You Need Feasibility Studies
You have a magnetic sensing application. You have a tentative idea as to the magnet and sensor design you'd like to use. One of the most important things you can do is run a feasibility study. This will help you decide how much chance of success your design has before you start working on it.
What is a Feasibility Study?
A feasibility study is simply a quick set of simulation runs or a few quick lab experiments. The idea is to quickly learn about what magnets and sensors might work (or not work) for your application. The purpose is to get guidance as to the best way to address your magnetic sensing situation before you invest much time or effort in a particular solution. In the rest of this article, a few examples are shown to illustrate how feasibility studies work.
Example: Proximity Sensing from a Large Distance
Your sensing requirement is to detect when one object moves within 2 inches of another object. Your idea is to put a sensor on one object and a magnet on the other object. Will this work? If so, what's the estimated size of the magnet needed?
One approach is to acquire a number of sample magnets and sensors and do some experiments. In practice, this will work. The catch is that it takes time to get samples. Also, if the samples you get do not work, you may have to repeat this procedure a few times.
A second approach is to do a few quick simulations using a basic magnetic field calculator. The graph to the left shows the field at a distance of 2 inches from a Neo35 cylinder magnet as a function of magnet diameter and length. Depending on the type of device you choose, a field on the range of 40 to 70 gauss will be needed. This will require a large magnet. If a ceramic material is chosen, the magnet would be much larger.
It might be challenging to find the mounting space for such a large magnet. Also, magnets of this size will be expensive.
This example shows what a feasibility study can do. With just a few minutes of simulation work, it's possible to estimate the size of the required magnet. If your application is cost sensitive and you require a magnet that costs perhaps 10 to 20 cents, there is no way this type of magnet and sensor design can work. If your application has constraints on the magnet size due to packaging limitations, it might be possible there is not room for a magnet. In such cases, this feasibility study tells you something must change.
Example: Proximity Sensing Using a Particular Magnet Shape
This type of situation can occur when an application has mounting limitations. The question to be answered might be what is the maximum sensing gap possible. For numbers, we will assume that the packaging limitation is a 5 mm diameter by 5mm deep cylindrical hole for the magnet. We'll assume that the application requires the use of a particular Hall-effect switch with a typical activation level of 60 gauss and with a max tolerance of 80 gauss for activation. If there were no limitation on the device, it would probably make sense to find the most sensitive one possible.
One method is of course to order 5mm D x 5mm L magnets of the strongest possible materials and do some experiments. This will give you a good idea of the typical behavior of this magnet and sensor.
Another method is to do a more detailed simulation including a few estimated tolerances. This can give an estimate of the worst case sensing distance accounting for tolerances.
In this case, we assumed a high powered Neo material. We took into account estimated temperature and manufacturing variation along with small assembly tolerances. The blue curve shows the estimated typical magnet field on the axis from the pole. The reddish region shows the estimated variation in the field. The two green dotted lines show the typical and tolerance activate levels of the device.
The blue dot shows the typical activate point. The red dot shows the estimated worst case activate point. As can be seen, there is about a 2.0mm to 2.5mm difference between the estimated and typical values. An experiment with a magnet of this design and this sensor would probably give a measured value of about 13mm in the lab.
A feasibility study of this sort gives a rough idea of what can be expected in this application if the magnet shape is restricted to 5mm D x 5mm L. The advantage of simulation over experiment is estimating the effect of manufacturing variation.
In some situations, there might be a target activate distance. In this case, if the target activate distance were 7mm, this magnet and sensor would probably work fine. If the target activate distance was 15mm, this magnet and sensor would not work. If the target activate distance were about 12mm, the design is borderline. More work would need to be done to determine if it would work. Ideally, something would be changed to improve the performance. For this type of situation, a feasibility study can give you a good idea of what the estimated performance is compared to the target performance.