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Application and Advantages of Allegro's ATS344LSP Magnetically Back-Biased Differential Linear Sensor IC

B.y Yannick Vuillermet,
Allegro MicroSystems Europe Ltd.

Introduction

This application note aims to give insight on typical use of the AllegroATS344LSP背部偏差差分线性传感器IC。该传感器的主要应用是测量线性运动，例如轴轴向位移。

For proper use, this sensor must be associated to a well designed moving ferromagnetic target. The back-bias arrangement and the differential sensing technique require a specific target shape to produce a useful magnetic signal.

The ATS344LSP includes a two-wire output interface and integrates a bypass capacitor into the package, which makes it suitable for decentralized sensing (typical in automotive applications) without the need for a printed circuit board.

The ATS344LSP offers unique performance benefits when compared to magnetic sensors generally used for linear position measurements.

In the following application note, the ATS344LSP sensing principle is described, advantages of its magnetic configuration are explained, and a typical user application is shown.

ATS344LSP Measurement Principle

The ATS344LSP comprises, in a single package, two Hall plates, HP₁and HP₂, separated by 3 mm, and a rare-earth magnet, positioned behind these sensing elements (see Figure 1).

**Figure 1: ATS344LSP Back-Bias Arrangement**

The magnet is magnetized along the y axis and both Hall plates measure the field strength along the y axis. The sensor measures the differential field ΔB = B₂- B.₁。B.₂是由HP测量的字段₂and B₁是由HP测量的字段₁。

In Figure 2, the ATS344LSP sensor is placed in front of a basic ferromagnetic target. As a reminder, a ferromagnetic material is a material that gets magnetized when placed in an external magnetic field. Ferromagnetic materials also tend to concentrate local magnetic field lines. Most steels are ferromagnetic.

在这种情况下，目标作为传感器背部磁体的结果获取磁化。该目标磁化产生了其自己的磁场，由霍尔板HP感测₁and HP₂。

霍尔板也看到来自磁铁的背景磁场（称为磁体基线）。然而，在理想情况下，在差分操作期间有利地减去磁体基线场。

B.ecause of the target shape in Figure 2, Hall plate 1 senses
more field than Hall plate 2: the differential field ΔB₁= B₂- B.₁is then negative and large.

在下文中，气隙被定义为目标到传感器的最近点与传感器封装的面部之间的距离（参见图2）。

Figure 2: Basic Principle in Front of a Ferromagnetic Target – Large Differential Field — Figure 2: Basic Principle in Front of a
铁磁靶 - 大差分场

When the target is moved to the left, as in Figure 3, the differential field ΔB₂is still negative but the difference is much smaller between B₁and B₂。The reason for this change in differential magnetic field is the nonlinear behavior between the measured magnetic field level on a single Hall plate and the distance of the sensor to the target.

This nonlinear function can be seen in Figure 4, which demonstrates the typical behavior (with arbitrary units) of the field sensed by a single Hall plate versus the distance between this Hall plate and a ferromagnetic target. This figure also gives in red the case of Figure 2 and in green the case of Figure 3.

Figure 3: Basic Principle in Front of a Ferromagnetic Target – Small Differential Field — Figure 3: Basic Principle in Front of a Ferromagnetic
Target – Small Differential Field

图4：典型的磁场行为与Distance to Target — 图4：典型的磁场行为与
到目标的距离

Consequently, the differential field ΔB sensed by the ATS344LSP is a direct measure of the unique position of the target (Figure 5).

Figure 5: Typical Differential Field versus Target Position – Based on Figure 2 System — Figure 5: Typical Differential Field versus Target
Position – Based on Figure 2 System

ATS344LSP与其他增效等优点etic Arrangements

ATS344LSP提供了一种独特而有利的方法，用于测量线性位移。下面描述用于测量线性位移的其他常见技术。

The first common technique uses a single field measurement (for example, a single Hall plate) in association with a zero-gauss (or 0 G) ring magnet (Figure 6). The zero-gauss magnet is a magnet
designed to have no field at the Hall plate position (i.e. magnet baseline is zero). The ring magnet is also magnetized along the y axis.

Zero-gauss magnets are used with single Hall plate ICs to limit the inaccuracy of the sensor that results from temperature variation (for example, a SmCo rare-earth magnet loses around 4% of
它的强度在150°C与20°C）相比。非零高斯磁体将具有高基线磁场，并且难以补偿该场的变化。

A corresponding Allegro IC for these types of linear displacement measurements would be, for example, the ATS341LSE.

The field sensed by the Hall plate of such zero-gauss systems is a nonlinear measurement of the distance between the sensor and the moving ferromagnetic target: the closer the target, the stronger
场。传感器响应如图4所示。

The main advantage of the 0 G arrangement is the simplicity of the concept. The drawbacks are mainly the expensive 0 G magnet (compared to a rectangular magnet) and the sensitivity to external
perturbing magnetic fields—any external field perturbation will be directly sensed by the single Hall plate. Note that it is also usually necessary to calibrate this type of sensor in the application to compensate for variations in the actual mounting air gap.

Figure 6: Cross-Sectional View of 0 G Magnet and Single Hall Plate Measurement — Figure 6: Cross-Sectional View of 0 G Magnet and
Single Hall Plate Measurement

用于测量线性位移的第二种常用技术使用安装在移动物体上的永磁体和能够测量角度的传感器
磁铁ic field generated by this magnet.

图7示出了该原理：移动磁铁沿X轴磁化。测量磁场角β并直接测量磁体位置。

Much more information on this principle can be found in the Allegro application note: “Linear Position Sensing Using Angle Sensor ICs” available on Allegro’s website. A corresponding Allegro IC for these types of linear displacement measurements would be, for example, theA1335。

The configuration in Figure 7 has a low sensitivity to air gap variations and, depending on the magnet design, is the only technique described in this application note that is able to reach large
air gaps (>4 mm) and long travel distances (>10 mm).

The main drawback of such a configuration is the need to mount the magnet on the moving object to be sensed in the system. The process of mounting the magnet is expensive, and there is always
the potential for the magnet to become displaced from the object.

另外，磁角度测量对外部扰动磁场敏感。

由于ATS3444LSP中使用的差分传感原理，该IC对外部磁场扰动非常不敏感。在IC中使用的差分处理电路自然地拒绝对霍尔板（即共模场）类似的扰动。ATS344LSP对两个大厅板上不同的扰动仍然敏感。例如，与SP封装引线平行的导线远离传感器40mm，携带500a，将产生在传感器输出上观察到的2g差分响应。但请注意，在这种情况下，单个或2D场测量值将感测25克变化。

ATS344LSP的差分测量技术还允许使用简单且经济高效的矩形磁体而不是复杂和昂贵的零高斯磁体。使用更简单的磁铁是可能的，因为磁体基线被ATS344LSP中的差分计算取消。

The use of a ferromagnetic target and an IC with an integrated back-bias magnet has many advantages, and there are also tradeoffs which must be considered. The main tradeoffs relate to the
operating air gap capability and the linear displacement sensing range of the IC. These parameters are limited by the size of the integrated magnet in Allegro SP package. For the SP package,
the typical maximum air gap is around 2 mm and the maximum sensed travel range is around 10 mm. In the case of a moving magnet technique, air gap capability and travel range can be much larger—at the cost of a very large and expensive magnet and reduced immunity to external perturbing fields.

在一些应用中，待亚博尊贵会员感测的移动物体是将线性移位的轴，但也可以围绕其轴线旋转。在这种情况下，移动磁铁方法需要磁铁
涵盖了完整的周长of the shaft. This would also lead to an excessively large and expensive magnet.

As already discussed, the use of the ATS344LSP and a steel target to measure linear displacement is often much easier and less expensive when compared to mounting a discrete magnet.

表1：不同应用架构进行线性位移测量的比较

	0 g偏见 and single measurement (ATS341LSE)	Moving magnet and 磁铁ic field angle measurement (A1335)	ATS344LSP back-biased differential measurement
Max air gap [mm]	≈2	>4*	≈2
Typical stroke length [mm]	≈10	Depends on moving magnet Up to tens of mm*	≈10
Typical accuracy	中等的	High*	中等的
Calibration inside application	Recommended	Can be avoided	Recommended
Immunity to external perturbing field	Low	Low	High
Magnet	Integrated Complex shape	Depends on application	Integrated Simple shape
Target	Ferromagnetic	Permanent 磁铁	Ferromagnetic
Target mounting	容易	Difficult	容易

* Having good air gap capability, long range, and/or good accuracy is always at the cost of a large and expensive moving magnet.

Data in Table 1 are typical values only. For more details regarding a specific application, contact a local Allegro engineer.

Typical Application Example

Note that all results below are derived from simulations, and may differ slightly from real world results.

In this example, the goal is to determine the position of a target (Figure 8). The target moves along the x axis.

**Figure 8: Global View of Moving Target**

To illustrate the performances of the ATS344LSP sensor, consider a typical application with the requirements below:

Static air gap: 1.35 ±0.45 mm
Dynamic air gap: ±0.05 mm
Temperature range: –40 to 150°C
Travel rangeR: 10 mm
2 point calibration is conducted by the user at the end points of the linear stroke: 10/90% PWM output expected at these positions

To have a proper input field range, a V-shaped target is used, which generates a bipolar differential field on the ATS344LSP sensor.

As indicated previously, the magnetic field does not decrease linearly with the applications air gap (Figure 4). Consequently, using a straight V-shape target (Figure 9) will intrinsically lead to
a nonlinear differential sensor output and to accuracy error. This error is called the target intrinsic nonlinearity.

However, target shape optimizations could compensate for this intrinsic nonlinearity. Indeed, the field tends to decrease very quickly at close air gaps and much more slowly at large air gaps. Therefore, a target having a larger slope in the middle of the V-shape (i.e. where the Hall plates actually sense a large air gap) could compensate for the nonlinear magnetic field behavior.

A proper target design must also account for other application parameters (dynamic air gap variation, for example) and the sensor IC errors (offset drift with temperature, sensitivity drift with
temperature, etc.).

Figure 10 shows a cross-sectional view of the optimum target for the application example. A target lengthLof 14 mm has been chosen to not only fit the travel range and the distance between
霍尔板（3毫米），但也有关于V形终点的边距。需要这种余量以避免V形区域外部的扁平区域的错误测量。这里已经采取了1毫米的余量。然后给出目标长度L：

L ≥ R + 4 mm

对于V形高度，建议在2到4毫米之间（3.5mm，如图10所示）。小于2毫米的高度会导致小差异字段，因此
higher position inaccuracy. A height larger than 4 mm would not increase the field substantially because the ferromagnetic material would be too far away from the sensor.

Figure 11 displays the differential field sensed by the ATS344LSP sensor in front of this optimum target versus target axial positionn and versus air gap. It can be seen that the differential field is linear at the nominal application air gap (1.35 mm) and at large air gaps, but deviates significantly at small air gaps. This is intentional: at small air gap the differential field sensed by the sensor is much higher (Figure 12) which makes the sensor much less sensitive to measurement errors (mainly IC offset drifts). Consequently, there is a compromise that must be made to obtain similar accuracy performance at small and large air gaps. At small air gaps, errors mostly come from intrinsic target nonlinearity, and at large air gaps, errors mostly come from sensor measurements errors.

**Figure 10: Cross-Sectional View of Application Target**

**Figure 11: Differential Field Sensed by ATS344LSP Sensor versus Target Position versus Air Gap**

图12：差分场峰值与峰值与峰值Air Gap on Full Travel — 图12：差分场峰值与峰值与峰值
Air Gap on Full Travel

现在，将评估该应用示例的预期的准确性。为了获得现实的值，进行了蒙特卡罗统计分析。在该模拟中，根据其统计分布规律，为不同的应用参数（例如，安装气隙和传感器偏移误差）进行建模成千上万的现实情况。对于这些情况中的每一个，评估传感器输出精度。

Results given are valid for the full IC temperature range and include sensor lifetime drift. The error reported here is the maximum position error for the full range of target displacement. The
offset drift over lifetime considered is ±12 G (based on the reduced temperature cycle testing that was performed on a similar product; this number will be confirmed by future testing on ATS344LSP).

以下机械分布假设for performing the Monte Carlo analysis:

Parameter	Distribution	Mean [mm]	标准 deviation [mm]
Mounting Air Gap	高斯	1.35	0.15
Max Dynamic Air Gap	高斯; 只有积极的保留值	0	0.05/3

Figure 13 shows the distribution of the maximum position error over the full travel range for all the simulation cases evaluated. It includes mounting air gap, dynamic air gap variation, temperature variation, sensor errors, and target intrinsic nonlinearity. Sensor errors include offset and sensitivity drift with temperature, offset and sensitivity lifetime drift, sensor resolution and nonlinearity. Note that % FS (% Full Scale) stands for the percentage of the full linear travel range.

The sensor is calibrated, after mounting in the application, such that the first end of the travel range returns 10% PWM and second end returns 90% PWM (see Figure 14).

平均误差约为4.9％FS，标准偏差约为1.3％FS。从错误分布分析，看起来大约3000ppm的样本具有最大误差
larger than 9.4% FS or 0.94 mm.

Although output linearization was not performed to compensate for intrinsic target nonlinearity, the final accuracy of the sensor is reasonably good.

**Figure 13: Maximum Error on Full Travel – Statistical Distribution**

Figure 14 shows, for one random simulation case, the expected envelope of the sensor output with respect to all varied parameters.

图15显示了典型的测量误差如何与安装气隙相比。正如所预期的那样，最小误差围绕标称空气隙，曲线大致对称
relative to the mounting air gap range (0.9 to 1.8 mm).

**Figure 14: Sensor Output Envelope Example**

**Figure 15: Typical Error versus Mounting Air Gap**

Conclusions

Allegro Microsystems ATS344LSP magnetically back-biased differential linear sensor ICs offer unique advantages when measuring linear stroke position of a target or shaft. When compared to conventional zero-gauss back-biased linear ICs, or to magnetic角度传感器IC.sensing a moving magnet, the ATS344LSP offers:

Elimination of magnets from the customer system
容易integration of a ferromagnetic target
Very low sensitivity to external perturbing fields

Consequently, the ATS344LSP is recommended for use:

在苛刻的磁环境中，
to simplify target mounting (cost reduction),
to improve mechanical reliability of the target fixture in the application.

For more details on howATS344LSPwould perform in a specific application,contact a local Allegro application engineer。