for Particle Accelerators

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24 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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Digital
Signal

Processing and Generation
for a DC
Current

Transformer

for
Particle

Accelerators

Silvia
Zorzetti

Contents


Introduction


Fermilab


Direct
-
current current transformer principles


Direct Current Current Transformer (DCCT)


Simulink Model


Specifications and Parameters


Hardware


Digital implementation


Open loop test


Closed loop test

Introduction


This activity was supported and accomplished at
Fermilab, in the Instrumentation Department of the
Accelerator Division


Main Injector (MI)


Rapid cycling synchrotron


150
GeV

as Injector for the
Tevatron



High intensity protons for
fixed target and neutrino
physics


Recycler


Permanent Magnetics


8
GeV


Antiproton cooling before
the injection into the
Tevatron



Proton storage


Tevatron


Superconducting
synchrotron


980
GeV


Circular Accelerators
at
Fermilab

Different

types

of
DCCTs

at FNAL


An analog, homebrew version was developed

at FNAL in the 80’s.


Installed in all the machines, except for the Recycler


Bandwidth: 2 MHz


A commercial DCCT, designed by K. Unser (
Bergoz
)


Entire system, i.e. pickup, electronics, cables, etc.


Only DC signal detection (narrow band).


In 2004 the system failed due to an asymmetry of
permeability between the
toroids
.


Temporary replaced with another commercial DCCT from
Bergoz
, will finally be replaced by the “digital” DCCT that is
now under development.


DCCT
Introduction


The DCCT is a diagnostics instrument, used to observe
the beam current.


Detection of DC and low frequency components of the beam
current


Non
-
Distructive

instrument


For the detection of high frequency components the classical
AC transformer is used.

Principle

of
Operation

-

AC Transformer


The classical AC transformer can be used to identify the
high frequency components of the beam current

Principle

of
Operation

of the DCCT



Single
Toroid


The modulator winding drives the toroid into saturation.


The total magnetic flux is shifted proportionally to the
DC current


The measured DC current is proportional to the
amplitude of the 2
nd

harmonic detected by the detector
winding

Principle

of
Operation

of the DCCT



Double
Toroids

Principle

of

Operation

of

the DCCT



Double

Toroids

Complete
System


Beam


DCCT


Modulator


400Hz digitally
supplied


Second

Harmonic

detector


AM
demodulator
on FPGA


AC Transformer


Sum and Feedback


Output

Second

Harmonic

Detector


Input
:

The input signal can be
viewed

as a low frequency
signal

modulated

(in
amplitude
) with
800Hz

Second

Harmonic

Detector


CIC1
:

Perform

the first
decimation

of the
signal

sampling

frequency


From
62.5MHz

to
500kHz

Second

Harmonic

Detector


NCO
:


Supplies

in
-
phase

and quadrature
-
phase

signals

of same amplitude and
frequency (800Hz), for
downconversion

to
baseband

Second

Harmonic

Detector


CIC2
:

Performs

a
second

decimation

of the
sampling

frequency
,

allows

a more efficient FIR filter


From
500kHz

to
2kHz

Second

Harmonic

Detector


FIR
:

Defines

the
overall

system

bandwidth

at

baseband


DC to 100Hz

Second

Harmonic

Detector


Some mathematics to format the
signal
, and
adjust

gain and
phase


There
is

no
phase

detector
required
, because the
signal

is

sufficiently

slow,
thus

a signum detector
is

implemented
.

DCCT Model


Analytic

study of the DCCT functionality


Simulink

Model of the complete system (AC+DC)


Toroids behaviour simulation


Filter Design


Feedback

Simulink

Model

Simulink Model


Flux at Ib=0 (a.u.)

Simulink Model


Output Voltage at Ib=0

Simulink Model


Flux

at

Ib
=1 (
a.u
.)

Simulink Model


Voltage Output
at

Ib
=1

Simulink Model



AC + DC Closed Loop

Required

Specifications

and
Parameters


Number of turns per winding


Current and Voltage to saturate the toroids


DCCT Bandwidth


AC Bandwidth

Parameter

Space


Toroids Saturation


I
sat
<3A , V
sat
=36V,


N
m
=22


AC and DC Sensor windings


B
DC
=100Hz


B
AC
=1MHz


N
s_DC
=100


N
s_AC
=200



Test Setup for
Toroid

Measurements

Output Voltage from the pick
-
up
windings of the toroids


There

is

a
mismatch

between

the
voltage

outputs

from the
two

toroids
.


Poor

matching

of the core
material

Complete System

VHDL Implementation


CIC

0


k
M
f
k
f
s
k


M:

Differential Delay


ρ
:
Decimation

factor


N: Filter Order


A: Gain


Notch

at
:

N
M
A
)
(


CIC
Filter



VHDL
Model


The
firmware
is

synchronized

with a single
clock


Integration
Section


Comb

Section


Gain


Number of bits:



)
(
log
)
(
log
2
2





M
N
B
B
in
out
Filters


Test Setup

VHDL Implementation and Test


CIC1


f
s
=62.5MHz,


f
d
=500kHz,


M=1


ρ
=125


N=2


f
1
=500kHz


A
=

15625

VHDL Implementation and Test


CIC2


f
s
=500kHz,


f
d
=2kHz,


M=2


ρ
=250


N=2


f
1
=1kHz


A
=

250000

VHDL Implementation and Test


FIR


b
i
: filter coefficients


N: filter order (127)

FIR Filter
-

VHDL Model


The firmware
is

synchronized

with a single clock


Counter


ROM


Serial Function


Number of bits

VHDL Implementation and Test
-

FIR


f
s
=2kHz,


f
c
=100Hz,


N=127

VHDL Impelementation and Test



AM Demodulator


With a
waveform

generator
a low
frequency

signal
,
modulated

at 800Hz
is

generated

and
digitized

by
the ADC


The
resulting

output
signal

is

observed

on an
oscilloscope
,
connected

to
the DAC.

VHDL Implementation and Test
-

Demodulator


Input:


Output:

t)
f
m(t)cos(2
0

)
m(t
Open Loop Test Measurement Setup

DC Dectector
-

Output signal

Before the Transition Board
-

Ib=0.4A


The signal is supplied
by the DCCT DC
Sense


Before the
transition board


There are both
odd and even
harmonics

DC Detector
-

Output Signal

After the Transition Board
-

Ib=0.4A


The signal is supplied by
the DCCT DC Sense


Passed by the
Transition Board


Has only the 2
nd

harmonic (800 Hz),
the 1
st

harmonic is
suppressed.

Open
Loop

Result

Closed Loop Test
Measurement

Setup

Closed Loop Results

Conclusions


At
this

stage a
preliminary

implementation

and test of the
DCCT has
been

successfully

realized
.


P control


τ
=0.05s


Resolution 0.01A



Next

steps


Implementation

of the AC
section


Faster

loop

control


Thank

you

for
your

attention

Silvia Zorzetti

Backup
Slides

Silvia Zorzetti