GB2213661A

GB2213661A - Method for monitoring working sequence of a machine tool

Info

Publication number: GB2213661A
Application number: GB8828366A
Authority: GB
Inventors: R Paul Kohler
Original assignee: JM Voith GmbH
Current assignee: JM Voith GmbH
Priority date: 1987-12-11
Filing date: 1988-12-05
Publication date: 1989-08-16
Anticipated expiration: 2008-12-05
Also published as: FR2624420A1; DE3741973C2; GB2213661B; JPH01205958A; ATA299988A; GB8828366D0; CH676341A5; DE3741973A1; FR2624420B3

Description

1 - 1 ME FOR MN1ICRING VIUMU SEQUEN= OF A D%C11M TOML The invention

relates to a method for monitoring the working sequence of a machine tool for the purpose of detecting harmful influences by detecting the pttern of current consurrption, whereby, in addition to the pattern of current consumption, there are also detected the parameters frcim which it can be concluded whether the respective process state is permissible or inpermissible, whereby measures are taken to switch off the wachine-tool if a determined maximum value is exceeded.

In very general terms this involves monitoring whether a machine tool is working satisfactorily or not, so that corrective measures can be taken if necessary. The causes of unsatisfactory operation are very varied. The most important factor is the bluntness of the tool.

The conditions mentioned become apparent through an increase in the current consumption of the appropriate feed drive. in this case the current generally increases before the influences have caused serious damage. If, for example, the tool becomes increasingly blunt while a workpiece is being machined, the current increases until it reaches a determined maximum value set arbitarily. When this maximum value has been reached, the machine tool is. switched off, which lan either be done manually or automatically.

Parallel to this a cut-out is generally incorporated as overload protection. After the machine tool has been switched off, the cause of the current increase and consequently of the harmful influence is generally easily determined and removed.

However in practice the same problem always arose, namely that the cutout did not respond at the correct moment. Generally speaking the cut-out is not meant to respond too early, nor too late either, in other words: The required value should not- be too hch, nor too low. if it is too high there is the danger that damage occurs. On I the other hand, if it is too low this frequently means a false alarm, i.e. the cut-out is triggered without a proper reason, as a result of which economy is impaired. This problem has not previously been solved; in particular time and time again the cut-out responded without there being an actual faulty condition, in fact all the components of the machine were in perfect working order, and the cutting edge of the tool was sharp.

The object of the invention is to develop a method in accordance w-Lth the generic concept so that monitbring is more precise than before, i.e. it is more selective, so that the tolerance range of the response of the cut-out can be reduced.

is This object is achieved by the characterising features of the main claim.

For the implementation of the method as specified in the inventionit is advantageous if the appropriate machine tool is fitted with a direct and/or indirect incremental path measurement system. For the performance of the invention it may be necessary to switch a precision resistance into the appropriate electric circuit of the drive motor, which may be a part of an axle drive or of a spindle drive. However such drives are frequently equipped with an outlet which supplies the current consumed. A microprocessor with detection logic will generally be required for incremental path measurement systems and an A/D converter will be required for current measurement.

The necessary parameters may be detected by the dual use of measurement system outlets. This means that the velocity and acceleration of the axle or spindle to be monitored are indirectly detected by the direct determinI- kz r 3 ation of the position without additional sensors. The A/D converter also measures current values.

Then a microcomputer is required which is able to record a current value for each axle or spindle in fixed time intervals, for example every hundred microseconds, and which forms every twenty milliseconds a mean value from this value by f iltering out "implausible" values.

Inpermissible states can be distinguished from permiss- ible states on the basis of all the parameters now detected in accordance with the invention, i.e. the current, position, velocity, acceleration, etc and also if necessary deformation values. The computer is used for this purpose, by selecting the values in accordance with specified retrieval strategies and comparing them with stored reference values.

As already mentioned, the method as specified by the invention thus enables the detection and distinction of inpermissible states, such as, for example wear to the tool chipping of a tool cutting edge total tool breakage faults in the WC software and hardware faulty operation, e.g. by incorrect selection of the axle or by too high feed velocity As the method specified by the invention can work without powered sensors, it is also suitable for the subsequent 35 equipment of machine tools already in existence.

if 4 The method specified by the invention is also suitable for diagnosing machine states, such as, for example, the state of the track support or the adjustment of the drive boosters.

of course the system is in principle able to deduce required values to be prescribed f rom the CNC automatic operation so as to confirm the specified recommended values therefrom. One example would be to differentiate between the wear to which a large drill would be subject as opposed to the wear to which a small drill would be subject. In this way it is possible to create required values from the reference values, and these required values are to be specified precisely for a particular tool.

In this connection the inventor has discerned the following:

Apart from the aforementioned factors which can have a damaging effect in any way (poor workpiece surface, unacceptably high forces acting on the machine tool, etc) and which increase current consumption, there are also other factors which change current consumption, but which are completely harmless. Amongst these harmless factors are changes in the velocity of the feed, changes in acceleration and material composition, etc.

From this realisation the inventor has come to the cor- rect logical conclusions, namely he has recommended that not just the current consumption should generally be determined, but that in addition those parameters which are harmless but which also affect current consumption should also be determined. The most important parameters are velocity and the acceleration of feed advancement.

In practice a machine displays completely typical move- 1 1 t 1.

1 ments. It has clearly been shown, for example, that acceleration and current consuption of a feed drive are constantly proportional. This means that the current increases sharply if feed is accelerated. Generally speaking braking results in a countercurrent in the feed motor. The machine is braked by the negative acceleration which occurs. Here too the current and acceleration have the same direction. The acceleration of the drive is quite normal in the operation of a machine tool. it is generally intentional. As demonstrated, it has an effect on current consumption. Consequently it is a typical contributory factor which has a harmless effect with respect to the invention and nevertheless influences current consumption.

The invention is described in more detail with reference to the drawings. They show the following in detail:

Figure 1 shows a diagrammatic representation of the paths of the velocity and acceleration of a machine tool and the current consumption, in which changes in current consumption are attributable to harmful influences; Figure 2 shows graphs similar to those in Figure 1, in which changes in current consumption are attributable to harmful influences.

Figure 3 shows an actually recorded diagram of velo city and acceleration of a feed mechanism and the current consumption of the feed drive.

Figure 1 shows the effect of the accleration of the tool (upper diagram) and of braking (lower diagram). Here acceleration is designated by a, velocity by v and current consumption by I. As can be seen from the upper 6 1 diagram, current consumption increases rapidly with constant acceleration, but increasing velocity, right at the beginning of the increase in velocity and remains at a constant value throughout the entire increase. Once maximum velocity has been reached and the velocity remains constant, current consumption drops slightly and then also remains constant.

The equivalent is true for the lower diagram.

The diagrams represented in Figure 2 show f actors which are designated as being harmful within the context of this invention. In the upper part the vertical broken line marks the moment when the drill has become unserviceable because the cutting edge is chipped. As can be seen, sudden braking occurs; acceleration is consequently reduced to a certain value at which it remains constant. Velocity falls steadily from this moinent. Current consumption steadily increases from this same moment.

Even more drastic is the effect of a collision as shown in the lower diagram.

The diagram represented in Figure 3 also shows the three variables a = acceleration, v = velocity and I = current.

Firstly in the diagram one can see to the left of point 1 the tool in the resting position. Acceleration and velocity are approximately close to zero, while the current fluctuates slightly so as to maintain the position.

After a short time the tool begins its travel, which is shown by a sharp increase in the three named variables. At point 2 the current and acceleration reach a maximum i;, 7 value, which also represents a turning point. Acceleration then falls to a minimum 3, and then again rises to a maximum 4, etc. This phenomenon shows an adjustment which is attributed to the fact that the drive booster was not optimally set in this case.

However the velocity rises in a first, increasing part even further than the acceleration and current. Finally at 5 it reaches a value which is more or less kept constant.

At point 6,6 something decisive happens: For the purposes of the experiment a collision is simulated by a tool' being struck at high velocity against a workpiece.

For this experiment measures were taken so that the machine could not suffer serious damage.

The result of this collision can be clearly seen: Acceleration decreases drastically and reaches a minimum at 7. Simultaneously the current rises drastically and reaches a maximum at 8. As can be seen, there is then an increase in acceleration to the value 9. This increase is attributed to the fact that the static friction of the workpiece on its support has been overcome, with the result that the workpiece has subsequently been repelled at substantially the same velocity. As can also be seen, this only lasts a certain time, as point 9 simultaneously represents a turning point; acceleration subsequently falls again, as can be seen.

It is only pointed out for the sake of completeness that the path of velocity v has not been represented as it actually occurred for reasons relating to measurement technology. However in this context velocity only plays 35 a subordinate role.

4 8 As can be seen, because of the invention it is possible to identify behaviour at resistance line 6,6. As a result it is naturally possible to introduce appropriate measures, which, of course, may even be automated. None of this was possible previously.

2 Y 9

Claims

1. Method for monitoring the working sequence of a machine tool f or. the purpose of detecting harmf ul influences by detecting the pattern of current consumption, whereby, in addition to the pattern of current consumption, there are also detected the parameters from which it can be concluded whether the respectiv&- process state is permissible or inpermissible, whereby measures are taken to switch off the machine tool if a determined maximum value is exceeded, characterised in that the maximum value is altered as a function of the process state.

2. Method according to Claim 1, characterised in that the position of an axle, the velocity, the acceleration, the deformation of the drive system, etc, are regarded as being further parameters, either in their own right or in conjunction with others.

3. Method according to Claim 2, characterised in that the difference between the measurement results of a direct and an indirect measurement system (path measurement detection system) is used as a further parameter.

A method for monitoring the working sequence of a machine tool substantially as hereinbefore described with reference to the accompanying drawings.

Publiahadl9egat IIep&te T, ()Moe, Bta;te HOUSe. W/71 High HOlbOM loondorL WClR4TP. Further eopies maybe obtained tro T. tant=ee. Sales Brancl, St Mary CrAY- 03TIngton, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, KentmCohnePI'L87