Choosing an IMU: FOG IMUs vs MEMS IMUs

 

Concept
The full name of IMU is Inertial Measurement Unit. The IMU contains a 3-axis accelerometer and a 3-axis gyroscope. It is a device that measures the three-axis attitude angle (or angular velocity) and acceleration of an object. The IMU can calculate the position and attitude of the carrier by measuring the angle and acceleration of the carrier in three-dimensional space. Advances in IMU technology are revolutionizing the positioning and navigation industry, and the gap between microelectromechanical systems (MEMS) gyroscopes and fiber optic gyroscopes (FOG) is closing as new MEMS gyroscope sensors achieve greater accuracy. The two most common types of IMUs today are built around microelectromechanical systems (MEMS) technology and fiber optic gyroscope (FOG) technology.

Concept of MEMS IMU & FOG IMU

1.MEMS IMU
The MEMS IMU contains a MEMS accelerometer and a MEMS gyroscope. MEMS accelerometers detect linear acceleration, which can then be used to calculate speed and distance. MEMS gyroscopes detect rotational motion and are typically used to determine heading and/or attitude (roll and pitch). When data from accelerometers and gyroscopes are combined over time, the position of an object relative to its origin can be calculated.

Because of their small size, mainstream manufacturing processes, use of common materials, and widespread adoption, MEMS IMUs are, on average, smaller, lighter, consume less power, and are cheaper than their FOG counterparts. However, because MEMS IMUs contain more micromechanical components and are more sensitive to temperature fluctuations, they tend to be less precise and noisier than their fiber optic counterparts. If accurate position data is required over an extended period of time, MEMS IMUs are often used in conjunction with GNSS receivers or other sensor technologies that provide supplementary position information.

MEMS inertial sensors are small, lightweight, low power and low cost, in other words they have all the most desirable characteristics of low SWaP-C (size, weight, power and cost). Applications include mobile phones, vehicle navigation systems, autonomous ground vehicles, flying drones and robotic systems.

Because MEMS sensors are smaller and lighter, they are more sensitive to unmodeled temperature and vibration effects due to their mechanical properties. These effects increase noise (angular random walks) and lead to biases in the modeled noise figure. The IMU independently developed by Ericco Inertial Systems Co., Ltd. in China provides high-reliability, low-cost FOG/MEMS inertial measurement units to global customers. Ericco not only provides standard IMU products, but can also customize FOG/MEMS according to customers’ special requirements. For example, ER-MIMU-01 uses MEMS accelerometer and gyroscope with high quality and reliability, RS422 and external communication, baud rate can be flexibly set between 9600~921600, through the communication protocol to set the user’s required communication baud rate. With X, Y, Z three-axis precision gyro, X, Y, Z three-axis accelerometer with high resolution, can be output by RS422 X, Y, Z three axis of gyroscope and accelerometer’s original hexadecimal complement data (including gyro hexadecimal complement the numerical temperature, angle, the accelerometer hexadecimal temperature, the acceleration hexadecimal complement number); It can also output float dimensionless values of the gyroscope and accelerometer processed by the underlying calculation.

2.FOG IMU
As the name suggests, FOG IMU is a type of IMU that uses optical fiber to measure the angular velocity or rotation rate of any object. Due to the low noise of fiber optic gyroscopes, this technology has been widely used in demanding navigation applications. FOGs are inherently more accurate and stable than MEMS-based systems, making them an alternative in scenarios where GNSS signals are unavailable (such as mining and navigation applications) or where GNSS denial may occur. Another notable feature of FOG is its rapid north-seeking capability. The FOG IMU can measure the Earth’s angular rotation and achieve heading in just minutes, even as the IMU itself moves.

Fiber optic gyroscopes use fiber optic loops and measure the interference of light beams in opposing loops to detect rotation about each axis. The hardware used is more expensive, larger and typically consumes more power, but its lack of moving parts makes it less sensitive to temperature changes and mechanical vibrations. And FOG gyrocompasses have no moving parts, making them more resistant to vibration and shock than MEMS gyrocompasses. This means they are ideally suited for applications that may experience intense vibration levels, such as demanding high temperature, high vibration environments such as mining, defence, surface ships and aerospace, and heavy equipment stabilization.

The ER-FIMU-50 FOG IMU is the most cost-effective inertial measurement device for navigation, control and dynamic measurements. This is the smallest FOG IMU developed by Ericco (same performance as KVH 1775). The system uses high-reliability closed-loop fiber optic gyroscopes and accelerometers, and uses multiple compensation techniques to ensure measurement accuracy. Strict technology is used in the manufacturing process to ensure that the angular and linear motion parameters of the carrier can be accurately measured under harsh conditions.

The product has a good user experience. In addition to wide voltage power supply, users can also configure the output bandwidth, data update rate, communication port baud rate and communication protocol as needed. It can be used in aviation track reference systems, guidance control systems, ship attitude measurement, inertial/satellite integrated navigation systems, drilling systems, mobile surveying and mapping systems, mobile satellite communications and other fields.

Differences between MEMS IMU and FOG IMU:
Due to the different gyroscopes they have, these two types of IMUs are very different. Their 10 differences will be introduced below.

1.Technology:
MEMS IMUs use microelectromechanical systems with accelerometers and gyroscopes based on micromachining technology.The FOG IMU uses a fiber optic gyroscope, which utilizes the principle of light interference in optical fibers.

2.Working principles:
MEMS IMUs use the deflection of microstructures due to acceleration or rotation to measure motion.
The FOG IMU measures motion by detecting the phase shift of light due to rotation in a fiber optic coil.

3.Accuracy:
Compared to MEMS IMUs, FOG IMUs have higher accuracy and precision. This makes them particularly useful in situations where users rely on the IMU for long periods of time: accurate systems can calculate a position close to the true position even after hours.

4.Size and shape:
MEMS IMUs are smaller and more compact, making them suitable for integration into smaller devices such as consumer electronics.
FOG IMUs are typically larger and heavier, and are often used in larger systems such as aerospace and defense applications.

5.Cost:
MEMS IMUs are generally more cost-effective than FOG IMUs, making them widely used in consumer products.
FOG IMUs are more expensive due to their greater precision, highly specialized internal components, and complex advanced manufacturing processes.

6.Energy consumption:
MEMS IMUs typically have lower power consumption, making them suitable for portable and battery-operated devices.
FOG IMUs consume more power, which is less important in applications such as land vehicles and aircraft where power supplies are readily available.

7.Application:
MEMS IMU can be used for north seeking in logging tools/gyro tools, pointing, steering and initial alignment in guided weapons/drone launch systems in advanced mining/drilling equipment, direction in satellite antennas, target tracking systems , pointing and tracking navigation-level MEMS guidance and navigation in weapon systems, directional railway train systems, navigation-level MEMS IMU/INS for precise attitude and position, measurement of north seeking and positioning in geodesy/land mobile mapping systems, Oil exploration, bridges, high-rise buildings, towers, dam monitoring, geotechnical monitoring, mining, etc.
FOG IMUs are commonly used in aerospace, defense, marine navigation and other high-precision applications, particularly where GNSS is unavailable or unreliable, such as underground mining or military environments.

8.Robustness:
MEMS IMUs can be susceptible to environmental factors such as extreme temperatures and high vibrations, which can affect their accuracy.
The FOG IMU is more robust and stable, making it better suited for particularly harsh and demanding environments.

9.Calibration and automatic north seeking:
Due to possible drift over time, MEMS IMUs may require more frequent calibrations to maintain accuracy. They are not sensitive enough to automatically find an accurate heading unless connected to a GNSS receiver.
FOG IMUs have better long-term stability and typically require less frequent calibrations to maintain accurate heading or position. The most accurate fiber optic gyroscope systems are so sensitive that they can determine where north is by detecting the Earth’s rotation.

10.Integration:
Because MEMS IMUs are smaller and have lower power requirements, they are easier to integrate into compact devices.
FOG IMUs are typically used in larger systems to accommodate their size and weight and their higher power needs.

Selection of MEMS IMU and FOG IMU
In summary, the choice of IMU depends on the application and environment. You can choose the IMU type that suits you according to different application scenarios, accuracy requirements, etc.

MEMS IMUs are ideal for: lightweight, small size, low power consumption, short-range pointing sensors, and GNSS integration in predictable dynamic environments.

FOG IMU is suitable for: absolute attitude accuracy, high temperature, high vibration, bias stability over time.

Read the reference: https://www.ericcointernational.com/inertial-measurement-units

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Capacitive Micro Accelerometer

 

Accelerometer, as the name suggests, are used to measure acceleration, and today’s science and technology requires inexpensive, fully functional, and large-scale processing and production of  accelerometer. In many fields, such as aerospace, machinery, and military industry, small and lightweight accelerometer are needed that can achieve many functions and are easy to test. The acceleration sensor integrated on the silicon chip has many advantages, micro acceleration sensor due to small and lightweight, conducive to mass production and other advantages, has been widely used in military, scientific measurement and other fields.

Micro accelerometer definition

Micro-electro mechanical systems (MEMS) are integrated miniature systems that combine electronic, mechanical, and other components, ranging in size from a few nanometers to micrometers, and are widely recognized in many fields. At present, micro acceleration sensor is one of the most widely used MEMS devices, which has the advantages of microfluidization, integration, intelligence, low cost, high performance and easy mass production. Applications range from smart phones, personal computers, the automotive industry and military defense. The micro-mechanical acceleration sensor based on MEMS technology has many advantages such as small size, light weight, fast start-up, low power consumption, easy integration, good reliability, strong anti-overload ability and low cost. According to the different types of sensitive signals, it can be divided into capacitive, piezoresistive and other micro acceleration sensors.

Structure and characteristics of capacitive micro-accelerometer

Capacitive micro acceleration sensor is the mainstream of MEMS acceleration sensor. Its basic structure is a capacitor composed of mass block and fixed electrode. When the mass block is displaced by acceleration, the capacitance pole area or distance is changed, and the acceleration is measured by measuring the capacitance. Capacitive micro accelerometers are the most common and mature products. The basic principle is to use the capacitor as a detection interface to detect the micro-displacement of the inertial mass due to the inertial force. The mass is supported by the elastic microbeam support on the substrate. One plate of the detection capacitor is generally disposed on the moving mass, and one plate is disposed on the fixed substrate. Figure 2 shows a typical sandwich-type flat capacitive micro accelerometer. Also, the capacitive micro accelerometer developed by AD uses a comb-tooth array capacitor as a detection interface. The capacitive micro accelerometer has high sensitivity and measurement accuracy, good stability, small temperature drift, low power consumption, and strong overload protection capability. It can realize feedback closed-loop control by using electrostatic force, which significantly improves the performance of the sensor.

For high-precision MEMS micro-accelerometers, performance stability is an important index of product quality. Under the same test conditions, such as temperature, humidity, etc., the repeatability of the same performance parameters of the device over a long period of time is called the stability of the device. Especially for capacitive MEMS micro accelerometers, due to changes in environmental factors or long-term storage, its zero drift and scale factors will change, thus reducing the accuracy of the device, resulting in long-term stability problems. Ericco introduced ER-MA-5,bias stability (Allan Curve)is 5μg, has the characteristics of small size, light weight, low energy consumption, can be widely used in vibration detection, attitude control, security alarm, consumer applications, motion recognition and state recording and other fields. The accelerometer is combined with a gyroscope and magnetometer to form an inertial measurement Unit (IMU).

At present, capacitive micro accelerometers have been gradually mature in the application of smart phone smart wearable medical and automotive industries. At the same time, with the continuous expansion of the market and technological upgrading, the application market of accelerometers continues to expand. Accelerometer application scenarios will have a broader space for development.

If you want to get more details about accelerometer,pls visit https://www.ericcointernational.com/accelerometer/

How to Correctly Select the Parameters of the Tilt Sensor

 

You need to choose your own tilt sensor according to your own needs.

Basic parameters of tilt sensor

1. Range

The range is the maximum range that can be measured by the sensor, which refers to the value of the difference between the upper and lower limits. Each sensor has its own measurement range, and when it is measured in this range, the output signal of the sensor has a certain accuracy. An acceleration sensor with a range of less than 1G is used as an inclination sensor, and a range of more than 1G is used as an acceleration sensor or vibration sensor. Because the larger the range, the less accurate. The biaxial tilt sensor ER-TS-4250VO can be selected in a range of +90°, while the uniaxial tilt sensor can be selected in a vertical direction of 360°.

2. Accuracy

In the process of testing and measurement, measurement errors are inevitable. There are two kinds of errors: systematic error and random error. The causes of system error, such as the inherent error of measurement principle and algorithm, inaccurate calibration, environmental temperature influence, material defects, etc., can be used to reflect the degree of influence of system error. The cause of random error is the gap of transmission components, aging of electronic components, etc. The precision can be used to reflect the influence of random error. Precision is a comprehensive index that reflects systematic error and random error, the higher the precision means the higher the accuracy and precision.

3. Zero offset

Zero drift means that when the input of the sensor is constant zero, the output value of the sensor will still change slightly to a certain extent. There are many reasons for zero drift, such as the change of the characteristics of the sensitive elements in the sensor with time, stress release, element aging, charge leakage, environmental temperature changes, etc. Among them, the zero drift caused by ambient temperature change is the most common phenomenon.

4. Input frequency

The frequency response characteristics determine the frequency range to be measured, and must allow the frequency range without losing the true measurement conditions, in fact, the sensor response will always have a certain delay. The higher the frequency response of the sensor, the wider the frequency range of the signal that can be measured, and the greater the interference. The lower the sensor frequency response, the narrower the frequency range of the signal that can be measured, and the lower the interference. In practical applications, a large number of measured signals are time changes, such as changes in current value, changes in object displacement, changes in acceleration, etc. This requires that the output of the sensor not only accurately reflect the size being measured, but also keep up with the speed of change being measured. In the frequency response range of the sensor, the amplitude of its output has a small change in the -constant range (maximum attenuation is 0.707). Therefore, when the input value is done sine

When changing, it is usually believed that the output value can correctly reflect the input value, but when the input value changes more frequently, the output value will produce significant attenuation, resulting in large measurement distortion.

5. Communication interface

RS232, TTL, RS485, RS232, CAN bus, voltage, current, SPI, IC, Profibus interface optional.If you want to learn more about tilt sensors or buy

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Application of Quartz Accelerometer in oil Drilling Field

 

With the sharp increase of energy consumption worldwide, oil and natural gas are still the first place in the world's energy consumption, and oil and gas will remain the irreplaceable main energy for economic and social development for a long time in the future. Therefore, the exploration and development of oil and gas resources has gradually entered deep, deep water, unconventional and other complex oil and gas, and complex oil and gas has become an important replacement energy in China and even the world.   

  In the process of drilling, drilling and completion fluid faces many challenges, such as insufficient temperature resistance of treatment agent and difficult control of rheological properties of drilling and completion fluid. These problems bring many difficulties to drilling work and put forward higher requirements for drilling equipment. Deep oil and gas drilling mainly has the following problems: bottom hole ultra-high temperature, high pressure, high stress environment.      

The ER-QA-03D is an accelerometer designed for applications in the oil and gas field with a measuring range of ±30g and zero bias stability of 50μg. The maximum operating temperature is 180℃and the impact resistance is 500-1000g 0.5ms. Aiming at the need of small installation space, the volume of ER-QA-03F is only Φ18.2×16mm and the page can ensure normal operation in high temperature environment. it's bias repeatability is 150-220μg and scale factor repeatability is 150-220 PPM.  

  Although there are many difficulties in the process of oil exploitation, with the progress of science and technology and the continuous improvement of quartz accelerometer technology, it can ensure that more resources are excavated in a safe operating environment.

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Do You Know What IMU is?

Gyroscopes and accelerometers are the main components of IMU, and their accuracy directly affects the accuracy of inertial system. In practical work, due to various inevitable interference factors, gyroscopes and accelerometers produce errors. From the initial alignment, its navigation error increases with time, especially the position error, which is the main disadvantage of inertial navigation system. Therefore, it is necessary to use external information to realize integrated navigation, so as to effectively reduce the problem of error accumulation with time. In order to improve reliability, more sensors can be equipped for each shaft. Generally speaking, IMU should be installed on the center of gravity of the measured object.

Generally, an IMU includes three single axis accelerometers and three single axis gyroscopes. The accelerometers detect the acceleration signals of the independent three axes of the object in the carrier coordinate system, while the gyroscopes detect the angular velocity signals of the carrier relative to the navigation coordinate system, measure the angular velocity and acceleration of the object in three-dimensional space, and calculate the attitude of the object based on this. It has very important application value in navigation.

IMU is mostly used in devices that need motion control, such as cars and robots. It is also used in occasions that require precise displacement estimation with attitude, such as inertial navigation equipment of submarines, aircraft, missiles and spacecraft.

🎀Summary

Using the three-axis geomagnetic decoupling and three-axis accelerometer, it is greatly affected by the acceleration of external forces. In the environment of motion / vibration, the output direction angle error is large. This field magnetic sensor has shortcomings. Its absolute reference is the magnetic line of force of the geomagnetic field. Geomagnetism is characterized by a wide range of use, but the intensity is low, about a few tenths of a Gauss, which is very easy to be disturbed by other magnets. If the instantaneous angle of the z-axis gyroscope is combined, It can make the system data more stable. The acceleration is measured in the direction of gravity. In the absence of external force acceleration, it can accurately output the roll/pitch two axis attitude angle, and this angle will not have cumulative error, which is accurate in a longer time scale. However, the disadvantage of the acceleration sensor in measuring the angle is that the acceleration sensor actually uses MEMS technology to detect the small deformation caused by the inertial force, and the inertial force is the same as gravity in essence, so the accelerometer will not distinguish the acceleration of gravity and the acceleration of external force. When the system changes speed in three-dimensional space, its output is incorrect.

The output angular velocity of the gyroscope is an instantaneous quantity, which cannot be directly used in attitude balance. The angular velocity and time integration are required to calculate the angle. The angle change obtained is added to the initial angle to obtain the target angle. The smaller the integration time DT is, the more accurate the output angle is. However, the principle of gyroscope determines that its measurement benchmark is itself, and there is no absolute reference outside the system. In addition, DT cannot be infinitely small, so the cumulative error of integration will increase rapidly with the passage of time, resulting in the output angle inconsistent with the actual, so the gyroscope can only work in a relatively short time scale.

Therefore, on the basis of no other reference, in order to obtain a more real attitude angle, we should use the weighting algorithm to develop strengths and circumvent weaknesses, combine the advantages of the two, abandon their respective shortcomings, and design an algorithm to increase the weight of the gyroscope in a short time scale, and increase the speed weight in a longer time scale, so that the system output angle is close to the real value.

🎀Working principle of IMU

IMU is a strapdown inertial navigation system. The system consists of three acceleration sensors and three angular velocity sensors (gyroscopes). The accelerometer is used to sense the acceleration component of the aircraft relative to the vertical line of the ground, and the velocity sensor is used to sense the angle information of the aircraft. This sub component is mainly composed of two a/d converters ad7716bs and 64K e/eprom memory X25650. The a/d converter uses the analog variables of each sensor of IMU, After being converted into digital information and calculated by CPU, the aircraft pitch angle, tilt angle and sideslip angle are finally output. The e/eprom memory mainly stores the linear curve diagram of each IMU sensor and the part number and serial number of each IMU sensor. When the part is just started, the image processing unit reads the linear curve parameters in e/eprom to provide initial information for subsequent angle calculation. Ericco provides FOG IMU and MEMS IMU solution, if you are interested, please feel free to contact us.

If you want to know more about IMU, please contact me through the following ways:

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Tilt Sensor for 360° Dip Angle Measurement Under Coal Mine

Ericco 360D tilt sensor is designed by Ericco sensor for measuring tilt angle in coal mines. The tilt sensor can also be used in mining, steel, chemical industry and other fields that need to measure or control perpendicularity or tilt angle. Tilt sensors can be safely used in coal mines, oil fields and other environments with explosive gases.

360° Tilt Sensor (ER-TS-4250VO) Parameters:

Protection: intrinsically safe circuit design

Output interface: RS232,TTL, RS485,RS422, CAN, 0-5V, 0-10V, 4-20mA, 0-20mA optional, maximum output current＜45mA, accuracy class ≤0.1°, basic error＜+0.1%FS, protection level IP67

Measurement angle range: X.Y dual axis 0°~360° in any specified angle.

2. Material of tilt sensor:

The shell material of the tilt sensor is brass, fully sealed, and the leading wire is the mine explosion-proof flame-retardant wire, with high safety.

Ericco 360D Tilt sensor Features:

Easy installation, shock resistance, vibration resistance, high reliability, oil and corrosive gas resistance, explosion proof, cost-effective.

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What is Sensitivity and Measurement Range in Quartz Accelerometer?

 A quartz accelerometer is an instrument used to measure acceleration, commonly found in physics, engineering, and other related fields. When  design and select the quartz accelerometer,we need to consider the main specifications are: measurement range, sensitivity. The sensitivity of acceleration sensor is one of the most basic indicators of sensor. The sensitivity of the sensor directly affects the measurement of vibration signal. The measurement range of the acceleration value sensor refers to the maximum measurement value that the sensor can measure within a certain nonlinear error range. The nonlinear error of the universal piezoelectric acceleration sensor is mostly 1%. As a general principle,the higher the sensitivity, the smaller the measurement range,and the smaller the sensitivity, the larger the measurement range.

The sensitivity of the  quartz accelerometer

Sensitivity refers to the degree of response of a quartz accelerometer to changes in acceleration, usually expressed by changes in output voltage or current caused by changes in unit acceleration. The higher the sensitivity, the stronger the quartz accelerometer’s ability to detect small acceleration changes. Quartz accelerometer sensitivity – generally expressed in units such as WV/g or pC/g.

The capacitance of quartz flexible accelerometer is an important factor affecting its sensitivity. Generally,the larger the capacitor, the higher its vibration sensitivity, but the higher the sensitivity, the noise immunity will be reduced accordingly. Therefore, the capacitance size of the quartz flexible accelerometer needs to be selected reasonably.

The measuring range of the quartz  accelerometer

The measuring range of the quartz accelerometer is the maximum acceleration that can be measured under the specified performance index, which is measured and evaluated from both positive and negative directions. The level of acceleration supported by the acceleration sensor output signal specification is usually expressed as ±g, which is the maximum acceleration that the device can measure and accurately represent through its output. The ER-QA-03B designed for this purpose has measurement range of ±70g and bias repeatability of 10-30μg and scale factor repeatability of 15-50ppm. It is of great significance to improve the measuring range of quartz flexible accelerometer. First, with the rapid development of strapdown inertial navigation systems in recent years, the range of accelerometers that are required to be sensitive to has increased exponentially. If the magnitude of the linear acceleration exceeds the measurement range of the accelerometer, noise can be found in the output of the accelerometer, in which case the accelerometer does not properly reflect the input motion acceleration. Therefore, expanding the measuring range of quartz flexible accelerometer is an important strategic means to meet the needs of model development. In addition, the application field of quartz flexible accelerometer has gradually expanded, and many application fields have wider and wider measurement range requirements.

Application characteristics

The two basic indicators of sensitivity and measurement range have different requirements for different application scenarios. In the selection of quartz accelerometer, it is necessary to consider these indicators according to the actual needs to choose the most suitable quartz accelerometer. For example, in the field of aerospace, quartz accelerometers are required to have high sensitivity and a wide frequency response range in order to detect small changes in acceleration, and ER-QA-03A is designed for applications in this field with zero bias repeatability of 10-50μg and scale factor repeatability of 15-50 PPM. The second-order nonlinear repeatability is 10-30μg/g2.In the industrial production process, the linearity and temperature stability of the quartz accelerometer are required to ensure the accuracy of the measurement results.

Quartz  accelerometer has been widely used in various inertial navigation systems, and is one of the important measurement sensitive components in inertial navigation systems. With the development of inertial navigation systems, the requirements for inertial devices are becoming higher and higher. Sensitivity and measurement range as the most basic indicators of quartz accelerometer, but also the breakthrough point of the future development of quartz accelerometer technology.

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