Understanding the "secrets" of Hall effect sensors in one article

Hall effect switch and instrument level傳感器It is becoming increasingly popular in industrial applications, and now product and manufacturing process designers can choose various highly integrated Hall effect devices. Although there are still many confusions regarding the required specifications and magnetic field measurements, these devices have been proven to be quite easy to apply.

In terms of usage, only temperature sensors have a slight advantage, but Hall effect sensors have also been used in a wide range of devices in domestic and commercial applications, including DVDs, CDs, memory drives, automatic toys, mobile phones, car compasses, and car ignition systems. You can also see their presence in linear, industrial rotating equipment, position detectors, and military/aviation equipment.

Manufacturing and testing engineers use various types of discrete Hall effect sensors and instruments to provide product information and monitor manufacturing process steps. Although there may be some overlap in measurement functionality with other types of sensors and instruments, Hall effect sensors are clearly the best choice for certain types of measurements, and in some cases, there may not be other types of testing equipment that can provide the required data, including measurements of DC current values, rotational position, gap, surface or leakage magnetic field values. The history section of Hall effect sensors provides some background knowledge about these sensors.

The working principle of Hall effect sensor

When a magnetic field passing through a material at a certain angle affects the current flowing in the material, a Hall voltage is generated. A Hall chip is typically a rectangular semiconductor material used as an active element or "active region" to generate Hall voltage (Figure 1). The Hall sensor has a given length l, width w, and thickness t.

Figure 1: Hall voltage can be generated and measured using a DC magnetic field.

Measure Hall voltage

For the magnetic flux vector orthogonal to the Hall plate, the maximum Hall voltage VH is the product of the Hall plate magnetic field sensitivity γ B and the magnetic flux density B, that is:

VH = γBB

This is the maximum Hall voltage that can be measured on a Hall sensor. When the surface of the Hall sensor is not orthogonal to the magnetic flux vector but forms an angle θ, the Hall voltage VH is equal to:

VH = γBB × sinθ

The current I flows through a Hall sensor with a length of l. The current flows between the contacts Ic (+) and Ic (-). The magnetic field is in the z-direction, which means it is orthogonal to the Hall plane. The force exerted by a magnetic field is called the Lorentz force, which forces charge carriers (holes or electrons) to move along the illustrated line curve towards the edge of the Hall plate. This force is a coefficient of carrier velocity and magnetic field strength. The Hall voltage measured between the contacts VH (+) and VH (-) of the material with a width of w is proportional to the flux density of the magnetic field.

Instrument configuration

The supporting equipment for Hall effect sensors includes a current source for providing current Ic and a voltmeter for testing the Hall voltage between contacts VH (+) and VH (-). Some schemes also use load resistance RL for voltage measurement, as shown in Figure 2. Many types of Hall effect instruments provide a certain part of the supporting circuit as an organic component of the measurement system. The voltage leads from contacts VH (+) and VH (-) can be directly connected to a high impedance voltmeter for reading, or connected to other circuits for amplification, adjustment, and processing. (More complex systems using AC sources and lock-in amplifiers can also be used, but it is not within the scope of this article.)

Figure 2: Typical configuration of Hall generator used in the instrument.

application

In industrial environments, Hall effect devices generally serve one of the following two main applications:

● Measure magnetic field strength

● Detect the proximity, position, and rotation parameters of moving objects

The following text will discuss each application and provide some tips for efficiently using Hall effect devices.

Instrument level sensors for magnetic field measurement

When an industrial application requires precise or certified magnetic field measurement, instrument grade Hall effect devices are often used. Some common instrument level applications include electromagnetic field control, semiconductor ion implantation beam control, incoming inspection of magnets or magnetic components, online magnetization confirmation, magnetic field mapping, current detection, and continuous magnetic field exposure monitoring. As an alternative to many of these measurements, commercial Gaussian meters can be used. However, in practical applications, physical or cost constraints often require the use of discrete Hall sensors and commercial electronic devices.

Instrument grade Hall device users typically desire precise values of magnetic fields in a space or gap, or from a surface. According to the measured spatial characteristics, it is necessary to use appropriate installation methods to place and maintain the detection components.

Typical Hall effect sensors typically come in two configurations: transverse or axial (Figure 3). Lateral sensors are generally thin rectangular shapes designed for magnetic circuit gap measurement, surface measurement, and open magnetic field measurement. Axial sensors are generally cylindrical and used for measuring the center hole of annular magnets, measuring the magnetic field of solenoids, detecting surface magnetic fields, and detecting ordinary magnetic fields.

Figure 3: Basic Geometry of Lateral and Axial Hall Sensors

Practical considerations

High quality sensors can provide high precision, excellent linearity, and low temperature coefficient. Suitable probes for specific measurements and instruments can usually be purchased, and manufacturers will provide certified calibration data.

Some important practical considerations for instrument level Hall effect sensors include:

Accuracy. Designers must determine the required accuracy for specific measurements. A reading accuracy of 1.0% to 2.0% can be achieved without signal conditioning. In many applications, microprocessor calibration can achieve an accuracy of 0.4%.

Angle. As mentioned earlier, the output of the Hall sensor is a sine function of the angle θ between the Hall plate and the magnetic field vector. When the magnetic field vector is perpendicular to the device plane (sin90 °=1.0), the output reaches its maximum value, and when the magnetic field vector is equal to the sensor plane, the output reaches its minimum value (close to 0). Manufacturers will calibrate the Hall sensor at maximum output, so it is necessary to consider the angle error of the testing fixture or probe.

Temperature. Many sensor solutions can support a wide range of temperature and magnetic fields. Instrument level sensors support a temperature range from 1.5K (-271 ° C) to 448K (+175 ° C) and a magnetic field range from 0.1 Gauss to 300000 Gauss. Hall sensors have two temperature coefficients: one is used for magnetic field sensitivity (calibration), and the other is related to deviation (zero) changes. The effect of temperature on calibration is a percentage of reading error, while the zero effect is a fixed magnetic field value error that depends on temperature. Deviation changes are more important in low magnetic field readings (less than 100 Gauss). Technicians should carefully study the two temperature coefficient indicators provided by the manufacturer, and then determine whether a specific application can maintain the desired accuracy within the target temperature range.

Input current limit. It is recommended that designers understand the required input current value and be careful not to exceed the specified maximum value. Remember, under normal circumstances, Hall effect devices are calibrated at a certain current value. Any deviation from the calibration current will alter the output of the sensor. However, this is also a feature that can be utilized. As long as it does not exceed the maximum current value, the current doubling output will also double accordingly.

As mentioned earlier, the basic instrument level Hall sensor is a low resistance material with four electrical contacts. The input and output circuits are not isolated from each other, so you must avoid using common connections in the input and output circuits. To meet this requirement, you can use an isolated current source or output differential input amplifier.

Alternative solutions for sensor installation

In some measurement applications, using standard probes is impractical or undesirable. On the contrary, Hall effect sensors are directly installed on mechanical components. The design of customized sensor installation methods is beyond the scope of this article. Here are some general guidelines that are useful in customization:

Fragility. Hall sensors are particularly fragile and easily damaged by bending stress. Therefore, it is necessary to avoid the contact of Hall plates with surfaces or devices that apply direct pressure. In some applications, non-conductive ceramics or other insulating materials are used as interface pads.

Binding. Careful selection of binding adhesive is necessary to avoid adding stress to the sensor. When the temperature change does not exceed room temperature ± 10 ℃, ordinary epoxy (such as 5-minute air drying type) is good. It is generally not recommended to seal the tank unless it is in highly corrosive environmental conditions. Other binding methods can also be used to reduce the stress on the sensor leads, such as binding them to the mounting substrate.

The cavity for processing. These cavities can be used for axial or transverse Hall sensors, with the top of the sensor recessed beneath the surface to help prevent pressure contact or wear.

Test tube installation. The test tube installation method (Figure 4) can be used to protect the axial Hall sensor.

The recommended method is to choose the most robust sensor for any customized installation application. Units encapsulated with ceramics or phenol are generally the most durable.

Figure 4: Axial sensors can be installed inside test tubes, where the sensors can be exposed or recessed in the cavity for protection. Lateral sensors are generally installed in recessed areas.

Integrated proximity and rotation sensors

Hall effect sensors have been widely used in various linear proximity detection devices to respond to changes in the magnetic field of the proximity device. For example, the detected magnetic pole may be close to the sensor perpendicular to the Hall sensor, or the magnet may pass through the plane of the sensor. This movement will cause a change in the generated voltage. The additional integrated circuit converts the Hall voltage into a significantly larger digital compatible signal.

Angle detection, rotation, and speed detection use the same Hall effect principle to test the repetitive physical changes in position. For rotation, speed, or angle sensors, the magnetic pole is connected to a rotating object, such as a motor shaft, and the Hall sensor is stationary. The well-known applications of angular coordinates include detecting the commutation of brushless DC motors and the rotation angle of engine crankshafts.

Various types of devices used for proximity, rotation, and current detection are some form of Hall effect "switches" that are triggered by Hall effect outputs and then fed into other integrated electronic circuits. This switch provides binary high and low outputs based on the detected magnetic field value or the nearest magnetic field value and polarity. When combined with a current carrying coil, Hall effect switches can also provide current value detection for overcurrent circuit breakers.

Switch working mode

There are three main types of work:

Bipolar Hall switch: It requires both the south and north poles to be above the specified amplitude in order to change state, also known as a lockout switch.

Single pole forward Hall switch: requires one pole. Change the state (low or high) based on whether the forward flux density is greater than a certain amplitude or less than a minimum value (usually without a magnetic field).

Single pole negative Hall switch: requires one pole. Change the state (high or low) based on whether the negative energy density amplitude is greater than a certain value or less than the minimum value (i.e. no magnetic field).

The magnetic field in which the Hall sensor is located determines the output state. The signal from the Hall effect detector is detected, amplified, and then used to control the solid-state switching element at the output terminal. The connection to external logic and control components (such as CMOS or TTL circuits) is a standard connection with an external pull-up resistor. Due to mass production, integrated Hall effect devices (Figure 5) typically have low costs.

Figure 5: Simplified schematic diagram of integrated Hall effect device.

The most commonly used packaging types are surface mount or compatible with printed circuit board lead types (Figure 6). The positive and negative magnetic field directions related to sensor packaging are defined in the specifications provided by the manufacturer.

Figure 6: Packaging types of Hall effect sensors.

To make these devices more useful in applications, please remember:

When precise magnetic field readings are required, instrument grade devices should be selected. It is best to use an integrated "switch" for proximity detection (angle or linear).

● Understand important parameters such as magnetic field amplitude, AC or DC magnetic field, AC frequency, temperature range, and external noise (magnetic or electrical noise)

● Choose more robust packaging as much as possible

If you are planning to use permanent magnets, please seek assistance from the magnet manufacturer.

History of Hall Effect Sensors

Since Dr. Edwin H. Hall's experiment with a piece of gold foil in 1879, knowledge of the Hall effect has been widely spread. Although the development of modern sensors has taken a lot of time and effort from scientists and engineers around the world, Hawking's development has played a role in sparking further discussions. Selecting appropriate materials is one of the reasons for the delay. Before the mid-1950s, bismuth was the best practical material for sensor development. Although still not ideal, bismuth can provide sufficient Hall voltage and stability, making it suitable for use as a sensor in devices such as electromagnetic field controllers.

During the 1940s, there was a breakthrough in materials science, when III-V semiconductors were the main research topic in the Soviet Union. Scientists from Siemens AG in Germany first realized that the newly discovered properties of these compounds could make excellent Hall effect devices (Hall generators).

This type of semiconductor has high carrier mobility and high resistivity required for Hall effect applications, and exhibits excellent stability under variable temperature conditions. By the late 1950s, researchers in Ohio, USA, had discovered the unique properties of indium arsenide and indium antimonide, giving rise to several companies producing products based on the Hall effect. As an instrument level sensor, the performance of indium arsenide devices in terms of stability, low noise, and minimum temperature system has not yet been surpassed by other materials.

For many years, integrated circuit manufacturers have been committed to providing silicon Hall effect devices to the market. Their large-scale production facilities and the ability to add other circuits to sensors bring hope for low-cost and highly versatile devices. By the late 1970s, silicon Hall effect switches had made significant progress. The addition of Schmitt triggers and output transistors has brought highly influential devices to the industry, which can provide large output changes related to the presence or absence of magnetic fields. However, there are still some issues in obtaining accurate and reproducible results. The measurement results are usually affected by high temperature coefficients and variable switch calibration. It was not until the 1980s that modern calibration and compensation circuits enabled today's integrated sensors to reach a considerable level of performance.

For more technical information, please contact the technical personnel of Weikewei:

Li Sheng 18576410868 (same WeChat account)