Fun with Magnets.

An Adventure into Magnet Field Prediction and Mapping

Bryan LaBarge

Background

• Applications of magnetism range from hard drives to security systems to sensor technology

• Magnets typically used to open (or close) electronic circuits

• Switches in these types of circuits are reed-based

Reed-Based?

• 1930’s – Bell Labs create the reed switch

• Glass tube, filled with inert gas, contains two magnetic reeds

• Reeds extend out of sealed glass tube

• Magnetic field forces the two reeds closed

How can I use a reed switch?

• Can be used in any electronic circuit where a switch is needed.

• Perfect for position sensing– Used with a magnet as a system, magnet OR

switch can be moved– Strength of magnetic field “pulls in” the switch

Position Sensors

• Typically used for location doors, windows, moving machinery, etc., etc

• Also used to monitor liquid level and flow– Switch remains stationary– Magnet(s) are placed in a float that moves

with the fluid– Sensors tell other electronics to do something

(open a valve, sound an alarm)

Magnets like Refrigerator Magnets?

• There are many magnet materials and geometries

• Not all kinds of magnets are suitable for every application

• Certain switches are more sensitive than others, so magnet selection is key

Sounds complicated…

• In this case, we’ll look at an existing system

• Limited to a single magnet of known quantity, and one reed switch

• Attempt to visualize magnetic field via FEA analysis, hand calculations, and empirical data gathering

The Case.

• With a switch fixed in a set location, the magnet will swing on an arc towards the switch

• Switch has a known sensitivity

• Need to calculate magnet position to close switch

COMSOL Model

• Magnet material = Alnico 5

• ½” long, 1/8” in diameter

• 2D Axisymetric model

• Magnet placed in 1 sq ft of air

-1.5 -1.4 -1.3 -1.2 -1.1-1

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.10

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.91

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

S1

S3

S5

45-50

40-45

35-40

30-35

25-30

20-25

15-20

10-15

5-10

0-5

A magnetic field might look like:

• Thanks to COMSOL not working tonight, this is an Excel representation

How do I interpret this?

• Magnetic field form loops (lines) of magnetic strength

• Flux lines are perpendicular to the field lines.• Flux Density is easured in Tesla, T (in SI) or

Gauss, G (in CGS)• Switches activate at a certain level of gauss• The graph is a contour plot of ranges of gauss

levels

On deck…

• Validation of model (empirically)

• Tracing Gauss points on an arc thru the magnetic field

• Calculating a distance from switch to magnet based on points on the arc