Closed-loop control of the blank holding force in sheet metal forming with a new embedded-type displacement sensor

Abstract

Imposing an adaptable blank holding force (BHF) on the flange is an effective technique used to suppress the tendency towards sidewall tearing and to restrain the flange from wrinkling during sheet forming. To this goal, a novel photoelectric displacement sensor was adopted to detect the movement of sheet metal on the die shoulder, and a LVDT (linear variable displacement transducer) placed on the upper die was used to measure the wrinkle height during the process. The output of the PDS can be substituted into a simple formula to calculate the real-time minimum thickness of the sidewall. Together with the readout from the LVDT, the result of the calculation serves as feedback parameters for the BHF control. A tearing-wrinkling algorithm with two modes has been developed in the paper: the fixed-variance mode and the fixed-ratio mode. Beginning with very different initial values of BHF, the proposed BHF controls can attain formability at least as good as the optimal result of the test with constant BHF. The fixed-ratio mode enables the servo-system to react fast in high-speed operations. However, the uniformity of wall thickness is inferior to that of the fixed-variance control. In both cases, the optimal uniformity is obtained under the relatively low initial BHF, while a more uniform sidewall results in a smaller flange area and a product of lower final height is accordingly expected.