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Technical Discussions
Michael Trinidad
Consultant,
LMATS Pty Ltd , Australia, Joined Jan 2003, 138

Michael Trinidad

Consultant,
LMATS Pty Ltd ,
Australia,
Joined Jan 2003
138
08:11 Oct-19-1998
Sensors (Hall effect and Coil)

Sensors (Hall effect and Coil)

A coil sensor for sensing magnetic field amplitudes acts on the same basis as a transformer and relies upon the density of the magnetic flux cut and the speed at which the flux lines are cut (Faraday's law). A decrease in speed of the coil cutting a given flux density would result in a lesser induced current into the sensor coil giving a smaller indication.

A hall effect sensor's output is governered by the deviation of electron carrying electrons in a semi conductor when moved through a magnetic field. My question is if a decrease in speed of the Hall effect sensor cutting a given flux density would result in either a greater electron deviation (higher output current/higher indication) or smaller deviation (smaller output current/smaller indication)?

Kindest Regards


Michael Trinidad



    
 
 
William Friedman
William Friedman
04:37 Oct-20-1998
Re: Sensors (Hall effect and Coil)
: Sensors (Hall effect and Coil)

: A coil sensor for sensing magnetic field amplitudes

: A hall effect sensor's output is governered by the deviation of electron

The coil sensor only responds to an AC component while a Hall effect device responds to the DC value of magnetic field. Thus velocity in terms of a movement of the transducer should not have any effect.

William Friedman


    
 
 
Paul O'Shaughnessy
Paul O'Shaughnessy
09:13 Oct-20-1998
Re: Sensors (Hall effect and Coil)

Faraday's Law simply states that a loop enclosing a space containing a magnetic field will experience an "electro-motive force" (a voltage) in proportion to the change with respect to time of the total magnetic flux enclosed by the loop. If there is no change in the enclosed flux, the voltage induced in the loop is zero. So, the statement that "A coil does not have to be influenced by an AC field" is not correct. The explanation that follows about a magnet moving in one direction being a "DC" effect doesn't help. "AC" in this context refers to things that are changing, not just classically sinusoidal signals. "DC" refers to things that are not changing, or "steady state." Any change in the field enclosed by a loop will result in a voltage induced on that loop, in proportion to the rate of change of the field, and that's it. No change, no voltage. A non-changing field will have no effect, regardless of how strong it is. Thus the "AC" nature of the coil sensor - it captures information about rateof change, not absolute values.

A Hall device works by comparison on a relatively simple "DC" principal, in that it directly measures the field intensity at all times. If the field strength doubles, the voltage output of the Hall device doubles accordingly and stays there.

I have reviewed the literature which I have on Hall devices and know of no magnetic "AC" effect in a Hall device. That is, the deflection of electrons in the device (and thus the output voltage) at any moment will be simply in proportion to the magnetic field strength at that moment. The rate of change of the field by itself has no influence. So, the speed at which a Hall device is moving will, by itself, have no influence on the electron deflection, thus no influence on the output voltage. The statement by William Friedman is correct.

I hope that this helps to clarify these effects.

Paul O'Shaughnessy
Panametrics


: : : Sensors (Hall effect and Coil)

: : : A coil sensor for sensing magnetic field amplitudes

: : : A hall effect sensor's output is governered by the deviation of electron

: : The coil sensor only responds to an AC component while a Hall effect device responds to the DC value of magnetic field. Thus velocity in terms of a movement of the transducer should not have any effect.

: : William Friedman


:
: I have to disagree. A coil does not have to be influenced by an AC field. The movement of a bar magnet through a wire loop induces a current in that wire travelling in one direction only (DC). Reversing the direction in which the magnet travels reverses the induced current's direction. Faraday's law states that the induced EMF in a conductor moving through a magnetic field will be proportional to the rate of cutting flux and the area of flux moved through (density).

: Magnetic Flux Leakage (MFL) NDT equipment uses Hall effect and coil sensors to detect the field strengths of the magnetic leakage fields caused by defects. The magnetic most commonly comes from rare earth permanent magnets, which saturate the component (steel plate, steel pipe or steel wire rope), and is a DC field.

: Now I know that speed affects the sensor output on both but the exact physical reason the Hall effect sensor output changes as in what effect it has on the deviation of the electron path and subsequent sensor output. If the speed is not constant for example in a steel wire rope inspection or tank floor inspection the defect response varies accordingly.


: Kindest Regards

:
: Michael Trinidad




    
 
 
Michael Trinidad
Consultant,
LMATS Pty Ltd , Australia, Joined Jan 2003, 138

Michael Trinidad

Consultant,
LMATS Pty Ltd ,
Australia,
Joined Jan 2003
138
02:11 Oct-21-1998
Re: Sensors (Hall effect and Coil)

I can agree that most texts and literature refer to a magnetic field 'varying' in intensity induces a current into the wire loop this still does not equate to an actual AC field. I am looking at these sensors in relation to magnetic flux leakage instruments such as rope testers and tank floor scanners therefore variations in outputs from both sensors resulting from speed changes are a result of:

Coil - Basic Faraday's law and the equipment sampling rate/software

Hall effect - If the response is instantaneous the variation in output signal is a sole result of the equipment sampling rate/software.

I am not including other factors such as temperature etc.

I can also agree most NDT literature does not explain how a Hall effect sensor works just what it does. The previous picture was from a company website that manufactures Hall effect sensors and the following is a quote from that site:

" Principle and Structure

Here the magnetic sensor is based on Hall effect. The effect is based on the interaction between
moving electric carriers and an external magnetic field. In metal, these carriers are electrons.
When an electron moves through a magnetic field, upon it acts a sideways force

F = qvB

where q is electronic charge, v is the speed of an electron, and B is the magnetic field.

Let us assume that a source of electric current is connected to the upper and lower ends of the
strip (Fig. M1). The force F shifts moving electrons toward the right side of the strip which
becomes more negative than the lift side. The sign and amplitude of the transverse Hall potential
difference VH depends on both magnitude and directions of magnetic field and electric current.
At a fixed temperature it is given by


where a is the angle between the magnetic field vector and Hall plate (Fig. M2), and h is the
coefficient of overall sensitivity whose value depends on the plate material, its geometry, and its
temperature." End quote
site http://www.mfg.mtu.edu/cyberman/machtool/machtool/sensors/magnetic.html

A further good information site is http://math.arsc.sunyit.edu/projects/vector/hall.html

And it also refers to the deflection of the charge carriers:

" A TECHNICAL APPROACH TO THE HALL EFFECT

Let's look at what the Hall effect is from a more technical point of view. If a magnetic field is applied perpendicular to the
direction in which holes drift in a P-type bar, the path of the holes tend to be deflected (see figure three). Using vector notation,
the total force on a single hole due to the electric and magnetic field is given by:

(1) F=q(E + V x B)

In the y direction the force is:

(2) Fy = q(Ey -VxBx)

The basic interpretation of equation 2 is that unless an electric field Ey is established along the width of the bar, each hole will
experience a net force( and therefore an acceleration) in the -y direction due to the qVxBz product. Therefore to maintain a
steady state flow of holes down the length of the bar, the electric field Ey, must just balance the product VxBz. Essentially:

Ey=VxBz

so that the net force Fy is zero. Physically this electric field is set up when the magnetic field shifts the hole distribution slightly in
the -y direction. Once the electric field Ey becomes as large as VxBz, no net lateral force is experienced by the holes as they
drift along the bar. The establishment of the electric field Ey is known as the Hall effect, and the resulting voltage VAB= Eyw is
known as the Hall voltage. The electric field Ey can be represented by:

Ey = RHJxBz

The current density J that results from the net drift or movement of holes is just the number of holes crossing a unit area per unit
time. B is the magnetic field, and RH is the proportionality constant known as the Hall constant or Hall coefficient.

RH is defined by:

RH = 1/qpo

po the concentration of holes in the valence band. q is the + charge. Another way of thinking of po is remembering that in n
type semiconductors electrons are the charge carriers and in p type holes are charge carriers. Think of a hole as an empty state
in the valence band.

Although the discussion here has been related to p type material, similar results are obtained for n type. A negative value of q is
used for electrons, and VAB and RH are negative." End quote


Attached is the applicable Fig. My purpose was to find out why output signals vary with speed on MFL equipment and appears to have been solved. I post the Hall effect information for your interest as I too could find no technical information in my NDT text books.


Kindest Regards


Michael Trinidad




    
 
 
Pim van Andel
Pim van Andel
07:11 Oct-24-1998
Re: Sensors (Hall effect and Coil)



    
 
 
Wisnumurti Kristanto
Wisnumurti Kristanto
04:59 Nov-01-1999
Re: Sensors (Hall effect and Coil)
I'm Sorry for give thie unrelated question.
I'm Asking about the application oh this hall effect on the motor.
I've observe one motor that using this hall element. To be honest, I still don't know the real fuction of the hall element.
I only look at the schematic of the system. It seems that the hall element is connected, both sides to two transistor, which drive the winding.
But, from what I have learned from hall element, it only has two output, positive and ground output.
The motor is comprised of four windings.
Of course the winding not only have hall element to drive them.
Thank you very much for the attention.


    
 
 
karthikeyan .B
karthikeyan .B
05:27 Nov-01-1999
Re: Sensors (Hall effect and Coil)
: : Sensors (Hall effect and Coil)

: : A coil sensor for sensing magnetic field amplitudes

: : A hall effect sensor's output is governered by the deviation of electron

: The coil sensor only responds to an AC component while a Hall effect device responds to the DC value of magnetic field. Thus velocity in terms of a movement of the transducer should not have any effect.

: William Friedman


    
 
 
Hyunchul Lim
Hyunchul Lim
03:39 Jan-22-2000
Re: Sensors (Hall effect and Coil)
: : Sensors (Hall effect and Coil)

: : A coil sensor for sensing magnetic field amplitudes

: : A hall effect sensor's output is governered by the deviation of electron

: The coil sensor only responds to an AC component while a Hall effect device responds to the DC value of magnetic field. Thus velocity in terms of a movement of the transducer should not have any effect.

: William Friedman




    
 
 
Thilagaraja Punusamy
Thilagaraja Punusamy
01:38 Nov-10-2000
Re: Sensors (Hall effect and Coil)
: I'm Sorry for give thie unrelated question.
: I'm Asking about the application oh this hall effect on the motor.
: I've observe one motor that using this hall element. To be honest, I still don't know the real fuction of the hall element.
: I only look at the schematic of the system. It seems that the hall element is connected, both sides to two transistor, which drive the winding.
: But, from what I have learned from hall element, it only has two output, positive and ground output.
: The motor is comprised of four windings.
: Of course the winding not only have hall element to drive them.
: Thank you very much for the attention.




    
 
 
sam
sam
02:36 Mar-02-2001
Re: Sensors (Hall effect and Coil)
: I'm Sorry for give thie unrelated question.
: I'm Asking about the application oh this hall effect on the motor.
: I've observe one motor that using this hall element. To be honest, I still don't know the real fuction of the hall element.
: I only look at the schematic of the system. It seems that the hall element is connected, both sides to two transistor, which drive the winding.
: But, from what I have learned from hall element, it only has two output, positive and ground output.
: The motor is comprised of four windings.
: Of course the winding not only have hall element to drive them.
: Thank you very much for the attention.

sujest you look up stepper motor



    
 
 

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