Nipun Mathur
National Instruments Corp.
Austin, Tex.
The European Organization
for Nuclear Research, more
commonly known as CERN,
hopes the LHC will soon reveal
the secrets of the universe. The
machine will recreate the conditions
right after the Big Bang on
a miniature scale by accelerating
and colliding two beams of microscopic
particles traveling at
99.9999991% of light speed.
Particles traveling at close to the
speed of light can have high levels
of energy. The LHC creates beams
with trillions of particles with energy
equivalent to that of a 400-ton
train traveling at 150 km/hr. Any
stray particles deviating from the
beam core can severely damage the
machine. So collimators eliminate
these particles to keep that from
happening.
There will eventually be over
150 collimators in the LHC. But
controlling them presents a challenge.
The problem isn’t how fast the collimators move or react —
collimator parts only move a few
millimeters at about 1 kHz at most.
The real difficulty is synchronizing
collimators that can be 13 km
away from each other. In all, there
are 500 axes of motion involved
over this large distance. The axes
are made up of motors that position
the collimators’ jaws. These
jaws are synchronized to within
100 μsec and move with accuracies
of ±10 μm.
The LHC collimators mainly
comprise several layers of jaws
that absorb stray particles. Each
jaw consists of a pair of blocks 1-m
long, 80-mm wide, and 25-mm
deep. The blocks can be CFC,
graphite, copper, or tungsten, depending
on their proximity to the
beam halo. Primary and secondary
blocks see high thermal loads and stresses and are made of lowdensity
material. Tertiary blocks
are metallic and stop low-energy
particles. But regardless of block
material, jaw positioning must be
accurate to one-tenth the beam
core diameter, so the required accuracy
is 20 μm.
Also, the jaws must be positioned
at a precise angle with respect
to the particle beam trajectory.
Two stepper motors, one at
either end of each jaw face, handle
the positioning. Any vibration during
motion can damage the blocks,
some of which are graphite brazed
on copper and quite fragile. So
both motors on each block are synchronized
to within 1 msec of each
other. And jaws must be almost
perfectly parallel to each other; the tolerable orientation angle is below
2 milliradians.
CERN engineers designed the
control system using National Instruments
Corp. LabView software.
It runs on a PXI programmable
automation controller platform,
also from NI. In all, there are
120 PXIs. Each controls between
one and three collimators. PXI
platforms can be synchronized using
external or internal clocks. But
for the LHC, a central clock coordinates
motion control. Specifically,
a central computer sends synchronization
signals over dedicated
optical fibers to a motion-control
supervisor. This signal then goes
to one of the PXI platforms which
houses a high-precision timing source. Signals from this source, in
turn, go to external timing inputs
on other PXI platforms.
There is a high level of radiation
near the collimators. This means
electronics can’t be placed anywhere
near them. So the PXI controllers
sit a few hundred meters motionfrom
the collimators, protected in
an underground area.
For reliability, CERN divided
the collimator controls between
two sets of PXI platforms. The first
set controls the stepper motors and
reads resolver feedback from motor
shafts at a 400-Hz rate. In the standard configuration, one PXI
platform controls up to 15 stepper
motors mounted on three different
collimators.
The second set of PXI platforms
reads LVDT sensors, backups for
the resolvers. These get read at a
rate of 1 kHz to check real-time
positioning of the collimator jaws.
If the PXI platform controlling the
motors fails, CERN can still determine
where the collimators’ jaws
are using LVDTs. And if the PXI
platform reading the LVDTs fails,
CERN can bring the collimators to
a known safe position.
Both kinds of PXI chassis run
LabView Real-Time on the controller.
And CERN used the NI
SoftMotion development module
and reconfigurable modules to create
a custom motion controller.
CERN computers interface with
collimator actuators and sensors
through reconfigurable I/O modules.
LabView FPGA runs on these
devices. All inputs/outputs are
completely reprogrammable with
LabView. The I/O card controlling
each collimator carries a 3M-gate
FPGA, providing eight analog inputs
and outputs, and 96 digital
input/outputs. The digital outputs
are through NI C Series modules
which provide isolation — necessary
because the control electronics
are some distance from the motors.
The I/O cards send step-anddirection
pulses to the stepmotors
and read limit switches, interlocks,
and triggers. Resolver signals are
read through cards made in-house
by CERN engineers.
Collimator Control
In normal operation, collimator
jaws go through a 20-min motion
profile. The LHC collimator motion-
control process starts when
the central controller issues a command.
The motion-control system
checks for these commands every
millisecond, acknowledges their
receipt, and signals that actions
have taken place.
Collimator jaw settings are defined
in normalized terms around
the beam center. So the motion-control
computers must convert
these into real jaw positions for
each location. The computers also
interpolate movement profiles
from the central control so they are
smooth and synchronize all axes of
the same collimator. The computer
generates trajectory setpoints that
take into consideration the speed,
acceleration, and jerk specified for
each motion profile. These setpoints
then go to the FPGA controller.
In addition, the motion
controller generates interlocks if it
detects problems.
A control loop runs on the
FPGA at 1 MHz. It generates step
and direction signals controlling
the stepmotors once it gets data
from the trajectory generator. The
I/O task for both digital and analog
signals runs on the FPGA every
5 msec. The resolver signals are
decoded in the FPGA and read at
400 Hz. The real-time processor
also reads the status of the FPGA
every millisecond to check for data
failures.
Meanwhile, as the motion controller
executes commands, the
collimator-position sensors double-
check that collimator jaws are
positioned properly. These sensors
use the LVDTs to gauge absolute
position of each jaw. Two LVDTs
read each jaw position, one for each
stepmotor. Two additional LVDTs
measure the distance between jaws
of the same collimator. PXI platforms
compare LVDT readings
with error thresholds from LHC
central control. These thresholds
may be different at different points
in the motion profile.
The position reading takes place
at 100 Hz. Readings from the seven
LVDTs on each collimator go to two
data-acquisition cards with eight
16-bit, 250 ksamples/sec simultaneous
differential analog inputs. The
acquired data then transfer via DMA
to the controller. A reconfigurable
I/O card with eight analog outputs
generates the stimulus for the
LVDTs.
As with the motion-control system,
the PXI position computers
receive commands from the LHC central controller every millisecond.
They translate these commands
into error thresholds for
each of the LVDTs. The resulting
positional accuracies are measured
in micrometers.
The LHC is in its final shakedown
stage and should be central controller every millisecond.
They translate these commands
into error thresholds for
each of the LVDTs. The resulting
positional accuracies are measured
in micrometers.
The LHC is in its final shakedown
stage and should be operating this summer.
Make Contact
National Instruments Corp.
NI.com
Video of CERN LHC collimators
tinyurl.com/4nwfrb