Thesis Demonstrates 3D Motion Tracking on Mobile Device
 
Thesis Demonstrates 3D Motion Tracking on Mobile Device

Amar Nikhanj, an engineering graduate school student at the University of Wisconsin – Milwaukee, published a thesis that demonstrates 3D motion tracking is possible on smart phones. Acceptance of this new approach in the marketplace would cause a paradigm shift for the motion capture industry and, in particular, kinesiology.  

 

In his thesis, Adaptation of Moiré Phase Tracking to a Mobile Device for Field 3D Data Collections, Nikhanj combined the successful history of moiré phase tracking (MPT) in studying biomechanics with the mobility provided by a mobile device to focus on an immediate market need: quantitative Rapid Upper Limb Assessment (RULA).   

 

The RULA is a survey method developed to perform risk analysis of upper limb injury in the workplace. It has been used to measure musculoskeletal risk, improve workstation designs, increase productivity, and educate workers on improving their motion to reduce their risk of injury.   

 

Nikhanj and his team adapted existing MPT technology from Metria Innovation to work on a mobile device. The 3D motion tracking device features camera controls, processing power, and calibration abilities. They applied the technology to the RULA to allow for a quicker and more precise instrumented RULA versus simply an observation analysis.   


The RULA  

RULA was developed to be completed visually, where an observer analyzes the worst-case posture, or in some cases multiple postures, in a motion by estimating the joint angles. Angles of concern are upper arm flexion or extension, upper arm abduction, elbow flexion or extension, wrist flexion or extension, wrist radial or ulnar deviation, neck flexion or extension, and trunk flexion or extension.   

 

The final score determines if the motion is acceptable and how quickly changes to the motion are required to prevent injury. But the standard RULA measurement has downfalls:  

-          The observer performing the RULA must use their judgement on which posture to use  

-          The observer must estimate the angles through which each limb is rotating   

 

RULA is a good candidate for 3D motion tracking: it consists of information in 3D space that must be decomposed into angles. Unfortunately, many 3D motion tracking technologies require more than one camera and have complicated calibration processes. RULA also is typically performed in the field in a potentially messy, enclosed, or dangerous environment — where having any camera system may not be realistic. This equipment barrier includes the Metria MPT Series 2 system since it requires a calibrated camera and a computer to calculate six degrees of freedom: forward/back, up/down, left/right, pitch, yaw, and roll.  


The Device  

Accelerated technologies in mobile devices such as tablets and phones provide an ability to perform high intensity calculations all while obtaining precise data, including through photogrammetry — making measurements through photographs. 

 

Nikhanj and his team adapted Metria's MPT software to work on a Google Nexus tablet. Manual focus and exposure control is absolutely necessary for MPT and ideally the settings are repeatable with a high level of accuracy. The team used the Android platform to develop MPT processing, camera control, camera calibration, and the algorithm to calculate limb angles.   

 

When tracking motion, there is no complicated setup. No tripods with expensive and specialized cameras on them. Just a subject with markers and an operator holding the mobile device. The operator/image collector has freedom to move around, as space allows, and can take images from several angles.   


The Results 

Nikhanj received Institutional Review Board (IRB) #16.137 approval to perform a study using the device. The RULA study required a total of six motion tracking markers placed in the room, and then on the subject’s torso, upper arm, lower arm, wrist, and head. The room marker was placed so the Y-Axis was aligned with gravity.   

 

A successful RULA measurement was captured in the study via the mobile 3D tracking device. Subjects were not required to remove jewelry or any other reflective objects.   

 

The table below shows the RULA angles calculated by MPT compared with three observed RULAs on the same motion. All of the angles are reasonably similar and most angles are within 15° of each other, but the quantitative calculations are precise where some of the observational calculations are ranges. And smaller angles can now be calculated, for example the Torso Rotation, which was normally rounded to 0° by two of the observers performing the RULA.  

 

Angle Under Observation 
MPT 
Angle  
Observer 1 
Angle
Observer 2 
Angle
Observer 3 
Angle
Upper Arm (Shoulder)
Rotation
48.3°  15-45°  15-45° 15-45°
Lower Arm (Elbow)
Angulation to Torso
 50.7°  60-100°  60-100°  60-100°
Wrist Rotation  
10.1° 
Downwards
0° to 15° 
Downwards
0° to 15°
Downwards
0° to 15° 
Downwards
Neck Rotation  18.4° Flexion 10° to 20° Flexion 0° to 10° Flexion 0° to 10° Flexion
Torso Rotation 4.8° Forward  0°-20° Forward 0° (Erect) 0° (Erect)

RULA angle comparison with three observed RULAs on the same motion. All of the angles are comparable.

 

 

The study determined that 3D motion tracking on a mobile device in the field is possible. Since the data capturing source can be mobile, this 3D motion tracking technology is suited for field use and does not need significant control of its environment.  

 

Nikhanj’s thesis lays the foundation for future research to improve upon accuracy, speed, and robustness of MPT on a mobile device. Much of the ground work has been established, with the basic challenges solved in his thesis such as processing, camera control, and calibration. Tracking motion in 3D space on a device that can fit in a pocket truly is a paradigm shift. 

 

Download the full thesis here.


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