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
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:
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.