Magnetic Field Sensing, the common practice and available added values

Sensing a magnetic field has immensely expanded, while the demand has been evolving the adoption of a variety of magnetic sensors, detecting presence, strength, or direction of magnetic fields, (from the Earth as well as from permanent magnets, fields generated from electric currents, vehicle interferences, etc.) One of the main added values of magnetic sensors is that the mentioned measurements can be conducted in a contactless manner.

The first documented usage of magnetic sensors dates over 2,000 years back, essentially for direction and navigation applications . Nowadays, even though these sensors remain one of the main means of navigation, a much larger variety of usage have evolved. A strong motivation for the growth of the knowledge and usability of these sensors is also the need for higher sensitivity, miniature dimensions, and the ease of compatibility with electronics.

Measuring magnetic fields is paradoxically not the main intent for using magnetic sensors. These systems make it accessible to calibrate and analyze physical quantities and parameters that are extracted from the changes in the magnetic field (distance, temperature, etc.).

Measurement methods

While the design and equipment of the measurement tools are evolving on an almost daily basis, most methods of magnetic measurements have remained practically unchanged . With the vast amount of tooling available per method, lets go over the typical magnetic sensor types:

Coiled sensors

Coiled magnetic sensors are some of the more simple, affordable magnetic field sensing methods.

The general principle lies in their response to magnetic fields (AC/RF) that are in the parallel axis of the coil. An analog output voltage is produced that reflects the strength of the magnetic field. These are the simplest commercially known sensors that can identify changes in the magnetic flux density. The process, simply put, is as follows: the moment when a magnet approaches the coil(s) the magnetic flux density increases in the coil (and vice versa, when moving the magnet away from the coil).

As simple as this system is, it unfortunately is not perfect for all the necessary applications, as the output voltage is dependent on the rate that a magnetic flux changes: it is rather challenging, or impossible for this system to measure a fixed magnet or one that changes the magnetic flux slowly.

Reed Switch sensors

Simply put, a reed switch is a glass capsule that has two ferromagnetic reed blades that overlap each other at the center of the class capsule or be off from each other at the center of the glass capsule, these blades pass through the tube from parallel ends of the capsule.

When a magnetic field is generated in the vicinity of the reed switch sensor, the reeds become carriers of magnetic flux (within the magnetic circuit), essentially being charged by opposite magnetic poles, the overlapping ends in the glass capsule attract each other. When the reed blades (reeds) are drawn to each other the reed switch sensor actuates.

These are hermetically sealed sensors, free from condemnation risks, thus useful in chemically aggressive, explosive environments, no electromagnetic discharge is effective on these sensors and these can create variety of flexible practical solutions, when accompanied with magnets and coils, however it may be a demand of specific use cases most of the time.

Unfortunately reed switch sensors are rather delicate and prone to breakage when in physically unstable environments, i.e. vibrating, harsh shock environments. Over time, mechanical reed switches also tend to wear out, which is why cell phone manufacturers had to replace these with Hall sensors for example.

Hall Effect sensors

In the presence of a magnetic field, the current (more precisely the charges that are carrying the current) are exposed to the Lorentz force , after which a charge is built up on one side of the conductor and on the other side the charge is decreased. The buildup of the charge results in Hall Voltage (difference between the electric field and voltage potential) that restricts the further buildup of the charge. The perpendicular components of the magnetic field and the bias current are proportional to the Hall Voltage (which also depends on the specific properties of the material property).

A Hall Effect Sensor is essentially a transducer, which, as a response to a magnetic field generated by a current flow, outputs a voltage. It is a beneficial system to monitor primary conductor current precisely, with no effect on the circuit.

Clear advantages of Hall effect sensors are their miniature size, which allows them to be placed in otherwise hardly accessible locations. Sure, there is additional electronics required for the clear detection, however a CPU can, as it is widely known, be miniaturized as well. Distance and mechanical changes however, are an essential specified part of these sensors, should there be a tiny misalignment when placed in the vicinity of a metal conductor for example, the accuracy of the measurements may greatly suffer.

What holds Magnetic Sensing back and what should the future hold?

When considering forces like external magnetic fields interfering and swaying the measurement of current flow (i.e. hall sensors), the temperature effects on the electrical resistance, the sensitivity ranges of these sensors, temperature/force resistance levels, the prices of i.e. MEMS sensors, compared to the durability of magnetic sensing systems that are commercially available nowadays – we willingly or not uncover the Achilles' heel for magnetic sensing.

A sensing system that incorporates magnetic principles without compromising the size, durability, cost, and overcoming the negative effects of external magnetic/electric fields, hazardous, alkaline, extreme temperature environments, adding to qualities like contactless, real-time measurements; the RVmagnetics MicroWire sensor.

MicroWire: Longevity, Sensitivity, Ease of implementation

The sensors: these glass-coated sensors commonly use soft magnetic materials – meaning they are easily magnetized and demagnetized – but also metals that exhibit magnetostriction, meaning they change their shape when subjected to a magnetic field. The sensing can happen through the magnetic field from inside almost any material.

The MicroWire sensor measurements have no hysteresis (unlike for example Hall sensors), it is linear (see on the diagram) from the principle of the measurement.

Linearity is valid up to the highest value of the range. The range can be adjusted to almost any value since the feature of MicroWire’s res­ponse (no hysteresis/li­nearity) works from very low to very high fields.

The sensing: sensing is contactless. It is important to mention that the MicroWire is a passive element with neither wiring attached to it nor data being gathered in it. The data itself is gathered through the sensing head and the electronics. The sensing head consists of two copper coils (excitation coil gives out the magnetic field, sensing coil “gathers” the MicroWires response in the local physical environment).

RVmagnetics technology employs switching between two stable magnetic states to sense different parameters. The technical solution allows. This data is gathered on a Central Processing Unit (CPU) which also acts as an AD converter.

The pure added value of increased longevity, saved expenses, durability, ease of production, etc, is of course clear to be able to choose this system over other magnetic sensors.

However, we must recognize that this may not be true for all of the applications, as, in the end of the day there is a wise saying “if it ain’t broke, don’t fix it”, and if your application strives by this, than a sensor providing a real-time data from otherwise impossibly inaccessible locations may not add much to your operations.Ne­vertheless, it is clearly a system which, if implemented in the right places, will solve issues that may bend whole economical perspectives into a more positive route.

Take for instance the Electric Engines – is there a sensor that may provide real-time extremely precise data on vibrations gathered directly from the internal particles of the engine rather than calculated based on the more external behaviour of the engine (external and internal vibrations are not likely to be homogeneous, the same is true for temperature).

Out of the whole idea of the future of magnetic sensors and eventually of this example on electric motors, we can conclude that RVmagnetics enables reaching the real data over having to conclude it.