| X-ray beam position measurement plays a very important role for synchrotronradiation light source. It means whether x-ray beam is stable, as one of theanticipation conditions to obtain high quality experiment data. The measured valuesenable users to monitor the change of the x-ray beam center position timely.The XBPM (x-ray Beam Position Monitor) detector is mainly used to measurex-ray beam center position in SSRF. They have been playing a great useful part in thedebugging and running for those established beam lines. In order to improve theaccuracy of the XBPM detector, the upgrading of the detector is required. And a newXBPM data acquisition and analysis system, which is based on a new generationdigital processor Libera Photon, is carried out and studied. And the existing XBPMdetectors will be improved by using the new system, so that better serves thecustomer.The hardware architecture of the new system is based on the embedded digitalsignal processor Libera photon. In order to maximize its performance, the detail of theLibera photon is analyzed and tested in hardware and software, and test results showsthat the Libera photon can meet the requirements of SSRF XBPM detector readoutsignal’s measurement. And the system software platform adopts EPICS, it’s to makemore convenient that the information exchange with other systems.There are many factors that can influence accuracy of the x-ray beam positionmeasurement for the undulator beam line. When the Insertion Device magnetic gapchanges, the x-ray energy and its power density distribution, it will be followed tochange. Photocurrent generated within the blade electrode is also going to change,thus the x-ray beam center position by calculated will be inconsistent with the x-raybeam actual position. In this paper, by means of data mining, software algorithm isgiven to compensate the measurement deviation caused by the change of the Gapvalue of the undulator.After analyzing and calculating a great deal data of the XBPM running records of BL15U1and BL17U1, the rule of influence between x-ray beam position and ID Gapvalue was revealed. For Gap value of the most frequently used, the changes of thecalculated position of the x-ray beam is corresponding to the magnetic Gap valuechange. Overall, the relationship is nonlinear, but close around this particular Gapvalue, the correspondence relation is approximately linear. The change relationshipbetween the calculated x-ray beam center position and Gap value is piecewise linearaccording certain Gap value.To make better use of the XBPM detector, input reconstruction algorithms isadopt to modify the mentioned influence. This can decrease the difficulty andcomplexity of operation and maintenance, because passing period of running time, thecoefficients k1and K2of calculating x-ray beam center position need recalibration.As input reconstruction algorithms identifies the relationship between Gap value andinput currents, and the relationship will be not changed if k1and K2are calibrated inthe same Gap value and there’s no major change in Insertion Devices. In this paper,data of the XBPM running records were selected along with several mostly used Gapvalues from beam line BL15U1and BL17U1. With the same k1and K2, inputcurrents are compensated by input reconstruction algorithm, the calculated X-raybeam center position agrees with the actual position. Input reconstruction algorithmscan revise the measurement deviation caused by the change of the Gap effectively.Finally, the whole system is summarized and the direction of improvement andoptimization is given. The thinking of unfinished work is also presented. |
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