NUMERICAL CONTROLLER

Abstract: A numerical controller has a semi-closed loop instruction and outputs a torque command (E6) to a servo motor, based on a motion command (MC(i)), a position signal (POS(i)) and a velocity signal (VEL(i)). A backlash correction data (BL(i)) is used to compensate a response time delay due to a backlash in the servo motor when the rotation is reversed. In addition, a torque correcticn signal (FR(i)) gives an offset value to a velocity command (E2), which compensates the effect of static and dynamic frictions.

Description: This invention relates to a numerical control apparatus, which has a backlash correction function for a move command, for effectively carrying out positioning in a semi-closed loop servo system. A semi-closed loop-type control circuit of the kind shown in Fig. 3 is employed as a CNC servo-control circuit. In the Figure, an information processing circuit X comprises a CPU and a memory and forms a command signal for a servomotor. A servomechanism Y is constituted by a comparator circuit a for comparing the command signal and a signal from a position detector e, a servo drive circuit b, a servomotor c, a velocity detector d and a position detector e employing a resolver or a pulse encoder, and is adapted to control a machine Z such as a table via a ball screw shaft f. As illustrated, the semi-closed loop system controls a load by performing position detection at a portion of the motor shaft or ball screw shaft in front of the machine Z, such as a table, which is the object to be finally controlled. Control is performed accurately up to the motor shaft or ball screw shaft, and from this point onward control depends on the precision of the machine. Fig. 4 is a block diagram of such a servo-control system, in which K denotes position gain, k₁ and k₂ gains, K T a torque constant, T L disturbance, and J m rotor inertia. In control of the machine by such a semi-closed loop system, a response delay due to backlash in the mechanical system arises when there is a change in the direction of servomotor rotation. In order to improve upon this delay with respect to a move command, the conventional practice is to supply the servo system with a backlash correction signal together with the move command signal. Fig. 2 is a block diagram illustrating an example of a control circuit in which a backlash correction signal BL(i) of this kind is inputted together with a move command signal MC(i) to the servo system to form a torque command. In Fig. 2, an integration term Z⁻¹ is expressed using a Z conversion (pulse transfer function conversion). VEL(i) indicates a fed back velocity signal, and POS(i) represents an amount of movement in a sampling period T, namely a fed back position signal. In this conventional control circuit, the response delay of the servo system is improved by inputting the backlash correction signal to the servo system. However, since frictional resistance is present in an actual servomotor and machine, positional displacement based on a response delay still remains when the servomotor reverses its direction of rotation. If a machine tool is to be controlled along, e.g., a circular arc by controlling a plurality of axes simultaneously, positional displacement caused by frictional resistance in the machine tool will hinder machining along a true circle. JP-A-60-116004 discloses a numerical control apparatus according to the preamble of attached claim 1, wherein the backlash of a controlled machine is compensated by a compensating means in the form of a pulse generator which outputs a minus or plus pulse each time movement commands are changed. The object of the present invention is to at least partly alleviate the aforementioned problems of the prior art by replacing the aforementioned pulse generator by a compensating means which compensates a torque command by taking frictional resistance into consideration. According to the present invention there is provided a numerical control apparatus for controlling a servo-control system which controls a servomotor, by forming a position command from a move command and a detected position signal, and a velocity command which is produced from the position command and then compared with a detected velocity signal to provide a velocity command for the servomotor, there being compensating means for providing a backlash correction command for backlash at a mechanical load, and a correction signal having a polarity which depends upon the backlash correction command, characterised in that the servo-control system is a semi-closed loop servo-control system and said correction signal is additional to the backlash correction command and is produced by a correcting means which discriminates, on the basis of said backlash correction command, which assists in the formation of said position command, a quadrant position prevailing when the direction of rotation of the servomotor reverses, and then forms said correction signal as a torque correction signal corresponding to frictional resistance of the mechanical load, this torque correction signal being employed in the formation by an arithmetic means of said velocity command for the servomotor. Accordingly, in the numerical control apparatus of the present invention, a position command regarding the servo system and a backlash correction command are applied to a comparator to which the servomotor position signal is inputted, and a torque correction signal corresponding to the frictional torque of the machine constituting the load is applied to the servo system as offset data in dependence upon the quadrant position which prevails when the direction in which the servomotor rotates reverses. Thus, accurate positional control is realized by eliminating the influence of friction with regard to the load. Brief Description of the Drawings Figs. 1(a), (b) are a block diagram and flowchart illustrating the general arrangement of the present invention, Fig. 2 is a block diagram illustrating an example of a circuit according to the prior art, Fig. 3 is a circuit diagram of a servomotor control apparatus using a semi-closed loop system, and Fig. 4 is a block diagram of a servo-control system. An embodiment of the present invention will now be described in detail with reference to the drawings. Fig. 1(a) is a block diagram illustrating the general arrangement of the invention. The move command MC(i) formed by an information processing circuit and the backlash correction command BL(i) are inputted to a comparator B₁ of a servo system, and an error signal E₁ is formed by comparing these commands with the amount of movement of a servomotor output shaft in a sampling period T, namely the position signal POS(i). The error signal E₁ is multiplied by the position gain K to form a velocity command E₂, which is applied to a comparator B₂. Also inputted to the comparator B₂ are the servomotor velocity signal VEL(i) and a torque correction signal FR(i) obtained from a correction circuit A by processing the backlash correction command BL(i) under predetermined conditions. Fig. 1(b) is a flowchart for forming the torque correction signal FR(i) in the correction circuit A. Solely the backlash correction command BL(i) is inputted to the correction circuit A, which outputs the torque correction signal FR(i) corresponding to frictional resistance in accordance with the quadrant position that prevails when the servomotor reverses direction. More specifically, a frictional resistance value Fa of the machine coupled to the servomotor is preset, and the following torque correction command FR(i) is computed in dependence upon the quadrant position, which is discriminated by the backlash command BL(i), prevailing when the servomotor reverses direction: (1) FR(i) = -Fa/(K₁T) for the third quadrant [BL(i) < 0]; (2) FR(i) = 0 for BL(i) - 0; and (3) FR(i) = Fa/(K₁T) for the first quadrant [BL(i) > 0]. In accordance with the correction circuit A of the control apparatus constructed as set forth above, the torque correction signal corresponding to frictional resistance is applied to the comparator B₂ as offset data, and a velocity command E₃ from the comparator B₂ is inputted to a comparator B₃ as a signal which compensates for frictional resistance. Subsequent processing is the same as in the example of the prior-art circuit of Fig. 2, in which the command signal is corrected by the integration term Z⁻¹ and the gains k₁, k₂ so that a comparator B₄ may form a torque command E6 for the servomotor. In accordance with the numerical control apparatus of the present invention, as clarified by the description of this embodiment, a backlash correction regarding the move command is carried out in the semi-closed loop servo system, and a torque correction can be achieved by taking the frictional resistance of the mechanical system into consideration. As a result, there is an improvement in positional displacement based on a response delay at the time of a change in the rotating direction of the servomotor. The numerical control apparatus of the present invention can be utilized as a semi-closed loop servo-control system adapted to effect a backlash correction with regard to a position command.

Claims: A numerical control apparatus for controlling a servo-control system which controls a servomotor (C), by forming a position command (E₁) from a move command (MC(i)) and a detected position signal (POS(i)), and a velocity command (E₂) which is produced from the position command (E₁) and then compared with a detected velocity signal (VEL(i)) to provide a velocity command (E₃) for the servomotor (C), there being compensating means for providing a backlash correction command (BL(i)) for backlash at a mechanical load, and a correction signal (FR(i)) having a polarity which depends upon the backlash correction command (BL(i)), characterised in that the servo-control system is a semi-closed loop servo-control system and said correction signal (FR(i)) is additional to the backlash correction command (BL(i) and is produced by a correcting means (A) which discriminates, on the basis of said backlash correction command (BL(i)), which assists in the formation of said position command (E₁), a quadrant position prevailing when the direction of rotation of the servomotor (C) reverses, and then forms said correction signal (FR(i)) as a torque correction signal corresponding to frictional resistance of the mechanical load, this torque correction signal (FR(i)) being employed in the formation by an arithmetic means (B₂) of said velocity command (E₃) for the servomotor (C). A numerical control apparatus according to claim 1, wherein said correcting means (A) is adapted to store a frictional resistance value of the mechanical load and form the torque correction signal (FR(i)) in accordance with a predetermined offset parameter.

Inventors: KURAKAKE MITSUO, SAKAMOTO KEIJI, KURAKAKE, MITSUO, SAKAMOTO, KEIJI
Applicants: FANUC LTD, FANUC LTD.
Application year: 1988
Main classification: G05B19
IPC codes: G05D3, H02P29, G05D17, G05B19