Hall-effect sensors measure fields and detect position

Transcription

1 Hall-effect sensors measure fields and detect position Jeffrey Dierker - March 06, 2014 Hall-effect switches and instrumentation-grade sensors have become more commonplace in industrial applications, with a wide range of Hall-effect devices now packaged for designers of products and manufacturing processes. While there is still some confusion about which specifications are needed and about magnetic field measurement in general, these devices have proved to be relatively straightforward to apply. Surpassed in number only by temperature sensors, Hall-effect sensors are found in a sizable range of devices in domestic and commercial applications. These devices include DVD, CD, and memory drives, automated toys, cell phones, auto compasses, and auto ignition systems. You'll also find them in linear, industrial rotary, and position transducers and military/aerospace equipment. Manufacturing and test engineers use various types of discrete Hall-effect sensors and instruments to provide product information and to monitor manufacturing process steps. While there may be some overlap with the measurement capabilities offered by other types of sensing and instrumentation, there are some types of measurement for which Hall effect sensing is clearly the best choice, and even a few instances where no other type of test equipment will provide the needed data. These include measurements on DC current levels, rotary position, and gap, surface, or leakage-field levels. Hall-effect sensors: A History provides some background on these sensors. Hall Effect Sensor Operation A Hall voltage is generated when a magnetic field directed through a sheet of material affects a current that flows within the material. The Hall plate is a generally rectangular sheet of semiconductor material that serves as the active component or "active area" for generating the Hall voltage (Figure 1). The Hall plate has a given length l, width w, and thickness t.

2 Figure 1. A Hall voltage can be generated and measured using a DC magnetic field. Measuring Hall Voltage For a magnetic flux vector orthogonal to the Hall plate, the maximum Hall voltage VH is simply the product of the magnetic sensitivity of the Hall plate γb and the magnetic field flux density B, that is: V H = γ B B This is the maximum Hall voltage value measured at the Hall plate. Where the Hall plate surface is not orthogonal to the magnetic flux vector but is at an angle, the Hall voltage VH is given by: V H = γ B B sinθ

3 A current I flows through the length l of the Hall plate. Current flow is between contacts Ic(+) and Ic(-). The magnetic field is in the z-direction, that is, orthogonal to the plane of the page. The force exerted by the magnetic field, termed a Lorentz force, urges the charge carriers (holes or electrons) toward the boundaries of the Hall plate along the curved lines shown. This force is a factor of the carrier velocity and the magnetic field strength. The resulting Hall voltage, measured across the width w of the material, between contacts V H (+) and V H (-), is proportional to the flux density of the magnetic field. Instrumentation Setup Support equipment for the Hall effect sensor includes a current source for providing current Ic and a voltmeter for measuring the Hall voltage between contacts V H (+) and V H (-). Some arrangements also employ a load resistor R L for the voltage measurement, as Figure 2 shows. Many types of Halleffect instruments provide some portion of this support circuitry as an integrated part of the measurement system. The voltage leads from contacts V H (+) and V H (-) can be connected directly to a high-impedance voltmeter for readout, or may be routed to other circuitry for amplification, conditioning, and processing. (More sophisticated systems using AC sources and lock-in amplifiers can be used, but are beyond the scope of this article.) Figure 2. Typical setup of the Hall generator used in instrumentation. Applications and instrumentation Applications In the industrial environment, Hall effect devices typically serve one of two main applications Measuring magnetic field magnitude; sensing proximity, position, and rotation of moving objects. We'll take a look at each of these uses and give some tips for using Hall effect devices effectively. Instrumentation Sensors for Magnetic Field Measurement When an industrial application requires accurate or certified magnetic field measurement, instrumentation Hall effect devices are often used. Some of the more common instrumentation applications include electromagnet field control, semiconductor ion implantation beam control, incoming inspection of magnets or magnetic parts, in-process magnetization confirmation, magnetic field mapping, current sensing, and continuous magnetic field exposure monitoring. As an alternative for a number of these measurements, a commercially available gaussmeter could be used. In practice, however, physical or cost restraints often dictate the use of a discrete Hall sensor and available electronic equipment.

4 The user of an instrumentation Hall device normally wants an accurate value for a magnetic field in a volume or gap, or from a surface. Depending on the spatial characteristics of the measurement, an appropriate mounting is used to hold and position the sensing element. Classical Hall effect sensors are normally supplied in transverse or axial configurations (Figure 3). The transverse sensor is generally thin and of rectangular shape, designed for measurement in magnetic circuit gaps, for surface measurement, and for open field measurements. The axial sensor is generally cylindrical and is used for measurements such as ring magnet center bore measurement, solenoid fields, surface field detection, and general field sensing. Figure 3. Basic geometries of transverse and axial Hall sensors. Practical considerations Good quality sensors offer high accuracy, excellent linearity, and low temperature coefficients. Appropriate probes for a particular measurement and instrument can often be purchased with certified calibration data provided by the manufacturer. A few of the more significant practical considerations for instrumentation Hall effect sensors are: Accuracy. The designer must decide the accuracy required for a particular measurement. 1.0% to 2.0% percent of reading accuracy is available without signal conditioning. 0.4% accuracy is achievable in many applications with microprocessor correction. Angle. As described earlier, the Hall sensor output changes as a sine function of the angle between the Hall plate and the magnetic field vector. The output is greatest when the field vector is perpendicular to the plane of the device (sin 90 degrees = 1.0) and minimum (approximately zero) when the field vector lies along the plane of the sensor. The manufacturer calibrates the Hall sensor at maximum output, so angularity errors for a test fixture or probe need to be considered. Temperature. Wide temperature and field operation is available for various sensor arrangements. Instrumentation sensors are available for use from 1.5K (-271 C) to 448K (+175 C), and from 0.1 gauss to 300,000 gauss. The Hall sensor exhibits two temperature coefficients: one for magnetic sensitivity (calibration), and the other as an offset (zero) change. The temperature effect on calibration is a percent of reading error and the zero effect is a fixed field value error depending on the temperature. The offset shift is of more importance at low magnetic field readings (below 100 gauss). The technician should study the manufacturer s specifications for both coefficients and determine whether desired accuracy can be maintained over temperature for a particular application. Input current limitations. The designer is advised to be aware of the input current value required and cautioned not to exceed the maximum values specified. Remember, the Hall effect device is normally calibrated at a certain current value. Any variation from calibration current changes the output of the sensor. However, this is also a characteristic to utilize; doubling the current doubles the output as long as maximum current is not exceeded. As noted earlier, the basic instrumentation Hall sensor is a sheet of low-resistance material with four

5 electrical contacts. The input and output circuits aren't isolated from one another, and you must avoid using a common connection in both the input and output circuits. To meet this requirement, you can use an isolated current source or differential-input amplification of the output. Mounting and roximity Sensor Mounting Alternatives In some measurement applications, the use of a standard probe is not practical or desirable. Instead, the Hall-effect sensor is directly mounted on a mechanical assembly. Design for customized sensor mounting is beyond the scope of this article. There are, however, some general pointers that can be of help when a custom arrangement is provided: Fragility. Hall sensors can be extremely fragile and can be damaged by bending stress. Avoid contact of the Hall plate with any surface or device that applies direct pressure. In some applications a non-conductive ceramic or other insulator is used as an interface plate. Bonding. Bonding adhesives must be selected carefully so they do not add stress to the sensor. General epoxies such as 5-minute curing-types are satisfactory when temperature does not vary more than ±10 C from room temperature. Potting is not suggested, except in cases of very corrosive ambient conditions. Bonding methods can also be used to provide strain relief for leads to the sensor, bonding them in place against the mounting substrate. Machined cavities. These can be used for axial or transverse Hall sensors, with the top of the sensor recessed below the surface to help prevent pressure contact or abrasion. Tube mounts. Tube mounts (Figure 4) can be used to protect axial Hall sensors. The recommended approach is to choose the most robust sensor possible for any custom mount application. Units in ceramic or phenolic packages are generally the most rugged. Figure 4. Axial sensors can be mounted in a tube mount with the sensor exposed, or recessed in a cavity mount for protection. Transverse sensors are typically mounted in a recess. Integrated Proximity and Rotation Sensors Hall-effect sensors have been adapted for use in a range of linear proximity detection devices, responding to changes in magnetic field near the device. For example, a detected magnetic pole might approach the sensor perpendicularly to the Hall plate or as the magnet passes along the plane

6 of the sensor. This movement causes the generated voltage to change. Additional integrated electronic circuitry converts the Hall voltage to a noticeably larger, digital compatible signal. Angular detection, rotation and speed detection use the same Hall-effect principles to measure recurring physical change in position. For a rotation, speed or angular sensor, the magnetic pole is attached to a rotating object, such as a motor shaft, with the Hall plate stationary. Well known applications of the angular position sensor include sensing for brushless DC motor commutation and engine crank shaft rotation angle. Each of these types of devices for proximity, rotation, and current sensing is in the form of a Hall effect "switch," triggered by the Hall effect output, that is then fed into other integrated electronic circuitry. The switch provides a binary high or low output depending on the value of the magnetic field sensed or the most recent magnetic field value and polarity. When combined with a current carrying coil, the Hall effect switch can also provide electrical current level detection for overcurrent circuit interrupters. Switch Operation Modes The three main types of operation offered are: Bipolar Hall Switch: Requires both north and south poles above specified magnitudes to change states and is considered a latching switch. Unipolar Positive Hall Switch: Requires one pole. Changes state (low or high) based on a positive flux density above a certain magnitude or below a minimum value (usually absence of a magnetic field). Unipolar Negative Hall Switch: Requires one pole. Changes state (high or low) based on a negative flux density magnitude greater than a certain value or less than minimum (i.e., absence of a magnetic field). The magnetic field at the Hall plate determines the output state. The signal from a Hall-effect detector is sensed, amplified, and used for controlling solid state switching components at the output. Connection to external logic and control components, such as CMOS or TTL circuitry, is standard, with external pull up resistors. Integrated Hall effect devices (Figure 5) are usually low cost due to mass production.

7 Figure 5. Simplified schematic for an integrated Hall effect device. The most common types of packages are surface mount or leaded printed wiring board compatible (Figure 6). Positive and negative magnetic field directions, related to the sensor packages, are defined by the manufacturer in their specifications. Figure 6. Types of Hall effect sensor packages. To make these devices more useful in an application, remember to:

8 Select instrumentation grade devices when accurate magnetic field readings are needed.plan on using integrated switches for proximity detection (angular or linear). Know the important parameters such as magnetic field magnitude, AC or DC field, AC frequency, temperature range, and external noise (magnetic or electrical). Choose a more robust package when possible. Consult a magnet manufacturer for help if a permanent magnet is to be used. Hall-effect sensors: A History Hall-effect sensors: A History Knowledge of the Hall effect has been around since Dr. Edwin H. Hall experimented with this behavior using a sheet of gold foil in His developments led to modern day sensors, though they took considerable time and effort by scientists and engineers around the world. Suitable materials were partly the cuase of the delay. Prior to the mid 1950s, bismuth was the best readily available material for sensor development. Though still not the ideal, bismuth provided sufficient Hall voltage and stability for use as a sensor in equipment such as electromagnet field controllers. A significant materials science breakthrough came during the 1940s when group III-V semiconductors were the subject of research in the Soviet Union. Scientists at Siemens Company in Germany were the first to recognize that the newly discovered characteristics of these compounds would make exceptional Hall-effect devices (Hall generators). Semiconductors in this class exhibited both the high carrier mobility and high resistivity needed for Hall effect utilization, along with a superior stability in variable temperatures. By the late 1950s, researchers in Ohio, United States became aware of the unique characteristics of indium arsenide and indium antimonide, and several companies were spawned from this research to produce products based on the Hall effect. The indium arsenide device is still unsurpassed as an instrumentation sensor due to its stability, low noise, and minimal temperature coefficient. For a number of years, integrated circuit manufacturers worked to offer silicon Hall effect devices to the market. Their high volume production facilities and the ability to add other circuitry to the sensor offered the hope of low cost, highly versatile devices. By the late 1970s, the silicon Hall effect switch was well developed. The addition of a Schmitt Trigger and output transistors offered industry a responsive device with a large output change related to the presence or absence of a magnetic field. Obtaining accurate and repeatable results presented some problems; measured results were typically affected by a high temperature coefficient and a variable switch calibration. It was not until the 1980s that modern calibration and compensation circuitry allowed the performance levels achieved by today s integrated sensors. Return to main article