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.
I can certainly agree with tekochip on this one.It would seem logical to test the entire installation over a period of days to ensure proper operation of all systems prior to going "live".Then again, I've been there.Management sometimes does not allow time for a complete "shake-down" and we end up bench testing complementary systems instead of testing the entire assembly.This is one of the great frustrations design engineering types have with right-to-left program management.
I can certainly agree with tekochip on this one.It would seem logical to test the entire installation over a period of days to ensure proper operation of all systems prior to going "live".Then again, I've been there.Management sometimes does not allow time for a complete "shake-down" and we end up bench testing complementary systems instead of testing the entire assembly.This is one of the great frustrations design engineering types have with right-to-left program management.
You are right William K. With the benefit of 20/20 hindsight, a lot of the technology on the early earth stations was overkill on a grand scale. However, we had only just progressed from low orbit fast-moving satellites to synchronous satellites and many of the operators wanted to be able to track fast-moving satellites too, "just in case". But I take issue over operating for "months". As satellite technology improved and smaller dishes (5 to 7 metres) came into use they were often driven by bought-in electric actuators with so-called "sealed for life" lubrication. This would be fine if they were operated at reasonable speed and over their whole travel to keep the lube grease distributed. In fact they would operate for months just inching to and fro over only a small distance which then became dry and very worn and the unused leadscrew deteriorated. Then when you wanted to move to another location it would sieze up and break the shear-pin and require complete overhaul or even replacement. That was particularly a problem on the Caribbean islands with corrosive salty atmosphere that defied all but the very highest levels of IP rating.
This is a case of the unintended consequences of somthing that seemed like a good idea at the time. Shutting down the tracking system instead of sending an alarm message seemed like a good thing, although the leadscrew and gears could hgave lived for months without the lubrication spray. But somebody thought that it was more important to shut down the auto-tracking to preserve the gears. Of course, if the protocol had been adequately documented the solution would have been clear much sooner.
Now you have put your finger on the root of the problem! The gearboxes and lubrication systems had been supplied and installed by a sub-contractor and the only "official" interface to our control system was the oil pressure signal. The timeswitch didn't appear on any of our system diagrams and was fed from a spur from the antenna domestic mains, installed on an ad hoc basis by a sub-contract "sparks" at the request of the gearbox supplier. That's how it got as far as it did - we were as surprised as anyone else when we found it. Later earth-stations projects paid much more attention to such integration issues and consequently the commissioning threw up fewer and fewer surprises. We still had some nasty experiences though, one in particular from an air-conditioning system. But that's another story...
Tekochip, it seems that integration testing had not happened. By component wise all the devices or parts may work fine and by integration testing we can find how the system is behaving in a complex atmosphere or in collaborative mode.
Rod, we had a similar experience while working with the rocket launch division. During the testing phase, when time counter is incremented from 23.59 to 00.00, the vernier engines stops fueling the booster rockets and hence there is change in thrust pressure. We had done all sort of testing to find out why this fuel injection missing happens at that particular time and finally found that a small bug in the Micro controller program. According to the program 00.00 is the resetting cycle, so whenever the clock reaches 00.00, it reads as a reset instruction.
I'm surprised this wasn't discovered during testing of the lubrication system. It's not intermittent, and there wasn't anything odd about 11:30, it's just that the lubrication system caused the machine to fail. It makes me think that the lubrication system was tested outside of the machine, or when it was at idle.
You have to remember that this was around 1968 and control system design was moving from traditional relay technology to use of early (and expensive!) integrated circuits. It just wasn't possible to latch every one of the hundreds of possible fault conditions and with hindsight, use of hard-wired printed circuit logic with IC's (mostly 7400 family in flat-packs) was smaller but no better than the relay version. Only with the arrival of the microprocessor and true Programmable Logic Controller or PLC would all this be resolved sensibly.
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