Those things about map collection vehicles | Inertial Navigation

1. Background

High-precision maps and high-precision collection vehicles are some common words that students in the field of maps and travel often talk about. However, students outside the circle may ask, what exactly is high precision?

High-precision refers to high-precision positioning , and high-precision map refers to a map with rich geographic information data and high-precision coordinates. Of course, a high-precision collection vehicle is a special operating vehicle that collects and produces high-precision map data.

Some curious friends will break the casserole and ask in the end, how is high precision achieved? How can it be called high precision?

In fact, high-precision standards are not quite sure, but the basic thought centimeter above accuracy can be regarded as precision. The realization of high-precision mainly relies on various sensors, the most important of which is the high-precision positioning and orientation system, which includes satellite positioning and inertial navigation.

This article mainly introduces some situations in this respect from the perspective of hardware and its application in actual work.

2. Term explanation

: POS system (Position and Orientation System, POS), refers to the high-precision position and attitude measurement system of inertial navigation + GNSS satellite navigation combination, using the satellite receiver installed on the carrier to accurately determine the spatial position, using The inertial measurement device measures the instantaneous sensor posture, combines the two through a precise clock, and finally obtains the carrier’s speed, posture, position and other information through calculations.

Inertial navigation system : abbreviated as inertial navigation, is a navigation parameter calculation system with gyroscope and accelerometer as sensitive devices. According to the output of the gyroscope/accelerometer, the navigation coordinate system is established to calculate the speed and the carrier in the navigation coordinate system. Location.

IMU : Inertial Measurement Unit (Inertial Measurement Unit), a device that measures the three-axis attitude angle (or angular rate) and acceleration of an object. IMU is part of the inertial navigation system.

GNSS: Global Navigation Satellite System (Global Navigation Satellite System) refers to all satellite navigation systems, including global, regional and enhanced, such as GPS in the United States, Glonass in Russia, Galileo in Europe, and BeiDou satellite in China Navigation systems, and related enhancement systems, such as WAAS (Wide Area Augmentation System) in the United States, EGNOS (European Geographical Navigation Overlapping System) and MSAS (Multi-Function Transport Satellite Augmentation System) in Japan, etc., also cover the construction and Other satellite navigation systems to be built in the future.

Remarks: Our internal habit is to refer to the positioning part as inertial navigation for short. In fact, it not only includes inertial navigation equipment, but also refers to a positioning and orientation system that includes a complete set of software and hardware.

Third, what does the high-precision positioning and orientation system include?

The high-precision positioning and orientation system on the acquisition vehicle generally consists of the following parts:

The composition of the positioning and orientation system

Algorithm process

The whole system is composed of hardware and supporting software and algorithms. As there are a lot of researches on combinatorial solving algorithms in the industry, various methods and ideas are also blooming. There are related students in AutoNavi doing development work in this area. Therefore, this article only focuses on the introduction from the hardware perspective.

The composition of the low-precision and high-precision positioning system is similar, the difference is only the accuracy level of the sensors (IMU, GNSS).

Fourth, the role and role of each sensor

1.GNSS

Similar to the GPS that we often said before, but with the advancement of science and technology, our country's Beidou system BD can also match GPS in accuracy and reliability, and it plays an important role in practical applications.

From modules of tens of yuan to high-precision GNSS boards of tens of thousands of yuan, the positioning principle is basically the same. The distance between the satellite with a known position and the user receiver is measured, and then the data of multiple satellites is integrated. Can know the specific location of the receiver. The satellite position can be found in the satellite ephemeris based on the time recorded by the on-board clock.

But in terms of signal-to-noise ratio, frequency bands, number of constellations, number of channels, signal capture and tracking, etc., high-precision GNSS boards are significantly better than ordinary GPS. For example, mobile phones generally only support the C/A code of GPS L1 band, but professional grade Basically all of the boards support L1/L2/L5 multi-band and multi-channel.

In addition to being significantly better than ordinary modules in real-time positioning accuracy, professional GNSS boards can also perform post-processing with an accuracy of millimeters. This is the value of professional receiver boards.

Major satellite receiver manufacturers in the industry include foreign Trimble, Novatel, Leica LeiCa, Topcon, etc., as well as domestic Beidouxingtong, China Test, China Haida, Sinan, etc.

The main function of GNSS in the system is to obtain the absolute coordinates of the current position. The advantage is that there is no accumulation of position errors, and the disadvantage is that the update frequency is low, generally not exceeding 10~50HZ.

2.IMU

In fact, the IMU is the core of the high-precision positioning and orientation system commonly known as inertial navigation, and the price also reflects this: For example, a device worth nearly one million yuan, the satellite receiver part only accounts for tens of thousands of yuan, and most of the rest are It is the IMU fee.

To some extent, when choosing a positioning and attitude fixation system, the focus is actually on choosing IMU, because the GNSS part selection is relatively simple and intuitive.

This article mainly focuses on IMU later in this article.

The IMU is usually composed of three single-axis accelerometers and three single-axis gyroscopes. The accelerometer detects the acceleration signal of the carrier independent of the three axes in the coordinate system, and the gyroscope detects the angular velocity signal of the carrier relative to the coordinate system. After the signal is processed, the attitude of the carrier can be calculated.

It is worth noting that the IMU and the inertial calculation algorithm provide a relative original positioning information. Its function is to measure the route of the movement relative to the starting point, so it does not provide information about your specific location. Therefore, it is often used together with GNSS. When some GNSS signals are weak or missing, IMU can play its role, allowing the carrier to continuously obtain absolute position and attitude information.

The update frequency of the IMU is relatively high, generally up to several hundred to 1KHz. Using three acceleration values, the displacement can be obtained through two integrations to achieve position positioning, and the angular velocity value integration can obtain attitude information, which can be combined to obtain the actual state of the object.

Don't look at the technology of IMU, which is relatively unfamiliar. In fact, the mobile phones we use every day, as well as cars, airplanes, and even spacecraft will use IMU. The difference lies in materials, cost and accuracy.

According to different usage scenarios, there are different requirements for the accuracy of the IMU. High accuracy also means high cost. Our high-precision cars, of course, use the highest grade.

IMU price and accuracy comparison

3.

The standard odometer is usually externally mounted on the wheel, and a rotary encoder is built-in, which is driven by the wheel to rotate together. The function is to measure the linear distance of the vehicle movement and help suppress drift errors when the satellite loses lock. There are many forms of odometer, except for rotary encoders, magnetic grid type, Hall type, etc. have applications.

4. Enhanced and auxiliary means

Mainly some land-based and satellite-based augmentation technologies, including RTK, RTD, PPK, PPP, DGPS, various SBAS, etc. We rarely use satellite-based augmentation systems in our daily operations, and mainly use land-based augmentation systems. . High-precision collection generally adopts the method of post-difference analysis and processing.

5. How is the inertial navigation made?

As mentioned earlier, the positioning and orientation system includes inertial navigation. The main hardware of inertial navigation is IMU, and IMU is composed of two parts: gyroscope and accelerometer.

1. Gyroscope

Let's take a look at what the top looks like? Yes, it's the stuff below.

What is the relationship between this gyro and the angular velocity sensor in the inertial navigation equipment that we usually understand?

In terms of production process, materials and principles, it doesn't matter much, they are two different things. But these things can all be used as rotational angular velocity sensors, and this kind of mechanical gyroscope-like products were first used and preconceived, so this angular velocity sensor was later called a gyroscope.

In fact, there are many types of gyroscopes, which can be roughly divided into mechanical gyroscopes, laser gyroscopes and fiber optic gyroscopes, micromechanical (MEMS) gyroscopes, and so on based on physical principles.

Mechanical gyroscope : It was used extensively in early aircraft. It is large in size, complex in structure, and poor in accuracy. There are some improved types in the later period, such as: ball bearing free gyroscope, liquid floating gyroscope and so on.

laser gyroscope : Use the optical path difference in Sagnac theory to measure the rotational angular velocity. The main point is: when the light beam travels in a circular channel, if the circular channel itself has a rotation speed, then the time required for the light to travel along the direction of rotation of the channel is longer than that of traveling in the opposite direction along the channel. It takes more time.

Sagnac experiment

This Sagnac theory is very interesting, and interested students can deepen their understanding.

The laser gyroscope is actually a ring laser. In the closed light path, the two beams of light transmitted in the clockwise and counterclockwise directions from the same light source interfere with light. By detecting the phase difference or the change of interference fringes, the rotational angular velocity of the closed light path can be measured. When the ring laser is in a static state, the optical path of the two laser beams is the same, so the frequency is the same, the difference between the two frequencies (frequency difference) is zero, and the interference fringe is zero.

When the ring laser rotates around the axis perpendicular to the plane of the closed optical path, the light length of the light that is consistent with the direction of rotation is extended, the wavelength increases, and the frequency decreases; the other light is the opposite, so there is a frequency difference, forming interference fringes.

The laser gyroscope has no internal moving parts, low data drift rate, high reliability, and high measurement accuracy. Due to the optical path difference, the laser gyroscope will have a certain speed threshold. Below this threshold, the angular velocity change may not be detected. In addition, because of the need to use a ring laser, the entire device is bulky and costly.

Fiber optic gyroscope : Strictly speaking, it is also a laser gyroscope. The principle is the same as that of a laser gyroscope, except that the ring laser is replaced by an optical fiber. The cost of the optical fiber is low, but it is susceptible to uneven thermal expansion and contraction caused by temperature changes and tension changes during winding, so the accuracy is slightly lower. The light of the laser gyroscope propagates in the resonant cavity and is less affected by the outside world, so the accuracy is high, but the resonant cavity is expensive. Because of its advantages in cost and size, fiber optic gyroscopes have been widely used in practice.

Micro-mechanical gyroscope : MEMS gyroscope, which is a combination of micro-mechanical structure etched by semiconductor technology and CMOS circuit technology. It is characterized by small size, low cost, and easy mass production. It is widely used in mobile phones, portable devices and other areas that do not require high performance. In addition, with the advancement of technology, some high-end MEMS gyroscopes have an accuracy comparable to fiber optic gyroscopes.

The principle of MEMS gyroscope is to use the Coriolis force-the tangential force that a rotating object receives when there is radial movement to sense the angular velocity. The vibrating object is suspended on the base by a soft elastic structure. The overall dynamic system is a two-dimensional elastic damping system, in which the Coriolis force induced by vibration and rotation converts the energy proportional to the angular velocity into the sensing mode, and calculates the output.

2. Accelerometer

The function of the accelerometer is to measure the acceleration force of the carrier to determine the position of the carrier in space and monitor the movement. The acceleration is a vector and the rate of change of speed.

There are many types of accelerometers, from material , there are quartz flexible, liquid floating accelerometer and MEMS.

According to the sensor element classification, it can be roughly divided into several types: piezoelectric accelerometer, piezoresistive accelerometer and capacitive accelerometer, in addition to heat flow type and resonance type.

piezoelectric accelerometer uses the piezoelectric effect (piezoelectric material generates electricity when subjected to physical stress) to sense changes in acceleration. Piezoelectric accelerometers are most commonly used for vibration and shock measurement. Piezoresistive accelerometers have much lower sensitivity than piezoelectric accelerometers and are more suitable for vehicle crash tests. The resistance of a piezoresistive accelerometer is proportional to the pressure applied to it.

The most commonly used accelerometer is the capacitive accelerometer , which uses changes in capacitance to determine the acceleration of an object. When the sensor experiences acceleration, the distance between its capacitive plates changes with the movement of the sensor diaphragm, thereby detecting the acceleration value.

3. Why do you want to do integrated navigation

In practical applications, it is difficult for a single navigation and positioning mode to meet the navigation performance requirements. An effective way to improve the overall performance of the navigation system is to use integrated navigation technology, that is, to use two or more dissimilar navigation systems to perform navigation information on the same navigation information. Measure and calculate to form measured values, and calculate and correct the errors of each navigation system from these measured values.

For example, a gyroscope measures angular velocity, and acceleration measures linear acceleration. The former is the principle of inertia, and the latter is the principle of force balance. The measured value of the accelerometer in a long time is correct, but in a short time due to the existence of signal noise, there is an error. The gyroscope is more accurate in a short period of time, but in a longer period of time, there will be errors due to drift. Therefore, the two need to be adjusted to ensure the correct signal.

For another example, in the entire system, the IMU provides the carrier attitude and can calculate the relative position. Its advantages are high update frequency, continuous output, stable data, and good short-term stability, but the disadvantage is that there is error accumulation, accuracy diverges with time, and long-term The stability is poor, because the position information and attitude information are obtained by integration. The advantages of GNSS satellite navigation are that there is no accumulation of position errors and good long-term stability. The disadvantage is that the update frequency is low and the signal may be discontinuous.

Therefore, GNSS and IMU can complement each other, and long-term absolute positioning can be achieved through GNSS. IMU can be used to calculate and position in the gap of GNSS position update, and GNSS can be used to correct errors.

Since IMU and GNSS complement each other in performance, the combination of these two devices as a positioning and orientation system design is the best solution recognized in the industry.

Six, the main parameters of IMU

For performance parameter requirements, it is necessary to start with two aspects of accelerometer and gyroscope. There are many related indicators. Generally, the following are mainly concerned:

Seven, error factors affecting accuracy

In the inertial navigation system, the hardware part that affects the accuracy is mainly the performance of the IMU. The error source is shown in the following figure:

Inertial navigation error source

1. Gyroscope influencing factors

As the core sensor on inertial navigation equipment, the importance of the gyroscope is self-evident. The posture estimation data largely depends on the data quality of the angular velocity, so the accuracy of the gyroscope will directly affect the degree of the solution. In other words, whether the IMU can correctly perceive the attitude of the carrier depends on the accuracy performance of the gyroscope. .

2. Accelerometer influencing factors

In the IMU, the influence of the accelerometer is mainly reflected in the stability and accuracy of the accelerometer. Among them, the high accuracy of the accelerometer is to ensure the accuracy of subsequent data processing, and the stability of the accelerometer is one of the key factors that directly affect the normal performance of the IMU.

3. Temperature influencing factors

When the temperature changes, the accuracy of the sensor will have a big difference. Under normal circumstances, the working environment of the inertial device cannot be a constant temperature environment, especially the accuracy of the gyro is seriously affected, so the influence of temperature cannot be ignored.

4. Error handling

There are many sources of error in the positioning and orientation system. The errors in the hardware of inertial devices are generally divided into two categories: systematic errors and random errors. The essence of systematic errors is to find regular errors, so they can be compensated in real time, mainly including constant value offset, scale factor, installation error, etc.

However, random error generally refers to noise, and it is difficult to find a suitable relationship function to describe the noise, so it is difficult to deal with. Generally, methods such as allan variance and time series analysis are used to perform error modeling analysis on the zero offset data. For example, the Kalman filter algorithm can be used to reduce the influence of random noise.

8. How to choose a positioning system

When designing a high-precision acquisition system, an important thing is the selection of inertial navigation equipment, because it is not only related to the hardware cost, but also related to the accuracy performance of the final product. When selecting a specific model, one of the main tasks is to examine the IMU. It is nothing more than paying attention to the following aspects, and then making choices based on product requirements.

• Whether the various index parameters of the sensor meet the demand.
• Whether the price of the sensor is reasonable and whether the supply chain is perfect.
• Whether the design difficulty of supporting software and hardware is acceptable.
• Whether the manufacturer’s technical support capabilities and supporting services are good.

The most important of these is the first and also a prerequisite. Only when the technical performance meets the requirements, the next cost and business considerations will be carried out.

1. Indicator analysis

We still analyze from the most important technical aspects and introduce several key indicators in the selection process of gyro:

Range

The range is first determined when selecting the sensor. What field the selected sensor is used for, generally for vehicles, the gyroscope should be selected within 300 degrees/sec and the accelerometer within 4G. Others are based on your own use. Choose the scene, for example, the airborne range needs to be larger, and the rail transit can be smaller. In the case of high accuracy, a smaller range corresponds to a higher accuracy.

Bias and Bias Stability

In principle, the gyroscope will drift when it is powered on or starts to work. It is divided into constant drift and random drift . The constant drift is called zero offset, which can also be called zero drift. It is °/h, °/s. By obtaining the zero offset of the gyroscope, we can compensate it in subsequent use, but the compensation is the average value of multiple measurements. After compensation, the constant value drift will still have some residuals in the output of the gyroscope. Therefore, The zero offset repeatability index in the gyro test appears, which characterizes the close repeatability of the gyro's zero offset each time. After calibration and compensation of the gyro with good offset repeatability, the residual constant drift is relatively small, and higher accuracy can be achieved. .

The bias stability is obtained by calculating the variance of the output data of the gyro during a power-on process. The constant drift mentioned above is deducted when calculating the variance. Therefore, the bias stability reflects the random drift index of the gyro, also known as random noise.

The zero bias and zero bias stability largely reflect the performance of gyroscopes and accelerometers. It has long been regarded as a key indicator of inertial device specifications. When selecting a model, the appropriate model should be selected according to the cost and accuracy requirements.

Angle random walk

When the gyroscope is in the zero input state, the output signal is a superposition of white noise and a slowly varying random function. Diffuse random function can be used to determine the zero bias and zero bias stability index.

The error of the attitude and velocity length value generated by the random walk of the gyro is zero, but there is a certain oscillation error in the speed and attitude, and the amplitude of the oscillation is related to the magnitude of the random walk of the gyro drift. Random walk will cause position error with larger oscillation amplitude, but the mean position error does not increase linearly with time, but presents a random walk process.

The random walk reflects the development level of the gyroscope, and also reflects the minimum detectable angular rate of the gyroscope.

physical conditions

(1) Dimensions

The evaluation of the external dimensions mainly needs to select the appropriate size according to the actual vehicle installation situation, and the IMU needs to have good environmental adaptability. For the IMU body, the position of its center of mass should be as close as possible to the physical center of the IMU.

(2) Electrical and interface requirements

The conventional vehicle power supply is 9-16V, and the selection should consider whether its working voltage range is in line with it, otherwise, a power conversion module needs to be added. In addition, it is necessary to investigate whether there are self-protection functions such as short circuit and overvoltage.

For the interface, it is necessary to clarify the interface type and cable form, such as USB/Ethernet/serial port, etc., whether the data rate matches the communication form, etc., to avoid data interruption and loss during use.

(3) Environmental requirements

Inertial sensors are very sensitive to temperature changes, so you need to pay attention to their temperature drift related indicators. For some devices with large temperature drift, you should pay attention to the installation environment and try to provide a stable working environment temperature, such as adding a fan/air conditioner for heat dissipation, or Heating device, etc.

In addition, pay attention to the protection level of the IMU, choose to install it outside or inside the car, and take appropriate waterproof and dustproof measures.

(4) Relevant standards and specifications

Including but not limited to:

• General specification for vehicle satellite navigation equipment (GB/T 19392-2013).
• Reliability test standard for in-vehicle electronic equipment (ISO 16750).
• International standard for functional safety of road vehicles (ISO 26262 2018).
• Environmental conditions and tests for electrical and electronic equipment of road vehicles (GB/T 28046).

2. Focus on testing

For products that focus on performance and single functions, such as inertial navigation equipment, you need to focus on their actual performance. You cannot fully believe the indicators in the technical manual. For example, the following two commonly used mainstream equipment produced by major manufacturers, you can see that the parameters are not different. , The accuracy is also very high, how is the actual performance?

Trimble POS LV 510 nominal accuracy index:

Novatel SPAN-CPT nominal accuracy index:

In fact, you get what you pay for, and the price naturally represents performance. The accuracy of POS LV510 is actually much better than CPT, so the application scenarios of the two are also different. 510 can be used for high-precision collection, and CPT is only used for update or ADAS collection. .

In addition, don't be confused by the indicators. The nominal accuracy of 0.02 meters is the result of ideal operating conditions.

In addition to looking at the indicators, how else to choose?

As the saying goes, a mule is a horse, and you can pull it out for a walk. This also applies to the selection. Our method is: actual measurement. After all, practice is the only criterion for testing truth.

At present, positioning and orientation system-level test methods mainly include: actual motion test, software simulation test, software-hardware combined semi-physical physical simulation test . Among them, the actual sports test is obviously the most intuitive and true. We usually use the sports car test to investigate the accuracy.

The test phase is generally divided into several steps: equipment preparation, route planning, data collection, and data analysis.

equipment preparation : To evaluate the accuracy of the positioning and orientation system, a reference standard is required, so that when driving the same route, the trajectory and attitude error of the tested equipment can be accurately quantified. In practice, a very high level is often used. The high equipment is used as the reference, that is, the true value, which is installed on the same carrier platform together with the tested equipment to collect the trajectory and carrier attitude of the same route.

During installation, the part containing the IMU needs to be firmly installed on the platform, and the measurement direction of the body should be kept orthogonal/parallel to the forward and horizontal direction of the vehicle, and can not be shaken freely; the installation position of the antenna should be guaranteed to be unobstructed, and the satellite should be in good condition and nearby No sources of interference. If necessary, the odometer needs to be connected. The power and data cables are connected in accordance with regulations.

Route planning : Different scenarios have different requirements for positioning systems, but for us, it is mainly road data collection, and collection of all road conditions, which is not only good satellite signals such as highways and rural roads , Road sections that do not require high inertial navigation, including urban ordinary roads, loop lines, viaducts, parking lots, ordinary national highways and other scenarios where satellite signals are obscured or even interrupted. Therefore, the test section selection should be as comprehensive as possible to cover these scenarios.

In addition to the requirements of the normal test scenario, because the inertial navigation often needs to be initialized, an open area and a place with good satellite signals should be selected at the starting and ending points of the test line.

data collection : data collection is relatively simple, abide by the relevant operating specifications, and collect the data of the device under test and the true value device according to the specified line. In order to ensure reliability and consistency and facilitate comparison, the same line is often collected multiple times. As for the reference station, you can set it up by yourself, or you can use data from service providers such as Chihiro.

data analysis : After the sports car is over, the collected inertial navigation, GNSS raw data, and reference station data enter the post-processing software for various preprocessing, format conversion, filtering, difference analysis, fusion, smoothing and a series of processes , Output trajectory and attitude data.

data analysis

Multiple trajectories can be selected and compared with the output of the true value device on indicators such as plane position, elevation, heading angle, roll and pitch angle, etc., to obtain the error range under various scenarios, and analyze the performance of the tested device based on this. Selection is the basis.

In addition to using higher-level equipment as the true value to verify the accuracy of the equipment under test, other methods can also be used, for example, a calibrated close-range photogrammetric acquisition system + control field method to estimate the accuracy.

Nine, summary

This article roughly introduces the basic situation of inertial navigation equipment and its sensors in the positioning and orientation system from the perspective of hardware. In fact, in the application of these sensors, there are still many links that need to be processed before they can reach the state of use by the end user. Environmental testing, aging, screening, turntable calibration, etc. before leaving the factory are also very important, and the combined calculation algorithm is even more critical. Only when a complete set of these combined punches are beaten, high precision can be achieved.

For students who are interested in positioning and inertial navigation, it is recommended to read Yongyuan , Yan series of books, written very systematically.