WO2017199035A1

WO2017199035A1 - Electroadhesive gripper

Info

Publication number: WO2017199035A1
Application number: PCT/GB2017/051387
Authority: WO
Inventors: David MANBY
Original assignee: Aylesbury Automation Ltd
Current assignee: Aylesbury Automation Ltd
Priority date: 2016-05-18
Filing date: 2017-05-18
Publication date: 2017-11-23
Anticipated expiration: 2018-11-18
Also published as: GB201608729D0; GB2552450A

Abstract

An electroadhesive gripper, comprising a substrate carrying a dielectric layer and at least two gripper electrodes;in which the dielectric layer comprises barium titanate and/or there is a capacitive proximity sensor on the substrate. The dielectric layer may comprise a carrier for the barium titanate, such as a resin, such as a cyanoresin.

Description

ELECTROADHESIVE GRIPPER

This invention relates to an electroadhesive gripper, and a method of manufacture thereof.

Electroadhesive grippers are known in the art. They comprise a pair of electrodes across which is applied either an alternating current (AC) or direct current (DC) drive signal. They generate an electric field. This generates an electrostatic charge in any material that may be nearby, especially where that material is non-conducting. The electrostatic charge generated is then attracted to the electrodes. Thus, the electroadhesive effect can be used to provide an astrictive, non-permeating gripper.

As such, the gripper avoids the problems of various other gripping techniques, such as impactive grippers (such as jaws or clamps) which may interfere with the structure of a delicate product, ingressive grippers which can puncture the structure of a product and contiguitive grippers which make use of chemical or thermal bonds and so may damage the product or leave residues thereon. Furthermore, unlike another common astrictive gripper, the electromagnet, it does not require that the material gripped to be magnetic.

Electroadhesive grippers have been used in wall-climbing robots such as shown in US Patent no 8 125 758. Sheet and hand-shaped electroadhesive grippers have been disclosed in the International (PCT) patent application published as WO201 1/100028.

According to a first aspect of the invention, there is provided an electroadhesive gripper, comprising:

• a substrate carrying:

o a dielectric layer; and

o at least two gripper electrodes;

in which the dielectric layer comprises barium titanate .

The use of barium titanate (BaTi0₃) as a dielectric has been found to surprisingly improve the performance of the gripper; it has been found that the adhesive force generated by the gripper increases by 10- 15% by using this dielectric. It can also reduce the residual charge held in the gripper after an applied voltage has been removed (thus improving the speed with which the gripper releases) .

Typically, the dielectric layer will comprise a carrier for the barium titanate . The carrier may comprise a resin, such as a cyanoresin or another form of support material. The dielectric layer may contain barium titanate and the carrier in a volume ratio of between 3 : 5 and 10: 5, preferably between 4: 5 and 9: 5, particularly between 7.5 : 5 to 8.5 : 5. Alternatively, by weight ratio, the ratio of barium titanate to carrier may be between 4: 1 and 12: 1 , or between 5 : 1 and 1 1 : 1 , or between 9: 1 and 1 1 : 1.

The dielectric layer may consist of the barium titanate and the carrier.

According to a second aspect of the invention, there is provided an electroadhesive gripper, comprising:

· a substrate carrying:

o a dielectric; and

o at least two gripper electrodes; and

• a capacitive proximity sensor on the substrate. Thus, by including a capacitive proximity sensor on the common substrate, it is possible to determine whether the gripper has picked up an item whilst making use of the substrate without having to provide a separate proximity sensor.

Typically, the capacitive proximity sensor will comprise at least two sensor electrodes on the substrate. The capacitive proximity sensor may also comprise a detection circuit arranged to determine changes in capacitance across the sensor electrodes. Typically, the detection circuit may be arranged to apply an electrical signal across the sensor electrodes, such as a periodic signal, and to determine a resonant frequency of a circuit comprising the sensor electrodes.

Typically, the gripper electrodes and the sensing electrodes will have been deposited upon the substrate using the same technique, for example printing using a metal- containing ink. The gripper may be in accordance with the first aspect of the invention. The following features can relate to either of the preceding aspects of the invention.

Typically, the dielectric layer will be on the substrate, and each gripper electrode will be on the dielectric layer. Alternatively, the gripper electrodes may be on the substrate, and the dielectric layer may be on the gripper electrodes.

There may be further substrate distal from the substrate and acting with the substrate to sandwich the gripper electrodes and the dielectric between the substrate and the further substrate .

There will typically be two gripper electrodes. At least one (if not all) of the gripper electrodes and/or the sensor electrodes may comprise metallic silver. At least one (if not all) of the gripper electrodes and/or the sensor electrodes may have been printed using a silver bearing material.

The gripper may comprise a drive circuit, which is electrically coupled to the gripper electrodes and is arranged to drive the gripper electrodes with a drive signal having a voltage, frequency and duration. The frequency will typically be greater than zero.

The gripper electrodes will typically be coplanar on the substrate, and may be interdigitated, for increased performance . The drive circuit may be arranged so that the drive signal is applied across the gripper electrodes, so that a potential difference is set up between the gripper electrodes, generating an electric field that acts to attract items to be gripped.

The substrate (and/or the further substrate) may comprise polyurethane (PU), polyethylene terephthalate (PET), polyimide (PI) or other suitable materials. The substrate (and/or the further substrate) may be flexible. The substrate and the further substrate may be formed of different materials, for example the substrate may comprise PU and the further substrate may comprise PET. In such a case, the gripper may be arranged so as to grip items on the PET surface . According to a third aspect of the invention, there is provided a method of forming an electroadhesive gripper in accordance with the first aspect of the invention, comprising:

• providing a substrate;

· applying the dielectric layer onto the substrate; and

• printing each gripper electrode onto the substrate .

Thus, this provides a simple method of manufacture. Typically, the printing step will comprise screen printing, typically through a calendered stainless steel mesh. The step of applying the dielectric layer may comprise a draw down coating.

In a preferred embodiment, each gripper electrode is printed on the substrate and then the dielectric layer is applied on top of each gripper electrode, although in an alternative, the dielectric layer is applied on the substrate before each gripper electrode is printed on top of the dielectric layer.

The dielectric layer may be applied using an amalgamate of a cyanoresin and barium titanate, typically dissolved in a solvent. The method may comprise distributing the barium titanate through the cyanoresin using sonication. The solvent may be a polar solvent, such as a ketone .

There now follows, by way of example only, description of an embodiment of the invention, described with reference to the accompanying drawings, in which:

Figure 1 shows a schematic view of an electroadhesive gripper in accordance with a first embodiment of the invention; Figure 2 shows a cross section through the gripper of Figure 1 ;

Figure 3 shows the sensing electrodes of the capacitive proximity sensors of the gripper of Figure 1 in more detail; Figure 4 shows the gripper electrodes of the gripper of Figure 1 in more detail; and

Figure 5 shows an alternative set of gripper electrodes in accordance with another embodiment of the invention.

An electroadhesive gripper in accordance with a first embodiment of the present invention is shown in Figures 1 to 4 of the accompanying drawings. The electroadhesive gripper 1 has a pair of electrodes 10a, 10b, which are mutually interdigitated, in that each electrode 10a, 10b is formed as a number of parallel fingers

12 separated by gaps 13, the fingers 12 of one electrode 10a extending into the gaps

13 of the other electrode 10b. Whilst the electrodes 10a, 10b are shown schematically in Figure 1 of the accompanying drawings, the electrodes are shown in more detail in Figure 4 of the accompanying drawings. Such electrodes have been found to provide an electroadhesive gripper with a good performance .

The electrodes 10a, 10b are provided on a substrate 9, which in this embodiment is polyimide (PI) . Over the electrodes 10a, 10b, there is provided a dielectric layer 15. This comprises a cyanoresin binder amalgamated with barium titanate (BaTi0₃) in a 8 :5 BaTi0₃ to cyanoresin weight ratio (although ratios as low as 4: 5 will function) . The cyanoresin is cyanoethyl pullulan available from Shin Etsu of Japan.

Finally, the gripper is finished with a further substrate 16, this time formed of polyurethane, which seals the gripper together so as to sandwich the electrodes 10a, 10b and dielectric layer.

Electrical connections 1 1 are provided to each electrode . These are coupled to a control module 6, which houses a drive circuit 8 and a control circuit 7 (which may be implemented as a microprocessor running computer program instructions). The drive circuit 8 is coupled to the electrical connections 1 1 , so as to provide a current signal across the electrodes 10. Typically but not exclusively voltages are in the region of 1 - 100 kilovolts ( 1 - l OOkV), alternating current. The control circuit 7 controls the drive circuit and the other functions of the apparatus, so that a variable voltage within the range - l OkV to + 10kV (resolution 5V) can be applied across the electrodes. As such, the drive circuit 8 drives the electrodes 10a, 10b with the drive signal, so that an electric field is created. This attracts items that may be nearby. By using the barium titanate in the dielectric layer, we have found that the attractive force generated by the gripper increases by 10- 15% compared to if it were not used.

In order to determine whether an item has been gripped, any number of capacitive proximity sensors 20 are provided; in this example, there are four, but the number will depend on the component being picked up. As shown in Figure 3 of the accompanying drawings, each sensor comprises a pair of electrodes 21 a, 21b, comprising each comprising a plurality of concentric part-circular circumferentially- extending interdigitated fingers, connected to a sensing circuit 22. The electrodes 21a, 21b are provided on the substrate 9 (or alternatively the further substrate 16) . The sensing circuit is arranged to determine the capacitance of each pair of electrodes. When an object is in proximity to the sensor 20, the capacitance will change, thus enabling the sensing circuit 22 to determine when there is an object in proximity (and so when the gripper has picked something up) . Such a sensor requires no moving parts and can very easily be fabricated on the substrate 9, typically at the same time and using the same method as the gripper electrodes 12a, 12b. Furthermore, we have found that it operates satisfactorily even when the gripper electrodes are energised to voltages of several kilovolts. In order to fabricate the gripper, the gripper electrodes 12a, 12b and the sensor electrodes 21 a, 21b are printed onto the substrate using a silver-laden ink through a stainless steel calendared mesh. This produced good quality prints with well-defined edges. The thickness of the electrodes was 10-20 um. In a second stage, the dielectric layer is prepared by forming an amalgamate of the barium titanate and the cyanoresin in the ratio set out above with the addition of a ketone solvent. Sonication is used to distribute the barium titanate through the cyanoresin. The viscosity of the resulting mixture is approximately 10-20 centipoise. The dielectric layer is then applied as a draw down coating. The coating is repeated until the dielectric layer is 10 um thick. Whilst the solvent can attack the underlying silver electrodes, the effect is minimal as the dielectric layer coating dries quickly. In fact, this may facilitate adhesion. In any case, the attack can be circumvented by employing a thin vacuum-deposited layer if required.

In a second embodiment of the invention, the gripper electrodes 10a, 10b are replaced with the electrodes 30a, 30b shown in Figure 5 of the accompanying drawings. Instead of the parallel fingers, the electrodes each have a plurality of concentric part- circular circumferentially-extending interdigitated fingers. As such, whereas the electrodes in Figure 4 can be referred to as "comb" electrodes, those of Figure 5 can be referred to as "circular" electrodes.

Examples

Electroadhesive grippers according to various embodiments were made and then tested in the same test apparatus. In each case, a 2.4kV voltage was applied to the gripper electrodes. There was no further substrate but otherwise the gripper was as discussed in the embodiments above. The material of the substrate (PET or Polyimide (PI)) and the presence of the dielectric layer containing barium titanate was varied, as was the side of the gripper on which the force was measured. The attractive force applied to a gripper pad was measured after 90 of signal being applied.

The forces measured were as follows:

Force

Substrate Substrate Electrode Dielectric measured

Test no measured material thickness pattern layer on which

(Newtons) side?

1 PET Thick Circular None Electrodes 1.25

2 PET Thick Circular Present Dielectric 1.84

3 PET Thick Circular None Substrate 1.96

4 PET Thick Circular Present Substrate 2.01

5 PET Thick Comb None Electrodes 1.23

6 PET Thick Comb Present Dielectric 2.04

7 PET Thick Comb None Substrate 1.44

8 PET Thick Comb Present Substrate 2.59 Force

Force

Substrate Substrate Electrode Dielectric measured

Test no measured material thickness pattern layer on which

(Newtons) side?

9 PI 75 um Circular None Electrodes 1.37

10 PI 75 um Circular Present Dielectric 1.58

11 PI 75 um Circular None Substrate 1.68

12 PI 75 um Circular Present Substrate 2.46

13 PI 75 um Comb None Electrodes 1.28

14 PI 75 um Comb Present Dielectric 2.26

15 PI 75 um Comb None Substrate 1.28

16 PI 75 um Comb Present Substrate 2.8

17 PI 125 um Circular None Electrodes 1.54

18 PI 125 um Circular Present Dielectric 1.8

19 PI 125 um Circular None Substrate 1.32

20 PI 125 um Circular Present Substrate 1.72

21 PI 125 um Comb None Electrodes 1.4

22 PI 125 um Comb Present Dielectric 2.4

23 PI 125 um Comb None Substrate 1.1

24 PI 125 um Comb Present Substrate 2.15 following conclusions can be made :

PI is slightly better than PET for achieving the maximum electroadhesive force when coated with the dielectric layer. There is a maximum relative increase of 22% in forces obtainable (test 12 vs test 4). However, when the dielectric layer is not present, PET is slightly better than PI.

When the dielectric layer is present, the comb shape design is better than the circular shape for achieving the maximum electroadhesive force (maximum about 43% relative increase, tests 14 vs 10). However, the circular shape is better than the comb shape design when the dielectric layer is not present (maximum about 36% relative increase, tests 3 vs 7).

The presence of the dielectric layer compared to its absence increases the force by a maximum of an about 1 19% relative increase (tests 16 vs 13).

Thinner pads (PI, 75 μιη) are better than thicker pads (PI, 125 μιη) at achieving the electroadhesive force (maximum about 30% relative increase, tests 16 vs 24) when the force was measured on the substrate PI side of the gripper. A further set of electrodes were the subject of testing. In these cases, rather than being printed, the barium titanate was applied to the electrode using a painting method. In this method, a mixture of water, barium titanate granules and a domestic emulsion paint (Dulux (RTM) Primer and Undercoat) were mixed in the quantities given below:

As can be seen, samples 1 , 2 and 3 have 1 , 2 and 3 parts of BaTi03 by weight respectively, with the remainder of the samples being the same. The mixture was then applied to the electrodes using a paint brush.

The results of various different electrodes tested with the above mixtures as the dielectric can be seen in Figures 6 to 12 of the accompanying drawings. These show the variation between when a sample is picked up and the subsequent release time for that sample to be dropped at difference voltages. These grippers were prepared with comb electrode . Several different grippers made to the same overall design were tested for repeatability. Figures 6 to 12 therefore represent:

As such, this alternative method of applying the dielectric appears to function satisfactorily.

Claims

1. An electroadhesive gripper, comprising:

• a substrate carrying:

o a dielectric layer; and

o at least two gripper electrodes;

in which the dielectric layer comprises barium titanate .

2. The gripper of claim 1 , in which the dielectric layer comprises a carrier for the barium titanate.

3. The gripper of claim 2, in which the carrier comprises a resin, such as a cyanoresin.

4. The gripper of claim 3, in which the dielectric layer contains barium titanate and the carrier in a volume ratio of between 3 : 5 and 10 : 5, preferably between 4: 5 and 9: 5, particularly between 7.5 : 5 to 8.5 : 5.

5. The gripper of any of claims 2 to 4, in which the dielectric layer consists of the barium titanate and the carrier.

An electroadhesive gripper, comprising:

a substrate carrying:

o a dielectric; and

o at least two gripper electrodes; and

a capacitive proximity sensor on the substrate

7. The gripper of claim 6, in which the capacitive proximity sensor comprises at least two sensor electrodes on the substrate .

8. The gripper of claim 7, in which the capacitive proximity sensor comprises a detection circuit arranged to determine changes in capacitance across the sensor electrodes.

9. The gripper of claim 7 or claim 8, in which the gripper electrodes and the sensing electrodes have been deposited upon the substrate using the same technique, for example printing using a metal-containing ink.

10. The gripper of any preceding claim, in which the dielectric layer is on the substrate, and each gripper electrode is on the dielectric layer.

1 1. The gripper of any of claims 1 to 9, in which the gripper electrodes are on the substrate, and the dielectric layer is on the gripper electrodes.

12. The gripper of any preceding claim, comprising a further substrate distal from the substrate and acting with the substrate to sandwich the gripper electrodes and the dielectric between the substrate and the further substrate .

13. The gripper of any preceding claim, in which there are two gripper electrodes.

14. The gripper of any preceding claim in which at least one of the gripper electrodes and/or the sensor electrodes where the claim depends on claim 7 comprise metallic silver.

15. The gripper of claim 14, in which at least one of the gripper electrodes and/or the sensor electrodes where the claim depends on claim 7 have been printed using a silver ink.

16. The gripper of any preceding claim, comprising a drive circuit, which is electrically coupled to the gripper electrodes and is arranged to drive the gripper electrodes with a drive signal having a voltage, frequency and duration.

17. A method of forming an electroadhesive gripper in accordance with any of claims 1 to 5, comprising:

• providing a substrate;

• applying the dielectric layer onto the substrate; and

• printing each gripper electrode onto the substrate .

18. The method of claim 17, in which the printing step comprises screen printing, typically through a calendered stainless steel mesh.

19. The method of claim 17 or claim 18, in which the step of applying the dielectric layer comprises a draw down coating.

20. The method of any of claims 17 to 19, in which the dielectric layer is applied using an amalgamate of a cyanoresin and barium titanate, typically dissolved in a solvent.

21. The method of claim 20, comprising distributing the barium titanate through the cyanoresin using sonication.