In the past few decades, a new generation of press, servo press, has been researched and developed widely, because its programmable punch motions offer many benefits on stamping, such as high accuracy, energy saving, reduction of stamping noise and extension of tool life. In the market, the present servo press machine is essentially similar to a mechanical press, but it is directly driven by a servomotor(s), for the purpose of generating free punch motions. No flywheel is applied for energy storing; therefore a high-power servomotor(s) may be required to great stamping and energy capability.
In this dissertation, a novel conceptual “hybrid-driven servo press” is investigated. This kind of servo press uses a two degrees-of-freedom (DOF) seven-bar linkage. Two inputs are required to generate the punch motion. In the study, the following arrangement is undertaken: one input is a normal AC motor connecting to a flywheel, while the other input is a controllable servomotor. Under this arrangement, the output punch motion is programmable by coordinating the two inputs, and using the flywheel is advantage for energy accumulation to cope with the issue of considerable stamping force and energy during stamping. Three main topics related to the hybrid-driven servo press are specifically studied in this work, including linkage dimensional synthesis, stamping capability evaluation, and motion control issues.
Firstly, the dimensional synthesis procedure of the employed seven-bar linkage for the hybrid-driven servo press is derived analytically. The considered constraints involve the stroke length, assembly feasibility, and the toggle property at the absolute bottom dead center (BDC). All potential dimensional solutions can be expressed on a specific plane which allows one to decide the favorable position of the ground pivot. The servomotor function for producing the desired punch motion is also formulated. In a case study, a set of solution is exemplified, which provides maximal stroke of 140 mm and possesses a double-toggle property at BDC.
Secondly, the motion control issue of the hybrid-driven servo press is dealt with. The control scheme consisting of the feedback control and the iterative learning control (ILC) is presented. The feedback control with a synchronizing controller is applied to generate the desired punch motion. In the meantime, the ILC is for improving the accuracy performance of the punch motion in the repetitive manner. In the design phase of the ILC controller, the sensitivity Jacobian is particularly introduced as the proportional learning factor to adapt to the kinematical non-linearity of the driving linkage.
Furthermore, to understand the mechanics of such new servo press, the stamping capacity and the energy distribution between the flywheel and the servomotor are investigated. The capacity is derived based on the principle of energy conservation, and a method of using a capacity percentage plane for evaluation is proposed. A case study is included to illustrate and interpret that the stamping capacity is highly dependent on different punch motions.
For validation purpose, an experimental machine of the hybrid-driven servo press has been built based on the synthesized dimensions. The machine is equipped with a PC-based controller, in which the proposed control scheme is employed. Drawing experiments have been carried out for validating the effect of the established controller, and the results show that the ILC controller effectively made the punch position’s root-mean-squared (RMS) errors converge to less than 0.2 mm within five iterations. The precision (standard deviation of errors) of the control points was also improved to less than 50 μm, which was equivalent to 35-40% to the original level without the ILC. Besides, blanking experiments have been conducted and testified the torque and energy distribution of the inputs. The results show that the servomotor provided 15% torque in comparison with the flywheel torque, and contributed only 12% to the total blanking energy.
In summary, an overall investigation of the conceptual hybrid-driven servo press is presented in this work. A series of studies on linkage dimensional design, mechanical analysis, performance evaluation and motion control are presented. In conclusion, the proposed control system is a practicable solution to the servo press motion control problem. Based on the theoretical evaluations and experiments, only a small portion of stamping is taken charge by the costly servomotor, therefore it is believed that the hybrid-driven servo press with flywheel has a great potential to fulfill the power-saving and economic profits.

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