Sabri Cetinkunt

Raman microspectroscopy analysis of pressure-induced metallization in scratching of silicon

A single-point diamond turning machine was used to make grooves on (111) p-type single-crystal silicon wafers at room temperature. Scratch tests have been performed with both sharp (Vickers and conical) diamond tools, and a spherical (Rockwell) diamond tool. Our results showed that material removal mechanisms differed between these tools. Pressure-induced metallization of Si allows the ductile regime mechanical micromachining of wafer surfaces. Raman microspectroscopy and electron microscopy were used to determine the machining parameters that do not introduce cracking or other types of damage. The surface of the groove, after machining, was covered by a mixture of metastable, high-pressure silicon phases and amorphous silicon. Further, these phases can be transformed into cubic silicon by annealing. The maximum depth of cut in the ductile regime has been determined for the given scratch test conditions and tools. The developed technique can be used to machine Ge, GaAs and other semiconductors. Applications drawing from this research are many. For example, channels for microfluidic devices can be engraved with a channel cross-section that is determined by the shape of the tool, which allows patterns that cannot be produced using etching. There are no limitations on the channel length or direction, and the channel width can vary from potentially a few nanometres to several micrometres.

Design, fabrication, and real-time neural network control of a three-degrees-of-freedom nanopositioner

A nanometric precision three-degrees-of-freedom positioner is designed and fabricated. Actuation is based on piezoelectric stacks. Capacitive gap sensors with less than 1.0-nm resolution are used for position feedback. In order to design a proper closed-loop controller, the open-loop characteristics of the nanopositioner (static stiffness, hysteresis, drift, frequency response, and the coupling effects) are experimentally investigated. A cerebellar model articulation controller neural network control algorithm was applied in order to provide real-time learning and better tracking capability compared to a standard proportional-integral-derivative control algorithm.

Performance limitations of joint variable-feedback controllers due to manipulator structural flexibility

The performance limitations of manipulators under joint variable-feedback control are studied as a function of the mechanical flexibility inherent in the manipulator structure. A finite-dimensional time-domain dynamic model of a two-link, two-joint planar manipulator is used in the study. Emphasis is placed on determining the limitations of control algorithms that use only joint variable-feedback information in calculations of control decisions, since most motion control systems in practice are of this kind. Both fine and gross motion cases are studied. Results for fine motion agree well with previously reported results in the literature and are also helpful in explaining the performance limitations in fast gross motions. >

Closed-Loop behavior of a feedback-controlled flexible arm: a comparative study

The dynamics of mechanical systems with distributed flexi bility are described by infinite-dimensional mathematical models. In order to design afinite-dimensional controller, a finite-dimensional model of the system is needed. The con trol problem of a flexible beam is a typical example. The general practice in obtaining a finite-dimensional model is to use modal approximation for distributed flexibility, retain a finite number of modes, and truncate the rest. In this approx imation, the appropriate selection of the mode shape func tions and the number of modes is not clearly known. Mostly standard pinned-free and clamped-free mode shapes are used for the flexible beam model, retaining only two or three modes and truncating the rest. The actual system, on the other hand, is infinite-dimensional, and the modes describing its flexible behavior under feedback control would be neither pinned-free nor clamped-free boundary condition modes. Rather, the mode shapes themselves are a function of the feedback control. The infinite-dimensional transcendental transfer functions for a flexible beam are formulated without any modal ap proximation. Finite-dimensional transfer functions with different shapes and numbers of modes are formulated. The closed-loop performance predictions of different models under the same colocated and noncolocated controllers, which attempt to achieve high closed-loop bandwidth, are compared. Results are surprisingly consistent in all cases; the predictions of clamped-free mode shape models are much more accurate than the predictions of the pinned-free mode shape models.

Symbolic modeling and dynamic simulation of robotic manipulators with compliant links and joints

Abstract The explicit, non-recursive symbolic form of the dynamic model of robotic manipulators with compliant links and joints are developed based on a Lagrangian-assumed mode of formulation. This form of dynamic model is suitable for controller synthesis, as well as accurate simulations of robotic applications. The final form of the equations is organized in a form similar to rigid manipulator equations. This allows one to identify the differences between rigid and flexible manipulator dynamics explixitly. Therefore, current knowledge on control of rigid manipulators is likely to be utilized in a maximum way in developing new control algorithms for flexible manipulators. Computer automated symbolic expansion of the dynamic model equations for any desired manipulator is accomplished with programs written based on commercial symbolic manipulation programs (SMP, MACSYMA, REDUCE). A two-link manipulator is used as an example. Computational complexity involved in real-time control, using the explicit, non-recursive form of equations, is studied on single CPU and multi-CPU parallel computation processors.

CMAC Neural Network Control for High Precision Motion Control in the Presence of Large Friction

Precision requirements in ultra-precision machining are often given in the order of micrometers or sub-micrometers. Machining at these levels requires precise control of the position and speed of the machine tool axes. Furthermore, in machining of brittle materials, extremely low feed rates of the machine tool axes are required. At these low feed rates there is a large and erratic friction characteristic in the drive system which standard PID controllers are unable to deal with. In order to achieve the desired accuracies, friction must be accurately compensated in the real-time servo control algorithm. A learning controller based on the CMAC algorithm is studied for this task.

Symbolic modeling of flexible manipulators

This paper presents a new systematic algorithm to symbolically derive the full nonlinear dynamic equations of motion of multi-link flexible manipulators. Lagranges-Assumed modes method is the basis of the new algorithm and adapted in a way suitable for symbolic manipulation by digital computers. It is applied to model a two-link flexible arm via a commercially available symbolic manipulation program. The advantages of the algorithm and simulation results are discussed.

Fast tool servo control for ultra-precision machining at extremely low feed rates

Abstract Diamond turning of brittle materials such as glass, ceramic, germanium and zinc sulfide has been of considerable research interest in recent years due to applications in optics and precision engineering systems. When diamond turning brittle materials, material removal should be kept within the ductile regime to avoid subsurface damage [1, 2]. It is generally accepted that ductile regime machining of brittle materials can be accomplished using extremely low depth of cut and feed rates. Nanometric level positioning accuracy of the machine tool axes is difficult particularly at low feed rates due to friction and backlash. Friction at extremely low feed rates is highly nonlinear due to the transition from stiction to coulomb friction, and as such is very difficult to model. In order to compensate the effect of friction and backlash of a machine tool stage, a nano-metric precision three degrees of freedom (DOF) positioner is designed and fabricated as a fast tool servo. The fast tool servo, which is an independently operated positioning device, would have a small range but high bandwidth and accuracy compared to the conventional lead-screw mechanism. The nano positioner as a fast tool servo and motor-driven feedback controller were combined in order to obtain large motion with high positioning accuracy. A CMAC neural network control algorithm was applied in order to provide on-line learning and better tracking capability compared to standard PID control algorithm. The learning controller was implemented using “C” language on DSP based PC-bus board to control both diamond turning machine and nano positioner.

Optimal design issues in high-speed high-precision motion servo systems

Abstract High speed and precision requirements in servo driven multi-axis motion control systems are such that optimal design consideration of not just the servo motor, but all of the system components is necessary in order to achieve the best possible performance from the available technology. In general, speed and precision are conflicting requirements in incremental motion control systems. The current state of the art in servo systems, their performance characteristics, ultimate performance limitations, optimal design and selection criteria are discussed for high-speed high-precision incremental motion control applications such as assembly machines and x-y table based machines.

Positive flow control of closed-center electrohydraulic implement-by-wire systems for mobile equipment applications

Digitally controlled electrohydraulic (EH) closed-center valves and load sensing hydraulic pump systems have been quickly replacing the older generation of hydro-mechanically controlled (pilot actuated, mechanically controlled) open-center valves and fixed displacement pump systems in mobile equipment applications such as earth moving and construction equipment (i.e. wheel loaders, excavators, harvesters). With carefully designed and coordinated motion control systems, the power efficiency of the hydraulic system can be improved using closed-center EH systems to the point that significant fuel savings can be achieved. Furthermore, the machine behavior can be optimized for different applications via real-time control software. Such capabilities were not possible in hydro-mechanically controlled systems. Energy efficiency, high performance, software based re-configurability and reliability are the main objectives in design of EH systems for mobile equipment applications.