Unreal-wheel deal
Tales From The Cube: Do all those rules for signal propagation, high-speed-digital design, and line terminations really apply to cables more than a mile long? Tough lessons you learn in a real-life application prove that they actually do.
By Jeff Fries, GE Transportation -- EDN, September 3, 2009
Awhile ago, I got involved with troubleshooting a field issue on one of our wheel-sensing products. The product used inductive methods to sense the presence or absence of a train wheel. The inductive sensor would then drive an analog signal over twisted-pair copper wires from the sensing point to a central-processing location. The issue in this application was that the sensors were detecting phantom wheels.
The source of the problem was not obvious from initial clues and surface investigations. To troubleshoot, we started out by generating a diagram to map out all possible root causes. The one that seemed the most obvious, given the clues we had, was noise on the power supply to the sensor electronics. After isolating the power supply from the rest of the neighboring electronics and floating the supply from ground, we learned that the power supply was definitely not the cause of the phantom-wheel detections. We then became fixated on the local ground reference for the central-processing system. We tested the ground and found it to be less than 1Ω—also not the problem.
We began to focus on capturing the actual waveforms coming into the central-processing system from the wheel sensors. We placed some analog data-acquisition modules on key signals coming from the wheel sensors. Once we captured the anomaly, we saw that there was a large noise disturbance on the analog signal after the signal was heavily filtered. Further dissection of the clues showed that the disturbances coincided with a train’s presence. We also noticed that the disturbances appeared to have a repetitive frequency of 100 Hz associated with them, as well. We began to suspect that we were seeing rectified noise from electric trains that used the overhead, 50-Hz electrification system for motive power. This idea sounded reasonable, but the question still remained about how this noise was getting into our system.
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The system includes some heavy hardware and software filtering, such that any noise that could affect the system would have to be in-band with the wheel-detection signal, which was approximately 50 kHz. It is well-known that electric-train propulsion systems emit a broad band of harmonic frequencies. Was it possible that a 50-kHz component of these harmonics was magnetically coupling into our cables between the wheel sensors and the central-processing system? Our first reaction was that this scenario was not possible because we always used shielded cables and grounded the cable shield at the receiving end of the signal.
After weeks of frustration, I came across an old textbook stating that, when the cable length exceeds one-twentieth of a wavelength, you should ground both ends of the cable shield instead of just the receiver end. Just out of curiosity, I ran the calculation for one-twentieth of a wavelength for my signal at 50 kHz and determined it was 300m. Hmmm. Our cables in some cases could be as long as 2000m. Could it be that these recommendations and formulas that I had reserved in my mind for high-speed digital design applied to a much lower-frequency analog signal with a nearly one-mile-long cable?
We modified the installations in which our cable lengths exceeded 300m to ground both ends of the cable shield, and we thus solved the problem.
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Hi Jeff
Its good to hear examples like these from real-life engineering tasks. This one emphasizes the role of engineer in applying various principles across the domains, leading to smarter engineering designs & solutions.
sistla vamsi krishna - 2009-19-9 21:20:00 PDT -
Unreal-wheel deal comment: Jeff, Can you help me with my dilemma? I found your tale very interesting as I have been struggling with wheather or not to ground Category 5-7 Ethernet cables at both ends. Based on your article Category 5 [100 MHz] cables over about 0.6 meters long should be grounded on both ends.
To complicate things, STP Ethernet cables, by default, ground at both ends when grounding clips are present in the receptacles at both ends. However, most of the references I've found recommend grounding at one end only. I don't have easy access to the TIA/EIA standards; but other references indicate that TIA/EIA specify grounding at both ends.
Thanks.
Bill
Bill Phillips - 2009-17-9 11:37:00 PDT -
Could you please give me the name/author of the textbook you are referring to about the lambda/20 shield grounding? Do you know of any other references to this grounding scheme? I am curious especially related to shield grounding on field bus systems. Please reply to my e-mail address
Tom Collen - 2009-15-9 09:50:00 PDT -
In fighting with noise in industrial machinery, our standard policy was always ti start with the cable shield only grounded near the downstream end, and if that did not work, then to try a different ground. Or multiple grounds, at both ends. This would have found the problem is a fairly short time, which usually makes a good impression on those watching.
William Ketel - 2009-3-9 16:30:00 PDT
