Measuring 20,000 square feet, and with 30-foot ceilings, PURT is the world's largest indoor motion capture facility. We support large and fast-moving vehicles, such as fixed-wing aircraft. We can also recreate scaled versions of large environments.
Our motion capture system tracks rigid bodies with active or passive markers. This enables:
- Mixed-reality environments
- Sensor emulation. We can mimic GPS, ultra-sonic, ADS-B, LIDAR, ToF, camera, and other inputs
- Real-time position feedback for closed-loop control
- Ground truth. For a typical UAS, we have position accuracy to the millimeter, and attitude precision to 0.1 degrees.
High-precision cameras in a large indoor space
The system consists of 60 Oqus 7+ motion capture cameras that can capture at 300 frames/sec at 12 megapixels resolution, or at 1100 frames/sec at 3 megapixels resolution. A large portion of the cameras are reconfigurable on tripods.
The dual capture rate and resolution make this facility's capabilities unique. The size of the facility is critical for supporting high-speed flight rotary and fixed-wing UAS flights that previously could not be adequately studied in a motion-capture environment. The combination of large facility and high camera resolution can also accommodate large drone swarms.
The motion capture (MoCap) system consists of 58 cameras from Qualisys which are used to track a rigid body inside an enclosed 3D volume whose dimensions are roughly 175 x 95 x 30 ft. There are two types of cameras being used – 56 Oqus 700+p MoCap cameras and 2 Myquis video cameras.
To track the motion of a rigid body, special markers are attached on its surface in a unique geometric configuration. The Oqus 700+p MoCap cameras then use optical tracking technology to identify and calculate the marker positions accurately, with very low latency. By tracking the position of the markers over time, the MoCap system in turn tracks the movement of the rigid body. The remaining 2 Myqus video cameras stream synchronized video and give perfectly aligned reference video to complement the movement data of the rigid body. The Qualisys Track Manager (QTM) software is used to visualize the MoCap tracking in real time and collect the positioning data. The arrangment of the camera system can be seen in the figure below.
Before tracking a rigid body, the MoCap system needs to be calibrated to determine the residual error in measurements and the coordinate system. A Wand kit provided by Qualisys, consisting of an L-shaped reference structure and a T-shaped calibration wand, is used for the process. Both these structures have markers attached to them and their fixed dimensions are fed into the QTM software. During the calibration process the L-shaped structure is used to denote the orientation of the x, y and z axes and is placed on the ground at the origin. The T-shaped wand is moved in spherical circles in front of all the cameras, such that the markers on the wand are always visible by a subset of cameras. This process takes about 5 minutes to complete.
After a successful calibration, the software returns the residual error in measurements for each camera, which is of the order of ~3mm. The figure below shows an example of a successful calibration, with an average standard deviation of 0.89mm for the T-shaped wand of length 601.8mm. The volume is not uniformly calibrated and there are patches of high and low precision areas due to the positioning of the cameras. However, markers of size 8mm or higher can be detected with relative ease even in the low precision areas.
Drone Swarm Test