Data acquisition and control system for reflectometer station

M.H.Modi, G.S.Lodha, K.J.S. Sawhney

Synchrotron Utilization Division

In modern laboratory environment computer has become an essential instrument for establishing interactive communication among different laboratory devices.  Nowadays most of the instruments are available with PC controllable feature.  With modern PC technology, the laborious data generation tasks have reduced significantly.  A PC based data acquisition and control system is developed for CAT-reflectometer station on Indus-1 reflectivity beamline.

In reflectometer station precision motions of sample and detector are required in wide angular range and in small step size (<0.01°).  Simultaneous readings of various inputs corresponding to reflected signal, incident beam signal, and ring current have to be recorded.  Computer controlled wavelength and angle scan modes are required.

Figure 1Schematic diagram of data acquisition and control system of reflectometer

Hardware description: In the existing reflectometer station three stepping motor drives are installed, two corresponding to q-2q rotary motion of sample and detector, and one for linear motion of sample.  These stepping motors are controlled using a PC plug-in card on ISA bus.  The card has a capability to drive unipolar 2-phase stepping motors in micro stepping mode.  The motors can be connected to the controller card directly which has integrated power stages for driving the motors.  Card draws the power from PC SMPS supply.  As per the requirement of motors, the nominal current can be set in 50-800mA range.  Depending upon the motors used, proper current selection is necessary.  This 3-axis controller card provides two limit switch inputs per axis.  The inputs are TTL type i.e. input voltage <0.4V means logical 0 (end switch active), and input voltage >1.2V means logical 1 (end switch inactive).

Figure 2: Flowchart of the reflectometer control program.

            Silicon photodiode detectors (AXUV-100, IRD USA) are used for soft x-ray detection. Detector photo current is monitored using Keithley electrometer model-6514.  The detector current varies, in range of few micro amperes to 1pA, depending upon photon beam intensity.  The model is highly sensitive and can measure current down to 100 aA (10^-18Amp).  Electrometer is connected to PC through GPIB interface using a GPIB board of National Instruments.  The GPIB bus is the IEEE-488 instrumentation data bus with hardware and programming standard adopted by the IEEE (Institute of Electrical and Electronic Engineers).  This standard follows a group of common commands and syntax for sending the data to and from instruments.  The primary address required for instrument identification on GPIB bus can be set in range of 0-30.

Figure 3: View of reflectometer control program

            Wavelength of TGM monochromator can be set using Jobin Yoven stepping motor drive, which is controllable through RS-232 serial communication port.  Absolute value of wavelength is available from Heidenhain VRZ 480 encoder.  The encoder is connected on serial communication port COM2.  Status of storage ring current is available in 0-5V signal.  A 4½ digit digital multimeter on serial communication port COM3 is connected.  Schematic of data acquisition and control is given in figure1.

Software description:  The reflectometer data acquisition and control program is written in VC++ that works in Microsoft Windows environment.  Flow chart of program is given in figure2.  The visual interface generated in VC++ makes the program user friendly.  Outlook of control program is given in figure3.  The program allows user to accomplish various modes of reflectivity scan 1) rocking curve scan 2) detector scan 3) q-2q scan and 4) wavelength scan. In Mode 1, sample rotates (Motor1), in a given range, and detector remains fixed (Motor2) at particular angle.  In mode2 i.e. detector scan mode the detector rotates and sample remains fixed.  q-2q scan mode is used to make the angle versus reflectivity measurements where sample and detector rotates in 1:2 ratio. First two modes of scan are useful for sample and detector alignment in respect of incident photon beam.  Angular range of scan can be set in 0-180° range.  The sample is mounted on linear translation stage that can also be controlled through software.  Linear position of sample can be set in ±25mm range in step of 1mm.  This feature allows in keeping the sample in and out of the beam.  The detector current is read, in each scan mode, as a function of angular position of either detector or sample and stored in data file.  Wavelength scan mode can be carried out with keeping both sample and detector at a fix angle.  The available wavelength range for the beamline is 40-1000Å that can be scanned using the software.  The status of storage ring current is always monitored and stored along with detector current.  The effect of natural decay of storage ring current is taken into account while processing the reflectivity data.

The program initializes communication port COM1, 2 & 3 at the beginning where monochromator, its encoder and digital multimeter is connected.  Along with serial communication devices the GPIB instrument is also initializes.  For online data plotting the Microcal Origin software is used.  A macro is define that calls stored data file and gives graphical display continuously.