Driving a synchro from digital logic? - Page 1
Forum section: electronics / projects

author: philpem
author_rank: Frequent Contributor
timestamp: February 11, 2015, 01:12:40 pm
content: So it turns out I have yet another crazy projectI was away visiting friends last weekend, and stumbled across a shop which sold old military parts. I just happened to fall in love with a pair of gauges from a Westland Wessex helicopter - the ADF gauge, and the engine (E1, E2 and rotor) tachometer. As near as I can tell the tachometer is fairly standard - three squirrel-cage three-phase AC synchronous motors (no idea of voltage) connected to a mechanical tachometer (the aluminium-cup eddy-current type) - but the ADF is something a little more interesting.It's based on a Smiths synchro, with two wires to an "R" coil (rotor) and three to a set of three Y-wired "S" coils (stator). From what I can gather, the R winding is powered at 110V AC 400Hz, while the S windings would have been connected to a synchro transmitter which would have sent a 0-90V (from where to where?) AC 400Hz signal on the S windings.The R winding is (in theory) fairly simple to drive. A 400Hz AC sine oscillator, some transistors and a ~150V HV power supply would do it. My problem is the S windings.What voltages do I need to apply to these? Am I correct in thinking that I'd need a set of three 400Hz AC oscillators with a variable output from 0 to 90V, or am I barking up the wrong tree? Or is there a phase component to this too?I've found references to how resolvers work, but very little on synchros, and nothing at all on the output voltages I'd see on the various windings an their relations to the reference. Just the "S" windings relations to each other, and how that'd be generated by a synchro transmitter (which I don't have).My goal with all this is to have a PCB which can control the synchro (or another -- I have my eyes on a few other similar displays which also use synchros) based on commands from, say, a PC. All in the name of fun.Thanks,Phil.

author: DanielS
author_rank: Frequent Contributor
timestamp: February 11, 2015, 03:40:52 pm
content: I am not familiar with how ADFs work but since what you have there looks practically like a synchronous motor, I suspect the phase relationship between rotor and stator is what determines the rotor's position. The voltage and frequency can be almost whatever you want within reason, as long as you drive enough magnetizing current to overcome static friction.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 11, 2015, 04:23:39 pm
content: If its a military aircraft they are usually 26 VAC or 110 VAC 400 Hz and the core is tuned for a specific frequency. A few are 60 Hz.You'll need 3 phase power at a a few tens of mA and one signal with a controled phase relationship to spin the dial. Ie the phase angle is the position control.There are a wide variety of synchros, so you need to figure out if you have a selsyn, synchro, synchro resolver, or synchro transformer.Find US NAVY NEETS and read up on synchros and resolvers. They are not a dead technology, Many car instrument panels use resolvers to move the dials.Steve

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 11, 2015, 04:41:13 pm
content: Quote from: LaserSteve on February 11, 2015, 04:23:39 pmYes, there are various ways to use them. You can have a lot of fun with them. If you have two equal of them, you can connect the rotors windings together and the stator windings one to one. If you power both rotors with the same appropriate AC voltage you have an "electrical shaft". If you turn the rotor of one, the other will followIf you reverse the connection of one of the rotor windings, the direction of the other following reverts too.

author: zapta
author_rank: Super Contributor
timestamp: February 11, 2015, 05:33:12 pm
content: These are also called selsyns. Used to transmit an angle via wires. For example of a rotating antenna.http://www.allaboutcircuits.com/vol_2/chpt_13/11.htmlThere used to be in the market converters to/from digitals but where not cheap. IIRC correctly the phase is identical in all three coils and the angle is determined by the ratio of amplitude. All the ones I saw worked on 400Hz.

author: SeanB
author_rank: Super Contributor
timestamp: February 11, 2015, 06:08:40 pm
content: Tacho is driven with a nominal 26VAC signal derived from a tacho transmitter on the engine reduction gearbox, typically it is a 3 phase signal with variable frequency ( what you want) generated by a rotating magnet with a saturating shunt to provide a coarse voltage limit with increasing rotor speed in the 3 phase coils. You can excite it with 20VAC variable from 100 to around 500Hz, and it will indicate.Simplest way to generate the 3 phase signal is to use a ring counter to give 3 pulses in sequence, generally a 4017 coupled as a count to six counter, taking the output from outputs 1,3 and 5 and using it to drive a 3 phase transformer ( or 3 regular ones) with a tuned output using a capacitor. Primary ( the original secondary) to match your supply and a 115VAC secondary( the original primary) will give a good enough 3 phase supply to drive them. For the tacho just omit the capacitor and use a 1:1 transformer off 12V.I made a single phase one to drive an hour meter using only a small transformer and a 555 timer with some adjustment to get the frequency to 400Hz for it. Worked till the meter itself wore out mechanically.

author: woodchips
author_rank: Frequent Contributor
timestamp: February 11, 2015, 08:14:22 pm
content: OK, don't hide it, where was this shop? Geetting quite rare now.A synchro transmits an angular position from somewhere, the engine or flap etc, to somewhere else, the cockpit. There are two basic types, one that doesn't use amplification called torque transmitter and receiver, and one that does need amplification. The torque type just move pointers in instruments.Simply work by feeding a 115V 400Hz signal to the rotors of both synchros, wire the three stator wires together and that is it. There are 26V versions but somewhat rarer. Easiest way to generate the voltage is an old audio amp feeding a suitable transformer.Best book is Synchro Engineering by Upson, seem to be £20 on abebooks.

author: zapta
author_rank: Super Contributor
timestamp: February 11, 2015, 08:41:08 pm
content: Quote from: SeanB on February 11, 2015, 06:08:40 pmIIRC they use a single phase and three signed amplitudes, not the different phases.

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 11, 2015, 09:04:18 pm
content: Quote from: zapta on February 11, 2015, 08:41:08 pmYes true, but looking at the static absolute angle of the shaft you can say the outputs are 120 deg shifted in amplitude.Feeding the rotor with an AC signal and synchronous rectifying the outputs of the stator gives you 3 DC signals shifted 120 deg absolute angle. That can be processed by a micro controller for an accurate indicator of the shaft rotation angle. That use is called a "resolver".

author: philpem
author_rank: Frequent Contributor
timestamp: February 11, 2015, 09:46:56 pm
content: Quote from: LaserSteve on February 11, 2015, 04:23:39 pmThis one is very definitely 115V 400Hz - I had the cover off the ADF indicator and it says so right on the synchro:SYNCHRO TORQUE RECEIVERCODE ACS 2/AFSMITHSMADE IN ENGLANDAnd on the other side:400 C/S[wiring diagram]115/90VQuote from: LaserSteve on February 11, 2015, 04:23:39 pmAhh - that makes sense! The stuff I'd read said things like "the voltage on the stator determines the position", but that didn't make any sense. Phase angle vs. the rotor winding makes a lot more sense, thinking in terms of magnetic fields!Thanks very much for the reference to the NEETS -- chapter 15 (http://jacquesricher.com/NEETS/14187.pdf). I read through that a few times and it's pretty much clicked now. Looks like you're right, I need a 3-phase supply and a single-phase AC supply, and some way of varying the phase between them. This is at least simpler than my last idea, which involved a board full of analog multipliers and the ominous thought of "what the hell am I thinking here? This can't possibly be right!"Looks like I'll have to dig through my box of ferrites and see if I have any suitable RM-cores and bobbins to make the transformers. I doubt I'll find 400Hz-capable transformers off-the-shelf. I guess that means I need to find some references on transformer design...Quote from: SeanB on February 11, 2015, 06:08:40 pmThanks! I figured it was something along those lines. Small 3-phase generator on the gearbox runs small 3-phase motor in the tacho, which runs the tacho mechanism (the aluminium eddy-current cup, spring and needle). The only things I was missing were the drive voltage and frequency.Time to bash together a transistor driver circuit and mate it to a PIC micro, I guess!Quote from: woodchips on February 11, 2015, 08:14:22 pmBirkett's, at the top of the hill in Lincoln. Absolutely full of aircraft instruments (broken and not so broken), ex-RAF parts, and all manner of other things.I really need to do another Saturday day-trip with my rucksack and some bubble-wrap and buy out half the contents of his window!Quote from: woodchips on February 11, 2015, 08:14:22 pmIs it even possible to wind a single-phase to three-phase transformer? I can't even get my head around how you'd do such a thing (though admittedly transformer design is one of many facets of electronic engineering which is currently on my "black arts I really should learn more about" list!)Thanks for the book reference - I might have a look for a copy if I get further into this.Thanks,Phil.

author: zapta
author_rank: Super Contributor
timestamp: February 11, 2015, 09:51:06 pm
content: Quote from: Pjotr on February 11, 2015, 09:04:18 pmThats' the A/D side, I think the OP asks about D/A.As for the A/D direction, you may need to consider also the sign of the three amplitudes, not just the absolute values, otherwise you will have angle ambiguity (rotating the transmitter's rotor by 180deg will give the exact same DC values).

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 11, 2015, 10:04:32 pm
content: Quote from: zapta on February 11, 2015, 09:51:06 pmYes true, rectifying is not the right description, synchronous demodulation is better. But why would youdrivesuch things of the past digitally? Probably only for some antique aircraft indicator with such a synchro in it. I got the impression OP looked for some purpose of what he collected.

author: zapta
author_rank: Super Contributor
timestamp: February 11, 2015, 10:19:02 pm
content: Quote from: Pjotr on February 11, 2015, 10:04:32 pmFor example for a test equipment or simulator that need to inject syncrho signal or for a drop in replacement for a legacy subsystem.Here is a good starting point for information.http://www.ddc-web.com/Products/Synchro_Resolver.html

author: brotherphil
author_rank: Newbie
timestamp: February 23, 2018, 05:24:00 pm
content: Glad I'm not the only one with such strange notions. I came across this looking for ideas on how to drive a Sperry compass repeater - which I also got from Birketts. They have a lovely selection of RAF surplus dials and gauges, and there's also a radio that I might pick up some time.I'm hoping to get at least some of it going to make a physical instrument panel, driven by a flight sim. I can get the digital data out from the sim; the fun but is figuring out how to drive original instruments from that data - ideally without just stripping out the works and replacing them with new servos.Thanks for the tips!

author: albert22
author_rank: Regular Contributor
timestamp: February 24, 2018, 03:08:31 pm
content: Here is a thread with similar requirements for driving a synchro.May be useful to youhttps://www.eevblog.com/forum/projects/driving-a-synchro-from-a-uc/msg1306130/#msg1306130

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 24, 2018, 06:52:25 pm
content: Phil, don't give up, and please don't replace synchros with RC servo crap.What you want to do can be done and you can drive the whole instrument from custom built controller.I can't offer much advice because it was more than 10 years since I have done last synchro project - it was fully operational Collins HSI conversion. The biggest problem was the pinout of two huge connectors on the back. I have also done conversions for Sperry FDs and altimeters.From memory - never needed any 110V 400Hz signal, I had 24V DC bus and all synchros (both transmitters and receivers) happily worked from 24V AC synthesised from 24V DC bus and a stack of PWM drivers.CheersLeoP.S. I have found some old PCB designs for Sperry HZ-6. It is full of synchros as you can imagine.Quote from: philpem on February 11, 2015, 09:46:56 pm

author: opus
author_rank: Newbie
timestamp: August 30, 2021, 09:54:05 am
content: I do know that is a fairly old thread, however it is just about the project I'm facing now.We ( some volunteers in a naval aviation museum) are "maintaing" ( as in presenting and explaining) a full mission simulator. By adding a projection system and a PC simulation (FSX) which takes the needed inputs from the controls ( by using a BU0836X board ) we managed to make the cockpit be sort of useable again.What we are lacking is feedback to all the indicators.They were driven by synchos and I THINK it should be possible to digitally mimic the needed signals/powers to drive those synchros again.The reply of @Leo Bodnar sounds interesting, however it is lacking detailed informations.Has anybody else done something like this and can share some information?

author: obsoletepower
author_rank: Contributor
timestamp: June 13, 2022, 04:13:47 am
content: Quote from: Leo Bodnar on February 24, 2018, 06:52:25 pmHello Leo, I also am interested in driving a Sperry HZ-6B using digital logic. I noticed in you post you added an image of a PCB you designed. Would you be willing to share the Eagle files and BOM for your design?