Resolvers, synchros, and inductive and capacitative encders, good resources? - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: May 01, 2023, 11:46:30 pm
content: I'm thinking of making a project out of trying to design some absolute encoders based on electrical (rather than optical or hall effect based) principles.I've been reading about the principles of resolvers/synchros and of inductive and capacitative encoders, but whilst a lot is said about the underlying principles and the high level arguments for when and where to use each, I'm a bit mystified as to precisely what the geometry involved is.I'm considering doing it all with coils and/or carefully shaped copper areas on PCBs, but can't seem to find much discussion of what the shapes involved for the coils are.Most stuff about resolvers/synchros discusses the commercial style units with wire wound coils, and looking at cut-away images and disassembly diagrams doesn't reveal much to the eye, you just see amorphous wire bundles as you would in any motor or transformer, no way to recognise how they are supposedly aligned 90 degrees apart. I also can't quite see how resolvers/synchros can have the read outs from the two sensing coils not totally swamped by picking up crosstalk from the coil which is serving to excite the rotating target coil.And inductive and capacitative sensors don't have so much about them either, except that it would seem they are a lot more vulnerable to shaft misalignment and wobble than resolvers/synchros would be, with anything which causes part of the rotating target to approach or recede from the fixed coils likely to drown out the true change in signal due to rotations of the target shape. I know the capacitative type is used as a linear encoder in digital calipers, but despite being such a common product there isn't that much I could find explaniing their operation.Is anyone aware of good websites explaining these topics, or anything else to do with rotary transformers as well?Thanks

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 02, 2023, 10:06:31 am
content: Do you want to measure linear displacement or rotation?For linear measurement, you can do a search for LVDThttps://en.wikipedia.org/wiki/Linear_variable_differential_transformeror "eddy current probe".A lot of the "classic" commercial resolvers have laminated steel cores and copper coils wound around them, and that would be difficult to diy.Maybe you are looking for something like the zettlex incoder?O a simpler version. The link below has some graphics with PCB tracks. It's mostly the form of a sinusoid wound around a circle, combined with a rotating round plate with "fingers" that cover half of each sinusoidal period.https://www.melexis.com/en/tech-talks/inductive-resolver-for-e-motor-e-brake-and-e-steering

author: fcb
author_rank: Super Contributor
timestamp: May 02, 2023, 10:42:10 am
content: +1 for Zettlex in Cambridge who makes a whole series based. Their patents are probably expired by now.

author: Infraviolet
author_rank: Super Contributor
timestamp: May 02, 2023, 08:06:36 pm
content: Rotary, not linear, I only mentioend calipers because I know that whilst they are linear the principle could probably apply to rotary too.I'd heard of IncOders, but all I can see is commercial adverts, not explanations of the underlying technology, especially not in terms of the geometries involved. I guess the manufacturer is trying their best to keep hold of their design secrets.

author: jmelson
author_rank: Super Contributor
timestamp: May 03, 2023, 02:11:31 pm
content: The classic brushless resolver has a rotary transformer to bring out the rotor without the use of brushes. These work exactly the same as if there were brushes bringing out the ends of the rotor winding, but without the wear. The rotor is wound with two coils at 90 mechanical degrees. One winding is shorted, the other is brought out as the rotor winding. The stator has two sets of windings that can be thought of as 90 degrees apart, mechanically. But, actually, these windings are spread out to get more even flux coupling to the rotor. As the rotor is turned, the coupling betwwen the stator winding and active rotor winding increases/decreases in a sine/cosine relationship. To see one way of reading out a resolver, take a look at the Analog Devices AD2S1200 chip. This converts a resolver to look like a quadrature encoder or absolute encoder. The datasheet has some theory of operation info on resolvers.Jon

author: TimFox
author_rank: Super Contributor
timestamp: May 03, 2023, 02:15:47 pm
content: Resolvers and synchros are very similar machines.Resolvers have outputs in phase quadrature (90 deg apart).Synchros are three-phase, with outputs 120 deg apart.The wikipedia article on synchroshttps://en.wikipedia.org/wiki/Synchromentions resolvers at the end.

author: Infraviolet
author_rank: Super Contributor
timestamp: May 06, 2023, 07:14:50 pm
content: jmelson, can you link to some very clear diagrams of that? That is to say ones which show where the windings are, but show small enough numbers of coils to be interepreted by eye rather than simply becoming a single massive of copper colour the waya photo of a disassembled nuti would be. Thanks

author: johansen
author_rank: Super Contributor
timestamp: May 07, 2023, 04:04:10 am
content: The same problem is present in any ac generator, you want a clean sin wave output from an electromagnet.When i rewound a 5kw generac generator for 3 phase, i used a 10 pitch mush or lap winding, as in, 24 identical coils that overlap themselves continuously (getting the last 10 windings under the first 10 is difficult and machines cant do it, which is why almost all motors are concentric wound with multiple different coils that are all inserted into the motor at thr same time. I was able to get 138v on each phase but only 230v line to line in Y. About 16 volts went missing iirc due to the third harmonic being subtracted. This is actually really good for a small generator.Because most synchros are very small, the coils are probably concentric , inserted by hand, and the exact ratio of the number of turns for the outer windings vs the inner windings is going to be a trial and error process assisted with fem and years of experience.If you want to start somewhere i would suggest a lap winding, the number of coils is the same as the number of slots and they span 35 to 40 percent of the core. So 9 or 10 slots if you have 24 total.

author: Dundarave
author_rank: Regular Contributor
timestamp: May 07, 2023, 05:45:00 am
content: Curious Marc recently did an “Intro to synchros” video on his YouTube channel, if you’re into his style of technical exposition:https://youtu.be/8QvoY4_xBjwHe also discusses the differences between synchros and resolvers towards the end.

author: Infraviolet
author_rank: Super Contributor
timestamp: May 08, 2023, 04:27:13 pm
content: Ok, I've been reading up about PCB resolvers and found a good paper from melexis:https://www.melexis.com/en/documents/documentation/whitepapers/whitepaper-high-speed-inductive-position-sensor-for-e-machinesBut I am having some difficulties understanding a few points, I did learn maxwells equations and the fundamental physics of EM radiation many years ago but have no intuition for "RF" stuff (this being not so much RF as near field effects, but still about fields between conductors rather than just currents within them).Apparently the varying current in the spiral coil round the cirumference induces currents in the rotor structure, which then induces currents in the receiving coils. The magic appears to be the use of a coil shaped according tox1=(R0+DelR*sin(N*theta))*cos(theta)y1=(R0+DelR*sin(N*theta))*sin(theta)andx2=(R0-DelR*sin(N*theta))*cos(theta)y2=(R0-DelR*sin(N*theta))*sin(theta)which gives each coil a form where half of it runs the same way as the outer spiral, and half of it runs the opposite way.This is supposed to cancel out any circularly symmetrical currents induced directly by the outer coil, but not cancel induced currents "reflected" off the asymmetric rotor metal.To my limited understanding induction of voltages in this way follows:V_induced=-M(dI/dt)so the rate of change of current (rather than of voltage) in the powered coil is inducing a voltage (rather than a current) in the recieiving coil, with M being a mutual inductance which is stronger for conductors which are close together and run together for long distances. Can I double check whether I am correct to consider than it is rate of change of current (rather than voltage) which matters for producing induced signals, and than in the receiving coil the induced signal acts as a "voltage source" rather than "current source"?But considering a 1 pole pair system (upper part of Fig 7 in the paper), the lengths of both the transmitting coiland the receiving coils are pretty short really. Several loops round the circumference for the transmitter, and effectively just two loops worth for the recieiver.Won't this make the mutual inductance between both the tx coil and the rotor, and between the rotor and the rx coils really tiny?I've looked at datasheets for various chips from various manufacturers designed as single piece solutions for driving and reading these kinds of coils, but none of them looks like it is designed to drive huge currents in to the coils, even just for brief instants at the peaks of a several MHz waveform. Yet a description I found from renesas about lab testing of such a PCB resolver said it was getting several volt amplitude readings back on the receiving coils. How can this be achieved when mutual inductance looks like it should be very small, and the chip surely isn't outputting multiple amps on its driving pins?I must be missing something, so would be very grateful for an explanation of what it is.I'm going to go ahead and get some PCBs of the coil structures made, I've been able to find enough information, plus those crucial equations above, to be able to procedurally generate the coil shapes and import as dxf files to eagle. But in the mean time I've been trying to get a breadboard understanding of the principles at work here.I produced a voltage controlled current source (push only for now) with an op amp, and fed in a 200KHz voltage waveform, I know that the commercial chips operate at about 10 times this, but I thought initial measurements might be a little easier at this somewhat lower frequency, and whilst inductive effects might be weaker at 200kHz than 2MHz they ought to still be detectable. The current output runs through a "coil" to ground, it appears as a roughly triangular wave peaking at 25mA or so. This "coil" is just 5 loops of wire circling in a radius similar to the tx coils in example designs I've seen described online. When I bring an inductor close to this coil, measuring voltage at one end of the inductor (10mH, cylindrical coil, <20 ohm DC resistance, assumedly several hundred loops of wire behind the heatshrink) and having the other end run through a large resistor to ground (common to the ground the coil and op amp are both relative to), I get a reasonably clear several hundred mV signal on it at sub-centimetre separations. This signal even changes, like more classical resolver technologies, phase and amplitude when rotated relative to the transmitting coil.But if I bring another "several loops of wire" coil, surely a better analogue for the PCB coils than an inductor, it doesn't pick up any signal from the coil. So the principle might work with inductors, but if a wire loop similar to the PCB coils won't get a measureable voltage induced in it, how can coils of so few loops as seen in the example designs ever pick up enough signal to be measured?Thanks

author: johansen
author_rank: Super Contributor
timestamp: May 08, 2023, 04:40:50 pm
content: i don't fully understand how it works, but i believe that the servos i worked on 15 years ago had similar method for generating a tachometer signal.the method was similar to what's in that paper except that a copper cup on the end of the drive shaft had a motor rotor type coil inside it, with a single phase excitation most likely.. and also a 3 phase winding for pickup.as the cup spins its going to drag the flux around with it due to eddy currents and produce a voltage in the 3 phase coil.you could very easily make one by removing the magnets from a small ESC motor and soldering a copper ring in place, and winding a second excitation coil around the 3 phase coil.

author: Infraviolet
author_rank: Super Contributor
timestamp: May 08, 2023, 04:45:07 pm
content: Johansen, isn't that sort of situation going to be one where the coil rotating with the shaft acts as a generator? It would induce signals when rotating, and the faster it was the stronger the signals, but no signals when static at a position??

author: Infraviolet
author_rank: Super Contributor
timestamp: May 08, 2023, 07:00:59 pm
content: I'mnow wondering if the effects a PCB resolver tries to exploit are precisely the same pysics as for crosstalk between wires? I'm doing some mroe reading up on that, but still a bit mystified as to those matters I asked about in reply #9. How can I get measurable amounts of "crosstalk" with such short lengths of trace?

author: johansen
author_rank: Super Contributor
timestamp: May 10, 2023, 02:11:53 am
content: Quote from: Infraviolet on May 08, 2023, 04:45:07 pmThe cup rotates around a stationary coil which had at least 5 wires coming out of it. So its generating a signal by dragging the flux somehow in proportion to rpm.Imagine for a moment, a kilowatt hour meter working in reverse. You spin the disk and read the voltage out of the current coil, which is going to be in or out of phase depending on the disk's direction. The current coil would have hundreds of turns instead of one.The servo was a 24000 rpm capable system that was driven by a 2 phase driver. One coil gets 400hz the other gets transistors shorting the other coils out, in or out of phase and 90 degrees offset, sort of like a transistor controlled shaded pole motor.. 1970's technology but it was fully explained in the navy manuals.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 10, 2023, 03:34:06 am
content: I don't think you will find the fundamental answer to your question anywhere. The basic idea of these is based around ideal coils (zero thickness). The resultant outputs are then indeed perfect sines and cosines assuming perfection in a few other ways.As soon as any of those perfection assumptions are violated errors arise. You haven't stated your goals in terms of resolution and accuracy, but those imperfections only matter in relation to those goals. When these originally came into use 1% accuracy was considered wonderful. Now many applications look for 14 or 16 bit accuracy.Coil winding techniques as mentioned by Johansen can help, but the details really matter in unobvious ways as you get to the higher accuracy levels. Shaping of the magnetic cores can also be used to improve the approximation. Field visualization software can help, but the only real way to find out what is happening is to build it.In some cases you can "calibrate" out the errors with a large enough error table. If your application involves continuous rotation you can generate "best fit" sinusoids using phase lock techniques and then use these for figuring actual angle. Changes in rotation speed wreak havoc with this approach, but it works for some applications.

author: Infraviolet
author_rank: Super Contributor
timestamp: May 10, 2023, 04:27:06 am
content: EDIT: I've realised I just need to go ahead and finish the PCB design, working from what I have been able to deduce from various designs. Then I can start testing it and see what can be detectable.It would still help though at this stage if anyone could guess whether the return signal is going to seem like a current source or voltage source, so I'd have a feel of the sort of op amp arrangement I'd need for buffering and amplifying that signal before further processing of it.My overall aim is relatively limited performance, 2 degree accuracy out of 360, and having any necessary calibration during initial setup of the board, and a fairly low maximum rpm rate. Ordering PCBs is cheap, so that will be easier than trying to guess too much in advance.

author: jonpaul
author_rank: Super Contributor
timestamp: May 10, 2023, 10:53:15 am
content: Farrand INDUCTOSYN since 1960s, used in gun and missile directors, submarines, industry.http://www.inductosyn.com/products.htmSuper accurate and super resolutionRotary and linear...http://www.inductosyn.com/PDF%20Files/Farrand%20Controls%20Brochure.pdfJon

author: jmelson
author_rank: Super Contributor
timestamp: May 10, 2023, 03:57:11 pm
content: Quote from: Infraviolet on May 06, 2023, 07:14:50 pmThe standard resolver manufacturers are Harowe Controls and Singer/Kearfott, maybe they have some theory documents available.Jon

author: jmelson
author_rank: Super Contributor
timestamp: May 10, 2023, 04:03:19 pm
content: Quote from: Infraviolet on May 10, 2023, 04:27:06 amThere is a document from Analog Devices for the 2S1200 chip, that has the schematic and layout for their demo board. The 2S1200 datasheet also has a LOT of deatils on how the chip works, etc.I used this as the basis for my product. It definitely uses the stator windings in voltage mode. I think Inductosyns are more likely to work in current mode. The resolvers I use for testing usually have a transformer voltage ratio somewhere between 2:1 and 1:2, so that is a step down or step up of half to double, from rotor to stator.Jon

author: johansen
author_rank: Super Contributor
timestamp: May 11, 2023, 03:48:48 am
content: The output of any resolver is voltage, but at high frequencies you will have to compensate for the impedance of the amplifier amd the transmission line.I belive my kwh meter analogy is accurate. You put a voltage and frequency in and the disk does not turn.You spin the disk and you get a voltage out of the currrent sense coil.Or you put a current through tbe coil and it produces torque on the disk.Making the primary coil have multiple coils and the disk is sectored just provides you (potentially) a higher frequency or quadrature output. But its fundamental the same magnetic property.. you are basically reflecting off the disk, the emf of the primary coil.

author: Infraviolet
author_rank: Super Contributor
timestamp: June 04, 2023, 08:59:21 pm
content: PCBs turned up and the good news is the principle works, I'll say more when I've got the system a bit more refined for tolerable performance, but I can already say getting return signals of 100s of mV (peak to centre) is pretty easy even with the rotor metal several mm away from the PCB coils, JUST SO LONG AS YOU ENSURE THE Tx AND Rx COILS BOTH HAVE THE RIGHT CAPS IN PARALLEL TO BE RESONANT AT YOUR DRIVING FREQUENCY. (the all caps note should help anyone who stumbles on this thread with similar issues in the far future).