Pressure-Distribution Methods for Estimating Lifting Force of a Swirl Gripper

The swirl gripper is a recently developed noncontact handling device that generates swirling air flow to create an upward lifting force. This force can be used to pick up a work piece placed underneath the swirl gripper without any contact. It is applicable, for example, in the semiconductor wafer manufacturing process, where contact must be avoided during the handling and moving of a work piece to minimize damage. For the purpose of indirectly estimating the lifting force of the swirl gripper, pressure-distribution methods are proposed and studied via theoretical analysis and experimental verifications in this paper. First, we show the design of the swirl gripper and briefly illustrate the mechanism by which it forms a negative pressure to create a lifting force. Then, we experimentally investigate the characteristics of the pressure distribution. Furthermore, we conduct a theoretical analysis of the pressure distribution and propose two methods for estimating the lifting force: one based on the rotation speed and a single pressure point (the ω-P method) and the other based on two pressure points (the P-P method). Then, we verify the proposed methods via a force measurement experiment. We find that the ω-P method causes considerable deviations in the calculated pressure distribution around the gap entrance of the swirl gripper, because of which the estimated force is smaller than the actual one. In contrast, we find that the P-P method can compensate for the error by directly detecting the pressure at the gap entrance with another pressure sensor. Our results show that the P-P method markedly improves the accuracy of estimating the lifting force as compared to the ω-P method. Finally, for the purpose of further improving the estimation accuracy, we increase the number of pressure points to estimate lifting force. It is made clear that the estimation accuracy is improved, and the effect is particularly significant for the P-P method.

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