What is an Inertial Sensor?

What is an inertial sensor?

Inertial sensors include accelerometers (also called accelerometers) and angular velocity sensors (also called gyroscopes), as well as their single, double and three axis combined inertial measurement units (also called IMUs) and AHRs.Here we focus on accelerometers and gyroscopes.

Accelerometer by detecting quality (also known as the sensitive quality), bearing, potentiometer, springs, dampers, and shell, is using the principle of acceleration calculation objects in space motion state, at first the accelerometer is induction surface vertical acceleration, early detection is only used in the aircraft in the instrument system overload. After the function upgrade, optimization, now can actually sense the acceleration of the object in any direction. At present, the mainstream is the 3-axis accelerometer, which measures the acceleration data of an object in the X, Y and Z axes in the spatial coordinate system, and can fully reflect the motion properties of the object translation.

Unlike accelerometer measurement dimensions, according to the definition of gyroscope, can easily understand the gyroscope is mainly by measuring the vertical axis of the gyro rotor in space coordinate system and the angle between the object, now the mainstream of the gyroscope is also a three axis, the measuring objects on the X, Y, Z axis rotation, pitch, horizontal and vertical wave respectively.

The earliest gyroscopes are mechanical gyroscopes with high-speed rotating gyroscopes built in. Because gyroscopes can maintain high-speed and stable rotation on the universal support, the earliest gyroscopes are used to identify the direction, determine the attitude and calculate the angular velocity in navigation. Later, they are gradually applied to aircraft instruments. But the mechanical type of machining precision requirements are very high, but also easy to be affected by external vibration, so the calculation accuracy of mechanical gyroscope has been not high.

Here have an example, during world war 2, the germans made a missile inertial guidance system, which is used by accelerometer and gyroscope, accelerometer to measure acceleration, calculation of missile flight distance and route, and determine missile direction and angular velocity gyroscope, but because of the precision of the gyroscope is insufficient, Germany’s missile wanted to bomb London. It nearly blew up every nook and cranny of Britain.

Later, in order to improve precision and applicability, the principle of the gyroscope has not merely mechanical, is now developing a laser gyroscope (optical path difference principle), fiber optic gyroscope (sagna stark effect, a kind of optical path difference) and MEMS gyroscope (namely, MEMS, it is according to the principle of coriolis force, its internal capacitance change is used to calculate the angular velocity, Micro electromechanical gyroscopes are the most common in smartphones).

A quick word about magnetometers. Magnetometers work in a similar way to compass-they measure the position of an object in four geographic directions, north, south, east, and west.

In general, an accelerometer tells how far an object has traveled, a gyroscope calculates how much it has rotated, and a magnetometer tells which direction an object is facing. Therefore, in many practical applications, accelerometers, gyroscopes and magnetometers are combined. The most commonly used 6-axis sensors are three-axis accelerometers and three-axis gyroscopes, which are collectively referred to as inertial measurement unit (IMU). A 9-axis sensor usually refers to a three-axis accelerometer, a three-axis gyroscope, and a three-axis magnetometer. There is also a six-axis accelerometer plus a three-axis gyroscope, or a three-axis accelerometer and a six-axis gyroscope.

It is worth mentioning that the movement of an object only has 6 degrees of freedom (DOF), that is, the three degrees of translation of the accelerometer and the three degrees of rotation of the gyroscope, so now some manufacturers boast that their products can measure 9 degrees of freedom is exaggerated.

In recent years, high-precision positioning and high-precision navigation have become well-known for autonomous driving and 5G commercial use. But before this, the satellite positioning (GPS) is the daily use of the function, the use of people know, in the signal is not good, the navigation position will occur elegant, roughly also about 2.5-5m, some people will say that the most wrong way or detour. That’s fine, but if you think about it on autopilot, such navigation could have even more serious consequences.

Therefore, at present, the mainstream positioning scheme in the fields of automatic driving is to use GPS+IMU together. Theoretically, GPS can roughly position the vehicle, and then the autonomous navigation system composed of IMU which is not disturbed by the outside world can accurately correct the vehicle. As the last positioning threshold of automatic driving, it will also be the most important threshold. The importance of the safety and stability of IMU is self-evident.

According to the relevant design of aircraft control system introduced in the previous part, IMU has become a standard component of aircraft control system. When sensors fail on an aircraft, the stability of flight is directly affected. According to the level, if the GPS or magnetometer fails, then the aircraft can choose to switch the safety level to ensure flight safety by using sensors such as gyroscope and accelerometer. However, if the gyroscope data in the IMU fails, there is only one outcome, waiting for the plane to crash.

Similarly, the autopilot control system in the future, once the last line of defense of IMU fails, GPS positioning are not allowed to be not happen, there’s hope for safe driving, but once the vehicles through the tunnel or poor reception area, so in the case of GPS and IMU can work normally, the result and the plane will be the same.

Not only in automatic driving, IMU has been widely used in robots. How to ensure the safety of IMU in high-risk work or human-computer cooperation will also be an important research topic in the future.

IMU is indeed applied in every aspect of daily life, and will be further applied in autonomous driving, unmanned aerial vehicles, industrial robots and other fields. At that point, the IMU will not only be a threat to privacy, but also to workplace and life safety.

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