While working on a satellite earth station, we had finished the majority of the commissioning and were doing a shakedown period prior to full operation. The so-called geostationary satellites actually execute a small, daily figure-eight path and slight drift, which is no problem for small dishes on the house for TV, but when you’re using a 90-foot dish to handle huge volumes of telephone traffic, that means the dish has to be in "autotrack mode" 24/7.
We realized over a few days that we kept finding the tracking servo system in "standby mode." Consequently, the satellite would slowly drift out of the narrow beam, and the channel noise alarms would sound. Eventually we realized that the problem happened every day at precisely 11:30 a.m.
After several days of posting engineers at various places to observe what actually happened on the site at 11:30 a.m., someone spotted a momentary blink on a lubrication control panel warning lamp. Tracing the interlock signals then led us to the main oil pressure switch for the elevation lead screw gearbox and a 24-hour time switch set to 11:30 a.m. Once a day, the time switch opened a solenoid valve to spray oil over the exposed surface of the lead screw for a few minutes, and due to rather inept design of the pipework, all the oil drained out during idle periods.
When the solenoid valve opened, the main manifold pressure dropped until the pipes filled up as far as the spray nozzles. The pressure drop tripped the main servo system for an apparent lubrication fault. A swift adjustment of the pressure switch setting and installation of check valves to prevent the oil draining back solved the problem.
This entry was submitted by Rod Hine and edited by Rob Spiegel.
Rod Hine, MA (Cantab) MIET MBCS, graduated from Churchill College in Cambridge, England. He worked in satellite communications, meteorological telecomms, and then general automation, machine tools, and industrial control systems.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
TJ McDermott; Yes, too many alarm messages can also be a problem. The ABB IRB 6400 S4c controller had several message lists, and a general list. And some alarms would generate a secondary message, or even a tertiary message; e.g. on a servo fault, each of the 6 axes would reply with a shutdown message. It took some time to sort through the alarms to get to the primary error.
Alarm history is vital when there is more than a handful of alarms to track. On the other hand, having a hundred alarms to sort through can be just as difficult as the intermittent one.
GlennA, I was working for a company that made simulators at one time. I was involved in a R&D project to track errors and predict faults. It was very interesting and an early application of AI in a real-time system. We acutally logged every fault, even intermittent ones. I learned about this working on satellites, actually. Having an indicator that just blinked is not really useful, you need to keep that information.
Intermittent or momentary faults can be very hard to find. On some machines, where possible, I added a latch to the alarm. Other times, you just had to watch for the error. One of the machines that I worked on had an air blow for spindle cleaning during tool change. If the air pressure dropped below the switch setting the machine Emergency-Stopped on low air. As soon as it stopped and shut off the air blow, the pressure built back up above the switch setting, and the error automatically reset . Machine operators were notorious for not paying attention, so the complaint was usually "the machine just stopped". Often the fix was just replacing the air supply hose with a shorter, larger diameter hose.
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