The inertial navigation system (INS) uses the inertial sensor (IMU) to measure the specific force and angular velocity information of the carrier, combined with the given initial conditions, and integrates with the information of the GNSS and other systems, so as to calculate the speed, position, attitude and other parameters in real time. navigation system. Specifically, the inertial navigation system belongs to a kind of reckoning navigation method. That is to say, from the position of a known point, the position of the next point can be calculated according to the continuously measured heading angle and speed of the carrier, so that the current position of the moving body can be continuously measured.
1.Inertial navigation system is an indispensable key component in autonomous driving
Inertial navigation is irreplaceable in automatic driving positioning system. Inertial navigation has the unique advantages of uninterrupted output information and no external interference. It can ensure that the vehicle motion parameters are output at high frequency at any time, and provide continuous vehicle position and attitude information for the decision center, which is unmatched by any sensor. .
Inertial navigation systems are the only devices that can output complete six-degree-of-freedom data.
Inertial navigation can calculate the translation (position, velocity, acceleration) and rotation (angle, angular velocity) of the three dimensions of x, y, and z, and can infer the measured values of other sensor states through the observation model, and then use the predicted value and The difference of the measured values is used for weighted filtering. To obtain real-time attitude, azimuth, velocity and position, inertial navigation is the only option.
The data update frequency of inertial navigation is higher, which can provide high-frequency positioning result output.
The frame rate of the camera is generally 30Hz, and the time uncertainty is 33ms; the GNSS delay is generally 100-200ms; and the shortest delay of the inertial navigation prediction state is only a few ms, so the inertial navigation can be used to estimate and compensate the delay of other sensors to achieve global Synchronize. When the vehicle is driving, the delay of GNSS is 100ms. When the camera shoots the environmental target, the actual position of the image will be inconsistent with the position reported by GNSS. Assuming the speed of the car is 120km/h, the delay of 100ms means the delay of the distance of 3.3 meters. At this time, the accuracy of map and target recognition is meaningless. However, if combined inertial navigation is used, the position delay will be about 2.5ms, and the resulting error is only 0.08m, which can better ensure the safety of driving.
Inertial navigation can be used as a fusion center for positioning information, integrating information from lidar, cameras, and body systems.
In L3 and higher-level autonomous vehicles, more sensors will be introduced to support the functions of the system. The inertial navigation system is the easiest subject of all positioning technologies to integrate with the positioning information provided by other sensors as positioning information. The fusion center integrates visual sensors, radar, lidar, and body system information at a deeper level to provide accurate and reliable continuous vehicle position and attitude information for the decision-making layer.
2.The composition of inertial navigation
The reason why inertial navigation is called [inertial] navigation is because it uses [inertial devices], that is, accelerometers, gyroscopes, magnetometers, barometers, etc.
The accelerometer is the core element of the inertial navigation system. Accelerometers measure acceleration, using the principle a=F/M to measure the “inertial force” of an object.
The accelerometer is used to measure the linear acceleration of the system in the inertial reference frame, but it can only measure the acceleration relative to the direction of motion of the system (since the accelerometer is fixed to the system and rotates with the system, it does not know its own direction). This can be imagined as a blindfolded passenger squeezing the seat back when the car accelerates, leaning forward when the car brakes, pressing down on the seat when the car accelerates uphill, and bouncing off the seat when the car goes downhill over a hill, based only on With this information, the passenger knows how the car is accelerating relative to itself, ie forward, backward, up, down, left or right, but not in the direction relative to the ground.
Classification of accelerometers:
According to the relationship between input and output, it can be divided into ordinary type, integral type and quadratic integral type;
According to physical principles, it can be divided into pendulum type and non-pendulum type. Pendulum accelerometers include pendulum integral accelerometers, liquid-floating accelerometers and flexible pendulum accelerometers. Non-pendulum accelerometers include vibrating beam accelerometers and electrostatic accelerometers. Accelerometer;
According to the degrees of freedom of measurement, it can be divided into uniaxial, biaxial and triaxial;
According to the measurement accuracy, it can be divided into three categories: high accuracy (better than 10-4m/s2), medium accuracy (10-2 m/s2–10-3 m/s2) and low accuracy (less than 0.1m/s2). ERICCO mainly researches and manufactures high-precision accelerometers.
A gyroscope in the traditional sense is an object mounted in a frame that rotates at high speed around the axis of symmetry of the body of revolution. The gyroscope has stability and precession, and these characteristics are used to make a rate gyroscope sensitive to angular velocity and a position gyroscope sensitive to angular deviation. Since optics, MEMS and other technologies have been introduced into the development of gyroscopes, it is now customary to refer to devices that can perform gyroscope functions as gyroscopes.
According to its accuracy range, it is roughly divided into ultra-high-precision gyroscopes, medium-high-precision gyroscopes, and low-precision gyroscopes. Ultra-high-precision gyroscopes mainly include liquid-floating gyroscopes, electrostatic gyroscopes, etc. At present, the highest-precision gyroscopes are electrostatic gyroscopes. Medium-high-precision gyroscope refers to the accuracy, and the most promising gyroscope at present is MEMS gyroscope. The MEMS gyroscope has been exported to more than 50 countries around the world and is widely used in medium and high precision applications. Although the low-precision gyroscope has low precision, its low price makes it have broad application prospects.
The magnetometer/geomagnetic field sensor, it has a popular name: electronic compass. When the acceleration sensor is completely horizontal, it can be expected that the gravity sensor cannot distinguish the rotation angle on the horizontal plane, that is, the rotation around the Z axis cannot be displayed. At this time, only the gyroscope can detect it. Although the dynamic of the gyroscope is very fast, because its working principle is integration, there will be accumulated errors in the static state, and the angle will always increase or decrease. So we will need a sensor that can confirm the orientation in the horizontal position. This is the third sensor necessary for IMU today, the geomagnetic field sensor. Through the mutual correction of these three sensors, we can finally get a more accurate theoretically. attitude parameters.
The barometric pressure sensor is an instrument for detecting atmospheric pressure, and the barometric pressure sensor can be used as an altimeter in practical applications. In inertial navigation systems, Z-axis dynamics and accuracy are sometimes enhanced by adding a barometer.
3.Inertial navigation classification
According to the realization form of mechanical arrangement, it can be divided into: strapdown inertial navigation system and platform inertial navigation system
Strapdown inertial navigation: the inertial measurement devices (accelerometers and gyroscopes) are directly installed on carriers such as aircraft, ships, and missiles. The strapdown is based on a mathematical platform and is directly connected to the carrier. It is easy to install, maintain and replace. Small, the defect is that the inertial measurement device is connected to the carrier, which leads to the deterioration of its working environment and the decrease of measurement accuracy.
Platform inertial navigation system: an inertial navigation system that connects inertial elements such as gyroscope and acceleration to the moving load through the gimbal angular motion isolation system. Its inertial measurement device (accelerometer and gyroscope) is installed on the electromechanical navigation platform, and the motion parameters of the carrier are measured based on the platform coordinate system. The platform inertial navigation system isolates the angular motion of the carrier through the frame servo system, so it can obtain higher system accuracy. The platform inertial navigation system adopts the frame servo system with high development level, and the cost and maintenance cost are also high. The defect is that the electromechanical platform controlled by the frame servo will affect its reliability.
4.The integrated navigation system (INS) composed of GNSS+IMU is the mainstream positioning system solution
The inertial navigation system is combined with the initial point of the vehicle obtained by satellite positioning, and real-time precise positioning can be obtained. The principle of the inertial navigation system is to obtain the relative displacement variable through the quadratic integration of the acceleration. However, the absolute position of the vehicle cannot be obtained only by relying on inertial navigation. Therefore, the initial point information of the vehicle obtained by GNSS must be added, that is, the method of original reference point + relative displacement can jointly achieve accurate and real-time position update.
GNSS can provide centimeter-level positioning when the satellite signal is good, but in scenarios where the satellite signal is lost or the signal is weak, such as between underground garages and urban buildings, the positioning accuracy provided will be greatly reduced. Inertial navigation can provide stable signals independent of the external environment.
The GNSS update frequency is too low (only 10Hz) to provide sufficient real-time location updates. The update frequency of the IMU can reach 100Hz or higher, which can fully make up for the lack of real-time performance of GNSS. The combined GNSS/IMU system can help autonomous driving complete positioning through global positioning and inertial update data at frequencies up to 100Hz.
When the satellite signal is good, the INS system can normally output the centimeter-level positioning of GNSS; when the satellite signal is weak, the inertial navigation system can rely on the IMU signal to provide positioning information.
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