A motion control system for fly scans synchronized to X-ray pulses

With the High Energy Upgrade project in the installation phase at the SLAC National Accelerator Laboratory Linac Coherent Light Source (LCLS), soon the repetition rate of the X-ray beam will exceed the capability of the present motion control system to reliably deliver samples. We describe the design of a motion control system which aims to deliver solid crystal samples within 5 um of the focal point of the X-ray beam at a rate of 1000 Hz using a fly scan. Throughout the fly scan, a triggered timestamping device is used with an EtherCAT based distributed clock to predict the position error at the time of each X-ray pulse. This position error is used in a feedback loop to bias the velocity command to each motion axis and restore synchronism. The design is flexible enough to expand to any number of synchronized axes up to the limits of the computing hardware. Additionally, with most of the software written in Structured Text language defined in IEC 61131-3, it is transferable to other EtherCAT based real-time systems with few modifications. Finally, we describe how the TwinCAT PLC programming environment can be used to develop and test almost all of the functionality of the software without hardware present, through a combination of unit testing and simulation axes.