MMA8451/2/3Q高通滤波与动态检测：去除静止误差的策略

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本应用笔记详细介绍了高通滤波数据和瞬态检测在MMA8451、2、3Q传感器中的应用。MMA8451和MMA8452Q配备了内置高通滤波器，能够有效地去除输出数据中的直流偏移和低频噪声。这种滤波器的截止频率可以根据输出数据速率（ODR）和过采样模式进行用户设置，以适应不同的应用场景，比如提高动态加速度测量的精度，减少误报。高通滤波器的作用在于筛选出高频信号，允许通过的是高于设定截止频率的加速度变化，从而更好地识别动态行为，如敲击、震动等。对于MMA8451，使用者可以选择将高通滤波后的数据通过去噪计数器进行处理，设定预设阈值，以确保事件发生的时间符合预设条件，这对于实时检测震动事件尤其有用。特别值得注意的是，MMA8453Q虽然具有高通滤波功能，但并不支持直接从滤波器读取数据，这意味着它的数据处理方式可能略有不同。然而，其内置的瞬态检测功能同样强大，能分析高通滤波数据，通过检测加速度的变化幅度来判断是否存在特定的运动状态或事件。该功能可用于触发中断，以便及时响应并记录震动的方向，这对于需要精确运动检测的应用非常关键。 MMA8451、2、3Q传感器的高通滤波和瞬态检测特性提供了高度灵活的信号处理能力，适用于需要区分静态与动态加速度变化的倾斜检测、游戏控制和振动监测等多种应用场景。通过合理的参数配置和算法设计，用户能够充分利用这些功能以实现高效准确的数据分析。

AN4071

Sensors

Freescale Semiconductor, Inc. 3

2.1 Output Data, Sample Rates and Dynamic Ranges of all Three Products

2.1.1 MMA8451Q

1. 14-bit data

2g (4096 counts/g = 0.25 mg/LSB) 4g (2048 counts/g = 0.5 mg/LSB) 8g (1024 counts/g = 1 mg/LSB)

2. 8-bit data

2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)

3. Embedded 32 sample FIFO (MMA8451Q)

2.1.2 MMA8452Q

1. 12-bit data

2g (1024 counts/g = 1 mg/LSB) 4g (512 counts/g = 2 mg/LSB) 8g (256 counts/g = 3.9 mg/LSB)

2. 8-bit data

2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)

2.1.3 MMA8453Q Note: No HPF Data

1. 10-bit data

2g (256 counts/g = 3.9 mg/LSB) 4g (128 counts/g = 7.8 mg/LSB) 8g (64 counts/g = 15.6 mg/LSB)

2. 8-bit data

2g (64 counts/g = 15.6 mg/LSB) 4g (32 counts/g = 31.25 mg/LSB) 8g (16 counts/g = 62.5 mg/LSB)

3.0 Comparison of Motion Detection and Transient Detection

The transient function (with HPF enabled) is different from the motion detection embedded function in MMA8451, 2, 3Q. Refer

to Freescale application note AN4070 for details on the motion detection. The intended use of these functions is different.

A. The embedded motion function triggers an interrupt when any of the enabled axes has exceeded a set acceleration

threshold.

• Motion detection is useful when the device may be tilted in an orie

ntation and the interest is to know if the

acceleration in one axis has crossed a specified threshold amount measured from 0g.The motion detection

analyzes change in threshold considering both static and dynamic acceleration.

B. The transient detection embedded function compares whether any o

f the enabled axes has exceeded a set change

in acceleration disregarding the static acceleration. The data that is compared to the threshold to be exceeded is

high-pass filtered data.

• Transient detection is useful wh

en the device may be tilted in an orientation (i.e., there is some non-zero reading

on one or more of X, Y and Z) and the interest is more on the dynamic acceleration crossing a specified threshold

amount in a short amount of time (frequency being the reciprocal of time) regardless of what the current value is.

Example

If a change in acceleration of 0.5g in any axe

s was the target for an application, then it would be challenging to use the em-

bedded motion detection function for detecting this change. The reason for this can be better understood with an example:

Assume that the device was oriented such that X = 0.24g, Y = 0.44g and Z = 0.82g; it would be challenging to detect a 0.5g

change in acceleration in any of the axes using the motion detect function since the new readings have to be outside the range

X = -0.26g to 0.74g, Y = -0.06g to 0.94g and Z = 0.32g to 1.32g. Because only a single threshold setting can be chosen for each

axes when using the motion detect embedded function and because of the need to detect acceleration outside a range, it would,

therefore, be seen as impossible to choose a threshold that would detect a 0.5 change in acceleration for any of the axes.

3.1 Use Case 1

Use Case 1 Transient: Unknown Orientation with Specific Acceleration Change for Detection

A use case example for transient detection may be when the device

containing the accelerometer is in an environment of an

unknown orientation or changing orientation and a small amplitude, but quick disturbance must be detected, e.g., when the de-

vice is accelerating through 0g to +1g to -1g and a subtle impact force (change in acceleration of 0.5g) is used as an indication

to wake the device or as a motion signature of an event. The accelerometer reading combines the gravity and the linear

acceleration reading. It is much simpler to decouple the two by eliminating the effect of gravity. Using the example above, assume

that the device was oriented such that at given time T, X = 0.24g, Y = 0.44g and Z = 0.82g. If transient detection is enabled on

all

3 axes with a threshold of 0.5g and an ODR of 100 Hz, any time that X goes outside of the range -0.26g to 0.74g within

T + 10 ms,

a transient detect interrupt would be generated. Eliminating the static acceleration on the device simplifies the condi-

tion to focus on the change in acceleration only, since the orientation is of no consequence in this situation. Figure 2 demonstrates

that the sensor detects both the static and the dynamic acceleration. Decoupling the static from the dynamic acceleration can

simplify the analysis.

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