The other day, while cleaning out some very old reports, I came across the mention of a piece of test gear I hadn’t thought about in ages: the Gertsch Ratio Transformer. Way, way back in the day, I was involved with interface circuitry for LVDTs, and this basic passive box was a key part of the test-and-validation process.
If you’re not familiar with the LVDT (linear variable differential transformer) it’s a very rugged, reliable, and precise position transducer that can be used over a wide measurement range — spans of 6 or even 10 inches are common. It’s completely passive. You excite the primary coil with an AC signal, and read back the secondary-side signals, then do some fairly straightforward demodulation to get a signal proportional to position.
To check out the accuracy, repeatability, and precision of our interface circuitry, we initially had a long-stroke LVDT set up in a fixture with a leadscrew on the movable core, along with a precision caliper for visual readout. We’d manually set the position at various settings via the leadscrew, and then check our electronics compared to the caliper reading. It worked, but it required a lot of manual setting, along with a need to pay attention when setting the position and reading the caliper.
Then one of the old-timers said, “Hey, get a ratio transformer; it’ll make your life easier.” A few hundred dollars later (the boss signed off, thankfully) and we were all set. The Gertsch Ratio Transformer is just that: a precision, multitap autotransformer box with big knobs to set the input/output ratio. To our electronics, it looked just like the LVDT, except it was much easier to consistently set to different values compared to adjusting the real LVDT.
with big knobs to set the input/output ratio.
Spurred by remembering the ratio transformer cited in that old report, I used the Internet to step into the “way-back” machine. Sure enough, there were several good links, including a site devoted to them here, some brand acquisition history here, as well as some units for sale (along with tutorial) on eBay here.
Seeing them for sale reiterated one point about them: Unless they have been electrically abused or left outside, there is nothing to go wrong with them. Even an old one that has been used repeatedly should still work, and the same is true for one that has been sitting in a storeroom and forgotten. No batteries or capacitors to leak or dry out, no operating system to go obsolete, no LCDs to decompose, just a combination of wire and switches. Perhaps the switch contacts have some modest corrosion that can be cleaned, but that's all there would be to worry about.
There’s a lesson here. Sometimes, simulating your real-world signal source or transducer is key to evaluating the performance of your analog front end, in design, debug, and qualification stages. That's why a modest investment in a thermocouple simulator, for example, is a great idea: You don’t want to be messing around and heating your thermocouples, for obvious reasons. But simulators for other real-work transducers and signal patterns — pressure, position, velocity, loudness; the list goes on — can really be worth their modest cost. Of course, some are even more sophisticated, as you can also get standardized, certified simulators for an entire array of cardiac signals and their permutations, very helpful for validating your ECG device or heart-rate monitor.
There’s another thing about that ratio transformer that I recall: the nice, solid, tangible click of its front-panel switches as you stepped through the various settings. It really made you aware that you were dealing with signals that live in the real world, even if you were only simulating them. Often, these signals are quickly digitized and then consumed within the system software, to be just another bunch of anonymous bits. Yet you always need to have some way to remind you that you are working with transducer and analog I/O signals, which represent motion, power, action, and perhaps even danger.
Have you ever used a basic yet effective simulator or some sort of hardware-in-the-loop (HITL) to move your project along? Did you ever have to build your own when you couldn’t buy what you needed?