Research of speed ripple reduction method for computed tomography

Abstract

This paper presents a study on the high per formance speed control of permanent magnet synchronous m otor for a computed tomography (e1).ln a CT system, many units are attached on the rotational part of the gantry, such as x-ray tube, detector, collimator, and data acquisition system. Therefore, it is difficult to balance many components of dif ferent weights; this is inevitable problem in CT systems.To reduce speed ripple caused by eccentricity, this study proposed modified adaptive eccentricity compensator (modi fied AEC) and verified its validity by comparing it with load torque obsernr (LTO). I. IJI.'TRODUCTIONAs shown in Figure I, computed tomography is composed of direct-drive SPMSM[I], X-ray tube, collimator, detector, data acquisition system, power supply, etc. The different weight of components attached to the rotator creates eccentricity, which causes speed ripple. In addition, tilting is made in CTs in order to get images from different positions. The range of tilting is generally between -25° -+30°, and the magnitude of eccentricity load is different according to the tilting angle. There have been researches to solve the problem of maintaining a constant speed in a system where speed ripple happens by load torque pulsation. These researches studied methods of reducing speed rip ple using eccentricity load position estimation [2], methods of compensating by estimating load torque using a state observer (3], methods of compensating by composing an adaptive observer on the assumption that load is eccentric [4], etc. Speed ripple reduction through estimating the position of eccentric load maintains a constant speed by investigating the shape of disturbance in advance and directly compen sating it. Because this method has to know in advance the position on the motor rotator to which the weight center is connected, it is difficult to apply the method when load po sition changes as in tilting.The method of redUCing speed ripple by designing a load torque observer (LTG) using a state observer sets an esti mated variable as load torque, and reduces speed ripple by feed-forwarding load toque to command torque. The method can respond actively to the change ofload during operation, but its fast response becomes a problem in CT, the object of the present study.Assuming that disturbance input is eccentricity load, an eccentricity adaptive compensator (AEC) composes an adaptive observer to reduce speed ripple. Because the shape of eccentric load is sinusoidal wave, the mechanical equation is nonlinear. Thus, this is a method of composing an adaptive 0-7803-8730

Adaptive internal model-based suppression of torque ripple in brushless DC motor drives

“…There are applications (e.g. computer tomography (Lee, Park, & Kwon, 2004), optics, machine tools (Jahns & Soong, 1996)) where very high precision is needed at low motor speeds.…”