rotating coils - giordana severino – 14.04.2014 rotating coils pacman meeting 14.04.14 printed...
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Rotating Coils - Giordana Severino – 14.04.2014
Rotating Coils
PACMAN meeting 14.04.14Printed Circuit Coils – Future
developments
Rotating Coils - Giordana Severino – 14.04.2014
Rotating coils
Rotating coils measurement systems are developed to characterize the magnetic field quality of magnets in accelerators facilities.
The field harmonics are computed knowing the rotating coil’s precise geometry.
The process of bucking allows improving the measurement accuracy.
Rotating Coils - Giordana Severino – 14.04.2014
Rotating coilsUseful background information
The system is based on Faraday law : U = -A magnetic flux measurement by means of the rotating coils thus
requires the integration of the voltage induced on the coil
Magnetic field quality in the aperture of accelerator magnets is commonly described by a set of Fourier coefficients known as field harmonics or multipole coefficients.
The field harmonics (multipoles) are derived using knowledge of the coil geometry.
Using an angular encoder
w
Rotating Coils - Giordana Severino – 14.04.2014
Rotating coilsUseful background information
Radial coil: Harder to build for small dimension, better the tangential coil
A rotating coil (loop of wires) measures the variation of the intercepted magnetic flux
multipole coefficients Bn(ref) and An(ref) are called normal and skew multipole coefficients
Rotating Coils - Giordana Severino – 14.04.2014
Vertical dipole Horizontal dipole Quadrupole
Tangential coil
Rotating Coils - Giordana Severino – 14.04.2014
Tangential coil: Blind eye
Rc = coil central radius
D = Opening angle
Rotating Coils - Giordana Severino – 14.04.2014
Rotating coilsMotion related issues
Vibration motion causes spurious harmonics
in the coil signal Torsion motion
Minimize (errors) “Bucking method” Effect
Coil system whose sensitivity
to the (n -1)- th harmonic is zero
Coil system whose sensitivity
to the n- th harmonic is zero
Vibration Torsion
Rotating Coils - Giordana Severino – 14.04.2014
Rotating coilsBucking
Analog bucking:
Digital bucking:
The Bucking Ratio is the ratio of the harmonic n in the absolute and compensated measurements
It is the «quality check» for bucking
For a quadrupole it should be tipically 100
Rotating Coils - Giordana Severino – 14.04.2014
Rotating coilsExample: How to calculate the gain factor
Example on a tangential coil setup: 3 FFT signals from the 3 coils Considering the sum of the dipole and quadrupole harmonics is zero:
Gain will be applied also to calculate the sensitivity factor Kn of the compensated coil
Kn_comp = Kn_coil1 + Kn_coil2* g1 + Kn_coil3 *g2
Rotating Coils - Giordana Severino – 14.04.2014
Rotating coilsSensitivity Factor
Calibrated parameters (coil central radius, axial tilt, magnetic surface)
a, R2, and R1
Construction drawings
General:
Tangential:
Radial:
Rotating Coils - Giordana Severino – 14.04.2014
Rotating coilsFabrication problems
Handmade systems: Time of fabrication is long, wiring is a complicated process.
Connectors
Manual coil winding facility
Rotating Coils - Giordana Severino – 14.04.2014
Printed circuit rotating coils
The PCB coil is based on the same principle as classic rotating coil.
PCB coils are made up by multiples layers boards on which are printed the coil loops traces on both sides.
Rotating Coils - Giordana Severino – 14.04.2014
Ideal PCB coil
A perfect PCB coil:
accuracy implies high stiffness and straightness of the coils
excellent knowledge of traces (windings) localization.
Coils fabrication should also be easy and low-cost, so that coil parameters (type, length, number of turns, radius, etc.) can be customized to the magnet requiring test.
Rotating Coils - Giordana Severino – 14.04.2014
Printed circuit rotating coils
The trace continues in loops of increasing width on the reverse side (dashed line) following the original traces and maintaining the same chirality to complete a winding:
Rotating Coils - Giordana Severino – 14.04.2014
Printed circuit rotating coilsDipole and Quadrupole bucking
Quadrupole and Dipole Bucking DQB(1+3) Also only Dipole bucking DB (1+2+3+4)
Rotating Coils - Giordana Severino – 14.04.2014
Printed circuit rotating coilsSensitivity factor
One method to compute it is:
The (-1) j gives the sign of the current flow of each wire and the (xj,yj) are the locations of the wires with respect to the rotation axis.
Multiple layers: The position of traces in not well known…
Hard to calculate with high precision
L length of a given wire
R reference radius
Rotating Coils - Giordana Severino – 14.04.2014
Printed circuit rotating coilsOpening angle
Tangential coils: Determinate the distances between traces according to the opening angle:
Rotating Coils - Giordana Severino – 14.04.2014
Printed circuit rotating coilsSensitivity factor
Comparison between tangential coil and pcb tangential coil (one layer) at probe radius of 16mm and 32mm
Rotating Coils - Giordana Severino – 14.04.2014
Printed circuit rotating coilsTo sum up: Advantages…
The number of turns can be increased by adding identical layers (PCB stack)
Precise trace positioning (<10 μm) Sensitivity comparable to traditional designs (with 32 or more
layers) Manufacture large series quickly, reproducibly, cheap Makes practical high-order compensation schemes
(B1=B2=B3=B4=0)
Rotating Coils - Giordana Severino – 14.04.2014
Coil Resistance is High [kW]: consequent instrumentation required
Cost of production when producing few coils
Human error on multiple layers assembly (stack of double layer)
Stiffness of the structure too low, flexible during rotation much harder to calibrate
Printed circuit rotating coils… and inconvenients
Rotating Coils - Giordana Severino – 14.04.2014
Printed circuit rotating coils… and inconvenients
Fabrication errors:
Rotating Coils - Giordana Severino – 14.04.2014
Some PCB coils-related R&D themes
General improvement of multi-layer PCB coils: track density (currently only ~1/3 of conventional coils)
Optimization of track layout to minimize sensitivity to production errors
Rotating Coils - Giordana Severino – 14.04.2014
Some PCB coils-related R&D themes
Improvement of the existing 8 mm rotating PCB coil shaft: mechanical stiffness of the assembly (materials, geometry, resins …) alignment and stability of ball bearings, scaling above and below 8 mm
Development of a more compact Mini Rotating Unit MRU-II
Rotating Coils - Giordana Severino – 14.04.2014
Some PCB coils-related R&D themes
Micro PCB connectors for multi-strand wire coils : Standard for all number of Brins Easy to use Avoid human errors
PCB fluxmeters : upper limits of current printing pressing and assembly techniques new possibilities offered by ELTOS
Rotating Coils - Giordana Severino – 14.04.2014
Some PCB coils-related R&D themes
Polyvalent PCB sandwich coil shaft advanced materials (foams, honeycombs etc … for higher stiffness-to-weight ratio)
Software tool to facilitate the design of new PCB coils, bypassing traditional CAD:
from geometry specifications layer design Gerber format file
Other techniques alternative to PCB: circuits printed on flexible rolls, inkjet circuit printers
Other tasks….