LANSCE-R Wire-Scanner Analog Frontend Electronics (AFE)

LANSCE-R Wire-Scanner Analog Frontend Electronics (AFE)
MOP232
Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA
LANSCE-R WIRE-SCANNER
ANALOG FRONTEND ELECTRONICS (AFE)*
Michael Gruchalla#, URS (EG&G Division), Albuquerque, NM 87107, U.S.A.
Phillip Chacon, J. Douglas Gilpatrick, Derwin Martinez, James Daniel Sedillo,
Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
c 2011 by PAC’11 OC/IEEE — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○
Abstract
A new AFE is being developed for the new LANSCE-R
wire-scanner systems. The new AFE is implemented in a
National Instruments cRIO module installed a BiRa 4U
BiRIO cRIO chassis specifically designed to
accommodate the cRIO crate and all the wire-scanner
interface, control and motor-drive electronics. A single
AFE module provides interface to both X and Y wire
sensors using true DC coupled transimpedance amplifiers
providing collection of the wire charge signals, real-time
wire integrity verification using the normal dataacquisition system, and wire bias of 0V to +/-50V. The
AFE system is designed to accommodate comparatively
long macropulses (>1ms) with high PRF (>120Hz)
without the need to provide timing signals. The basic
AFE bandwidth is flat from true DC to 50kHz with a true
first-order pole at 50kHz. Numeric integration in the
cRIO system provides real-time pulse-to-pulse numeric
integration of the AFE signal to compute the total charge
collected in each macropulse. This method of charge
collection eliminates the need to provide synchronization
signals to the wire-scanner AFE while providing the
capability to accurately record the charge from long
macropulses at high PRF.
INTRODUCTION
One of the systems being replaced in the LANSCE
upgrade are the wire-scanner systems.
Both the
mechanical actuators and the data-acquisition and control
electronics are being replaced.
The National Instruments (NI) CompactRIO (cRIO)
system [1] has been selected for the wire-scanner systems
as well as a number of other LANSCE systems. The
basic cRIO crate is shown in Figure 1.
The majority of the wire-scanner system is comprised
of commercial off-the-shelf (COTS) cRIO modules. The
wire-scanner analog frontend electronics (AFE) cRIO
module however is a custom LANL design developed to
meet the specific needs of the LANSCE wire scanners.
WIRE-SCANNER CHASSIS ASSEMBLY
The LANSCE electronics is to be packaged in a
standard 4U 19-inch rack chassis. The BiRa BiRIO cRIO
chassis system [2] has been selected for LANSCE-R to
house the wire-scanner electronics. This chassis system is
shown in Figure 2.
Figure 2: Wire Scanner System BiRIO Chassis.
The BiRIO chassis allows for convenient mounting of
the cRIO crate in the foreword volume, and provides
substantial wiring area in the rear volume providing both
convenience and well-managed lead dress.
This same chassis system is also being used in several
other LANSCE-R applications.
AFE DESIGN GOALS
The operational parameters of the LANSCE accelerator
are bounded with comparatively narrow operational limits
since LANSCE is a production accelerator rather than a
purely experimental system. This simplifies the AFE
requirements somewhat.
AFE Dynamic Range
Figure 1: NI cRIO Crate.
___________________________________________
*Work supported by US Department of Energy, LA-UR-11-10200
#
gruch@lanl.gov
542
The dynamic range of the wire signals for the
LANSCE-R application is comparatively small. The total
collected charge is a function of the location of the wire
scanner in the accelerator.
The maximum charge
collected by the sense wire varies over nominally a factor
of 100 along the accelerator for a given beam
configuration. The dynamic range needed in any specific
profile measurement is nominally 100:1. Therefore a
Instrumentation and Controls
Tech 03: Beam Diagnostics and Instrumentation
Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA
AFE Bandwidth
The sense wire remains at each scan position for only a
single macropulse, and is moved to the next position
between macropulses to minimize the time required to
collect a profile. This operational profile requires the
AFE to have sufficient bandwidth and recovery speed to
collect the wire signal on a pulse-by-pulse basis.
The charge collected during each macropulse is to be
integrated to provide the total collected charge in each
macropulse at each wire position through the scan. Also,
a design goal is to utilize an RC integrator that recovers to
baseline between macropulses. This topology eliminates
the need to provide integrator reset commands to the
AFE.
Typically the integration function would be an analog
function integral in the AFE. Classically this is provided
by setting a low AFE bandwidth, e.g. a 1Hz to 5Hz
response pole, with a well-defined first-order response to
nominally 10kHz.
However, the maximum pulse-repetition frequency
specified is 120Hz, 8.33ms period. And the longest
macropulse that is to be accommodated is nominally
700µs. A simple RC integrator cannot provide accurate
integration of a 700µs pulse and recover sufficiently in
8.33ms.
The collection of wire charge in the LANSCE-R AFE is
implemented in a hybrid analog and digital dataacquisition structure. The AFE bandwidth is set at
nominally 50kHz with a well-behaved first-order
response to nominally 1MHz. The AFE is therefore an
integrator, but with a 50kHz pole.
The AFE signal is digitized at 500k samples per
second, and the full 50kHz data record recorded. This
wide-bandwidth data is a temporal representation of the
actual beam current during the macropulse captured with
a 50kHz bandwidth. The 50kHz data record is then
numerically integrated in the cRIO system with a
response pole at nominally 5Hz and a zero at 50kHz.
This provides an integral response from 5Hz to nominally
the sample rate of the digitizer. This approach of widebandwidth analog data collection and numeric integration
provides a very accurate integral of the charge in each
macropulse without the need to provide integrator reset
signals, and additionally provides temporal beam-current
characteristics within each macropulse.
Wire Bias
The charge collection is optimized by applying a bias to
the sense wires. The AFE provides the means to apply
bias potentials up to ±50V to each sense wire.
Wire Integrity
A requirement of the AFE design is to provide in the
data acquisition system the means to validate the integrity
of each sense wire. Also, since a false indication of a
failed wire that could be caused by failure of the integrity
system itself could result in needless, and costly, machine
down time, the wire integrity monitoring system must
also provide a verification of its own integrity.
The AFE effects the wire integrity verification by
applying a known potential to one end of the wire and
recording the signal at the other end of the wire. A
portion of the interrogation signal is also applied directly
to the AFE input to provide an output signal even when
the wire it totally open. A competent wire is witnessed by
a specific signal level during the integrity test. A failed
wire is witnessed by a reduced but well-defined output
signal level. A failure of the wire-integrity system is
witnessed by no signal during integrity verification.
Grounding
Careful attention must be given to the grounding and
shielding topology due to the low signal levels that must
be collected at a substantial distance from the sensors in
an industrial environment having numerous equipments
generating nuisance electromagnetic fields and coupling
nuisance currents on the facility ground structure.
The grounding design developed for the LANSCE-R
wire-scanner systems has been quite successful. The
complete details of the grounding topology utilized are
quite detailed due to the number of noise sources which
must be considered. Reporting the complete details of the
grounding and shielding design is well beyond the space
allotted to this paper.
However, the most troublesome noise source
encountered was the motor driver driving the actuator
stepper motor. The full motor drive must remain active
during the entire scan due to the scan speed required.
Therefore the motor drive current cannot be disabled
during the collection of the wire signal. A combination of
shielding,
guarding
and
grounding-configuration
management successfully eliminated all motor-drive
artefacts.
The basic simplified AFE circuit diagram is shown in
Figure 3. The wire signal is collected from each end of
the wire and applied to the input of two transimpedance
amplifier stages through coaxial cables. Although this
amplifier configuration appears to be a differential
amplifier, it is simply two similar stages with summed
outputs. These amplifier stages are referenced to the bias
potential. The shields of the wire-signal cables are
returned to the bias supply rather than ground to provide a
true guard configuration for each signal.
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c 2011 by PAC’11 OC/IEEE — cc Creative Commons Attribution 3.0 (CC BY 3.0)
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total dynamic range of nominally 10,000:1 is required.
This dynamic range may be provided in the AFE without
the need for gain switching.
An NI cRIO-9222 digitizer module is used to digitize
the analog output from the two AFE wire-signal channels.
The 9222 provides full 16-bit precision plus an additional
sign bit. Since the basic resolution of this digitizer is
65,000:1, no scale switching is required to meet the
needed dynamic range. A third channel of the 9222 is
used to digitize the wire bias potential as a verification of
the wire-bias potential.
MOP232
MOP232
Proceedings of 2011 Particle Accelerator Conference, New York, NY, USA
An overall shield isolated from the individual signal
coax cables is provided over the coaxial cables to provide
an overall ground shield to the signal cable.
The overall shield is connected between the equipment
grounds of the beam line and the wire-scanner
electronics. It is reasonably expected that there will be a
difference in potential between these two grounds with
frequency components within the bandwidth of interest in
the sense-wire data. If the shields of the wire-signal coax
were simply connected between the two equipment
grounds, any ground noise would be coupled into the
signal path due to nuisance currents introduce into the
shield and the less than ideal shielding effectiveness of
the coax.
The overall shield carries all of the nuisance currents
due to inter-ground potentials, and provides shielding
from electromagnetic fields.
The input to a transimpedance amplifier is a low
impedance, so the signal potential at the input terminal is
effectively zero with respect to the amplifier signal
reference over the operational bandwidth of the amplifier.
The signal reference for this AFE design is the bias
potential. Therefore, connecting the signal coax shields to
the bias potential forces the shield potential to be
identically the same as the potential at the
transimpedance-amplifier input. Noise coupled to the
signal coax shield is therefore communicated to the bias
source and prevented from coupling into the wire-signal
path.
with the number of data point equal to the total number of
macropulses during the scan.
Initial Experimental Results
The design of the new LANSCE-R wire-scanner AFE
meets all the of design specifications. The specified
sensitivity, dynamic range and bandwidth are achieved,
and nuisance noise is successfully rejected.
The first-article wire-scanner AFE installed in the
complete prototype cRIO wire-scanner electronics system
was integrated with the first prototype of the new actuator
system and installed on the LANSCE beam line in late
December 2010. All elements of the new LANSCE-R
wire-scanner system functioned as expected and excellent
data were collected. A typical profile collected during
this initial experimental period is shown in Figure 4. The
data of Figure 4 were collected on a pulse by pulse basis
LEBLG PROJECTED DISTRIBUTION
Bias On, Integrator On, 5-kHz Bandwidth
1.E-01
Projected Hor.
Projected Vert.
Proj. Hor. - Log.
Proj. Vert. - Log.
0.149
1.E-02
0.099
1.E-03
0.049
1.E-04
-0.001
Relative Charge - Log Scale
0.199
Relative Charge - Linear Scale
c 2011 by PAC’11 OC/IEEE — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○
Figure 3: Wire Scanner AFE Analog Configuration.
1.E-05
-40
-30
-20
-10
0
10
20
Wire Location (mm)
Figure 4: X and Y LBEG Beam Distributions.
These initial results confirm that the new LANSCE-R
wire-scanner system and specifically the AFE meet all of
the design goals. Specifically, the specified dynamic
range is provided with the specified response
characteristics.
CONCLUSIONS
REFERENCES
[1] National Instruments, Inc., Austin, TX 78759-3504,
www.ni.com/compactrio.
[2] BiRa Systems, Inc., Albuquerque, New Mexico,
USA, www.bira.com.
Instrumentation and Controls
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Tech 03: Beam Diagnostics and Instrumentation
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