1. Photolithography Fabrication of a Clamshell Surface Current Coil for the nEDM Experient
Hunter Blanton, Ali Frotanpour, Andrew Mullins, Christopher Crawford
Department of Physics and Astronomy, University of Kentucky
Motivation
Robotic Control of Laser Etching
The electric dipole moment (EDM) changes its sign as time is reversed. This is directly related to CP-violation. The CP-violation present in the standard model is insufficient to explain the abundance of matter and the lack of antimatter in the universe.
The SNS nEDM collaboration aims to measure the electric dipole moment of the neutron to a precision of e cm by measuring the Larmor precession of the neutron in a magnetic field modulated by a reversing electric field.
An essential requirement for this measurement is precision magnetic fields to guide the spins of polarized neutrons and 3He atoms into the precession cell. These coils must have uniform fields, zero fringes, and may not contain any magnetic materials.
Abstract: We are developing a prototype spin transport magnet for the SNS nEDM experiment. The coil is designed by solving a numerical boundary value problem of the magnetic scalar potential and fabricated by CNC photolithography using a 6-axis industrial robot.
Two lasers are used in our etching process:
a laser displacement sensor (LDS) scans the surface to be etched.
a UV laser sensitizes the photoresist.
We calibrate the orientation of both lasers with respect to the robot.
LDS: we measure the distance to a reflective sphere of known radius from multiple orientations to completely determine the rotation and translation transformation from the robot to the tool frame.
UV Laser: Both lasers were tracked separately using a standard webcam focused on a flat, semitransparent sheet. The XY offsets between the LDS and UV laser on this sheet were calculated to less than half of a pixel width. For a 1080p image with the camera 5 cm away from the filter, this gives a resolution of less than 100 microns.
STEP 1: Substrate
The 3-d coil surface substrate is fabricated using stereolithography (SLA) using a high-temperature ABS-like resin. This method is quick, less prone to human error, with feature detail of 0.1 mm. We ordered two printed pairs from stratasys.com and quickparts.com Both were within specified tolerance. 1 1
Magnetic Scalar Potential Method
Surface Current
Magnetic flow sheets
(scalar equipotential)
Magnetic flux lines
STEP 2: Copper Plating
We developed a method to design and construct precision surface current coils at the University of Kentucky based on a novel physical interpretation of the magnetic scalar potential. It is a source potential in the sense that the corresponding magnetic field is generated the source currents running along its equipotentials. If one wire is wrapped around around each equipotential contour of the magnetic scalar potential along the boundary of the coil, it will generate the gradient field inside the volume of the coil.
This interpretation results from the boundary conditions of Ampere's law
We use this principle to design coils as follows:
We solve the scalar potential U using Neumann exterior boundary conditions, specifying the magnetic flux.
We create a 3-d printed circuit with traces following the equipotential contours of U along the boundary.
It is necessary to construct such a coil around the boundary of each region with a non-zero field.
0.125 mm of copper using electroless electroplating at epner.com
Laser tool
Scanning
The RX130 can be moved to an
arbitrary position with repeatability
of mm. Combining the robot
end-effector transformation with the
LDS read out, we can generate a point
cloud of any arbitrary 3d object within the
robot workspace. The surface is naturally parameterized in cylindrical coordinates.
Etching
Once a transformation matrix has been found, it is
simple to generate workspace vertices and normals for
an arbitrary path on the 3d mesh representation of the
workspace object. With accurate normals, ZYZ euler rotations
are generated to align the UV laser not only to pass through a
specific 3d point, but with any rotation within the robot limits.
STEP 3: Etching
Photolithography is widely used for device fabrication such as integrated circuits and thin film patterning. Ultra-violet (UV) laser light is used to sensitize a photoresist mask for etching. One of the benefits of this method is its non-destructive patterning: before etching, we can verify our traces. Another benefit is the precession and fine detail (100 micron).
Positive type photoresist is coated on the electroplate surface using a spray coating.
A UV laser attached to the robot traces the desired lines, sensitizing the photoresist. It is removed with a developer solution.
Exposed copper is etched with ferric chloride .
Coil design of winding pattern
This project is to demonstrate practical CNC construction techniques for surface current coils. The prototype is a tapered double cos-theta coil with a transverse field which decreases axially down the cylinder. The coil is constructed of two half-cylinder clam shells which clamp together around the 3He transport tube.
The equipotential contours were obtained by numerically a Laplace boundary value problem in the finite element software package COMSOL. The boundary conditions were: a) dU/dn=0 on the outer surface; b) dU/dn=G z cos(θ) on the inner surface. Contours were numerically extracted from COMSOL in Matlab and passed along to the robot control software for etching of the circuit traces.
Future Work
3D printing offers the potential of a simpler fabrication process. This image shows a solid model with raised patterns marking the equipotential contours. Once printed and coated, we will sand away the raised contours, leaving the designed circuit traces. This not only simplifies the process dramatically, but also allows for arbitrarily complex objects. This is a significant generalization of our current method.