introduction
Tractor-modified, aerial guided, with pockets – this was the prototype of the world's first AGV in 1953. Under the mainstream trend of industrial automation, the AGV industry has ushered in rapid development, and has now been widely used in factory handling, warehousing and logistics, medical equipment transportation, automatic parking and other scenarios. After more than 60 years of development, the AGV that originally used wire navigation has basically been eliminated, and has evolved a variety of ways to navigate.
This article will introduce ten mainstream AGV navigation methods. And the mainstream development direction of navigation in the future is briefly discussed.
1. Magnetic nail navigation
Magnetic nail navigation requires a magnetic stripe sensor to locate the left and right deviation of AGV relative to the path, and the difference between magnetic nail navigation and magnetic stripe navigation is that the magnetic stripe is laid continuously, and the magnetic nail is discretely laid.
If you need to use magnetic nails to navigate, you need to lay a large number of magnetic nails, and magnetic nail guidance is more used in terminal AGVs.
Advantages: low cost, mature technology, good concealment, more beautiful than tape navigation, strong anti-interference, wear resistance, acid and alkali resistance.
Disadvantages: The AGV path is susceptible to the influence of ferromagnetic substances, and the construction of the path change is large, and the construction of magnetic nails will have a certain impact on the ground.
2. Electromagnetic navigation
Electromagnetic guidance is a more traditional guidance mode, the realization form is to bury a metal wire on the driving path of the automatic guided vehicle, and load a low-frequency, low-voltage current on the metal wire, generate a magnetic field, realize navigation through the identification and tracking of the strength of the guided magnetic field through the vehicle-mounted electromagnetic sensor, and completely specify the task by reading the RFID card that is embedded in advance.
Electromagnetic guidance has a wide range of applications in manufacturing (such as automobile manufacturing) where the route is relatively simple and requires 24-hour continuous operation.
Advantages: the lead is hidden, not easy to be polluted and damaged, the guiding principle is simple and reliable, easy to control and communicate, no interference with sound and light, and the manufacturing cost is low.
Disadvantages: The path is difficult to change and expand, the limitation of complex paths is large, and the requirements for RFID hardware are high.
3. Tape navigation
The principle of magnetic tape guidance is relatively similar to that of electromagnetic guidance, and it is also to lay magnetic tape on the driving path of the automatic guided vehicle, and realize the guidance mode through the identification of the magnetic field signal by the vehicle-mounted electromagnetic sensor.
Magnetic stripe guidance is suitable for ground-embedded, light-load traction, and can be used in non-metallic ground, non-degaussing indoor environment, and can be stable and long-lasting operation.
Advantages: Flexibility, easy to change or expand paths, simple and easy to lay magnetic stripes.
Disadvantages: The navigation method will be affected by the mechanical damage of hard objects such as metal through the loop, which has a certain impact on navigation.
4. Laser navigation
Laser navigation generally refers to laser navigation based on reflector positioning, and the specific principle is to install a reflector with precise position around the AGV driving path, and the laser scanner will be installed on the AGV body.
The laser scanner emits a laser beam along the AGV's walk, and the laser beam is directly reflected back by multiple sets of reflectors laid along the AGV's driving path, triggering the controller to record the angle of the rotating laser head when it encounters the reflector. Based on these angular values that match the actual position of the reflectors, the controller calculates the absolute coordinates of the AGV, and based on this principle, very precise laser guidance can be achieved.
Advantages: AGV positioning is accurate, no other positioning facilities are required on the ground, the driving path can be flexible and changeable, and can be adapted to a variety of on-site environments, which is the advanced navigation mode preferred by many foreign AGV manufacturers.
Disadvantages: high manufacturing cost, relatively harsh environmental requirements (external light, ground requirements, visibility requirements, etc.)
5. QR code navigation
QR code guidance, the sign of coordinates is realized through a QR code on the ground. The two-dimensional code guidance is similar to the magnetic nail guidance, but the coordinate markers are different.
The principle of QR code navigation is that the AGV scans the ground QR code through the camera, and obtains the current location information by analyzing the QR code information. QR code navigation is often combined with inertial navigation to achieve precise positioning.
QR code navigation is currently very hot in the market, the main reason is that Amazon acquired the KIVA QR code navigation robot at a high price, and its chessboard-like working mode is impressive. The cost of a single machine of a QR code-guided mobile robot is low, but a large number of QR codes need to be laid at the project site, and the QR codes are easy to wear and tear, and the maintenance cost is high.
Advantages: Flexibility, easy to change or expand paths, simple and easy to lay magnetic stripes.
Disadvantages: The navigation method will be affected by the mechanical damage of hard objects such as metal through the loop, which has a certain impact on navigation
6. Optical ribbon navigation
Ribbon guidance is a method of setting optical signs (pasting ribbons or painting) on the form path of an automated guided vehicle, and realizing guidance by acquiring image signals from the vehicle's optical sensor. Optical guidance is similar to tape guidance, but the main advantages are that the road surface is easier to pave, the expansion and change of the path is easier than that of tape guidance, and the cost is low. The disadvantage is that the ribbon is more susceptible to pollution and damage, the requirements for the environment are high, the reliability of the guidance is subject to the ground conditions, and the stopping positioning accuracy is low.
The ribbon guide is suitable for occasions with clean working environment, good ground flatness, and low requirements for AGV positioning accuracy.
Advantages: Flexibility, simple and easy ground route setup.
Disadvantages: Sensitive to ribbon contamination and mechanical wear, too demanding for the environment, poor navigation reliability, and difficult to achieve precise positioning.
7. Inertial navigation
Inertial navigation is the use of internal sensors of mobile robots to obtain postures. Mainly use optical encoders, gyroscopes, or both.
The wheels of the mobile robot are equipped with photoelectric encoders, and in the process of movement, the mobile robot uses the pulse signal of the encoder to carry out rough dead reckoning to determine the posture of the mobile robot. The three-axis angular velocity and acceleration of the mobile robot can be obtained by using the gyroscope, and the pose information can be obtained through the integration operation, and the two dead reckoning can be fused. In general, inertial navigation is used as an auxiliary positioning for other navigation methods.
Advantages: advanced technology, high positioning accuracy, strong flexibility, easy to combine and compatible, wide range of applications, has been adopted by many foreign AGV manufacturers.
Disadvantages: The manufacturing cost is high, and the accuracy and reliability of the guidance are closely related to the manufacturing accuracy and service life of the gyroscope.
8. GPS navigation
GPS (Global Positioning System) navigation is a method of navigating by acquiring location and heading information from GPS sensors. The navigation accuracy of GPS navigation is low, and the position error is about 10 meters.
GPS navigation is mainly used in the positioning of automobiles, ships, mobile phones, etc., and is less used in indoor AGV positioning with high accuracy requirements.
Advantages: It is usually used for outdoor long-distance tracking and guidance, and its accuracy depends on factors such as the fixed accuracy and number of satellites in the air, as well as the surrounding environment of the control object.
Disadvantages: The manufacturing cost of ground facilities is too high and unacceptable to the average user.
9. Visual Navigation
This is a method of navigating by acquiring image information of the circumference of the operating area by the on-board vision sensor of the automated guided vehicle. The hardware needs a down-looking camera, a fill light and a hood to support the implementation of this navigation mode, which can use rich ground texture information, calculate the displacement and rotation between the two images based on the phase correlation method, and then obtain the current position through integration.
In this method, the ground texture is automatically mapped by the camera of the mobile robot during the movement, and then the ground texture information obtained during operation is registered and compared with the texture image in the self-built map, so as to estimate the current pose of the mobile robot and realize the positioning of the mobile robot.
Visual navigation AGV is currently less used in the market, and the advantages of visual texture navigation are lower hardware cost and accurate positioning. The disadvantage is that the ground needs to have texture information, and when the running site area is large, the time to draw the navigation map is longer than that of laser navigation.
Advantages: The combination of image recognition technology and laser navigation technology will provide unexpected possibilities for automation engineering, such as the accuracy and reliability of navigation, the safety of driving, and intelligent memory recognition.
Disadvantages: The technology is not mature enough, and it is difficult to achieve commercial application at present.
10. Composite Navigation
Composite navigation refers to the method of applying two or more guidance (or navigation) methods to realize the operation of automatic guided vehicles. For example, the combination of two-dimensional code navigation and inertial navigation uses the characteristics of inertial navigation with high short-distance positioning accuracy, and the navigation blind area between the two two-dimensional codes uses inertial navigation. The combination of laser navigation and magnetic nail navigation is used to use magnetic nail navigation at the platform position with high positioning accuracy requirements to increase the stability of AGV positioning. Composite navigation is to adapt AGV to various common navigation methods in various use scenarios, and it will also be more and more widely used in various AGVs.
Advantages: high navigation accuracy and good effect.
Disadvantages: high cost, difficult design and installation.
The mainstream development direction of future navigation
Compared with other navigation methods, visual navigation has obvious advantages and is expected to become the mainstream development direction of navigation in the future, and its advantages are mainly reflected in the following three aspects:
(1) Low cost: The low cost of visual navigation is mainly reflected in the low cost of sensors and the low cost of operation and maintenance. The world's top brand of industrial-grade monocular camera plus lens is only 3000 yuan, and the price of ordinary commercial cameras is about 100 yuan, which is lower than the cost of laser sensors that cost 10,000 yuan at every turn. In addition, VisionNav Robotics' visual navigation technology in the natural environment does not require the use of any markers in the environment, reducing the operation and maintenance costs of the customer's site. Combining the above two points, the return on investment time for customers to purchase AGV equipment will be greatly shortened.
(2) No need to transform the on-site environment: The core of VisionNav Robotics is the visual positioning and navigation technology in the natural environment, which does not need to make any changes to the customer site, does not need artificial markers such as magnetic wires, magnetic nails, laser reflectors, etc., and can achieve efficient positioning and navigation only through visual natural features. Reduce the difficulty and time of project implementation.
(3) High performance: In principle, the visual capture of two-dimensional image information, the image contains not only contour information, but also color information. Among them, the color information cannot be obtained by other sensors. The color information is very valuable for the autonomous positioning, control, collision avoidance and correction of mobile robots. Through abundant visual information, VisionNav Robotics' AGV products and modification solutions can achieve efficient (linear speed of 2m/s, cornering speed of 1.5m/s (cornering without stopping)), high precision (position error <1cm), and high stability (error rate <0.1%).
Of course, pure visual navigation also has disadvantages, mainly because the current maturity of pure visual navigation technology is average, and commercial application is more difficult. A better way to do this is to use a combination of multiple navigation methods.