As an emerging motion capture technology, the emergence of inertial motion capture has broken the industry structure dominated by optical motion capture and is considered as a new force in the motion capture industry. Motion capture technology based on inertial sensor system is a comprehensive technology that integrates sensor technology, wireless transmission, body dynamics, computer graphics and other disciplines. The technical threshold is very high. Although inertial motion capture technology has not been around for a long time, its excellent performance has quickly been demonstrated with its use in various industries.
Inertial motion capture is a new type of human motion capture technology. It uses a wireless motion gesture sensor to acquire the position and position of the body part, and uses the principle of human kinematics to recover the human motion model. At the same time, it uses wireless transmission to present data in computer software. .
Prior to the appearance of inertial motion capture systems, optical motion capture technology was the most common. It is to mark the point on the actor, and then use a high-speed camera to capture the exact displacement of the marker point, and then transfer the captured data to the computer equipment, thus completing the whole process of motion capture. The cost of an optically-acquired complete set of equipment is extremely expensive, it is cumbersome to set up, and it is susceptible to obstruction or light interference, which brings a lot of trouble to the post-processing work. For some severely obstructed movements, optical motion capture can not accurately restore actions such as squatting, cuddling, and scuffing in real time. The emergence of motion capture technology based on inertial sensor systems has greatly improved this situation.
Compared with optical motion capture technology, inertial motion capture technology has a high adaptability to capture the environment. Its technical advantages, cost advantages, and ease of use make it have excellent performance in various industries. In motion picture animation, experiential interactive games, virtual studios, live simulation exercises, sports training, medical rehabilitation and other fields, the inertial motion capture system has obvious advantages over other devices.
Inertial motion capture system principle
The general structure of motion capture system is mainly divided into three parts: data acquisition equipment, data transmission equipment, data processing unit, inertial motion capture system that is applied to the data acquisition end of inertial sensors, data processing unit through the principle of inertial navigation The collected data is processed to complete the attitude measurement of the moving target.
Wearing inertial sensor devices such as accelerometers, gyroscopes, and magnetometers at the important nodes of the moving object. The sensor device captures the motion data of the target object, including the posture and azimuth information of the body part, and transmits these data to the data transmission device. In the data processing equipment, after the data is corrected and processed, a three-dimensional model is finally established, and the three-dimensional model is made to move with the moving object truly and naturally.
After processing, the mobile capture data can be applied in animation production, gait analysis, biomechanics, ergonomics and other fields.
The role of accelerometer, gyroscope and magnetometer in inertial motion capture system
The accelerometer is used to detect the magnitude and direction of the acceleration received by the sensor. The accelerometer measures the force acting on the component in an axial direction to obtain the result in the form of axial acceleration and direction (XYZ), but it is used to The accuracy of the measurement device relative to the ground is not high, and the defect can be compensated by the gyroscope.
The working principle of the gyroscope is to measure the angle between the vertical axis of the gyro rotor in the three-dimensional coordinate system and the device, and calculate the angular velocity, and determine the motion state of the object in three-dimensional space through the included angle and angular velocity. Its strength lies in measuring the rotational motion of the device itself, but it cannot determine the orientation of the device. And just the magnetometer can make up for this shortcoming. Its strength lies in the orientation of the positioning equipment. It can measure the angle between the current equipment and the four directions of the northeast and southwest.
In motion capture systems, gyro sensors are used to handle rotational motion, accelerometers are used to handle linear motion, and magnetometers are used to handle directions. It's easy to understand—your gyroscope knows “what if we turn around?†The accelerometer knows “how long we move†and the magnetometer knows “our direction of motion.â€
In the motion capture system, the three sensors make full use of their respective characteristics to track the movement of the target object.
The advantages of inertial motion capture technology
Technical advantages
The inertial motion capture system has a small amount of signal, which can be used to complete the attitude tracking task in real time. The obtained gesture information has a large range, high sensitivity, and good dynamic performance. It has high adaptability to the capturing environment and is not subject to external environment such as light and background. Interference, and overcome the shortcomings of the camera surveillance area of ​​the optical motion capture system is limited; overcome the VR device often occluded problems, can be restored in real time, such as squatting, hugging, wrestling and other actions. In addition, the inertial motion capture system can also achieve multi-target capture.
Convenient advantages
Easy to use, the device is small and light, easy to wear.
Cost advantage
Compared with the low cost of optical motion capture, it not only can be used in film and television, games and other industries, but also help promote VR devices into the public life faster.
In general, inertial motion capture technology has a high adaptability to capture the environment, its technical advantages, cost advantages, and ease of use, making it in film and television animation, interactive experience games, virtual studios, live simulation exercises There are excellent performances in sports training, medical rehabilitation and other fields.
Inertial motion capture system disadvantages and solutions
Under normal circumstances, inertial motion capture system uses MEMS three-axis gyroscope, triaxial accelerometer and three-axis magnetometer inertial measurement unit (IMU, Inertial Measurement Unit) to measure the sensor's motion parameters. However, the sensor motion parameters measured by the IMU have serious noise interference, and the MEMS devices also have significant bias and drift. The inertial motion capture system cannot accurately track the human body posture for a long time. Only by solving this problem can the inertial motion capture system be fully utilized in the VR industry.
Solution for Inertia Capture Technology Weaknesses
First, the sensor motion data measured by the IMU is pre-processed to filter out noise interference that is doped in the original inertial data;
And then continue to carry out calibration and calibration, that is, the corresponding inertial devices are constantly compensated to solve the MEMS device's bias and drift, and improve its data accuracy and reliability;
Next, the attitude calculation is performed, and the attitude reference system is used to verify the accuracy of the attitude angle data, and ultimately achieve the entire inertial motion capture.
In addition, the difference is that domestic G-Wearables use IK + indoor positioning technology as the main motion capture algorithm, using inertial motion capture as an auxiliary algorithm. In this solution, indoor positioning technology is used to calibrate the inertial motion capture technology in real time, which avoids the constant calibration troubles.
IK algorithm
IK is the abbreviation of Inverse Kinematics, which is inverse kinematics. In the human layered structure, the joints and bones actually constitute the kinematic chain. For example, shoulder joints, elbow joints, wrist joints and their sub-skeletons are a kinematic chain, which is a branch of the entire human kinematic chain. The body uses the kinematic chain. Control the movement. The movement is divided into forward movement and reverse movement. Knowing the rotation angle of each joint on the chain, find the position information of each joint and the position information of the end effector, which is the problem of forward kinematics; and knowing the position information of the end effector, to determine its ancestral joints. The rotation angle and position, which is the inverse kinematics.
Inverse kinematics determines the motion of the entire skeleton based on the final angles of the main joints that determine the motion. It is usually used to link objects, and joints that are constrained by different movements are connected to form a hierarchical structure skeleton. Hierarchical structure skeletons are composed of a number of chain links that use hierarchical groups, including hierarchical joints or chains, motion constraints, and effectors that are driven by effectors to move all parts simultaneously. However, certain hierarchical relationships must be followed in order to prevent the individual components from spreading in different directions during transformation. Such as: pitching action, only stipulates the starting position, the ending position and the path of the ball. The rotation of the arm following the joint can be automatically calculated according to the inverse kinematics. Inverse kinematics alleviates the tedious work of forward kinematics to a certain extent and is one of the best ways to generate realistic joint movements.
Application of IK Algorithm in Motion Capture System
If we know the position information of the end effector, we can reverse the rotation angle and position of its ancestor joint. This is the inverse kinematics. That is, we use indoor positioning technology to obtain the position information of the end effector, and then use IK algorithm to calculate the rotation angle and position of the ancestor joint, so as to know the movement information of the athlete, and then use the motion information to realize the real-time motion tracking display.
Laser positioning technology is used to scan the position tracking sensor (namely the end effector in the IK algorithm) on the body worn by the wearer through a laser transmitter on the wall to obtain position and orientation information. Specifically, this indoor positioning technique uses lasers and light sensors to determine the position of a moving object. A number of laser emitters will be placed diagonally to form a rectangular area that can be adjusted based on the actual size of the space. Each laser emitter is designed with two scanning modules, which respectively emit horizontal and vertical laser scanning and positioning space in the horizontal and vertical directions in the positioning space. There is a photosensitive sensor on the athlete's body. The precise position of the photosensitive sensor is calculated by the time the photosensitive sensor receives the laser light.
After obtaining the precise position of the sensor by the laser indoor positioning technology, the IK algorithm can be used to inversely calculate the rotation angle and position of the ancestor joint, thereby knowing the movement information of the athlete. However, there may be occlusion problems in the laser positioning process, such as squatting, cuddling, and scuffing. Therefore, the inertial sensor is used for supplementary tracking. That is, when an occlusion situation occurs, the motion capture technology combined with IK+ indoor positioning cannot be completely accurately implemented. At this time, inertial motion capture technology is used as a supplement. In turn, indoor positioning technology can be used to perform real-time calibration of the inertial motion capture technology without the need for additional calibration, thereby solving the occlusion problem and avoiding the disadvantages of inertial motion capture that cannot work accurately for a long time.
The application of inertial capture technology
Application in film and television production, animation production and game production
In terms of film and television production, the addition of inertial motion capture equipment has not only greatly improved the shooting efficiency, but also reduced the difficulty and cost of post-processing.
We know that optical motion capture equipment can not take into account both real-time and reductive, even some special actions can not be restored in real time, but also by space constraints. In contrast, inertial sensor motion capture systems are quite different. Inertial motion capture is not only more identifiable scenes, but also more intelligent management. The use of full wireless sensors to complete the motion capture in the movie shooting process does not affect the wear of the actors, but also ensures that the actors move in real and smooth motions, so that the motion can be restored naturally, and even some large dynamic actions can be successfully captured. As a high-performance, easy-to-use and accurate motion capture system on the market today, motion capture devices based on inertial sensor systems have gained popularity on the market.
In animation production and game production, many of the character's action settings in game production, such as the action of slashing in martial arts, are acquired through motion capture. Because inertial motion capture has good real-time and various technical advantages, this technology is widely used in animation and games. It makes the animated picture more realistic and natural, and the action of the game character is more natural and delicate. The improvement of animation quality is of great benefit.
Application in the field of virtual reality interactive experience and game interaction
The real-time interaction between real people and virtual characters is a major application of motion capture technology. For example, virtual studios, real-time rehearsals of movies, interactions between real people and virtual game characters.
The virtual studio is now very mature. Similar scenes can be seen in the columns of many TV stations. The basic operation method is to let the host shoot under a green curtain, and then use the backpack to pin down characters in real time. Attached to the virtual scene. The general inertial trapping can't realize the interaction between the host and the virtual character due to the displacement of the virtual character. However, with the advancement of technology, the excellent inertial motion capture device has been able to do this, not only the dynamic trapping displacement error is extremely small. , And can do dozens of meters back and forth or casual exercise for ten minutes, fully able to meet the virtual studio host and the interaction of virtual characters.
About the application of this technology, the latest example is about the 2014 World Cup program. CCTV5's "I Love the World Cup" and CNTV's "Super World Cup" two-stage programs all use inertial motion capture technology. In the studios of these two programs, virtual players with extremely high fidelity were set up. In the broadcast programs, virtual players interacted with live guests and presenters in a real-time manner. The way they played was refreshing for the audience. .
From a more cutting-edge perspective, areas where motion capture technology can truly produce revolutionary value will be born in virtual reality games. For example, the combination of motion capture and head-mounted display devices using inertial sensors can extend the game from the living room or fixed scene to more free scenes, and the precision of the motion can also produce a qualitative improvement. Currently, many game industry leaders are willing to try inertial sensor motion capture devices, and according to industry predictions, more than 70 games may use this device to experience. This indicates that inertial motion capture technology will usher in a broad market prospect in the future.
Application in Multiplayer Simulation Exercises
The inertial motion capture system can provide virtual simulation environments, field exercises, and role-play training for military training and fire drills. Virtual military environment simulation can enable a large number of teachers and trainees to complete training without entering the real field environment, which can greatly save manpower and material resources and other aspects of consumption. At present, inertial motion capture technology has been applied to the development of the world's leading military simulation training system, and it has played a very good simulation exercise results.
Application in sports training and sports analysis
In the field of motion analysis and sports medicine research, researchers need to analyze large amounts of motion data, such as speed, acceleration, and angular velocity. These data are not only very large, but they are often influenced by factors such as larger perspectives, longer distances, and artificial biases. The inertial sensor motion capture system completely solves the above problem in principle, it can accurately capture the motion details of the moving human body, and provides a basis for athletes and coaches to analyze the sports situation. There are no shortages of successful cases in this area. For example, for the golfer's posture correction product MySwing. When using this product, the user can capture the swing rhythm, speed, head trajectory, angle and other data by clamping the product on the club. These data can help exercisers improve their posture and improve their skills.
In addition, the use of inertial motion capture can calculate the number of dribbles, the number of passes, the number of shots, the number of rebounds, etc. when playing basketball; it can also calculate the number of hitting shots and the probability of scoring when playing billiards. Training entered the digital era and greatly improved the training level. At the same time it makes remote teaching possible, especially suitable for special sports such as sailing and rock climbing. In general, this kind of real-time and accurate analysis and evaluation data acquired by the inertial motion capture device has great help in improving athletic performance, preventing injuries, and recovering the state.
Application in medical health virtual experience
Inertial motion capture technology can also be applied to medical diagnosis, attitude correction, rehabilitation aids, kinematics and dynamics simulation evaluation, etc. It not only has significant effects, but also provides medical institutions with efficient and low-cost solutions. For example, for patients with bone disease or mobility problems, the inertial motion capture system can provide measurement and report of whole body, half body and individual parts. The doctor analyzes the data transmitted by the sensor and can provide a quick and concise solution for the patient.
At present, some domestic medical laboratories have begun to use inertial motion capture to help patients with epilepsy for diagnosis and rehabilitation training. When the doctor judges the condition of the patient, the traditional approach is to draw conclusions by visually observing the patient's gait and physical signs, and the device for motion capture is more precise and intelligent. It uses digital precision measurement methods to more objectively perform data statistics and diagnosis, and Provide objective and accurate data to reflect the patient's treatment.
In addition, through quantitative analysis of gait, inertial motion capture technology can provide real-time kinematics and dynamics data, which provides more accurate basis for the development, optimal installation and safe use of artificial limbs.
Application in industrial measurement and design
Inertial motion capture provides accurate data for virtual industrial measurements, creating an interactive, non-intrusive design environment for design users, and can be applied in industrial measurement design fields such as workplace design, automotive testing, and aerospace research, for example in the field of automotive design. The researcher can use it to study the handling performance of the car's cab, the distance of the steering wheel, the convenience of entering and exiting the car door, and the height of the car. In the field of mechanical design, it can be used in the installation of the console, the design of the height of the washing machine, etc. Aspect; can effectively improve productivity and save production environment costs. In the operation of the production line, it can also provide users with professional and precise positioning, so as to solve the problem of repetitive motion during production and control.
With the continuous improvement of inertial motion capture technology, its accuracy is getting higher and higher, and its application range is more extensive. In addition to the above seven aspects, inertial motion capture can also be used in stage effects, robot control, ergonomics, chemical and chemical online simulation, vehicle crash and vibration analysis...etc.
At present, the motion capture industry market still dominates the optical motion capture technology. Manufacturers who can use inertial sensors to develop human body motion capture systems are only a handful, which indicates that the inertial motion capture system has enormous potential for immeasurable results. The inertial motion capture technology headed by the company uses technical strength as a weapon to fill gaps in the Chinese market through the practice of internal cultivation and gradually take the lead in the market. It will surely pick up the action and capture the banner of the industry, and completely reverse the pattern of the domestic dynamic arrest industry. It will become an important technology that equals the optical world and even more market competitiveness.
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