DIGITAL GLOBAL ORBIT FEEDBACK SYSTEM DEVELOPING IN SRRC C. H. Kuo, K. K. Lin, C. J. Wang, Jenny Chen , J. S. Chen C. S. Chen , and K. T. Hsu Synchrotron Radiation Research Center No.1 R&D Road VI, Hsinchu Science-Based Industrial Park, Hsinchu, Taiwan, R.O.C. Abstract The digital global orbit feedback system for the storage ring of SRRC has been upgraded in terms of its feedback bandwidth extension by increasing its data acquisition sampling rate and compensating eddy current effect of vacuum chamber with filter. This orbit feedback system has been applied incorporate with the insertion devices operation, such as U5 undulator and engineering model adjustable phase undulator. Eliminate orbit drift and low frequency oscillation is to continue effort. 1 INTRODUCTION Work to improve beam stability continues during 1996 with improvement of the orbit feedback system. New BPM and data acquisition system is installed in the storage ring. In this paper , we will discuss the results of global beam position feedback experiments conducted on new insertion device in Synchrotron Radiation Research Center. Any vibrations and orbit drift that lead to distortions in the closed orbit will result in a larger effective emittance. Together with the brightness reduction, beam motion induced incident light position and angle varying can degrade the advantages of using synchrotron light. Insertion devices are essential to produce high brilliance synchrotron radiation, however it influences the electron orbit and the lattice of storage ring. Global feedback system is used to eliminate these undesirable effects. From control points of view, global feedback is an typical multiple input multiple output (MIMO) problems. Technical, it is difficult to implement an analog matrix operation consisting of large amount of BPMs and correctors. Consequently, digital processing was used here to implement global feedback system. BPM resolution has to be better than 3 um, decoupling the interference between global and local feedback loops [7], integrating these two feedback loops for better tunability, bandwidth of 10 - 100 Hz is necessary to suppress vibration and power supply ripple related beam motion, etc. The global feedback system is integrated with the existed control system. BPMs data and correctors readback are updated into control system dynamic database in the period of 100 msec. Digital global feedback system is bounded on I/O as well as computation. It is important to arrange the real time task and to arbitrate computer bus properly in order to optimize system performance. 2 CONTROL ALGORITHM The global orbit feedback system includes 19 BPMs and 18 correctors in the vertical plane for this study. The response matrix of the system was measured by vertical beam displacement while sequentially varying the corrector strengths, and then is inverted by SVD [1, 2] skill. This method is part of the extension of local feedback technique. The advantage of the method used in this system is that it isn sensitive to the beam instability while measuring the response matrix. A schematic diagram of the feedback system is shown in figure 1. The position error vector [∆y] is filtered by a LPF [4] in order to compensate the system response dominated by eddy current effect of vacuum chamber. Filters are also used to extend close loop bandwidth and to eliminate processing noise. Storage Ring [∆ y] Mx1 [y ref ] Mx1 [y] Mx1 Data Acquisition System BPM + - PID parameter Response Matrix G [R] -1 NxM Figure 1: Block diagram of digital global feedback. Then control algorithm is applied to position error vector. Control algorithm of the digital global feedback is executed in corrector and computation VME crate. The conventional PID controller function G(z) is given by G(z) = K + K 1-z + K (1-z ) p i -1 d -1 where K p , K i , K d are the proportional, integral, and derivative controller gains, respectively. The gain 2374 0-7803-4376-X/98/$10.00  1998 IEEE