GB2266808A - Electrostatic pick-up devices - Google Patents

Abstract

The device comprises a planar support electrode 5 adapted to support a sheet of material to be picked-up, a planar pick-up electrode 1 carrying a surface layer of dielectric material which is to be brought into face-to-face contact with the sheet material, and power means to apply a voltage between the electrodes so as to induce electric charges in the dielectric layer which causes the sheet material to be attracted to the pick-up electrode. The pick-up electrode 1 comprises a planar array of electrode elements adjoining one another and which can be selectively energised by the voltage from the power means under the control of a controller 4 so that only electrode elements overlying the sheet material are energised. The device enables a piece of cut carbon fibre fabric to be lifted away from its surrounding fabric and deposited on a table 6. <IMAGE>

Description

ELECTROSTATIC PICK-UP Technical Field This invention relates to a method and means for electrostatically picking-up sheet material.

It is known that a high voltage electrode separated by a thin dielectric layer from sheet material will induce an electric charge in the sheet material which will cause it to be attracted towards the electrode. The electrode may be a single planar electrode on one side of the sheet material with a voltage applied between it and the sheet material or a second planar electrode on the other side of and in contact with the sheet material. This arrangement is rather similar to a parallel plate capacitor. In an alternative arrangement, two electrodes are arranged adjacent one another on one side of the sheet material and the voltage is applied between them so that the electric field produced between the electrodes polarises the dielectric and induces charges at the interface with the sheet material.Much work has been done on the form of the electrodes, with interdigitated electrodes preferred.

In one particular application, electrostatic pick-up means is used to support a semi-conductor wafer during a manufacturing process. Two electrodes or sets of electrodes are incorporated in a pick-up head covered by a dielectric layer.

The total area of the electrodes is approximately equal to the area of the semi-conductor wafer being picked-up, and typically comprises a circular area which conforms to the circular outline of the wafer. The pick-up is therefore designed to pick-up wafers of one particular size and shape.

In order to ensure a sufficiently large electrostatic force, the dielectric layer is kept as thin as possible and the applied voltage is relatively high. However, this can cause arcing problems between the electrodes and can make it difficult subsequently to release the sheet material because of retained charge. Additional release means may therefore be required to positively remove the sheet material from the electrodes.

Disclosure of the Invention An object of the present invention is to provide electrostatic pick-up means which can be used efficiently to pick-up different sized and/or shaped articles of sheet material.

According to the present invention, electrostatic pick-up means comprises a planar support electrode adapted to support a sheet of material to be picked-up, a planar pick-up electrode carrying a surface layer of dielectric material which is to be brought into face-to-face contact with the sheet material, and power means to apply a voltage between the electrodes so as to induce electric charges in the dielectric layer which causes the sheet material to be attracted to the pick-up electrode, characterised in that the pick-up electrode comprises a planar array of electrode elements adjoining one another and which can be selectively energised by the voltage from the power means under the control of a controller so that only electrode elements overlying the sheet material are energised.

By selectively energising only those electrode elements overlying the sheet material, the picking-up of any adjacent unwanted pieces of sheet material on the support electrode is avoided.

Preferably, all of those electrode elements overlying the sheet material are energised so that all portions of the sheet material are attracted towards the pick-up electrode. This is especially important with edge portions of pliable sheet material which, if not held electrostatically, may tend to peel away from the pick-up electrode, taking the rest of the sheet material with it.

This same phenomenon, however, is beneficial in preventing adjacent unwanted pieces of sheet material from being picked up by an outer one of the energised electrode elements with also partly overlies an adjacent unwanted piece.

Preferably, the voltage applied to the electrode elements is reduced to a low level that will still ensure reliable pick up of sheet material, but will also allow ready release of the sheet material when the electrode elements are de-energised. Also, a lower voltage enhances the selective pick up action of the electrode elements in producing an adequate electrostatic pick up force only when all of those overlying the sheet material are energised.

Preferably, the voltage is applied to the electrode element so that it is increased progressively in a predetermined manner to a peak voltage, thereby avoiding charging current surges that would be created by a step voltage and would cause arcing. Further, the voltage is reduced progressively in a similar manner to release the sheet material from the electrode element.

Preferably, the support electrode is earthed and a positive voltage is applied to the pick-up electrode. In one embodiment, the pick-up electrode is carried by transfer means between the support electrode and a horizontal receiving surface so as to transfer sheet material between the two, and both the support electrode and receiving surface are earthed.

The invention is especially applicable to sheet material which is electrically conductive, for example, carbon fibre fabric.

Description of the Drawings The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of electrostatic pick-up means according to the invention for transferring carbon fibre fabric between horizontal supporting surfaces, Figure 2 is a schematic diagram of the pick-up electrodes of the pick-up means in Figure 1, Figure 3 is a plan view of the pick-up electrode of Figure 2, and Figure 4 is a diagram showing variation with time in the charging voltage and current applied to the pick-up electrode of Figure 3.

Mode of Carrying Out the Invention The pick-up means shown in Figure 1 comprises an electrostatic pick-up head 1 which is carried at the end of an arm 2 of robot transfer means 3. A programmable controller 4 controls operation of the transfer means 3 so as to move the electrostatic pick-up head 1 between a support table 5 and a receiving table 6. At the support table 5 the pick-up head assumes a predetermined pick-up position in contact with sheet material on the table and is energised to pick up the material electrostatically, as described hereinafter. At the receiving table 6, the pick-up head assumes a predetermined release position in which it places the sheet material on the receiving table and is de-energised to release the material.In a particular application the support table 5 may be a cutting table on which a piece of carbon fibre fabric is cut from a larger sheet ready to be transferred to the receiving table for assembly into a composite item.

The electrostatic pick-up head 1 comprises a rigid structure 7 which carries a planar array of electrode elements 8 which are insulated electrically from one another and contact a dielectric layer 9 over their lower surface.

Typically, the dielectric layer 9 is acetate sheet 0.25mm thick, and the electrode elements 8 comprise aluminium panels attached to the upper face of the acetate sheet. The rigid structure 7 may comprise a framework. As shown in Figure 3 the electrode elements 8 are square panels of side lOcms, and sixteen of them are arranged in a square array of 4 x 4 with their adjacent edges spaced slightly apart and separated by electrical insulation material 10.

The pick-up head 1 is energised to pick up a piece of sheet material, e.g. carbon fibre fabric, from the support table 5 by the application of a positive voltage of 2kV to one or more preselected electrode elements 8 under the control of the controller 4 when the head 1 is lowered into contact with the fabric. Those electrode elements energised are all of those which overlie a portion of said fabric, for example, the three electrode elements 8t in Figure 3 which overlie the L-shaped piece of fabric 11. This L-shaped piece of fabric 11 is shown as a cut-out from a larger sheet of fabric 12 which is located in a predetermined position on the table 5 and is cut in place on this table.The pick-up position of the pick-up head 1 is predetermined by the controller 4 and thus the alignment of the electrode elements 8' relative to the L-shaped piece of fabric 11 is assured.

The support table 5 has an electrically conductive upper surface which is earthed so that the positive voltage applied to the electrode elements 8' generates an electrical field across the dielectric layer 9 and induces electrical charges therein which are transferred to the upper surface of the fabric opposite the energised electrode elements 8'. As a result of the opposite polarities of the induced charges on the engaging surfaces of the dielectric layer 9 and fabric 11, 12, an attractive electrostatic force is produced therebetween.This force, which is applied over the whole of the surface area of the L-shaped piece of fabric 11, is sufficient to hold this piece of fabric in place on the lower surface of the pick-up head as it is lifted away from the support table 5, but the attractive force applied to the edge portions of the fabric 12 around the cut-out shape is insufficient to hold and lift the remaining fabric 12 away from the table. Any tendency to lift these edge portions off the table 5 is resisted by the surrounding fabric 12 adjacent non-energised electrode elements 8 and the edge portions peel away from the dielectric layer 9 as the pick-up head is lifted.

Once the pick-up head 1 has picked up the piece of fabric 11, it is moved by the transfer means 3 to a predetermined release position over the receiving table 6 and the electrode elements 8' are de-energised. The receiving table 6 also has an electrically conductive upper surface which is earthed, and the conductive nature of the piece of fabric 11 helps the induced charge to be dissipated, thereby removing the electrostatic force at least to a level where the piece of fabric 11 is released under its own weight and remains on the table 6 when the pick-up head 1 is lifted away for another operation.

In order to avoid surges of the charging current that might cause arcing between electrode elements 8, 8' or across the dielectric layer 9 when the electrode elements 8' are energised, the controller 4 is adapted to apply the 2kV voltage as a ramp voltage, as shown in Figure 4, over a period of 200ms. Curve C1 shows the corresponding charging current. Comparing this with the charging current for the application of a step voltage of 2kV to the electrode elements, shown by curve C2, it is clear that a high peak current is avoided, although the actual charging time needs to be lengthened slightly to achieve the same charge level.

In the illustrated embodiment, all of those electrode elements 8' overlying the piece of fabric 11 are energised, but it will be appreciated that in alternative embodiments of the invention, especially where edge portions of the piece 11 only slightly overlap an electrode element 8, the latter may not be energised provided that the electrostatic force generated is otherwise sufficient to allow the piece to be picked up.

It will also be appreciated that the number and size of electrode elements 8 employed in any particular pick-up array is a matter of choice depending on the size and shape and variations of the pieces of sheet material to be picked up.

In order to assist the speed of release of fabric 11 from the pick-up head 1, the applied voltage is reduced from 2kV to 0 at a constant rate over a period of 200ms, which produces a correspondingly controlled discharge current.

The controlled use of a ramp voltage for both the pick-up and release of fabric 11 by the pick-up head 1, also assists in ensuring that the lateral position of the fabric is disturbed to a minimum extent during these operations.

Claims (22)

1. Electrostatic pick-up means comprising a planar support electrode adapted to support a sheet of material to be picked-up, a planar pick-up electrode carrying a surface layer of dielectric material which is to be brought into face-to-face contact with the sheet material, and power means to apply a voltage between the electrodes so as to induce electric charges in the dielectric layer which causes the sheet material to be attracted to the pick-up electrode, characterised in that the pick-up electrode comprises a planar array of electrode elements adjoining one another and which can be selectively energised by the voltage from the power means under the control of a controller so that only electrode elements overlying the sheet material are energised.

2. Pick-up means as claimed in claim 1 in which the controller is such as to energise all of those electrode elements overlying the sheet material.

3. Pick-up means as claimed in claim 1 or 2 in which the pick-up electrode is carried by a movable transfer means.

4. Pick-up means as claimed in claim 3 in which the transfer means comprises an arm which carries the pick-up electrode at its end.

5. Pick-up means as claimed in any one of the preceding claims in which the support electrode comprises a substantially horizontal support surface for the sheet material.

6. Pick-up means as claimed in any one of the preceding claims in which the support electrode is earthed.

7. Pick-up means as claimed in any one of claims 3 to 6 which includes a receiving surface to which the sheet material is transferred by the pick-up electrode.

8. Pick-up means as claimed in any one of claims 3 to 7 which includes a programmable controller which controls movement of the transfer means.

9. Pick-up means as claimed in any one of the preceding claims in which the power means is adapted to apply a voltage to an electrode element so that the voltage increases progressively in a predetermined manner to a peak voltage.

10. Pick-up means as claimed in any one of the preceding claims in which the power means is adapted to reduce the voltage progressively in a predetermined manner to zero when releasing the sheet material.

11. A method of electrostatically picking up sheet material comprising supporting the sheet material on a planar support electrode, providing a pick-up electrode with a surface dielectric layer so that it contacts the sheet material in face-to-face contact, and applying a voltage between the electrodes so as to induce electric charges in the dielectric layer which causes the sheet material to be attracted to the pick-up electrode, characterised in that the pick-up electrode is formed as a planar array of electrode elements adjoining one another and these elements are selectively energised with said voltage so that only electrode elements overlying the sheet material are energised.

12. A method as claimed in claim 11 in which all of those electrode elements overlying the sheet material are energised.

13. A method as claimed in claim 11 or 12 in which the controller is programmable and the sheet material is located in a predetermined orientation on the support electrode.

14. A method as claimed in any one of claims 11 to 13 in which the pick-up electrode is transferable between the support electrode and a receiving surface.

15. A method as claimed in claim 14 in which the controller controls energisation and de-energisation of the pick-up electrode in a programmable manner so as to pick-up the sheet material from the support electrode and deposit the sheet material on the receiving surface.

16. A method as claimed in claim 14 or 15 in which the support electrode and receiving surface are both substantially horizontal.

17. A method as claimed in any one of claims 14 to 16 in which the support electrode and receiving surface are earthed.

18. A method as claimed in any one of claims 11 to 16 in which the voltage applied to an electrode element is increased progressively to a peak voltage in a predetermined manner.

19. A method as claimed in any one of claims 11 to 16 in which the voltage is reduced progressively in a predetermined manner to zero when releasing the sheet material.

20. A method as claimed in any one of claims 11 to 19 in which the sheet material is electrically conductive.

21. Electrostatic pick-up means substantially as herein described with reference to the accompanying drawings.

22. A method of electrostatically picking up sheet material substantially as herein described with reference to the accompanying drawings.