Introduction to Acceleration Sensors

Catalog

I Working Principle

Figure 1. Acceleration Sensors

The principle of the linear acceleration sensor is Newton's second law, namely the law of acceleration:

A (acceleration) = F (inertial force) / M (mass)

We only need to use the electromagnetic force to balance this force to get the relationship between F and current.

Most acceleration sensors work according to the principle of the piezoelectric effect. The so-called piezoelectric effect is "the external force applied to a heteropolar crystal without asymmetric center will not only deform the crystal but also change the polarization state of the crystal and establish an electric field inside the crystal. The polarization phenomenon caused by the mechanical force is called the direct piezoelectric effect.

Figure 2. Piezoelectric Effect

The general acceleration sensor utilizes the characteristic of crystal deformation due to the acceleration inside. Since this deformation generates a voltage, as long as the relationship between the generated voltage and the applied acceleration is calculated, the acceleration can be converted into a voltage output. Of course, there are many other methods to make acceleration sensors, such as piezoresistive technology, capacitive effect, bubble effect, and photo effect, but the most basic principle is that a certain medium is deformed due to acceleration. Each technology has its own advantages and problems.

II Classification of Acceleration Sensors

1. Piezoelectric

The piezoelectric acceleration sensor is also called a piezoelectric acceleration sensor, which belongs to inertial sensors. The principle of the piezoelectric acceleration sensor is to use the piezoelectric effect of piezoelectric ceramics or quartz crystal. When the acceleration sensor is vibrated, the force applied to the piezoelectric element by the mass block changes accordingly. And when the measured vibration frequency is much lower than the natural frequency of the acceleration sensor, the change in force is proportional to the measured acceleration.

Figure 3. The Cross-section of a Piezoelectric Accelerometer.

2. Piezoresistive

Based on the world's leading MEMS silicon micromachining technology, the piezoresistive acceleration sensor has the characteristics of small size and low power consumption and is easy to be integrated into various analog and digital circuits. It is widely used in automobile crash experiments, testing instruments, equipment vibration monitoring, etc...

3. Capacitive

The capacitive acceleration sensor is a capacitive sensor with great changes based on the principle of capacitance. The capacitive accelerator is irreplaceable in certain fields, such as airbags, mobile devices, etc. It usually adopts the micro-electromechanical system (MEMS) technology, which is economical in mass production, thus ensuring lower cost.

4. Servo

The servo acceleration sensor is a closed-loop test system, which has good dynamic performance, a large dynamic range, and good linearity.

The vibration system of the sensor is the "m-k" system, which is the same as the general acceleration sensor, but there is an electromagnetic coil on the mass m. When there is acceleration input on the base, the mass deviates from the equilibrium position, and the displacement is detected by the displacement sensor, amplified by the servo amplifier, and converted into a current output. This current flows through the electromagnetic coil and generates an electromagnetic restoring force in the magnetic field of the permanent magnet, trying to keep the mass in the original equilibrium position in the instrument case, so the servo acceleration sensor works in a close loop.

Figure 4. Servo Type Accelerometer

Because of the feedback function, it enhances the anti-jamming capability, improves the measurement accuracy, and expands the measurement range. The servo acceleration measurement technology is widely used in inertial navigation and inertial guidance systems. It is also used in high-precision vibration measurement and calibration.

III Technical Parameters

1. Output Type

This is the first thing to consider, which depends on the interface between your system and the acceleration sensor. Generally, the voltage and acceleration of the analog output are proportional, for example, 2.5V corresponds to an acceleration of 0g, and 2.6V corresponds to an acceleration of 0.5g. Digital output generally uses pulse width modulation (PWM) signals.

Figure 5. Pulse Width Modulation(PWM)

If the microcontroller you are using has only digital inputs, such as BASIC Stamp, then you can only choose the acceleration sensor with digital output, but the problem is that you must occupy an additional clock unit to process the PWM signal, which could burden the processor.

If the microcontroller you use has an analog input port, such as PIC/AVR/OOPIC, you can simply use the acceleration sensor with the analog interface. All you need is to add a command like "acceleration=read_adc()" to the program, and the speed of processing this instruction is only a few microseconds.

2. Number of Measuring Axis

For most projects, two-axis acceleration sensors are already sufficient for most applications. For some special applications, such as UAV, ROV control, a three-axis acceleration sensor may be suitable.

3. Maximum Measured Value

If you only need to measure the inclination of the robot relative to the ground, then a ±1.5g acceleration sensor is sufficient. And if you need to measure the dynamic performance of the robot, ± 2g should be enough. If your robot will suddenly start or stop, you need a ±5g acceleration sensor.

4. Sensitivity

In general, the more sensitive the better. The more sensitive the sensor is, the more sensitive it is to the changes in acceleration within a certain range, and the greater the changes in output voltage. This makes it easier to measure and obtain more accurate measurements.

The minimum acceleration measurement value is also called the minimum resolution. Considering the noise problem of the post-amplifier circuit, we should avoid the minimum available value to ensure the best signal-to-noise ratio. The maximum measurement limit must consider the non-linear effect of the acceleration sensor itself and the maximum output voltage of the subsequent instrument.

The estimation method is:

maximum measured acceleration×sensor charge/voltage sensitivity

For whether the above value exceeds the maximum input charge/voltage value of the supporting instrument, it is recommended that if the measured acceleration range is known, we can look up the "reference range" in the sensor index. At the same time, if the frequency response and weight allow, the sensitivity can be higher to improve the input signal of the subsequent instrument to increase the signal-to-noise ratio.

5. Bandwidth

The bandwidth here actually refers to the refresh rate, which means how many readings the sensor will produce per second. For applications that generally measure inclination, 50 Hz bandwidth should be sufficient, but for the detection of dynamic performance, such as vibration, you will need a sensor with hundreds of Hz bandwidth.

6. Resistance/Cache Mechanism

For some microcontrollers, to perform A/D conversion, the resistance of the connected sensor must be less than 10kΩ. For example, the resistance value of the acceleration sensor is 32kΩ, which can not work properly on the PIC and AVR control boards, so it is recommended to read the manual carefully before purchasing the sensor to ensure it can work normally.

7. Accumulative Error

The acceleration sensor measures the acceleration once in a period and then calculates the total displacement and terminal speed of the previous period based on the previously accumulated speed (including velocity and direction) and position.  Repeating such calculations can get the results.

Obviously, the shorter the sampling time, the higher the accuracy. But this will be subject to some technical limitations, such as the speed of computer operations is not fast enough; the acceleration sensor itself has a response time, etc. In addition, since the speed and position are always cumulative, there is a cumulative error, and as time goes on, the overall accuracy decreases greatly.

IV Application of Acceleration Sensors

1. Vehicle Safety

The acceleration sensor is mainly used in safety performance as automobile airbags, anti-lock brake systems, traction control systems, and so on.

In security applications, the fast response of the acceleration sensor is very important. When the airbag should be released must be determined quickly, so the acceleration sensor must respond in an instant. Therefore, we can adopt a sensor that can quickly reach a stable state rather than vibrate endlessly, and the response time of the device can be shortened. Among them, piezoresistive acceleration sensors have developed fastest due to their wide application in the automotive industry.

Figure 6. Acceleration Sensor in Automobile

2. Game control

The acceleration sensor can detect the change of the inclination angle. Therefore, it is easy to control the directions of the objects in the game by tilting the handheld device forward and backward.

3. Automatically Flipped the Image

Use the acceleration sensor to detect the rotation action and direction of the handheld device to flip the image to the correct direction.

4. Tilt Correction of Electronic Compass

The magnetic sensor determines the direction by measuring the magnitude of the magnetic flux. When the magnetic sensor is tilted, the geomagnetic flux passing through the magnetic sensor will change, resulting in an error in the direction. Therefore, if we use an electronic compass without tilt correction, we need to place it horizontally. However, since the acceleration sensor can measure the tilt angle, it can compensate for the tilt of the electronic compass.

Figure 7. Electronic Compass Acceleration Sensor Module

5. Compensation for Dead Angle of GPS Navigation System

The GPS determines the position of the object by receiving three satellites’ signals distributed at 120 degrees. On some special occasions and landforms, such as tunnels, tall buildings, and jungles, the GPS signal will become weak or even completely lost. This is the so-called dead end.

By installing an acceleration sensor and inertial navigation, we can measure the dead zone of the system. And after the measured value is integrated once, it becomes the amount of speed change per unit time, thereby we can measure the movement of objects in the dead zone.

6. Pedometer Function

The acceleration sensor can detect the AC signal and the vibration of the object. When a person walks, a certain regular vibration will produce. The acceleration sensor can detect the zero-crossing point of the vibration, so as to calculate the number of steps taken by the person or the number of steps taken by the runner, thus calculating the displacement of the person. We can also calculate calorie consumption with a certain formula.

Figure 8. Pedometer with Built-in 3D Acceleration Sensor

7. Image stabilization

Use the acceleration sensor to detect the vibration/shake amplitude of the handheld device. When the vibration/shake amplitude is too large, it can lock the camera shutter so that the image is always clear.

8. Flash SMS

When we wave the handheld device, the text can display in the air, and we can also edit the displayed text. This flash SMS function is using people's visual persistence, and the acceleration sensor can detect the swing period and accurately positioning the displayed text.

9. Hard Disk protection

The acceleration sensor is used to detect the free-fall state, thereby protecting the mini hard disk. As we all know, when the hard disk is reading data, the distance between the magnetic head and the disc is very small. Therefore, slight vibrations from the outside world will have a bad influence on the hard disk, causing data loss. The acceleration sensor can detect the free-fall state. When the free-fall state is detected, the magnetic head is reset to reduce the damage to the hard disk.

10. Terminal Pose Detection

Acceleration sensors and gyroscopes are usually called inertial sensors, which are often used in various devices or terminals to achieve pose detection, motion detection, etc., very suitable for people who play motion-sensing games. The acceleration sensor uses gravity acceleration to detect the inclination of the device, but it will be affected by the motion acceleration, making the inclination measurement inaccurate, so it is usually necessary to use gyroscopes and magnetic sensors to compensate.

Figure 9. Motion-Sensing Games

At the same time, the magnetic sensor also uses the geomagnetic field when measuring the azimuth. When the current in the system changes or there is a permeability magnetic material around it, and when the device is tilted, the measured azimuth is not accurate. In this case, an acceleration sensor and gyroscope are also required for compensation.

11. Smart Products

The application of the acceleration sensor in WeChat is innovative. The integrated acceleration sensor in the mobile phone can measure the acceleration values of X, Y, and Z respectively. X represents the horizontal movement of the mobile phone, Y represents the vertical movement, and Z represents the vertical direction in space. The direction towards the sky is positive, and the direction towards the earth is negative. Then the relevant acceleration value is transmitted to the operating system. By judging the change in this value, you can know the friends playing WeChat at the same time.

Figure 10. 3 Directions of Acceleration Sensor in Smartphones

V Installation Precautions

The natural frequency of the acceleration sensor is determined by the coupling degree of the adhesive. Choosing the correct adhesive will be an important step.

Some important issues that must be considered are the weight of the acceleration sensor, the frequency, bandwidth, amplitude, and temperature during the test. Also, some problems will occur during the test like the limitation of the sine curve and the random vibration. Generally, engineers and technicians will choose the appropriate adhesive to bond the acceleration sensor according to the different needs of the test.

Adhesives used for the acceleration sensor are generally cyanoacrylate, magnet, double-sided tape, paraffin, hot glue, etc. The key issue is how to use these adhesives effectively. During the bonding process of the acceleration sensor, the amount of adhesive used plays a key role in achieving a good frequency response. Using as little adhesive as possible to bond the acceleration sensor on a small film will make the acceleration sensor have the best frequency response transmission.

Figure 11. Cyanoacrylate Adhesives

Before installing the sensor, we should use a hydrocarbon solution to clean the surface. Then evenly apply the cyanoacrylate, magnet, double-sided tape, paraffin to the bonding surface. The adhesive should not be too thick or too thin. The appropriate thickness will play a good bonding effect. Besides, we should also pay attention to the setting time of the hot adhesive during installation.

In addition, it is best not to use adhesive when the temperature is close to the maximum limit value. Instead, use it after a while, otherwise, the adhesive itself will be damaged, and the tensile strength of the adhesive will be reduced.

In any case, when we want to install the acceleration sensor by bonding, all the limiting factors must be taken into account. Similarly, the installation of the above acceleration sensor is only for most cases, and the most suitable sensor installation method for measuring acceleration on some special equipment depends on the specific situation.

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