The 'role' of a pickup pole piece?

[peskywinnets]

@Steve: The shape matters whether it is raw magnet or steel pole -- some poles are designed to have greater fringe fields, particularly the Lace Sensor. More fringe fields means more flux lines for the guitar string to cross.

@Mike: Poles and blades are commonly made of soft (= low carbon) steel like 1010 or 1018, depending upon builder preference. Mu metals and similar, while superb magnetic conductors up to a point, saturate at a lower field than cheep steel and actually pass _less_ magnetic field for a given magnet.

Just curious - has anyone ever bothered with magnetic (FEMM) modelling - changing the pole piece shapes/size/material etc? And if so, care to post up the associated jpegs?

[David Schwab]

So how in the world can I make one of those.
It sure looks neat if nothing else.
I wonder what the pole pieces look like out of the bobbin.
I don't think too many folks are going to Mimic this one!
Rock on!
Terry

You have two years to wait before you can legally make one for sale. The patent expires in 2012.

[salvarsan]

Just curious - has anyone ever bothered with magnetic (FEMM) modelling - changing the pole piece shapes/size/material etc? And if so, care to post up the associated jpegs?

Yes.


Steve Kersting over at SK Guitars posted his images a few years back.

[ken]

@Mike: Poles and blades are commonly made of soft (= low carbon) steel like 1010 or 1018, depending upon builder preference. Mu metals and similar, while superb magnetic conductors up to a point, saturate at a lower field than cheep steel and actually pass _less_ magnetic field for a given magnet.

If a magnetic conductor like mumetal saturates easily and passes less total magnetism through it than 1018 steel, then why even bother to use it? Wouldn't you just be better off by simply using a weaker magnet? The last time I checked 1018 was lots cheaper and easier to find than mumetal.

ken

[salvarsan]

If a magnetic conductor like mumetal saturates easily and passes less total magnetism through it than 1018 steel, then why even bother to use it? Wouldn't you just be better off by simply using a weaker magnet? The last time I checked 1018 was lots cheaper and easier to find than mumetal.

Good question and good answer.


If I absolutely HAD to use mu metal, I'd put a short rod on top of a ceramic magnet button and see how well it worked.



But you're right: low carbon steel is cheaper and better for pickup purposes. *1008 steel sounds different from 1018, so there are choices within the idiom.

[Mike Sulzer]

@Steve: The shape matters whether it is raw magnet or steel pole -- some poles are designed to have greater fringe fields, particularly the Lace Sensor. More fringe fields means more flux lines for the guitar string to cross.

That brings up this question : does the guitar string crossing flux lines induce a voltage in the coil? If the flux lines point in the direction of the axis of the pole piece (that is, vertical, if the guitar is sitting on the bench), then if the guitar string moves horizontal, it crosses flux lines. That induces a voltage along the guitar string, but not in the coil.

If we want to induce a voltage in the coil, we have to increase/decrease the density of the component of flux pointing through the coil. The permanent field induces magnetization in the string. These field lines spread out as they move down from the string and eventually loop back to the string.

Therefore, if you move the string vertically the flux density through the coil changes. You can also think of this as flux lines crossing the wire of the coil. Note that the flux lines cross in opposite directions on opposite sides of the coil. But the voltages add because the wire on opposite sides of the coil also points in opposite directions. (This is a bit hard to see without a drawing.)

If you move the string horizontally, the field through the coil does not change very much and there is little induced voltage. You can also think of this as flux lines crossing the wire of the coil. But in this case the voltage induced on one side of the coil cancels with that induced on the other (because the flux lines move in the same direction, but the wire on the two sides points in opposite directions), and so you get very little.

[Rick Turner]

Mike, one way to demonstrate that is to drive a string with an E-Bow and try it on two axes. No, it's not easy to replicate the force, but it's possible if you carefully set the distance...at least it's good enough to prove your exact point.

[peskywinnets]

Mike, one way to demonstrate that is to drive a string with an E-Bow and try it on two axes. No, it's not easy to replicate the force, but it's possible if you carefully set the distance...at least it's good enough to prove your exact point.

The problem here is one of control - how could we be sure that an 'Ebow stimulated string' only moves in one perfect axis (I've an inking that the string would end up vibrating in a semi-rotational manner). To get to the bottom of this one, it would be far better to keep the guitar string stationary but actually mount/attach the guitar pickup to something that mechanically moves up/down in one axis (& something that moves fast enough to get a meaningful scope trace) - then change the axis, rinse ....repeat.

[Steve Conner]

This is an oversimplification IMO, to say that the pickup is sensitive to vertical motion but not horizontal motion.

It's sensitive to the component of motion perpendicular to the field lines, and the field lines aren't necessarily perpendicular to the face of the pickup. The angle of them depends on the place you're measuring in, and the shape of the pole pieces.

[Mike Sulzer]

It's sensitive to the component of motion perpendicular to the field lines...

No, it is sensitive to the changing component flux along the axis of the coil (integral of B dot da). That does not completely exclude horizontal motion but it makes it much lets sensitive to it. It is the coil (or rather each turn) that does the selection.

Is it possible that there is some confusion with the case of a conductor moving perpendicular to magnetic field? In the normal pickup (not Joeseph's) the string is not a conductor, but rather a magnet, since it is magnetized by the permanent field. Think of it as a vibrating temporary magnet.

[Steve Conner]

OK, well let me put it another way. With rod pole pieces, the field that they conduct through the coil is along the axis of the coil. But when it leaves the ends of the poles, it spreads out, on account of the different permeabilities of steel and air.

Therefore in the air above the pole pieces, the string can encounter field lines that were along the axis of the coil when they were in the coil, but are now at some other angle.

To my mind, motion of the string parallel to these won't change the reluctance of the system, but motion perpendicular to them will.

[David Schwab]

OK, well let me put it another way. With rod pole pieces, the field that they conduct through the coil is along the axis of the coil. But when it leaves the ends of the poles, it spreads out, on account of the different permeabilities of steel and air.

Therefore in the air above the pole pieces, the string can encounter field lines that were along the axis of the coil when they were in the coil, but are now at some other angle.

To my mind, motion of the string parallel to these won't change the reluctance of the system, but motion perpendicular to them will.

The two Bill Bartolini patents deal with this. Bil wanted to pickup the up/down motion of the string, and wanted to exclude the side-to-side motion, as he feels this is more musical sounding, and emulates the top of an acoustic guitar.

So his idea is that rod magnets produce a tall field with the lines of flux being more vertical, while his Hi-A (high asymmetry) design used a low field with the lines more parallel to the top of the pickup.

[Mike Sulzer]

OK, well let me put it another way. With rod pole pieces, the field that they conduct through the coil is along the axis of the coil. But when it leaves the ends of the poles, it spreads out, on account of the different permeabilities of steel and air.

Therefore in the air above the pole pieces, the string can encounter field lines that were along the axis of the coil when they were in the coil, but are now at some other angle.

To my mind, motion of the string parallel to these won't change the reluctance of the system, but motion perpendicular to them will.

I agree with your first two paragraphs, but I do not see that you have any reason for asserting the truth of your last sentence. It is fine to say that a guitar pickup is a variable reluctance device, but it is altogether another matter to analyze it.

The analysis method I have used above breaks the problem into two parts:
1. The magnetization of the string.
2. The induction of voltage around the coil turns due to the changing flux (pointing through the coil) from the vibrating magnetized string.

Do you have any doubts that this method is correct?

[Steve Conner]

Well, look at it this way.

If the string is moving parallel to a field line, then the magnetic field it sees is not changing. (Field lines are contours of equal field strength.)

I reason from that, that if the magnetic field seen by the string doesn't change when it moves, then the motion of the string doesn't change the magnetic field seen by any other part of the system.

I guess that is the assertion you're challenging, and now I think about it myself, I'm not sure it is right, because it doesn't take into account distortion of the field lines by the string itself. (Your "magnetized".)

[Mike Sulzer]

Well, look at it this way.

If the string is moving parallel to a field line, then the magnetic field it sees is not changing. (Field lines are contours of equal field strength.)

I reason from that, that if the magnetic field seen by the string doesn't change when it moves, then the motion of the string doesn't change the magnetic field seen by any other part of the system.

I guess that is the assertion you're challenging, and now I think about it myself, I'm not sure it is right, because it doesn't take into account distortion of the field lines by the string itself. (Your "magnetized".)

Field lines show the direction that the magnetic field points. The density of field lines shows the strength of the field. So if you follow a field line out from the pole of a magnet, you move from a region of high density of lines to a region of lower density. This agrees with the observation that the magnetic field gets weaker as you move away from the pole of the magnet.

When the string becomes magnetized by the permanent field you can indeed think of this as a modification of the total field. But it is equally correct to think of the total field as made up of the sum of two parts (by linearity), that is, two fields. The field we are interested in is the one resulting from the magnetization of the string. The reason we are interested in that field is that if we look at it inside the coil, this field changes when the string vibrates, inducing a voltage around each turn of the coil.

This field changes most when the string moves towards or away from the coil because the field of the string weakens with distance from the string. There is a second effect, too. The magnetization of the string changes with its distance from the pole piece. This is because the strength of the permanent field decreases with distance. These two effects multiply together, and it means, as Joseph has pointed out, that the pickup generates some harmonics. How big this effect is depends on the excursion of the string.