This paper deals with precision positioning in the presence of friction. The object studied is an aerostatic slide system driven by a DC motor with brushes that introduce friction to the system. For such systems, models that do not account for friction can only be used to describe the macrodynamic behavior. The microdynamic behavior is significantly different. Instead of designing two controllers for the different dynamics, a single-step precision positioning using a high-gain controller designed according to the macrodynamics alone is proposed. A proportional-integral-derivative (PID) controller with finite-gain derivative and a modified anti-windup integral reset is designed. Controller parameters are obtained by pole placement rather than manual tuning or other rules of thumb. Identification of the microdynamics is avoided. Single-step precision positioning and uniform response to different sizes of step input are achieved. This method is particularly effective when the friction has uncertainty. Experimental and simulated results indicate that the PID controller can provide a sufficiently high loop gain. In point-to-point positioning for step inputs from millimeter size down to submicrometer size, the positioning error is within ±2 nm and the response dynamics is satisfactory.

Precision positioning of a DC-motor-driven aerostatic slide system

Indirect fractional order pole assignment based adaptive control

In order to achieve good performance in precision positioning systems where friction is present, a straightforward approach is using single-integrator controllers to suppress the effect of friction. In this paper, double-integrator (DI) control, in which two integrators are involved, is proposed to enhance controller gain at lower frequencies. A new integral anti-windup scheme is developed for the DI control and is shown to be effective. Performance of a DI controller is compared with that of a proportional-integral-derivative controller (I-PD) and a pole-placement-with-integration controller (PPI) by experiment on a slide system driven by a dc motor. In the system, the dc motor has brushes that give rise to friction, and all other frictional contacts are removed by frictionless aerostatic support. Because of the unmodeled high frequency dynamics caused by aerostatic dynamics, closed-loop bandwidth with the I-PD controller is limited at a lower value than in the cases with the DI and PPI controllers. With the three controllers having parameters for similar closed-loop bandwidths, the DI controller gives uniform closed-loop responses to step inputs of different heights, while for the other two controllers the responses are obviously dependent on the height of the step inputs.

Double-integrator control for precision positioning in the presence of friction

Control of process parameters is one of the important problems in process industry. The process considered for modeling is spherical tank liquid level system. Control of liquid level in a spherical tank is nonlinear due to the variation in the area of cross section of the level system with change in shape. Nonlinear process control is a difficult task in process Industries. Spherical tank is a nonlinear system due to its variation in the area of cross section. In this project the modeling of Spherical tank system is done. The nonlinear parameters are linearised by using the Taylor-series. The controller is designed using Model Reference Adaptive Controller based on MIT rule. Adaptive Controller for Spherical tank system is simulated in matlab simulink environment.

http://inpressco.com/wp-content/uploads/2012/12/Paper8374-3762.pdf

Design and Development of Model Based Controller for a Spherical Tank

This paper presents an innovative design method of polynomial control laws by mean of pole placement which are actually smart solutions to many industrial applications. Even if this category of controllers is very popular in industry, most of their applications concern problems of constant reference signals. In this paper, we propose an indirect adaptive controller by fractional order pole placement, and show using a simulation example where this real time fractional order control strategy is applied to an air-lubricated capstan drive for precision positioning the improvement in system's behavior when compared to the classical control scheme with integer order poles.

Fractional order self-tuning control

Because friction-induced nonlinearities in positioning systems are generally range of motion-dependent, dual-model or dual-stage strategies are often adopted to deal with the inconsistencies encountered when a system moves from submicrometer steps (micro mode) to larger scale strokes (macro mode). Although good performance is usually obtained when each model/stage operates in its designed range of motion, a system frequently performs less satisfactorally when operating near the switching point between models or stages. An air-lubricated capstan drive was used in this study to minimize the discrepancy between macro and micro modes, and a single-mode MRAC was developed to control the capstan drive system for precision positioning. Accuracy better than ±15 nm with no overshooting was achieved in all conditions tested (including 50 nm, 500 nm and 10 μm steps). Disturbance resistance of the system also proved to be satisfactory.

Model reference adaptive control of air-lubricated capstan drive for precision positioning

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Model reference adaptive control of air-lubricated capstan drive for precision positioning