Bendix Air Data Computer - Part 3: Intro to Synchros

[@nealelliott1360]

Wow, watching this video and you mentioned "a very flat A", and it clicked, that's the sound I hear when I'm boarding an aircraft. It all makes sense now, thanks Marc!

[@ropersonline]

10:35, if you too were wondering. So that sound is the result of old-timey analog instruments being made to sing as a side-effect, thanks to the 400V onboard electricity supply that drives them? Are more modern airplanes with glass cockpits quieter in that regard?

[@150flyer4]

400Hz is such a legacy that modern avionics still sing along. Cooling fans and transformers make enough noise on their own. Probably the biggest downside is that 400Hz is in the audio range and is hard to filter out. It leaks into everything audio and communication related. Many newer aircraft do away with constant speed generator drives and run the generators directly at varying speed. Wild frequency systems will run anywhere from 300Hz to 600Hz. You can tell by hearing the frequency change with engine speed.

[@chromosundrift]

@@150flyer4 pretty much all AC frequencies are in the audio range though right? Even the ubiquitous 50Hz or 60Hz. I can often tell if a youtuber is in north america before they open their mouth if there's a stray mains hum. Apparently the cold war spies were totally on top of this when analysing audio signals they acquired.

[@redherring3110]

"My breath seems to have no humidity" "it's because of your dry humour" comedy cold 😂

[@EdwinSteiner]

Marc: "That's the error in minutes." Ken: "I don't have time for that."

[@Brian-L]

Marc and Ken need to take their show on the road. 😂

[@tekvax01]

I work at a TV station, and we use to have a mechanical teleprompter that had several synchros motors to move the paper scripts through the machine, so the camera could photograph it, and place the video from the camera on the screen, for the talent to read. you would have a hand controller, and it would move the other motors in the prompter convertor belt. You could also gang several units together as well. it was quite cool, back in the day!

[@GrantWyness]

I can sense Ken’s inner Hulk biology bubbling into being every time Marc come close to that machine with a probe or finger 😂

[@ludmilascoles1195]

Or a screwdriver 😁

[@chrisdickens4862]

“Error in minutes” “I don’t have time for that!” I love Ken’s sense of humour.

[@jc5445]

Neither did the mechanic replacing these parts.

[@RingingResonance]

I love these. I remember playing with the little differentials at my uncle's house. He was the mad-scientist of the family. Sadly, he passed away about a decade ago. I think the parts might still be in that house somewhere as there are still many places that haven't been cleaned out yet. Every time we clean there, it's like traveling back in time as he was a hoarder of old electronics and what ever other junk he thought was neat. The other day I pulled an IBM5150 and behind the desk was a bunch of old security cameras; the kind that used a vacuum tube for it's image sensor. I haven't bothered pulling those out yet as I haven't had time. Speaking of old things; someone reached out to me and mailed me all of the schematics to my DT80! I'm so excited to scan the packet he gave me, but I haven't yet gotten a way to scan it all properly. It's at least 150 pages.

[@ignaciomenendez8672]

These were used in IBM, with the name of ‘RESOLVERS’ with an accuracy of 20 seconds of arc. The IBM-2321 data Cell, AKA ‘noodle snatcher’ used one of these, on an analog servo circuit to position the bin to the exact address to pick a selected strip, out of 200. (The old BB CC HH address)

[@williammanganaro2022]

This episode hits home for me. I have been working with radar and EW systems for nearly 40 years. Our sea surveillance radar antenna platforms use synchros to determine antenna array azimuth position and tilt orientation. Just about all synchros I've seen used in airborne radar systems are 26 VRMS 400 HZ excitation. This is a great series of videos. Look forward to seeing more. Thank you for all the work and research your team does.

[@ifitsrusteditsmine]

The sense of humor of these guys is second to none.

[@kaitlyn__L]

That intro music always makes me so happy :) Oh my, I figured it all out once I saw the oscilloscope! (That the magnitudes of each phase encodes the rotation angle, and the motion of the stator induces current in the circuit - a slow AC motor/generator pair!). But your elevator music explanation was nice to confirm my suspicions :D That's very elegant indeed. Always vaguely wondered how the dials in eg a Spitfire were driven. This is a bit like discovering the temperature gauges in Victorian steam engines were thermocouples connected to volt-meters! It's always nice to see how old some of these commonplace electrical techniques actually are.

[@Rob2]

Those synchros are so much fun! The fact that they work in both directions... modern electronic solutions often do not do that.

[@carpetbomberz]

I have heard that name Bendix so many times, it's like the TRW of aircraft systems. Anything they couldn't buy they made themselves. And milspec, oh man I can remember visiting my Dad's workplace at IBM. They had a small library full of manuals for all the milspec relating to stuff they contracted/sub-contract for IBM FSD back inna' day.

[@wtmayhew]

I remember seeing a film from the early 1960s from IBM. They talked about their involvement in SAGE making the vector display scopes. They also mentioned making airborne computer parts for B-52s. The B-52 system appeared to be cards similar to the ones in a 1401, but each card was in a black hermetically sealed cylinder. Not exactly high density packaging, but it could work from -40 degrees up to an altitude of 100,000 feet while being rained on.

[@acmefixer1]

Eric Likness failed to tell us what the FSD meant. I believe it means Federal Systems Division, basically anything to do with the government, and costing many times what other divisions would typically cost.

[@Brisbane_Astro]

Used to maintain syncro systems on a River class destroyer escort in the RAN. They were used extensively for the gyro compass to transmit to repeaters around the ship, and to get details on the position of radar antennas for some of the older radar displays.

[@Edisson.]

Hi, absolutely amazing series of videos, I look forward to each new episode. The motors you refer to as "Synchro" were supplied under the name "Selsyn" in the former Czechoslovakia. In my 15th year, I got hold of one pair and made an antenna rotator tracker out of it. Thank you for these videos, thanks to which I have expanded my knowledge in an area that I would not have gotten into otherwise. Nice day 🙂 Tom

[@nickm8134]

This series is very interesting. Been involved over 60 years in electronics and computing, but I was never aware of synchros or resolvers until now - amazing precision engineering!

[@HanMasho]

That "dry sense of humor" line was very quick-witted. Didn't miss a beat on that one! Edit: The comedy was on point with all of you guys! If you get tired of all this sciency stuff, I bet you could find success by starting the Electrical Engineers Comedy Tour.

[@utp216]

The engineers that designed and built this stuff pre solid state sensors, etc were some clever people!

[@chromosundrift]

Looks like they have cute little indicator lights in them. I look forward to maybe seeing them lit and mounted in something pretty.

[@juango500]

So excited for the in-depth look into these historical pieces of technology!

[@jlwilliams]

Okay, so lately around the CuriousMarc Secret Lair we have been seeing an almost complete Apollo S-band communication system, a bunch of resurrected 8” floppy drives, a mechanical air data computer and ancillaries, and a Globus. I'm trying to think where all this could be leading… it's got to be either world domination or an awesome steampunk R2-D2…

[@martinhow121]

Brilliant. I had never heard of these until a couple of weeks ago when a pair turned up in a donation form someone's workshop to our Mens's Shed, and a military design guy explained them, to me. They were heading indicators for a naval vessel. Simple concept when you know how.

[@ChristianWSG]

Thank you for another episode of explaining complex equipment. I was hooked since your AGC project. Better than any book or movie. Like your style and those of your equally competent buddies.

[@ericwilner1403]

Cool! I had an idea for a semi-novel application for synchro-like tech many years ago, but never got around to playing with it. This is a good explanation of the tech, and a stimulant to the imagination!

[@jimstanley_49]

They were also used in the film industry to sync audio recording with the camera. As part of the pre "Action!" exchange, the director would call out "roll sound!" and the engineer in the recording booth would respond with "sound to speed!" indicating the recorder had matched speed with the camera and was recording.

[@jpjude68]

3:00 - "error in minutes..." - "i don't have time for that!" absolutely destroyed me

[@johnclawed]

Where the receiver needs to drive a lot of torque, like in the B-29 fire control computer or battleship guns, feedback from the receiver creates an error voltage if it's different from the transmitter position. That drives an alternator coil which generates the power to drive the actuator motor, which brings the receiver into position and nulls the error voltage. I'd love to see an episode on that error voltage, what it looks like, and what you can do with it.

[@DerekHerbst747]

I am an avionics technical instructor and grew up with synchros and servos. It's quite nostalgic watching this video. The Resolver is still in use on more modern airplanes like the 747 to give position feedback to controllers in many systems and for position indication in the cockpit. Really enjoying this!

[@mynung7008]

Makes sense - you get absolute angular position for an unlimited number of rotations, without any sliding or exposed contacts or resistors that might degrade. The only other way I know of to get close would be an optical encoder with Gray code, but that provides digital signals and usually has a fragile glass disk. The synchros and resolvers are marvelous devices!

[@steveschulte8696]

Another unit of synchro systems is the Differential Synchro Transformer. This unit can do addition and subtraction of angles, say between the fire control director and the gun mount. The differential transformer has 3 windings on the stator and 3 on the rotor.

[@dmacpher]

The Twitter thread on this is phenomenal!

[@CuriousMarc]

I had not even seen it! Here it is: twitter.com/kenshirriff/status/1650565976294055937?s=61&t=UXfboHQEfahjAKeGVe1J8w

[@KeritechElectronics]

Again, a lovely explanation of how these things work :).

[@dosgos]

Fantastic video; I know it took a lot of time and effort to make. We learned a lot today.

[@jamesgreer7615]

I haven't seen one in 20 years! Worked on these when I was in the Navy. They were used to repeat signals from the inertial navigation system (gyro compass) 120V at 400 HZ. On one of the ships I was on the backup 400 Hz converter was in our shop. When that thing was on you wanted to die.

[@rav3nx33]

I bought a surplus military aircraft parts box a few years ago; and to my supprise found a large box full of little boxes of these; like 40 in total, amazing little things; had no end to fun of playing with these on the bench. Smart stuff in the world of simple and reliable reigns king.

[@wtfgreg1246]

Man this is my new favorite channel

[@jamesburke2759]

Thanks for showing. as a modern Pc tech and Engineer i love this old stuff to give me inspiration for future projects. It always makes me sit and marvel at the fact that this seems like such simple technology today that a smart kid could understand but there were the top minds of the time working hard to make it work. To me its the thought if we had some sort of disaster and had to start all over again technology wise, it probably wouldn't take us that long to get back to where we are using mechanical or steam PC's and Media.

[@cambridgemart2075]

Analog devices used to manufacture a synchro to digital converter , we used them in a 1980s designed satellite communications system.

[@michaelmiller641]

Fastest flying bench ever! Hilarious! Once it's explained how a synchro works, it makes sense! Thanks Marc and Ken!

[@neillthornton1149]

The Navy still uses sychros and resolvers! Synchros send highly accurate 3 axis compass readings all over the ship for everything from fire control to navigation. Meanwhile resolvers work on SATCOM systems so that antenna control units can keep gimbaled antennas locked on to geo-sync birds while the ship is bobbing around in the ocean.

[@paulw3182]

Wow the synchros are amazing! Great Video.

[@lwilton]

BTW, those synchros and resolvers are mounted with what is called "servo mounts". This is a semi-triangular "nut" with a tab that fits into the groove on the front of the servo device, and is then screwed down with a #4-40 screw (or maybe #2-56 screw, I forget) for little machines like this. You can see one in use in the picture of the static air temp gauge. Like motor mounts, servo mounts come in various frame sizes.

[@gonzinigonz]

Talking of 4-40 screws. Would a 8-32 UNC be near size to a metric M4? Its come up in the past and struggled to find the UNC equivalent size. Obviously thread is different but was looking for replacements, not easy to get in the UK and got to know what your ordering if you do find something.

[@lwilton]

@@gonzinigonz Looking at some screw charts, an 8-32 is about 4% larger than an M4, so is approximately an M4.2x0.8. They likely have very similar torque tightening specs and holding power. Can you order from McMaster-Carr? If so you could get US screw sizes.

[@bigjd2k]

Very interesting! I can now play with the Muirhead ones I have somewhere! Be sure to check aircraft instruments for radium paint with a Geiger counter - you don’t want to open up spicy ones!

[@fragdude]

The puns being thrown around …. So good Gotta love the products of those fat defense budgets

[@acmefixer1]

I was fiddling with a synchro training setup that was not damped, when oscillations started building up and the much larger pointer started spinning wildly. I grabbed it to stop it, and got whacked hard on my knuckles, hard enough to almost draw blood. So learn from your mistakes and cut the power rather than get injured!

[@RoamingAdhocrat]

ooh, I appreciate the description of how mach numbers were mechanically calculated from pitot and static pressure. Back in the day, X-Plane couldn't give indicated mach, just actual mach from the flight model and insulated from any pitot/static failure. Might be fixed now, but if not, this would allow a better mach gauge to be simulated!

[@ajdkjfakdjaldjhfaldf]

You are spoiling us now!

[@genetomblin2883]

Old ship board radars used syncro motors to transfer the motion of the rotating antenna to he yoke on the CRT on the display thus the rotating display was always the same as the antenna position. A simple switch on the antenna that closed when the scanning antenna was pointed dead ahead provided a brighter sweep line at the ahead position , heading flash. Gyro compas repeaters made good use of syncro motors to relay the compass rose position to places that they were needed. Most radars had syncro gyro position to rotate a ring around the ppi display that provided a quick accurate way of determining positions to true north in degrees.

[@gonzinigonz]

Used to have a small collection of aircraft instruments when i was a kid. We used to go to the airshows and there would normally stalls setup selling parts and associated junk. All cheap from what i remember, that would of been in the 70's. No idea what happened to it all, probably all got dismantled to see how it worked. But some avionics have crept back into the house the last few years 😆

[@efx200z7]

Synchros reminds me of that wheel used in ships to communicate from the bridge to the engine room, moving the lever on one causes the other indicator to move

[@Ariccio123]

If I remember correctly, synchros had some role in converting the outputs from analog naval fire control computers to actual power assisted gunlaying mechanisms for battleships

[@johnyoungquist6540]

You can run a synchro pair on low voltage 60 Hz. You can convert synchro to resolver using a scott-T transformer. If you synchronously demodulate the output and take the 4 quadrant arctan you get heading in degrees. A simple equation can also convert synchro 120 degree data to resolver 90 degree data. A simple CMOS analog switch and couple of Rs and Cs are all you need for denod. I used this in aircraft instrument designs. We make air data computers no gears though. Really neat piece of technology. Great video

[@juliussokolowski4293]

Resolvers were used to make the sweep on an old school (CRT) radar PPI scope.

[@leon545b]

Just like a 3-phase ac motor, if you want to reverse the direction of the receiver w/r/t the transmitter, just swap two phases... ;-)

[@confuseatronica]

i think the pcbway xmas ornaments are breeding

[@joealig7019]

There are a couple of good books that I would recommend to anyone interested in servo motors. The first is published by Analog Devices "Syncro and Resolver Conversion " by Geoffrey S Boyes, and the second is "Theory of Servomechanisms" Vol 25 of the Radiation Laboratory Series. Both books can be found as pdf's online.

[@dosgos]

Thanks for the recommendations!

[@clytle374]

Liked the "what kind of witchcraft is this?" line. I always say "What sorcery is this?" Some line out of a Johnny Depp movie.

[@Debraj1978]

BTW, every body wants to work on digital domain. So there are R2D converter, that converts resolver analog output to digital format (usually 12 bit or higher bits of SPI data), to be handled by a microcontroller. These are manily used to avoid the arc-tan computation, which is needed in case of sin-cos signals from resolver.

[@random007nadir]

"Error in minutes. I don't have time for that." Peak engineering humour.

[@I967]

Lovely mechanics. It is much more interesting to look at gears, levers and linkages than at digital technology - not much exicement looking at a static PCB populated with strange metal and plastic insects.

[@ydonl]

They were used in the Palomar 5 meter telescope mount control!

[@snugglebunnyhaven7258]

These have been used in wind direction systems of wind measuring equipment that are analog for many many years. Mounted inside the weather vane housing, a 5 wire cable would be run down inside to the gauge with a compass. They would run with 6VAC across the rotors. Accurately displaying real time wind direction. Unfortunately these little gizmos are becoming very hard to find, as many have been discontinued manufacturing.

[@SkyhawkSteve]

So fun! I wonder if you could change the direction of the Mach indicator by swapping two stator phase wires? In my avionics days, that was done to change the direction of the 3 phase motors. Might work with synchros too?? BTW, I've got a spare resolver if anyone has a use for it.

[@lwilton]

Yes, it does.

[@acmefixer1]

That's why the wires are all color coded - to prevent things from going the wrong way!

[@SarahKchannel]

I would call that "Spooky Action at a Distance" !?

[@b43xoit]

Or phase modulation.

[@acmefixer1]

Synchro's were a large part of analog computing before digital, because analog was realtime, digital wasn't fast enough to keep up with things like the positions of flight controls - rudder, etc. I assume that Marc will get into this as we see more episodes after this one.

[@TheEmbeddedHobbyist]

The fun to be had when trying to modify a navigation switching system to include a new device is very overrated in my opinion. synchros and current driven signals where you have to keep the impedance constant as units are switched in and out runs rings around your head. Arinc429 and Mil1553 was so much easier. 🙂

[@deviljelly3]

I think master Ken may have some admirers in the military.

[@b43xoit]

If there is any resistive torque applied to the receiver, the motors act like pulleys linked with a rubber band. The more torque, the more deviation in the angle.

[@ropersonline]

I still would love to see the reverse/inner side of the faceplate, the one with the big round connectors on it (that wasn't shown in this video. Next video, maybe? Pretty please? I'd love to see how that all connects to the internal D-sub connectors that are part of the Bendix's innards.

[@c1ph3rpunk]

Congrats on the major award, it’s French, it says fra-gee-lee on it and everything.

[@computer_toucher]

Frazheel-uh?

[@150flyer4]

From all my avionics work, 400Hz brings a smile. But at the same it has driven me to virtual insanity after having to listen to it for hours on end! Apparently 400Hz became a standard as it was the fastest a generator could turn and yet still have some reliability at the time. I was taught that it has the sound of A flat, but I have zero musical ability.

[@lmiddleman]

400Hz is nearly a whole tone flat of A440, perhaps more of a sharp G (~392 Hz).

[@MCW1955]

I don’t have time for that! That’s pretty funny.

[@mikebarushok5361]

It makes me feel old when a part made when I was 30 years old is treated like an an ancient artifact. Synchros were used for such a long time and for so many systems because they work well and have very simple wiring. They're very robust because there's only one moving part and even when wires are shorted or reversed nothing burns up. The Mach Indicator having an output is so that the Mach number can go as an input to the autopilot, the flight data recorder and sometimes to other places like fuel computers or cabin displays.

[@AppliedCryogenics]

Looks like a tailor-made solution for interconnecting mechanical computers with electronic computers. Maybe on some US navy ships in the 1950's, they had such hybrid systems doing firing solutions.

[@computer_toucher]

en.wikipedia.org/wiki/Mark_I_Fire_Control_Computer

[@MrMilarepa108]

I Do, I do follow the channel and know that you're doing that!!!

[@RussBootlegMan]

Oh! the Synchros !!!

[@wesleymays1931]

Less than 5 minutes in, I'd like to make a guess about how these synchrons work: They're built like motors that don't spin on their own. The rotor has an exciter winding that the 26Vac is fed into, and there's a three-phase stator (wired up in the delta configuration?) that picks up that signal. The synchro being moved acts like a resolver, and the angle is encoded in the 3 phase outputs, which is picked up in the other synchro, and converted to a magnetic field with varying angle, which the rotor in that synchro spins around to match up with. It's kind of similar to field-oriented control (method of driving brushless motors, which are technically 3-phase permanent magnet AC motors IIRC), where you rotate the magnetic field generated by the stator, and the rotor follows.

A bidirectional position transmission system? Just a fancy drive belt 🙂

[@CrawfordMethod]

My jet engine has a similar arrangement for engine speed. There is a “tach sender” on the auxiliary shaft which connects to the “N1” engine speed indicator gauge. The connection is 3 wire between the two, no excitation or outside power. I assume that no excitation is needed, as the gauge probably is a spring return to zero and requires a constant rotation input from the sender to produce the needle position. So maybe they are just 2 synchronous 3 phase motors?

[@iamdarkyoshi]

Such cool stuff!

[@edgeeffect]

I love the milspec packaging.... once you've done too much - do a little bit more.

[@cprogrck]

Omg!!!! Of course! I was in the air force for 6 years and could never understand why C130s and indeed most airplanes make that sound with power turned on!!! 400hz now I know.

[@wd9dau]

That’s frageelee so it must be Italian! Learned about synchros in the late 60s at the original Devry. In Chicago.

[@physnoct]

Fresh: Dessican When expired: Dessican't :)

[@colingale]

for bonus kudo's could you explain and recreate chuck yega's historic flight on the mach +alt +air temp meters ? awesome video ..need more !!

[@davidchipman1964]

Marc, you made me Curious with the "very flat A" observation. I tried to match the pitch of a simple tone generator to the sound in your video using my ear as the guide. My result was something like 389 Hz. How close was the actual output frequency? Am I due for calibration?

[@kh40yr]

At First I was like,, "oh, a sensored brushless motor instead of sensorless brushless motor". Nope. Quite much more.

[@sashimanu]

A motor that *is* the sensor

[@wtmayhew]

[old] Syncros have slip rings to supply the excitation voltage to the rotor.

[@kh40yr]

@@sashimanu Yep.

[@jamesdecross1035]

Quick question: Can you link one sender to two receivers?

[@oberguga]

So if we connect two syncrhos, mechanically fix one and power second one with different PSU, then it's output should indicate difference in phase and frequency?

[@bobowzki]

What flight deck poster is that in the background at about 1:11?

[@ludmilascoles1195]

In my day we called these repeaters

[@TechGorilla1987]

@3:20 and the gang is all about dry humor. That made me stop and type.

[@ansgaryeysymontt7155]

Internal structure looks simolar to car alternator. Any one tested if 2 alternatolrs with connceted windings and supplied with ac have similar abilities?

[@eduardoschubert5738]

Hi all. Once I connect 2 step motors to each other and they behave pretty like the syncros. Of course without the power, they do not syncronize like the syncros do. Just a curiosity.

[@deniskirin6007]

Does the car fuel level indicator work with the same principle?

[@michaelproust7891]

Thanks, could you explain how Sperry could have invented instruments so early after the first aircraft flew?

[@JerryBiehler]

you connected them with flying leads?

[@Rob2]

The mach number indicator will require a "x5" sticker...

[@drstrangelove09]

part # = ? (wanted to look one up for the price) how much did they cost?

[@rsmrsm2000]

This work is exciting. bring some of the refined technology of the cold war thanks for showing

[@dine9093]

"I don't have time for that" - lol