Electronic Artisans ELS Article

I'm confused, I thought vector control converted the rotating three phase vector to a quasi-stationary two dimensional vector, D and Q, via the Park-Clarke transform, and controlled the stator currents rather than direct controlling the speed? Of course if you have a speed, or rotation, sensor you can have full torque at zero speed, which is what we used for electric vehicle drives.

Andrew

That's why I mentioned it Mark. VF speed control included in vector control gives decent speed regulation with load and as consequence a more constant speed. Adding slip compensation to that and it improves things further but I have no idea by how much. The manufacturers reckon it can make the speed constant enough for some applications that couldn't use inverter speed control before.

The other mode they have, constant torque will give speed variations with load. I believe it's mostly intended for water pumps and things like that.

One of the problems with ELS's is variation in motor speed especially with a 1 line encoder but it will also cause problems compensating for with more lines. I've no idea how it worked out but I came across some one who switched to a dc motor and I assume and encoder and tried to keep the speed constant that way. It's needn't be an expensive option - just find a cheap motorised treadmill. One problem though may be the cost of good quality servo motor drivers. Just to make it more difficult these motors are generally 180v with the power ratings well overstated. There may be brushless ones about now. Not that this would help either really.

John

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Andrew, I don't know what goes on with the binary imps inside the inverter but the net result of the benefits you mention is that you also get much tighter speed control - if you ask for 100 rpm you get 100 rpm not 100 rpm ish.

John, I was not certain about the VF aspect. I checked the handbook for an inverter I am using that has vector capability (Durapulse). VF is listed as a basic control method. Vector control is an advanced option (although I suppose the very nature of these drives means there will be manipulation of voltage and frequency).

In the picture previously from JS the thread looks to show accumulated errors as he mentioned. I don't see how speed regulation is to fault here - that would cause a (random?) widening of the thread all along rather than the pitch error developing at one end?

Mark

Think we are missing the point here.

Speed control shouldn't be needed.

Any system that can use multi-line encoder and threads correctly does it with good software and real time.

The BBC never had a closed loop vector controlled motor, neither has the Tormach or even the basic LinuxCNC

We don't need no steenking smoke and mirrors

JS, I agree - all the binary imps need to do is put the tool point in the right place on the surface of the bar at the correct time, over and over...... how hard can it be?

Mark

PS. I now have a mental image of a chief imp whipping the lesser imps as the speed gets turned up more and more........perhaps its time for bed.

PPS. If you think that is hard, I am currently trying to get a HMI running a couple of control macros to co-ordinate a PLC running two motors at varying speeds to provide a formed metal part that is springy. The PLC uses both 16 and 32 bit registers. The addressing is octal, the registers are binary and the number formats are either binary, octal, hex, BCD or real with implied decimals. I still don't know if the system is big or little indian!

Edited By Mark C on 28/03/2016 00:36:26

I have been following this thread (pun not intentional) with fascination. I think I have learnt quite a bit and a lot of things I was going to ask about have been covered. However one or two things puzzle me.

I could never understand how a one pulse per rev could provide enough control to achieve a constant pitch over each turn of thread (I still can't). I accept if the thread was relatively long then with the flywheel effect and the relatively constant cutter load the end result would be acceptable. What happens though if one was cutting a large diameter very short blind thread up to a shoulder . When I do this conventionally its generally at a very low spindle speed but when the cut first starts the spindle noticeably slows. That would surely wreak havoc with simple ELS?

With a high count spindle encoder the above becomes irrelevant so a perfect thread could be cut every time.

Even with an all singing all dancing ELS is it practical to cut say, a 4mm long internal M75x1.0 thread? (even with a runout groove)?

Ian P

Ian, the slowing down you refer to is the reason single pulse timing does not work for some people. A system using a single pulse, such as Mach3, uses the time between two pulses to calculate the rpm of the spindle. It then calculates the feed rate required for the cutting of the required thread pitch. If the spindle slows when cutting starts the system does not detect this soon enough so the pitch of the thread is longer than it should be. If your spindle kept a constant rpm whether cutting or not then a single pulse per rev will work fine. I think with Mach3 that once the travel speed is set it is not varied until the next code block which is the retract operation of the tool. That is why you get a result like the one in an earlier photo where the pitch stretches after a time as a slowly slowing spindle speed is not detected and acted on. The systems that use a multi line encoder and constantly respond to the input from the encoder should have much more accurate pitch control. I will not say perfect because there will always be some control lag but it should be so low that it is either undetectable or competely acceptable.

The discussion has veered towards how good speed control can be with a vfd. This is because single pulse threading needs this good speed control and so makes either ELS or Mach3 useable for threading in more situations such as large diameters or long lengths of thread where the spindle is more likely to slow down.

Martin

Ian, I missed answering you last question. If your spindle slows when cutting the thread you describe then you would need a multi-line encoder system that responds constantly to the encoder input.

Martin

I agree with you, Ian, I reckon that from a standing start, you would need 10 turns of the thread , before the pitch would become stabilised. These ten turns would have to be removed before the thread could be used. Any change in the drag of the saddle along the ways would end up in the pitch getting finer until the spindle has pumped enough correction pulses into the system (another 10 turns of the thread?).

Frank

I understand that completely Martin and if I was going to pursue fitting a motor driven leadscrew I would only do it with a high count spindle encoder.

Whilst an ELS might be able to create every pitch of thread as well as position the cutter repeatable for every cut it seems to me the whole ELS system would be of little value unless a cutter retraction method was incorporated.

Since that retraction may as well be the cross slide feedscrew one may as well call it a CNC lathe. It seems to me that the whole ELS idea was predicated on the 'answer looking for a question' principle!

I assume the big difference between CNC and ELS is one of backlash?

Screwcutting conventionally is always in one direction with the operator ensuring the (significant) half nut etc backlash is taken up before the cutter engages, presumably a basic ELS does the same.

I did seriously consider an ELS on my current lathe (Harrison M250) whilst it can cut metric and imperial threads with its screwcutting gearbox, it frequently needs different changewheels for each pitch which rather defeats the object of the gearbox.

Ian P

I may be in a very small minority here, but:

For my purposes [being to have full set of 'virtual' ratios available], I would like to see all reference to spindle speed taken out of the equation ... i.e. the ELS should work properly when the lathe is being hand-cranked.

This is particularly relevant to short threads, such as found on various optical instruments; but I suggest that it might be the purer starting point in any case.

... I think this demands a high resolution encoder on the headstock spindle.

MichaelG.

I have seen comments about a very coarse pitch being cut by hand cranking just due to the pitch. This particular "project" died and the demo's had some strange aspects. He used an arduino and a relatively low count encoder so it wouldn't have worked that well hand cranking anyway. The resolution needed for that would need to be enormous and would still likely to cause problems unless it was turned very steadily. Think about maintaining a fine pitch to microns by turning a leadscrew very slowly in relationship to the spindle speed.

Gears can do that easily even if the actual pitch is slightly out the variation will be tiny.

No need anyway - providing thread length can be specified. I tried to buy a used lathe from Italy that did this by specifying pitch and the number of them which would be ideal for many pieces of work but a means of specifying long lengths would also be useful so could do the same thing. It was a modified Taiwanese / Chinese lathe by the look of it. Maybe a variation on the phase lock approach I posted earlier. It was set up via BCD switches. Maybe it used rate multipliers.

John

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PS There is also a need to think about people taking a fine finishing cut.

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Edited By Ajohnw on 28/03/2016 12:39:10

Michael,

I think that (a) this makes the problem much harder and (b) is not necessary.

With the normal CNC/ELS approach you at least try to have the work and the leadscrew turning continuously at the right speed ratio and in sync, start the tool moving at the same point for each pass, with the same acceleration profile each time. Keeping sync comes down to having enough power and inertia in the system and if possible fast sensing of the spindle position to correct the tool velocity if the spindle slows down with the cutting force. The example of Linux CNC seems to show that this can be done even though the user interface and set-up may be ghastly.

But sensing just how far you have turned the spindle by hand, and moving the leadscrew by just the right number of turns, starting it and stopping it in exact sync, when the speed range may be rather large (stationary to say a rev per second), possibly stopping very quickly when you reach a shoulder, would need a really complex servo system. But you're right, it would need a high resolution, probably absolute encoder on the spindle.

On the other hand, if you are cutting short threads to a shoulder, you can do this under power and trust the CNC to stop the feed and withdraw the tool, probably more accurately than you could by hand. I do this with Mach3, and while I accept JS' reservations about the one pulse per rev encoder and possible inaccuracies in the thread, it does work.

Just as a matter of fact, I have cut threads successfully using Mach3 on a Myford Super 7, at least quite small diameter, at the lowest non-back-geared speed which is about 200 rpm. But I am thinking about moving to LinuxCNC on the lathe at least, to get better threading. It should be perfectly feasible to use the workshop PC in dual-boot, with Windows + Mach3 for milling and possibly plain turning; and Ubuntu + Linux CNC for threading. If this is successful then maybe I'll produce an article...

What I have gathered so far seems to indicate that a simple (one pulse/rev) ELS would have limited practical use (from my point of view). Many of the threads I cut are short and up to a shoulder so I dont see how the ELS can hack it.

Presumably a multiline encoder would be needed for multistart threads, or are the same methods used as when conventional screwcutting?

Ian P

Michael, I agree with that. Ian hits the mark regarding CNC, if you go to the trouble of getting it all working you can't be very far from a CNC system (I think JS more or less said the same thing?). If you want to cut a LOT of threads then there are easier options (or just get a real CNC). If you want to cut the odd thread on a lathe and single point tooling is a challenge then buy some dies etc

Mark (who is still managing to mix his number formats up....)

John, I would not contemplate trying to do any of this with anything other than absolute encoders. You need to track the relative position of the tool point on the surface of the work at all times. This is inline with Michael appraisal? The problem then moves to how fast you can move and still keep up - this is where JS came in with very large drives on machine centers. they can move and position at blindingly fast speeds (too fast to see without help) and still start and stop at the exact point on the work (probably within a few microns?)

Mark

The existing ELS uses a microswitch as a stop so shoulders and thread length aren't a problem. Less of a problem than on full cnc after a fashion as it can be referenced to the work and no need to know how far it is from the tool.

John

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A KIS approach is needed Mark C and as a result it can only be so good. Lathes with gears aren't that good anyway for various reasons. There will be limitations. By most account Linuxcnc copes anyway as have other cheap machines without absolute positioning.

John

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Ajohnw said "I'd be surprised if some one coded a system that just said the spindle is running at this speed so cut the thread and forget it."

That is exactly what Mach3 running through a Smooth Stepper does. If you stop the spindle when the thread is being cut the motion continues to the end of the thread before retracting for return to the start. There may be a difference when using other set-ups but since the index pulse is picked up and sent back to Mach3 by the Smooth Stepper I suspect it is the same with the parallel port.

Martin

John, KIS (KISS = keep it simple stupid ?) might be what is needed but in reality anything that is really usable is not going to be. You are going to get a glorified variable speed feed from the lead screw - might as well just stick a motor on the end with a control pot and cross your fingers.....If you need the help of electronics then the system is not going to be simple, there is going to be jiggery pokery going on - its just the nature of the thing.

Mark