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….

Rotating Coils - Giordana Severino – 14.04.2014

Scientific Planning for 2014