Today Scouting New York ran some photographs of the mechanicals of an old, working elevator that dates back to the 1920s.
I was especially intrigued by this amazing picture of the control system:
Can anybody explain what the heck is going on there?
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12\$\begingroup\$ I must say, that's a well-exposed picture for the combination of extreme lights and darks! \$\endgroup\$– tybluDec 9, 2010 at 7:58
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2\$\begingroup\$ Tut tut Joel, you should really accept an answer! ;) \$\endgroup\$– Mr. HedgehogFeb 12, 2011 at 17:21
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\$\begingroup\$ I'll do something better... add a bounty! \$\endgroup\$– Joel SpolskyFeb 13, 2011 at 0:59
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1\$\begingroup\$ I've always wondered how an electro-mechanical (pre-computer) elevator control system worked. What happens when you press a floor number? How does it decide which car to send? How are doors, indicator lights, etc. controlled? Why doesn't the DOOR CLOSE button do anything? \$\endgroup\$– Joel SpolskyFeb 13, 2011 at 20:26
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\$\begingroup\$ People that say the "Door Close" button does nothing are only sometimes correct. For one, it depends on the elevator system, and two, it depends on the context in which you're pressing it. For example, in my office's elevator, the door close button will immediately close the door if it stopped at a floor because someone inside requested it. In some override mode, I've seen it used to keep the door open despite the cab moving (used when we needed to transport something that was ~3 inches too long for the elevator) \$\endgroup\$– Nick TFeb 18, 2011 at 19:43
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2 Answers
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I found a description of a similar arrangement on p. 333 of Electric Journal, Volume 9.
The relays close in series to gradually accelerate the car and to come to a smooth stop.
This truth table shows how each relay is closed as the car accelerates and decelerates up and down.
This is just for a simple elevator control that does "up", "down" and "stop". Fancy modern elevators have pushbuttons for each floor, so there would be a more complex system, with relays operated by switches on each floor, and relays keeping themselves energized, a form of one-bit memory.
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It looks like a relay panel capable of handling a serious amount of current.
Way-back-when logic circuits were built out of relays. They are big, bulky (a gate is a relay or two), and costly, so not as common today, but near impossible to kill and can handle all the current you have the space for.
The coils of various relays would be connected to the buttons and position sensors, while the contacts of the large relays appear from the video to carry the main motor current. I would imagine there are some additional low-power relays elsewhere for more logic; I doubt what's seen just there could provide control for more than a couple floors.
Relay logic is still used today (in spirit) in PLCs in the form of ladder logic. There are seldom any physical relays, save the occasional one on the logic's output.
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\$\begingroup\$ Relay logic was being used at least as recently as the mid 90's in trains (locomotives, trams etc), at least in Europe, using the train battery supply, typically 48 to 110V d.c. This was in addition to microprocessor logic. It's bulky but reliable, at least it can be designed to avoid dangerous states when it fails, which was/is almost impossible with software. (An example of a dangerous state might be allowing the train to depart with the doors open). \$\endgroup\$– MartinDec 9, 2010 at 11:26
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1\$\begingroup\$ The photo reminds me of a "computer" made from relays at my school in the '70s. I believe it was a binary adder and had been made a decade or so earlier by some enterprising students. It caught fire around '76. Seems like only yesterday :o{ \$\endgroup\$– MikeJ-UKDec 9, 2010 at 11:32
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\$\begingroup\$ One of the first examples of relay logic was telephones. electromechanical relays replaced the telephone operator and her plug board. \$\endgroup\$– Jim CDec 9, 2010 at 13:44
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\$\begingroup\$ When I was younger I worked doing security/fire systems and I tore out several old fire alarm panels that were just massive cases with 100+ relays in them hand wired to build all the alarm logic. Pretty crazy what they used to do with a box full of relays and a spool of wire. We'd replace a 6' x 4' x 2' enclosure filled with relays with a 12" x 8" x 3" modern panel, and the only reason new panels that big is for all the screw terminals and wire management. \$\endgroup\$– MarkFeb 13, 2011 at 22:05
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\$\begingroup\$ The number of relays on the main panel wouldn't need to increase with the number of floors if each control button had a couple of multi-contact relays located behind it. \$\endgroup\$– supercatMay 10, 2018 at 19:59