FIELD OF THE INVENTION

The invention relates to an apparatus for a workpiece with a plane surface, wherein the workpiece is held by a pressure difference between the upper and lower surface, said pressure difference being generated by a gas stream. The invention further relates to a method for lifting a workpiece by using the apparatus according to the present invention.

BACKGROUND OF THE INVENTION

From the prior art, vacuum lifting cylinders for simultaneously aspiring and lifting of thin, porous and plane workpieces are known. Such a lifting cylinder is connected to a vacuum generator and a vacuum is applied thereto. When a low pressure develops upon contact of the sucker with the workpiece, the workpiece is aspired and thus held. The aspiring effect is only exerted on the top workpiece and is not exerted on workpieces lying thereunder. The workpiece is held until the vacuum is switched off. In order to be able to lift a workpiece with such a lifting cylinder, the cylinder has to be moved over the plane workpiece and has then to be lowered onto the workpiece until the sucker comes into contact with the workpiece. Thus, for transporting a workpiece a movement being parallel to the surface of the workpiece as well as a movement being vertical to the surface of the workpiece is necessary. Furthermore, especially when transporting thin and delicate workpieces, there is the risk of damaging the workpiece by the exerted low pressure and the physical contact between the sucker and the workpiece. Additionally the use in case of workpieces having a rough surface and dirty objects is problematic, since in these cases there is the risk that the sucker cannot contact the surface hermetically and thus, the holding force is reduced considerably or eliminated entirely.

From DE 35 36 432 A1 a holding device is known with which semiconductor wafer can be held and transported without contact. This holding device comprises a blow tube for introducing protective gas, the blow tube being mounted coaxially to a diffuser; the diffuser comprises a coaxial distribution cone being opposite to the end of the blow tube. The physical effect of the hydrodynamic paradox used here can be described by Bernoulli's equation.

The pressure difference between the outer environmental pressure and the developing static pressure results in that the workpiece can be lifted and transported or hold. That means that an equilibrium of forces between the weight of the semiconductor wafer and the pressure difference forms.

In order to avoid a radial sliding and tilting of the workpiece, the diffuser comprises four guiding pins and knobs at its circumference.

The holding device described is indeed suitable for holding and transporting plane workpieces, but during transport of the workpiece with this holding device no high accelerations in the direction of the effective direction of the holding force, i.e., in a direction parallel to the surface of the workpiece, are possible, since the workpiece is held by the holding device only by the low pressure between the holding device and the workpiece contact-free, i.e., without any surface contact. The guiding pins can only avoid a lateral sliding of the workpiece when the sliding is directed vertically to the holding force, i.e., in the direction of the surface of the workpiece if the distance between workpiece and device during holding and transporting is accurately adjusted and maintained so that the guiding pins are flush with the workpiece. If the distance between the workpiece and the diffuser is too broad or the workpiece is pushed away from the device or tilts due to blows during transport the guiding pins cannot avoid a lateral sliding of the workpieces. Thus, in case of an acceleration in a vertical direction to the lifting force, the workpiece remains due to its inertia whereby the low pressure between diffuser and workpiece will get lost and the workpiece cannot be hold any longer.

A similar device is known from DE 25 39 036 A1.

The known holding device is completely unsuitable for holding and transporting a workpiece having a through-going central opening, like in the case of an optical data carrier, e.g. a CD, DVD or a Blu-ray disk, since it is not possible to generate a sufficient low pressure in the gap between the workpiece and the device as a considerable part of the introduced gas would vanish through the opening of the workpiece.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus and a method for holding a workpiece by using the Venturi effect and for transporting a workpiece in the direction of the holding force and/or vertically thereto.

This object is achieved by the features of the claims.

The apparatus according to the present invention also allows accelerations being directed vertically to the holding force, i.e., parallel to the surface of the workpiece to be held.

An apparatus according to the present invention and a method according to the present invention allows the lifting of workpieces without the apparatus itself having to be moved vertically, i.e., in the direction to the workpiece.

The invention allows the lifting, holding, transporting and depositing of workpieces comprising a plane surface with an impression or a through-going opening.

The apparatus is configured such that using the Venturi effect, a low pressure is generated in a gap between the apparatus and the plane surface of the workpiece by means of a suitable air stream. As the low pressure is lower than the outer environmental pressure an equilibrium of forces is formed between the weight of the workpiece on the one hand, and the force resulting from the difference in pressure, on the other hand. Thereby a so-called lifting force being vertical to the workpiece, is generated by means of which the workpiece can be lifted and held. Additionally, the apparatus can comprise a holding plug which engages into the impression or through-going opening when the workpiece is lifted.

The apparatus according to the invention has the advantage that very thin and delicate workpieces can be transported. The surface of the workpiece is only held by the pressure difference between the upper and lower side of the workpiece. Thus, the workpiece and particularly its surface is not influenced by the transport.

Preferably, the introduced gas cools down the workpiece during transport in case the workpiece has a raised temperature after production or a preceding processing step. In order to achieve a cooling of the side facing away from the apparatus and, thus, to avoid deformation by cooling uniformly the workpiece on both sides a means for guiding an air stream to the lower side of the workpiece can be provided.

The holding plug of the apparatus according to the invention allows that the workpiece can be reliably transported even if accelerations vertical to the lifting and holding force generated by the low pressure occur. Here, the holding plug ensures that the workpiece does not slip from the apparatus due to its inertia even at high accelerations and that the low pressure generated between the apparatus and the workpiece is maintained and thus the workpiece is reliably hold.

Here, preferably a sufficient play between holding plug and impression or through-going opening of the workpiece is provided. The play is chosen in such a manner that the workpiece can easily, i.e., without additional forces, be taken up by or removed from the holding plug.

Preferably, the workpiece is an optical data carrier like a CD, DVD or a Blu-ray disk, wherein the holding plug which is preferably arranged in the middle of the apparatus, is adapted to the dimensions of the impression or the opening of the workpiece.

Furthermore, the invention provides a method for lifting a workpiece using the apparatus according to the invention.

According to the present invention, the workpieces can be lifted a considerable distance against the gravity or transported in a direction vertical to the surface of the workpiece by using the Venturi effect. In tests, e.g., a vertical lift of 50 mm has been achieved.

The method according to the invention has the advantage in that the apparatus which can be provided with a holding plug if the workpiece comprises an impression on the surface or an opening, only has to be moved horizontally or parallel to the surface of the workpiece in a certain distance above said workpiece in order to be positioned in a suitable position vertically above said workpiece.

Contrary to the prior art, it is no longer necessary to move the apparatus vertically to the surface of the workpiece in order to pick up the workpiece. According to the present invention, the apparatus is, e.g., only horizontally moved over the workpiece and the approaching between apparatus and workpiece as well as the holding effect is exclusively achieved by the lifting force generated by the low pressure.

Thus, considerable saving of costs can be achieved: on the one hand, the apparatus can be configured in a simpler manner since no additional means for a movement vertically to the surface of the workpiece have to be provided; furthermore, the cycle period for lifting, holding and transporting, and depositing can be considerably reduced with regard to the state of the art, since no additional vertical lift is necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail by means of the drawings:

FIG. 1 shows a cross-sectional view of an apparatus according to the present invention for holding and transporting a workpiece,

FIG. 2 shows a partial cross-sectional view of the apparatus according to the present invention according to FIG. 1,

FIG. 3 shows a cross-sectional view of an apparatus according to the invention for lifting, holding and transporting a workpiece,

FIG. 4 shows a cross-sectional view of an alternative embodiment of an apparatus according to the invention,

FIG. 5 shows a cross-sectional view of an apparatus according to the invention for lifting and depositing a workpiece, and

FIG. 6a and 6b show a cross-sectional view of an apparatus according to the invention having an ejecting means.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 schematically show a cross-sectional view or a partial cross-sectional view, respectively, of an apparatus 1 according to the invention for holding and transporting a workpiece 2. In the condition shown, the workpiece 2 rests on a support 15 which is connected to a machine part not shown via a clutch 16 indicated in broken lines. From this support 15 the workpiece 2 should be lifted and transported by means of the apparatus 1. For this purpose, first the apparatus 1 has been coaxially aligned with the workpiece by being moved in the direction of arrow A by means of a transport arm 12 indicated in broken lines and to which apparatus 1 has been fixed, and has then been lowered in the shown position by being moved in the direction of arrow B so that a gap 10 remains between the holding surface facing the workpiece 2 and the apparatus 1. In this position, a centering or holding plug 6 engages in an opening 3 of the workpiece 1. Now gas or a gas mixture is inserted into the apparatus 1 over a gas supply 8. The pressure, at which the gas or the gas mixture is supplied, is, e.g., about 1 to 7 bar, depending on the force necessary for the reliable holding or lifting of the workpiece 2.

The gas or gas mixture should preferably not comprise any solid or liquid components, in order to avoid a contamination of the apparatus as well as of the held workpiece 2. Through the gas supply 8, the gas is introduced into a space 9 between the centering plug 6, a plane surface 4 of the workpiece 2 and the plane parallel holding surface 7 of the apparatus 1, so that a radial gas stream through the gap 10 forms.

The space 9 is preferably configured as a ring space and the gap 10 as a ring-gap so that the gas radially flows from the ring space through the ring gap 10 from the inner side to the outer side.

The effect of the hydrodynamic paradox used here has the effect that the static pressure in the area of gap 10 and space 9 is lower than the environmental pressure. The velocity v of the gas (density p) decreases radially from the inner to the outer side along the gap 10 while the static pressure increases from the inner to the outer side in the area of the gap 10.

This dependency is shown by Bernoulli's equation $\begin{matrix} P & + & \frac{Ï}{2} Â\cdot v^{2} & = & P & = & Constant \\ static & â€ƒ & dynamic & â€ƒ & total â ¢ â€ƒ â ¢ pressure & â€ƒ & â€ƒ \\ pressure & â€ƒ & pressure & â€ƒ & â€ƒ & â€ƒ & â€ƒ \end{matrix}$

The holding plug 6 serves for centering the workpiece 2, when said workpiece is held and transported by the apparatus 1 due to the Venturi effect. The length of the holding plug 6 is preferably chosen in such a manner that it sufficiently extends through the opening 3 of the workpiece 2 or into the impression of workpiece 2 to reliably hold the workpiece 2 when the workpiece 2 is lifted. Thus, the workpiece 2 remains secured at the apparatus 1 against slipping or tilting even if pushes or other outer impacts occur during the transport. A further advantage is that variations in the distance between the workpiece and the apparatus during holding and transporting, e.g., due to fluctuations in the gas supply, can be compensated for and, thus, the reliability of the apparatus is increased. Depending on the length of the holding plug 6 and the kind of the workpiece to be transported, the distance between the workpiece and the apparatus can be varied. This can also be achieved by a stop 6â€² indicated in FIG. 2 by broken lines, at holding plug 6. Said stop 6â€² guarantees a predetermined distance d between the holding surface 7 of the apparatus 1 and the surface 4 of the workpiece 2. Thus the dimension of gap 10 can be predetermined.

A radial play is present between holding plug 6 and the impression or through-going opening of the workpiece 2, wherein said play is preferably selected in such a manner that the workpiece can easily be picked up and deposited by the holding plug. The play is preferably adjusted that high that no friction between holding plug 6 and workpiece 2 occurs when the workpiece 2 is picked up or deposited. The play is preferably so small that when the workpiece 2 is picked up a sufficient low pressure between workpiece 2 and apparatus 1 is guaranteed, i.e., it should only leak a small gas amount through the gap between holding plug 6 and opening 3 of the workpiece 2. If the stop 6â€² shown in FIG. 2 by broken lines is provided as plane element, wherein when being picked up, the surface 4 of the workpiece 2 entirely contacts the lower side thereof essentially no air can leak between the holding plug 6 and the workpiece 2. Alternatively, the stop 6â€² can also be configured, for example, in a ripped-shape so that air can flow between the stop 6â€² and the surface 4 of the workpiece 2. In this case, too, it should be guaranteed that only small gas amounts leak through the gap between holding plug 6 and the opening 3 of the workpiece 2.

The holding plug 6 can comprise in an embodiment at least one recess, so that it can engage into a centering with a corresponding projection when the workpiece 2 is taken over.

Furthermore, the holding plug 6 can be biased by a spring arrangement and configured in such a manner, that it can be inserted behind a contact surface 11 for picking up the workpiece 2 which, for example, can be held in position by another holding plug. The holding plug 6 can also be configured in such a manner that it operates an opposite holding plug, not shown, which, for example, can be biased by a spring arrangement. Furthermore, the holding plug 6 can also be arranged interchangeably at the apparatus 1, so that it can easily be replaced by another holding plug having different dimensions, for the transport of another workpiece.

Within the scope of the invention, the apparatus can also be configured without an central holding plug and having instead holding plugs at the outer circumference of the holding surface 7, said holding plugs having the form of distance pieces which are suitable to prevent a lateral slipping of the workpiece and to limit the gap 10. Thus, it is possible to lift, hold and transport workpieces without a central opening 3 or impression.

Generally the present invention provides also a method for lifting a workpiece 2 using the apparatus according to the invention.

It has been shown that the apparatus according to the invention is capable of lifting or moving the workpiece 2 over a transport distance vertically to the surface of the workpiece by generating a sufficiently great gas stream and by using the Venturi effect between the apparatus and the workpiece, so that a mechanical movement of the transport apparatus 1 vertically to the surface of the workpiece 2 is no longer necessary, but that only a movement in a plane parallel to the surface of the workpiece is sufficient for the entire transport between pick up and deposit position.

In the start position shown in FIG. 3, the workpiece 2 rests in a hutch 18 in a rest area 19 where said workpiece is centered with its opening 3 by a second holding plug 17. The apparatus 1 has been coaxially aligned with its holding plug 6 to the holding plug 17 and thus with the opening 3 of the workpiece 2, by means of the transport arm 12 in the direction of arrow A parallel to the surface of the workpiece 2. In contrast to the embodiment of FIG. 1 a lifting movement of the apparatus in an axial direction is not necessary; rather, in the embodiment according to FIG. 3, the workpiece 2 is lifted from the holding plug 17 by inserting gas into the apparatus 1 using the Venturi effect, and thereby plugged on the holding plug 6; in FIG. 3, the thus lifted workpiece 2 is indicated by broken lines. Subsequently, the horizontal transport of workpiece 2 can be effected by a simple horizontal movement in the direction of arrow A, without the need of a movement of apparatus 1 being vertical to the direction of the horizontal movement and being vertical to the surface of the workpiece. The same applies in the reversed direction for the depositing of workpiece 2 from the apparatus 1 onto the holding plug 17 in the hutch 18 in the rest area 19.

According to FIG. 3, in this transport cycle the apparatus 1 is positioned with its holding plug 6 coaxially above the impression or opening 3 of the workpiece 2. The holding plug 6 preferably comprises a beveled edge 11 (see FIG. 2) in order to guarantee a guiding of the workpiece to be lifted and to compensate for possible inaccuracies in the horizontal positioning of apparatus 1.

By inserting a gas into the gas supply 8 and generating a gas stream through the space 9 and the gap 10 in a radial direction from the inner to the outer side the workpiece 2 is lifted automatically. Equal to play 5 between the opening 3 and the holding plug 6, a predetermined play between the opening 3 of the workpiece and the holding plug 17 allows that the workpiece can be picked up and later deposited without additional forces to be applied, e.g., to overcome friction between the workpiece and the holding plug.

FIG. 4 shows schematically a cross-sectional view of an alternative embodiment of the apparatus according to the present invention. In the embodiment shown in FIG. 4, the holding surface 7 of the apparatus 1 is configured as a great plate, the area of which is greater than the area of the workpiece 2. The great plate 7 has the effect on the one hand that the air consumption during lifting and holding of the workpiece 2 is reduced. Furthermore, the great plate prevents that scratches form on the surface of the workpiece 2 facing the apparatus 1 when the workpiece is lifted. Even if the workpiece 2 does not remain parallel to the plate 7, but raises, e.g., tiltedly, the workpiece 2 only contacts with its outer edge 21, the conical, beveled area 7â€² of the plate 7, and not with the surface of the workpiece 2 facing the holding surface 7 of the apparatus 1 according to FIG. 1, as in the case of the embodiments shown in FIGS. 1 and 2. In contrast to the embodiment shown in FIG. 3, which also comprises a great plate 7, the area of which also extends over the area of the workpiece 2, in the embodiment shown in FIG. 4 the edge 21 of the workpiece 2 contacts the conical, beveled edge 7â€² of the plate 7 and is not hold in the area of the central opening by a stop 6â€² (see FIG. 2).

Additionally, in FIG. 4 a further preferred feature of the invention is shown. When hot workpieces, e.g., hot disks directly after casting, are lifted, held or transported by the apparatus according to the present invention, there is the risk that the workpieces are permanently deformed by being subjected to blowing of generally cool air on one side. In order to avoid said deformation, a ring channel 81 being coaxially to the workpiece 2 and the apparatus 1, can be provided in the area of the holding plug 6 below the holding position of the workpiece 2; through said ring channel, air being directed inclinedly upwards according to FIG. 4, can also be guided to the surface of the workpiece 2 facing away from the apparatus 1. Thus, a uniform cooling of the workpiece is guaranteed. By this additional air stream, the capability to aspire a workpiece is not affected, but even supported; only the air consumption is slightly increased.

Within the scope of the invention, it is also possible to omit one or both holding plugs 6 and/or 17, particularly, if the workpiece 2 does not comprise an opening 3 or impression. Here, the apparatus preferably comprises holding plugs which are arranged at the outer circumference of the workpiece, in order to avoid a slipping of the workpiece parallel to the surface of the workpiece.

For depositing the workpiece 2, both, in the case of the embodiment shown in FIGS. 1 and 2 and the embodiment shown in FIGS. 3 and 4, only the gas supply has to be interrupted or sufficiently reduced so that the low pressure is sufficiently reduced and the workpiece 2 either falls down onto the rest area due to gravity or be released.

When the workpiece is deposited by interrupting or reducing the gas supply this will not create any problems in the case shown, for example, in FIG. 1, in which the workpiece lies on or is held at its outer circumference after being dropped.

In other embodiments not shown, the workpiece is dropped onto holding plugs, like, for example, holding plug 17 shown in FIG. 3, the diameter of which is hardly greater than the diameter of the inner opening of the workpiece. In this case it may happen that the workpiece tumbles and that it takes a longer period of time until the workpiece lies steadily. This problem can be avoided by the embodiment shown in FIG. 5.

FIG. 5 schematically shows a cross-sectional view of an apparatus according to the invention, which is particularly advantageous when the workpiece is deposited on a holding plug. In the apparatus shown in FIG. 5, the holding plug 6 is connected to an axially movable piston 32, which divides a pressure chamber into an upper compartment 30 and a lower compartment 31. By means of a means 8â€² compressed air is supplied to the apparatus via a switch 42, the compressed air, on the one hand, being used for aspiring the workpiece 2 as described above, but in the present embodiment also supplies compressed air to the upper and lower compartment of the pressure chamber over unidirectional valves 43, 44, which open towards the pressure chamber when compressed air is supplied. When the compressed air supply is switched on, a pressure equilibrium forms between the upper and lower compartment of the pressure chamber, i.e., the pressures p₁and p₂are equal. Thus, the piston 32 does not move. That means, as long as compressed air is supplied to the system and the aspiring unit is activated, both chambers are supplied with the same pressure and the spring 23 keeps the piston 32 in its upper final position. When the supply of compressed air is switched off, using switch 42, both unidirectional valves 43, 44 close. The piston 32 and, thus, also holding plug 6 should now move downwards against the spring force of the spring 23. For this purpose, the upper restrictor 40 is adjusted stronger than the lower restrictor 41, so that the air can leave more rapidly the lower compartment 31 over the restrictor 41 and the channel 8 than it can leave the upper compartment 30 over the restrictor 40 and the channel 8. Simultaneously, the aspiring effect on the workpiece 2 decreases and a difference in pressure develops between the two compartments of the pressure chamber: the piston 32 is pressed downwards thereby, so that the workpiece 2 is brought nearer to the rest area 15 and is deposited there when the aspiring effect has stopped.

Subsequently, the apparatus should be ready for a new cycle in its upper start position as soon as possible; for this purpose it is necessary, that the piston 32 with the piston bar 33 and the holding plug 6 moves back upwards immediately after the workpiece 2 has been deposited. Due to the unidirectional valve 43 and the restrictor 40, the air can only comparatively slowly leave the upper compartment 30 of the pressure chamber, so that the upward movement of the piston 32 effected thereby is only relatively slow. According to the invention, a rapid upward movement of the piston 32 can be achieved by a rapid deairing (venting) of the upper pressure chamber 30, by which the superfluous pressure can be reduced rapidly. The slot 34 at the upper end of the piston bar 33 above the piston 32 serves as deairing. When the piston 32 reaches its lowest position, the deairing slot 34 gives free an opening to the channel 8: the superfluous pressure can leave the upper compartment 30 through said opening. The pressure spring 23 then resets piston 32 in its upper start position. Additionally, the speed at which the piston 32 moves can be adjusted by the two restrictors 40 or 41.

Additionally, as shown in FIGS. 6a and 6b, an ejecting means 61 surrounding the holding plug 6 can be provided, the ejecting means being configured in form of an ejecting ring surrounding the holding plug 6 or individual ejecting plungers and being flush with the holding surface 7 when the ejecting means 61 is retracted. When needed, i.e., when an aspired or held workpiece should be deposited, the ejecting means 61 is extended (see FIG. 6b) and pushes the workpiece away from the holding surface 7 until the distance between holding surface 7 and workpiece becomes that great that even when the air stream is maintained no aspiring effect can be achieved.

Preferably, the movement of the ejecting means 61 is controlled by a means being similar to the means described with regard to FIG. 5. Instead of connecting the holding plug 6 with the piston bar 33 and the piston 32â€”as shown in FIG. 5â€”the ejecting means 61 is connected to a piston 62 such that the movement of the ejecting means 61 can be controlled as described with regard to FIG. 5. Particularly, when the held workpiece is deposited on a centering plug or a holding plug 17, there is the risk that the movable holding plug 6 shown in FIG. 5 knocks onto the opposing holding plug (not shown) without being able to prevent the tumbling of the workpiece. This can be prevented by the ejecting means 61.