GB2224586A

GB2224586A - Numerical control of machine tools

Info

Publication number: GB2224586A
Application number: GB8919582A
Authority: GB
Inventors: Takahiro Yamaguchi; Kenji Ito
Original assignee: Okuma Tekkosho KK; Okuma Machinery Works Ltd
Current assignee: Okuma Corp
Priority date: 1988-08-31
Filing date: 1989-08-30
Publication date: 1990-05-09
Anticipated expiration: 2009-08-30
Also published as: GB8919582D0; DE3928547A1; GB2224586B; JPH0266604A; JPH0752365B2

Description

2 2 24 5 8 6 NUMERICAL CONTROL METHOD

BACKGROUND TO THE INVENTION

This invention relates to a numerical control method which controls a driving section of a numerical control machine tool to limit path errors caused by the discrepancy between an instructed path of a machining program and an actual tool path of a machine tool within tolerable path errors entered previously.

In prior art numerical control (NC) machine tools, an input machining program instructs the machining shape, feed speed, tools to be used, etc. for a machining process. However, in actual practice, there are often caused deviations (or path errors) between the instruction path instructed by the machining program and the tool path actually taken by the tool due to time lag and so on in the machine servo system. The path errors are particularly conspicuous in curved shapes which fluctuate when the cutting speed is high and especially in corners where the change rate for curves is drastic.

In order to overcome such problems, there has recently been proposed a numerical control method which calculates the feed speed based on an instructed instruction path so as to limit its path errors within the tolerable range, and controls the driving section of the NC machine tool based on the calculated feeding speed.

FIG.1 is a block diagram to show an embodiment of the NC system which realizes a prior art numerical method.

In the figure, the NC system which realizes the conventional numerical control method comprises a machining program 1 stored on a sheet of paper tape, an outside input device 2 which is inputted with a tolerable path error Et, a machining program interpreting section 3 which interprets the content of the machining program 1, an instructed shape evaluating section 4 which calculates the machining shape data SD, a function generating section 5 which calculates displacement tsf per unit time, a feed speed correcting section 9 which corrects the inputted feeding speed Fc, a servomotor (M) 7 which drives machine tools, a servo controlling section 6 which controls the servomotor 7, and a position detector (D) 8 which detects the position of the tools and so on.

The above prior art NC system controls numerical controlled machining in the following manner.

The machining program 1 is inputted to the NC system via a tape reader or the like, and the data in each block of the machining program 1 is read out in the machining program interpreting section 3. The machining program interpreting section 3 which has been read out with the data for each block analysis the data to calculate the instruction path Pc and sets the instructed feed rate Fc. The instruction path PC is inputted to the function generating section 5 as well as in the instructed shape evaluating section 4, which subsequently calculates the machining shape data SD based on the input. The machining shape data SD indicates the shape of the instruction path which is to be used to calculate the data such as feed speed attributable to the shape of the instruction path. Shapes may be a radius of an imaginary arc which passes through an instructed point in an arbitrary block and instructed points in blocks before and after the block, an angle of a corner forming an arbitrary block and another subsequent block, or a slope formed by approximating an instructed point in an arbitrary block and instructed points in the blocks before and after the block to a curve and measuring the inclination thereof. The machining shape data SD is fed into a feed speed correcting section 9, which calculates a follow-up lag to limit the path errors within the tolerable path error Et instructed by an operator via the outside input device 2. The followup lag is determined by the tolerable path error Et, the machining shape data SD, constants in the servo system, etc. The tolerable path error Et may be instructed from the machining program 1 instead of the outside input device 2.

The feed speed correcting section 9 calculates the follow-up lag mentioned above as well as the actual follow-up lag from the displacement af per unit time obtained from the function generating section 5 and the value Pa detected by the position detector 8 mounted on the servomotor 7. The feed speed correcting section 9 also calculates the difference between the calculated follow-up lag and the actual follow-up lag, corrects the instructed feed speed Fc based on the difference, and calculates the feed speed Fex- In this time, the feed speed correcting section 9 makes the feed speed Fex less than the speed Fc when the difference is a positive in order to narrow the difference, and suspends the correction operation when the difference becomes zero 1 Lastly, the function generating section 5 calculates the displacement Af per unit time based on the instruction path PC and the feed speed Fex, and the servo controlling section 6 drives the servomotor 7 based on the displacement Af.

As mentioned above, the prior art system limits the path errors to be within the tolerable path error range by comparing the follow-up lag calculated from the instruction path by the machining program and the tolerable path inputted in advance with the actual follow-up lag, and correcting the instructed feed speed to remain within the tolerable follow-up lag.

However, the prior art numerical control method mentioned above is inconvenient in that even if the machining program instructs a high cutting speed to finish the machining in a short period of time, cutting cannot be performed at high speed at locations where the machining shape rapidly changes such as a' curved portion or at corners where the rate of changes is high. Therefore, this inevitably increases the total machining time. The increases of machining time poses a particularly grave problem in metal die machining which often takes a long time. SUMMARY OF THE INVENTION

This invention was conceived to eliminate such inconveniences encountered in the prior art and aims at providing a numerical control method which can obtain a path error within a tolerable path error, and further which can prevent an increases in the machining time due to the reduction in feed speed and which can achieve stable machining precisious allowing errors which are caused by unexpected factors.

According to one aspect of this invention, for achieving the objects described above, there is provided a numerical control method of the type where a driving section of a numerical control machine tool is controlled based on an instruction path and instructed feed speed instructed from a machining program which is characterized in that path errors between the actual path taken by the driving section when it is controlled based on said instructed feed speed and said instruction path is predicted, positional correction is calculated based on the predicted path error and the tolerable path error which is inputted in advance, said instruction path is corrected based on the calculated positional correction, the tolerable follow-up lag is calculated based on the corrected instruction path and said tolerable path error, the actual follow-up lag is calculated based on the data obtained by detecting the position of said driving section, said instructed feed speed is corrected based on the calculated follow-up lag and said tolerable follow-up lag, and said driving section is controlled based on corrected instruction path and corrected instructed feed speed.

The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG.1 is a block diagram to show an NC system which realizes a prior art numerical control method;

FIG.2 is a block diagram to show an NC system which realizes this invention numerical control method; FIG.3 is a flow chart to show the operation thereof; FIG.4 is a view to explain the method of calculation of the instruction pati, Pex; FIG.5 is a block diagram to show another embodiment of the NC system of this invention numerical control method; FIG.6 is an explanatory view to describe the positional correction Dc and changes in the feed speed Et; and FIG.7 is a block diagram to show still another embodiment of the NC system which realizes this invention method.

PREFERRED EMBODIMENTS OF THE INVENTION An numerical control method according to this invention can secure machining precision allowing errors which may generate due to unpredictable factors without the necessity of drastically lowering the feed speed by corresponding an instruction path instructed from a machining program based on a tolerable path error which has been inputted in advance, and correcting the feed speed with information obtained by detecting the-corrected instruction path and the position of the driving section.

FIG.2 is a block diagram of an embodiment of the NC system according to this invention method shown correspondingly to FIG.1 where in the same parts are denoted with the same reference codes and numerals and their description is omitted to avoid duplication.

In the figure, the NC system of this invention is additionally provided with a feed speed calculating section 11 which calculates the feed speed Ft based on the instruction feed speed Fc, the shape data SD and the tolerable path error Et, a positional correction calculating section 10 which predicts path errors caused by the lag or the like in the servo system at the feed speed Ft based on the shape data SD, and which calculates optimal positional correction Dc in accordance with the instruction path Pc based on the predicted path errors and the inputted tolerable path error Et, and an instructed shape evaluating section 4' which calculates the shape data SD' based on the corrected instruction path Pex which has been corrected with the positional correction Dc.

FIG.3 is a flow chart to show the operation of this invention numerical control method which will be described below referring to the chart.

A machining program 1 is inputted to the NC system via a tape reader or the like, and further the data for each block in the machining program 1 is read out in the machining program interpreting section 3 (Step Sl). The machining program interpreting section 3 with the read out data analyses the data to calculate the instruction path Pc and the instructed feed speed Fc (Step s2). The feed speed calculating section 11 sets a feed speed Ft, and the positional correction calculating section 10 predicts path errors at the feed speed Ft caused by the lag and so on in the servo system based on the shape data SD, and calculates optimal positional correction DC corresponding to the instruction path Pc with the predicted path error and inputted tolerable path error Et (Step S3). The instruction path Pc is corrected based on the positional correction Dc thereof, and an instruction path Pex is newly calculated (Step S4). The instruction path Pex is sent to the function generating section 5 as well as to the instructed shape evaluating section 4', and the instructed shape evaluating section 4' calculates the shape data SD' based on the instruction path Pex in a process similar to the instructed shape evaluating section 4.

The feed speed correcting section 9' calculates the tolerable follow-up lag based on the shape data SD' in a manner so that the path error I.

caused when the function is generated based on the instruction path Pex by the function generating section 5 remains within the sum of the tolerable path error Et and the positional correction Dc (Step S5). The feed speed correcting section 9' also calculates actual follow-up lag based on the displacement &f per unit time outputted from the function generating section 5 and the detected value Pa from the position detector 8. Further, the feed speed correcting section 9' corrects the feed speed Ft so that the actual follow-up lag remains within the tolerable follow- up lag and calculates a newly instruction feed speed Fex (Step S6). Description will be omitted for the method of correcting the feed speed as it is similar to the aforementioned prior art method.

The function generating section 5 calculates the displacements &f per unit time by generating a function based on the instruction path Pex and instruction feed speed Fex. The displacement Af per unit of time is sent to the feed speed correcting section 9' as well as to the servo controlling section 6. The servo controlling section 6 drives the servomotor 7 based on the inputted displacement isf.

FIGA shows a calculation method of the instruction path Pex- In the figure, the letter 6e denotes a path error which is predicted based on the shape of the instruction path Pc instructed from the machining program within the positional correction calculating section 10. The path error ce may be calculated by such methods as the one wherein an actual tool path Ps is simulated permitting a lag in the servo system as shown in the figure or the one wherein it is calculated with a function having the servo constant, feed speed and shape data as variables. The positional correction Dc is calculated 1.3az>t--U Ull LAIC 1 % predicted path errors ee and the tolerable path error Et. The amount of the positional correction Dc is calculated so that is becomes substantially equal to the value obtained by subtracting the tolerable path error Et from the predicted path error 6e in the stationary state where the rate of shape changes stays constant, and becomes larger than the value obtained by subtracting the tolerable path error Et from the predicted path error 6. in the transient state, taking into consideration the rate of changes in the machining shape or rate of changes in the predicted path errors. The instruction path Pc is corrected based on the above positional correction Dc to calculate a new instruction path Pex- In the figure, the letters dr denote a tolerable follow-up lag calculated by the feed speed correcting section 9'. The follow-up lag which is generated actually when the instruction path Pex is executed at the feed speed Ft becomes substantially equal to the tolerable follow-up lag dr while the rate of changes in machining shapes and the path errors can be limited to the scope of the tolerable path errors Et at a high cutting speed without correcting the feed speed. Even when a dynamic factor such as an unpredictable lag in a mechanical system is involved, and as a result, the actual follow-up lag exceeds the tolerable follow-up lag and the path error exceeds the tolerable path error Et at a corner where the rate of shape changes is rapid, the path errors can be limited within the tolerable path error Et simply by slightly correcting the feed speed by the feed speed correcting section 9'.

The feed speed Ft which is used for predicting the path errors by the positional correction calculating section 10 and which is 1 w k -ff corrected by the feed speed correcting section 9' may be either one of the feed speed Fc instructed by the machining program (1) and the value which is predicted in advance to make the path error become n (n > 1) times of the tolerable path error Et (2). The former method (1) is the one which holds the relation Ft = Fc irrespective of the shape data SD of the instruction path Pc in the feed speed calculating section 11 while the latter method (2) is a method which determines the feed speed Ft based on the shape data SD so that the path errors should remain within n (n > 1) times of the tolerable path error Et. The letter n denotes an internal parameter of the feed speed calculating section 11.

The path error predicted by the positional correction calculating section 10 increases in proportion to the feed speed Ft while the positional correction Dc increases in proportion to the feed speed Ft as it is substantially identical to the value obtained by subtracting the tolerable path error Et (constant) from the predicted path error in the stationary state. Moreover, since the path error predicted by the positional correction calculating section 10 increases at the corner where the rate of changes in the machining shape becomes large, the positional correction increases at the corner shapes.

As described above, the higher is the speed of instruction feed speed Fc instructed from the machining program, and the higher becomes the rate of shape changes of the instruction path Pc instructed from the machining program, the positional correction Dc in the former method (1) becomes larger. In-the latter method (2), the path error predicted is clamped to be less than Et X n (n > 1). The positional correction Dc is substantially equal to the value l- 11 1 obtained by the tolerable path error Et from the path error. If it is assumed that Et X (n - 1) is a tolerable positional correction Ep, the positional correction Dc could be clamped to be less than the tolerable positional correction Ep irrespective of the shape on the instruction path Pc instructed from the machining program and of the instruction feed speed Fc.

Another embodiment will now be described wherein such the tolerable positional correction E P is inputted by an operator via an outside input device so that the upper limit of the positional correction could be set by the operator.

FIG.5 is a block diagram to show another embodiment realizing this invention numerical control method in correspondence to FIG.2 wherein the same parts are denoted with the same codes and numerals and their descriptions are omitted to avoid duplication.

In the figure, this NC system is additionally provided with an outside input device 13 to which a coefficient m is inputted in order to clamp the path error in size, and a tolerable positional correction calculating section 12 which calculates a tolerable positional correction Ep from the tolerable path error Et and the coefficient m.

The tolerable positional c-orrection calculating section 12 is capable of calculating the tolerable positional correction Ep by multiplying the tolerable path error Et with the coefficient m as an operator can input the coefficient which is equivalent to the coefficient (n - 1) described in relation to the embodiment in FIG.2 through the outside input device 13. If it is assumed that he sum of the 1 1 j 11 calculated tolerable positional correction Ep and the tolerable path error Et is a feed speed calculation error Ev, the value Ev becomes the equivalent to the value Et X n described in relation to the embodiment shown in FIG.2, and the internal perameter thereof can be set from outside with (m + 1). The feed speed calculating section 11 calculates a feed speed Ft which allows the path error to remain within the scope of the calculated feed speed calculation error Ev based on the shape data SD.

FIG.6A shows the changes of the positional correction Dc based on the tolerable positional correction which is set by the operator. FIG.6B shows the feed speed Ft calculated by the feed speed calculating section 11 based on the feed speed calculation error Ev. In FIG.6A, the solid line represents the instruction path Pc instructed by the machining program, the broken line the actual tool path Pa, and the dot-and-chain line the instruction path Pex after the positional correction.

The operator is assumed to set the tolerable positional correction Ep to be m-times (m > 1) of the tolerable path error Et in the section (a) on the instruction path to be equal to Et (m - 1) in the section (b), and to be zero (m = 0) in the section (c). The feed speed calculation error Ev in the section (a) is equivalent to the (m + 1) times of the tolerable path error Et, two times of the tolerable path error Et in the section (b) and equal to the tolerable path error Et in the section (c).

In the positional corrections Dc are denoted as Dcl in the section (a), Dc2 in the section (b) and Dc3 in the section (c), and the feed speeds Ft in the sections are assumed to be Ftj, Ft2 and Ft3 respectively to hold the relation shown by the following formula (1) A X Dcl < MEt, Dc2 < Et, Dc3 0 Dcl > Dc2 > Dc3...... (1) Ftl > Ft2 > Ft3 The operator thus becomes capable of adjusting the positional correction while observing the actual movement of the tool and based on the machining state and type of machining by arbitrarily clamping the positional correction Dc to make the path error remain within the scope of the tolerable path error Et. When the operator wishes to suspend the machining operation temporarily during a work, he can suspend it simply by making the tolerable positional correction Ep equal to the tolerable path error Et (m = 1). The deviation caused by the correction from the instruction path of the machining program would remain constantly within the tolerable path error Et no matter where the operator suspends the machining. If manual cutting operation is required for a section by manual interruption, it can be smoothly made by simply making the tolerable positional correction Ep zero (m = 0). When the work does not require any suspension or manual interruption, the maximum speed can be attained in cutting /feeding simply by setting the tolerable positional correction Et at maximum. In order to avoid an increase of the actual tolerable error beyond the predicted path error because of some unpredictable reasons exceeding the tolerable path error Et, the positional correction should be adjusted smaller.

Although description was made in the foregoing embodiments for the method which designates an information for the tolerable positional correction calculation from the outside input device 13, it may be designated by the machining program 1. Although the information necessary for calculating the tolerable positional correction Ep is described as a coefficient m which is to be multiplied with the tolerable path error Et in the foregoing statement, it may be the tolerable positional correction Ep instead.

in the second embodiment, although the size of the positional correction Dc which is to be added to the instruction path PC instructed by the machining program is arbitrarily modified based obn the tolerable positional correction Ep which can be set from outside, the positional correction D. may be modified in accordance with the machining shape of the instruction path. In other words, it becomes difficult to predict the path errors at sections such as curves where the rate of shapes changes on the instruction path PC is high or corners where the rate of changes is high as acceleration changes at driving section and an unpredictable lag in the servo system tends to occur to thereby lower precision of the positional correction Dc calculated in the feed speed calculating section 11. The precision, however, is increased in machining if the positional correction Dc is adjusted in accordance with the shapes on the instruction path PC as the positional correction Dc is made smaller for the sections where the rate of changes in shapes is high and the path error prediction is more difficult while it is maximized for the sections where the rate of changes in shape is substantially constant and prediction of the path error is relatively easy.

The method will now be described in relation to the third embodiment. FIG. 7 is a block diagram to show the third embodiment of this invention numerical control method in correspondence to FIG.5 wherein the -1 t? same parts are denoted with the same codes and their description is omitted to avoid duplication.

In the figure, this embodiment NC system is provided anew with an error prediction coefficient calculating section 13 which calculates an error prediction coefficient Kr based on the shape data SD outputtted from the instructed shape evaluating section 4.

The error prediction coefficient calculating section 13 analyses the shape of the instruction path Pc insturcted from the machining program 1 based on the shape data which is outputted from the instructed shape evaluating section 4 and calculates and error prediction coefficient Kr. The error prediction coefficient Kr satisfies the condition of 0 < Kr < 1 in accordance with the shape data SD. The error prediction coefficient Kr assumes a value close to zero for the shapes such as corners where path errors are influenced easily from unpredictable factors while it assumes a value close to "1" for the machining shapes which are not easily influenced by inpredictable factors. The error prediction coefficxient Kr is sent from the tolerable positional correction calculating section 14, which in turn calculates the tolerable positional correction Ep by multiplying the tolerable path error Et which the coefficient m and the error prediction coefficient K. which are inputted from the outside input device 2. Therefore, at corners where path errors are not fully predictable, the tolerable positional correction Ep becomes close to zero and positional correction is not executed. Conversely., at machining shapes where path errors are easily predicted, the tolerable positional correction Ep becomes substantially equal to the value inputted by the operator, and positional correction is =.& = %_ LL I- R-- U.

C_ Although the error prediction coefficient Kr is sent to the tolerable positional correction calculating section 12 in the above embodiment, it may be sent to the positional correction calculating section 10, which in turn multiplies the error prediction coefficient Kr with the predicted path error, and calculates the positional correction Dc based on the path error to thereby modify the positional correction Dc in accordance with the machining shapes of the instruction path Pc.

According to this invention numerical control method, stable machining precision can be secured allowing path errors which may be caused due to unpredictable dynamic factors. Therefore, this invention enables high speed and highly precise machining irrespective of shapes. This invention also enables simple interruption for manual cutting by an operator.

It should be understood that many modifications and adaptations of the invention will become apparent to those skilled in the art and it is intended to encompass such modifications and changes in the scope of the claims appended hereto.

1 C

Claims

WHAT IS CLAIMED IS:

1. A numerical control method of the type where a driving section of a numerical control machine tool is controlled based on an instruction path an instructed feed speed instructed from a machining program which is characterized in that path errors between the actual path taken by the driving section when it is controlled based on said instructed feed speed and said instruction path is predicted, positional correction is calculated based on the predicted path error and the tolerable path error which is inputted in advance, said instruction path is corrected based on the calculated positional correction, the tolerable follow-up lag is calculated based on the corrected instruction path and said tolerable path error, the actual follow-up lag is calculated based on the data obtained by detecting the position of said driving section, said instructed feed speed is corrected based on the calculated follow-up lag and said tolerable follow-up lag, and said driving section is controlled based on corrected instruction path and corrected instructed feed speed.

2. A numerical control method as claimed in Claim 1, wherein a feed speed is calculated based on the instruction path instructed by said machining program so that said path error would not exceed a multiple of said tolerable path error by a predetermined number, path error between the actual path taken by said driving section when it is controlled with the calculated feed speed and said instruction path is predicted, and the calculated feed speed is corrected.

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3. A numerical control method as claimed in Claim 1, wehrein the path error prediction coefficient is calculated based on said instruction path instructed by said machining program, said positional -correction is modified based on the calculated path error prediction coefficient, and said instruction path is corrected based on the modified positional correction.

4. A numerical control method as claimed in Claim 2, wherein a feed speed is calculated so that said path error would not exceed the sum of the tolerable positional correction set by the machining program or inputted from an outside device and said tolerable path error.

5. A numerical control method as claimed in Claim 4, wherein the path error prediction coefficient is calculated based on said instruction path instructed from said machining program, said tolerable positional correction is modified based on the calculated path error prediction coefficient, the feed speed is calculated so that said path error would not exceed the sum of said tolerable positional correction and said modified tolerable path error.

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