A young engineer upstages an older co-worker while investigating the
source of an ear-piercing tone.
I once worked with a test system for certifying and sorting 8-inch
It is a source of many interesting stories (including this Engineering Investigation
) because of its
unique combination of electronics, mechanics, and raw power.
The main component was a sorting shuttle that zipped up and down an 8
ft. track. The motor could move this five-pound shuttle the length of
the track in about 2 seconds.
When our senior analog engineer fired up the prototype for the first
time and commanded the shuttle to move, it reached its destination and
began to "sing." Examining the position control error voltage reveled a
3KHz oscillation that just happened to resonate nicely with the motor
The result was an ear-piercing tone that put anyone exposed to
it into a migraine in a few minutes. Add to that, the shuttle moved
just enough from this to appear blurry. Your eyes would constantly try
and bring it in focus but just could
not quite manage. It would have sent anyone with even mild epileptic
tendencies into seizure.
If you just rested your finger on the motor housing, the oscillation
would stop, so our analog and mechanical engineers spent a week looking
for a resonance in the motor and track design. But nothing could bring
this demon under control. I was eventually asked to have a look at it.
My first impression was this was an unstable control loop--not a
mechanical problem. So I asked the senior engineer for his design
What I found was that the bench prototype shuttle and track were
very different mechanically from the model we were now working on. So I
got out a spring scale and measured the mass and friction for the
shuttle. It was nearly 2X different from the values in the notes. The
mechanical pole (mass x friction) is the dominant pole in this kind of
position control loop. So I doubled the value of the capacitor in the
lead / lag compensation and the oscillation stopped. This all took me
about 2 hours to sort out.
I hated to upstage our senior engineer
(being a fresh out 20-something), but he made a fundamental mistake
(literally). He forgot to re-examine his fundamentals. He was so sure
of the integrity of his
original work, it just never occurred to him there could be a problem
there. The one thing he did do right was document his original work in
an engineering notebook. This diligence saved probably several days
having to re-do those calculations for myself. This is something I
never see young engineers do today as a regular practice. And it quite
often costs a lot of time retracing things when debugging in the lab.
Now that I am one of the ‘old guys,’ I try never to let some young
engineer show me up this way, by remembering this lesson. But it
doesn’t always work. Never underestimate the creativity of young minds.
Author Chuck Hill has 30 years engineering design experience.