As an emerging Motion Capture technology, the emergence of Inertial Motion Capture, breaking the absolute dominance of the optical motion capture industry pattern, is considered a new force in the motion capture world. Motion capture technology based on an inertial sensor system is a comprehensive technology that integrates sensor technology, wireless transmission, human dynamics, computer graphics, and other disciplines, with high technical threshold requirements. Although inertial motion capture technology has not been around for long, its superior performance is rapidly emerging as we use it in various industries.

Inertial motion capture, a new type of human motion capture technology, which uses wireless motion posture sensors to collect the posture orientation of body parts and use the principles of human kinematics to recover the human motion model, while using wireless transmission to present the data in the computer software.

Before the emergence of inertial motion capture systems, the most common is the optical motion capture technology. It is through the actor’s body paste marker points, and then use high-speed cameras to capture the exact displacement of marker points, and then transfer the capture data to the computer equipment, thus completing the whole process of motion capture. The cost of the entire set of optical motion capture equipment is extremely expensive, cumbersome to set up, vulnerable to blocking or light interference, bringing a lot of trouble to the post-processing work. For some serious obstruction of the action, optical motion capture can not accurately restore the action of, for example, squatting, hugging, twisting and so on in real time. The emergence of motion capture technology based on inertial sensor system has greatly improved the status quo.

Compared with optical motion capture technology, inertial motion capture technology has a high adaptability to the capture environment, its technical advantages, cost advantages and ease of use advantages, making it has excellent performance in various industries. In the fields of film and television animation, experiential interactive games, virtual studios, live simulation exercises, sports training, medical rehabilitation, inertial motion capture system has significantly better than other equipment.

Principle of an inertial motion capture system

The general structure of the motion capture system is mainly divided into three parts: data acquisition equipment, data transmission equipment, data processing unit, inertial motion capture system is the application of inertial sensors to the data acquisition end, the data processing unit through the inertial navigation principle of the collected data processing, so as to complete the motion target attitude angle measurement.

In the important nodes of the moving object wear integrated accelerometers, gyroscopes and magnetometers and other inertial sensor devices, sensor devices to capture the motion data of the target object, including the body parts of the posture, orientation and other information, and then these data through the data transmission equipment to the data processing equipment, after data correction, processing, and finally establish a three-dimensional model, and make the three-dimensional model with the moving object really and natural movement.

The processed motion capture data can be used in animation, gait analysis, biomechanics, ergonomics and other fields.

The role of accelerometers, gyroscopes and magnetometers in inertial motion capture systems

The accelerometer is used to detect the magnitude and direction of the acceleration to which the sensor is subjected, and it gets the result by measuring the force on the component in a certain axial direction, expressed as the magnitude and direction of the acceleration in the axial direction (XYZ), but it is not very accurate when used to measure the pose of the device relative to the ground, and this defect can be compensated by the gyroscope.

The gyroscope works by measuring the angle between the vertical axis of the gyro rotor and the device in a three-dimensional coordinate system and calculating the angular velocity, and discerning the motion of the object in three-dimensional space by the angle and angular velocity. Its strong point is to measure the rotational motion of the device itself, but it cannot determine the orientation of the device. And it just so happens that the magnetometer can make up for this deficiency, its strong point is to locate the orientation of the equipment, and can measure the current equipment and the four directions in the southeast and northwest angle.

In a motion capture system, the gyroscope sensor is used to handle rotational motion, the accelerometer is used to handle linear motion, and the magnetometer is used to handle direction. In layman’s terms – the gyroscope knows “whether we turned”, the accelerometer knows “how far we moved”, and the magnetometer knows “the direction of our motion”.

The three sensors in the motion capture system make full use of their respective specialties to track the motion of the target object.

Advantages of inertial motion capture technology

Technical Advantages

Inertial motion capture system collects a small amount of signal, easy to complete the task of posture tracking in real time, the solution to get a large range of posture information, high sensitivity, good dynamic performance; high adaptability to the capture environment, not subject to light, background and other external environmental interference, and to overcome the shortcomings of the optical motion capture system camera monitoring area is limited; overcome the VR equipment often have the problem of obscuration, can accurately restore the following in real time squatting, hugging, twisting and other actions. In addition, the inertial motion capture system can also achieve multi-target capture.

Advantages of ease of use

Easy to use, the device is small and lightweight, easy to wear.

Cost advantage

Compared to optical motion capture low cost, making it can be applied not only in film and television, games and other industries, but also help to promote VR equipment faster into the life of the public.

In general, inertial motion capture technology has a high adaptability to the capture environment, and its technical advantages, cost advantages and ease of use make it excellent in film and television animation, experiential interactive games, virtual studios, live simulation exercises, sports training, medical rehabilitation and other fields.

Disadvantages and solutions of inertial motion capture system

Inertial motion capture systems generally use an Inertial Measurement Unit (IMU) consisting of a MEMS 3-axis gyroscope, 3-axis accelerometer and 3-axis magnetometer to measure the sensor motion parameters. The sensor motion parameters measured by the IMU have severe noise interference, and the MEMS devices have significant zero bias and drift, so the inertial motion capture system cannot accurately track the human pose for a long time. Only by solving this one problem can the inertial motion capture system play a full role in the VR industry.

Solutions for the disadvantages of inertial capture technology

Firstly, the sensor motion data measured by the IMU is pre-processed to filter out the noise interference mixed with the original inertial data;

Then continuously calibrate and calibrate, i.e., continuously compensate each inertial device accordingly to resolve the zero bias and drift of MEMS devices and improve the accuracy and reliability of their data;

The next step is to perform attitude solving and verify the accuracy of the attitude angle data using the attitude reference system to finally realize the whole inertial type motion capture.

In addition, different from this, the domestic G-Wearables uses IK + indoor positioning technology to do the main motion capture algorithm, using inertial motion capture as a secondary algorithm. This program uses indoor positioning technology to do real-time calibration of inertial motion capture technology, avoiding the trouble of constant calibration.

 IK algorithm

IK stands for Inverse Kinematics, or reverse kinematics. In the hierarchical structure of the human body, joints and bones actually constitute the kinematic chain, for example, the shoulder joint, elbow joint, wrist joint and its sub-bones are a kinematic chain, which is a branch on the whole body kinematic chain, and the body that is using the kinematic chain to control the movement. Motion is divided into forward motion and reverse motion. If we know the rotation angle of each joint in the chain, we can find the position information of each joint and the position information of the end effector, which is the problem of forward kinematics; while we know the position information of the end effector, we can find the rotation angle and position of its ancestor joint, which is the problem of reverse kinematics.

Examples of IK algorithm

Inverse kinematics determines the motion of the entire skeleton based on the final angles of several master joints that determine the motion, and we usually use it for link objects, which consist of joints with different motion constraints connected into a graded structural skeleton composed of links. The hierarchical structural skeleton consists of a number of link chains grouped in a hierarchical manner, including hierarchical structural joints or chains, kinematic constraints and effectors, with the effectors driving all parts to move simultaneously. However, we must follow a specific hierarchical relationship in order to prevent the individual parts from dispersing in different directions during transformation.

For example, the pitching action, only the starting position, end position and path of the ball is specified, and the rotation of the arm, etc., i.e., following the joint can be automatically calculated according to the reverse kinematics. The inverse kinematics method alleviates the tedious work of the forward kinematics method to a certain extent and is one of the best methods to generate realistic joint motion.

Application of IK algorithm in motion capture system

If we know the position information of the end effector, we invert the rotation angle and position of its ancestor joint, which is the reverse kinematics. In other words, we obtain the position information of the end effector through indoor positioning technology, and then use IK algorithm to deduce the rotation angle and position of the ancestor joint, so as to know the motion information of the movers, and then use the motion information to realize real-time motion tracking display.

The laser positioning technology is used to obtain position and orientation information by scanning the position tracking sensors (i.e., end effectors in the IK algorithm) on the body worn by the wearer through a laser emitter on the wall. Specifically, this indoor positioning technology relies on lasers and photosensitive sensors to determine the location of moving objects. We will place several laser emitters diagonally to form a rectangular area, and adjust it according to the actual space size. We design two scanning modules within each laser emitter to scan the positioning space by emitting horizontal and vertical lasers in horizontal and vertical rotation respectively.

Application areas of inertial capture technology

Application in the field of film and television production, animation production, game production

In terms of film and television production, the addition of inertial motion capture equipment, not only greatly improve the efficiency of filming, but also reduce the difficulty and cost of post-processing.

We know that optical motion capture devices can not balance real-time and restoration, we can not even restore some special movements in real time, but also by the space constraints. Compared with this, inertial sensor motion capture system is very different. Inertial motion capture not only can identify more scenes, but also more intelligent management. The use of full wireless sensors to complete the film shooting process of motion capture, does not affect the actor’s wear, and can ensure that the actor’s feet move real smoothly. As the current market performance, easy to use and accurate motion capture system, based on inertial sensor system motion capture equipment is quite popular once listed.

In animation production and game production, the action settings of characters in many games production nowadays, such as the action of chopping and slashing in martial arts games, we obtain it through motion capture. It is because inertial motion capture has good real-time and various technical advantages, so we use this technology widely in animation and games, which makes the animation picture more realistic and natural, and the action of the game character is more natural and delicate, which is beneficial to the improvement of animation quality.

Application in the field of virtual reality interactive experience and game interaction

Real-time interaction between real people and virtual characters is a major application of motion capture technology. Examples include virtual studios, real-time previews of movies, and interaction between real people and virtual game characters.

Virtual studio is now very mature, we can see it in many television programs in a similar scene, the basic operation is to first let the host in the green curtain under the shooting, and then in real time with the key back machine to snap the character attached to the virtual scene. General inertial motion capture due to virtual character displacement, there is no way to achieve the interaction between the host and the virtual character, but with the progress of technology, excellent inertial motion capture equipment has been able to do this, not only motion capture displacement error is very small, and can do back and forth movement of tens of meters or random movement for ten minutes, fully able to meet the interaction between the host and the virtual character in the virtual studio.

Application in the field of multi-person simulation exercises

Inertial motion capture systems can provide virtual simulation environments, field exercises, role-play training, etc. for military training and fire drills. Virtual military environment simulation can enable a large number of instructors and trainees to complete training without entering the real field environment, which can greatly save human and material resources and other aspects of consumption. Currently, we use inertial motion capture technology in the development of the world’s leading military simulation training system and has played a very good simulation exercise effect.

Application in the field of sports training and motion analysis

In the field of motion analysis and sports medicine research, researchers need to analyze a large amount of motion data, such as velocity, acceleration, angular velocity, etc. These data are not only very large, but also often affected by factors such as larger viewing angles, longer distances, and biases in human understanding. Inertial sensor motion capture system in principle completely solve the above problems, it can accurately capture the details of the movement of the moving human body, providing the basis for athletes and coaches to analyze the movement situation. There is no shortage of successful cases in this area. For example, MySwing, a posture correction product for golfers, can capture data on swing tempo, speed, club head trajectory, angle, etc., which can help exercisers improve their posture and improve their game.

In addition, using inertial motion capture can calculate the number of dribbles, passes, shots, and rebounds when playing basketball; it can also calculate the number of shots and probability of goals when playing billiards, making physical education and training enter the digital era and dramatically improving training levels. At the same time it makes distance teaching possible, especially suitable for sports with special requirements, such as sailing, rock climbing, etc. In general, such real-time and accurate analysis and evaluation data obtained by using inertial motion capture equipment can be of great help in improving sports performance, injury prevention, and condition recovery.

Application in the field of medical and health virtual experience

We can apply Inertial motion capture technology in medical diagnosis. Which is not only effective, but also provides an efficient and cost-effective solution for medical institutions. For example, inertial motion capture systems can provide patients with site measurements and reports, and physicians can analyze the data transmitted by the sensors to provide a quick and concise solution for patients.

Currently, some medical laboratories in China have started to experiment with inertial motion capture development. While doctors traditionally rely on visual inspection of a patient’s gait and signs to draw conclusions when judging a patient’s condition, motion capture devices are more accurate and intelligent, providing more objective data statistics and diagnosis with the help of digital precision measurements, and providing objective and accurate data to reflect the patient’s treatment outcome. In addition, inertial motion capture technology can provide real-time kinematic and kinetic data.

Applications in industrial measurement and design

Inertial motion capture provides accurate data for virtual industrial measurement. It also creates an interactive and non-invasive design environment for design users. For example, in the field of automotive design, researchers can use it to study the distance of the steering wheel. We can use it in the installation of operating tables, etc. It can effectively improve productivity and save costs in the production environment. And in the operation of production lines, it can also provide users with professional and precise positioning. Thus solving the problem of repetitive motion in the production and control process.

With the continuous improvement of inertial motion capture technology, it has become more and more accurate. In addition to the seven aspects introduced above, inertial motion capture can also be useful in the fields of stage special effects, robot control, ergonomics, chemical and chemical online simulation, automotive collision and vibration analysis …….

At present, optical motion capture technology still dominate the motion capture world market. Manufacturers that can use inertial sensors to develop the human body motion capture system are few. It is clear that the inertial motion capture system has an immeasurable potential. And the inertial motion capture technology led by China’s Noiton company, technical strength as a weapon, through the cultivation of internal strength, to fill the gap in the Chinese market, and gradually occupy the first opportunity in the market. It will certainly carry the flag of the motion capture industry, a comprehensive reversal of the domestic motion capture industry pattern, and become an equal share of the world with the optical motion capture, and even more market competitiveness of the heavyweight technology.

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