Brief Introduction of Accelerometer

 An accelerometer is an instrument that measures acceleration. Acceleration measurement is an important subject in engineering technology. When the object has a large acceleration, the object and the instruments and equipment carried by it and other objects without relative acceleration are subjected to forces that can produce the same large acceleration, that is, dynamic loads. To know the dynamic load, you need to measure the acceleration.

There are many types of accelerometers. Micro-mechanical accelerometers, also known as silicon accelerometers, are now widely used. The principle of sensing acceleration is the same as that of general accelerometers. According to the different reading elements, micro-mechanical accelerometers are classified into piezoresistive accelerometers, capacitive accelerometers, resonant beam accelerometers, and electrostatic force balanced accelerometers. The micro-mechanical accelerometer is small in size, easy to install, simple in measurement method, low in cost and strong in anti-overload capability, and satisfies the requirements for the structure and space limitation of the micro-mini aircraft.

Accelerometers consist of test masses (also called sensitive masses), supports, potentiometers, springs, dampers, and housings.  The detected mass is constrained by the support and can only move along the axis, which is often called the input axis or the sensitive axis. According to the number of input shafts, there are single-axis, dual-axis and triaxial accelerometers.

With the development of MEMS technology, inertial sensor is one of the most widely used MEMS devices, and micro-accelerometer is an outstanding representative of inertial sensor. The theoretical basis of the microaccelerometer is Newton’s second law, according to the basic principles of physics, within a system, the speed cannot be measured, but its acceleration can be measured. If the initial velocity is known, the linear velocity can be calculated by integrating, and the linear displacement can then be calculated. Combined with a gyroscope (used to measure angular velocity), the object can be precisely positioned. The high-precision MEMS accelerometer ER-MA-5 has a bias stability of 5 ug and a monthly bias repeatability of 100-300 ug.

Application

Car safety system

Accelerometers play an important role in automobile safety system. For example, when a car is in a collision, the accelerometer can detect changes in the vehicle’s acceleration and send signals to the airbag system to inflate it at the appropriate time, protecting the driver and passengers. In addition, accelerometers can also be used in vehicle stability control systems to help vehicles in emergency situations. Keep it steady.

Aerospace

Accelerometers are also widely used in the aerospace field. During a rocket launch, for example, an accelerometer can measure changes in the rocket’s acceleration to help control the trajectory of the rocket. In addition, the accelerometer can also be used in the aircraft’s autopilot system to help maintain stability.For example, the ER-QA-03A accelerometers commonly used in the aerospace field have a  bias stability of 10-50μg, and the Scale factor repeatability is 15-50ppm.

The existing problems and development trend 

The advancement of MEMS technology and the improvement of technological level also bring new opportunities to the development of micromechanical accelerometers. By understanding the research dynamics of micromechanical accelerometers at home and abroad, there are several development trends of micromechanical accelerometers in the future:

1. The  micromechanical  accelerometer with high resolution and large range has become the focus of research. Because the inertial mass block is relatively small, the inertial force used to measure acceleration and angular velocity is correspondingly small, and the sensitivity of the system is relatively low, so it is particularly important to develop a high-sensitivity accelerometer.

2. The development of multi-axis accelerometers has become a new direction. The inertial measurement combination has six output variables, three of which are mutually positive accelerations on the X, Y, and Z axes. There have been literature reports on the development of triaxial micro-silicon accelerometers, and the methods used are different, but its performance is still a long way from practical, and the structural design of multi-axis accelerometers is still a difficult point.

3. small temperature drift, small hysteresis effect has become a new performance target. The accuracy of micromachined accelerometers can be greatly improved by selecting suitable materials, adopting reasonable structure and applying new low-cost temperature compensation link.

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Easy to Understand IMU Explanation

IMU is the abbreviation of Inertial Measurement Unit. It is a sensor composed of an accelerometer, a gyroscope and a magnetometer, which is used to measure the motion status of an object such as angle, speed and acceleration. The accelerometer is used to measure the object’s acceleration and gravity, the gyroscope is used to measure the object’s angular velocity and gravity, the gyroscope is used to measure the object’s angular velocity, and the magnetometer is used to compensate for the interference of the geomagnetic field on the gyroscope.

As the core component of the inertial navigation system, IMU can help the system achieve autonomous navigation and positioning without being restricted by GPS, and has wide application value. It can play a role in a variety of fields such as drones, robots, missiles, aircraft, ships, camera stabilizers, etc. The magnetometer is used to compensate for the interference of the geomagnetic field on the gyroscope. The technology and parameters of IMU will be introduced below.

Features of IMU technology:

1.High accuracy: IMU can achieve high-precision measurement and prediction of motion status, and can achieve high measurement accuracy.

2.Compact and lightweight: The IMU is very small and can be equipped on various devices, such as drones, robots, etc.

3.Degree of freedom of motion: IMU can measure acceleration, angular velocity and magnetic field in three axes, and can achieve various combinations of six axes, nine axes, etc. to meet the needs of different applications.

4.Low energy consumption: The IMU has very low power consumption and can meet the needs of long-term continuous use.

Parameters of IMU technology:

1.Accelerometer: It can measure the acceleration of an object (including static and dynamic acceleration) and the acceleration of gravity, and can also measure the inclination angle of the object.

2.Gyroscope:It can measure the angular velocity of an object, also known as the rotational speed or rotation rate of the object.

3.Magnetometer: It can measure the geomagnetic field and can be used to help solve the direction and angle of objects.

4.Data frequency:IMU can output data at different frequencies, such as 100Hz, 200Hz or 1000Hz. The higher the data frequency, the stronger the real-time nature of the data.

Application areas

IMU technology is widely used, from satellites to subways, from aircraft to spacecraft, from drones to robots, everywhere. It can help devices achieve autonomous navigation and positioning, and can also be used in areas such as attitude sensing, tilt compensation, and vibration suppression. IMU technology is widely used in military, civilian, scientific and medical fields.

The emergence of IMU technology makes up for the shortcomings of GPS positioning. The two complement each other and enable autonomous vehicles to obtain the most accurate positioning information. It is worth noting that the IMU provides relative positioning information. Its function is to measure the path of an object relative to a starting point, so it cannot provide specific location information about your location. So often used with GPS. When in some places where the GPS signal is weak, the IMU can play its role, allowing the car to continuously obtain absolute position information to avoid getting “lost.” In fact, although the IMU technology seems strange, in fact, IMU is used in the mobile phones, cars, airplanes, and even missiles and spacecraft that we use daily. The difference is cost and accuracy.

According to different usage scenarios, there are different requirements for the accuracy of IMU. High precision also means high cost.

Low-precision IMU: Used in general consumer electronics, this low-precision IMU is very cheap. Widely used in mobile phones and sports watches, often used to record steps. ​

Medium-precision IMU: used in driverless cars, with prices ranging from a few hundred to tens of thousands of dollars, depending on the positioning accuracy requirements of the driverless car.

High-precision IMU: for missiles or space shuttles. Take missiles for example. From the time the missile is launched to hitting the target, the aerospace IMU can achieve very high-precision calculations, and the error can even be within one meter.

Conclusion

IMU technology has been developing rapidly. With technological innovation in various fields, the application scenarios of IMU technology will become more and more extensive. ERICCO INERTIAL SYSTEM has conducted more in-depth research on IMUs. For example, ER-MIMU01 can independently seek north and uses high-quality and reliable MEMS accelerometers and gyroscopes. It is equipped with X, Y, Z three-axis precision gyroscopes, X, Y, Z three-axis accelerometer, with high resolution, can output the original hexadecimal complement data of X, Y, Z three-axis gyroscope and accelerometer through RS422 (including gyroscope hexadecimal complement) numerical temperature, angle , accelerometer hexadecimal temperature, acceleration hexadecimal complement); it can also output floating point dimensionless values of gyroscopes and accelerometers processed by underlying calculations. If you want to better study and master IMU technology, it is necessary to study subjects such as physics, mathematics, and computer science. I hope this article will help you understand IMU. Ericco also has a variety of high-precision IMUs, north seekers, and navigators. Welcome to learn more!

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The Advantages and Disadvantages of the Transmission Mode of Tilt Sensor

In today’s industrial and commercial environment, tilt sensors are increasingly used to measure and monitor the tilt angle of equipment or structures. Depending on the data transmission method, tilt meters can be divided into wired and wireless types. In this article, we’ll delve into both types of tilt sensors, including the pros and cons, and how to choose the best solution for your specific needs. 

1. Advantages&disadvantages of wired tilt sensor

Wired inclination sensor built-in high performance MCU built-in algorithm, through a high oversampling rate, improve the high frequency characteristics of the data, through the data filtering algorithm to remove unreasonable accidental error data, Kalman filter algorithm for higher precision data processing, suitable for monitoring the structure deformation monitoring field with high monitoring frequency requirements. Wired tilt meters usually use RS485 bus or other similar bus protocols to transmit inclination signals. RS485 is a serial communication protocol widely used in the field of industrial automation, which has the advantages of noise suppression and high signal quality. ER-TS-3160VO is a linear inclination sensor with an accuracy of 0.01°, its cable standard length is 1.5m, and it can output RS232 at the same time, RS485 can be customized, the measurement range can reach 0~±180°, and the impact vibration resistance is strong.

The main advantage of the wired tilt meters is that the signal stability is high, and the signal quality is not easy to be disturbed because of the wired transmission mode. In addition, wired sensors have a long service life, lower maintenance costs, and a lower failure rate. However, this sensor also has some disadvantages, such as the need to lay cables, high requirements for the field environment, may exist in some application scenarios wiring difficulties. 

2. Advantages&disadvantages of wireless tilt sensor

Wireless tilt meters have become more and more popular in recent years, and common wireless inclination sensors include NB-IoT wireless inclination sensors and LORO wireless inclination sensors. These sensors transmit tilt signals via wireless communication technology without cable connections, making them highly flexible.

The main advantages of wireless tilt sensors are their flexibility and convenience. Since no wiring is required, the sensor can be easily installed anywhere it is needed, without considering the laying of cables. In addition, wireless sensors also have the advantages of high mobility, easy expansion and maintenance. However, wireless sensors also have some disadvantages, such as signal quality may be affected by radio interference, and signal stability and reliability may not be as good as wired sensors. ER-TS-12200-Modbus is a high-precision tilt sensor using Bluetooth and ZigBee wireless transmission technology of the internet of things, eliminating the complicated wiring and noise interference caused by long cable transmission, using lithium battery power supply, good long-term stability, zero drift, can automatically enter low-power sleep mode, get rid of the dependence on the use of environment.

Generally speaking, the mass of the building structure is huge, and the rate of change of inclination is relatively small, and there is a development process. The sampling frequency of conventional structural health monitoring is not high, and once a day can meet the requirements. In this case, the choice of the wireless inclination sensors with its own battery is the most appropriate, and the installation is very convenient.

3. How to choose the most suitable tilt sensor for you according to the transmission mode

When choosing a tilt meter, you need to consider the following factors:

(1) Application scenarios: Different application scenarios have different requirements for sensors. For example, in some scenarios where long-term stability measurements are required, wired sensors may be preferred; Some use cases, such as house monitoring, basically have 220V mains power in the field, and low cost wired tilt sensors can be used. Of course, for monitoring scenarios with high acquisition frequency requirements, choosing a wired inclination sensor is a wise choice. In scenarios that require flexible deployment, easy scaling and maintenance, wireless sensors may be more suitable.

(2) Signal quality: For some application scenarios that require high-precision measurement, such as precision equipment monitoring or large-scale structure monitoring, it is necessary to choose a wired sensor with higher signal quality. For some application scenarios with low precision requirements, such as logistics and transportation, agricultural monitoring, etc., wireless sensors may be enough to meet the needs. 

(3) Cost and maintenance: For some application scenarios that require a large number of sensors to be deployed, such as large-scale facility monitoring or logistics tracking, the deployment and maintenance cost of wireless sensors may be lower. For some cost-sensitive scenarios, such as small device monitoring or small structure monitoring, the cost of wired sensors may be lower.

(4) Durability and reliability: For some scenarios that require long-term continuous operation, such as equipment monitoring and fault warning systems in petrochemical, electric power and other fields, it is necessary to choose wired sensors with higher durability and reliability. For some scenarios that require portable and temporary use, such as construction sites, agricultural monitoring, etc., wireless sensors will be more suitable. 

In short, when choosing an inclination sensor, it is necessary to choose according to actual needs and specific scenarios. Both wired and wireless sensors have their own advantages and scope of application. Only by fully understanding the characteristics and application scenarios of various sensors can we make the most appropriate choice.

Application of IMU in UAV Flight Control System

Nowadays, with the development of chip, artificial intelligence and big data technology, UAV has begun the trend of intelligence, terminal and clustering. A large number of professional talents in automation, mechanical electronics, information engineering and microelectronics have been invested in UAV research and development. In a few years, UAVs have flown from military applications far away from people’s vision to ordinary people’s homes. It is undeniable that the development of flight control technology is the biggest driver of UAV changes in this decade.

Flight control is the abbreviation of flight control system, which can be regarded as the brain of aircraft. The flight control system is mainly used for flight attitude control and navigation. For flight control, it is necessary to know the current status of the aircraft, such as three-dimensional position, three-dimensional velocity, three-dimensional acceleration, three-axis angle and three-axis angular velocity. There are 15 states in total. The current flight control system uses an IMU, also known as inertial measurement unit, which is composed of three-axis gyroscope, three-axis accelerometer, three-axis geomagnetic sensor and barometer. So what is a three-axis gyroscope, a three-axis accelerometer, a three-axis geomagnetic sensor, and a barometer? What role do they play in the aircraft? What are the three axes?

The three axes of the three-axis gyroscope, three-axis accelerometer and three-axis geomagnetic sensor refer to the left and right of the aircraft, and the vertical up and down in the front and back directions, which are generally represented by XYZ. The left and right directions in the aircraft are called roll, the front and rear directions in the aircraft are called pitch, and the vertical direction is the Z axis. It is difficult for a gyroscope to stand on the ground when it does not rotate. Only when it rotates, it will stand on the ground. This is the gyro effect. According to the gyro effect, smart people invented a gyroscope. The earliest gyroscope was a high-speed rotating gyroscope, which was fixed in a frame through three flexible axes. No matter how the outer frame rotates, the high-speed rotating gyroscope in the middle always maintains a posture. The data such as the degree of rotation of the external frame can be calculated through the sensors on the three axes.

Because of its high cost and complex mechanical structure, it is now replaced by the electronic gyroscope. The advantages of the electronic gyroscope are low cost, small size and light weight, only a few grams, and its stability and accuracy are higher than those of the mechanical gyroscope. Speaking of this, you will understand the role of gyroscope in flight control. It is used to measure the inclination of the three XYZ axes.

So what does the three-axis accelerometer do? It was just said that the three-axis gyroscope is the three axes of XYZ. Now it goes without saying that the three-axis accelerometer is also the three axes of XYZ. When we start driving, we will feel a thrust behind us. This thrust is acceleration. Acceleration is the ratio of speed change to the time of occurrence of this change. It is a physical quantity describing the speed of object change. Every second power of meter. For example, when a car is stopped, its acceleration is 0. After starting, it takes 10 seconds from 0 meters per second to 10 meters per second. This is the acceleration of the car, If the vehicle travels at a speed of 10 meters per second, its acceleration is 0. Similarly, if it decelerates for 10 seconds, from 10 meters per second to 5 meters per second, its acceleration is negative. The three-axis accelerometer is used to measure the acceleration of the three axes of the aircraft XYZ.

Our daily travel is based on landmarks or memories to find our own direction. The geomagnetic sensor is a geomagnetic sensor, which is an electronic compass. It can let the aircraft know its flight direction, nose direction, and find the position of the mission and home. The barometer is used to measure the atmospheric pressure at the current position. It is known that the higher the altitude, the lower the pressure. This is why people have plateau reactions after arriving at the plateau. The barometer obtains the current altitude by measuring the pressure at different positions and calculating the pressure difference. This is the whole IMU inertial measurement unit. It plays a role in the aircraft to sense the change of the aircraft attitude, such as whether the aircraft is currently leaning forward or left and right, What is the role of the most basic attitude data, such as nose orientation and altitude, in flight control?

The most basic function of flight control is to control the balance of an aircraft when flying in the air, which is measured by IMU, sense the current inclination data of the aircraft and compile it into an electronic signal through the compiler. The signal is transmitted to the microcontroller inside the flight control through the new time of the signal. The microcontroller is responsible for the calculation. According to the current data of the aircraft, it calculates a compensation direction and angle, and then compiles the compensation data into an electronic signal, It is transmitted to the steering gear or motor. The motor or steering gear is executing the command to complete the compensation action. Then the sensor senses that the aircraft is stable, and sends the real-time data to the microcontroller again. The microcontroller will stop the compensation signal, which forms a cycle. Most flight controls are basically 10HZ internal cycles, that is, 10 refreshes per second.

This is the most basic function application of IMU in the flight control system. Without this function, once an angle is tilted, the aircraft will quickly lose balance and cause a crash.

Ericco’s MEMS IMU ER-MIMU-03 and ER-MIMU-04, ER-MIMU-07 and ER-MIMU-08 have built-in high-precision advanced MEMS gyroscopes and high-performance accelerometers, which can measure linear acceleration and angular velocity of rotation from three directions, and obtain carrier attitude, velocity and displacement information through analysis. They are specially designed for high-performance applications of inertial navigation equipment such as UAV flight control. Provides excellent stability in the temperature range of – 45° C to 80° C. The advanced gyro sensor design suppresses the linear acceleration effect of shock and vibration, enabling ER-MIMU-04, ER-MIMU-07 and ER-MIMU-08 to operate in harsh environments.

In addition to the application of ERICCO’s MEMS products in UAVs, its popularity in oil drilling, mining and other application markets is also growing. MEMS technology is developing into a huge industry. Just like the great changes brought to mankind by the microelectronics industry and computer industry in the past 20 years, MEMS has also bred a profound technological change, which has had a new round of impact on human society.

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Function and Test Analysis of Tilt Sensor Detection System

 At present, tilt sensors are widely used in construction machinery, road machinery, port machinery, lifting machinery and other construction machinery fields, which can realize real-time monitoring and control of angle-related state and attitude in the operation process of construction machinery. However, the inclinometer sensor often has defects such as nonlinear error and temperature drift error in practical application, so that the accuracy is easily affected and the measurement error is too large, which can not meet the requirements of practical application of construction machinery equipment. In addition, in the mass production process of the tilt gauge, it is easy to have the acceleration sensor installed tilt, so that the measurement angle is offset. Therefore, in order to improve the measurement accuracy of the inclinometer sensor, the product needs to be calibrated, temperature drift compensation and accuracy detection before leaving the factory to ensure that the product meets the factory standard.

1.Inclination sensordetection system

The high-precision inclinometer sensor detection system CAN intelligently control the temperature change of the thermostat, the position rotation of the servo motor and the CAN communication control with the tilt gauge. The test data can be displayed and controlled in real time through the interface designed by Labview software. The angle sensor can automate 0° angle calibration, temperature scale aging, angle linear calibration and angle error detection.

2.Inclinometer sensordetection system function

2.1 0° angle calibration function

The tilt measurement sensor detection system uses smooth marble as the cornerstone, which is not easy to shake or deform, and the plane is adjusted to the standard 0° plane by the measuring instrument. When the tilt gauge is located at 0° water level, the system controller SYMC sends 0° calibration instructions to correct the original 0° data.

2.2 Temperature bleaching aging function

The purpose of temperature drift aging test is to solve the problem that the performance of tilt sensor components is unstable with temperature changes. The specific reason is that the output of tilt sensor and speed sensor is PWM signal, and the waveform is shown in Figure 1.

Figure 1 PMW signal of acceleration sensor

The formula for calculating the output acceleration of the inclination sensor is a = (T1 /T2-0g output)/sensitivity

The output angle of tilt gauge is calculated by θ=sin-1 (a).

In the formula, 0g output and sensitivity are constants in theory, but in fact, they will change due to temperature changes in component performance. To realize temperature drift compensation correction for 0g output and sensitivity values of the tilt measurement sensor, the detection system needs to sample at least 3 typical temperature points (such as -30 ℃, 25 ℃, 60 ℃) under 0g output values and sensitivity values. The output values of the acceleration sensor at 0°, 90°, 180° and 270° were collected at each temperature point, and the output values of the acceleration sensor at other temperatures and positions were linearly corrected by piecewise linear compensation method, so as to calculate the 0g output and sensitivity values of the tilt gauge at different temperatures.

2.3 Angular linear calibration function

The inclinometer sensor adopts linear fitting linearization measures to make the input and output signals have a linear relationship. The specific algorithm adopts two-stage quantization method, such as: take 24° and 26°, and use a straight line connection to replace the original curve; 26 degrees and 28 degrees are also connected by a straight line. The piecewise linear fitting method is adopted for the whole 360°, so that the relationship between the measured data and the actual angle is close to linear relationship. In the algorithm, the more test angle points collected, the more the curve calculated by the algorithm approximates the linear relationship. The inclinometer sensor detection system can randomly select 120 angle sample points, start from 0° angle, every 3° interval, respectively, to carry out piecewise linear fitting of the tested inclination sensor, angle calibration, and maximize the nonlinear error of the inclination sensor.

2.4 Angle detection function

The angle detection system can compare the angle value measured by the inclination sensor with the corresponding angle value detected by the encoder in the detection system, so as to calculate the measurement error of each angle position of the tilt gauge. angle detection selects 60 test sample points, and the selected angle is between the selected point of calibration angle, so as to effectively detect the accuracy of linear fitting algorithm of tilt measurement sensor. The angle resolution of the inclinometer sensor product is ±0.1°, and the accuracy of the angle encoder selected by the detection system is ±0.003°, which can make the detection error resolution reach the thousandth grade, effectively ensuring the accuracy of the system angle detection.

3.Check system test requirements

The requirements of the high-precision inclination sensor detection system in the test: ① The ambient temperature in the thermostatic box should be constant, and the change is less than 0.5 ℃; ② Ensure that the positioning of the structure shaft is consistent with the position of the sensor on the turntable, and the deviation of the angle position of 0°, 90°, 180°, 270° is less than 0.1°; ③ Ensure that the turntable will not shake when rotating, and the sensor can be firmly installed on the turntable; ④ Ensure that the support components can be adjusted in height and match the size of the thermostat; The whole device should have sufficient stiffness and will not be deformed in high and low temperature environments.

4.Check the system test results

The high precision tilt sensor detection system can test 96 board tilt sensors at a time, and the entire test cycle is 6 hours. After the 0° calibration is completed, the system conducts temperature drift aging test. Figure 2 shows the temperature drift effect data of the single-board tilt sensor at 180.5° from -30 ℃ to 80 ℃. It can be seen that the temperature drift compensation front and rear tilt sensor has obvious temperature drift performance effects.

Figure 2 Data Curve of Temperature Coefficient

After the temperature bleaching, the system calibrates 120 test angles of the inclination sensor, and the angular resolution of the inclination sensor is ±0.1°. After the calibration of the inclinometer sensor is correct, the detection system randomly selects 60 angles within 360° (which do not coincide with the angle selected by the calibration), and calculates the measurement angle of the tilt measurement sensor and the feedback angle error of the system encoder. The angle error curve formed by the deviation between the measurement angle of the inclination sensor and the feedback angle of the system encoder is shown in Figure 3.

Figure 3 Error curve of angles

After the test is completed, it can be identified from the figure that the angle error of the white curve is greater than -0.3°, which is a unqualified product and needs to be returned to the factory for processing.

Conclusion: The measurement accuracy of tilt sensor can be greatly improved by calibrating, temperature drift compensation and accuracy detection. Ericco’s ER-TS-3160VO (accuracy 0.01°) and ER-TS-12200-Modbus (0.001°) have been calibrated at 0° angle, temperature drift aging function test, angle linear calibration and angle detection before leaving the factory, so their accuracy is not easily affected, resulting in excessive measurement errors.

The application of high precision inclination sensor detection system realizes intelligent testing instead of manual testing. Production practice has proved that the test cycle of the tilt gauge is reduced from the original 9 h to 6 h, and the unmanned test can still be carried out at night, to achieve two batches of test a day, and the number of test pieces per cycle is increased from the original 60 pieces to 96 pieces, which greatly saves labor costs and improves production efficiency and product quality.