JPH0430945A - Lost motion detecting for feed mechanism of machine tool - Google Patents

Abstract

PURPOSE:To warn the nearing of an overhaul time with the increase of a lost motion previously detected by determining a difference, between the rotation movement quantity of a feed motor and the actually moved quantity of a moving body from the point of time of transmitting a reversing command, to compare it with a reference value to make judging and detecting. CONSTITUTION:A scale 5 is fixed to a table 11 integrated with a nut 4 threadedly engaged with the screw 1 of a ball screw feed mechanism, a pulse signal corresponding to the displace quantity is outputted, an encoder 6 is fitted to a motor 3, and a pulse signal corresponding to rotation is transmitted. When a reversing command is transfered to the motor 3, pulse signals A and B are outputted to a subtraction circuit 8 with the encoder 6 and the scale 5, and the difference D of the pulse signals A and B is outputted to a comparator circuit 9 from the subtraction circuit 8 at efery fixed time. In the comparator circuit 9, when a reference value M and a difference signal D, inputted at every reversion of the motor 3, become D > M, it is supposed that the lost motion of the feed mechanism is enlarged to reach a threshold value imminently, and an alarm is given with an alarm sounding means 10.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for detecting lost motion in a feed mechanism of an NC machine tool.

[Conventional technology]

A moving body such as a table or head of an NC machine tool is precisely displaced by a feed mechanism such as a ball screw mechanism, and performs cutting work on a workpiece. However, play or lost motion gradually increases due to wear of the screw during use or increased play in the thrust direction of the hair ring supporting the screw, so if this becomes large, repair the feed mechanism. - It is necessary to replace it so that accurate positioning can be obtained.

[Problem to be solved by the invention]

In the past, the machining accuracy of the workpiece was observed and the feed mechanism was repaired or replaced if the accuracy deteriorated, but with this method, planned overhauls could not be carried out and there was a risk that production would be disrupted. was there.

The present invention solves the problems in the prior art, detects in advance the increase in lost motion of the feed mechanism, and
It is an object of the present invention to provide a method of warning that the time for overhaul of a feed mechanism is approaching.

[Means to solve the problem]

The purpose of this is to find the difference between the amount of rotational movement of the feed motor and the actual amount of movement of the moving object from that point on when a reverse command is issued to the feed motor, and compare it with a preset reference value to see if it exceeds it. This is achieved by determining and detecting whether or not this is the case.

More specifically, it has a scale that detects the amount of feed movement of the moving body and issues a pulse signal, and an encoder that detects the amount of rotational movement of the feed motor and issues a pulse signal.
In a feed mechanism of an NC machine tool that performs feedback control of the feed amount of the moving body based on a feed command from the device, the encoder issues a signal starting from the point in time when a reversal command in the feed direction is issued from the NC device. Pulse signal A and pulse signal B emitted from the scale
Start counting, end the counting at the time after the pulse signal B is emitted from the scale, find the difference between the counted value of the pulse A and the counted value of the pulse B, and calculate this value. A feed mechanism for a machine tool, characterized in that it is configured to issue an alarm when a preset reference value M is exceeded, and automatically detects that the lost motion of the feed mechanism exceeds a predetermined value. A lost motion detection method is provided.

(Operation) When a command to reverse the feed direction is issued, the motor reverses and drives, for example, a screw in a ball screw mechanism in the opposite direction.Then, the nut attached to this screw Even though the screw itself is rotating due to the backlash that exists between the screws and the play in the thrust direction of the hair ring that supports the screw, it remains in its position for the time T to overcome the loose space. Therefore, the table, etc. fixed to the nut also remains stopped during this period. After the time T has elapsed, the nut and screw come into contact, and both move in conjunction, and the table, etc. The displacement is started. This is called a lost motion. That is, the total number of rotations of the motor within this time T corresponds to the magnitude of the lost motion.

In the present invention, counting of encoder pulse signals corresponding to the rotation speed of the motor is started at the same time as a reverse rotation command is given to the feed mechanism, and at the same time, counting of pulse signals of the position from the scale corresponding to the actual feed amount is also started. . Of course, as mentioned above, the pulse signal from the scale is actually counted with a delay of the lost motion amount from the pulse signal from the encoder. Therefore, if the two counting results are compared after a predetermined period of time has elapsed (after the pulse signal is always emitted from the scale), there should be a difference corresponding to lost motion.

In the present invention, this difference is compared with the base $M set with a margin, and if D exceeds M, an alarm is immediately issued to notify that the time for overhaul of the mechanism is approaching. has been done.

Hereinafter, the present invention will be explained in more detail based on preferred embodiments shown in the drawings.

[Embodiment] FIG. 1 shows a block diagram when the present invention is applied to a ball-cage type feeding mechanism.

A screw 1 of the ball screw feeding mechanism is supported at both ends by a hair ring 2 fixed to a stationary frame, and is rotationally driven by a reversible motor 3 connected to one end of the hair ring 2. A nut 4 is screwed into the screw 1 and is displaced left and right as the screw 1 rotates. A scale 5 is fixed to a table 11 integrated with the nut 4, and moves along with the displacement of the NAND 4, indicating the amount of displacement by a pointer fixed to the frame, and outputs a pulse signal corresponding to the displacement. is configured to be

An encoder 6 is attached to the motor 3 and is configured to emit a pulse signal according to the rotation of the motor 3.

The ball screw mechanism is driven based on an NC command given to the motor 3 via a servo circuit (not shown) from an NC device 7 consisting of an arithmetic control circuit.

The resulting displacement amount is measured by the scale 5 by the NC device 7.
are sequentially input and feedback-controlled so that the desired positioning is performed. This is a closed-loop system feed control system.

The present invention detects an increase in lost motion of the feed mechanism based on such a closed-loop feed control system.

In a feeding mechanism such as a ball screw mechanism, when the feeding action is being performed, the threaded surface a of the screw 1 is aligned with the threaded surface of the nut 4, as shown in FIG. 2(a). A play space S is formed on the opposite side. When the feeding direction is reversed, even if the screw 1 rotates, at first only the play space S moves and the nut 4 does not move, as shown in Fig. 2 (b).
), the nut 4 begins to displace only after the surface b of the thread of the nut 4 comes into contact with the thread a' of the screw l.

The principle of the present invention is to quantify the magnitude of the lost motion of the feed mechanism by measuring the number of rotations of the screw 1 required to move the idle space S when the feed direction is reversed. This lost motion includes bank lash caused by wear of the threads, play in the thrust direction of the bearing 2, and elastic distortion of the screw 1 itself during startup, and is only a degree that occurs even under normal conditions. should be allowed.

However, if this increases beyond the limit, it will affect the machining quality of the workpiece, so countermeasures are required.

In the present invention, during the feeding operation, a pulse signal A from the encoder 6 representing the rotation speed of the screw 1 (that is, the rotation speed of the motor 3) and a pulse signal B from the scale 5 representing the feed amount are used.
A subtracting circuit 8 is provided which receives and integrates each of the signals, and outputs the difference between the two after each predetermined period of time has elapsed. Furthermore, a comparison circuit 9 is provided which receives the difference signal output from the subtraction circuit 8, compares it with a pre-stored reference value M, and outputs an operation signal P only in the case of DAM. This comparison circuit 9 is connected to alarm generating means 10 such as a buzzer or a lamp.

Next, the operation of the apparatus of the present invention having such a configuration will be explained.

When a reversal command for the feed direction is transmitted to the motor 3 during a feed operation based on an NC command from the NC device 7, the encoder 6 and the scale 5 each send the pulse signal AB to the subtraction circuit 8 in accordance with this command. Start output.

These signals are each calculated within the subtraction circuit 8, and the difference between them is outputted to the comparison circuit 9 at regular intervals. As can be seen from the above-mentioned principle, the pulse signal A representing the rotation speed of the screw 1 is generated at the same time as the reversal command issued from the NC device 7, but the pulse signal B representing the displacement of the scale 5 is related to lost motion. occurs with a slight delay. This difference shows a value that gradually increases in the amount of output until the lost motion is overcome, but converges to a constant value after a short period of time. This indicates that the lost motion has been completely overcome; the pulse signal from scale 5 acts as a subtraction operation completion signal, and the value of D at this point corresponds to the magnitude of the lost motion. Become.

The comparison circuit 9 compares the stored reference value M with a difference signal input every time the motor 3 reverses. This reference value M is a value determined with a margin by multiplying the lost motion limit value determined based on the permissible limit of machining accuracy of the workpiece by a predetermined safety factor. This comparison may be continued at least until D reaches the aforementioned convergence value. During this comparison, if D does not exceed M, the lost motion is determined to be within safe limits.

If D>M, it is expected that the stroke motion of the feed mechanism will become thick (and reach the limit value soon), so a command will be issued to the alarm generating means 10, and the operator will A warning will be issued to that effect.Based on this, a reasonable overhaul will be carried out, taking into consideration the production plan.

Although the above explanation has been made regarding a pole and pinion type feeding mechanism, it can be similarly applied to other rank and pinion type feeding mechanisms.

〔Effect of the invention〕

The play detection operation of the feed mechanism is started by a reverse feed direction command in the NC command, and ends after the pulse signal B is issued from the encoder, that is, when the above-mentioned value of D converges to a constant value. Therefore, the amount of lost motion can be monitored virtually all the time, and it is possible to reliably determine the timing of overhaul with plenty of time to spare.

This makes it possible to formulate a comprehensive overhaul plan linked to the production plan, improving work efficiency.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an outline of the stacking system for implementing the present invention, and Fig. 2 (a) and (b) show the state of play between the screw and nut of the feeding mechanism when the feeding direction is reversed. FIG. 1 - Screw 2 - Bearing 3 - Motor 4 - Nut 5 - Scale 6 - Encoder 7, - NC device 8 - Subtraction circuit 9 - Comparison circuit 10 - Alarm generating means 11 - Table A, B - Pulse signal D - Difference signal M - reference value (Figure (a) Figure (1))

Claims (1)

[Claims] 1. It has a scale that detects the amount of feed movement of the moving object and issues a pulse signal, and an encoder that detects the amount of rotational movement of the feed motor and issues a pulse signal, and the said movement is based on the feed command from the NC device. In the feed mechanism of an NC machine tool that performs feedback control of the feed amount of the body, the NC
Start counting the pulse signal A emitted from the encoder and the pulse signal B emitted from the scale starting from the time when a reversal command in the feed direction is issued from the device, and after the pulse signal B is emitted from the scale. The counting is finished at the point of time, the difference D between the counted value of the pulse A and the counted value of the pulse B is determined, and an alarm is generated when this value D exceeds a preset reference value M. A method for detecting lost motion in a feed mechanism of a machine tool, comprising: automatically detecting that lost motion in the feed mechanism exceeds a predetermined value.