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Figure 5: Placement of the micro-work part

Figure 5 Placement of the micro-work part

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Figure 1: Comparison of forces acting in the micro-range as modelled by Fearing [5] and Bohringer et al [7] COMPARISON OF ADHESIVE FORCES WITH RESPECT TO SEPARATION DISTANCE The three adhesive forces and vacuum forces are modelled with respect to separation distance between the interacting surfaces. Micro-objects range from 10 mm down to 1 um in dimensions (Lambert [2]) and these should be of mass not exceeding 1x10°kg (Sanchez [8]) in cases where gravity becomes less significant. Therefore, in this comparison, a flat micro-part (not a sphere as examined by Bohringer et al [7]) having a uniform cross- section and weighing 1x10°kg is considered. The materials compared are lead zirconate titanate (PZT) ceramic, polystyrene (PS), gold, copper, silicon, aluminium oxide (Al,0O3), silver and diamond. The adhesive forces were modelled using analytical equations. The formulae used are as follows:
Figure 1: Comparison of forces acting in the micro-range as modelled by Fearing [5] and Bohringer et al [7] COMPARISON OF ADHESIVE FORCES WITH RESPECT TO SEPARATION DISTANCE The three adhesive forces and vacuum forces are modelled with respect to separation distance between the interacting surfaces. Micro-objects range from 10 mm down to 1 um in dimensions (Lambert [2]) and these should be of mass not exceeding 1x10°kg (Sanchez [8]) in cases where gravity becomes less significant. Therefore, in this comparison, a flat micro-part (not a sphere as examined by Bohringer et al [7]) having a uniform cross- section and weighing 1x10°kg is considered. The materials compared are lead zirconate titanate (PZT) ceramic, polystyrene (PS), gold, copper, silicon, aluminium oxide (Al,0O3), silver and diamond. The adhesive forces were modelled using analytical equations. The formulae used are as follows:
Figure 2: Comparison of gripping forces in the micro-range The parameters used by Zesch et al [1] in the design of a pipette vacuum gripper are used for modelling as shown in Figure 2. The parameters are: air density, p - 1.22521 kg/m’; air flow rate, Q = 1.2 x 10%m?/s; internal radius of gripper, rint - 2.5x10°m; and external radius of gripper, rex: - 5.0x10°m (Zesch et al [1]) . It is evident from the modelling results in Figure 2 that vacuum forces predominate over others in the whole range of 1x10*m to 1x10%m, occupying the first position in magnitude and electrostatic the second. However, they cannot be used for a micro- workpart whose minimum dimension is equal or less than the internal radius of a gripper of size, 2.5x10°m.
Figure 2: Comparison of gripping forces in the micro-range The parameters used by Zesch et al [1] in the design of a pipette vacuum gripper are used for modelling as shown in Figure 2. The parameters are: air density, p - 1.22521 kg/m’; air flow rate, Q = 1.2 x 10%m?/s; internal radius of gripper, rint - 2.5x10°m; and external radius of gripper, rex: - 5.0x10°m (Zesch et al [1]) . It is evident from the modelling results in Figure 2 that vacuum forces predominate over others in the whole range of 1x10*m to 1x10%m, occupying the first position in magnitude and electrostatic the second. However, they cannot be used for a micro- workpart whose minimum dimension is equal or less than the internal radius of a gripper of size, 2.5x10°m.
Experiments proved that both grades of polyurethane could pick different types of micro- materials from several types of surface (which include wood, glass, metals, Perspex, polyvinyl chloride (PVC) and other polymer materials). Square micro-parts of 10 mmx10 mm with thickness less than 500 um, and made of silicon coated with copper were picked. It was observed that polyurethane of 60 shore hardness exerted less van-der-Waals forces than that of 30 shore hardness. Therefore, in a picking-and-placing cycle the former was used for the gripper’s interactive surface and the latter for the releasing place as shown in Figures 3 to 5. The figures show a micro-part picked by a micro-gripper (made of polyurethane of shore hardness value of 60) from an anti-static mat (supplied by RS Components Company) and released onto the polyurethane interactive surface of 30 shore hardness. The anti-static mat and crocodile clip discharged the pick-up place, micro-part, releasing place and the micro-gripper of any electrostatic charge which could interfere with the van-der-Waals forces. Figure 3: Initial arrangement before picking
Experiments proved that both grades of polyurethane could pick different types of micro- materials from several types of surface (which include wood, glass, metals, Perspex, polyvinyl chloride (PVC) and other polymer materials). Square micro-parts of 10 mmx10 mm with thickness less than 500 um, and made of silicon coated with copper were picked. It was observed that polyurethane of 60 shore hardness exerted less van-der-Waals forces than that of 30 shore hardness. Therefore, in a picking-and-placing cycle the former was used for the gripper’s interactive surface and the latter for the releasing place as shown in Figures 3 to 5. The figures show a micro-part picked by a micro-gripper (made of polyurethane of shore hardness value of 60) from an anti-static mat (supplied by RS Components Company) and released onto the polyurethane interactive surface of 30 shore hardness. The anti-static mat and crocodile clip discharged the pick-up place, micro-part, releasing place and the micro-gripper of any electrostatic charge which could interfere with the van-der-Waals forces. Figure 3: Initial arrangement before picking
Figure 7: Retracting curve for Cu 20
Figure 7: Retracting curve for Cu 20
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