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The Variable Permeability Method
A brand new isotropic approach to earth’s field sensing has been newly championed by Precision Navigation. Surprisingly full tech details appear in their patent #4,851,775.The inductance of any winding is proportional to the permeability of its core. Normally, you will want your permeability to be a constant. One which is independent of the applied field or bias currents. Fail to do this in an audio transformer and you will get mild to severe distortion. One location where a variable or nonlinear permeability has been used for years is as a swinging inductor in dc power supply filters. A partial air gap gets used so that you’ll end up with additional inductance (and more filtering) at low currents. And faster response at higher currents. A unique new class of MetGlas magnetic materials manufactured by Allied Signal purposely goes out of its way to provide you a variable permeability which changes with the applied field strength or bias current.
This new material has a high permeability with low applied fields and a much lower permeability with high fields.Note particularly the fairly linear permeability shift with applied field above and below the bias point. You can bias to this point by running some dc current through an overwound sensing coil. The earth’s magnetic field (or some other mag source) will add to or remove from this magnetic bias level. Raising or lowering the coil’s inductance! You thus end up with a plain old coil whose inductance varies with the applied field strength. Put this in any suitable oscillator circuitry, and your output frequency should follow the strength and direction of the earth’s applied field. With proper design, as much as a 2:1 frequency change can be caused when you rotate the sensor through the compass points. What is really unique here is that a single ultra cheap solenoid winding over an ordinary core bar or rod acts both as a field sensor and the control bias setter. The sensing gets done by measuring the inductance. And the biasing by inputting a dc current Note that this is not a fluxgate and that your core material never really gets into hard saturation. Instead, you have a variable permeability sensor that progressively saturates. The rest is easy. Place the coil in a relaxation oscillator. Add some dc bias and shove the variable frequency output into your microcontroller.Figure four shows us one possible circuit. Unlike fluxgates, one simple winding over the magic core material is all you’ll need. To calibrate your sensor, rotate it through 360 degrees or else drive around the block.The falling slope is chosen for the following reason: An increasing mag field will decrease the permeability.Which in turn decreases inductance.And the decreasing inductance will increase frequency in most oscillator circuits. Thus, your output frequency should linearly track your input field strength on the falling slope...
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Don Lanaster's article
A brand new isotropic approach to earth’s field sensing has been newly championed by Precision Navigation. Surprisingly full tech details appear in their patent #4,851,775.The inductance of any winding is proportional to the permeability of its core. Normally, you will want your permeability to be a constant. One which is independent of the applied field or bias currents. Fail to do this in an audio transformer and you will get mild to severe distortion. One location where a variable or nonlinear permeability has been used for years is as a swinging inductor in dc power supply filters. A partial air gap gets used so that you’ll end up with additional inductance (and more filtering) at low currents. And faster response at higher currents. A unique new class of MetGlas magnetic materials manufactured by Allied Signal purposely goes out of its way to provide you a variable permeability which changes with the applied field strength or bias current.
This new material has a high permeability with low applied fields and a much lower permeability with high fields.Note particularly the fairly linear permeability shift with applied field above and below the bias point. You can bias to this point by running some dc current through an overwound sensing coil. The earth’s magnetic field (or some other mag source) will add to or remove from this magnetic bias level. Raising or lowering the coil’s inductance! You thus end up with a plain old coil whose inductance varies with the applied field strength. Put this in any suitable oscillator circuitry, and your output frequency should follow the strength and direction of the earth’s applied field. With proper design, as much as a 2:1 frequency change can be caused when you rotate the sensor through the compass points. What is really unique here is that a single ultra cheap solenoid winding over an ordinary core bar or rod acts both as a field sensor and the control bias setter. The sensing gets done by measuring the inductance. And the biasing by inputting a dc current Note that this is not a fluxgate and that your core material never really gets into hard saturation. Instead, you have a variable permeability sensor that progressively saturates. The rest is easy. Place the coil in a relaxation oscillator. Add some dc bias and shove the variable frequency output into your microcontroller.Figure four shows us one possible circuit. Unlike fluxgates, one simple winding over the magic core material is all you’ll need. To calibrate your sensor, rotate it through 360 degrees or else drive around the block.The falling slope is chosen for the following reason: An increasing mag field will decrease the permeability.Which in turn decreases inductance.And the decreasing inductance will increase frequency in most oscillator circuits. Thus, your output frequency should linearly track your input field strength on the falling slope...
from
Don Lanaster's article
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