Introduction
BEST
User’s Manual
10
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 -
PreDAC
device (i.e. up to 4 DAC channels) from one single
BEST
central unit.
