There is often “too much magnetism” to make a reed switch or proximity sensor work the way it is desired. These undesired magnetic fields can come from motors, electric solenoids, magnetic components (from inside capacitors or other discrete components), nuts, bolts or other hardware. These stray and undesired fields can often be negated by the overall design of the sensor assembly.
The problem with nuts bolts and other discrete components is their ferrous materials are not suitable for magnetic ‘control’. By control we mean they can be magnetically charged in a different polarity by whatever stray field happens by. Typically in a given application they will accept the field that is present, hopefully that is consistently in the same magnetic orientation. In the positioning of the sensor or switch relative to the moving magnet, their orientation can be set in a perpendicular alignment and the influence of the unwanted field is minimal or none. Increasing the physical spacing from these components is very helpful as their magnetic field strength tends to decrease rapidly with distance.
In the case of motors or electric solenoids, their fields are measurable and predictable. The voltages applied are typically very consistent. Their orientation is also consistent. AC motors and solenoids are a bit more difficult to position a sensor near because the magnetic field oscillates north and south synchronized with the frequency. However, the magnetic field from electric devices can also be minimized.
The key to working around all these unwanted magnetic influences is three-fold: proper shielding material, shield shape & design and specific magnetic sensing.
First and foremost is shielding with the right material. The shield must be made of ferrous material, annealed dead soft so it will not retain any magnetism and become a magnet itself. This shield is positioned between the sensing device and the unwanted magnetic fields. It can have holes or ‘windows’ that allow the correct or specific magnetism to influence the sensor inside the shield. These shields can be made of flat steel stock, cylinders, box type housings or other custom shapes suitable for the application.
The second element is the magnetic shield design. The overlapping and crisscrossing magnetic fields may seem inundating in a given application. The key is to position the sensing device, in our case a custom reed switch, inside the shield in the right spot to react only to the target magnetic field and ignore all the other counterfeit magnetism. The shield redirects unwanted magnetic fields and only allows the desired magnetic field to influence the sensor.
Third is the magnetic sensing device itself. Sometimes shielding the counterfeit magnetic fields can reduce the magnetic influence from the intended target, or certainly reduce the detected change in magnetism. Detecting this subtle change in a magnetic field requires a special device. The switch inside the sensor assembly must have what is called a “close differential” between operate and release.
These three factors, the shield material, the shield shape and the internal sensor specifications all make up the elements required for a fully functional proximity sensor in an area where there is “too much magnetism’.