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How Does It Work? -- Magnetic Pickups

Magnetic pickups are used in antiskid brake systems to detect wheel speed, in engine control systems to detect engine RPM and engine position, in automatic transmissions to detect output shaft RPM, and in many other devices which require knowing the rotational speed of an object.

They provide a simple method of determining the rotational speed of an object, without the use of any mechanical counters, and outputting an electronic signal which may be processed by any number of control units.

Contents

  1. Overview
  2. Basics of magnets
  3. Magnetic lines of force
  4. Atoms
  5. Basic electricity
  6. Electromagnets
  7. Current generation
  8. Magnetic pickups

Overview

 The following is an overview of how magnetic pickups work. Do not expect to understand it immediately; that's what the rest of this document is for.

Magnetic pickups output an AC voltage signal occurring from a magnetic field passing through a stationary conductor. The moving magnetic field comes from the distortion of a stationary magnetic field by a ferromagnetic object moving through the field. The amplified voltage surge is then signalled to the operator audibly or visually.

In a nutshell, that is how magnetic pickups work.

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Basics of magnets

 A magnet is generally identified as a lump of metal which attracts iron and attracts/repels other magnets. How do iron atoms act magnetic if they've been magnetized?

A single iron atom is an extremely small magnet. In a non-magnetic lump of metal, the small magnets point in all directions, cancelling themselves out. In a magnetic piece of metal, the small magnets are all aligned in the same direction. When all of the small forces are aligned in the same direction, the resultant force is the sum of the weaker forces. Here is a picture of the difference between metal in which the little atom-magnets are aligned and a piece in which they are not.

A normal bar shaped magnet has to areas where the concentration of magnetic force is greatest. These are called poles, and are assigned the names North Pole and South Pole.

As many people have found out by experimentation, the North Pole of a magnet is repelled by the North Pole of another magnet and attracted to the South Pole of another magnet. Similarly, South is repelled by South and attracted to North.

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Lines of force

 To begin understanding magnetism, you must be able to visualize the magnetism surrounding a magnet.

A magnet has a force field surrounding it. The following pictures should help you visualize this force field and how it interacts with objects around it.

Pictures with explanations

  1. Lines of force around a single bar magnet.
    The bar magnets being used are Alnico (an alloy of aluminum, nickel, and cobalt which retains a permanent magnetic charge) bar magnets about 1.5 X 0.5 X 0.375". This picture is of a single bar magnet, and shows the lines of force coming from the poles and wrapping around the magnet.

  2. Lines of force between two bar magnets with opposite ends near each other.
Here, two bar magnets of the above dimensions have been laid side by side with the North pole of one next to the South pole of the other, and vice versa. You can see the intense concentration of force directly between the poles, and also the lines of force coming out of the poles and circling back into the neighboring pole. This is a graphical illustration of the attractive force existing between opposite magnetic poles.

  • Lines of force between two bar magnets with similar poles near each other.
    Again, two bar magnets have been laid side by side; however, this time the South pole of one magnet is near the South pole of the other and the North is next to North. The lines of force radiating out from the poles do not meet, but rather assume the same shape as would be found in a single magnet.

  • Lines of force around a magnet with a piece of iron near one of the poles.
    A single bar magnet's lines of force as affected by a piece of iron near one of the poles is here imaged. Note how the lines of force around the magnet have been stretched out and flow into the piece of iron. This means that as the iron was bought near the pole, the lines of force had to move to flow into the iron. This is important to remember.

  • Lines of force around a magnet with a piece of iron between the poles.
    Here again the lines of force of a single bar magnet are shown as they are affected by a piece of iron between the poles. Note how the lines of force bend to meet the piece of iron. They also flow through the piece of iron and radiate outwards from it.
    As the pictures above illustrate, when a piece of magnetic material, such as iron or steel moves into a magnetic field, the magnetic field bends to flow into the material. If the magnetic material is moving, this means that for the lines of force to continue to bend towards the material, the lines of force must themselves move. This is important to remember.

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    Atoms
    Disclaimer
     Atoms consist of three basic particles;
    Neutrons
    Neutrons are neutral particles that reside in the nucleus, or center, of the atom.
    Protons
    Protons are positively charged particles that reside in the nucleus with the neutrons. They are 1800 times as heavy as electrons.
    Electrons
    Electrons are negatively charged particles that orbit the nucleus.
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    Basic Electricity

     The various primary properties of electricity can be summed up as the interactions between three components of any electrical circuit:
    Current
    Current is defined as the flow of electrons through a conductor.
    Voltage
    Voltage is the force pushing electrons through a conductor (such as copper wire).
    Resistance
    Resistance is the force opposing the movement of electrons through a conductor.