Serie Precision labs: sensores magnéticos

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Hello. My name is Gloria Kim, and I am a product marketing engineer for Texas Instruments current and magnetic sensors. In this video, I will introduce the fundamentals of Hall effect sensing and the basics behind hall effect sensors. So what is the Hall effect? As shown in this picture, current flows from left to right through a conductor. And that means the electrons responsible for the current flow right to left.

An external magnetic field is applied, which needs to be in a perpendicular direction to the current for the Hall effect to happen. This magnetic field applies what's called the Lorentz force on those electrons in motion. And that force pushes them towards one side of the conductor. This causes a buildup of electrons on the bottom area, and that causes a voltage shown here.

As the magnetic field straightens, additional electrons build up on the bottom creating a larger voltage. There's always an equilibrium between the Lorentz force and the electrostatic charge force. Furthermore, the voltage that develops, which is called the Hall voltage, is directly proportional to the amount of current, and it's dependent on the geometry of the conductor. This voltage is usually in the microvolt range and will need to be amplified.

Primarily, in Hall effect applications, magnets are attached to objects that move. And the movement is sensed based on the Hall voltage that develops. Every magnet has two poles, a north and a south, and it creates a magnetic field as shown. Permanent magnets are created by applying a strong magnetic field to a ferromagnetic material. The three most common ferromagnetic elements are iron, nickel, and cobalt. The most common, permanent magnet materials on the market are neodymium-iron-boron, ferrite, aluminum-nickel-cobalt, and samarium-cobalt.

Neodymium is known for having the highest flux density and resiliency to being demagnetized. Ferrites are known for being low cost but have weak magnetic properties. Aluminum-nickel-cobalt has widespread use and has excellent temperature consistency. Samarium-cobalt is most similar to neodymium, but it has better temperature consistency and supports far higher temperatures. As temperature increases, all of these materials produce a reduced flux density. The appropriate shifts per material are shown. If temperature goes to high, all magnets will demagnetize at some point.

There are two main sources that generate magnetic force fields-- magnets and current. Magnetic flux refers to the total sum of magnetic field lines passing through a specified area. For example, the region shown in green has some amount of magnetic field passing through it. The unit for this is weber or maxwell.

As far as Hall sensors are concerned, the sum of total lines isn't important but rather the flux density. Magnetic flux density is the amount of flux per area. The unit for flux density is gauss or tesla. Tesla is the official SI unit, but gauss is also very commonly used. We usually talk in terms of millitesla for values involved here. And 10 gauss is equal to 1 millitesla.

Now there are basically three main concepts that you should know here. First, flux density is greatest at the surface of a magnet, and it reduces with distance. Second, at a given distance, physically large magnets will produce a larger flux density. And that's another way of saying that fall off rate for big magnets is less than for small magnets. Third, different magnet materials produce different flux densities. They have different amounts of change with temperature, and they have different coercivities, which means different materials are easier to demagnetize than others.

Determining the flux density that a given magnet produces given its size, shape, and composition at a distance can be challenging. Some magnet suppliers provide this information in the magnet data sheet. But if this information isn't provided, it can be determined in a few other ways. One, a simulation. Although magnetic fields are a three-dimensional vector problem, there are some free, simple simulators available that can quickly give you reasonable results.

Another way to determine flux density at a given distance from a given magnet is to measure it. There are are handheld instruments called gaussmeters or teslameters that provide a flux density readout. An alternative way of measuring it is to swap the Hall sensor in your system with a linear Hall sensor, which outputs an analog voltage proportional to flux density. Finally, you can use our magnetic field calculator on ti.com that will make this calculation for you.

The two main classes of Hall effect devices are discrete elements and integrated circuits, or ICs. Starting with discrete element shown on the left, these must be biased with a certain amount of current. The equivalent circuit is basically a resistor, and a differential voltage forms on the other two terminals when a magnetic field is present. The main benefit of discrete element is their low cost, but they also require external signal conditioning. Their characteristics also change a great deal over temperature, and elements are easy to damage.

ICs, on the other hand, have a Hall element integrated with thousands of transistors that will handle all of the analog signal conditioning. These devices are incredibly robust and reliable. For example, TI's Hall sensor ICs support a Vcc range of 2.5 volts to 38 volts. And they can also tolerate down to negative 22 volts, which is there for reverse battery production in vehicles. Also, the performance across temperature is very consistent.

And with the ICs, there are many different classes of devices, including those with analog versus digital outputs, those with serial interfaces, such as I squared C, and devices with low power modes. Amongst the various magnetic sensor types, there are three basic Hall effect position sensors-- Hall effect latches, Hall effect switches, and linear Hall effect sensors. Throughout this session, I will briefly introduce each of these Hall sensors.

A Hall effect latch indicates that most recently measured magnetic flux density. Alternating north and south poles are required to toggle latch devices. These are used in rotary applications, such as BLDC motor sensors and incremental encoding. Shown here is the output graph of a Hall effect latch.

Applying a south magnetic pole near the top of the device causes the output to drive low. A north pole causes the output to drive high. And the absence of a magnetic field causes the output to continue to drive the previous state whether low or high. For more information on considerations behind the common rotary encoding sensing technologies, check out our tech now at the following link.

Hall effect switches indicate the presence or absence of magnetic flux density compared to a defined threshold. Unipolar Hall effect switches only respond to this direction of magnetic field and have a BRP that's greater than zero. With most industry devices, a positive B field is defined as when a south pole is brought close to the top of the device.

This means that once a magnet has moved away, the sensor returns to its off state and brings the output to either a high voltage or high impedance, depending on the device output. It will have the same response if a north pole is held to the top of the device. Omnipolar Hall effect switches respond to both directions of flux. The main advantage of this is that it simplifies the assembly process for the magnet because north versus south does not need to be discerned.

Linear Hall sensors provide an output that's proportional to the magnetic flux density present. Most devices on the market have an analog output. Some have a pulse with modulated output, and some even have a built in ADC along with an I squared C interface to assess the sensor data. Below are output graphs of TI single access linear Hall effect sensor family.

The bipolar linear Hall sensor with analog output detects both north and south magnetic poles and outputs a voltage corresponding to the detected magnetic flux density. The unipolar linear Hall sensor with analog output responds linearly to the south magnetic pole incident to the top of the package. This enhances the resolvable accuracy by providing double the voltage range per change in magnetic field.

The bipolar linear Hall sensor with pulse width modulation output outputs a 2 kilohertz clock that varies its duty cycle, depending on the magnetic flux density it reads. This device is immune to voltage noise and enables more robust transmissions over distance, while also providing a constant signal to verify that the device is powered and operational.

SOT-23 and TO-92 packages are industry standard packages. Most Hall sensors are sensitive to magnetic fields that are perpendicular to the face of the package. One soldered onto the PCB, the two packages offer two different sensing directions. A correct magnet approach must be used for the sensor to detect the field. The correct and incorrect approaches are shown here. To find more magnetic position sensing technical resources and search products, visit ti.com/halleffect.