3-DIMENSIONAL MAGNETIC MAPPING FOR REEDSENSOR ACCURACY

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Introduction
Purpose› Introduce the concept of magnetic mapping andhow it helps the reed sensor designer
Objectives› Introduce magnetic mapping technology› Define the key functions and key terms› Define how magnetic mapping can help the Sensor
designer

Defining the Reed Sensor››››

The reed sensor is an ideal method of sensing anddetecting movement.
Typically a permanent magnet is the moving
member in the magnetic system
A hermetically sealed reed switch is generally
mounted to a PCB or hard wired to an electricalcircuit
The reed switch senses the physical movement ofthe magnet and the reed contacts will close or open.

Key Terms

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When a reed sensor’s contacts close it’s called thepull-in or closure point
When a reed sensor’s contacts open it’s called thedrop-out or opening point
Reed sensor hysteresis is defined as the ratio of the
Drop-out/Pull-inKey Terms - Hysteresis›››

Understanding Hysteresis in a reed sensor isimportant
Sense points in liquid level sensing can be unstableparticularly when the liquid level is in a movingvehicle
Under this condition with no hysteresis the closure
point would continue to fluctuate as well as theopening point with any small changes in the liquidlevel

Key Terms - Hysteresis

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Reed sensors can be selected for varying degrees ofhysteresis
A typical wide hysteresis would be about 50%.
So if the closure point is 1.0 inch (2.54 cm) awayfrom the reference point, the drop out point would
be 0.5 inches (1.27 cm).
Or Hysteresis = Dropout/Pull-in x (100%)

Key Terms›››

The magnetic fields we will be talking about are
generally produced by permanent magnets
Ferromagnetic materials are those metals that affectthe flow of magnetic lines of force
Ferromagnetic materials are generally iron, steel,nickel, and cobalt.

3-D MAGNETIC FIELD MAPPINGAPPLICATION EXAMPLE AND WHY IT’S IMPORTANT

Magnetic Field Mapping›››

Magnetic mapping is the method of incrementallymeasuring the pull-in and drop-out points
The movement is carried out in all three dimensions.
Software is then used to bridge all the points

3-D Field Mapping›››Example using only one magnet and one reed sensor
This example has the magnet in a slight verticaloffset relative to the reed sensor sitting on a PCB
The pull-in and drop-out fields are shown 3
dimensionally3-D Field Mapping Example›

Pull-in and Drop-out fields from a frontal view
MagnetDrop-out field
Pull-in field3-D Field Mapping Example›

Pull-in and Drop-out fields from a frontal view
MagnetDrop-out field
Pull-in fieldHysteresis

3-D Field Mapping Example›

Pull-in and Drop-out fields from a side view
MagnetDrop-out field
Pull-in field3-D Field Mapping Example›

Pull-in and Drop-out fields from a side view
MagnetDrop-out field
Pull-in field
Hysteresis

3-D Field Mapping Example››

Displaying pull-in and drop-out mapping boundaries.
Three-dimensional viewing is critical to optimize
parameters
MagnetDrop-out field
Pull-in field

Front View

Side View

Mapping Results›››

In the mapping example if a maximum sensing
distance is required the design must change
If the magnet and reed sensor position can not bechanged then a more sensitive reed sensor needs tobe used
Or you will have to use a stronger magnet - usually
this will add costWhy Magnetically Map?

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In Sensor applications it is important to understandthe exact pull-in and drop-out fields.
This information then allows one to properly
position the magnet and sensor well withinappropriate guard bands and avoid any tolerance
issues.
Mapping allows the designer to solidify his design
before finalizing all design constraints.

Summary››››

Adequate operate and deactivate points
Operation well within the magnetic envelopes toavoid tolerance issues
Acceptable hysteresis between the operate anddeactivate points
Sensor and magnet costs optimized

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