EP2852472A2

EP2852472A2 - Method for grinding workpieces, in particular for centring grinding of workpieces such as optical lenses

Info

Publication number: EP2852472A2
Application number: EP13723403.5A
Authority: EP
Inventors: Joachim Diehl; Steffen Moos; Achim Schmidt
Original assignee: Satisloh AG
Current assignee: Satisloh AG
Priority date: 2012-05-22
Filing date: 2013-04-25
Publication date: 2015-04-01
Anticipated expiration: 2033-04-25
Also published as: HK1202489A1; WO2013174468A3; EP2852472B1; DE102012010004A1; CN104321163A; WO2013174468A2; CN104321163B; US9278421B2; US20150093967A1

Abstract

The invention relates in particular to a method for centring grinding of workpieces such as optical lenses by means of a grinding tool using an actuator (34) for generating a relative advancing movement between the grinding tool and the workpiece, wherein the actuator is integrated in a current regulator (48) for an actuator current which determines an advancing force of the actuator in a position control loop (40) which is run through using a predetermined control cycle. In the method, for each control cycle: (i) a desired direction of movement (R_soll(n)) of the advancing movement and an actual direction of movement (R_ist(n)) of the advancing movement are ascertained; then (ii) the ascertained actual and desired directions of movement are compared to one another; and finally, (iii) when the comparison results in a deviation between the actual and desired directions of movement, a predetermined current limit (I_Sollmax) for the actuator current emitted via the current regulator is decreased in a defined manner in order to reduce the advancing force of the actuator. As a result the advancing movement and material machining can be carried out quickly and efficiently without overstressing the tool or workpiece.

Description

METHOD FOR GRINDING WORKPIECES, ESPECIALLY FOR CENTERING GRINDING OF WORKPIECES SUCH AS OPTICAL LENSES

TECHNICAL AREA

The present invention generally relates to a method for 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 integrates with a current regulator for an actuator current determining actuator force in a position control loop is, 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.

PRIOR ART Lenses for objectives 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 by machining a defined geometry both in the plan view of the lens - peripheral contour of the lens

- as well as seen in radial section - contour of the edge, such as straight training or education with stage (s) / facet (s)

- given. This is done especially in the case of glass lenses by a grinding process. If, in connection with the invention before ^¬ general "loops" is mentioned, but this should also include "fine grinding" and "polishing" with, where equally worked with geometrically indeterminate cutting.

As far as the mechanisms used to center the relative feed movement between the grinding tool and the workpiece are concerned, the older ones were cam-controlled. Centering machines "LZ 80" of LOH Optikmaschinen AG, Wetzlar, Germany (legal predecessor of Satisloh GmbH), the two grinding spindle for the rotating drive of the grinding tools (grinding wheels) by means of adjustable weights delivered via a cable. The maximum feed motion of the grinding spindles itself was in this case controlled by means of slowly rotating cams on which a coupled to the respective grinding spindle ^¬ scanning roller expired as a fixed stop. Even ^though this very simple mechanical solution had advantages with regard to 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 disk had to be provided for each workpiece geometry ,

Solutions are also known (see, for example, document EP-A-1 693 151, which, however, does not concern a centering machine) in which the grinding force is adjusted by means of the pretensioning of springs which act on the grinding spindle. The use However, the use of springs in setting 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. A remedy here, of course, programmed safety distances create, for example, such that the feed rate up to a predetermined distance between see tool and workpiece is set high and is switched to this distance to a lower feed rate. However, such security mechanisms inevitably require longer processing times. Finally, so-called "adaptive control" solutions are known (see, for example, the document US-A-2006/0073765), in which the current consumption of the grinding spindle and / or the rotary drive for the workpiece or signals from specially provided force transducers as input for a forward thrust limit can be used. 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, if necessary reacts too late. The use of a force sensor, however, has the particular disadvantage that it must always be arranged between the tool and machine, or workpiece and machine, which functionally leads to a certain flexibility ^¬ speed of the machine, which can be detrimental to a high workpiece quality and accuracy ,

OBJECT The invention is based on the object of providing a method for

To provide grinding of workpieces, especially 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.

SUMMARY OF THE INVENTION This object is solved by the features specified in patent 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 Using an actuator for generating a relative feed movement between the grinding tool and the workpiece, which is integrated with a current controller for an actuator force of the actuator determining actuator current in a position control circuit, which is traversed with a predetermined control cycle, for each control cycle first (i) determines a desired direction of movement of the feed movement and an actual direction of movement of the feed movement; 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) given 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 conditions and as a result of the feed force is influenced process-dependent on the motor current, In particular, the removal rate during grinding, especially when centering off non-circular workpieces, is optimized. This results in comparison to the prior art, significant shortening of the process times, a loss of

Safety distances, a simple edge detection and a safe 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 yield ^¬ opportunities are thus avoided.

Preferably, for determining or determining the directions of movement of the advancing 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.

In view of a good exercise influence possible on the behavior of the current change, it is further preferred if, in the comparison of the determined actual direction of motion of the feed movement with the determined target direction of movement of the feed movement in the above step (ii), a comparison signal is generated ^¬ riert, which generates a current reduction signal via a PI or PID transfer element, wherein in step (iii) then a signal for the predetermined current limit is reduced by the respective current reduction signal 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 capability, 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

In the following the invention will be explained in more detail by means of a preferred embodiment with reference to the attached, simplified drawings. In the drawings show:

Fig. 1 is a front view of a centering machine only schematically shown for optical lenses in particular, in which the grinding method according to the invention

Application can find;

Fig. 2 is 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;

3 shows a simplified block diagram of a position control circuit for a feed drive of the centering machine according to FIG. 1, with superordinate current control or limiting for carrying out the inventive grinding method; Fig. 4 is a schematic representation of a centering

Grinding process with inventive method, which is carried out 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 rotation angle-dependent changing distance of the engagement point between

Grinding tool and workpiece to the workpiece axis of rotation and then correspondingly reduced feed ^¬ force; and

Fig. 5 is a diagram are plotted against time t in the way of example for a zentrie ^¬ leaders grinding process with the novel procedure the feed quantity x (top) and the permitted due to the limitation of the actuator current lag error (below).

DETAILED DESCRIPTION OF THE EMBODIMENT

In Fig. 1 is a CNC-controlled centering machine 10 for

Grinding of workpieces, in particular optical lenses L only shown 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 simultaneously submitted German

Patent Application DE 10 2012 XXX XXX. Are taken X of the present application ^¬ rin, is incorporated herein by reference genome ^¬ men.

Can be seen in Fig. 1 to the left, two with respect to a centering ^¬ axis C arranged in alignment centering spindles 12, 14, whose Zentrierspindelwellen 16, 18 independently in Drehwin ^¬ angle position controlled rotationally driven are (workpiece rotation axes Cl, C2). A synchronous operation of Zentrierspindelwellen 16, 18 is effected in a conventional manner CNC technically. At the ends facing each other, the Zentrierspindelwellen 16, 18 each for receiving a clamping bell 20, 22 ausgebil ^¬ det, 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 clamping devices that allow a defined movement or force application to one of the centering ^¬ spindles 12, 14 along the centering axis C, are not shown in Fig. 1. 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 rotatably driven according to the arrow in Fig. 1 in the rotational speed driven (tool rotation axis A) to provide with its peripheral 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 to the right or left linearly movable (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 the drive of the X-carriage 28, a linear motor 34 serves as an actuator, of which in Fig. 1, the machine-fixed stator 36 can be seen with its magnet. The rotor (coils) of the linear motor 34 is mounted under the X-carriage 28 and not visible in Fig. 1. In Fig. 1 above the X-carriage 28, a linear displacement measuring system 38 is arranged, by means of which the axial position (x ± _s t) of the X-carriage 28 can be detected in a known per se.

In Fig. 1 above the linear position measuring system 38 and the centering spindle 14 are finally indicated on the right acting in the direction of the centering C feed force F _v , which applied by means of the linear motor 34 on the X-carriage 28 can be and whose size is proportional to the applied to the rotor of the linear motor 34 current I, and left side 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 / mating), the material and the geometry of the workpiece L, the material, the geometry and the wear state of the grinding wheel G, the cooling and lubrication (friction) at the point of engagement between the workpiece and grinding wheel G , Etc.

FIG. 2 illustrates a centering grinding process in general form. Via the linear motor 34, a feed movement V of the grinding wheel G rotating about the tool rotation axis A is effected via the linear motor 34. In this case, the X-axis is to be adjusted in such a way that the axis (C) rotatably driven about the centering axis C (workpiece axis of rotation C) optical lens L, which may have an arbitrary outer contour AK ^¬ (in the example shown octagonal), on a by an NC Program defined final contour EK is centered. In a non-circular end contour EK, such as slightly elliptical final contour EK shown here, the feed axis X is additionally coordinated at itself be ^¬ known manner with the workpiece rotation axis Cl, (see Fig. 1) for which the latter with a high resolution angle measurement system WM provided is , It can be seen that the grinding wheel G can not be continuously moved in a feed direction, ie in FIG. 2 only to the left in a non-circular machining of workpieces L, but rather - at least at the end of processing - in dependence on the rotation angle of the workpiece L about the centering axis C has to be moved along the feed axis X before and ^¬ back to the non-circular end contour EK to generate ^¬ Center.

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 of FIG. 1, a special

Stromsteuer- or limiting circuit, short current limit 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) and in the frame The position feedback is a summation point 50 for the desired position x _so ii and the actual position- The linear position measuring system 38, which provides the actual position x _is , is shown in Fig. 3 as little as the NC-Steue - tion, which specifies the desired position x _S0 n. Also under ^¬ stored speed and current feedbacks are not shown, which may be provided as part of a cascade control. The position control circuit 40 as usual through with a particular prior ^¬ control cycle, for example, with a cycle time or sampling rate of 2 ms.

It should be mentioned at this point, finally, that I _so ii in the position control loop 40 of FIG. 3, the target current specification for the current controller 48 indicates that - possibly after current feedback - in the position control loop 40 in the endeavor is given, the

To control linear motor 34 so that the actual position value (actual position ^¬ Xist) as a control loop output the position setpoint (setpoint position Xsoii) ^a ls control loop input follows as error-free as possible. The output via the current controller 48 actuator current I, however, defined limited, namely at the expense of larger lag errors, for which purpose the current limit 42 is pre ^¬ see, which will be described below.

As input variables for the current limitation 42, the NC control for the feed axis X can be used. The desired position x _so ii, the actual position x detected by the linear position measuring system 38, _is the feed axis X and also a maximum setpoint feed force Fvsoiimaxr predetermined by the NC control, from which a predetermined current limit I _S oiimax is generated and which will be explained later in more detail.

In the upper functional element 52, which is in FIG. 3, the desired positions x _S oii (n), Xsoii (ni) of the linear motor 34 are derived from the current control cycle (n) and from the preceding control cycle (n-1) evaluated a Signumfunktion ("Sgn"). The abbreviation "d / dt" (time derivative) stands for the following relation: d / dt - (Xsoll (n) "Xsoll (nI)) / (t (n)" t ( _n- i))

Since the sampling rate is constant, this can be done with (t ₍ " ₎ - t _(n -i ₎ ) = const.

be simplified to: d / dt - (Xsoll (n) - Xsoll (n-I))

The result of the formed Signumfunktion is the target direction of movement Rsoii (n) of the feed movement V in the current Re gelzyklus (n). The following three cases are possible (x _S oii (n) - Xsoii (ni))> 0 -> Sgn (d / dt) = R _so n _(n) = +1

(Xsoii (n) - Xsoii (ni)) ⁼ 0 -> Sgn (d / dt) = R _so n ( _n ) = 0

(Xsoii (n) - soii (ni)) <0 -> Sgn (d / dt) = R _S oii (n) = -1

In an analogous manner, in the upper right-hand functional element 54 in FIG. 3, the detected actual positions x _are (n); -Xist (ni) of the linear motor 34 from the current control cycle (s) and from the previous control cycle (n-1) evaluated by means of a Signumfunktion. Where: d / dt - (Xist (n) - Xist (n-1)) / (t (n) "t ( _n -i))

With (t (n) - t ( _n-1 )) = const., This expression is again simplified to: d / dt = (Xist (n) - Xist (n-1))

Accordingly, the following three cases are possible for the actual movement direction Rist ₍ n _{) of} the feed movement in the current control cycle (s):

(1.) (Xist (n) - Xist (n-1))> 0 -> Sgn (d / dt) = R _is t (n) = +1

(2.) (Xist (n) - is (ni)) = 0 -> Sgn (d / dt) = R _is t _(n ) = 0

(3.) (Xist (n) - Xist (ni)) <0 -> Sgn (d / dt) = R _is t ₍ n) = -1 In other words, in the first case, (1.) with respect to Centering axis C tends to forward movement of the grinding wheel G instead, in the second case (2.), the distance of the grinding wheel G to the centering axis C does not change, ie the grinding wheel G is (no movement) and in the third case (3.) is related on the centering axis C, a rearward movement of the grinding wheel G tends to occur.

The thus determined direction values (1, 0 or -1) for the desired direction of movement Rsoii and the actual direction of movement Ri _{St of} the feed movement V are then respectively switched to a proportionally acting transmission element (P element) 56 and 58, respectively outputs respective signal with an adjustable gain. This gain can be varied to weight the influence of each signal.

The thus amplified signals for the desired direction of movement Rsoii and the actual direction of movement R _is the feed movement V are then switched to a summation point 60, the ver ^¬ means of a difference (setpoint minus actual value) a Comparison of the determined actual movement direction Ri _St of the feed movement ^¬ V with the determined target movement direction R _so ii of the feed movement V causes. Parts in this case the target and detected actual movement directions R _so and R ii _is the feed movement V match -

(a) should be (n) = +1 = Rist (n) or (b) Rsoll (n) = -1 = Rist (n)

- that is, (a), the grinding wheel G is to be with respect to the centering axis C to move forward tends and moves did ^¬ neuter also forward, or (b) the grinding wheel G intended to refer to the centering axis C to move back tends based and moves, in fact, also back, the output of the summation point 60 is zero. The same applies to the limiting case of the intended feed axis X -

(C) Rsoll (n) ⁼ 0 = Rist (n)

- i. 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.

The possible deviation cases in the above-described comparison in the summation point 60 include in particular the states:

(d) R _S oii (n) = +1 * Rist (n) = 0 and (e) R _S0 || (n) = +1 + Rist (n) = -1 In the former case (d), the Grinding wheel G move in the direction of the centering axis C (feed movement V in Fig. 2), but does not do this (blocking the feed axis X). Accordingly, the process force component F _P counteracting the feed force F _v is at least equal to the feed force F _v (see FIG. Disc 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 case of deviation (e) can be found in the

Abrasive machining of a non-circular geometry on the workpiece L result if the process force component F _P exceeds the feed force F _v , due to the angle-dependent changing engagement point for magnitude and Wirkrichtungsänderun- conditions of the grinding force, the workpiece L, the grinding wheel G due to the non-circular outer contour AK of the workpiece L pushes against the feed direction. This is illustrated in FIG. 4: The rotating workpiece L, with its radius which changes over the circumference to the centering axis C or its "protruding" contour sections extending in the radial direction, pushes the grinding wheel G to the right counter to the feed direction in FIG. 4 by an amount Δχ , 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 Rist ₍ n _{) of} the feed movement V with the determined desired direction of movement R _so ii (n) of the feed motion 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 _re d (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 similarly to a PI controller.

The current reduction signal I _re d (n) is applied as subtrahend a white ^¬ direct summation point 64th The minuenden at the summation point 64 forms the predetermined current limit, ie a signal for a maximum target current I _so iimax _/ which results from a further proportional acting transfer member (P member) 66 from the above-mentioned maximum target feed force Fvsoiimax, which is specified by the NC control. With this specification for the maximum set feed force ^c Vsollmax

(eg 100 N) takes into account on the one hand, what advance thrust for the actual grinding process is desired, which can be entered 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 at the working space covers (not shown), which are determined once only exem ^¬ plarily and as an additive value in the Nominal feed force Fvsoiimax.

The summation point 64 finally outputs a current limiting signal I _ma x (n) (maximum nominal current I _S oiimax minus the respective current reduction I _re d (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 _ma x (n), ie despite possibly higher current setting I _so ii (n ) in the management ^¬ loop 40 outputs the current controller 48 from only the limited current I _m ax (n) to the linear motor 34th In the above movement direction deviations cases (d) and (e), this leads to a reduction of the feed force F _V (n) of the linear motor 34 (illustrated with differently long force arrows for the forward displacement). Thrust F _v in Fig. 4 top and bottom right). In the above cases (a) to (c), however, where no deviation of the actual and desired directions of movement of the feed motion exists V, the predetermined current limit, that is, the maximum target current I _so IImax not reduced because the summation point 60 outputs zero and consequently the current reduction signal I _re d (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 _re d (n) increases correspondingly via the PI element 62; after the summation point 64, the permitted current I _m ax (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. Advantageously, depending on the roundness or angularity of the article to be sanded workpiece geometry of the grinding process for grinding process various ^¬ dene parameter sets for the gain K _P and the reset time T _N is used, but then for the respective grinding process continuous. So be for a square, eg square

Outer contour AK gain K _P quite high, the reset time T _N but rather small, the gain K _P rather low, the reset time T _N, however, tends to be higher pre ^¬ selected for a round or corner-less, for example elliptical outer contour AK. 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. If, in the comparison of the actual and desired directions of movement at the summation point 60, there is no longer any deviation, the actuator current I is again supplied via the current regulator 48 up to the preset value. th current limit I _so iimax increased, whereby the feed force F _{v of} the linear motor 34 grows again accordingly.

FIG. 5 shows, in a diagram by way of example, a zoning grinding process with the above-mentioned - optionally switched on or off - Aktuatorstrom- or force limit on the linear motor 34 plotted over the time t above the feed path x (solid or dashed line) X-carriage 28, thus the tool spindle 24 with the grinding wheel G and below the resulting due to the limitation of the actuator current I drag error (dot-dash 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 set position x _so ii to the point b substantially error-free follows, "fall" actual position x _is

(solid line) and target position x _so n (dashed line) after that "apart"; a following error (dash-dotted line below) arises. At point b it is a short ^¬ term blockade of the feed movement V expected (in the graph can not be seen), the limitation on the Strombe- as described above 42 induces a reduction of the feeding force F _v, so that overstressing of the workpiece L or Tool G did not take place. As a result, the position control loop 40 "endeavors" to compensate for the following error, however, when the linear motor 34 is energized, despite current command I _s oii at the current regulator 48, this is due to the current limitation 42

(Limited in _ax ). Only from the point c, when the final value of the desired position x _so ii 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

Graphs) merely as a consequence of the feed limit F _v permitted via the current limitation 42. The latter will be so large between the points b and d as a result of the current limit 42 that no longer deviation between the actual direction of movement Ri _S t and the desired direction of movement R _thus ii of the feed movement V, is always within the permissible maximum be great. 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.

While the deviation case (d) described above (b) is expected to be present at (b) in FIG. 5 (blocking of the feed axis X), the detail D _v in FIG. 5, which is greatly enlarged in the x and t directions, illustrates FIG Situation in the deviation case (e) explained above with reference to FIG. 4, when the rotating workpiece L pushes the grinding wheel G counter to the feed direction by an amount Δχ. Here, the point e in detail corresponds to D _v the state in Fig. 4 above, while the point f in detail D _{v represents} the state in Fig. 4 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 _so ii output by the speed controller 46 may be limited anyway in the current controller 48 during processing (Imax). Thus, also with different preselected feedforward speeds be worked, for example, with a fast rapid traverse for fast approach of tool G and workpiece L and a contrast 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 the 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 in Fig. 5 after the point b rapidly building

Following error). A common in the prior art security ^¬ distance to the workpiece L, which involves a considerable loss of time by "air abrasion in operation" with it, is not necessary since there by the force of reducing the linear motor 34 to no 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, the actuator having a current controller for an actuator current determining a feed force of the actuator in a position control loop is integrated, 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) if the comparison results in a deviation between the actual and desired directions of travel, a predetermined current limit for the actuator current output 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 machine

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-slides

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 speed control

48 current regulator

50 summation point

52 functional element

54 functional element

56 P member

58 P member

60 summation point

62 PI member

64 summation point

66 P member

A tool rotation axis (speed-controlled)

AK outer contour

Cl, C2 Workpiece rotation axis (angular position controlled) C centering axis

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 Movement direction of the feed movement t Time

U circumferential surface of the grinding wheel

V feed movement

WM angle measuring system

x position of the grinding tool

Δχ amount of tool offset

X feed axis / linear axis grinding tool

(position controlled)

Claims

CLAIMS:

1. A method for grinding workpieces (L), in particular for centering grinding workpieces such as optical lenses, by means of a grinding tool (G) using an actuator (34) for generating a relative feed movement (V) between the grinding tool (G) and workpiece ( L), wherein the actuator (34) with a current regulator (48) for a feed force (F _v ) of the actuator (34) determining the actuator current (I) in a position control loop (40) is integrated, which with a predetermined control cycle (s) and for each control cycle (s):

(i) a target direction of movement (R _so ii (n) = ~ lr 0 or 1) of the feed movement (V) and an actual direction of movement

(Ri _S t (n ₎ = -l 0 or 1) of the feed movement (V) determines ^¬ who;

(ii) the determined actual direction of movement (Ri _S t (n>) of

Feed movement (V) is then compared with the determined desired direction of movement (Rsoii (n)) of the feed movement (V);

and,

(iii) if the comparison is a deviation between the actual direction of movement (Ri _S t ₍ n>) of the feed movement (V) and the desired direction of movement (R _so ii (n)) of the feed movement (V) he ^¬ gives a predetermined current limit (I _so IImax) f ^for the output via the current regulator (48) actuator current (I _(n>) is reduced defined to the feed force (F _V _(n)) to reduce the actuator (34).

2. The method of claim 1, wherein for the determination of the directions of movement (Ri _S t ₍ n>; Rsoii (n)) of the feed movement (V) in step (i) the desired and actual positions (x _so ii (n) , XsoiKn-n / "

Xist (n), ^x is (n)) are evaluated of the actuator (34) from the current control cycle ^¬ (s) and from the preceding control cycle (n-1).

3. The method of claim 1 or 2, wherein in the comparison of the determined actual movement direction (Ri _S t (n>) of Vorschubbewe ^¬ tion (V) with the determined target movement direction (R _S oii (n)) of the feed movement ( V) in step (ii) a comparison signal is generated, which via a PI or PID transmission element (62) generates a current reduction signal (I _re d (n)), and wherein in step (iii) a signal for the predetermined current limit

(Isoiimax) ^to the respective current reduction signal (I _re d (n)) ver ^¬ reduced as a current limiting signal (I _ma x (n)) the current regulator (48) is switched.

4. The method of claim 3, wherein depending on the shape of the workpiece to be ground (L) different parameter sets for the proportional component (gain K _P ) and the integral part (reset time T _N ) of the PI or PID transfer member (62) be used.

5. The method according to any one of the preceding claims, wherein as an actuator for generating the relative feed movement (V) between the grinding tool (G) and the workpiece (L), a linear motor (34) is used.