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Measuring Freefall using Freescale's MMA7360L 3-Axis AccelerometerIntroductionFreefall SensingFigure 1. Example of a 3-axis AccelerometerMMA7360L AccelerometerFigure 2. Pin Out of MMA7360LPin Out of MMA7360LFigure 3. 0g Detect Circuitry in the MMA7360L for Pin 9Equations of MotionTable 1. Calculated Time and Distance for a Linear FallLimitations of Linear Freefall Detection for MMA7360L0g Offset ShiftsFreefall Detection Errors from Shaking or Possible NoiseFigure 4. 0g Detect Modified Circuit OutputProjectile and Rotational FallsDetecting Freefall Using a MicrocontrollerLinear Freefall Using a MicrocontrollerTable 2. Sampling Frequency and Freefall Detection TimesRotational and Projectile Type Fall ConditionsFigure 5. Graphical Display of a Linear Freefall and a Rotational Fall EventConclusionRelated ArticlesHow to Reach Us© Freescale Semiconductor, Inc., 2007. All rights reserved.AN3459Rev 0, 04/2007Freescale Semiconductor Application NoteMeasuring Freefall using Freescale’s MMA7360L 3-Axis Accelerometerby: Kimberly Tuck Accelerometer Systems and Applications Engineering Tempe, AZINTRODUCTIONThis is an article which describes the use of Freescale’s MMA7360L 3-axis accelerometer as a freefall detection sensor. This is also a discussion of different types of freefall scenarios, the detection method using the MMA7360L accelerometer, and the limitations of the linear freefall detection. The MMA7360L can detect freefall which can be used as a device protection mechanism. A well known example of this is for disk drive head protection. Damaging freefall events can happen very quickly. Freefall must be detected by the accelerometer and the disk drive must be signaled to park the drive into safety all within an extremely short window of time. The challenge is to accomplish this algorithm within this short time period of a fraction of a second. A robust freefall algorithm should be able to sense various different types of freefall scenarios.FREEFALL SENSINGA 3-axis accelerometer offers a more accurate result when monitoring for freefall than a 2-axis accelerometer. In a 2-axis solution, when horizontally oriented, the X-axis and Y-axis outputs are the same (0g) regardless of whether the accelerometer is stationary or falling. When a 3-axis accelerometer is stationary, the total magnitude of the acceleration on the sensor is equal to 1g = - 9.8 m/s2, at any orientation.Figure 1. Example of a 3-axis AccelerometerWhen the sensor is in freefall, assuming a linear freefall with no initial acceleration or angular acceleration, the output acceleration will approach 0g. The distance of the fall and the time of the freefall can be calculated from known equations of motion. If there is an angular acceleration on the sensor as it is falling the output of the accelerometer can be quite high. There are limitations of the linear freefall that must be understood, some of which can be compensated for using different algorithms and additional circuitry.MMA7360L ACCELEROMETERThe MMA7360L is a low power, low profile capacitive micromachined accelerometer featuring signal conditioning, a single pole low pass filter, temperature compensation, self test, 0g detect which detects linear freefall, and g-Select which allows for the selection among 2 sensitivities. The zero-g offset and sensitivity are factory set and require no external devices. There is also a sleep mode pin on the accelerometer which makes it ideal for handheld battery powered electronics.1g +x +y +zAN3459Sensors 2 Freescale SemiconductorFigure 2. Pin Out of MMA7360LPIN OUT OF MMA7360LPin 9 on the MMA7360L is the 0g detect digital logic output pin. In the 0g detect circuit there is a window comparator, latch, and other control logic circuitry. When the output is in the equivalent voltage range of -0.4g to +0.4g the comparator output goes to a digital logic high and is latched. Each output applied to the window comparator is multiplexed. When the 3 latch outputs are high at same time, the 0g detect output, pin 9, goes high. It will be kept high until one of the outputs goes out of the range of -0.4g to +0.4g. Note: This threshold range is for the MMA7360L device.Figure 3. 0g Detect Circuitry in the MMA7360L for Pin 9EQUATIONS OF MOTIONThe following are the equations of motion for the case of constant acceleration. By integrating acceleration one can solve for velocity. Then to solve for position, a second integration is required as shown below in the following equations.A = constant = -9.81m/s2(Integrate acceleration to get the velocity equation)(Integrate velocity to get position.), solving for time: (Assuming the initial conditions are equal to zero)Based onTable 1 one can determine the time that a fall will take based on the distance or visa versa. When designing a freefall protection algorithm the typical height of the fall or a range of heights should be considered. Then the time the system will take to realize the device is in freefall, along with the time required to implement a protection mechanism, must be considered. For example it may take 10 ms to realize that a freefall is occurring, and then it takes typically 60ms to park a disk drive head.LIMITATIONS OF LINEAR FREEFALL DETECTION FOR MMA7360L0g Offset Shifts0g offset errors occur device to device based on offset variations from trim errors, mechanical stresses, from the package mounting, shifts due to temperature and due to aging. All of these variables can change the 0g offset value which ultimately will affect the ability for the 0g detect to function within the full +0.4g to -0.4g range. There is no way to internally compensate for 0g offset errors. The only way to resolve this problem would be to use an A/D converter on a microcontroller, sample the x, y and z outputs and run a 0g calibration routine.1Freefall Detection Errors from Shaking or Possible NoiseIt is possible that the accelerometer could have forces acting on it from shaking or certain motions which would set off the 0g detect pin by measuring in the detection range. This would cause a false freefall detect. This is depicted below in Figure 4 in the left callout bubble. Shaking or noise can cause the 0g detect pin to spike to Vdd (indicating a false freefall). In order to overcome this error one could add a single pole RC filter on the output of this pin. This filter will help vAtd∫v0At+==Table 1. Calculated Time and Distance for a Linear FallTime Distance Distance Time1ms 4.91µm 1cm 45ms10ms


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