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Introduction

BEST

User’s Manual

The current from the phBPM are acquired by the

TetrAMM

and sent to the

BEST

Central Unit using a very low latency SFP connection.

The

BEST

Central Unit takes care of all the calculations to obtain the beam position

and intensity information. The

BEST

Central Unit calculates the correction

necessary  to  stabilize  the  intensity  and  position  of  the  beam  at  the  desired  setpoint,
using a fast PID algorithm. The correction setpoints are then sent to the actuator block
using a low-latency SFP interface. The critical tasks are performed in hardware using
a FPGA device in order to have a deterministic computing time, maximum calculation
speed and high reliability. The X-ray beam intensity is constantly monitored and can
be  used  to  automatically  enable  or  disable  the  PID  controller,  by  determining  if  the
beam  is  ON  (intensity  higher  than  a  specific  threshold)  or  if  the  beam  is  OFF
(intensity lower than a specific threshold). The control and interface unit offers a local
graphical  interface  (Local  GUI),  which  allows  to  fully  monitor,  manage  and  control
the beam position and intensity. A standard 10/100/1000 TCP-IP Ethernet link allows
remote  control  and  configuration  of  this  system,  hence  it  is  possible  to  connect  the
control unit directly to the beamline control system.

The actuator device, called

PreDAC

, receives the correction/compensation data

calculated by the

BEST

Central Unit and drives the beamline optics, using its

internal high precision digital-to-analog converters to generate an output voltage
signal capable of driving piezoelectric actuators acting on the optical elements. In this
way it is possible to close the control loop and to stabilize the X-ray beam.

The FPGA-based hardware architecture allows performing the control algorithms at a
maximized speed and with very low latency in order to guarantee full effectiveness of
the

BEST

correction performance over a frequency spectrum up to several kHz. The

slower and non-critical tasks (i.e. configuration commands) are separately performed
on an embedded industrial PC running a Linux OS with dedicated software.

The distributed architecture was selected in order to maximize the performance, both
in terms of speed as well as of sensitivity and accuracy, of the whole system. The

TetrAMM

readout system should be placed as close as possible to the phBPM and

the

PreDAC

system as close as possible to the actuator driving the beamline optics

in order to reduce the noise pickup on the analog part of the feedback system. The
internal

BEST

computation and communication between the three system blocks

are  all  performed  in  a  fully  digital  way,  therefore  excluding  all  additional  noise
sources  that  can  strongly  affect  the  controller  and  stabilization  loop  performances  at
high  speed.  All  three  building  blocks  are  interconnected  via  low-delay  fast
communication SFP links running a proprietary protocol.  The SFP links on the back
of  the

BEST

Central Unit are directly interfaced to a powerful FPGA board that

performs the position and control algorithms and sends correction values to the DACs
embedded in the

PreDAC

device.

The

BEST

system was designed with one of its main focuses on configurability

expandability and flexibility, being able to control and monitor up to two readouts -

TetrAMM

devices (i.e. up to 8 picoammeter channels) and one multichannel

actuator -