Monday, 1 September 2014

Simple NMR Part 3

Coil Winding

One of the big improvements in the new design was increasing the sample volume. This means that large coils are needed. I decided to use 20 AWG wire for the pre-polarization coil, and 30 AWG wire for the T/R coil.

The left spool is about 3000 ft of 30 AWG, and the right spool is about 300 ft of 20 AWG. This wire was wound on to ABS plastic piping like this.

The pre-polarization coil was wound onto a 4'' ABS pipe, and the T/R coil was wound onto a 3'' ABS pipe. To keep the coil from spreading out along the length of the pipe, I used a threaded female fitting like this on the end of each pipe.

They are kept on the ends of the pipe by some ABS cement. 

I am a patient guy, but thousands of feet of wire needs to be wound around this pipe. The solution was to construct a makeshift coil winder. I took threaded ABS caps.

And I drilled a hole down the middle such that it could be placed on threaded rod like this.

Since the cap is threaded, I was easily able to thread the coil form assembly onto the threaded rod as well. The result looks something like this.

I added a washer and bolt on each end. This immobilized the coil form on the threaded rod. Next I needed something on which this form could spin. I used a power drill on one end.

And on the other end I used a piece of copper pipe attached to a piece of wood with some plastic (Basically use whatever you have laying around).

To automate the turning of the coil, I hooked up the power drill  to the 5V line on my DIY power supply breakout board.

I then tied some rope around the power drill trigger so that if current is delivered to the drill, it will always turn. But this is not what we want, I wanted an easy way to control the coil winder. The solution was to create this foot pedal out of some scrap plastic and a pushbutton switch.

So we have a way of turning the coil, but where does the spool go? I mounted a hand drill sideways using a vice. I placed a rod in the hand drill. I wrapped a bunch of layers of tape around the rod, and the feeder was born!

The next problem to be solved was, how do I keep tension in the wire that is being fed? The solution was to use a friction based tension system. And by that I mean two pairs of garden gloves and a few hand tools:) The finger on each glove is wrapped around the wire, and then the vice grips are clamped on top to provide pressure on the wire. The result is that it is difficult to move the wire through the glove.

It took a few tries to master the art of coil winding, but the last coil I was able to wind in a matter of minutes!

New Coil Design

Humbucking Configuration

One of the biggest problems in a system like this is noise. To try and cut down on noise, I will actually have two identical pairs of coils arranged in a humbucking configuration.

The idea is that you have two identical coils, one wound clockwise, and the other wound counter clockwise. When you connect them together, any external EMI that is induced in Coil A, is cancelled by the EMI that is induced in Coil B. This is great, but then how do we get the sample signal? The trick is to only put a sample in one of the coils. This would technically mean that I only need one pre-polarization coil, and two T/R coils. However the pre-polarization coil provides a certain level of EMI shielding to the sample coil, therefore we need a second pre-polarization coil to create the exact same environment for the bucking coil.

Pre-polarization Coils

Each pre-polarization coil was wound on 4'' ABS. I do not recommend ABS as it is a very expensive material compared to some of the PVC variants. I could probably go back and redo the entire build for half the price if I were to use cheap PVC instead.

The coil is actually two layers that will be connected in parallel, this keeps the resistance of the coil low, meaning that we can get higher currents at low voltages. The highest voltage that I have on my power supplies is 12V. The resistance of each layer of the coil is 2.1 Ohms. This means that we can expect a maximum current of.

I = V / R
I = 12V / 2.1 Ohms
I = 5.714 Amps

This is good, because the maximum current that 20 AWG wire can handle is about 7-11 Amps. In practice, I get about 4.6 Amps and the coil gets warm after a minute. The maximum current that the power supply can provide is 9 Amps on the 12V rail, so this will not work because we would exceed the specifications on the power supply. I could instead run the coil off of the 5V Rail as follows.

I = V / R
I = 5V / 2.1 Ohms
I = 2.38 Amps

In practice I get a current of about 2.17 Amps, and the coil does not heat up as fast, or as much.

What magnitude of a magnetic field do we get from each layer?

B = u N I / L

B = (4 * 3.1415926535 * 10 ^ -7 T / Amp m) * 167 Turns * 2.17 Amps / 0.1524 m

B = 2.988 mT

There are two layers, so we can expect double the magnetic field.

B = 5.976 mT

This isn't that great, but if we want to scale up, we could buy a better power supply, move to the 12V rail, and add two more layers. This would bring the total field strength up to.

B = 28.69 mT (Future Improvement)

Here is the first layer of the coil.

Here is the finished coil, wrapped in electrical tape. This picture shows my digital callipers that I use religiously.

The length of the pre-polarization is 6 inches.

Transmit/Receive Coils

Each T/R coil is wound on a 3'' diameter section of ABS. The length of the T/R coil is about 4.38 inches. I made the T/R coil shorter because there are fringe effects towards the end of the solenoid. That being said, this is only a pre-polarization coil, so the Bp field does not have to be too homogeneous.

In order for the hum bucking configuration to do it's job, each of the T/R coils must be identical. I have used my multimeter to ensure that both coils have the exact same resistance.