In order to analyze a skill such as the golf swing, we have to measure it. To do that we need one or more reference frames, also known as coordinate systems. Reference frames consist of a zero point and three perpendicular axes, usually called, the X, Y and Z axes, which originate from this origin. There is always one global reference frame from which everything else is measured. It is also known as the origin or the zero point. The small “sugar cube” with red, green and blue arrows, that is located behind the golfer avatar, in the diagram, is the global reference frame. In the AMM3D system, this is the transmitter, and is the source of the electromagnetic waves.
All of the motions of each body segment can be measured with respect to this global reference frame. Notice in the middle diagram we have lots of these reference frames. These are called local reference frames because they are local to the golfer’s body. These local reference frames are directly derived from each sensor that we have placed on each limb. But they are not the same. Each sensor receives the electromagnetic signal from the transmitter, and from that they can measure where they are in space with respect to the transmitter, which I said is also the global reference frame.
When we put the sensors on the body we don’t place them on the joints, we place them on a convenient position on each segment. We then place one sensor into a plastic pen. We know the offset from the sensor to the pen tip, so as we move the sensor the computer also knows the position of the pen tip.
Once all the sensors are strapped on the golfer and are secure we touch this digitizing pen to important body land marks. We need at least three points per segment to create an anatomically relevant local reference frame that is suitably aligned with the body segment on which it is situated; we call this anatomical alignment. For example, the shin; a sensor is securely strapped to the lower shin and the pen tip is touched to the outside and inside knee and to the outside and inside ankle. The computer now knows where the center of the knee is with respect to the shin sensor and of course where the ankle is as well. So we have the knee and ankle centers as being half way between the outside and inside points. But we are not satisfied with a point. We want a local reference frame in the knee. We create the three axes for the knee by joining a line from center knee to center ankle; this is the Z axis of the reference frame (blue in the diagram). Then we can join a line between the two knee points and that allows use to create a plane through the shin using that line and the shin axis that we just created. Using a mathematical procedure called “the cross product” we create a line perpendicular line to that plane. That becomes the Y axis (green in our case). Now we go back and create the final axis, the X axis (red in our case), by doing the cross product between the already existing Y and Z axes. Now we have an anatomically aligned local coordinate system based in the knee and representing the shin. This is the motion we want to measure, not the sensor placed on the shin. The computer is fast enough to do all these calculations just mentioned in real-time so that you can see the robot moving as the golfer moves.
Using this method we can “relocate” the real sensors to be in the center of each joint; for example, the shoulder, the elbow, the wrist, the hip, the knee, the ankle and so on. We can of course repeat this procedure with the club as well. Once the alignment procedure is finished and as the golfer moves we will know the position and orientation of each body segment at any instant during the swing; provided none of the sensor slip.
With the system now aligned, we can make measurements that are global, such as the position of the head at any instant, or we can make measurements that are local, such as the position of the shin with respect to the thigh, hence getting the motion of the knee joint. This is the procedure that the AMM3D system uses to track the body with the electromagnetic sensors. It can do this at 240 times per second for the whole swing.