铁磁靶磁性磁性磁性磁性对后偏置传感器输出的影响
铁磁靶磁性磁性磁性磁性对后偏置传感器输出的影响
由Yannick Vuillermet，
亚博棋牌游戏Allegro MicroSystems欧洲有限公司
介绍
本申请说明旨在描述对Allegro反向偏置磁传感器输出的目标相对磁导率影响。
传感器性能在目标机械几何形状上高度取决于高度。在速度应用的情况下，牙齿和谷几何形状是关键亚博尊贵会员的 - 但这些机械性能不是本申请说明的主题。在这里，假设目标是为客户应用设计的精心设计。相反，本申请说明侧重于目标铁磁性材料特性，尤其是磁导率。
The practical goal of this application note is to define the minimum target material relative permeability to guarantee optimum sensor performance in the application. This application note applies to any applications using a back-biased sensor associated with a ferromagnetic target: speed sensors (cam, crank, transmission, etc.), position sensors (linear, angle, etc.), etc.
铁磁材料特性
当倾向于在放置在外部场（从永磁体，从线圈中的电流从地球场等）时，倾向于获得磁化的材料是铁磁性的。在铁磁材料中，材料磁化与所得到的内部场对齐。与永磁体相反，铁磁材料的剩余磁化时非常小，当没有
应用外部场。
Figure 1 is a simplified way of representing the above properties. In this figure, it is assumed that the material behavior is purely linear in low field and that there is no hysteresis (this is equivalent to no remanent magnetization here).H是磁场，j是磁极化，js是饱和时的极化，以及μ是磁性渗透性。磁极化j与磁化相关联mwith this relationship:
J =μ0×m（1）
The relative permeability is defined as the permeability of the material versus the permeability of free spaceμ0.：
在下文中，假设材料仅用于线性范围。此假设在Allegro传感器目标的大多数应用程序中都是完全有效的。亚博尊贵会员在这种线性条件下，μ – μ0.是斜坡J（h）曲线，和：
b =μ.0.× μr × H（3）
因此,唯一的磁参数,垫ters for the target material is the relative permeability,μr.。基本上，渗透率表示由外部场磁化的材料能力。
Figure 2 shows the measured data of steel1010, which is a classical material used in combination with Allegro sensors. It appears that the relative permeability of this material is always larger than 600 in the linear range of the material, that is to say, forh <1000 a / m。
This 1000 A/m field value in the material, equivalent to ~12.5 Oe (oersted)—and which looks very small—must not be compared to the magnetic field in air, for example, produced by the back-biased magnet. A magnet can easily produce B-field of a few hundred gauss in air. However, a ferromagnetic material placed in this large B-field will have a much smaller internal H-field. As an example, for a magnet producing a 600 G field in air, a ferromagnetic material which has a relative permeability of 300 will typically only see a 5 Oe (or ~400A/m) H-field, according to equations 3 and 4, and to a typical form factor of 0.4 (see next section). This behavior is due to the demagnetizing field or, otherwise said, from the field that the material generates on itself. In summary, it is important to keep in mind that a large field in air from the back-biased magnet (few hundred gauss) does not necessarily imply that the ferromagnetic material works in its nonlinear mode.
This table gives the magnetic relative permeability of some common materials.
| 材料 | 磁性相对渗透率 |
| Air | 1 |
| Copper | 1 |
| 钕磁铁 | 1.05 |
| 钢* | 1至4000 |
| Permalloy | 8,000 |
| μ-metal | > 20,000 |
资料来源：https://en.wikipedia.org/wiki/Permeability_(electromagnetism)
*请注意，一些钢变体不是磁性的，例如不锈钢。
渗透率与形状因素
铁磁性材料的磁化由两个主要参数驱动：磁导率和物体的形状（形状因子）。
下面通过一个非常简单的例子说明这两个参数是如何影响磁化的。
在椭圆体对象的情况下，磁化在材料内部是均匀的，无论施加到物体的均匀外部场。注意，该椭圆体可以被视为速度目标齿的非常粗略近似。
图3显示了放置在均匀场中的椭球体Ho沿着X和均匀的磁化j。
在这种情况下，假设没有材料磁饱和度，磁化由：
In this equation,Nx公司是椭圆体的形状因素X。此参数取决于椭圆形形状，始终低于1.伸长的物体Xdirection will have a smallNx公司(for exampleNx公司= 0.1）。特定情况是具有的球体Nx公司= 1/3.
图4显示了对象极化与少量形式因子的相对渗透率。显然似乎在外部场方向上伸长的物体更容易磁化。更有趣的是，人们可以注意到，高于给定水平的渗透率，物体极化仅取决于物体形状。这在1 /（μr.–1）相对于形状系数Nx变得可以忽略不计。
Figure 5 shows the same plot but with normalized polarization to better see the permeability level. It appears that, whatever the object shape, at least 95% of the maximum magnetization is reached as soon as the relative permeability is larger than 300.
此数字将在现实应用中的下一段中确认。
典型应用示例：带ATS699LSN速度传感器的Allegro 60X参考目标
现在，考虑使用典型的速度应用程序ATS699LSN公司transmission part placed in front of the Allegro 60X reference target (Figure 6). ATS699LSN is a differential part which has three Hall plates (Left, Center, and Right) and two differential channels (Left-Center and Center-Right). The output of only one channel is considered in the following.
此零件的典型工作气隙为1 mm和2 mm，气隙由传感器的标记面和目标齿顶部之间的距离定义。
Figure 7 gives the normalized output of one channel when the target is passing in front of the sensor over one and half period. This graph shows that the differential field waveform is almost not dependent on the relative magnetic permeability. It can be observed that there is only a (small) difference between the waveforms whenμr.=10，位置在3左右°. 位置在0左右° 无论相对渗透率是多少，都有类似的行为，因为这些位置对应于目标的一个谷。
Figure 8 and Figure 9 give the peak-to-peak differential field of the channel versus relative permeability at 1 mm and 2 mm air gap respectively. These figures confirm what was seen earlier: to guarantee optimum performances, the target material relative permeability should be at least 300. Any further increase of relative permeability has a marginal impact on the magnetic signal measured by the sensor.
If the ferromagnetic target material has a relative permeability smaller than 300, it does not mean that the back-bias arrangement will not work. It will only work with degraded performance
与渗透率大的靶相比。例如，可以减小应用的最大工作气隙。
结论
最后，这个应用程序说明了这个问题的简单答案，“我的目标材料是适合反向偏见的应用吗？”：为了具有最佳性能，目标材料的磁相对渗透性必须至少为300-Field <2000 A / m。
但是，这是必要但不是充分的条件;具有适当的目标机械设计也是强制性的，以实现应用所需的性能。
Allegro工程师可以帮助评估目标的材料是否适应背部偏置的布置。如果该材料具有较低的相对渗透性，则Allegro还可以提供支持来估计对应用程序性能的影响。