Abstract

Magnetic servo levitation (MSL) is currently being investigated as an alternative to drive fast-tool servo systems that could overcome the range limitations inherent to piezoelectric driven devices while operating over a wide bandwidth. To control such systems, a feedback-linearized controller coupled with a Kalman filter has been previously described. Performance limitations that degrade tracking accuracy suggest the use of a more robust controller design approach, such as sliding-mode control. Current literature on sliding mode deals almost exclusively with systems that are affine on the input, while the magnetic fast-tool servo is nonlinear on it when the control action is current command. This paper discusses a sliding-mode-based controller that overcomes the aforementioned problem by defining a modified sliding condition to calculate control action. Experimental results demonstrate the feasibility of achieving long-range fast tracking with magnetically levitated devices by using sliding-mode control.

Introduction

MAGNETIC SERVO levitation (MSL) [4], [7], [8], [20]–[22] represents a promising but very challenging alternative to actuate positioning systems where both long range and fast response (wide bandwidth) are required. Several applications require high-speed, high-resolution, high-stiffness positioning actuators. For these applications, piezoelectric actuators have been used extensively, providing excellent performance, but they are severely restricted in range of travel. Piezoelectric stacks are also difficult to mount and pose problems related to mechanical design, cost, and cooling. For longer ranges of motion, linear motors have been used, but the maximum force and bandwidth available are very limited. MSL is a form of electromagnetic actuation that uses the attractive forces rather than the Lorentz (shear) forces utilized by most electromagnetic motors (see Fig. 1). Much higher forces are possible, but the range of motion is smaller and the system is inherently unstable and highly nonlinear. It is important to emphasize the need of both an accurate model and a robust control law if high positioning accuracy is expected. An active tool holder (fast-tool servo) based on MSL is depicted in Fig. 2. This type of actuator can be used in machining operations such as diamond turning of nonrotationally symmetric surfaces.
Most of the currently existing magnetically levitated devices are used for regulation tasks and not for tracking (maglev vehicles, magnetic suspension systems). Long-range wideband tracking clearly represents a more difficult problem.

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