David E. Hardt

High bandwidth force regulation and inertia reduction using a macro/micro manipulator system

A robots ability to maintain desired interface forces during constrained motion is governed by its driving-point impedance and primarily by its inertia. Negative force feedback is a means of reducing this driving-point impedance, but the system becomes unstable at high bandwidths. A macro/micro manipulator system, consisting of a large (macro) robot carrying a small (micro) high-performance robot, alleviates this problem by physically reducing the endpoint inertia as well as providing an inherently stable physical configuration for high bandwidth force control. A robust controller design based on physical equivalence and impedance matching is proposed. It is shown that interface force regulation at bandwidths higher than the structural frequencies of the macromanipulator can be achieved with only minimal knowledge of the structure.<<ETX>>

Design and analysis of reconfigurable discrete dies for sheet metal forming

Abstract Discrete dies have been investigated for sheet metal forming since the early part of the 20th century. The reconfigurable nature of these dies lends itself well to flexible manufacturing systems; unfortunately, the state of knowledge on how to design and analyze discrete dies consisting of densely packed pins is very limited, thereby hindering industrys acceptance of this type of tooling. This paper addresses the design and analysis issues involved with movable die pins, turning a matrix of die pins into a rigid tool, and the pin matrix containment frame. A generalized procedure for designing discrete dies is developed and then applied to the design and fabrication of a pair of high-resolution sheet metal forming dies. These dies are set to shape and then used to stamp benchmark parts out of steel sheet.

The macro/micro manipulator : an improved architecture for robot control

Abstract A macro/micro manipulator system, consisting of a large (macro) robot carrying a small (micro) high-performance robot, has been proposed as a means of enhancing the functionality of a manipulator. The objective of the research presented in this paper was to investigate the inherent features in dynamic performance of such a system, and to evaluate its feasibility. The effect of a micromanipulator on the stability and performance of a robot system was investigated. It was found that a macro/micro manipulator is a stable and well-suited physical architecture for endpoint control. A robust controller desingn based on physical equivalence and impedance matching proposed. It is shown that both endpoint position and force control bandwidths higher than the structural frequencies of the robot can be achieved with minimal knowledge of the structure. A five degree-of-freedom micromanipulator that can accelerate a 22 kg mass at 45 G was designed, fabricated, and attached to an experimental one-axis robot. Using this system, a force-control bandwidth of 60 Hz (32 times higher than the first structural mode of the robot) was achieved against an environment that is five times stiffer than the robot structure. An endpoint position control bandwidth of 28 Hz (15 times higher than the first structural mode of the robot) was also achieved. This can improve endpoint accuracy, reduce cycle-time, and enhance the robots capability to successfully interact with a large class of environments.

Weld Pool Impedance Identification for Size Measurement and Control

A primary indicator of weld quality is the size of the bead produced, and for full penetration welds this size parameter can be reduced to the width of the backbead. A method for determining this width in real-time is proposed that measures the natural frequency of pool motion when driven by a time varying arc plasma force. This method is developed analytically and verified experimentally for stationary weld pools. A control system based on this measurement scheme is also developed and simulation results are presented.

Metallic glasses: viable tool materials for the production of surface microstructures in amorphous polymers by micro-hot-embossing

Metallic glasses possess unique mechanical properties which make them attractive materials for fabricating components for a variety of applications. For example, the commercial Zr-based metallic glasses possess high tensile strengths (?2.0 GPa), good fracture toughnesses (?10?50 MPa) and good wear and corrosion resistances. A particularly important characteristic of metallic glasses is their intrinsic homogeneity to the nanoscale because of the absence of grain boundaries. This characteristic, coupled with their unique mechanical properties, makes them ideal materials for fabricating micron-scale components, or high-aspect-ratio micro-patterned surfaces, which may in turn be used as dies for the hot-embossing of polymeric microfluidic devices. In this paper we consider a commercially available Zr-based metallic glass which has a glass transition temperature of Tg ? 350??C and describe the thermoplastic forming of a tool made from this material, which has the (negative) microchannel pattern for a simple microfluidic device. This tool was successfully used to produce the microchannel pattern by micro-hot-embossing of the amorphous polymers poly(methyl methacrylate) (Tg ? 115??C) and Zeonex-690R (Tg ? 136??C) above their glass transition temperatures. The metallic glass tool was found to be very robust, and it was used to produce hundreds of high-fidelity micron-scale embossed patterns without degradation or failure.

A Transfer Function Description of Sheet Metal Forming for Process Control

Three-dimensional forming of sheet metal parts is typically accomplished using one or two shaped tools (die) that impart the necessary complex curvature and induce sufficient in-plane strain for part strength and shape stability. This research proposes a method of applying closed-loop process control concepts to sheet forming in a manner that automatically converges upon the appropriate tooling design. The problem of controlling complex deformation is reduced to a system identification problem where the die-part transformation is developed as a spatial frequency domain transfer function. This transfer function is simply the ratio of the measured change in spatial frequency content of the part and the die. It is then shown that such a transfer function can be used to implement closed-loop process control via rapid die redesign. Axisymmetric forming experiments are presented that establish the appropriateness of the linear transfer function description (via a test of superposition) and demonstrate the convergence properties of the proposed control method

Rapid Tooling for Sheet Metal Forming Using Profiled Edge Laminations—Design Principles and Demonstration

Sheet metal forming dies constructed of laminations offer advantages over more conventional tooling fabrication methods (e.g. CNC-machining) in terms of tooling accessibility, reduced limitations on die geometry and faster fabrication with harder die materials. Furthermore, the recently introduced Profiled Edge Lamination (PEL) tooling method improves upon other lamination-based tooling methods. Adoption of this promising rapid tooling method by industry is being hindered by the lack of formal analysis, design principles, and manufacturing requirements needed to construct dies in such a manner. Therefore, the propensity for delamination of the die is discussed and preventive measures are suggested. The basic machining instructions, i.e., an array of points and directional vectors for each lamination, are outlined for both compound and planar profiled-edge bevels. Laser, AWJ and flute-edge endmilling are experimentally identified as the most promising methods for machining bevels. Development of a stand-alone PEL fabrication machine is suggested over retrofitting commercially-available 5-axis machines. Finally, the general procedure for creating PEL dies is implemented in the construction of a matched set of sheet metal forming tools. These tools are used to successfully stamp a sheet metal part out of draw-quality steel.

Controller design in the physical domain

Abstract “Design in the Physical Domain” is proposed as a means of integrating control systems design with mechanical systems design. This approach facilitates separation of design issues from implementation issues through high-level abstraction, and provides guidance in selecting the proper physical architecture for a given control task. As an example it is shown how this philosophy may lead to alternative robot architectures that are inherently stable and well-suited for high performance end-point control.

Application of Adaptive Control Theory to On-Line GTA Weld Geometry Regulation

This study addresses the uses of adaptive schemes for on-line control of backbead width in the Gas Tungsten Arc (GTA) welding process. Open-loop tests using a step input current confirm the validity of a nominal first order process model. However, the time constant and gain prove highly dependent upon welding conditions including torch speed, arc length, material thickness, and other material properties. Accordingly, a need exists for adaptive controllers that can compensate for these process nonlinearities. The performance of two adaptive controllers is evaluated: Narendra and Lin’s Model-Referenced Adaptive Control (MRAC/NL), and Self-Tuning Control with Pole Placement (STC/PP). The addition of a quadratic term to the adaption mechanisms of MRAC/NL is proposed and preliminary simulations and experiments clearly demonstrate the stabilizing effect of this added term. The main experiments compare the performance of the modified MRAC/NL controller and the STC/PP controller with each other and with linear PI controller and the STC/PP controller with each other and with linear PI controller under four experimental conditions: first, where welding conditions are nominal; second, when conditions are disturbed by a step-wise increase in the torch velocity, and third, when conditions are disturbed by a step-wise increase in material thickness. In each case the experimental demonstrates the superiority of the adaptive controllers over the linear PI controller. However, the STC/PP controller exhibits high frequency control action in response to severe disturbances of material thickness and the parameter estimates it generates drift during steady-state operations. The MRAC/NL controller proves more robust under these circumstances. Analysis demonstrates that the superior performance of the MRAC/NL is due both to the inherent normalizing effect of the quadratic feedback terms and to the noise filtering properties of the adaptive mechanism.

Sheet Metal Die Forming Using Closed-Loop Shape Control

Summary The shape of compound curvature sheet metal parts is determined primarily by the geometry of forming dies and secondarily by interfacial and other forming environment conditions. Consequently the die geometry design step is critical to successful part manufacture. Existing analytical techniques do not permit sufficiently accurate prediction of die shape because of the inherent complexity of the three dimensional plasticity problem, the difficulty of characterizing interface forces and the need to continuously recalibrate the model as material properties vary. In this paper the use of a variable configuration, discrete element die in a closed-loop shape control system is proposed as a solution to this problem. The system forms a workpiece by an iterative procedure involving forming, measuring, and die shape change until shape convergence occurs. A critical experiment was performed on such a system that confirmed the validity of this concept.