EP2852472B1 - Method for grinding workpieces, in particular for centring grinding of workpieces such as optical lenses - Google Patents

Description

TECHNICAL AREA

The present invention generally relates to a method of grinding workpieces by means of a grinding tool using an actuator for generating a relative feed movement between the grinding tool and the workpiece, wherein the actuator is integrated with a current controller for an actuator current determining a feed force of the actuator in a position control loop, which is traversed with a predetermined control cycle.

In particular, the invention relates to a method for centering grinding of workpieces from the fields of fine optics (optical lenses), watch industry (watch glasses) and semiconductor industry (wafers), where workpieces are centered by means of centering initially tensioned and to be subsequently sanded on the edge.

STATE OF THE ART

Lenses for lenses or the like. are "centered" after the processing of the optical surfaces, so that the optical axis, whose position is characterized by a straight line passing through the two centers of curvature of the optical surfaces, also passes through the geometric center of the lens. For this purpose, the lens is initially aligned and tensioned between two aligned centering spindles such that the two centers of curvature of the lens coincide with the common axis of rotation of the centering spindles. As a result, the edge of the lens is processed in a defined relationship to the optical axis of the lens, as will be seen later for the Installation of the lens in a socket is necessary. In this case, the edge is shown by machining a defined geometry both in the plan view of the lens - the peripheral contour of the lens - as well as in radial section - contour of the edge, about rectilinear training or education with stage (s) / facet (s) - given. This is done especially in the case of glass lenses by a grinding process. However, when referring to "grinding" in the context of the present invention, it is intended to include "fine grinding" and "polishing" where equally geometrically indeterminate cutting is used.

With regard to the mechanisms used to center the relative feed motion between the grinding tool and the workpiece, LOH Optikmaschinen AG, Wetzlar, Germany (legal predecessor of Satisloh GmbH), used the two grinding spindles for the rotary drive of the older LZ 80 cam - controlled centering machines Grinding tools (grinding wheels) delivered by means of adjustable weights via a cable. The maximum feed movement of the grinding spindles themselves was controlled by slowly rotating cams on which a scanning roller coupled to the respective grinding spindle ran as a fixed stop. Although this very simple mechanical solution had advantages in terms of the possible process speed, because the feed largely depends on the performance of the grinding wheels and the ground substrate material itself, a serious disadvantage was that a separate cam was provided for each workpiece geometry.

Also solutions are known (see, for example, the document EP-A-1 693 151 which, however, does not concern a centering machine) in which the grinding force is adjusted via the bias of springs acting on the grinding spindle. The usage However, the use of springs in adjusting the grinding force has disadvantages when it comes to grinding non-circular, in particular angular, geometries on rotating workpieces. Namely, at the corners, the workpiece tends to push the grinding wheel against the direction of advance, increasing the preload of the spring acting on the grinding spindle. This in turn causes an undesirable increase in the grinding force, which can lead to a depression, that is to say a shape error, in the region of the corner of the workpiece which presses against the grinding wheel.

In modern CNC-controlled centering, which enable the grinding of any workpiece shapes via a corresponding web guide of the tool and / or workpiece, usually a forced feed control is provided. However, if the feed rate is chosen too fast, it may lead to an overload of the grinding tool and possibly also to "burning" of the workpiece in the contact point between the tool and workpiece, which in particular when using mineral oil as a cooling lubricant to deflagration and significant consequential damage ( not only) on the centering machine can lead. Of course, this can be remedied by programmed safety distances, e.g. such that the feed rate is set high up to a predetermined distance between tool and workpiece and is switched to a lower feed rate when this distance is reached. However, such security mechanisms inevitably require longer processing times.

Finally, so-called "adaptive control" solutions are known (see, for example, the publication US-A-2006/0073765 , on which the preamble of claim 1 is based), in which the power consumption of the grinding spindle and / or the rotary drive for the workpiece or signals from specially provided force transducers are used as input for a feed limitation. A disadvantage of one of the Current consumption of the grinding spindle dependent feed control is that the latter is sluggish due to the high cutting speeds required for grinding due to the inertia of the grinding spindle and tool and therefore only delayed, possibly too late reacts. The use of a force sensor, however, in particular has the disadvantage that it must always be arranged between the tool and machine, or workpiece and machine, which functionally leads to a certain flexibility of the machine, which can be detrimental to a high quality and accuracy workpiece.

TASK

The invention has for its object to provide a method for grinding workpieces, namely for centering grinding workpieces such as optical lenses, which addresses the above-mentioned in the prior art problems. In particular, in this case, the feed movement between the grinding tool and the workpiece should be such that on the one hand during grinding neither an overload of the grinding tool still a "burning" or a shape defect on the workpiece occurs / arises, on the other hand, the feed movement and Materialzerspanung be performed as quickly and efficiently.

PRESENTATION OF THE INVENTION

This object is achieved by the features specified in claim 1. Advantageous or expedient developments of the invention are subject matter of the claims 2 to 5.

According to the invention, in a method for grinding workpieces, in particular for centering grinding of workpieces such as optical lenses, by means of a grinding tool using an actuator for generating a relative feed movement between the grinding tool and the workpiece, which is integrated with a current regulator for an actuator current determining actuator force in a position control loop which is traversed with a predetermined control cycle, for each control cycle first (i) Direction of movement of the feed motion and an actual direction of movement of the feed motion determined; then (ii) the determined actual direction of movement of the feed movement is compared with the determined desired direction of movement of the feed movement; and finally (iii), if the comparison results in a deviation between the actual direction of movement of the advancing movement and the desired direction of movement of the advancing movement, a predetermined current limit for the actuator current delivered via the current regulator is definedly reduced in order to reduce the advancing force of the actuator.

By this method, in which the feed motor (actuator) specified a variable feed force on the motor current, closed on the basis of the desired and actual directions of the feed movement on the current force ratios and as a result the feed force is influenced process dependent on the motor current, in particular the removal rate optimized during grinding, especially when centering off non-circular workpieces. This results in comparison to the prior art significant shortening of the process times, a loss of safety distances, a simple Anschnitterkennung and a secure prevention of overload conditions of workpiece and tool due to excessive feed speeds or collisions. The actual feed rate is ultimately determined by the removal rate of the tool, which can change in the process by, for example, blunting or clogging of the abrasive coating or a change in the coolant and lubricant properties. By evaluating the setpoint and actual directions of the feed motion and the use of the power / current dependence of the feed motor are finally external force transducer or the like. dispensable; The workpiece quality and accuracy possibly detrimental resiliency are thus avoided.

Preferably, for determining or determining the directions of movement of the feed movement in step (i) above, the setpoint and actual positions of the actuator are evaluated from the current control cycle and from the preceding control cycle, which can be tapped without problems on the position control loop.

With regard to a good possibility of influencing the behavior of the current change, it is further preferred if a comparison signal is generated in the comparison of the ascertained actual direction of movement of the feed movement with the determined desired direction of movement of the feed movement in step (ii) above, via a PI - or PID transmission element generates a current reduction signal, wherein in step (iii) then a signal for the predetermined current limit to the respective current reduction signal is reduced as the current limiting signal is applied to the current regulator.

In order to optimize the grinding process for the machining of non-circular geometries, which may be more or less "angular", preferably depending on the shape of the workpiece to be ground, different parameter sets for the proportional component (gain K _P ) and the integral component (reset time T _N ) of the PI or PID transmission link.

Although arbitrary actuators can be used as a feed drive for the grinding method according to the invention, as long as they have a defined force / current dependency, it is finally preferred, in particular with regard to high control sensitivity, fast reaction behavior, ease of movement and self-locking freedom, etc. when a linear motor is used as the actuator for generating the relative feed movement between the grinding tool and the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1

eine Vorderansicht einer lediglich schematisch dargestellten Zentriermaschine für insbesondere optische Linsen, bei der das erfindungsgemäße Schleifverfahren Anwendung finden kann;

Fig. 2

eine prinzipielle Darstellung zu einem zentrierenden Schleifprozess, wobei im oberen Teil der Figur der Beginn der eigentlichen Schleifbearbeitung und im unteren Teil der Figur das Ende der eigentlichen Schleifbearbeitung gezeigt ist;

Fig. 3

ein vereinfachtes Blockschaltbild eines Lageregelkreises für einen Vorschubantrieb der Zentriermaschine gemäß Fig. 1, mit übergeordneter Stromsteuerung bzw. -begrenzung für die Durchführung des erfindungsgemäßen Schleifverfahrens;

Fig. 4

eine prinzipielle Darstellung zu einem zentrierenden Schleifprozess mit erfindungsgemäßer Verfahrensweise, der an einem Werkstück mit unrunder Außenkontur durchgeführt wird, zur Veranschaulichung der Änderung des entgegen der Vorschubkraft wirkenden Prozesskraftanteils infolge des sich drehwinkelabhängig ändernden Abstands des Eingriffspunkts zwischen Schleifwerkzeug und Werkstück zur Werkstück-Drehachse und der sodann entsprechend reduzierten Vorschubkraft; und

Fig. 5

ein Diagramm, in dem exemplarisch für einen zentrierenden Schleifprozess mit erfindungsgemäßer Verfahrensweise der Vorschubweg x (oben) und der infolge der Begrenzung des Aktuatorstroms zugelassene Schleppfehler (unten) über der Zeit t aufgetragen sind.

In the following the invention with reference to a preferred embodiment will be explained in more detail with reference to the accompanying simplified drawings. In the drawings show:

Fig. 1

a front view of a centering machine only schematically shown for optical lenses in particular, in which the grinding method according to the invention can be applied;

Fig. 2

a schematic representation of a centering grinding process, wherein in the upper part of the figure, the beginning of the actual grinding machining and in the lower part of the figure, the end of the actual grinding machining is shown;

Fig. 3

a simplified block diagram of a position control loop for a feed drive of the centering according to Fig. 1 , with superordinate current control or limitation for carrying out the inventive grinding process;

Fig. 4

a schematic representation of a centering grinding process with inventive method, which is performed on a workpiece with non-circular outer contour, to illustrate the change in the counter to the feed force acting process force component as a result of the angle of rotation changing distance of the engagement point between the grinding tool and the workpiece to the workpiece axis of rotation and the then correspondingly reduced feed force; and

Fig. 5

a diagram in which, for example, for a centering grinding process with the inventive method of the feed path x (top) and the permitted due to the limitation of the actuator current lag error (below) are plotted over time t.

DETAILED DESCRIPTION OF THE EMBODIMENT

In Fig. 1 a CNC-controlled centering machine 10 for grinding workpieces, in particular optical lenses L is shown only schematically and only insofar as it appears necessary for the understanding of the present invention. Further details on the structure and function of the centering machine 10 can be submitted at the same time German patent application DE 10 2012 XXX XXX.X are taken from the present applicant, which is hereby incorporated by reference.

In Fig. 1 On the left, two centering spindles 12, 14 arranged in alignment with respect to a centering axis C, whose centering spindle shafts 16, 18 can be driven in rotation independently of one another in the rotation angle (workpiece rotation axes C1, C2). A synchronous operation of Zentrierspindelwellen 16, 18 is effected in a conventional manner CNC technically. At the ends facing each other, the centering spindle shafts 16, 18 are each designed to receive a clamping bell 20, 22, as is known from the German standard DIN 58736-3. Between the tensioning bells 20, 22, the optical lens L for firmly gripped the grinding of its edge. The necessary lifting and tensioning devices that allow a defined movement or force application to one of the centering spindles 12, 14 along the centering axis C, are in Fig. 1 Not shown. In a direction perpendicular to the centering axis C, the centering spindles 12, 14 are fixed, ie not movable.

On the tool side (at least) a tool spindle 24 is provided with a rotary drive for a tool spindle shaft 26, on which a grinding wheel G is held as a grinding tool. The grinding wheel G is thus according to the arrow in FIG Fig. 1 Controlled rotationally driven in the rotational speed (tool rotation axis A) in order to provide with its circumferential surface U for a material removal on the workpiece L.

The tool spindle 24 is further mounted on an X-carriage 28, the CNC-position controlled in Fig. 1 is linearly movable to the right or left (linear axis X, feed movement). For this purpose, the X-carriage 28 is guided over guide carriages, not shown here, on two parallel running guide rails 30, 32 (not shown) on a machine bed. For driving the X-carriage 28 is a linear motor 34 as an actuator, from the in Fig. 1 the machine bed fixed stator 36 can be seen with its magnets. The rotor (coils) of the linear motor 34 is mounted under the X-carriage 28 and in Fig. 1 not to be seen. In Fig. 1 above the X-carriage 28, a linear displacement measuring system 38 is arranged, by means of which the axial position (x _is ) of the X-carriage 28 can be detected in a known per se.

In Fig. 1 Finally, the feed force F _V acting in the direction of the centering axis C, which is applied to the X-carriage 28 by means of the linear motor 34, is indicated above the linear displacement measuring system 38 or the centering spindle 14 can be and whose size is proportional to the current applied to the rotor of the linear motor 34 current I, and left the along the x-direction of the feed force F _V counteracting process force component F _P , which is dependent on the speed and direction of the workpiece L, the speed and direction of the grinding wheel G (synchronism / countercurrent), the material and the geometry of the workpiece L, the material, the geometry and the state of wear of the grinding wheel G, the cooling and lubrication (friction) at the point of engagement between the workpiece and grinding wheel G, etc ,

The Fig. 2 illustrates a centering grinding process in general form: Via the linear motor 34, a feed movement V of the rotating around the tool rotation axis A grinding wheel G is effected according to the arrow. In this case, the X-axis is to be adjusted in such a way that the axis (C) rotatably driven about the centering axis (workpiece rotation axis C1) optical lens L, which may initially have any outer contour AK (octagonal in the example shown), defined by an NC program Final contour EK is centered. In the case of a non-round final contour EK, such as the slightly elliptical final contour EK shown here, the feed axis X is additionally coordinated in a known manner with the workpiece rotation axis C1, for which the latter is provided with a high-resolution angle measuring system WM (see Fig. 1 ) is provided. It can be seen that the grinding wheel G in a non-circular machining of workpieces L is not continuous in a feed direction, ie in Fig. 2 can only be moved to the left, but rather - at least at the end of machining - must be moved back and forth in dependence on the angle of rotation of the workpiece L about the centering axis C along the feed axis X to generate the non-circular final contour EK.

The Fig. 3 shows the position control circuit 40 for the linear motor 34 (feed drive) with the aid of a simplified block diagram the centering machine 10 according to Fig. 1 to which a particular current control or limiting circuit, short current limiting 42, is assigned for the actuator current I for carrying out the inventive grinding process. The position control loop 40 comprises in a manner known per se - cf. for example the Textbook "Machine Tools Volume 3, Automation and Control Technology" by Prof. Dr.-Ing. Manfred Weck, 3rd edition 1989, VDI-Verlag, Dusseldorf, p. 195 , Figure 8-3 - a position controller 44, a speed controller 46, a current regulator 48 and the actuator driven therefrom (the linear motor 34 in the present case) as well as in the context of bearing recirculation a summation point 50 for the desired position x _soll and the actual position x _is . The linear displacement measuring system 38, which provides the actual position x _ist , is in Fig. 3 shown as little as the NC control, which specifies the desired position x _soll . Also subordinate speed and current feedbacks are not shown, which may be provided as part of a cascade control. The position control loop 40 is traversed as usual with a predetermined control cycle, eg with a cycle time or sampling rate of 2 ms.

It should be mentioned at this point, finally, that I _should in the position control loop 40 according to Fig. 3 the target current specification for the current controller 48 is indicated, which - if necessary after current feedback - in the position control circuit 40 in the endeavor to control the linear motor 34 so that the actual position value (actual position x _is ) as the control loop output the position setpoint (target position x _soll ) as a closed-loop control input follows as error-free as possible. The output via the current regulator 48 actuator current I, however, defined limited, namely at the expense of larger lag errors, for which purpose the current limit 42 is provided, which will be described below.

The input variables for the current limitation 42 are the ones predefined by the NC control for the feed axis X. Target position x _soll , the actual position x detected by the linear position measuring system 38 _is the feed axis X and also a maximum setpoint feed force F _Vsollmax predetermined by the NC control, which results in a predetermined current limit I _sollmax and which is even closer later will be explained.

In the in Fig. 3 The left upper functional element 52 is the setpoint positions X _{soll (n)} , x _{soll (n-1) of} the linear motor 34 from the current control cycle (n) and from the previous control cycle (n-1) by means of a Signumfunktion ("Sgn"). evaluated. The abbreviation "d / dt" (time derivative) stands for the following relationship: $$\mathrm{d}/\mathrm{dt}=\left({\mathrm{x}}_{\mathrm{should}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{should}\left(\mathrm{n}-\mathrm{1}\right)}\right)/\left({\mathrm{t}}_{\left(\mathrm{n}\right)}-{\mathrm{t}}_{\left(\mathrm{n}-\mathrm{1}\right)}\right)$$

Since the sampling rate is constant, this can be simplified with (t _(n) -t _(n-1) ) = const. $$\mathrm{d}/\mathrm{dt}=\left({\mathrm{x}}_{\mathrm{should}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{should}\left(\mathrm{n}-\mathrm{1}\right)}\right)$$

$$\left({\mathrm{x}}_{\mathrm{soll}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{soll}\left(\mathrm{n}-\mathrm{1}\right)}\right)=\mathrm{0}->\mathrm{Sgn}\left(\mathrm{d}/\mathrm{dt}\right)={\mathrm{R}}_{\mathrm{soll}\left(\mathrm{n}\right)}=0$$

$$\left({\mathrm{x}}_{\mathrm{soll}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{soll}\left(\mathrm{n}-\mathrm{1}\right)}\right)<\mathrm{0}->\mathrm{Sgn}\left(\mathrm{d}/\mathrm{dt}\right)={\mathrm{R}}_{\mathrm{soll}\left(\mathrm{n}\right)}=-\mathrm{1}$$

The result of the formed Signumfunktion is the target direction of movement R _{soll (n) of} the feed movement V in the current control cycle (s). The following three cases are possible: $$\left({\mathrm{x}}_{\mathrm{should}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{should}\left(\mathrm{n}-\mathrm{1}\right)}\right)>\mathrm{0}->\mathrm{sgn}\left(\mathrm{d}/\mathrm{dt}\right)={\mathrm{R}}_{\mathrm{should}\left(\mathrm{n}\right)}=+\mathrm{1}$$

$$\left({\mathrm{x}}_{\mathrm{should}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{should}\left(\mathrm{n}-\mathrm{1}\right)}\right)=\mathrm{0}->\mathrm{sgn}\left(\mathrm{d}/\mathrm{dt}\right)={\mathrm{R}}_{\mathrm{should}\left(\mathrm{n}\right)}=0$$

$$\left({\mathrm{x}}_{\mathrm{should}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{should}\left(\mathrm{n}-\mathrm{1}\right)}\right)<\mathrm{0}->\mathrm{sgn}\left(\mathrm{d}/\mathrm{dt}\right)={\mathrm{R}}_{\mathrm{should}\left(\mathrm{n}\right)}=-\mathrm{1}$$

In an analogous manner, in the in Fig. 3 right upper functional element 54, the detected actual positions x _{is (n)} , x _{is (n-1) of} the linear motor 34 from the current control cycle (n) and from the previous control cycle (n-1) evaluated by means of a Signumfunktion. Where: $$\mathrm{d}/\mathrm{dt}\left({\mathrm{x}}_{\mathrm{is}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{is}\left(\mathrm{n}-\mathrm{1}\right)}\right)/\left({\mathrm{t}}_{\left(\mathrm{n}\right)}-{\mathrm{t}}_{\left(\mathrm{n}-\mathrm{1}\right)}\right)$$

With (t _(n) - t _(n-1) ) = const., This expression is simplified to: $$\mathrm{d}/\mathrm{dt}=\left({\mathrm{x}}_{\mathrm{is}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{is}\left(\mathrm{n}-\mathrm{1}\right)}\right)$$

$$\left({\mathrm{x}}_{\mathrm{ist}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{ist}\left(\mathrm{n}-\mathrm{1}\right)}\right)=\mathrm{0}->\mathrm{Sgn}\left(\mathrm{d}/\mathrm{dt}\right)={\mathrm{R}}_{\mathrm{ist}\left(\mathrm{n}\right)}=0$$

$$\left({\mathrm{x}}_{\mathrm{ist}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{ist}\left(\mathrm{n}-\mathrm{1}\right)}\right)<\mathrm{0}->\mathrm{Sgn}\left(\mathrm{d}/\mathrm{dt}\right)={\mathrm{R}}_{\mathrm{ist}\left(\mathrm{n}\right)}=-\mathrm{1}$$

Accordingly, the following three cases are possible for the actual direction of movement R _{ist (n) of} the feed movement in the current control cycle (s): $$\left({\mathrm{x}}_{\mathrm{is}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{is}\left(\mathrm{n}-\mathrm{1}\right)}\right)>\mathrm{0}->\mathrm{sgn}\left(\mathrm{d}/\mathrm{dt}\right)={\mathrm{R}}_{\mathrm{is}\left(\mathrm{n}\right)}=+\mathrm{1}$$

$$\left({\mathrm{x}}_{\mathrm{is}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{is}\left(\mathrm{n}-\mathrm{1}\right)}\right)=\mathrm{0}->\mathrm{sgn}\left(\mathrm{d}/\mathrm{dt}\right)={\mathrm{R}}_{\mathrm{is}\left(\mathrm{n}\right)}=0$$

$$\left({\mathrm{x}}_{\mathrm{is}\left(\mathrm{n}\right)}-{\mathrm{x}}_{\mathrm{is}\left(\mathrm{n}-\mathrm{1}\right)}\right)<\mathrm{0}->\mathrm{sgn}\left(\mathrm{d}/\mathrm{dt}\right)={\mathrm{R}}_{\mathrm{is}\left(\mathrm{n}\right)}=-\mathrm{1}$$

In other words, in the first case (1), forward movement of the grinding wheel G tends to take place with respect to the centering axis C; in the second case (2), the distance of the grinding wheel G from the centering axis C does not change, that is. the grinding wheel G is stationary (no movement) and in the third case (3) there is a tendency, with respect to the centering axis C, for a rearward movement of the grinding wheel G.

The thus determined direction values (1, 0 or -1) for the desired direction of movement R _{is intended} and the actual direction of movement R _is the feed movement V are then each connected to a proportional-action transfer member (P term) 56 and 58, which outputs the respective signal with an adjustable gain. This gain can be varied to weight the influence of each signal.

oder $${\mathrm{R}}_{\mathrm{soll}\left(\mathrm{n}\right)}=-\mathrm{1}={\mathrm{R}}_{\mathrm{ist}\left(\mathrm{n}\right)}$$

- d.h. (a) die Schleifscheibe G soll sich bezüglich der Zentrierachse C tendenziell vorwärtsbewegen und bewegt sich tatsächlich auch vorwärts, oder (b) die Schleifscheibe G soll sich bezogen auf die Zentrierachse C tendenziell zurückbewegen und bewegt sich in der Tat auch zurück, so ist der Ausgang der Summationsstelle 60 gleich Null. Gleiches gilt für den Grenzfall der gewollt stehenden Vorschubachse X - $${\mathrm{R}}_{\mathrm{soll}\left(\mathrm{n}\right)}=0={\mathrm{R}}_{\mathrm{ist}\left(\mathrm{n}\right)}$$

- d.h. wenn (c) keine Vorschubbewegung V der Schleifscheibe G erfolgen soll und auch nicht vorliegt. Der Schleifprozess läuft bei diesen Fällen wie gewünscht; die Schleifscheibe G ist scharf.The thus amplified signals to the intended direction R _to and the actual direction of movement R _is the feed movement V are thereafter switched to a summing point 60, which by means of a difference (target value minus actual value) a Comparison of the determined actual direction of movement R _is the feed movement V with the determined target direction of movement R _{soll of} the feed movement V causes. If the determined nominal and actual directions of movement R _soll or R _are equal to the feed movement V, $${\mathrm{R}}_{\mathrm{should}\left(\mathrm{n}\right)}=+\mathrm{1}={\mathrm{R}}_{\mathrm{is}\left(\mathrm{n}\right)}$$

or $${\mathrm{R}}_{\mathrm{should}\left(\mathrm{n}\right)}=-\mathrm{1}={\mathrm{R}}_{\mathrm{is}\left(\mathrm{n}\right)}$$

that is, (a) the grinding wheel G should tend to move forward with respect to the centering axis C and, in fact, also move forward, or (b) the grinding wheel G should tend to move back relative to the centering axis C and in fact also move back so the output of the summation point 60 is equal to zero. The same applies to the limiting case of the intended feed axis X - $${\mathrm{R}}_{\mathrm{should}\left(\mathrm{n}\right)}=0={\mathrm{R}}_{\mathrm{is}\left(\mathrm{n}\right)}$$

- ie if (c) no feed movement V of the grinding wheel G is to take place and is not present. The grinding process runs as desired in these cases; the grinding wheel G is sharp.

und $${\mathrm{R}}_{\mathrm{soll}\left(\mathrm{n}\right)}=+\mathrm{1}\ne {\mathrm{R}}_{\mathrm{ist}\left(\mathrm{n}\right)}=-\mathrm{1}$$

The possible deviation cases in the above-described comparison in the summation point 60 include in particular the states: $${\mathrm{R}}_{\mathrm{should}\left(\mathrm{n}\right)}=+\mathrm{1}\ne {\mathrm{R}}_{\mathrm{is}\left(\mathrm{n}\right)}=\mathrm{0}$$

and $${\mathrm{R}}_{\mathrm{should}\left(\mathrm{n}\right)}=+\mathrm{1}\ne {\mathrm{R}}_{\mathrm{is}\left(\mathrm{n}\right)}=-\mathrm{1}$$

In the former deviation case (d), the grinding wheel G should move in the direction of the centering axis C (feed movement V in Fig. 2 ), but does not (blocking the feed axis X). Accordingly, at this time of the feed force F _V counteracting process force component F _{P is} at least equal to the feed force F _V (see. Fig. 1 ), causing the grinding wheel G is prevented from its further feed movement V. The reason for this can be, for example, a truncated / worn grinding wheel G or an insufficient supply of cooling lubricant.

The second-mentioned deviation case (e) can result in the grinding of a non-circular geometry on the workpiece L, if the process force component F _P exceeds the feed force F _V , after it comes due to the angle-dependent changing point of intervention for magnitude and effective direction changes of the grinding force, the workpiece L the grinding wheel G pushes away due to the non-circular outer contour AK of the workpiece L against the feed direction. This is in Fig. 4 illustrates: The rotating workpiece L pushes with its circumferentially changing radius to the centering axis C and its radially "protruding" contour sections the grinding wheel G by an amount .DELTA.x counter to the feed direction in Fig. 4 to the right.

In the described deviation cases, there is a risk of overstressing / overloading of workpiece L and / or tool G, which can lead to a "burning" at the point of engagement, in the non-circular machining also the risk of "digging" of the grinding wheel G in the workpiece L and Thus, of form errors on the workpiece L. In order to facilitate evasion of the feed axis X in these cases and to eliminate the associated breakaway torque of the linear guides 30, 32, the force limit of the feed axis X is dynamically reduced via the actuator current I.

More precisely, in the comparison of the determined actual direction of movement R _{ist (n) of} the feed movement V with the determined desired direction of movement R _{soll (n) of} the feed movement V in the summation point 60, a comparison signal is generated, which via a proportional integrating acting transfer element (PI element) 62 generates a current reduction signal I _{red (n)} . Alternatively, a fast PID element with, for example, a differential or derivative time T _V of zero or almost zero can be used here, which acts in a similar way to a PI controller.

The current reduction signal I _{red (n)} is applied as a subtrahend of a further summation point 64. The minuenden at the summation point 64 forms the predetermined current limit, ie a signal for a maximum target current I _sollmax , which results from a further proportional acting transfer member (P-member) 66 from the above-mentioned maximum target feed force F _Vsollmax , which is specified by the NC control. With this specification for the maximum setpoint feed force F _Vsollmax (eg 100 N), on the one hand, consideration is given to which feed force is desired for the actual grinding process, which can be input by the operator; On the other hand, force fluctuations of the feed axis X are taken into account by the influence of cogging torques of the linear motor 34 and force losses due to friction in the linear guides 30, 32 and the working space covers (not shown), which are determined once by way of example and as an additive value in the desired feed force F _{Vsollmax inflow} .

The summation point 64 finally outputs a current limiting signal I _{max (n)} (maximum nominal current I _sollmax minus the respective current reduction I _{red (n)} ), which is applied to the current regulator 48. As a result, the output from the current controller 48 to the linear motor 34 actuator current I, which determines the feed force F _{V of} the linear motor 34, dynamically limited to the current I _{max (n)} , ie despite possibly higher current setting I _{soll (n)} in the position control loop 40th the current regulator 48 outputs only the limited current I _{max (n)} to the linear motor 34. In the above moving direction deviation cases (d) and (e), this results in a reduction of the feed force F _{V (n) of} the linear motor 34 (illustrated with feed force force arrows of different lengths) F _V in Fig. 4 top and bottom right). In the above cases (a) to (c), however, in which there is no deviation of the actual and desired directions of movement of the feed motion V, the predetermined current limit, ie the maximum target current I _{sollmax is} not reduced because the summation point 60th Zero and consequently the current reduction signal I _{red (n) is} zero.

If there is a movement direction deviation according to cases (d) and (e) over several control cycles n, the current reduction signal I _{red (n) increases} correspondingly via the PI element 62; after the summation point 64, the permitted current I _{max (n)} accordingly becomes smaller and smaller from one control cycle to another. The control behavior of the PI member 62 - such as fast, "hard" or "soft" - can be influenced here by the parameters for the proportional component (gain K _P ) and the integral component (reset time T _N ) and also with regard to the material being processed be optimized. Depending on the roundness or the angularity of the workpiece geometry to be ground, it is advantageous to use different parameter sets for the reinforcement K _P and the reset time T _N from grinding process to grinding process, but then to be consistent throughout the grinding process. Thus, for a square, eg square outer contour AK, the gain K _{P is} quite high, but the reset time T _N is rather small; for a round or cornerless, for example elliptical outer contour AK, the gain K _P tends to be lower, but the reset time T _N tends to be higher preselected. The actual values for the controller parameterization are to be optimized individually for the respective centering machine 10 and the respective grinding process, so that a quantification should not take place here. Finally, if there is no longer any deviation in the comparison of the actual and desired directions of movement at the summation point 60, the actuator current I is again set via the current regulator 48 to a maximum of the preset value Current limit I _sollmax increases, whereby the feed force F _{V of} the linear motor 34 increases again accordingly.

The Fig. 5 shows in a diagram by way of example for a centering grinding process with the above - optionally switched on or off - Aktuatorstrom- or force limiting the linear motor 34 plotted over the time t above the feed path x (solid or dashed line) of the X-carriage 28, thus the tool spindle 24 with the grinding wheel G and underneath which builds up as a result of the limitation of the actuator current I following error (dotted line). At point a, the X-carriage 28 starts moving at a preselected feed rate, which does not have to be coupled to the removal capability of the tool and is preferably selected to be as high as possible from the grinding removal with a view to the fastest possible and efficient material cutting. At point b, the grinding wheel G encounters the workpiece L. While the actual position x _is the target position x _should follow essentially error-free up to the point b, "fall" actual position x _is (solid line) and target position x _soll (dashed line) thereafter "apart"; a following error (dash-dotted line below) arises. At point b, a short-term blockage of the feed motion V is to be expected (not visible in the graph) which, as described above, induces a reduction of the feed force F _V via the current limitation 42, so that overstressing of workpiece L or tool G does not take place , As a result, the position control loop 40 "endeavors" to compensate for the following error is limited in the energization of the linear motor 34, despite appropriate current input I _soll on the current controller 48 through the current limit 42 (I _max ). Only from the point c, when the end value of the setpoint position x _{soll is} reached, the following error builds up until the actual position x _{is reached} its final value at point d. In other words arising between the points b and d, the actual positions of the grinding wheel G _is x, and the speed of the feed movement V (slope of the graph) only as a result of the allowed over the current limit 42 feed force F _V. The latter is between the points b and due to the current limit 42 be d so large that _is no longer deviation between the actual movement direction R and the target traveling direction R _to the feed movement results in V, is therefore always have a maximum size within the permissible , The described force grinding process can be terminated if, at d, an adjustable limit value for the following error (eg 0.01 mm) is exceeded during a complete revolution of the workpiece L.

During (ua) at point b in Fig. 5 the deviation case (d) described above is to be expected (blocking of the feed axis X), illustrates the detail D _V in greatly enlarged in the x and the t direction Fig. 5 the situation in the above with reference to the Fig. 4 explained deviation case (e), when the rotating workpiece L, the grinding wheel G against the feed direction by an amount Δx pushes away. In this case, the point e in detail corresponds to D _V the state in Fig. 4 above, while the point f in detail D _v the state in Fig. 4 represented below. Accordingly, it comes to sawtooth repetitive increases in the following error (not shown repeatedly).

When the current limit 42 is activated, the amount of the preselected feed rate basically does not matter, because the desired actuator current I _soll output by the speed controller 46 may be limited in the current controller 48 during the processing anyway (I _max ). Thus, it is also possible to work with different preselected feed rates, for example with a rapid rapid traverse for fast approach of tool G and workpiece L and, on the other hand, a slower operation during machining. The switching point between rapid traverse and operation can be easily and safely found by continuous evaluation of the following error of the feed axis X. Be (edge detection) because at the moment of contact between tool G and workpiece L of the following error of the feed axis X by the lack of power reserve or limited feed force F _{V of} the linear motor 34 increases rapidly and sharply (see Fig. 5 the following error quickly builds up after the point b). A conventional safety distance to the workpiece L, which entails a considerable loss of time due to "air grinding in the work process", is not necessary since the force reduction on the linear motor 34 does not lead to any critical overloading or destruction of the tool G and / or workpiece L can come.

A method is disclosed, in particular, for centering grinding workpieces such as optical lenses by means of a grinding tool using an actuator for generating a relative feed movement between the grinding tool and the workpiece, wherein the actuator is integrated with a current regulator for an actuator current determining a feed force of the actuator in a position control loop which is traversed with a predetermined control cycle. In the method, for each control cycle: (i) a desired direction of movement of the feed movement and an actual direction of movement of the feed movement are determined; then (ii) the determined actual and desired directions of movement are compared with each other; and finally, (iii) when the comparison results in a deviation between the actual and desired directions of travel, a predetermined current limit for the actuator current delivered via the current regulator is definedly reduced to reduce the advancing force of the actuator. As a result, the feed movement and material cutting are carried out quickly and efficiently, without being able to overuse the tool or workpiece.

LIST OF REFERENCE NUMBERS

10

Centering

12

lower centering spindle

14

Upper centering spindle

16

lower center spindle shaft

18

Upper center spindle shaft

20

lower clamping bell

22

upper clamping bell

24

tool spindle

26

Tool spindle shaft

28

X slide

30

guide rail

32

guide rail

34

linear motor

36

stator

38

linear displacement measuring system

40

Position control loop

42

current limit

44

position controller

46

cruise control

48

current regulator

50

Summation point

52

functional member

54

functional member

56

P element

58

P element

60

Summation point

62

PI element

64

Summation point

66

P element

A

Tool rotation axis (speed-controlled)

AK

outer contour

C1, C2

Workpiece rotation axis (angular position-controlled)

C

centering

EK

final contour

F _P

Process force component in x-direction

F _V

feed force

G

Grinding tool / grinding wheel

I

actuator current

L

Workpiece / optical lens

R

Direction of movement of the feed motion

t

Time

U

Peripheral surface of the grinding wheel

V

feed motion

WM

Angle measuring system

x

Position of the grinding tool

Ax

Amount of tool offset

X

Feed axis / linear axis grinding tool (position-controlled)

Claims (5)

1. Method of grinding workpieces (L), particularly for centered grinding of workpieces such as optical lenses, by means of a grinding tool (G) with use of an actuator (34) for producing a relative advancing movement (V) between grinding tool (G) and workpiece (L), wherein the actuator (34) together with a controller (48) for an actuator current (I), which determines an advance force (F_V) of the actuator (34), is integrated in a position control circuit (40) which is run through with a predetermined control cycle (n), and characterized in that for each control cycle (n):

   (i) a target movement direction (R_soll(n) = -1, 0 or 1) of the advancing movement (V) as well as an actual movement direction (R_ist(n) = -1, 0 or 1) of the advancing movement (V) are determined;

   (ii) the determined actual movement direction (R_ist(n)) of the advancing movement (V) is then compared with the determined target movement direction (R_soll(n)) of the advancing movement (V); and

   (iii) if the comparison gives a difference between the actual movement direction (R_ist(n)) of the advancing movement (V) and the target movement direction (R_soll(n)) of the advancing movement (V) a predetermined current limit (I_sollmax) for the actuator current (I_(n)) delivered by way of the current controller (48) is subject to defined reduction in order to reduce the advance force (F_V(n)) of the actuator (34).
2. Method according to claim 1, wherein for determination of the movement directions (R_ist(n); R_soll(n)) of the advancing movement (V) in step (i) the target and actual positions (x_soll(n), x_soll(n-1); x_ist(n), x_ist(n-1)) of the actuator (34) are evaluated from the present control cycle (n) and from the preceding control cycle (n-1).
3. Method according to claim 1 or 2, wherein for the comparison of the determined actual movement direction (R_ist(n)) of the advancing movement (V) with the determined target movement direction (R_soll(n)) of the advancing movement (V) in the step (ii) a comparison signal is generated which produces a current reduction signal (I_red(n)) by way of a PI or PID transfer element (62) and wherein in the step (iii) a signal for the predetermined current limit (I_sollmax) reduced by the respective current reduction signal (I_red(n)) is applied as current limitation signal (I_max(n)) to the current controller (48).
4. Method according to claim 3, wherein different parameter sets for the proportional component (amplification K_P) and the integral component (reset time T_N) of the PI or PID transfer element (62) are used depending on the shape of the workpiece (L) to be ground.
5. Method according to any one of the preceding claims, wherein a linear motor (34) is used as actuator for producing the relative advancing movement (V) between grinding tool (G) and workpiece (L).

Images