Align up: The case of the out-of-sync synchro amps

-November 13, 2008

A previously published Tales from the Cube column (Reference 1) brought to memory a similar experience I had with a relay-release time and an attempt to reduce inductive spikes.

As a young engineer in the late ’60s, I was working on the Apollo instrumentation ships, which NASA used to track and communicate with the Apollo lunar-excursion module. The ships had C- and S-band radars, a satellite-com antenna, a telemetry-tracking antenna, a VHF/UHF command-and-control communication antenna, and a large data-processing center. Each of the antennas provided azimuth- and elevation-position-synchro data to a central network.

This network allowed selection of any antenna’s position data for making one antenna act as a slave to another antenna. Large synchro amplifiers drove 23 small synchro amps to amplify the synchro-position data. The 120V, 60-Hz synchro amps required an electromechanical-alignment procedure. The amps could align to approximately 0.1°.

After successful system tests, we noticed that the amplifiers were out of alignment by large amounts—in many cases, 20° or more. Realignments were tedious and time-consuming. We resorted to pinning the shafts to keep them aligned, thinking it was a mechanical problem. Over time, the pins became loose and even bent, however.

The three-wire outputs of the synchros went into a relay-switch network. The relays allowed selection of which antenna would act as a slave. The relay coils each had suppression diodes to reduce inductive spikes, the noise on control lines, and switch-contact arcing. This procedure was standard and accepted.

While monitoring the switching transients, I saw that the relays’ dropout time was a lot longer than their pull-in time. This excess time allowed the outputs of two or more synchro amps to connect across each other during the dropout time of the relay. During switching, the amplifiers also made loud gear noises. This pull-in- and dropout-time difference was new to me, but checking relay characteristics confirmed my observation. Some studying of the use of suppression diodes indicated that diodes would even lengthen the relays’ dropout time. I tried capacitors and RC networks to no avail.

Then, in a diode catalog, I found a Thyrector device from General Electric. It was basically two back-to-back diodes for reducing inductive spikes. I tried using the device to correct the time difference, and it greatly reduced the relays’ dropout time.

My boss was reluctant to make any change because the customer had accepted the system. We had no recording scopes back then, so I set up a fast strip-chart recorder to monitor the relay pull-in and dropout times during typical switching sequences and showed him the two connected amplifier outputs. I repeated the test with the Thyrectors in the circuit, which showed a great reduction of the overlap time and no gear noises during switching. Success!

I replaced the suppression diodes with Thyrectors, and subsequent tests during sea trials showed no synchro-amp misalignments.

Arnold N Simonsen is an electrical engineer in Tucson, AZ. Like Arnie, you can share your Tales from the Cube, find out how.

  1. Lindenbach, Walter, “Time bomb,” EDN, Sept 27, 2007, pg 26.

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