Accuracy of the Ariel Performance Analysis System
The Ariel Performance Analysis System (APAS) considered as the most accurate motion analysis system in the world. The following are third party studies and comments on the APAS accuracy:
There are detailed studies to verify the accuracy of the APAS system. In addition there are responses to questions posted by Biomechanists. The following are few of the studies and the responses.
Response No 1
I have an APAS & MacReflex system and use them both .
APAS uses the DLT so it's accuracy is dependent on the accuracy of the calibration. I
constructed my own frame which consisted of aluminum rods with reflective balls attached
hanging from the ceiling. I then had the frame surveyed and locations determined with
respect to my floor mounted force plate. The engineering firm said the accuracy of the
centroid locations are +/- .5 mm. By having the frame coordinated with respect to my force
plate coordinate system I eliminated having to take extra steps to determine where under
the foot my center of pressure was. I checked by system using a second calibration frame I
had constructed for outdoors and found the accuracy to be roughly +/- 0.5cm for a volume
of (1.23m X 2.45m X 1.84m) using 3cm diameter markers.
Response No. 2
We've had the Ariel system for a few years and we did check the accuracy when we first got it. We concluded that the accuracy was acceptable as far as the location of points in 2D. We didn't test the 3D.
Our tests showed that in 2D, the
joint coordinates as well as the segmental kinematics
The first test you can do is to take a grid of points on a flat surface and determine how well those points are located. You can place the grid in different orientations. That is for static analysis. For dynamic analysis take a record-player turntable (hard to find now with CD's but they're around). Place one or more markers on the turntable and turn it on. For 2D analysis test the system by orienting the turntable perpendicular to the camera. For 3D analysis, you can do different trials where you put the turntable in different planes relative to the camera. In either case, because you know the speed of the turntable and location of the points, you know how the kinematics should come out. Check that against Ariel's output. An old way to test kinematic analysis is to drop an object. The position, velocity and acceleration of the object is predictable based on Newton's law.
Response No. 3
Many of our users have performed accuracy studies for
static, dynamic, angular measures, wide-angle-lens, and even underwater. Several of these
are posted on the Ariel Dynamics, Inc. Home Page at the following address:
http://www.arielnet.com. You can access these studies under the heading of Product Comparison
Response No. 4
The better systems will also calculate lens corrections for your cameras. The others will just try to incorporate these problems in the DLT, which is not designed to handle such effects.
A simple(ish) way to *verify* the basic accuracy of a system is to attach a pair of markers to a rigid bar of fixed length. Film the bar from a number of positions (I would refer to the ASME B89 standard for these: it suggests 20 positions for a cubic volume that encompass the various edges and diagonals of a cube). Of course, the bar should be the same length in all 20 positions. It won't be, but deviation from the fixed length will give you a good idea as to how good or how poor the unit is. It will also be obvious if there is a problem with some positions of the bar.
Response No. 5.
I have worked extensively with APAS for the last four or five years with numerous sport and occupational applications. I've also done a number of accuracy checks on various aspects of the system:
I digitised various frames (2 cubes and a triangular pyramid) of known dimensions to check the accuracy of displacement data. Errors of less than 1% were averaged inside the calibrated boundaries. Outside these boundaries, however, errors started to increase markedly (so now we calibrate all over the place).
Linear velocity and acceleration Only basic tests done on this parameter as it is pretty hard to set up a test rig that is accurate to the level required. We used a ball drop experiment as well as a comparison of ball velocity with the results recorded from a system we use for golf (which is very accurate). Errors in the ball drop test were fairly small in terms of acceleration (about 9.81 m/s/s ) we achieved. The ball speed comparison returned differences of about 1%. Once again, set-up was important to these results with a high speed video image achieving better results than a normal speed image.
Response No. 6.
We purchased a NTSC APAS system in ... and a PAL System in..... I used both systems extensively and was - of course - concerned with the 'accuracy'. Whilst the topic is too lengthy to put it in one mail, I briefly relate a few thoughts to you: We tested APAS by placing objects in the gym (Hurdles, standards, boxes ...). Then we determined the precise 3-d- coordinates of points on these objects using a theodolite connected to an HP 'totalizer' (I'm not sure of the English word, however it is a system used by surveyors which is highly accurate). We also determined the camera positions. Then we digitized points, the rationale being that the fundamental measure to compute derivatives of are the x,y,z coordinates - hence it doesn't matter whether these points are moving or stationary. We also considered using mechanically defined systems such as a pendulum or a free falling mass but saw no advantage (assuming the time base is correct) over stationary points. We also had the advantage of using these known points either as calibration or as 'unknown' points.
Now, 'accuracy' has to be clearly defined for the context. This means, you should consider your object space size, your pixel resolution and the quantity to be derived. We found for an object space of 6x6x3 meters mean position errors of 0,2 to 1,5 cm. I consider this good.
Let me highlight just a few of points:
My bottom line is that the system is as accurate as one can expect from a video based DLT system. We used it successfully in a variety of laboratory and field situations