GB2354114A

GB2354114A - Micro-scale electrostatic gripper

Info

Publication number: GB2354114A
Application number: GB9921403A
Authority: GB
Inventors: Gareth John Monkman
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-09-13
Filing date: 1999-09-13
Publication date: 2001-03-14
Also published as: GB9921403D0

Abstract

An electrostatic gripping device comprises an arrangement for providing an electrostatically charged surface for gripping objects of millimetre and sub-millimetre dimensions. The device may have a planar or curved gripping surface. The device may also include a micro-scale pulsed or vibration actuator to assist in the acquisition and/or ejection of the object. The device may have one or more actuators which may involve acoustic wave propagation or a resonant cavity. The device may be used in a robotic system to automatically manipulate an object.

Description

1 2354114 ELECTROADHESIVE ROBOTIC NUCRO-GRIPPER

Description

Impactive gripping methods (such as claws, jaws or clamps) are traditionally employed in the prehension of macro-scale objects (where the largest physical dimension exceeds one millimetre) where localised grasping forces are allowable and where extraneous adhesion forces (such as Van der Waals forces) do not present a problem. Where micro-scale (submillimetre sized) objects are concerned such unwanted effects play a significant role often causing disorientation of the object during acquisition and transport and introducing further difficulties when ob ect release is required.

j Astrictive prehension methods which are capable of comparatively weak retention pressures at macro-scales can offer much higher retention pressures at micro-scales. For example: a thousand fold reduction in volume for a material of constant density results in an equivalent order of weight reduction. However, a thousand fold reduction in the volume for a symmetrically proportioned three dimensional cuboid leads to a hundred fold reduction in a single surface area (a cubic to quadratic relationship). This means that only a tenth of the retention pressure is needed with each order of magnitude size reduction. The use of electrostatic force (electroadhesion) is a well known method of astrictive prehension which works well at micro-scales and requires no complicated mechanisms. Furthermore, work at micro-scales is often carried out under vacuum conditions making other astrictive handling methods (such as suction) impossible.

This invention relates to an electroadhesive surface as depicted in figure I comprising two or more electrodes 1, 2 mounted on an electrically insulating substrate 3 over which a permanently bonded mechanically compliant dielectric layer 4 to a thickness 5 has been applied. The electrodes are deposited in pairs separated by a distance 6 which may not be less than the maximum applied voltage 7 divided by the breakdown electric field strength of the substrate 3 or dielectric layer 4. When used to hold electrically conducting objects the maximum applied DC voltage 7 is given by twice the product of the electrical breakdown field strength of the dielectric material 4 and the dielectric layer thickness 5 or by the product of the electrical breakdown field strength of the dielectric material 4 and the inter-electrode distance 6, whichever is the smaller. When applied to electrically insulating material objects (such as glass lenses) the applied voltage must be enough to polarise the dielectric material 4 but less than that needed to exceed the inter-electrode breakdown potential which is given by the twice product of the electrical breakdown field strength of the dielectric material 4 and the dielectric layer thickness 5 plus the product of the electrical breakdown field strength of the prehended object (if less than that of normal air) and the inter-electrode distance 6. The thickness of the dielectric layer 5 will depend on the degree of compliance required according to the topology of the object to be acquired. Due to this compliance, the distance 5 may also alter slightly during object acquisition.

I Though shown explicitly (for purposes of example) in figure 1, the polarity of the applied voltage 7 is not relevant and reversing it will not assist object ejection. Reducing the applied voltage 7 to zero by means of a "bleed" resistor or by switching a short-circuit across the terminals I and 2 will serve to discharge capacitively stored energy but depolarisation of the dielectric will follow only gradually (depending on the dielectric relaxation time of the material 4). In order to assist object ejection a further enhancement in the form of a n-iicro-actuation element (or elements) 8, above and physically connected, via an intermediate member 9, to the upper surface of the substrate 3, must be included. The micro-actuation element 8 is typically a piezoelectric element (though other methods of micro-actuation may be employed). The intermediate member 9 is included to ensure maximum power transfer between the actuator 8 and the substrate 3. The member 9 may be constructed from rigid or flexible material depending of the exact dynamic properties of the chosen design. In more advanced designs the member 9 may be an acoustically resonant cavity.

The inclusion of the micro-actuator element 8 is pivotal to this invention and its uses are twofold: As illustrated in figure 2, when the gripping head 10 (components 1, 2, 3, 4, 8 and 9) is used to acquire an object 11 from a work surface 12 an electrical pulse applied to the actuator 8 causes the gripping surface to displace rapidly over a very short distance in a vertical direction thus exploiting the mechanical compliance of the dielectric material 4 to ensure a good physical contact between the object surface and the gripping surface. Depending on the exact nature of the grasping process, the voltage 7 may be applied to the terminals 1 and 2 of the electroadhesive surface before or during the operation of the actuator 8. During lifting and transport of the object 11 the actuator element is out of use but the voltage 7 must be maintained. Figure 3 shows the reverse procedure on arrival of the gripping head 10 and object 11 at the required destination. Here, the voltage 7 must be removed and if possible, a short circuit placed across the terminals 1 and 2. The micro-actuator element 8 may then be again pulsed, or driven by a continuous AC voltage of suitable frequency (or frequencies), to ensure rapid ejection of the object 11 from the surface of the gripper dielectric 4 onto the work surface 12. The choice of AC frequency (and/or waveform) will depend on the dynamic characteristics of the combined gripping head 10 and object mass at the time of separation of the two. At ultrasonic frequencies the sound wave can be made to propagate in a manner which causes bending and/or flexing of the substrate and gripper surface thus augmenting object release. This effect may be further enhanced by a plurality of actuator elements 8 distributed across the surface of the substrate 3 in order to set up travelling or standing waves on the gripping surface 4.

ELECTROADHESIVE ROBOTIC IGCRO-GRIPPER

Claims

1. A micro-scale gripping device intended for the automated prehension and retention of electrically conducting and/or electrically insulating objects of millimetre and submillimetre size using electrostatic force in an astrictive manner.

2. A device as claimed in claim I whose gripping surface is planar or curved.

3. A device as claimed in claims I and 2 to which a micro-actuator is physically attached, either directly or by means of a mechanical linking member.

4. A device as claimed in claims I and 2 whereby object acquisition is assisted by means of pulsed vertical movement delivered by a microactuator as claimed in claim 3.

5. A device as claimed in claims I and 2 whereby object ejection is assisted by pulsed or vibratory means from a micro-actuator as claimed in claim 3.

6. A device as claimed in claims I and 2 where the micro-actuator mentioned in claim 3 causes bending and/or flexing of the gripper surface due to acoustic wave propagation.

7. A device as claimed in claims 1, 2 and 3 where the mechanical linking member between the actuator and gripper substrate is a resonant cavity.

8. A device as claimed in claims 1, 2 and 3 where a plurality of microactuators are employed.

9. A device as claimed in claims 1, 2 and 3 whereby the vibration source is electromagnetic, magnetostrictive, electrostatic, electrostrictive, piezoelectric, micropneumatic, microhydraulic, thermal, chemo-, opto-, or bio-kinetic in nature.

i Amendments to the claims have been filed as follows ELECTROADHESIVE ROBOTIC MICRO-GRIPPER Claims 1. A micro-scale gripping device, the underside of which is capable of prehension and retention of electrically conducting and/or electrically insulating objects of rnillirnetre and submillimetre size usina electrostatic force 'n an astrictive manner.

Z:' 1 1 2. A device as claimed in claim I whose gripping surface is planar or Curved.

3, A device as claimed in claims I and 2 where the gripping surface may be tilted or deformed.

1 1 4. A device as claimed in claims I and 2 to which a micro-actuator is physically attached, either directly or by means of a mechanical linking member.

5. A device as claimed in claims I and 2 whereby object acquisition is assisted by means of pulsed vertical movement delivered by a rnicroactuator as claimed in claim 3).

6. A device as claimed in claims I and 2 whereby object ejection is assisted by pulsed or vibratory means from a micro-actuator as claimed in clairn I 7. A device as claimed in clairris I and 2 where the rnicro-actuator mentioned in claim ') causes bending and/or flexing of the gripper surface due to acoustic wave propa(.1yation.

8. A device as claimed in claims 1, 2 and 3 where the mechanical linking member between the actuator and gripper substrate is a resonant cavity.

9. A device as claimed in clairris 1, 2 and 3 where a plurality of microaCtLiators are employed.

10. A device as claimed in claims 1, 2 and 3 whereby the vibration source is electromagnetic, magnetostrictive, electrostatic, electrostrictive, piezoelectric, rnicropneumatic, microhydraulic, thermal, cherno-, opto-, or bio-kinetic in nature.