The best size of a feedback coil?

[farmersplow]

I am an absolute beginner in the lathe field. I have little experience in electronics and no experience in construction of cutting heads. But I am very ambitious and have very good knowledge and good craftsmanship. My strongest trait is my perseverance and the will to solve all problems.

I'm from Austria and my English is not very good. But I'll try anyway and ask for your understanding if my expression is not perfect.

I am currently building a lathe and a cutter head with feedback coil. Now that I've solved most of the problems for the lathe, I'm just focusing on the feedback from the cutter head. I wanted to find out what properties such coils have and how they can be changed.

In theory, I've learned a lot about it, but the practice is mostly different. I want to find out how big and how long the coil should be. What diameter and how many turns it should have. And how it should be positioned. What resistance would such a coil give and is the resulting signal strong enough?

Those who know nothing must believe everything. So I want to test and try it.

This test does NOT take into account or measure the resonance of a feedback coil! I do not check resonances here (even if there are certainly resonances in the test setup).

This test is only intended to show the differences between feedback coil dimensions, magnets and windings. Take into account the positioning and possibly shielding.

I want to find out which feedback coil I need.

Thomas

[farmersplow]

MY FEEDBACK COIL‘s

Here I would like to introduce what I have already found out:

How I made my test coils (feedback coils):

Coil 1: First I disassembled an old speaker relay to get the coil wire.
Then I made a spool carrier out of paper mache on a 3mm tube.

Attached a short video that shows how i have done the spool carrier.

Thomas

[farmersplow]

In the meantime I have built a winder using an Arduino and a stepper motor to ensure the number of turns.
Then I prepared everything for the wrapping process.
After winding, I glued the coil with nail polish.

Attached video: Winding process preparation
Attached video: Winding process

[farmersplow]

One more video

[farmersplow]

At the end some glue

[farmersplow]

Coil-1 is ready and has a weight of 0.16 grams and a resistance of 10.4 ohms

[farmersplow]

I made Coil-2 (with different dimensions) in the same way.

Coil-3 and Coil-4 were created on a plastic spool carrier. Each spool carrier weighs 0.09 grams without the wire.
Coil-3 was spooled in the same way.
Coil-4 was spooled in other direction!

[farmersplow]

FEEDBACK COIL‘s OVERVIEW

[farmersplow]

MAGNET‘s & WASHERS

I have a variety of different ring magnets and washers. This allows different scenarios to be carried out.

[farmersplow]

Here some details to the magnets I used:

[farmersplow]

Loudspeaker (Drive Coil)

The loudspeaker is used as a drive coil.
It is a BOSE 30 Watt loudspeaker from the "Sound-Dock" series.

[farmersplow]

Preparing of the Loudspeaker:

I cut out the inner membrane area and replaced it with a recording stamp. The acceptance stamp comes from my wife's knitting. (Don't tell her)!

Attached two videos of the preparing process.

20210809_142343.mp4

[farmersplow]

Carbon Rod

The carbon rod also comes from my wife's knitting. (Don't you say that either)!
(You don't believe at all which high-tech elements can be found in knitting accessories. Stainless steel fine-thread parts, rods made of all different materials, Carbon Rod‘s from 2 to 8mm diam. in 0,5 steps,
connecting elements, etc.)

[farmersplow]

Diagnostic Instruments

4-Cannel Digital Storage Oscilloscope
Voltcraft DSO-1084E
80MHz 1GSa/s


Function Generator
PeakTech 4100A

[farmersplow]

Experimental setup
Part 1

The loudspeaker is fixed on a wood plate.
The wood plate has a hole in the middle and is fixed on the ground.
A plastic holder is glued in the middle of the loudspeaker to take the carbon stick.
A carbon stick with 3mm (diameter) is fixed in the plastic holder.
A sinus signal at the loudspeaker (coil) moves the carbon stick forward and reverse.

[farmersplow]

Experimental setup
Part 2

The feedback coil is fixed on the carbon stick.
A second moveable wood plate has a hole in the middle to pick up a magnet.
The plate with the centered magnet is moved over the coil.

[farmersplow]

This is how it looks like in real

[farmersplow]

Experimental setup
Part 3

[farmersplow]

This is how it looks like in real (Example at 11Hz to see the movement)

20210905_190332.mp4

[farmersplow]

TEST 1 a

TARGET of this test:
How does the signal strength change with different overlaps between coil and magnet

MAGNET & COIL:

MAGNET M 2 (15 x 8 x 6 mm)
COIL 1 200 turns
d 5,3mm 0,16 gram
L 6mm 10,4 Ohm
MAGNETIC AIR GAP : 0,6 mm

INPUT: SINUS ppV 1V (+0,5V / -0,5V)

OUTPUT: SIGNAL ppV test mV

2021-09-07 22_35_12-RECUT FeedBack Coil 1.3.pptx - PowerPoint.png

RESULT:
The strongest signals can be measured when the middle coil protrudes slightly over the edge of the magnet.