Yongseob Lim

Multi-Input Multi-Output (MIMO) Modeling and Control for Stamping

The binder force in sheet metal forming controls the material flow into the die cavity. Maintaining precise material flow characteristics is crucial for producing a high-quality stamped part. Process control can be used to adjust the binder force based on tracking of a reference punch force trajectory to improve part quality and consistency. The purpose of this paper is to present a systematic approach to the design and implementation of a suitable multi-input multi-output (MIMO) process controller. An appropriate process model structure for the purpose of controller design for the sheet metal forming process is presented and the parameter estimation for this model is accomplished using system identification methods. This paper is based on original experiments performed with a new variable blank holder force (or variable binder force) system that includes 12 hydraulic actuators to control the binder force. Experimental results from a complex-geometry part show that the MIMO process controller designed through simulation is effective.

Advances in the Control of Sheet Metal Forming

This paper presents a review of research on control of the sheet metal stamping process, and its effect on the quality of stamped parts. Section 1 of the paper introduces key quality considerations in the sheet metal stamping process, including new challenges for industrial needs. Section 2 presents the evolution of control strategies for the forming process. Section 3 describes the different types of active blank holder force (BHF) systems from previous research. Finally, Section 4 reviews in-process sensor technologies to monitor the process variables used in machine or process controllers for the sheet metal stamping process.

Improved part quality in stamping using Multi-Input Multi-Output (MIMO) process control

The binder force in sheet metal forming controls the material flow into the die cavity, and maintaining precise material flow characteristics is crucial for producing a high-quality stamped part. Process control can be used to adjust the binder force based on tracking a reference punch force trajectory to improve part quality and consistency. The purpose of this paper is to present a systematic approach to the design and implementation of a suitable MIMO process controller. The approach includes modeling of the sheet metal forming process, and the design of the process controller based on simulation. Experimental results from a complex-geometry part show that the MIMO process controller, designed through simulation, is effective.

Auto-Tuning and Adaptive Control

This section describes the design and implementation of automatic controller tuning and model reference adaptive control (MRAC) to improve part quality in stamping and extends previous work on a manually-tuned fixed-gain process controller. Automatic tuning is described with a discussion of implementation issues in the presence of plant disturbances. Design of a direct MRAC, whose controller gains are continuously adjusted to accommodate changes in process dynamics and disturbances, is investigated, including simulation-based robustness analysis of the adaptation law and a consideration of constrained estimation in the recursive least squares algorithm to address practical implementation issues. The performance of the MRAC process controller designed through simulation is experimentally validated. Good tracking of the reference process variable (i.e., punch force), and significant part quality improvement in the presence of disturbances, is achieved.

Recent Advances in Stamping Control

This chapter presents a review of research on control of the sheet metal stamping process, and its effect on the quality of stamped parts. First the evolution of control strategies for the forming process is presented. Next the different types of active blank holder force systems from previous research are described. Finally, a review of in-process sensor technologies to monitor the process variables used in process controllers for sheet metal stamping is given.

Direct and Indirect Adaptive Process Control

This chapter compares the design, implementation and performance of direct and indirect adaptive control (AC) to improve part quality in the stamping process in the presence of disturbances. First, previous work on the design and performance of a direct AC approach (i.e., model reference adaptive control or MRAC) is summarized. The direct AC filter uses nominal process parameters, and so requires some knowledge of the process. Consequently, an indirect AC approach, which estimates process parameters on-line, was also considered. However, due to the simple proportional plus integral (PI) control structure selected, the computation of the controller gains from the estimated parameters requires an optimization procedure, which is not amenable to real-time implementation. Thus, the indirect AC is implemented using a look-up table, with controller gains that are pre-computed off-line via optimization. The indirect AC with the look-up table is compared to the direct AC via simulations and experiments in terms of tracking performance as well as part quality, in the presence of plant variations. The indirect AC, with a sufficiently high level of discretization in the look-up table, performs well in simulations. However, due to extensive memory requirements, a smaller look-up table is used in the experiments, where it is outperformed by the direct AC.

Equipment and Material Flow Control

Process control ensures that stamped part quality is maintained in the presence of operational variations and disturbances. Control objectives are achieved by adjusting the flow of metal into the die cavity in response to these variations and disturbances. This chapter provides an overview of the process and equipment used for sheet-metal stamping, and describes the methods through which material flow control can be effected during the stamping process. Simplified kinematic and dynamic models for press motion are derived, followed by a description of hydraulic actuation to implement variable binder force for material flow control.

Laboratory Development of Process Control

In sheet metal forming processes the blank holder force controls the material flow into the die cavity, which is critical to producing a good part. Process control can be used to adjust the blank holder force in-process based on tracking a reference punch force trajectory to improve part quality and consistency. Key issues in process control include process modeling as well as process controller and reference punch force trajectory design. In this chapter a systematic approach to the design and implementation of a suitable process controller and an optimal reference punch force trajectory is presented. The approach includes the modeling for controller design of the sheet metal forming process, design of the process controller, and determination of the optimal punch force trajectory. Experimental results from U-channel forming on a laboratory forming simulator show that a suitable process controller can be designed through simulation and an optimal reference punch force trajectory can be synthesized through experiments. The proposed development will be useful in designing and implementing process control in sheet metal forming processes as described in subsequent chapters.

Direct and Indirect Adaptive Process Control of Sheet Metal Forming

This paper compares the design, implementation and performance of direct and indirect adaptive control (AC) to improve part quality in the stamping process in the presence of disturbances. First, previous work on the design and performance of a direct AC approach (i.e., model reference adaptive control or MRAC) is summarized. The direct AC filter uses nominal process parameters, and so requires some knowledge of the process. Consequently, an indirect AC approach, which estimates process parameters on-line, was also considered. However, due to the simple proportional plus integral (PI) control structure selected, the computation of the controller gains from the estimated parameters requires an optimization procedure, which is not amenable to real-time implementation. Consequently, the indirect AC is implemented using a look-up table, with controller gains that are pre-computed off-line via optimization. The indirect AC with the look-up table is compared to the direct AC via simulations and experiments in terms of tracking performance as well as part quality, in the presence of plant variations. The indirect AC, with a sufficiently high level of discretization in the look-up table, performs well in simulations. However, due to extensive memory requirements, a smaller look-up table is used in the experiments, where it is outperformed by the direct AC.Copyright