The Control Performance of Elliptically Polarizing Undulator at SRRC C.S.CHEN, JENNY CHEN, K.T.HSU, K.T. PAN Synchrotron Radiation Research Center, Hsinchu, Taiwan

Abstract The controller consists of four processes. The dynamic database upload process uploads data in 10 There are two elliptically polarizing undulators (EPUs) times per second. The setting service process deals with with a period length of 56 mm at SRRC. One is a setting command from console computer. The general engineering prototype 1-meter-long EPU which is being reading process reads from analog inputs and digital installed for beam test. The other is a final version 4- inputs in 100 times per second. The motion control meters-long EPU ( EPU5.6) which will be installed in the related process is for motion control including residual end of this year. The requirements of the control field compensation mechanism. The insertion device (ID) performance are a fast polarization switch, good magnet service process supplies the beamline users can directly gap control resolution for getting precise photon energy access ID information. For local testing and debugging and multiple axes tracking during changing gap and proposes, a program can be run in controller. It uses a adjusting phase. There are four axes horizontal magnetic common share memory located by local controller. Thus, array to adjust phase position for 1 m EPU. Due to some of the ID information can be uploaded to console magnet force, the adjacent and non-adjacent two axes will computer when the program is run. The relationship of possess different effects during moving magnetic array. processes is shown in Figure 1. Residual field compensation mechanism function to provide to preclude the beam orbit from drift. For

Beamline ID achieving a high performance, it should be fine tuned the Main Control Access Program Programs proportional-integral-derivative (PID) parameters of the Console Computer servo loop. The results will be reported in this paper. Local Controller Setting Service ID Service DDB Upload 1. INTRODUCTION Process Process Process

From the viewpoint of beamline users, the requirements Dynamic of the EPU5.6 need the good reproducibility of the gap Database General Motor Information Reading and phase, the minimum deviation between two gaps or Update & Corrector Process Follow Gap /Phase phases during movement, the good position accuracy of Control the gap drive system and fast response. Therefore, the Digital Analog Motion Digital Analog Output Output Controller Input Input structure design considerations of hardware and software of the control system are to meet the specifications of the

Motor Power Supplies Tilt EPU5.6 [1]. Drivers sensors ... etc

Figure 1. Software structure on EPU controller 2. STRUCTURE OF THE VME CRATE CONTROLLER 3. HIGHLIGHT OF THE EPU CONTROL 2.1 Hardware The controller of the EPUs is based on VMEbus system. 3.1 Motion control The key module is PowerPC CPU running with LynxOS. Motion control is an important issue on ID control. For Because the controller is a diskless computer, the control getting fast response and better tracking, the servo loop programs saved in server computer will be downloaded control is used. The proportional-integral-derivative (PID) automatically to the controller. The embedded VMEbus control method is supplied by motion control module. The motion controller is used to improve system response. The maximum speed of the gap and phase movement are other components are absolute linear encoder readout limited by driven mechanism, that is 1 mm/sec and 6 module and general purpose input/output module. mm/sec, respectively. Because the absolute linear encoder is with incremental and absolute signal output at same time. Therefore, the incremental signals are used for gap control servo loop 3.2 Residual field compensation mechanism purpose, the absolute values are read by absolute linear and orbit control strategies encoder readout module with the bidirectional A residual field compensation mechanism is built in synchronous-serial (EnDat) interface. controller. The machine physicist can build or update the 2.2 Software residual field compensation table which the end-pole correctors power supplies setting follow the gap change

2231 and phase change by himself. The power supplies setting reproducibility and two axes tracking have been achieved can be done in 100 times per second. The table can be to about 1ôm. Figure 4. and Figure 5. show the control accessed by controller via the network file system (NFS). resolution and dynamic tracking, respectively. The residual field compensation table builder user Table 1. Performance of the 1 m EPU and EPU5.6 interface is shown in Figure 2. Item encoder two axes gap/phase Device s resolution reproducibility tracking resolution 1m EPU (phase) 0.5 um 1 um 2 um (p-p) 1 um

EPU 5.6 (gap) 0.4 um 1 um 2 um (p-p) 1 um

1m EPU Phase Control ( step change : 10 um ) 0.12 dl.encoder

0.1

0.08

0.06

0.04 Encoder Position ( mm )

0.02

0

-0.02 0 500 1000 1500 2000 2500 Time ( x 100 ms ) (a) 4 m EPU Gap Control ( step change : 10 um ) 160.12 gap

160.1

Figure 2. The residual field compensation table builder 160.08 user interface 160.06 Gap ( mm ) 160.04 160.02

Two approaches can be used to compensate the orbit 160

159.98 distortion due to the gap or phase change. One is the 0 500 1000 1500 2000 2500 3000 3500 4000 4500 Time ( x 100 ms ) feedforward orbit compensation scheme that should build (b) the table which correctors power supplies setting follow Figure 4. (a) 1 m EPU phase control resolution the gap change and phase adjustment in order to keep the (b) EPU5.6 gap control resolution orbit at certain orbit. The disadvantage is that it can not

1 m EPU Phase Control ( two axis tracking ) eliminate the uncertain disturbance and need more 2 tracking iterations to generate the table. If the gap and phase were 1.5 1 needed to be changed at the same, then the table would be 0.5 a little complexity to be generated. The other is orbit 0 -0.5 feedback that can eliminate other disturbance except Encoder Difference ( um ) -1 caused by the gap change or phase adjustment of the -1.5 -2 200 250 300 350 400 450 insertion device. Time ( x ~= 10 ms ) (a) 4 m EPU Gap Control ( two axes tracking ) 2 3.3 Homing tracking 1.5 The homing procedure is executed on phase axes 1 0.5 using the incremental encoders. However, the absolute 0

-0.5 linear encoders are used on gap control. The advantage Encoder Difference ( um ) is to eliminate homing procedure when the controller is -1 -1.5

-2 initially started or power loss. 3000 3200 3400 3600 3800 4000 Time ( x~=1 ms ) (b) 3.4 Protection and interlock Figure 5. (a) 1 m EPU two axes tracking (b) EPU5.6 two Hardware, software and firmware interlock are axes tracking considered for system safety. The hardware interlock components include the tilt sensors, the limit switches, 5. OPERATOR INTERFACE and hard stops. The software interlock includes tilt The 1 m EPU is phase control only and is ready for checking and torque limitation. The slip and over torque beam test, whereas the EPU5.6 is gap and phase control limit are firmware interlock functions. and in construction phase. For 1 m EPU, the operator interface is shown in Figure 3. However, the operator 4 . CONTROL PERFORMANCE interface of the EPU5.6 will like as a real EPU system and show the devices related position. The control performance is tested on 1 m EPU phase control and EPU5.6 without magnetic blocks gap control [2]. The control achievable performance of two systems is shown in Table 1. Currently, the performance of the

2232 from console computer, the motor will response in about 0.1 second. Then, the controller acknowledges the information to console computer within 0.2 second [3]. 7.4 Error reporting and recovery The motion control module supplies commands which can automatically record data about motion at 2 msec intervals. The recorded data are helpful for system maintenance if the system malfunction. The error recovery procedure functions are supported due to tilt or limit switch active.

Figure 3. 1 m EPU phase control user interface 7.5 Mechanical deflection 6. BEAMLINE USER INTERFACE The tilt sensor is a dual-axis solid aluminum housing containing precision electrolytic transducers. One axis is Control system supports two different approaches to to measure the girder tilt, the other is to measure the C- beam-line users to change the gap or phase of EPU5.6. frame deflection during the gap change. End-station users can send command to machine control system or to VME crate controller directly to request the 7.6 Mechanical problem gap change. Relationship among main control system, EPU5.6 controller and the beamline end-station is shown There is a abnormal moving occurred on 1 m EPU in Figure 5. phase control. It always happens the same quantity of the overshoot on the same axis and same position. However,

PC-Based machine status broadcast system the overshoot can be compensated due to the servo DECServer VAXstation DECstation / ALPHAstation / PC Windows NT control loop working well. It is shown in the solid line Step 2 : Step 2 : acknowledged granted by Console by Console Computer Computer curve of Figure 7. From the point curve reading from

Control System Step 1 : requested resolver of Figure 7, it shows out the mechanical driver VME VME VME •by crate crate Endstation I/O I/O I/O I/O I/O I/O I/O I/O

I/O I/O I/O I/O crate CPU CPU CPU Computer Update : 100 msec Router problem. The movement trajectory is shown in Figure 7. or Switching Setting/Acknowledge: 20-30 msec Step 3 : Network controled by Endstation Decices Computer

m 1 m EPU Phase Control ( UR axis ) Beamline Endstation 2 VME ur.enc crate EPUs I/O I/O I/O I/O 1.8

CPU ur.rev Experimental Area Ethernet Optional 1.6 Beamline Control 1.4 Computer Ethernet PC, VME, ... etc. to 1.2 IEEE-488 IEEE-488 Adapter RS-232 CAMAC 1 Direct I/O Endstation Peripheral Hardware Computer 0.8 for EPU Monochromator Controller Vacuum Subsystem Optional

Diagnostics (Io, ...) Encoder Reading and Resolver (m 0.6 Interlock IEEE-488/RS-232 ... etc 0.4

Global Interlock 0.2 Interlock (Accelerator, ...etc.) Vacuum, Optics, ..etc. 0 SR Endstation Equipments 50 100 150 200 250 e - Time ( x~ = 10 ms) Monochromator

Figure 6. Relationship among main control system, EPU Figure 7. Line represents the encoder reading value; point controller and the beamline end-station represents the resolver reading value 7. DISCUSSION 8. ACKNOWLEDGMENTS 7.1 Dynamic tracking Thanks for H. H. Chen to help to solve problem during system testing and expand to colleagues of the C. If the dynamic tracking were worse during the two axes H. Chang and C. S. Hwang to let the project to progress gap control or two axes phase control, it would affect the smoothly. experiment of the beamline users due to the photon energy shift. REFERENCES [1] C.H. Chang, et al “ Optimization design for SRRC 7.2 Overshoot Elliptically Polarizing Undulator”, MT-15 Conference, Beijing, China, 1997 Fine tuned of PID can minimize overshoot and steady [2] Bahrdt, A. Gaupp, G. Ingold, M. Scheer “Status of the state error. A smoothing function supported by motion Insertion Devices for BESSY II”, Proceeding of the 5th controller. It can continuous and smooth acceleration to EPAC, Barcelona, June 1996 reduce overshoot. [3] K.T.Pan,et al, “Control for Elliptically Polarizing Undula- tor at SRRC “, Proceeding of the ICALEPCS’97 7.3 Response time When the controller receives the moving command

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