Signal and Noise

It took many years into my geotechnical instrumentation career to realize just how lucky I had gotten with my industry’s commitment to vibrating wire strain measurement.  What the heck does that even mean?  Basically, vibrating wire strain gages (VWSGs) are pretty darn easy to use.  With the right hardware to sample the frequency, strain is very simple to measure based on the tension in the wire.  It’s like an electric guitar with its steel string and electromagnetic pickup.   Although vibrating wire instruments are analog, the frequency sampler is a digital counter, so like most digital electronics, they either work properly or not at all and it’s easy to tell either way.

This point was driven home for me when I first started dealing with resistance strain gages, well into my instrumentation career.  This was a backward sequence.  Most engineers learn about Wheatstone Bridge resistance gages (the simplest and most common kind) first, then move on to more sophisticated instruments.  Lucky me, I’d never had to deal with signal conditioning or noise filtering, voltage drift or thermal compensation for the first 12 years of my career.

We were contracted through an engineering company to instrument a utility tunnel at NASA’s Kennedy Space Center.  It is located beneath the crawlerway that leads from the Vehicle Assembly Building (VAB) to the launch pads.  NASA had concerns that the new SLS moon rockets currently being developed are heavier than anything that has ever been transported by the crawlers, and that the utility tunnel would be over-stressed.  Aaron King and I spent several days in the tunnel, wading through a foot of murky, foul-smelling water in shrimping boots, bolting strain gages to the walls and ceiling.  The gages were resistance-type, designed for measuring dynamic strains (specifically, pile-driving).  We connected all the gages to a logger and got ready for the big load test – the Space Shuttle Endeavour (mission STS-134) being rolled out of the VAB, over the tunnel and out to the pad.

The big moment arrived.  The VAB doors opened majestically.  The crawler with its billion-dollar cargo emerged and began its ponderous journey.  We fired up the logger and started recording.  As the crawler and shuttle passed by, we watched the signal traces of our gages – flatline.  Every one of the traces continued to plot low-level noise but there was no change in strain.  The signal wasn’t conditioned, the voltage wasn’t right, something was amiss and the gages were producing garbage data.  Panic time.  Hey NASA, would it be possible to, um…, maybe stop and back up?  No, that would not be possible.  The Shuttle has a rendezvous with the heavens, and it is not going to go into reverse just because you tunnel-rat engineers don’t know how to read your own instruments.  What a stupid question!

At LTC, we focus on what we know best – properly instrumented bi-directional testing.  We use vibrating wire gages because of years of verified performance.  The only possible similarity I can think of between a drilled shaft and a space mission is that do-overs are extremely expensive, at best.  When the client asks for something we are not 100% familiar with, for example thermal profiling or fiber optic instrumentation, we partner with specialists to help us make sure we do not have to ask for second chance to get it right.

Luckily, back at NASA, we did get another chance.  Because there was one more Shuttle mission (Atlantis STS-135) that launched before the fleet was retired.  So we did get our data in the end, and it wasn’t rocket science.  It just needed an experienced hand.