How does magnetic sensor work?

Answer 1

The answer depends on what type of magnetic sensor you are speaking of.

One type might be a reed switch. In the presence of a magnetic field the switch changes state, either opened to closed or closed to open. I'd consider this one the simplest type of sensor. Either the field is strong enough to cause the state change or it isn't.

Another type I'd consider to be more of a transducer. According to physics' proof and the ideas of a guy from long ago named Faraday, a changing magnetic field (this field is also said to be in a state of "flux" when changing) in the presence of a conductor will induce a voltage proportional to the rate of change of the field.

Mathematically stated this is written as

e = k do/dt

where e is the voltage induced, k is a constant of proportionality, and do/dt (the derivative of phi, o, with respect to time) is the rate of change of the flux.

(Note: o is supposed to be the Greek letter phi, but I haven't a convenient means of entering it, and I don't remember the ASCII code.)

Anyway, so this e voltage that's developed can be measured with a voltmeter or via an oscilloscope and is proportional to the rate of change of the magnetic field. Rarely would a magnetic field remain constant. Therefore, one can always expect some amount of potential to be developed no matter how small.

However, you may be asking, 'but what about the field itself. How would the field's strength be detected?'

The do/dt in the above equation is the key. Just the same way that o can be differentiated (called "taking the derivative" of) to obtain do/dt, do/dt can be integrated (the opposite/inverse of differentiation) to obtain o. However, it should be understood that there must be a time period involved, because integration is, simply stated, the area under the curve. You can't have an area without a width. Therefore, when integrating we're taking a bunch of infinitessimally small-width rectangles and summing them over a period of time (because time is the independent variable in the above equation and not something else.)

So, leaving the Calculus aside except for the above equation, the way the magnetic field is detected is the voltage e that's developed must then be applied across a load. For example, we could just measure it with a voltmeter, but that voltmeter itself is a load, a very high resistance load. When that load is applied, a current flows due to more physics and the theories of another guy long before television named Ohm (if there is a closed loop for the current to flow in.)

Ohm's Law (actually, I'm pretty sure it's still a theory, but they call it a law) states:

voltage = current x resistance

or

e = i x r

or

The potential, e, developed across a resistance is proportional to the current flowing through the resistance. (Notice that e gets larger as i gets larger?)

Hence, rearranging Ohm's Law to solve for i we have (and I'm going to write the equation downward this time):

e

=

ir

therefore

e

--

r

=

ir

--

r

therefore

i

=

e

--

r

So (saying it like a normal person now) we have

i = e/r

What this says is we can produce a current proportional to the voltage developed by applying that voltage across a load.

Hence, to sense the strength of the magnetic field, we can integrate the rate of change of the flux over time, or, if we're only interested in whether the field is present or not, we can read the voltage developed whenever the field changes. Either way we have sensed the magnetic field.

There's one other way to sense a magnetic field and it's by way of another phenomenon of physics called "The Hall Effect." However, I regret that I don't remember enough about it to explain it very well. In a nutshell, a voltage is developed in the presence of a magnetic field within (usually) a semiconductor material by the magnetic force causing drifting positive charges to go one way and negative charges to go the opposite way thus producing a difference of potential across the material. That difference of potential is called the "Hall voltage" or "Hall emf" (electromagnetic force, voltage). However, with this short explanation I really do not do Edwin Hall (another guy who didn't watch tv) justice. And I'd really appreciate it if someone more knowledgeable will post an update to this answer which does explain it.