The helmet system consist out of a Multi Camera System (MCS), a laserscanner and a rigid body. The MCS includes three fisheye cameras with diverge viewing directions. Due to their large field of view, the images of the fisheye cameras slightly overlaps. Further a laserscanner is as well mounted at the helmet. The laserscanner has multi-echo capability, but only the first echo is considered for processing. Therefore 1080 single range values are recorded with the laserscanner per scanline. To track the helmet system a rigid body with three spherical rigid body markers is utilized.

In order to establish the pose the helmet system, an explicit assignment of the three fisheye cameras and the spherical rigid body markers is specified. Therefore it is important to specify the helmet system with labels. For the MCS the labels left/right/back are specified. The LaFiDa acquisition software also needs an explicit assignment, thus numbers are assigned in brackets.

Rigid Body

An improved rigid body was build out of three spheres which are covered by retro reflective tape, mounted on an aluminum bar.

According to the rigid body from OptiTrack a range extended rigid body was designed by enlarged markers to improve the visibility of the retro reflectivity markers for outdoor measurements and to operate in large distances. It includes retro reflectivity markers in form of three spheres which are completly covered by retro reflective tape (Typ 3, 3M). The reflective tape was selected after an experimental phase. For further information please get in touch with us.

For the improved rigid body has a aluminum bar of the size 270 mm x 20 mm x 7 mm with four drilled holes. One hole in the center of the bar links the rigid body with a spacer directly to the helmet. The remaining three holes have each a thread (6 mm diameter) inside, as well as the three wooden spheres. With two aluminum rods (45 mm) and one aluminum rod (95 mm) the spheres are mounted on the aluminum bar as shown above. The distance between the holes of sphere 1 and sphere 2 is 100 mm and the distance between of the  holes of sphere 0 and sphere 1 is 170 mm.

Initial measurements have shown a good visibility and stable tracking of the rigid body. OptiTrack produces a location accuracy of few tenth of a millimeter for a stable setup. For kinematic measurements the accuracy is about a few millimeters.

Rigid Body Coordinate System

A coordinate system has to be defined to orientate the rigid body. The rigid body is composed of three reflective spheres, called spherical rigid body markers. The tracking system OptiTrack cannot recognize each sphere apart. One sphere has to be determined as coordinate origin. First step is to distinguish each sphere. With the tracking system it is possible to obtain the distance between each marker. With the distance an explicit assignment can be done by testing each case. When each marker has a defined position, a coordinate system can be calculated (see image helmet with rigid body labels).

Synchronization

The synchronization between the MCS and laserscanner is accomplished by time stamps.

The LaFiDa acquisition software base on two different time stamps, by one time stamp label for the MCS data and one time stamp label for the laserscanner data. To be able to analyse the data the corresponding time stamps are relevant. Therefore the data output of both systems is arranged in the same structure. Further a high resolution temporal counter, the function HiResTimer in C# is implemented. It is a temporal counter that counts continuously and starts by zero, when the computer starts. The unit is in milliseconds [ms]. Each image is assigned to the current time stamp and each laserscanner rotation (including 1080 single measurements) has also a single unique time stamp. For this dataset a time stamp with 7 digit numbers is chosen.