Taking out heterodyning from multi-channel LVDT measurement systems: Page 2 of 2

December 27, 2016 //By John Matlack
Taking out heterodyning from multi-channel LVDT measurement systems
With its DIN-rail-mounting smart LVDT signal conditioner module S1A, H. G. Schaevitz LLC, Alliance Sensors Group has solved the age old problem of cross talk between sensors even if the master signal conditioner fails. When LVDT signal cables are bundled together or laid close to each other in a wire trough, even for short distances, the result can be a mutual interference phenomenon called heterodyning or “beating”.

The idea is that there cannot be any beat frequency interference if all the LVDTs are operating at the exact same frequency.

This practice works quite well but has a major drawback, because the "redundant" system is no longer fully redundant. The system is now dependent on one oscillator in one signal conditioner to supply all the LVDTs with the proper excitation frequency. If the master fails, two things could happen: either your other channels stop operating, or they revert back to their original oscillator frequencies and the multiple channel DC outputs now display beat- frequency-produced noise, ripple, or output drifting. Many companies offer LVDT signal conditioners with a master/slave function, but they all are fully susceptible to the beat frequency problem if the designated master oscillator falters or fails. Sometimes when LVDTs are used in a redundant pair this will be evident on only one channel of the system. In multiple channels it can cause similar effects in one or multiple channels or none and depends on many factors like cable layout, grounding, shielding along with various cable lengths etc.

Alliance Sensors Group has considered this beat frequency problem with multiple LVDT installations and developed a successful solution to this problem. Because the S1A and its variants have a digital address for RS485 communication capability, we are able to use this digital feature to maintain the full redundancy of multiple LVDT systems. If the "master" were to fail, another "master" having a different digital address would instantly come on stream to maintain a single excitation frequency. In this approach the only thing lost in a "master" oscillator failure is the former master channel itself.  For multiple channel LVDT systems, the integrity of the output signals from the other channels is fully maintained.  This "auto-mastering" feature is unique to the Alliance Sensors S1A and its variants.


About the author:

John Matlack is VP of Sales and Marketing at Alliance Sensors Group - www.alliancesensors.com

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