[Repost] Popular Science: Principles and Applications of Acceleration Sensors

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[Repost] Popular Science: Principles and Applications of Acceleration Sensors [Copy link]

Accelerometers can help your robot understand its environment. Is it climbing a hill? Walking downhill? Did it fall? Or for flying robots, it is also crucial to control its posture. What's more, make sure that your robot does not carry a bomb and go to a crowded place by itself. A good programmer can use accelerometers to answer all of the above questions. Accelerometers can even be used to analyze engine vibrations. By measuring the acceleration caused by gravity, you can calculate the tilt angle of the device relative to the horizontal plane. By analyzing dynamic acceleration, you can analyze how the device moves. But at first, you may find that simply measuring inclination and acceleration doesn't seem very useful. However, engineers have come up with many ways to get more useful information. Some laptops have built-in accelerometers that can dynamically monitor the vibration of the laptop during use. Based on these vibration data, the system will intelligently choose to shut down the hard drive or let it continue to run, which can maximize the protection of the hard drive from vibration, such as bumpy working environment, or accidentally dropping the computer, and maximize the protection of the data inside. Another use is that there are accelerometers in the current digital cameras and camcorders, which are used to detect hand vibrations when shooting, and automatically adjust the camera focus based on these vibrations. 25)]Acceleration sensors can be used in control, handle vibration and shaking, instrumentation, automobile brake start detection, earthquake detection, alarm systems, toys, structures, environmental monitoring, engineering vibration measurement, geological exploration, railways, bridges, dam vibration testing and analysis; mouse, high-rise building structure dynamic characteristics and security vibration reconnaissance.

Acceleration sensor working principle

The principle of linear accelerometer is the inertia principle, that is, the balance of force, A (acceleration) = F (inertia force) / M (mass) We only need to measure F. How to measure F? Use electromagnetic force to balance this force. You can get the relationship between F and current. You only need to use experiments to calibrate this proportional coefficient. Of course, the signal transmission, amplification, and filtering in the middle are the circuit's business. Modern science and technology require that accelerometers be cheap, have superior performance, and be easy to mass produce. In fields such as military industry, space systems, and scientific measurement, small, light, and stable accelerometers are needed. Accelerometers manufactured using traditional processing methods are difficult to fully meet these requirements. Therefore, micro-accelerometers made using emerging micromachining technology came into being. This type of sensor is small in size, light in weight, has low power consumption, fast startup, low cost, high reliability, and is easy to digitize and intelligentize. Moreover, because the micromechanical structure is precisely manufactured, has good repeatability, is easy to integrate, and is suitable for mass production, it has a high performance-price ratio. It can be foreseen that in the near future, it will dominate the accelerometer market. Micro-accelerometers are available in piezoresistive, piezoelectric, capacitive, and other forms.

Piezoelectric sensors use the principle of spring-mass system. The mass of the sensitive core generates a force proportional to the acceleration after being acted upon by vibration acceleration. The piezoelectric material forms a charge signal proportional to the force along its surface after being acted upon by this force. Piezoelectric accelerometers have the characteristics of large dynamic range, wide frequency range, durability, low external interference, and the piezoelectric material generates charge signals by itself when it is subjected to force, without any external power supply. They are the most widely used vibration measurement sensors. Although piezoelectric accelerometers have a simple structure and a long history of commercial use, their performance indicators are closely related to material properties, design and processing technology. Therefore, the actual parameters of the performance of similar sensors sold on the market, as well as their stability and consistency, vary greatly. Compared with piezoresistance and capacitance, their biggest disadvantage is that piezoelectric accelerometers cannot measure zero-frequency signals.

The entire sensor assembly is mounted on an original base and enclosed in a metal shell. In order to isolate any strain of the test piece from being transmitted to the piezoelectric element, the base is large in size. During the test, the base of the sensor is rigidly connected to the test piece. When the vibration frequency of the test piece is much lower than the resonant frequency of the sensor, the sensor output charge (or voltage) is proportional to the acceleration of the test piece, and the acceleration can be measured by a charge amplifier or a voltage amplifier.

The sensitive core of the strain piezoresistive accelerometer is a resistance measurement bridge made of semiconductor materials, and its structural dynamic model is still a spring-mass system. The development of modern micro-machining manufacturing technology has made the design of piezoresistive sensitive cores very flexible to suit various measurement requirements. In terms of sensitivity and range, there are piezoresistive accelerometers from low-sensitivity and high-range impact measurement to DC high-sensitivity and low-frequency measurement. At the same time, the measurement frequency range of piezoresistive accelerometers can also range from DC signals to high-frequency measurements with high stiffness and a measurement frequency range of tens of kilohertz. Ultra-miniaturization is also a highlight of piezoresistive sensors. It should be pointed out that although the design and application of piezoresistive sensitive cores are very flexible, the scope of use of a piezoresistive core of a specific design is generally smaller than that of a piezoelectric sensor. Another disadvantage of piezoresistive accelerometers is that they are greatly affected by temperature, and practical sensors generally require temperature compensation. In terms of price, the cost price of piezoresistive sensors used in large quantities has great market competitiveness, but the manufacturing cost of sensitive cores for special uses will be much higher than that of piezoelectric accelerometers.

The structure of capacitive accelerometers generally adopts a spring-mass system. When the mass moves under the action of acceleration, the gap between the mass block and the fixed electrode changes, thereby changing the capacitance value. Compared with other types of accelerometers, capacitive accelerometers have the characteristics of high sensitivity, zero-frequency response, good environmental adaptability, etc., especially less affected by temperature; but the disadvantages are that the input and output of the signal are nonlinear, the range is limited, it is affected by the capacitance of the cable, and the capacitive sensor itself is a high-impedance signal source, so the output signal of the capacitive sensor often needs to be improved by the subsequent circuit. In practical applications, capacitive accelerometers are mostly used for low-frequency measurements. Their versatility is not as good as that of piezoelectric accelerometers, and their cost is much higher than that of piezoelectric accelerometers.

Acceleration sensors can be used in control, handle vibration and shaking, instrumentation, automobile brake start detection, earthquake detection, alarm systems, toys, structures, environmental monitoring, engineering vibration measurement, geological exploration, railways, bridges, dam vibration testing and analysis; mouse, high-rise building structure dynamic characteristics and security vibration reconnaissance.

Currently, most devices provide accelerometers that can detect all directions. Taking iOS devices as an example, we use the characteristics of its three-axis accelerometer (x, y, and z axes represent directions as shown in the figure) to analyze. They are used to detect the acceleration changes in three directions when people walk.

Reasonable step counting algorithm

Because users may hold the device flat with their hands or put it in their pockets during exercise, the placement direction of the device is uncertain. To this end, by calculating the vector lengths of the three accelerations, we can obtain a sinusoidal curve trajectory of walking motion.

The second step is peak detection. We record the last vector length and movement direction. By changing the vector length, we can determine the current acceleration direction and compare it with the last saved acceleration direction. If it is the opposite, that is, it has just passed the peak state, then enter the step counting logic to count steps, otherwise it is discarded. By accumulating the number of peaks, the user's walking pace can be obtained. Finally, we need to remove interference. Handheld devices may have some low-amplitude and fast twitching states, or what we commonly call hand shaking, or a prank user may want to simulate human walking by shaking the device quickly and repeatedly for a short period of time. If these interference data are not removed, it will affect the accuracy of step counting. For this kind of interference, we can filter it out by adding thresholds and step frequency judgment to the detection. Source: Internet, please delete if infringed.

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For this kind of interference, we can filter it out by adding a threshold and a step frequency judgment to the detection. For this kind of interference, we can filter it out by adding a threshold and a step frequency judgment to the detection.