Messages

1821

General Mach Discussion / Re: microsteps should not be included in resolution calculations

« on: April 07, 2008, 08:32:05 AM »

Thanks John

For the sake of fun... (will I ever learn?)

Say you're using morphing capable drives and you're cutting a diagonal straight line in XY and suppose the angle or gradiant of the line is such that the X axis is moving at a rate above where the drive morphs and the Y axis is moving at a rate below where the drive morphs.

Are you going to get the X axis moving in full step increments and the Y axis moving in microstep increments and therefore the detail of the path may be not quite what you'd expect? i.e. is the "staircase" going to be coarser than you'd have otherwise expected from the microstep resolution of the system? FWIW I think the answer's - no.

Answers on a postcard please - cos I don't know 

And no I don't have too much time on my hands - it's lunch time!

1822

General Mach Discussion / Re: microsteps should not be included in resolution calculations

« on: April 06, 2008, 08:52:41 AM »

Thanks Mark - better explanation than I'd have managed.

We need to take care of not making the common mistake of confusing accuracy with resolution. Clearly microsteps by definition are smaller than a step (their resolution) but their closeness to, and their consistancy with their ideal is poorer than it is for a step (their accuracy). Therefore microsteps are less accurate than steps.

As has already been said effectively, the errors in a microstep being significant in the grand scheme of things should'nt be overstated. I mentioned it in the other thread in passing simply in an attempt to be "accurate" in what I was saying. Wow - you have to be careful what you say with you lot 

To answer your last question John, my best shot would be that a motor will stop best it can on a microstep but if the dynamics of the machine pull or push it slightly away from the microstep's "holding" point it may well stop there instead - but that's just how I see it - like I say you have to be carefull with you lot

LOL - however when you turn off the power - again depending on the machine's friction level the motor may "cog" itself to a full step - if its a permamnent magnet type motor - OMG - let's not go there...

Ian

1823

General Mach Discussion / Re: How to wire steppers???

« on: April 06, 2008, 08:13:09 AM »

Hi Jim - I've been doing a bit more thinking on this.

It all comes down to the fact that steppers are generally over driven in terms of Voltage in order to extend their torque range over the increased speed. However they should not be overdriven in terms of current.

All drives must therefore have some mechanism for limiting the current through the motor regardless of the Voltage. I know how this is achieved in chopper drives but not so clear on how in non-chopper drives.
 
Anyway - The bottom line is that IF your drive is a chopper, then you should be able to set the current it at its output to the current it "delivers" to the motor. If it isn't a chopper, then at the moment I don't know. However IF you can limit your drive to its max current of 2.5Amps or 3Amps (you've stated both so I'm not sure which it is), then you WILL be able to wire your motors in parallel with no ill effects on your drive.

The advantage of this is that if your drive is 2.5A max then the motors will still only get 2.5A but the current will be less than the rated parallel current of your motor - therefore you'll get less motor heating than with single coil wiring - not that I suspect you get much heating anyway.

IF indeed your drives are max 3A then you win even more - that is still less than your motors rated parallel current but obviously more than the max 2.5A of your motors single coil current.

Obviously the ideal would be to run your motors at their rated parallel current but given your driver isn't up to this then this is probably the best you can do with what you have.

Ian

1824

General Mach Discussion / Re: "Holding" vs "Running" torque

« on: April 06, 2008, 06:32:58 AM »

I knew I should have been more careful of how I phrased my answer. 

Of course I fully agree that the only reliable way to predict a motor's possible performance is to use the torque curves. (discounting previous experience of any given motor). I also agree that a motor's torque can be to an extent, maintained over higher speeds by increasing the voltage. This we all know. But of course it cannot actually be increased by doing this.

So at the risk of digging my hole even deeper - I'll re-phrase my original answer: As a rule of thumb - the MAXIMUM dynamic torque of a motor will generally be in the order of around 70% of its holding torque.

Now I'm going to run for cover 

Ian

1825

General Mach Discussion / Re: "Holding" vs "Running" torque

« on: April 05, 2008, 06:11:09 AM »

As a rule of thumb I think you can say dynamic is generally about 70% of holding for low speed - or at least that's what I've read - not sure why!!!.

1826

General Mach Discussion / Re: How to wire steppers???

« on: April 04, 2008, 09:11:14 AM »

Jim - That's a shame - I bet you could have fried an egg on your drive output transistors. Talk about destructive testing  . You've been lucky - but if you do this again you're running the risk of considerably shortening the life of your drivers.

1827

General Mach Discussion / Re: How to wire steppers???

« on: April 04, 2008, 05:57:20 AM »

OK - coffee made - where were we?

Hopefully when you've wired them single coil you should see quite an improvement in torque (should be around double). Now ideally (given the spec of your drivers) we'd like to up your voltage from 24V to 30V which I think is the max your driver allows so that we can maintain as much of the the torque as we can while we up the speed. However there's no quick fix to your PS to do that. You could get/build a new PS but frankly you're not going to get much extra for your money. You'd need 48V to double your speed at whatever the current stall speed of your motors is with your new torque (~8in/min?).

So first off I'd get rid of your 3:1 reduction. You'll increase your speed by 3 obviously (~24in/min). But I think I remember you saying you were about to get some new ballscrews? well if you do - consider a larger lead than your current 0.1 inch. In my first post I asked if you really needed the theoretical resolution of your current system. Decide what resolution you can work with and spec your screws accordingly.

So for example - if you go to 1:1 reduction your res will be 0.00016 * 3 or 0.00048. Double your lead and you'll be around 0.00096 - still within a thou. (@ ~48in/min) Anyway you get the idea.

EDIT: I'm just going to throw this in to possibly spoil a nice party - sorry  - You're motors are currently doing up to ~3 revs/sec. With these changes we'll hopefully see that increase. I'm now thinking we'll see how good these Chinese drivers are at mid band resonance compensation. 

Cheers

Ian

1828

General Mach Discussion / Re: How to wire steppers???

« on: April 04, 2008, 05:14:58 AM »

Sorry Jim - had to be quick off the mark there - don't want to see you trashing things unnecessarilly 

I think as Hood said, I could do with a bit more torque to get over the sticky patch.

My motors are wired in BiPolar series, i.e. coil A to coil C (A - A' - C - C') and B to D. The coil resistance is (across two coils in series) 5.8 ohms, across one coil is 3 ohms. This relates to the information about the motors - 2.5amps at 7.5 volts - which is on spec exactly.

My problem is, of course - if I now wire them in parrallel the net resistance will drop to 1.5ohms.

This is good news. Now to get that torque. Wire your motors in "bipolar single coil" configuration.
Basically this means you only wire 1 coil of each phase and forget the other two coils. Doing this you'll have a phase resistance of your 3 ohms (actually it should theoretically be 2.8 but it's near enough)

You'll effectively double your phase current and yet still be (just) inside or at your limit (of the driver) of 2.5Amps at around 2.3 to 2.5 Amps. (actually you should be fine because the manufacturers allways give a little leeway).

You should now see quite an improvement but next we'll look at how to use that extra torque to get even more speed.

BTW the reason your meter is not giving you the true story is that if you're measuring the current "drawn" by the drivers from the PS then remember what I said in the other thread about chopper drivers drawing less current than they output. Well this is part of it. But also, the current is being drawn in high frequency pulses and no "regular" meter can read this oscilating current fast enough so it gives you a much lower reading. BUT the real killer is that it's the power transistors that are delivering the FULL motor current that will be maxed out.

Cheers

Ian

1829

General Mach Discussion / Re: How to wire steppers???

« on: April 04, 2008, 04:48:37 AM »

Jim - don't do this - you'll likely blow your driver. I'm currently composing a solution for you.
Your meter is not telling you the truth.

1830

General Mach Discussion / Re: Why do large files cause lost steps?

« on: April 03, 2008, 11:44:18 AM »

multiple tiny moves in Z at the same time as it rotates in A

Sounds to me like you don't have CV mode turned on. For "multiple tiny moves", without it your machine will make a pretty good impression of a machine gun about to self destruct :-)