Optomechanics – The Trap of “Kinematics” vs “Kinetics”

Colleagues:

A while ago I got a call from a sponsor who wanted me to go to a design review in Texas on very short notice.  On arrival I found my name was on the attendance list but no one knew why.  There were a lot of peculiar looks around the registration desk.

I took a seat at the back of the auditorium and quietly made notes.  At coffee break the manager who was funding the subcontract being reviewed came back to introduced himself.  He tried to “talk shop” but I had very little I could say as I’d not been briefed by my sponsor.  I was learning as I listened.

It was mid-morning of the second day that I discovered why I was there.  They had made the hottest doubled-YAG laser I’d seen, but it was unstable.  The laboratory system worked fine but the flight system lost power whenever a door in the room was closed.  Hmmm.

Well, at lunch break I chatted with the laser scientists, who were bewildered by the problem.  So I talked to the mechanical engineers and asked them what they did for the flight system that was different from the laboratory system.  They said, uniformly, “Nothing.”

After lunch, back in the design review, I discovered what “nothing” was.  The mechanical engineers had been directed to put the resonant cavity on a “kinematic mount,” which they did.  It worked great.  In the flight environment however the “kinematic mount” would fly apart so they bolted it together for flight.  The “stiction” in the “bolted kinematic mount” prevented the cavity’s return to its original geometry after a disturbances such as the closing of doors.

In my report to my sponsor I suggested that a simple redesign to replace the failed “kinematic mount” with a “kinetic mount” (i.e., flexures) would probably fix the problem.  But it was too late.   Within a short time the whole project was cancelled.

A warning to optomechanical engineers:  “Kinematic mount” is just a figure of speech.  “Kinematics” is defined as “the study of motion without regard to forces or masses.”  “Kinetics” is the study of motions of masses under the influence of forces.  When asked for a “kinematic” mechanism we should request the allowable motions.  We can usually work out the “kinetics” from there.  If you cannot find out the allowable motions be very, very careful.

I had a good class of students for my tutorial on Thursday at SPIE’s Defense, Security and Sensing Symposium.

Wasn’t Springtime in Baltimore gorgeous? 

Joy to all.

Al H.
5-14-13

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