magnetic pick-up sensors
The magnetic field around a permanent magnet changes when a ferrous object passes through the field.
This is the basic principle behind the function of a magnetic pick-up sensor.
In this lesson we will take a closer look these common speed sensors, their construction and operation, and the types of signal output they generate.
We will touch on some common applications.
The Inside
A magnetic pick-up sensor is normally a self-contained unit, making it hard to guess what is inside.
Let's open it up!
Magnet
Body
Inside the sensor body there is a permanent magnet with an attached pole piece. There is also a wire coil connected to the sensor outputs.
The pole piece is needed to extend the magnetic field of the permanent magnet, allowing it to both encompass the coil and extend beyond the end of the sensor body itself.
Operation
Magnetic pick-up sensors are most often used to detect the speed of a rotating ferrous target.
The sensor must be placed close enough to the object being detected that the target object will pass through the sensor's magnetic field.
A ferrous object passing through the magnetic field changes the shape of the field. In this example, each gear tooth is ferrous, so each tooth disrupts the field.
And because the magnetic field is drawn to the gear tooth it distorts slightly back and forth.
The magnetic field is also affected by the size of the change in the air gap between the sensor and the object being sensed.
Air gap is the term for the space between the sensor and the object it is sensing.
A large change in the size of the air gap will have a greater effect on the magnetic field than a small change.
All this changing in the shape of the magnetic field induces a voltage in the coil.
The magnitude of that voltage is related to how fast the magnetic field changes shape.
A faster change will induce a larger voltage.
And the constant changing of the voltage creates an alternating current!
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