake, the center of gravity axis is previously raised by a distance larger than the amount of lowering before the mechanical brake is activated. Therefore, in JP 2003-131 701 A, the amount of lowering of the center-of-gravity axis is not reduced.In contrast, the object of the invention in JP 2010-215 369 A is to prevent vibration, noise, and reduce machine downtime due to backlash (flash), and is not to reduce the amount of lowering of the center-of-gravity axis. Further, in JP 2010-215 369 A, it is described that the output torque of the motor during passing through the predetermined period is limited according to the time required for the second transmission gear to move by the backlash width. However, this document does not describe in concrete terms to which value the torque is limited.US 5 692 733 A can be designed as a disclosure of a method for stopping a winch operated to wind a load up and down due to a torque command to an electric motor generated by a speed control circuit in accordance with a speed command by a speed command generating circuit. Upon stopping the apparatus, the engine control is terminated when the engine speed is zero after the torque command to the engine is brought to zero for a predetermined period of time, following the issuance of a brake command to an electromagnetic brake directly coupled to the engine.Summary of the InventionTherefore, an object of the present invention is to provide a servo control device having a function of reducing an amount of lowering of a center-of-gravity axis provided with a machine such as a machine tool or an industrial machine.According to the invention there is provided an apparatus according to the independent claim. Developments are set forth in the dependent claims.Brief Description of the DrawingsThe above and other objects, features and advantages of the invention will become more apparent from the following description of the preferred embodiments of the invention with reference to the accompanying drawings. The following are shown: FIG. 1 is a schematic configuration of a center-of-gravity axis and a mechanical brake to which the invention can be applied; FIGS. 2a-2d show the movement of the center of gravity axis and a friction plate at a lost motion section of the mechanical brake according to FIG. 1; FIG. 3 is a graph showing a time change in a torque of the servo motor, a speed and a position of the center of gravity axis when a conventional servo control device is used, with respect to the movement shown in FIGS. 2a to 2d; FIG. 4 is a block diagram showing a schematic configuration of a servo control device according to the invention and a schematic configuration of a machine including the center-of-gravity axis; and FIG. 5 is a graph showing a time change in a torque of the servo motor, a speed, and a position of the center-of-gravity axis when the servo control device of the invention is used.DETAILED DESCRIPTIONFIG. 1 is a view showing a schematic configuration of a mechanical brake with a backlash (backlash) to which the present invention can be applied. The mechanical brake 2 includes a friction plate 6 that is integrally rotatable with an axis (shaft) 4 of a machine tool or an industrial machine, such as a feed axis, etc., driven by a servo motor (not shown); and a brake pad 8 that is movable toward or away from the friction plate 6. Since the friction plate 6 is sandwiched between the brake pad 8, a predetermined holding force acts on the friction plate 6.On the other hand, the shaft 4 and the friction plate 6 are engaged with each other by a concave-convex structure as shown in an enlarged portion enclosed by a broken line 10. Therefore, when the friction plate 6 is stopped, the shaft 4 is also stopped. In this regard, there is a certain length of backlash 12 structurally between shaft 4 and friction plate 6 (or in the concave-convex structure shown). Furthermore, in addition to the backlash 12 between the shaft 4 and the friction plate 10, there may be another backlash between the brake pad 8 and a mount (not shown) for the brake pad 8.FIGS. 2a to 2d show the conventional movement of the shaft 4 and the friction plate 6 at a lost motion portion of the mechanical brake of FIG. 1, and FIG. 3 is a graph showing a time change in a torque of the servo motor, a speed and a position of the shaft with respect to the movement shown in FIGS. 2a to 2d. The shaft 4 is a center-of-gravity axis on which gravity acts. During an operation for stopping the shaft 4, a stopping operation using the torque of the servo motor (or by controlling the servo motor) is switched to a stopping operation using the mechanical brake 2. Specifically, first, as shown in FIG. 2 aand a portion (a) in FIG. 3, when the servo motor for driving the shaft 4 is stopped by the servo control, the shaft 4 is held at a certain position by the holding torque of the servo motor. On the other hand, the friction plate 6 is at a free position within the backlash 12, at which point, when the mechanical brake 2 is activated (or the brake pad 8 sandwiches the friction plate 6), the friction plate 6 is locked at the present position (or at the free position within the backlash), as shown in FIG. 2 bunder the portion (b) in FIG. 3.Next, in order to switch a device for stopping the shaft 4 from the servo motor to the mechanical brake 2, current to the servo motor is cut off (or servo control is cut off), and then the torque of the servo motor discontinuously becomes zero. Accordingly, the shaft 4 is accelerated downward within the backlash 12 due to gravity, whereby the shaft 4 descends or descends by a distance (for example, a distance L 1 in FIG. 3 ) that is not more than the width of the backlash, as shown in FIG. 2 cand a portion (c) in FIG. 3.In this regard, as exemplarily shown in FIG. 2 c, a convex portion 14 of the shaft 4 comes into contact with a backlash end portion (or a shoulder portion) 16, whereby the friction plate 6 can slip downward relative to the brake pad 8 (for example, the friction plate 6 continues to drop by a distance L 2 in FIG. 3 ). Accordingly, the shaft 4 further lowers. Due to such a movement, in the related art, as shown in FIG. 2 d and a portion (d) in FIG. 3, the center-of-gravity axis lowers by a relatively long distance when decelerated, whereby the accuracy of a machine tool or an industrial machine including the center-of-gravity axis can be lowered.Thus, a servo control device 20 according to the present invention has a configuration shown in FIG. 4. The servo control device 20 is configured to control a servo motor 24 for driving a center-of-gravity axis 4, such as a feed axis, provided on a machine 22, such as a machine tool or an industrial machine on which gravity acts. The servo control device 20 includes a brake signal output section 26 configured to output a brake activation signal for activating the brake 2 and a brake release signal for releasing the brake 2 to the mechanical brake 2 for holding (stopping) the center-of-gravity axis 4; a torque command generation section 28 that generates a torque command value for controlling the position of the servo motor 24; and a torque limit value generation section 30 that generates a torque limit value for limiting the torque command value generated by the torque command generation section 28 (i.e., the torque limit value is an upper limit that does not exceed the torque command value). The torque limit generating section 30 continuously decreases the torque limit from a first value larger than a torque corresponding to the gravity force (i.e., a torque acting on the center-of-gravity axis due to the gravity force) to a second value smaller than the torque corresponding to the gravity force after the brake signal output section 26 outputs the brake enable signal.Next, with reference to FIG. 5 similar to FIG. 3, the function of the servo control device 20 of the invention will be described. As has been described using Figs. 2a to 2d, the shaft 4 is the center of gravity axis on which the gravity acts. During an operation for stopping the shaft 4, a stopping operation using the torque of the servo motor (or by controlling the servo motor) is switched to a stopping operation using the mechanical brake 2. Specifically, first, as shown in FIG. 2 aand in the section (a) in FIG. 5, when the servomotor 24 for driving the shaft 4 is stopped by the servo control, the shaft 4 is held at a certain position by the holding torque of the servomotor 24. On the other hand, the friction plate 6 is at a free position within the backlash 12, at which point, when the mechanical brake 2 is activated (or a brake pad 8 sandwiches the friction plate 6), the friction plate 6 is locked at the present position (or at the free position within the backlash) as shown in FIG. 2 band in the section (b) in FIG. 5.Next, in the invention, in order to switch a means for stopping the shaft 4 from the servomotor 24 to the mechanical brake 2, without cutting off the current to the servomotor 24 (or cutting off the servo control), the torque limit value generating section 30 continuously (or monotonically) decreases the torque limit value for limiting the torque command value of the servomotor 24 from a first value larger than a torque corresponding to the gravity acting on the shaft 4 to a second value smaller than the torque corresponding to the gravity.In the example of FIG. 5, the torque corresponding to the first value is equivalent to a torque (or a holding torque) by which the shaft 4 does not drop due to the torque corresponding to the gravity force even when the brake 2 is not activated. When the torque limit value is gradually decreased from the first value, the shaft 4 starts to decrease at the moment when the torque limit value decreases below the torque corresponding to the gravity force (at a point of "start of lowering" in FIG. 5 ). However, in the invention, since the torque of the servomotor 24 is not gradually decreased thereafter, the lowering speed of the shaft 4 is not rapidly decreased as in the related art (portion (c) in FIG. 3 ). Therefore, the shaft 4 is moved within the backlash 12 at a comparatively low speed (see a portion (c 1) in FIG. 5 ).Therefore, in the invention, the lowering or the slipping of the friction plate 6 does not occur as shown in Fig. 2d, and the state shown in Fig. 2c is maintained. In other words, in the invention, the lowering speed (or acceleration) of the shaft 4 is significantly decreased relative to the related art, and the collision between the friction plate 6 and the center-of-gravity axis 4 does not occur, and also little the slip of the friction plate 6 relative to the brake pad 8 occurs. Therefore, the amount of lowering of the center-of-gravity axis is limited within a distance (L 1) corresponding to a backlash width (a portion (c 2) in FIG. 5 ). According to the invention, the noise and the vibration due to lowering of the center-of-gravity axis during braking are reduced, and an operation accuracy of the machine tool or the industrial machine including the center-of-gravity axis is improved.In the present invention, a period in which the torque limit value is decreased from the first value to the second value (i.e., a period from the "start of decrease in the torque limit value" to the "servo control off" in FIG. 5 ) is not particularly limited, and the period is preferably selected to be between 50 milliseconds and 500 milliseconds.Preferably, the torque limit value generating section 30 does not limit the torque limit value to that immediately after the brake enable signal is output by the brake signal output section 26. instead, as shown in a section (b) in FIG. 5, it is preferable that the torque limit value generating section 30 decreases the torque limit value after a period of time has elapsed from the time point when the mechanical brake 2 receives the enable signal to the time point when the mechanical brake 2 has actually been activated (i.e., after waiting until the brake signal output section 26 outputs the brake enable signal and then the mechanical brake 2 is activated). Thus, the amount of lowering of the center of gravity axis can be more surely reduced.As shown in section (c 2) in FIG. 5, the second value, which is smaller than the torque corresponding to the gravity force, may be zero. By this, the torque of the servomotor 24 can be prevented from being varied discontinuously with certainty. In this regard, after the movement (or lowering) of the center-of-gravity axis 4 within the backlash is completed (section (c 2) in FIG. 5 ), the servo control may be turned off before the second value is decreased to zero.Further, the first value may be any value as long as it is larger than the torque according to the gravity. The first value may be the same as the torque command value immediately before the start of the decrease of the torque limit value (the holding torque in the example of FIG. 5 ), for example. By means of this, the movement of the center of gravity axis 4 can be smoothed.According to the invention, by decreasing the lowering speed of the center of gravity axis when the center of gravity axis is braked, the noise and vibration due to the collision between the center of gravity axis and the mechanical brake are reduced. Further, by reducing the amount of lowering of the center-of-gravity axis, the operation accuracy of the machine tool or the industrial machine including the center-of-gravity axis can be improved.

Claims

A servo control device (20) for controlling a servo motor (24) that drives a center-of-gravity axis (4) of a machine tool or an industrial machine (22), the servo control device comprising: a brake signal output section (26) configured to output a brake activation signal and a brake release signal to a mechanical brake (2) having a backlash (12) for holding the center-of-gravity axis; a torque command generation section (28) configured to generate a torque command value for controlling a position of the servo motor; A torque limit value generating section (30) configured to generate a torque limit value for limiting the torque command value generated by the torque command generating section, wherein the torque limit value generating section is configured to continuously decrease the torque limit value from a first value larger than a torque corresponding to a gravity force to a second value smaller than the torque corresponding to the gravity force after the brake signal output section outputs the brake activation signal, wherein the center-of-gravity axis (4) starts decreasing at the torque at which the torque limit value falls below the torque corresponding to the gravity force, and an amount of lowering of the center-of-gravity axis (4) is limited within a distance (L1) corresponding to a width of the backlash (12).The servo control device according to claim 1, wherein the torque limit value generating section (30) is configured to decrease the torque limit value after waiting until the brake signal output section (26) outputs the brake activation signal and then the mechanical brake (2) is activated.The servo control apparatus according to claim 1 or 2, wherein the second value is zero.The servo control device according to at least one of claims 1 to 3, wherein the first value corresponds to the torque command immediately before the beginning of the decrease of the torque limit value.The servo control device according to at least one of claims 1 to 4, wherein the mechanical brake (2) includes a friction plate (6) that is i) integrally rotatable with the center of gravity axis (4) and ii) is engaged with the center of gravity axis (4) by a concave-convex structure, the backlash (12) being structurally present in the concave-convex structure.

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