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Feb 13 2019

How to Choose the Right Application Sensor?

Warm hints: This article contains about 4000 words and reading time is about 18 mins.

Introduction

In order to obtain information from the outside world, people must resort to the sensory organs.

And by relying on people's own sensory organs, their functions in researching natural phenomena and laws and production activities are far from enough. To accommodate this situation, sensors are needed. Therefore, it can be said that the sensor is an extension of the human five senses, also known as electric five senses.

With the advent of the new technological revolution, the world has entered the information age. In the process of using information, the first thing to be solved is to obtain accurate and reliable information, and the sensor is the main way and means to obtain information in the natural and production fields.

In modern industrial production, especially in automated production processes, various sensors are used to monitor and control the various parameters of the production process, to operate the equipment in a normal or optimal state, and to achieve the best quality of the product. Therefore, it can be said that without many excellent sensors, modern production will lose its foundation.

How To Choose an IoT Sensor for Your Application

In the basic research, the sensor has a prominent position. The development of modern science and technology has entered many new fields: for example, to observe the universe of thousands of light years on a macroscopic level, to observe the particle world as small as fm on the microscopic level, and to observe the evolution of celestial bodies for hundreds of thousands of years in the vertical direction. , an instant response as short as s. In addition, there have been various extreme technological studies that have an important role in deepening material understanding, exploring new energy sources, and new materials, such as ultra-high temperature, ultra-low temperature, ultra-high pressure, ultra-high vacuum, super-strong magnetic field, and ultra-weak magnetic field. Obviously, it is impossible to obtain information that is not directly accessible to human senses, and that there is no suitable sensor. The obstacles of many basic scientific research are firstly that there are difficulties in obtaining object information, and the emergence of some new mechanisms and high-sensitivity detection sensors often leads to breakthroughs in this field. The development of some sensors is often the pioneer of some marginal discipline development.

Sensors have long penetrated into areas such as industrial production, space development, ocean exploration, environmental protection, resource surveys, medical diagnostics, bioengineering, and even cultural relics protection. It is no exaggeration to say that from the vast space, to the vast ocean, to various complex engineering systems, almost every modern project can not be separated from a variety of sensors.

It can be seen that the important role of sensor technology in developing the economy and promoting social progress is very obvious. All countries in the world attach great importance to the development of this field. It is believed that in the near future, sensor technology will have a leap to reach a new level commensurate with its important position.

Article Core

Sensor

Purpose

Introduce how to choose the right application sensor.

Application

Consumer electronics, automobile, industry and so on.

Keywords

Sensor


Accelerometers measure acceleration, tilt, vibration or shock and are therefore suitable for a wide range of applications from wearable fitness devices to industrial platform stabilization systems. There are hundreds of accelerometer devices on the market to choose from, with varying costs and performance. The latest MEMS capacitive accelerometers are used in applications traditionally dominated by piezoelectric accelerometers and other sensors. The new generation of MEMS accelerometers provides solutions for applications such as CBM, Structural Health Monitoring (SHM), Asset Health Monitoring (AHM), Vital Signs Monitoring (VSM) and IoT wireless sensor networks. However, with so many accelerometers and so many applications, choosing the right accelerometer is not easy.

There are no industry standards that define which category an accelerometer belongs to. The general classification of accelerometers and their corresponding applications are shown in Table 1. The illustrated bandwidth and g-value ranges are typical values that accelerometers use in the listed end applications.

Accelerometer level

Main application

Bandwidth

Range of g values

Consumer Electronics

Motion, static acceleration

0 HZ

1 g

Automobile

Collision/stability

100 HZ

<200g/2g

Industry

Platform stability / tilt

5 HZ to500 HZ

25g

Tactics

Weapon/aircraft navigation

<1 KHZ

8g

Navigation

Submarine/aircraft navigation

>300 KHZ

15g

Table 1. Accelerometer ratings and typical application areas

Figure 1. Application map of ADI's selected MEMS accelerometer

Figure 1. Application map of ADI's selected MEMS accelerometer

Figure 1 shows a snapshot of various MEMS accelerometers, categorizing each sensor based on the main performance metrics and intelligence/integration levels of the particular application.

Main criteria for tilt detection: bias stability, offset drift, low noise, repeatability, vibration correction, cross-axis sensitivity. For MEMS capacitive accelerometers, accurate tilt detection is a demanding application, especially in the presence of vibration. It is very difficult to achieve a tilt accuracy of 0.1° using a MEMS capacitive accelerometer in a dynamic environment – 1° is easier to implement. In order for the accelerometer to effectively measure the tilt, a good understanding of the sensor performance and the end application environment is required. Compared to dynamic environments, static environments are more advantageous for tilt measurements because vibration or shock can corrupt the tilt data and cause serious measurement errors. The most important characteristics of tilt measurement are temperature coefficient offset, hysteresis, low noise, short/long term stability, repeatability and good vibration correction.

0g offset accuracy, 0g offset drift due to soldering, 0g offset drift due to PCB housing alignment, 0g bias temperature coefficient, sensitivity and temperature coefficient, nonlinearity, and cross-axis sensitivity are observable It can be reduced by the post-assembly calibration process. Hysteresis, 0g offset drift during lifetime, sensitivity drift during lifetime, 0g drift due to humidity, and PCB bending and torsion caused by temperature changes over time. These error terms cannot be solved by calibration or other methods. It can be reduced by a certain degree of in-situ maintenance.

Analog Devices' accelerometers are available in MEMS (ADXLxxx) and iSensor® (ADIS16xxx) special-purpose devices. iSensor or Smart Sensors are highly integrated (4 to 10 degrees of freedom) and programmable devices for complex applications in dynamic environments. These highly integrated plug-and-play solutions include comprehensive factory calibration, embedded compensation, and signal processing that address many of the above-mentioned in-situ repair errors, greatly reducing design and verification burden. This comprehensive factory calibration provides sensitivity and bias characteristics over the entire sensor signal chain over the specified temperature range (typically −40°C to +85°C). Therefore, each iSensor device has its own unique compensation formula that produces accurate measurements after installation. For some systems, factory calibration eliminates system-level calibration and greatly simplifies operation.

iSensor devices are specifically developed for certain applications. For example, the ADIS16210 shown in Figure 2 is specifically designed and customized for tilting applications, so it can be supplied.

Figure 2. ADIS16210 Precision Triaxial Tilt

Figure 2. ADIS16210 Precision Triaxial Tilt


The latest generation of accelerometer architectures (such as the ADXL355) offer more features (tilt, status monitoring, structural health, IMU/AHRS applications), and include integrated modules that are less specific to specific applications but feature-rich, as shown in Figure 3.

Figure 3. Low noise, low drift, low power 3-axis MEMS accelerometer ADXL355

Figure 3. Low noise, low drift, low power 3-axis MEMS accelerometer ADXL355


The following is a comparison of the ADXL345 general-purpose accelerometer and the ADXL355, a new generation of low-noise, low-drift, low-power accelerometers, which are ideal for a wide range of applications, such as IoT sensor nodes and inclinometers. This comparison looks at the sources of error in tilting applications and the errors that can be compensated or eliminated. Table 2 lists the expected performance specifications for the consumer grade ADXL345 accelerometer and estimates of the corresponding tilt error. Some form of temperature stabilization or compensation must be used when attempting to achieve optimum tilt accuracy. In the example below, the constant temperature is assumed to be 25 °C. The main causes of error that cannot be fully compensated are temperature drift offset, offset drift, and noise. Bandwidth can be reduced to reduce noise because tilting applications typically require bandwidths below 1 kHz.

Sensor parameter

Performance

Condition / comment

Typical application error g tilt °

Noise

X/y axis

290μg/(HZ)

Bandwidth is 6.25HZ

0.9mg

0.05°

Offset drift

Allan deviation

Short-term X/Y axis

(Example: 10 days)

1mg

0.057°

Initial imbalance

35mg

No compensation

35mg

Have compensation

0mg

Total deviation

No compensation

Bandwidth is 6.25HZ

36.9mg

2.1°

Table 2. ADXL345 Error Source Estimates


Table 3 lists the same standards that apply to the ADXL355. The short-term offset values are estimated from the Allan variance plot in the ADXL355 data sheet. At 25 ° C, the estimated tilt accuracy of the universal ADXL345 after compensation is 0.1 °. The industrial grade ADXL355 has an estimated tilt accuracy of 0.005°. By comparing the ADXL345 and the ADXL355, it can be seen that the error caused by the significant error contribution factor has been significantly reduced. For example, the noise-induced error is reduced from 0.05° to 0.0045°, and the error caused by the offset drift is reduced from 0.057° to 0.00057°. This shows that MEMS capacitive accelerometers have made a huge leap in performance such as noise and offset drift, providing higher levels of tilt accuracy under dynamic conditions.

Sensor parameter

Performance

Condition / comment

Typical application error g tilt °

Noise

X/Y axis

290μg/(HZ)

Bandwidth is 6.25HZ

78μg

0.0045°

Offset drift

Allan deviation

Short-term X/Y axis

(Example: 10 days)

<10μg

0.00057°

Initial imbalance

25mg

No compensation

25mg

1.43°

Have compensation

0mg

Total deviation

No compensation

Bandwidth is 6.25HZ

25mg

1.43°

Table 3. ADXL355 Error Source Estimates


Choosing a higher level accelerometer is critical to achieving the desired performance, especially if the application requires a tilt accuracy of less than 1°. Application accuracy depends on application conditions (temperature fluctuations, vibration) and sensor selection (consumer and industrial or tactical). In this case, the ADXL345 will require a large amount of compensation and calibration to achieve tilt accuracy of less than 1°, increasing the overall system effort and cost. Depending on the magnitude of the vibration in the final environment and temperature range, it is even impossible to achieve the above accuracy. The temperature coefficient offset drift in the range of 25 ° C to 85 ° C is 1.375 °, which has exceeded the requirement that the tilt accuracy is less than 1 °.

sensor

The maximum temperature coefficient offset drift of the ADXL355 is 0.5° from 25°C to 85°C.

sensor

The repeatability of ADXL354 and ADXL355 (±3.5mg/0.2° for X and Y axes and ±9mg/0.5° for Z axis) is a 10-year life prediction value, including high temperature working life test (HTOL) (TA=150°C, VSUPPLY = 3.6V, 1000 hours), temperature cycle (−55°C to +125°C and 1000 cycles), random walk, speed of broadband noise and temperature hysteresis. These new accelerometers provide repeatable tilt measurements under all conditions, eliminating the need for extensive calibration in harsh environments to achieve minimum tilt errors and minimizing post-deployment calibration needs. The ADXL354 and ADXL355 accelerometers guarantee temperature stability with a zero offset factor of 0.15mg/°C (max). This stability minimizes the resources and cost associated with calibration and testing, helping equipment OEMs achieve higher throughput rates. In addition, the product is hermetically sealed to ensure repeatability and stability of the final product consistent with its specifications.

In general, repeatability and suppression of vibration correction errors (VRE) are not shown in the data sheet as these parameters may expose product performance to a lower level. For example, the ADXL345 is a universal accelerometer for consumer applications, and VRE is not an important concern for designers. However, in certain environments where high-demand applications such as inertial navigation, tilting applications, or vibrations are frequent, the ability to suppress VRE may be a key concern for designers, so the ADXL354/ADXL355 and ADXL356/ADXL357 data sheets will give such parameters. 

As shown in Table 4, VRE is the offset error introduced when the accelerometer is exposed to broadband vibration. When the accelerometer is exposed to a vibrating environment, the VRE causes significant errors in the tilt measurement compared to the 0g offset caused by temperature drift and noise. This is one of the main reasons why the data sheet is no longer used, as it is easy to cover other major specifications.

VRE is the response of an accelerometer to AC vibration (converted to DC). These DC rectified vibrations can skew the accelerometer offset, causing serious errors, especially in tilted applications where the target signal is a DC output. Any small change in dc offset can be interpreted as a change in tilt angle, resulting in system level errors.

Device

Maximum tilt error 0g offset and temperature(°/°C)

Noise density(°/ HZ)

Vibration correction

(°/g²rms)

ADXL354

0.0085

0.0011

0.023

ADXL355

0.0085

0.0014

0.023

Table 4. Errors expressed in terms of slope

The 1g azimuth offset caused by 12.5g rms vibration is in the range of ±2g.

Filters in various resonant and accelerometers (in this case, the ADXL355) can cause VRE because VRE has a strong dependence on frequency. These resonances amplify the vibration, the amplification factor is equal to the Q factor of the resonance, and the vibration is suppressed at higher frequencies because the resonator has a second-order dipole response. The higher the resonance quality factor of the sensor, the greater the vibration amplitude and the larger the VRE. A larger measurement bandwidth will include vibrations in the high frequency band, resulting in a higher VRE, as shown in Figure 4. Choosing the right bandwidth for the accelerometer to suppress high frequency vibrations can avoid many vibration related problems.

10.png

Figure 4. ADXL355VRE test at different bandwidths

Static tilt measurements typically require a low g accelerometer of ±1g to ±2g with a bandwidth of less than 1.5kHz. The analog output ADXL354 and the digital output ADXL355 are low noise density (20μg 25Hz and 25μg √Hz, respectively), low 0g offset drift, low power triaxial accelerometer, integrated temperature sensor, measurement range is optional, as shown in Table 5. Show.

Device

Measuring range(g)

Bandwidth(kHZ)

ADXL354B

±2, ±4

1.5

ADXL354C

±2, ±8

1.5

ADXL355B

±2, ±4, ±8

1

ADXL356B

±10, ±20

1.5

ADXL356C

±10, ±40

1.5

Table 5. ADXL354/ADXL355/ADXL356/ADXL357 Measurement Range

The ADXL354/ADXL355 and ADXL356/ADXL357 are packaged in a hermetic package for excellent long-term stability. Performance improvement is usually positively related to the package, as shown in Figure 5. Packages are often overlooked, and manufacturers can use packages to achieve better stability and drift performance. This is a key area of interest for Analog Devices, and we offer a wide range of sensor packages to suit different applications.


High temperature and dynamic environment

Some designers have had to use standard temperature ICs far beyond the limits of the data sheet before accelerometers suitable for high temperatures or harsh environments are available. This means that the end user is responsible for the responsibility and risk of verifying the quality of the device at high temperatures, which is costly and time consuming. It is a well-known fact that sealed packages can withstand high temperatures, preventing corrosion through a barrier against moisture and pollution. Analog Devices offers a wide range of sealing devices with enhanced temperature stability and performance. ADI has also been researching the performance of plastic packages at high temperatures, especially the ability of leadframes and leads to accommodate high temperature soldering processes, making them robust and reliable in high shock and vibration environments. As a result, Analog Devices offers 18 accelerometers rated for −40°C to +125°C, including the ADXL206, ADXL354/ADXL355/ADXL356/ADXL357, ADXL1001/ADXL1002, ADIS16227/ADIS16228, and ADIS16209. Most competitors do not offer MEMS capacitive accelerometers that can operate over the temperature range of −40°C to +125°C or under harsh environmental conditions such as heavy industrial machinery and downhole drilling.

Figure 5. Example of performance improvements with advanced packaging techniques and calibration

Figure 5. Example of performance improvements with advanced packaging techniques and calibration

Tilt measurements in dynamic environments with vibrations (such as agricultural equipment or drones) require accelerometers with a high range of g values, such as the ADXL356/ADXL357. Accelerometer measurements in the finite g range may clip, resulting in increased output offset. The cause of the clipping may be that the sensitive axis is in the 1 g gravitational field, or the impact that the rise time is fast but the decay is slow. A higher range of g values reduces accelerometer clipping, which reduces offset and provides better tilt accuracy in dynamic applications.

Figure 6 shows a measurement with a limited range of g values for the ADXL356Z axis, which already exists in 1g. Figure 7 shows the same measurement, but the g value range extends from ±10g to ±40g. It can be clearly seen that the g-scale extension of the accelerometer significantly reduces the offset caused by clipping.

Figure 6. ADXL356VRE, Z-axis offset with respect to 1g, ±10g range, Z-axis direction = 1g

Figure 6. ADXL356VRE, Z-axis offset with respect to 1g, ±10g range, Z-axis direction = 1g

Figure 7. ADXL356VRE, Z-axis offset with respect to 1g, ±40g range, Z-axis direction = 1g

Figure 7. ADXL356VRE, Z-axis offset with respect to 1g, ±40g range, Z-axis direction = 1g

The ADXL354/ADXL355 and ADXL356/ADXL357 offer excellent vibration correction, long-term repeatability, and low noise performance, and are small in size, making them ideal for tilt detection applications in both static and dynamic environments.

Main criteria for stability: noise density, random walk at speed, bias stability during motion, offset repeatability, and bandwidth. Detecting and understanding motion can benefit many applications. It is beneficial to take control of a system's movements and then use this information to improve performance (short response time, improve accuracy, speed up), enhance safety or reliability (system shut down in hazardous situations), or gain other value-added features. of. Due to the complexity of motion, there are a large number of stability applications that require the use of a combination of gyroscopes and accelerometers (sensor fusion, as shown in Figure 8), such as UAV monitoring equipment and onboard antenna pointing systems.

Figure 8. Degree of freedom IMU

Figure 8. Degree of freedom IMU

The degree of freedom IMU uses multiple sensors to compensate for each other's weaknesses. It may seem like a simple inertial motion on one or two axes, which may actually require the fusion of an accelerometer and a gyro sensor to eliminate vibration, gravity, and other influencing factors that cannot be accurately measured by accelerometers or gyroscopes alone. Accelerometer data includes gravity components and motion accelerations. The two cannot be distinguished, but the gyroscope can be used to remove the gravity component from the accelerometer output. In order to determine the position based on the acceleration, integration is required, and after this process, the error caused by the gravity component of the accelerometer data may rapidly become large. Due to the cumulative error, the gyroscope alone is not sufficient to determine the position. The gyroscope does not detect gravity and can therefore be used as an auxiliary sensor for accelerometers.

In stability applications, MEMS sensors must accurately measure the orientation of the platform, especially during motion. Figure 9 is a block diagram of a typical platform stabilization system that uses servo motors to correct angular motion. The feedback/servo motor controller converts the direction sensor data into a servo motor correction control signal.

Figure 9. Basic platform stabilization system

Figure 9. Basic platform stabilization system

The final application will determine the level of accuracy required, and the quality of the selected sensor (consumer or industrial) will determine if it can be achieved. It is important to distinguish between consumer and industrial devices. Sometimes the difference between the two is subtle and may require careful consideration. Table 6 shows the main differences between consumer-grade accelerometers and mid-range industrial accelerometers integrated in the IMU.

Table 6. Industrial MEMS devices are fully measurable for all known potential sources of error, with levels of accuracy that are orders of magnitude higher than consumer devices.

Accelerometer parameters

Typical industrial specifications

Equivalent to the improvement of typical consumer devices

Dynamic Range

The highest is 40g

3×

Noise density

25μg/ HZ

10×

Speed random walk

0.03m/sHZ

10×

Bias stability during exercise

10μg

10×

Offset repeatability

25mg

100×

Table 6

Performance requirements can be met with low precision devices where certain conditions are favorable and less accurate data can be accepted. However, the demand for sensors capable of operating in a dynamic environment is rapidly increasing, and lower precision devices are greatly affected by the inability to reduce vibration effects or temperature effects in actual measurements, and it is difficult to achieve pointing accuracy of less than 3° to 5°. Most low-end consumer devices do not provide parametric specifications such as vibration correction, angular random walk, and these specifications are probably the largest source of error in industrial applications.

In order to achieve 1° or even 0.1° pointing accuracy in a dynamic environment, the designer's device selection must focus on the ability of the sensor to suppress temperature drift errors and vibration effects. Sensor filtering and algorithms (sensor fusion), while a key element in improving performance, do not eliminate the gap between consumer and industrial sensors. The performance of ADI's new industrial IMU is close to that of the previous generation of missile guidance systems. Devices such as the ADIS1646x and the announced ADIS1647x offer precision motion detection in standard and mini-IMU form factors, breaking into special applications in the past.


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