Position and Level Sensing Using Hall-Effect Sensing Technology

Position and Level Sensing Using Hall-Effect Sensing Technology

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By Gary Pepka, Allegro MicroSystems, LLC

抽象的

Hall-effect (magnetic field) sensing applications have become practical recently through advancements in supporting technologies. This paper introduces Hall-effect technology, and then explores how it has been applied, in particular, differentiating between the primary types of Hall sensor ICs, and the highly differentiated range of sensing behaviors they can support. In addition, it explores some of the enabling technologies, such as advances in signal processing, that have made this technology so much more robust than in its earliest days. This allows the application of the extreme high-reliability benefit of contactless Hall applications in a broader range than ever before.

In addition to the improvements in supporting technologies, the Hall-effect devices themselves have advanced, contributing to the designs of complete solutions. These advances include power and space reduction, as well as integration of diagnostic and protection functions that allow Hall sensor ICs to provide the advanced data-driven features that are becoming more in demand in miniaturized portable consumer electronics, automobiles, and other growing industries.

Introduction

With the extensive variety of solutions available for position sensing and level sensing, designers can select optimum technologies and packages to meet their commercial and engineering goals. Of these solutions, Hall-effect technology, with its application of contactless magnetic sensing, provides exceptional value and reliability. This application note examines the benefits of Hall-effect technology and how the latest developments in these devices enhance position and level sensing results.

Hall Effect Benefits

There may be almost as many means of sensing position and level as there are applications requiring these functions. Inductive, capacitive, mechanical, magneto-resistive, Hall effect, and optical, to name just a few, are all viable sensing options and the list continues to expand. Yet for a designer, there always remains the same critical elements that need to be addressed and that inevitably mate the requirements of the application to the appropriate sensing technology.

Critical requirements, such as: cost, distance of travel (effective operating air gap), resolution, accuracy, and often times cost again, all need to be determined to effectively and efficiently select the proper sensing technology. Of course, constructing answers for each of these elements is not always a straightforward task. Here, though, the flexibility of Hall-effect sensing technology is most advantageous. High reliability, small size, production-viable cost, wide operating voltage ranges, variety of output options, and ease of implementation allow Hall-effect sensing technology to service applications in most every market.

Overview of Hall Technology

First, a brief tutorial on how Hall-effect technology works. Simply stated, the Hall effect, so named after Sir Edwin Hall and discovered in 1879, refers to the measurable voltage across a conductive material, for example silicon (Si) or gallium arsenide (GaAs), that occurs when an electric current flowing through a conductor is influenced by a magnetic field (see figure 1). This transverse force created by the magnetic field is known as the Lorentz force. Therefore, a Hall-effect device requires a magnetic field in order to actuate the device.

Figure 1. In the Hall effect, magnetic flux perpendicular to the flow of an electrical current results in a measurable voltage.

虽然今天很常见，但霍尔效应技术并没有真正开始在20世纪80年代获得批量接受。这是因为霍尔元素跨越电压的电压是微小的，并且可能受到外力的影响，例如温度和封装应力。如图2所示，除了使用片上的胶印取消技术之外，还包括扩增信号的能力的进步，该装置允许允许霍尔效应感测技术允许在极端环境条件下采用霍尔效应感测技术，如汽车欠罩应用。亚博尊贵会员此外，霍尔效应IC的“非接触”操作在致动和切换方面为用户提供了几乎无限的寿命。

Figure 2. Modern Hall-effect sensor ICs integrate signal conditioning and amplification techniques to make practical devices.

Hall Device Options

Further investigating the elements that require consideration for a position or level sensing application, Hall-effect ICs provide the designer with a multitude of features and variations, including either digital or analog output. The former option is optimal for sensing discrete positions, while the latter affords the user a relatively infinite number of positions for greater resolution. Some examples of applications requiring discrete position or level sensing are: automotive shift selectors, seat belt buckle switches, seat position sensors, cellular flip phones, brushless dc motor commutation, windshield wiper fluid reservoirs, and gas tanks, to name just a few. Due to its high reliability, Hall-effect technology is used to replace reed switches and mechanical switches in these applications.

大多数霍尔效应开关具有开路漏极的输出结构，提供低电阻，从而简化了大多数微处理器和其他数字电子设备的接口（阈值比较器，多路复用器，基本TTL门等）。典型的开漏输出，一旦切换“开”，霍尔效应装置的输出电压从高电平到低电平。据说，霍尔效应IC有丰富的变化，以便为血腥的位置和水平传感应用服务，每个都有自己的细微差别。亚博尊贵会员这些变化包括：微功耗，磁极无关的传感，用户可编程选项，双线电流源输出装置，用于感测亚铁靶的磁偏压和反相输出的功能。这些不能在一次坐着中充分讨论，并且为了本文的目的，重点将在标准设备上：其操作和应用程序使用。

标准大厅设备特点

There are three common variations of standard digital position and level sensor ICs: unipolar, latching, and bipolar. With unipolar switches the actuation is caused by a magnetic field of sufficient strength to turn the device "on." Typically B_south(B indicates magnetic flux density) must be greater than the magnetic operate point, B_OP, of the device in order to turn these devices on. Once the magnetic field is reduced below the magnetic release point, B_RP，设备的这些设备返回“关闭”状态。

Latching devices turn on in a manner similar to unipolar switches. However, latching devices can only be turned off (unlatched) when the device sees sufficient magnetic field strength of the opposite polarity, B_north.

Bipolar switches are akin to latching devices in that they use opposing magnetic polarities to turn on and off. But due to the high sensitivity of these devices, they cannot be guaranteed to operate as a latch. In some cases, bipolar switches can have switchpoints (B_OPand B_RP) that cause them to function as a standard unipolar switch or even as a negative switch (switching only in the presence of sufficient north magnetic polarity).

低分辨率应用亚博尊贵会员

An excellent example of an application that uses discrete position sensing is an automobile shift selector. In shift selectors there are commonly as few as five discrete positions (Park, Reverse, Neutral, Drive and Low). With a unipolar switch placed at each individual position (P, R, N, D, and L), each switch only turns on when the magnet in the shifter is moved directly adjacent to the switch, as shown in figure 3.

Figure 3. Hall devices can be used as proximity switches, matched 1-to-1 with sensed positions, or arrayed to provide additional sensing positions through analysis of magnetic cross talk using multiple devices.

Should the designer require additional positions, the spacing between the devices can be reduced to create "cross talk" between the devices. In this manner additional positions are obtained when the magnet is sufficiently close to two devices such that they are both turned on, thereby increasing the number of positions from, for example, five to nine. Simple Binary Coded Decimal (BCD) systems, or more advanced systems such as Gray code or Densely Packed Decimal (DPD), can be used to decode the logic and acquire positional information.

Similarly this tactic could be used to sense fluid levels in a tank by means of a flotation device with a magnet inside, as illustrated in figure 4. As the magnet floats up and down with the changes in the level of the fluid, discrete levels are determined by which sensor IC is in the on state.

图4.流体箱中的电平传感应用;球形浮子用纽扣磁铁在流体表面上骑在骑行，而霍尔设备和接线完全隔离在单独的室内。

High Resolution Applications

It can be seen very quickly from the shift selector example that discrete position or level sensing is ideal when only a few positions are required. This method of adding a device for each position very quickly becomes cost prohibitive and spatially challenging when the application requires finer resolution.

Enter the linear Hall-effect device with an analog output. Similar to the digital switches, there is an abundance of features available in linears; for example, ratiometric outputs, user programmability, digital outputs (such as PWM), and unidirectional or bidirectional sensing. Like in the preceding description of devices for discrete positions or levels, this discussion will concentrate only on standard linear Hall-effect sensor ICs: their means of operation and application uses.

Most standard linear Hall-effect sensor ICs have ratiometric outputs (0.5 × V_DD) that respond proportionately to magnetic field strength. These devices generally require a regulated 5.0 V supply and the QVO (quiescent voltage output, V_出（q）) is 2.5 V when there is no significant magnetic field present (see figure 5). The output voltage increases when sensing an increasing magnetic field from the south pole of a magnet, approaching 5.0 V. Conversely, the output voltage will decrease when sensing an increasing magnetic field from the north pole of magnet, approaching 0 V.

图5。采用霍尔线性器件throu回应ghout the range of sensed magnet flux, outputting a ratiometric analog signal.

There are two common configurations for applications of linear devices, which form the foundation for most designs. These techniques are termed slide-by and head-on.

Slide-By Configurations

在标准逐件施加中，磁体在封装的面上移动，使得霍尔元件感测一个或两个磁极，如图6所示。可以有效地有三个位置，电压输出为零：（a）在磁铁足够接近的情况下，例如由装置感测到的场，（b）磁极之间的零交叉（b = 0）直接与霍尔元件相邻，并且一旦磁体有通过该装置移动超过设备，即在元件处不再可检测到足够的场景。有效地，输出电压的变化为2.5到0 V（假设V._DD由于磁场的北极通过了包装的底部，并且在南极通过了包装的面部，从2.5到5.0 V.这通常标记为双向感测。

Figure 6. Slide-by application configuration and response curve, showing separate nodes for the peaks at the north pole and at the south pole.

当然也可以在设备上感测仅一个极点的变化，尽管这可能会限制可用范围。被称为单向感测，然后将输出的变化限制为仅为2.5V的标准线性。为了获得全方位的操作，必须使用此功能使用用户可编程线性。然后，可以使用从霍尔效应IC输出的电压的变化作为脸部的场发生变化来确定移动磁体的相对位置。然后可以采用标准微处理器上的A-TO-D转换器和简单的查找表来传达实际位置。在这种情况下，分辨率（可以检测的位置的数量）是关于A-TO-D转换器的解析能力的预测，但模拟信号提供了相对无限的位置。

An example of an application that can use slide-by sensing is valve position, diagrammed in figure 7. In this application often the magnet is a two-pole ring magnet that rotates in front (slides by the face) of the Hall-effect package. As the opposing magnetic fields pass in front of the element, the voltage output changes proportionately to the change in field strength. By means of precise sensing, the position of the valve can be controlled to dictate more accurately the flow of a substance through a carrier.

Figure 7. Valve position sensing is a proven application for slide-by Hall IC configurations.

Head-On Configurations

头部位置感测与逐个配置的单向传感非常相似。实质上，线性霍尔IC仅区分一个磁极的磁场强度的变化，这可以是北方或南极极性。检测图案是简单的。当磁体接近器件时，由IC检测的场增加，并且在移除磁体时，场强降低，如图8所示。

Figure 8. Head-on application configuration and response curve, showing a monotonic characteristic regardless of pole orientation.

Detecting the height of the deck on a treadmill illustrates well the uses of a head-on sensing technique. When the height of the deck is altered to change the gradient for the runner, a linear Hall IC can be used to detect the displacement of the deck. Typically the magnet is attached to the deck itself while the sensor assembly remains stationary. As the runner increases or decreases the gradient of the deck, the sensor IC provides feedback to the control module as to the relative displacement, by means of the change in field strength witnessed by the Hall element.

Determining Field Specifications

As with any technology, there are some specific considerations when designing an application using a Hall-effect sensor IC. Careful selection of the magnet is of the utmost importance, including shape and placement, as shown in figure 9. Magnetic field strength decreases exponentially over distance. Furthermore, magnets have temperature coefficients that need to be considered.

Therefore for discrete position sensing, it is always good practice to determine the effective air gap, from the face of the package to the magnet, at the required switching position, and then determine the maximum and minimum field strengths, over the rated temperature range, at that distance. This value should then be compared to the maximum rated operating switchpoint for each alternative device.

A chart and formula for estimating field degradation by effective air gap is provided in figure 10. This change can be calculated using the formula below:

where:

• Br = Residual Magnetic Inductance of the material, in G,
• L = Length of the magnet, in mm,
• X = Distance between the surface of the magnet and the device, in mm, and
• r =磁铁的半径，mm。

The chart reflects typical results for a button magnet, similar to that used for figure 9, composed of NdFe, rated at 30 MOe (oersted; 1 Oe = 100 microtesla, micro;T), with 2 mm radius, and 1 mm thickness.

设计师的拇指很好的规则是确定，在设备的所需位置，在最大额定切换点处需要比所需的现场强度更多。例如，如果使用B的单极开关是必需的_OP(max) of 50 G to turn on at a certain distance, then the field strength at that distance should be no less than 55 G under all conditions.

Designing Linear Applications

Unlike digital Hall-effect switches, which require only a certain strength and polarity of field in order to actuate, linear devices require a little more application specification in order to achieve satisfactory results. The gain of a linear IC determines the resolution at a given distance. Therefore regardless of whether the application is slide-by or head-on, one must select the appropriate gain.

In order to do this, two known end points and the required resolution (number of data points) must be established. The following is a brief example for determining the appropriate gain.

Assuming that the requirements for the application are as depicted in figure 11, the useable linear range would be 3 V. The full range as the magnet travels across the device would be 200 G (gauss; 10 G = 1 millitesla, mT). Dividing the change in output voltage, V_OUT应用领域的变化,Bapplied提供es the appropriate gain of the linear Hall-effect device for this application.

For greater clarity, here are the equations and the results for this example. The general equation is:

要使用示例数据，请先转换v_OUTfrom V to mV.

Then:

V_OUT= V_–V_着干活

= 4000 mV – 1000 mV

= 3000 mV (full linear range),

and

B_applied（g）= b_最大限度– B_min

= 100 G – (–100 G) = 200 G.

注意:algebraic convention applied is: positive values for B denote south polarity, and negative values for B denote north polarity.

Entering these into the general equation:

Gain (mV/G) = 3000 mV / 200 G

= 15 mV / g。

Of course, in real world applications the transfer functions are not perfectly linear and there can be an inherent offset in the system. For this reason, further consideration must be given to the accuracy required by the application, as well as the resolution capabilities of the A-to-D converter or similar device that must read the output, and the temperature coefficient of the magnet.

It is helpful in these situations to consider:

• The change in the quiescent output voltage as a function of temperature, V_出（q）(TA),
• The change in sensitivity (gain) as a function of temperature, V_sens(Q)（ta），和
• The linearity of the device over a given range of magnetic field strength.

Linear Hall-effect ICs can be back-biased with a magnetic field in order to sense ferrous targets. For example, Hall IC-based sensors are widely accepted in the automotive industry to accurately sense the position of cam lobes and the speed of crankshafts in engines, in order to improve timing and thereby grant more efficient consumption of fuel. The high bandwidth capability of many Hall-effect linears allows them to be used to sense changes in current for DC-to-DC converters and battery management systems in hybrid vehicles.

Summary

Obviously these are simplified examples of applications that can employ Hall-effect sensing, and very compressed descriptions of capabilities and features offered by this technology. Other interesting examples of important Hall technology options include:

• The current source outputs of two-wire devices are ideal for safety-critical applications, such as seat position and seat belt buckle sensors. This is because these devices output two distinct current levels to indicate the on and off states. Any output that deviates from these levels is a fault condition, affording the user with inherent diagnostics.
• Extremely low current draw (<5 W) permits Hall-effect ICs to be used in open/closed circuit sensors. This is particularly valuable in battery-operated applications that are sensitive to power loss, for example: cellular flip phones, laptop computers, and pagers.
• The flexibility of these sensor ICs is further enhanced by the assortment of package options. Some micro-leaded packages (MLP, also known as leadless DFN or QFN packages) are as small as 2.0 × 2.0 × 0.5 mm, while others are large enough to include a samarium cobalt magnet to back-bias the IC.

It is the myriad of applications that can be served by Hall-effect technology that drives the ever-increasing diversity of these devices. As a result, the technology continues to evolve. The ongoing reductions in size and continual increase in capabilities lend Hall technology to be a viable solution to almost any position or level sensing application.