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Position Based Firing

Laser seam welding implantable medical devices such as pacemakers requires unique motion system control solutions, in addition to precise control over the delivered laser energy.  The principle task in this case is to seam-weld the two halves of the device together, achieving a reliable hermetic seal.  Equipment built for this purpose usually consists of a rotary stage holding the implantable device while applying pressure to clamp the two halves together and a XYZ robot that aims the laser beam at the edge seam, maintaining orthogonality to the seam as well as tight control of the focus offset distance.  Operation of the coordinated four-axis motion control system is deceptively simple: just turn the rotary stage and keep the laser beam pointed at the seam.  But a number of practical concerns need to be addressed. 

For one, the contour of the seam to be welded is most often not a simple circular path as the sample parts depicted in figure 1 illustrate.  By studying the kinematics of the motion system needed to follow a typical welding contour at constant speed, one finds that there are points in the motion of some of the stages that require theoretically infinite deceleration / acceleration.  These stages must instantaneously reverse direction without decreasing speed. See the polar acceleration plot in figure 2 as an example. Unless managed properly, this motion results in a large reactive force transmitted through the machine structure that is felt as a strong impulse force (“bump”) in the otherwise smooth operation of the servos.  The impulse causes the holding fixture to flex or vibrate and can cause irregularities in the spot to spot spacing in the seam, often resulting in a hermeticity failure. 


Pacemaker showing contours for position based firing

 
Polar acceleration - position based firing


In the past, a typical solution has been to slow down the whole operation so the force impulse is low enough to not affect the performance of the weld, but this means a lower production rate and higher product cost.

 A better approach is to use variable speed contour welding with “position-based firing.”  The motion system is set up to minimize the magnitude of the sharp changes in acceleration by slowing down as needed at these inflection points and then speeding up along the benign segments of the contour.  Using special software to achieve “position-based firing” along the contour, it becomes a simple matter to fire the laser not at a constant repetition rate, but rather in response to its actual position along the contour at any point in time.