PMSM Motor Controlled Using FOC for HiFi Turntable Project - Page 1
Forum section: electronics / projects

author: dellsam34
author_rank: Regular Contributor
timestamp: October 05, 2020, 07:30:19 am
content: For a an experimentation project I'm building a very large and slim BLDC type PMSM Motor that will be controlled using Field-Oriented control, The motor is an outrunner of a size of 30 cm in diameter and 1.5 cm in thickness, the stator uses at least 120 coils, The goal of the project is to reach a near zero torque ripple and a very stable angular/linear speed.I have few questions:1- What is the coil # to magnetic pole # ratio ? If I'm aiming for a large number of them, like I said about 120 for the best torque ripple and speed stability.2- What is the maximum phase # that can be driven by today"s technology? 40 phases sounds great but I know it is not realistic.3- What should I use to determine the motor outrunner platter position? A high resolution encoder or back EMF, didn't think hall effect sensors will be good for this application but still on the table.While I can do the mechanical design and fabrication, I have no idea on how to proceed due to the above questions, Any help is really appreciated.

author: Dave
author_rank: Super Contributor
timestamp: October 05, 2020, 02:36:45 pm
content: 1. There is no set ratio.Hereis a reasonably nice explanation on how stators are wound.2. Depends on how much you're willing to spend on hardware. You could implement hundreds of phases with a massive FPGA, but I don't think there would be any real benefit to it.6-phases can be done using readily-available microcontrollers on the market (TI Delfino, Microchip SAM, STM32).3. Sensing back EMF is fine if you don't need full control from 0 RPM.If you do, you're going to need some sort of absolute position encoder. If you can access one end of a shaft and place a magnet on there, you can use something likethis. Hall sensors are absolutely useless for this purpose.Keep in mind that the number of pole pairs is the multiplier which tells you how the rotational frequency translates to electrical frequency. For this reason, the number of PP practically divides your effective encoder resolution (for example, with 4 pole pairs, one electrical revolution equals just 90° of shaft rotation, so just 2048 digital steps of a 13-bit encoder). Getting good resolution positional feedback is crucial for torque linearity.

author: dellsam34
author_rank: Regular Contributor
timestamp: October 05, 2020, 09:10:02 pm
content: Thanks for the reply Dave;What I meant by magnetic pole to coil number ratio is if I have 120 coils, should the number of magnets will be exactly 120, less than 120 or greater? I don't think having equal numbers is good because you want the coils to be pulled or pushed at any position but I could be wrong with my assumption.While the normal working speed is anywhere from 40 RPM to 80 RPM, I do need one function which is hold still, it basically rotates the platter until something locks and senses the sudden stoppage of the platter and holds it there.If someone willing to go down with me on the electronic part of it, I'm willing to fully explain what this project is all about and a reward for the time spent will be given.I can initially use off the shelf parts but at the end the stator coils and all electronic components will be mounted on a 29 cm in diameter PCB.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 05, 2020, 11:28:23 pm
content: If you have no clue about how many magnets to use for a motor with 120 coils, then you're not ready to build a motor like that.I recommend to start much less ambitious, and make more iterations.Do you have some old HDD's lying around? These (nearly?) always have BLDC motors if it's less then 20 years old. Heck, you can buy a 2nd hand HDD for less than a loose BLDC motor, and they've also got a completely integrated BLDC motor driver somewhere on the PCB.Old washing machines can also be a source for pretty high powered BLDC motors. Some of these motors are regular sized, and have a multi-V belt to drive the drum. Others are "direct drive" and can have a BLDC motor with the same diameter as the drum.On sites like Ali / Ebay / Banggood / Etc. you can buy small BLDC motors for less then EUR10. Get some experience with motors like that and get back to the bigger motors when you've learned from the small ones what you can.About the driver electronics:How do you want to implement FOC? Do you want some pre-built thing, or do you want to build your own hardware and firmware?You may also want to have a look athttps://hackaday.io/search?term=bldcLot's of projects with BLDC motors and drivers.If you like video tutorials... There is an excellent series of these (I think 4 parts) from either (or both?) Texas instuments and ST Microelectronics on youtube.

author: jbb
author_rank: Super Contributor
timestamp: October 06, 2020, 12:00:05 am
content: Can people recommend some technical books on machine design? The design ofgoodelectrical machines is quite involved.It sorta sounds like you want some kind of precision mechanical control? Lowish speed, no gearbox to avoid backlash, low torque ripple.That sounds like a hell of a project for your first electrical machine design.I don’t see why you’d need many many phases. 3 phases can - in principle - give nice constant torque. You connect coils in series to make a repeating pattern, which allows your number of phases to be less than the number of coils.Do you need continuous rotation? Or only part of a turn? Because some kind of rotational voice coil actuator (as used to move the heads on an HDD) could be very good for smooth movement.If you don’t need a lot of torque, you could consider an ironless design which will have no cogging torque.You might also consider more advanced winding schemes to provide the most even magnetic coupling. For quick and easy manufacture, each winding slot normally has only one phase in (eg 20 turns of phase A), but this doesn’t give the most even torque. There are methods to improve torque ripple by having multiple phases go through one winding slot (eg 2 turns of phase C, 16 turns of phase A, 2 turns of phase C). This is quite complicated, but diligent engineers have worked on the problem extensively since the 19th century and someof them have written reference books.

author: Cerebus
author_rank: Super Contributor
timestamp: October 06, 2020, 12:20:19 am
content: A copy of "Brushless Permanent Magnet Motor Design" by Dr. Duane Hanselman (ISBN 1-881855-15-5) will answer all your questions and then more. Expect to (re-)learn some physics if you're not up to speed on magnetics. The examples chapter has examples of up to 32 pole motors with winding parameters, back emf harmonics etc. etc.

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 06, 2020, 01:35:46 am
content: Holy bookworms Batman!I was all ready to buy me a book on pm motors until I found a copy of that thing. Must have gold leaf pages. $349.00. I already flattened my book budget with all three volumes of the AOE earlier this year. Maybe I can put it my Amazon cart for Christmas!Seriously, thank you for the reference book.

author: Cerebus
author_rank: Super Contributor
timestamp: October 06, 2020, 02:03:31 am
content: You might find it *ahem* elsewhere, if you take my drift.

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 06, 2020, 02:39:11 am
content: Yes, quite easy if you search using the text of the title rather than the ISBN. DuohThank you. Oh boy more covid reading

author: dellsam34
author_rank: Regular Contributor
timestamp: October 06, 2020, 03:26:08 am
content: Thank you guy for the replies, I'm not trying to re-invent the wheel here, I will be using the exact same technology but with different dimensions and specifications and needed the above questions answered, I can handle the mechanical design from CAD to CNC, I was just hoping that someone has the electronic skills to help me design a custom driver for the motor. Let me try to answer some of the above replies:QuoteYes but none works for my projectQuoteYes I could, I jut wasn't too sure if having more phases is equivalent to having repeated same phases, Again the goal is to have as many coils as possible to reduce the torque ripple.QuoteYes continuous motion, but there is a function that requires the platter to spin about half a turn and then hold its position for few seconds.QuoteNot a lot of torque, 12-14V application, but if this approach is used I can make up for the needed torque by having more coil windings, What materials are used for this?

author: Cerebus
author_rank: Super Contributor
timestamp: October 06, 2020, 03:02:01 pm
content: This is beginning to look like a classic"XY Problem"question.What is your actual problem? What torque do you require, what angular precision, and what is your acceptable torque ripple? If you can't put numeric answers to those then I suspect you're not far enough along in your problem analysis to even be ready to pick a solution, let alone at the point where you need to ask detailed questions about a custom motor design.

author: dellsam34
author_rank: Regular Contributor
timestamp: October 06, 2020, 05:34:20 pm
content: Quote from: Cerebus on October 06, 2020, 03:02:01 pmLet me sketch this on Solid Works so we can have something to talk about, You're rights just words won't cut it.

author: dellsam34
author_rank: Regular Contributor
timestamp: October 07, 2020, 06:53:10 pm
content: Ok, I sketched the stator, The center hole shown is the rotor hub and bearing as well as some other mechanism that is not related to the topic.As I mentioned above the RPM range is from 40 to 80 where 4 pre programmed fixed speeds are selected via a switch and 4 master variable resistors for speed calibration for each selection, For torque, the load is the platter weight itself which will be cast aluminum plus the Neodymium magnets bonded on its inner edge, spinning around two brass ring bearings, So not a lot of torque needed.The picture doesn't show the mounting holes for the rotor hub and PCB, Will add them later when I finish the hub design.So next before I sketch the rotor, will the number of magnets be the same as the coils (which is 120) or less? Some of you who are familiar with motor control should know what is the best coil to magnet combination.

author: dellsam34
author_rank: Regular Contributor
timestamp: October 11, 2020, 07:46:09 pm
content: MIT used more magnetic poles than coils (42 magnets and 36 coils), it is usually the opposite:https://youtu.be/Mhxz2Bj2RXA?t=601

author: dellsam34
author_rank: Regular Contributor
timestamp: October 25, 2020, 03:44:20 am
content: Seems like 36/42 is a common coil to magnet ratio, so what would be best for 120 coils? Any volunteer who can assist in designing the FOC drive for 3 phase with that many coils? Any help appreciated.Oh, and this is a turntable record player design. that's why speed and torque ripple are important factors for the design.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 26, 2020, 02:17:15 pm
content: You probably do not want a BLDC motor, but:https://en.wikipedia.org/wiki/Reluctance_motorThese do not get much attention, maybe because they are not "fashionable" or maybe because the power density is lower.They do have some inherent advantages for use in a turn table, and this is the same reason why they are also used in HDD's, and that is that they have 0 torque ripple, and that is in turn because they do not have permanent magnets.Did you also change the title of this thread?I responded earlier, but had not noticed then it was for a record player.There is nothing special about a motor with 120 coils, They are still just 3 phase motors, with a number of inductors in series or parallel, (or combinations of that).I also am in doubt if it would be useful to cram in as many inductors as you can.High mass of the rotor, combined with low low friction are probably at least as important.After spin up, the motor itself only has to deliver enough torque to overcome friction losses and prevent rpm from decreasing.With the right motor geometry, it can be driven by a perfect 3-phase sine wave. If the motor geometry is not perfect, then it can be compensated for by pre-distorting the 3-phase sine waves as part of the FOC algorithm.Have you considered air bearings?"applied science" has a nice ecplanation, and I think even a tutorial for making them:

author: ConKbot
author_rank: Super Contributor
timestamp: October 26, 2020, 04:05:02 pm
content: Are there any concerns with leaking magnetic fields from the motor coupling into the record needle's coil? I.e 45rpm, 120 coils would be a 90Hz rate of the poles passing by, and higher for the motor drive frequency. Noting that couldn't be fixed by adding more steel, but it would drive torque requirements, bearing friction, etc.

author: Cerebus
author_rank: Super Contributor
timestamp: October 26, 2020, 05:40:55 pm
content: Quote from: Doctorandus_P on October 26, 2020, 02:17:15 pmYoureallyshould read your own references before making pronouncements.Quote from:https://en.wikipedia.org/wiki/Reluctance_motorA reluctance motor is probably theworstchoice of motor technology for a direct drive turntable - stepper motors are  reluctance motors which should make the torque characteristics of a reluctance motor clear. Even synchronous reluctance motors have significant torque ripple.A BLDC motor is a good choice. The Technics SL1200, the standard turntable in broadcast and recording studios for more years than I care to count, used a 12 pole brushless DC motor - in the earlier models they placed the rotor permanent magnets on the back of the turntable itself, later models opted for a more conventional BLDC motor with the turntable attached to it with screws.

author: langwadt
author_rank: Super Contributor
timestamp: October 26, 2020, 06:16:26 pm
content: Quote from: Cerebus on October 26, 2020, 05:40:55 pmit's older brother but,https://youtu.be/jobfx7wePh4

author: Benta
author_rank: Super Contributor
timestamp: October 26, 2020, 07:31:36 pm
content: Quote from: dellsam34 on October 25, 2020, 03:44:20 amThe basic thing is to have an odd/even or even/odd relationship between rotor and stator. 2/3, 3/4, 4/5 etc. Otherwise the motor won't start.Your example of 36/42 is exactly that: it reduces to 6/7.I'm not sure that increasing the number of magnets will give better a result, it will probably just increase the ripple frequency.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 27, 2020, 01:41:06 am
content: Quote from: Cerebus on October 26, 2020, 05:40:55 pm

author: dellsam34
author_rank: Regular Contributor
timestamp: October 27, 2020, 03:03:37 am
content: Quote from: langwadt on October 26, 2020, 06:16:26 pmI have the Technics SL-5300 and surprisingly they look very similar.Quote from: Benta on October 26, 2020, 07:31:36 pmThe goal is to increase the ripple frequency and reduce it with FOC control instead of trapezoidal commutation.

author: Cerebus
author_rank: Super Contributor
timestamp: October 27, 2020, 03:28:29 am
content: Quote from: Doctorandus_P on October 27, 2020, 01:41:06 amYou categorically stated about reluctance motors that "they have 0 torque ripple". But suddenly you change tack and that becomes only under low power, according to you. In fact, as the Wikipedia article states, they have large torque ripple. A google search for "reluctance motor torque ripple reduction" come up with "About 1,260,000 results" because techniques to reduce torque ripple in reluctance motors is a 'thing'. I don't think it would be that way, somehow, if they are "low torque ripple" motors.Here's an illustration from "ELECTRIC MOTORS AND DRIVES Fundamentals, Types, and Applications Fourth Edition by AUSTIN HUGHES AND BILL DRURY" demonstrating how reluctance motors develop torque, the thick lines are the steady state torque curve for when the motor is running (a) with zero load (b) with a medium load and (c) with the highest load that can be applied before pull out commences. Each oscillation in torque is down to the salience of the rotor reluctance combined with the angle of the rotor and the advancing stator field. That sure looks like torque ripple to me in all three curves.Why not just admit you mis-spoke, or even just keep quiet about it and move on - when in a hole stop digging. Don't double down by making silly statements like "FOC is designed to get optimize for minimum motor current, and therefore minimum torque" - the point of FOC, as far as current and torque are concerned, is to try and getmaximumtorque for minimum current compared to other methods of motor control - evenIknow that. Optimizing a motor for minimum current with minimum torque is easy, you don't need any fancy control strategy, you just don't turn it on - zero current and zero torque.Please, if a relative motor control neophyte such as myself can poke holes in what you're saying perhaps you ought to reassess how you go about makingcategoricalandauthoritative soundingstatements as if you're highly knowledgable about the subject, you're misleading people.

author: dellsam34
author_rank: Regular Contributor
timestamp: October 27, 2020, 03:44:49 am
content: It's okay Cerebus, All opinions are welcome. Anyway I came across this publication and into the first few pages my brain started to hurt.

author: fmkit
author_rank: Contributor
timestamp: October 27, 2020, 06:45:30 am
content: use any motor made for camera gimbal, torque ripple canceled out by MEMS sensor feedback/PWMhere is my DIY gimbal using 40g motors(BGM2804 12N14P)recorded footagegimbal mounted on quadcopter

author: Haenk
author_rank: Super Contributor
timestamp: October 27, 2020, 08:05:15 am
content: For the best stability in rpm (wow and flutter), even more mass to the platter is better than more mass. This only requires a simple motor concept and a rather slow feedback (Some top notch player use a rubber wheel and a simple motor to drive the platter). Downside: Depending on the torque, it will take some time to meet the desired rpm.Your massive motor concept will most likely not increase the stability compared to a massive platter - but reaching target rpm will be quick. The mentioned SL-1200 has a rather lightweight platter and starts/stops pretty fast, plus the feedback seems to work quite well - I guess they knew what they were doing (actually they started building direct drive turntables years earlier - the 1200 became famous due to the speed fader, which allowed for easy disco music mixing...). I assume there is not much room for improvement, unless you are able to combine a massive motor with a massive platter...

author: Cerebus
author_rank: Super Contributor
timestamp: October 27, 2020, 03:32:57 pm
content: The Technics' direct drive turntables real claim to fame was the 'instant start' capability rather than the continuously variable speed, that only became the selling point later in its life. The SL10 was the first in 1970 and didn't have the continuously variable pitch/speed. Before the direct drive turntables took over 'instant start' tables for broadcasting used a mechanically coupled massive flywheel and lightweight platter where the flywheel was always turning and a electromechanical clutch connected the platter to start it turning - they were huge. In the case of the Technics' turntables 'instant start' equated to 0.7s or1/4turn to get up to full speed and stability, which is good enough for playing a pre-cued LP track over the air on fader start. They also had active braking, so you could hit the start/stop button to quickly stop a record just before your cue point. All this, of course, required that the motor had high torque available to do all the stopping and starting of the 2kg platter.The feedback mechanism on at least some the Technics' turntables is interesting. They used three transformers driven by a 50 kHz sine wave to build a set of three linear variable transformers (something somewhat like a Linear Variable Differential Transformer without the differential bit in rough principle) with the permanent magnet flux on the motor modulating the 50kHz to derive the speed and position of the turntable. The exact details are difficult to discern, there's not a 'Principles of Operation' section in the service manuals, but you can guess how it worked if you know what an LVDT is and how position feedback is used in PMSM/BLDCs.

author: dellsam34
author_rank: Regular Contributor
timestamp: October 27, 2020, 03:55:07 pm
content: The goal is to build a high resolution encoder on the same PCB that houses the coils and controls, with a bigger circumference PCB the number of encoder steps can be in the range of 10-20 steps from one coil to the next, so the cogging can really be eliminated by modulating the current according to the position of the coil against the magnetic pole. Most of PMSM used in cars and steering wheel control have very few sensors to determine the motor position or use the back EMF which requires a complex mathematical calculation.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 28, 2020, 11:51:36 am
content: Quote from: Cerebus on October 27, 2020, 03:32:57 pmLooks a bit like:https://en.wikipedia.org/wiki/SynchroOn a youtube vid I saw recently (of a repair of such) it looked like there was a copper cage very much like a squirrel cage in the center of the motor, in between the windings, so instead of separate connections for the rotor, it looks more like they're using induction.Quote from: dellsam34 on October 27, 2020, 03:55:07 pmDo you already have a clear idea on what type of encoder you want to use? The way it's implemented in the technics is very dependent of mechanical tolerances of stamped parts, which is not good for accuracy.Encoders like these:https://www.celeramotion.com/zettlex/high-resolution-encoder/go to redicilous high resolutions, (Upto 22 bit/revolution) and they're relatively easy to make with a CAD program and a regular PCB factory. Similarly to this are the digital calipers. Rip one of those EUR10 calipers apart to see how they work. They have 2 PCB's with metallic strips with different pitch, and are measured capacatively in a similar way that the noninus on analog calipers work. (The more expensive "oil and water resistant" types work with induction)Quote from: dellsam34 on October 27, 2020, 03:55:07 pmThis is not really what you want (but can be added as a last step)It looks like Cerebrus misinterpreted my previous post (and probably thinks I'm an idiot).I'll try to explain again:There are different kinds of cogging in motors. First you have cogging causes by permanent magnets, which you can often feel when no electrical connections are made to the motor. Then there is cogging caused by non lineairities of the motor coils and interaction between the rotor and stator.The first kind of cogging can be completely eliminated by not using any magnets in the rotor at all (which lead me to the reluctance motors)The second kind of cogging can be reduced by reducing the friction of the motor.As long as the record is rotating at constant speed, the only torque that the motor needs to deliver is to overcome friction. When friction is very low, the drive current of the motor can be in the mA range, This makes the torque ripple of the coils according to "conventional motor theory" mostly irrelevant (whether you use BLDC, PMSM, or reluctance motor) And of course can be further reduced by shaping the the drive current.I also think that using a very high pole count for the motor is not (very) useful. With higher pole count you can deliver more torque at low RPM (thus faster starts) but you also need higher drive frequency to the motors, which leaves less room tho shape the motor currents. The best trade of is not very clear to me.Quote from: dellsam34 on October 27, 2020, 03:55:07 pmThis kind of feedback is not useful for a turntable. Because of mechanical tolerances you will always have non-linearity over a single revolution, which is completely irrelevant for them, but will completely kill the record player.It also helps to know what sort of quality and price range you're aiming at.Is a 10cm thick granite disk on air bearings within the realm of your goal, or does this sound exaggerated?[Edit:]Quote corrected.

author: Cerebus
author_rank: Super Contributor
timestamp: October 28, 2020, 03:29:54 pm
content: Quote from: Doctorandus_P on October 28, 2020, 11:51:36 amThat was on the early ones (the specific example I was looking at the schematic for was the SL1200 Mk 2). They seem to have used several different speed/position sensing techniques over the life of the product and its many revisions. I saw a photo of one version of the motor where they were clearly using something like you're describing, I've seen others that look like they have optosensors.QuoteThat wasn't me, you're getting your quoting muddled.However, I don't see sensorless control as being implausible for a turntable. If someone has indeed managed to use sensorless control for a car's power steering (I'm a bit sceptical) which is aservoapplication, then it'd be fine for a pure speed control application like a turntable. Sensorless control needs tuning, but that's trivial for a practically zero load system like a turntable. In fact I'm having a hard time thinking of an easier motor control problem than a record turntable - virtually constant inertia (record mass is swamped by the platter), virtually constant load (how much load torque can stylus drag generate), completely constant speed demand except when stopping and starting (even then speed demand is simple either zero or rated speed).Where do you think the non-linearities would come from? We're pretty good at turning slabs of metal on lathes to very high precision and very high tolerances - 12 microns is quite achievable by any machine shop at basic costs (that's half a thou in imperial) and works out at 1 part in 25,000 on the diameter, 0.004%. Cheap bearings come with TIR figures of a few microns. There aren't really any other sources of non-linearity that I can see beyond those inherent in a motor with windings and poles.

author: dellsam34
author_rank: Regular Contributor
timestamp: October 28, 2020, 04:52:45 pm
content: Quote from: Doctorandus_P on October 28, 2020, 11:51:36 amNot air bearings, Actually in my design I'm using a very ancient mechanical bearing system that uses 3 articulating points, two radial low contact surface brass bearings with different diameters to fit on the tapered spindle and one axial steel ball and steel disc for thrust load, an adjusting screw is added to raise the center spindle to have zero clearance and zero load on the radial bearings and all the load will be on the steel ball, A very low viscosity fluid like oil is added to reduce friction, I have bench tested this and the platter span for several minutes.As to how I'm planning to implement the encoder on the same PCB? Well just thin copper traces shaped in a square wave and a metallic or magnetic pickup is affixed to the rotating platter to excite those traces. The problem with a stand alone encoder is that they have their own bearings, own mounting screws and that can add to the complexity of the design let alone the cost.But the icing on the cake is not the spinning platter, I'm working on linear magnetic contact less H slider bridge that holds the cartridge and can follow the groove with no groove wall pressure. Also I'm adding a disc self centering system, the disc is held automatically with clamping claws so no 45 adapter required, You can literally put a CD and it will self center it.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 29, 2020, 12:58:54 am
content: Quote from: Cerebus on October 27, 2020, 03:32:57 pmAnything not 100% symmetrical in the motor can introduce non-linearities, and record players are extremely sensitive to small deviations. Some things like:* irregularities in the stamped stator form, so one pole has more iron in it then the others.* Differences in plate thickness of the laminate.* Eccentricity of the rotor.* Irregularities in the motor windings which results in changes of magnetic field strength.* A magnet mounted slightly out of position.* A magnet with a slightly different field strength.* Maybe far fetched: but even grain direction in the steel can influence magnetisation strength.* ...When using Back-EMF for position measuring, a high pole count motor is probably preferable.Steel ball & disk is a pretty classic axial bearing, but even steel balls deform, which increase contact area. These days harder materials are readily available. For example flat pieces of tungsten carbide as either inserts for lathe cutting tools or bars for grinding your own tools. Ceramic ball bearing balls are also easily obtainable for reasonable prices.https://html.duckduckgo.com/html?q=aliexpress+zirconium+oxide+ballCeramic ball bearings also have extremely low friction coefficients, but it may change over a rotation or introduce vibration, especially the cheap versions. High quality ceramic bearings are ... not very cheap. Just a brass sleeve bearing is probably nearly as good.The Zettlex Incoders I mentioned consist of 2 parts, and most of it is PCB material, as can clearly be seen in this video:If you want to go real fancy, then add an zero position index dot, let the platter spin without any motor drive, log the angle and compensate for any non-linearities.Maybe a weird Idea.Maybe worth thinking about...Have you thought about a homopolair motor?https://www.youtube.com/results?search_query=homopolar+motor

author: dellsam34
author_rank: Regular Contributor
timestamp: October 29, 2020, 06:04:59 am
content: Still it's much simpler and cheaper to have the encoder in the main board, It is just a tone generator nothing fancy about it, I can do up to 1000 steps in 360 degrees that should be more than enough for the angle position.That homopolair motor is not good enough for the application, you can't have more than one coil, similar designs exist using a plain disc instead of one coil.

author: dellsam34
author_rank: Regular Contributor
timestamp: October 29, 2020, 05:57:01 pm
content: I'm thinking about getting this evaluation board for testing, Anyone used one of these before? What software is needed? And what other test equipment needed?https://www.st.com/en/evaluation-tools/steval-spin3202.html#overviewAnd when I asked them what board is suitable for my project they suggested this:https://www.st.com/en/evaluation-tools/steval-ihm042v1.html

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 30, 2020, 07:43:00 pm
content: When you to to the "tools & software" tab you get a link to firmware that works with their board.A quick look suggests it's only 6-step drive though.As far as I know the peripherals of all the STM32 chips are very similar, so you should be able to get it to work with nearly any firmware written for STM32. (I guess that you'll find close to 100 projects when looking at sites like hackaday or github).I am a big fan of open source stuff. One of the biggest open soure (&Hardware) projects I know is O-Drive:https://odriverobotics.com/It also has software for communicating with the motor, and plotting motor parameters in graphs, which is very useful for tuning motor parameters such as PID loops. They also use an STM32, to that firmware may run on the ST development board with minimal changes.Another popular project is "VESC" "VESCTool" is a gui program for adjusting and plotting motor parameters.Vesctool has been under development of quite a lot of years now and seems pretty mature and has many features. Just have a look at:https://www.youtube.com/results?search_query=vesctoolI think the ST Microelectronics demo has some similar functionality, but I've not made a direct comparison.Another quick note:Motor drivers like the L6234 may be interesting to you. This chip has 3 half bridges and goes upto 50V 5A. Here is a simple project that uses it:https://hackaday.com/2015/04/20/driving-a-brushless-dc-motor-sloooooooowly/And there are also evaluation board with it, even including a uC. For example:https://www.digikey.com/catalog/en/partgroup/stm32f051-and-l6234-demonstration-board/44913And I would be very surprised if ST does not have some firmware to go with it.===============Just wondering if you are aware of the magnetic encoders as used on lathes and mills.They consist of a magnetic strip which has poles every 10mm or so and it usually has a stainless steel cover, and a reading head that can interpolate between magnet poles with around 10um accuracy.Multi pole magnet rings for these sensors are also commercially available.For example:https://www.aliexpress.com/item/4000945079428.htmlOr you can make it yourself by drilling a number of holes on a circle and inserting small magnets, and then use lineair output HALL sensors to interpolate between the magnets poles. I think you're already doing something similar.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 30, 2020, 09:56:16 pm
content: Quote from: Cerebus on October 27, 2020, 03:28:29 amI want to expand a bit on the above, as it was clearly misinterpreted, and I also made a typo.FOC maximizes the torque for a certain motor current, which is the same as saying:FOC minimizes the current needed for a certain amount of output torque.A HiFi turntable like this is a quite specialized application.If you have a heavy table and want to bring it up to rpm quick you want a fairly beefy motor, but when final RPM is reached you only have to overcome friction, which is very low.When using normal FOC control the motor coils will be driven with a very low duty cycle once final RPM is reached. With a low duty cycle, you loose resolution and thus controlability in fine tuning the motor. A possible solution is to not use the normal FOC angle of 90 degree phase difference, (which maximizes efficiency) but use a lower phase angle, for example 20 degrees. This lowers efficiency and increases current through the motor, but it also increases the PWM duty cycle, and with it, the resolution with which the motor can be driven.For clarity I'll give an example with some numbers:1. The motor starts with full torque, so high duty cycle (90% or more)2. When RPM is reached Duty cycle is reduced to 0.1%.3. With 16 bit timer resolution this is a pulse width count of 65535 * 0.001 = 66.4. A change of 1 in the pulse with then results in 1.3% of torque change.Another, way is to use 2 motor voltages. For example start the motor from a 36V power supply, then switch to a 5V power supply when only compensating for friction losses at constant RPM.By reducing the power supply voltage with a factor of 7, the motor controller needs to generate pulses which are (approx) 7x wider, which increases the control resolution also with a factor of 7. Or better: Use both: Reduce FOC angle and lower power supply voltage, you can even modify the FOC angle as part of the control algorithm.When optimizing for controlability, PWM duty cycle of around 70% is ideal. It leaves some headroom for generating more torque, wile making pulses a few ticks wider or smaller has a very small influence.The base problem is that you want a very wide range (or 2 ranges) of motor control.Motor control is easy when you stay with a range of 1:20 to 1:40 between max torque (rpm) between max and min. For this application the wanted control range is probably much bigger. High torque for starting and negligible torque to overcome friction and needle drag.Just curious:You wrote you had a disk spinning on it's own inertia for several minutes on your low friction bearings. How much does the drag of the needle reduce that time?Back of envelope calculation:If the disk spins up in 1/4 revolution that is approx 0.5s.If drag through friction stops the disk in 3 minutes, (180s) then the difference in torque is 180/0.5 = 360Having good control over a 1:360 range is not "very easy", but needs special attention as with modifying the control as described above.(This calculation suggests I was not far of when estimating a duty cycle of 0.1% in step 3. above).Feature creep:You can keep a log of the motor parameters needed to overcome friction, and light a LED (or whatever) when friction starts to increase, and it's time to oil the spindle.

author: dellsam34
author_rank: Regular Contributor
timestamp: October 31, 2020, 04:49:36 am
content: I though FOC uses true sinusoidal current not PWM like the 6 step trapezoidal method. Anyway I haven't figured out how many magnets and coils I will be using, Any suggestions? Also, should the magnets be polarized thru their faces or from one edge to the other? In other words, do they have to have one face N and the opposite face S, or one half is N and the other half is S?

author: dellsam34
author_rank: Regular Contributor
timestamp: October 31, 2020, 07:08:36 pm
content: This study suggests that the higher the phase number the lower the torque ripple. and for speeds lower than 100 rpm the higher the number of coils the better:http://www.jimhendershot.com/Jim_Hendershot/Articles_files/brushlessdcmotorphasepoleslotconfigurations.pdf

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 31, 2020, 09:11:55 pm
content: FOC is an algorithm to attempt to keep a (usually) 90 degree phase difference beween the magnet fields of the rotor and stator, as that is the area to get the most efficiency.Or: FOC is just a software algorithm. There is no inherent connection to the hardware it controls.It is possible to use FOC with 3 DA converters and analog amplifiers to (approximate) true (or better: distorted, to compensate for motor "weirdness") sine waves, but this is very inefficient, and not done in practice.Normally the transistors in the Half- Bridges are either on or off, and just use as switches to minimize dissipation in the transistors, and with FOC, PWM is used and the induction of the motor coils are used as low pass filters to approximate sinusoidal current waveforms.If you're still struggling with basic concepts such as the polarisation of the magnets you really should start with some small BLDC motor and a controller to get some experience with them. As long as you just keep reading literature with no practical experience it's difficult to close the gaps in the base knowledge of motor control. Get some real experience, then expand upon it.I think I already suggested to start with a simple small motor just to get some eperience in one of my first posts, but I'm not sure.I did a quick search for what sort of motor would be a good starting point for you to get some experience:https://html.duckduckgo.com/html?q=low+torque+ripple+gimbal+outrunnerThe "gimbal" type of motors are used for positioning of cameras and static position keeping, and I thought those probably have low torque ripple.However, one of the first links that popped up was for the type of motor that seems to be exactly what you want for your turntable:https://www.celeramotion.com/applimotion/products/direct-drive-frameless-rotary-motors/(I have not looked at prices though)

author: dmills
author_rank: Super Contributor
timestamp: November 01, 2020, 12:47:20 am
content: I have a cutting lathe project on the go, so this area is of some interest to me.There is really no advantage to more then three phases, and for that application 20 poles seems common.Usually you have something that amounts to three hall sensors used to provide phase information during initial startup, but once the thing is spinning and the software has lock on the rotors electrical position then a PLL fed with a high resolution encoder takes over in terms of tracking the rotor angle.The SP10 is an interesting circuit, 50kHz excited AC sensor rectified and the resulting AC used to control the three phase amplifiers. Current control is from the PLL that takes a signal from an electromagnetic encoder on the shaft, it works brilliantly, for all that it is a bit of a swine to adjust.Mad complexity by todays standards of course, because most of it is discrete transistors!Incidentally, look into ferrofluid torque dampers, possibly quite useful technology.

author: dellsam34
author_rank: Regular Contributor
timestamp: November 01, 2020, 05:29:15 am
content: I have a bunch of BLDC from VCR capstans and video drums to motor fans and electric motors, I would really like to design the thing if I just know how many coils and magnets should I need for a lower torque ripple, But I guess without a partner with in depth knowledge in electronics and coding my engineering degree alone is not going to be enough for such a complicated project. Until then I keep posting in this thread hoping that someone may hop in and give it a push, Full disclosure, I'm willing to finance the project up to a working prototype, then a crowd funding will be started for manufacturing and sales stages.For now I focus all my energy on the contact-less magnetic linear cartridge carriage, hoping that I can get some results.

author: dellsam34
author_rank: Regular Contributor
timestamp: November 04, 2020, 06:18:05 pm
content: Hi folks,What do you guys think of an axial flux motor? These type of motors have no cogging, I've made an illustration below to summarize the idea, Basically two sandwiched rings of permanent magnets with an air gap, A multi-layer PCB in a form of a disc acts as a stator and sits in the air gap and carries the electric windings, A control scheme of switching current in the windings makes the rotor spin based on the right hand rule as seen in the illustration.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 04, 2020, 07:12:55 pm
content: That kind of motor should also work, but is quite difficult to make.Reminds me a lot of how lineair motors are usually made, and it's also very similar to the way heads in HDD's are moved.A problem with this type is to get enough copper between the magnets to get a high enough starting torque, but a short burst of overload may be acceptable, and during normal operation the current will be low anyway.Also reminds me of:https://hackaday.com/2018/11/04/stepper-motor-mods-improve-cnc-flat-coil-winder/Motors like this also used to be used in floppy disk drives, but then the coils were also glued to the PCB instead of using copper tracks. Have a look at some of the pictures in:https://duckduckgo.com/?q=floppy+disk+drive+motor&t=h_&iax=images&ia=imagesYou did make an obvious error in drawing the coils though. Same as with any other motor, you need a phase shift between the inductors to get a starting torque.

author: dellsam34
author_rank: Regular Contributor
timestamp: November 05, 2020, 12:50:04 am
content: Yes the illustration is not perfect, The windings have to be phase shifted to avoid a dead spot, more phases the better. As to the complexity and cost, this is actually easier and cheaper to make than a star shape poles, Multi-layer traces are used as windings, so no winding machine needed.

author: dmills
author_rank: Super Contributor
timestamp: November 05, 2020, 06:01:21 pm
content: Pretty sure the magnets and drive coils that close to the pickup cartridge will be a problem unless you make the thing out of steel to close the magnetic circuit, in which case you might be back to cogging torque.The pickup is awful sensitive to magnetic fields.

author: dellsam34
author_rank: Regular Contributor
timestamp: November 05, 2020, 06:55:24 pm
content: It won't be any cogging even with metal, The two rows of magnets are spinning together as one rotor assembly. I can use a thin layer of steel on top just to divert the flux patterns, and the rest will be aluminium to reduce excessive weight.