与Allegro位置传感器IC的操纵杆

与Allegro位置传感器IC的操纵杆

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By Christophe Lutz and Andrea Foletto,
Allegro MicroSystems Europe Ltd

介绍

Joysticks are widely used human–machine interfaces (HMI)that simultaneously report information on direction and amplitude. Stick tracking is realized by use of a magnet and a magnetic
位置传感器。

This document explains how to implement a 2D or 3D magnetic sensor to obtain a joystick with a well-defined behavior. This note provides insights on two tracking methods:direct trackingandratio tracking。Direct tracking offers a straightforward implementation, while ratio tracking offers excellent robustness to stick mechanical play. Finally, this application note assesses the relative robustness of these techniques to parameter variations (mounting and in-life).

操纵杆描述

Mechanically, a joystick consists of a stick that pivots through a ball joint on its base. Figure 1 provides a cross-sectional view of a joystick.

为了追踪杆的位置，磁铁集成在球的底部，以使球和磁体随着杆被致动时移动。磁性位置传感器应放置在磁铁下方合适的距离，表示为air gap。

Stick Tracking

Action on the stick of a joystick will affect the magnetic field as sensed by the sensor. In this application note, the magnetization of the magnet is axial and pointing down (south pole up, north pole down). The information of stick position is contained in the sensed magnetic field in x and y directions, as indicated in Figure 2.

增加杆的倾斜度增加了感测信号，因为面内磁场分量增加。专注于操纵杆对倾斜的响应，θ., it is convenient to exclude directional information.

The stick position point in the position plot (represented by a black dot) is expected to move according to the tilt angle and in the same direction as the stick. Responsivity,Resp, shall be considered the distance from the stick position point to the center, as shown in Figure 3 and expressed as:

rcan represent x (when φ = 0°) or y (when φ = 90°) or any combination of both when the direction is arbitrary. The responsivity of the stick position point in the position plot is defined as:

在实践中，响应性也取决于杆，φ的取向，但通常可以将这种依赖性排除在诸如气隙之类的其他参数之外。

作为将在下一节中,响应nsivity is closely related to the distance from the magnet to the sensor since it can exacerbate or dampen magnet border effect, short-scale
不对称等。这个距离是通常被称为ir gap (AG).

对于操纵杆应用，空气隙无倾亚博尊贵会员斜，θ.= 0°.

气隙Constraints

Air gap as defined in Figure 2 is a key parameter in the application that will both affect the selection of sensor and the final responsivity of the stick. This parameter must comply with the following mechanical and magnetic constraints.

机械约束将为不嵌入操纵杆的球中的圆柱形磁体提供下限的气隙。该约束确保旋转之间没有接触
part and the sensor.

通过考虑图4中的极限接触案，可以推导出最小气隙，AGMIN（机械）。

The mechanical lower bound should be considered when using a low sensitivity device.

磁铁ic constraints arise from the signal level requirements. The sensor is typically able to sense a given range of magnetic field without experiencing saturation. For correct behavior, it is important to ensure the sensor does not saturate during implementation. In practice, this non-saturation condition provides an additional constraint on air gap, AG_MIN（MAG），取决于传感器的灵敏度，磁体的形状和常离磁场，以及最大倾斜角，θ_最大限度。When considering a joystick consisting of a ball joint of 10 mm diameter, a cylindrical magnet of 1 T, diameter 5.4 mm, length 1 mm, and which can be tilted ofθ._最大限度= 25°, simulations lead to the minimum air gap values shown in Table 1.

Table 1: Joystick Magnetic Limitations on Air Gap

Sensing Range (G)	AGMIN(MAG)
	No Saturation on x/y	没有饱和z
±500	1.5毫米	2.1 mm
±1000	0.9毫米	1.1毫米
±2000.	0.5 mm	mechanically limited

Generally, for joysticks that use only small tilt angles (θ._最大限度≪ 25°), the non-saturation constraint is more restrictive on the z-axis with respect to the x/y axes. For this purpose, Allegro has developed sensors such as ALS31300 with a different sensing range on the z axis.

Since the air gap sets the level of the signal, it defines the signalto- noise ratio (SNR). The application defines a minimum value of SNR which consequently defines an upper bound to the air gap, AGMAX(MAG).

Note: A safety margin should be considered to ensure the air gap stays within its allowed range despite any parameter variation due to fabrication, lifetime drift, etc.

直接和比率杆跟踪

如前所述，杆位置信息包含在X和Y轴上的感测磁场中。

Direct stick tracking plots stick position by using the data sensed in x and y directly. The simplicity and general accuracy of this technique is sufficient for most applications. Its major drawback is its vulnerability to dynamic air gap variations that may occur during the lifetime of the product. This variation is typically from vertical play of the stick. For instance, pressing on the stick may cause the stick position point in the position plot to jump to another value. A dynamic air gap reduction will always lead to an increase of the magnetic field sensed.

To counter this unwanted effect, the ratio stick tracking technique can be implemented. The values sensed on x and y will more or less have the same change as the value sensed on z-axis when air gap varies. Thus, using x/z and y/z instead of solely x and y will significantly decrease air gap dependence. Although ratio stick tracking is more robust, it does influence the responsivity curves.

该转换只需重新缩放位置图（见图5）。直接杆跟踪包含的所有结果都可以通过X / Z（分别的Y / Z）代替X（分别Y）来直接地转换为比率杆跟踪。作为示例，距离杆位置点到位置绘图中心的距离变为：

Because of this, all joystick corrective behavior post-processing can be applied to both tracking methods. The difference in responsivity and relative robustness to variations differentiates the two tracking methods.

Responsivity of the Joystick

Responsivity of the joystick describes the correlation between the mechanical movements of the stick and its stick position point on the position plot as output by the sensor. The air gap affects this relationship.

为了解释气隙的效果，用于操纵杆的仿真，由直径为10 mm的球接头，圆柱形磁体为1 T，直径为5.4mm，长度为1mm，可以倾斜θ_最大限度= 25°, gives results as shown in Figure 6 and Figure 7 for direct and ratio stick tracking, respectively.

From Figure 6, the following properties of direct tracking can be deduced:

• 大气隙导致倾斜角度范围几乎线性响应。

• Low air gap leads to a joystick that responds linearly with small θ angles while the characteristic becomes nonlinear for large angles. This feature is interesting in application requiring both precision and range (high responsivity at high angle).

From Figure 7, the following properties of ratio tracking can be deduced:

• The effect of the air gap has been tremendously decreased as shown by the superposition of the curves.
• 无论气隙如何，操纵杆响应都是线性的，对于小θ角度，而特征变为大角度的非线性。此功能在需要精度和范围（高角度高响应度）的应用中很有趣。

The response curve nonlinearity is due mainly to the nonlinearity of the magnetic field with position and not to the sensing of the sensor. Nonlinearities can be neglected for small values of θ_{最大限度。}

Joystick Robustness to Variations

从先前考虑（约束和行为）寻址的空气隙，传感器的位置完全确定。

现在，两种跟踪技术可以根据稳健性对抗因子而面临：

• 安装精度
• Mechanical play

Due to physical limitations, the sensing elements of multi-axis position sensors cannot sense the magnetic field components at the exact same location. This tiny built-in asymmetry leads to different responses in different directions. Likewise, error plots may reflect this asymmetry.

The following parameters drifts have been considered:

• 传感器相对于杆轴移位。
• 磁铁displaced with respect to the stick axis.
• 相对于其参考值，气隙小或更大。

The error is quantified as the distance between the ideal and drifted position of the stick position point. To compare direct and ratio stick tracking techniques, their errors have been respectively expressed as a percentage of their full-scale (FS) values, namely r_最大限度and r_RATIO(MAX)。

从这些地块中，可以进行若干观察结果：

• 由于传感器位移，大的倾斜角将始终加剧错误。
• 比率跟踪对气隙变化更加坚固。
• Direct tracking is more robust to in-plane displacements than ratio tracking.

Table 2 summarizes the maximum error and describes qualitatively the repercussion of error on the position plot.

The sensor raw data can be post-processed to reduce systematic errors (due to sensor or magnet mounting), but will not prevent drift over lifetime (due to mechanical plays).

Error per unit displacement:

请注意，最大误差取决于最大倾斜角θmax和操纵杆的尺寸。

Table 2: Maximum errors due to parameters drifts, no post-processing

Error %FS/0.1 mm	Direct 追踪	比率 追踪	定性效果
气隙 0.1 mm in z	10.8	1.6	改变响应性
Sensor 0.1 mm in x 0.1 mm in y	7.0 7.0	16.5 16.5	Adds offset in position plot
磁铁 0.1 mm in x 0.1 mm in y	5.5 4.6	15.5 4.4	改变响应性; φSENSEDdrifts from φ

The previous table leads to the following total error for an optimally compensated joystick with stick vertical play much greater than horizontal plays:

Table 3: Maximum errors due to parameters drifts, with post-processing

错误，％fs	Direct Tracking	比率跟踪
气隙 0.1 mm in z	10.8 × vertical play	1.6× vertical play
Sensor 0.1 mm in x 0.1 mm in y	〜0 横向戏剧有限公司	〜0 横向戏剧有限公司
磁铁 0.1 mm in x 0.1 mm in y	〜0 横向戏剧有限公司	〜0 横向戏剧有限公司

Generally, the direct stick tracking method will exhibit sufficient immunity to misplacements during mounting, though control of air gap is required.

假设由于补偿后处理而导致的误差减少。一旦纠正了这种系统错误，系统只能由于机械播放而具有错误。在实践中，
操纵杆部分没有t likely to move from each other horizontally, e.g. the sensor location with respect to the stick axis will not vary during product’s life. What can change is the air gap value when
the user applies pressure on the stick either intentionally (“crouch”) or not. Ratio stick tracking is therefore desirable to dampen the air gap variation error and have a highly accurate joystick.

结论

The joystick is a device having a stick tracked by a magnetic sensor through a magnet attached to a ball joint.

Several joystick behaviors can be generated from the joystick structural characteristics (regardless of post-processing). As discussed, air gap will be a key parameter for linearity and signal
level. Air gap cannot be smaller than the threshold defined by both mechanical and magnetic properties.

已经提出了直接和比率杆跟踪技术;表4总结了其关键特性：

表4：跟踪方法对比表

追踪	Direct	比率
Position Plot	x, y	x/z, y/z
AG Min.	没有 saturation on x and y	没有 saturation on x,y and z
AG Max.	受到SNR的限制	受到SNR的限制
Linearity with Tilt	Improves at high AG	需要帖子 - Processing
精度和范围	Improves at low AG	全部AG
机械约束 Without Post- Processing	AG Control	Sensor and Magnet 放置
机械约束 With Post-Processing	Limit horizontal and 垂直戏剧	Limit horizontal plays
AG dependence	Yes	No

Generally, for an application that does not require extreme precision, a direct stick tracking method will be sufficient. To make a precision joystick, it might be necessary to use a ratio stick
tracking method with post-processing (if the mounting precision is not already sufficient). This option provides low air gap dependence and creates a joystick that is very precise and robust over lifetime.

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