servo control vs stepper control

[Peter.]

I've been gathering parts to build a CNC milling machine capable of milling aluminium and steel. I have the base of a warco round-column mill to which I'm going to mount a very robust 6" square column and figure on using a heavy linear rail arrangement to carry the head, which I'm going to make myself.

I have some redundant axis control hardware coming - not sure what just yet - I got a call from a friend who is stripping it out of a machine for me.

I already have this hardware, which I'm told is a servo motor with a driver that can use step-and-direction inputs.

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The stuff I'm told I have coming is similar motor type but with servo drivers and encoders. What I'm not clear about is how, or even whether, it's prudent to combine the two different types of drive/motor combinations in one installation, what the pitfalls might be and whether a program like Mach3 can even deal with encoder-resolved steppers, and if not, what my software options are.

You can gather that I'm awfully green in the world of CNC. I'm not daunted but I'd appreciate some direction from those in the know.

Pete.

[JAZZCNC]

What I'm not clear about is how, or even whether, it's prudent to combine the two different types of drive/motor combinations in one installation, what the pitfalls might be and whether a program like Mach3 can even deal with encoder-resolved steppers, and if not, what my software options are.

Ok first yes it's fine to combine different motors on same machine, at worst it may mean having more than one PSU if they are so different.

Regards Mach3 then it can read the encoders to display position but it can't do anything with them.? By that I mean it can't command the servo to go back or forward if it runs out of position.
Mach sends out pulses to the servo drive and just takes it on faith it's happened and got to position, it doesn't check if it's actually got there or not.
With "normal" stepper drives that's the end of it and if it runs out of position you'll never know untill it shows in the work.

Now with servos the Drive can read the encoders and check it's within an acceptable error tolerance and if it falls out of that tolerance then it will fault the drive, which can send a signal back to mach which then e-stops the software. But again the Servo drive can't command Mach to move if out of position only report that it is.!!

Not sure but I think Linux cnc can talk to the servo's but I don't know anything about it so can't help here.!

Now here's where it's starting to get interesting regards the new digital stepper drives, some have a built in step error and can fault if fallout of tolerance sending a signal to Mach3 or any control software so working just like servo drive in this respect. . . . BUT . . . Here's where it's really starting to get very very good in the digital stepper world.?
The new Lead shine drives have positional error detection built into them and won't let the stepper run out of position.? By that I mean they correct positional error internally in the drive so don't need the Drive and control software to talk to each other.!!
They take the commanded position and make sure it gets there and if it goes to far they move it back or vise versa. Affectively you can't lose steps because the drive corrects any error and makes sure it arrives safely at the intended destination.

I'm currently playing with one at the minute and can tell you they are an amazing leap forward in stepper technology that will drive the price of servos down and in some cases thru there ease of use replace them all together.

Anyway I digress.!! Yes you can use mixed motors and yes Mach will work with them like said above.

One thing about that drive I've seen thou is it says 24V signal.? I'm sure that refers to the signal it expects to receive and the parallel port works on 5V.
It's common in professional industrial machine controls to use 24V signals so I suspect you won't be able to use that drive unless you find a breakout board that can change the signal from 5V to 24V.? I don't know of any.?

Hope this helps.

[boldford]

. . . . . I suspect you won't be able to use that drive unless you find a breakout board that can change the signal from 5V to 24V.? I don't know of any.?

Hope this helps.As these BoBs breakout board | eBay (http://www.ebay.co.uk/itm/breakout-board-/300776539546?pt=UK_BOI_Industrial_Automation_Contr ol_ET&hash=item4607adc59a) have complete isolation betwixt I/P and O/P and are fitted with nothing other than EL817s I would think they could be used for the 5v to 24v translation.

[Peter.]

Thanks Jazz & Boldford. Seems i have the choice of either similar to the similar unit to above or smaller 0.34Kw Berger Lahr motors, or itsubishi motor with J2s-40A drive, which on doing some quick research seems like it might accept 5v signals anyway.

Please tell me, aside from the drive controllers and breakout board, what other fundamental hardware am I looking for?

[JAZZCNC]

Please tell me, aside from the drive controllers and breakout board, what other fundamental hardware am I looking for?

Presume you mean electrical hardware.? If so then Nothing really other than PSU(s).

[boldford]

Presume you mean electrical hardware.? If so then Nothing really other than PSU(s).Stating the obvious; a PC running Mach3 or LinuxCNC would be useful too.

[Peter.]

Psu for the drives? The one I have is mains powered, so just a 5v for the breakout?
Pc is no problem, but linux is alien stuff to me.

[boldford]

Psu for the drives? The one I have is mains powered, so just a 5v for the breakout?
Pc is no problem, but linux is alien stuff to me.
The BoB draws such a tiny current you can tap off a spare HDD (or similar) connector inside the PC.

[Peter.]

Cool, thanks again. I'll get some details of the various drives and post then up to garner your opinions on the way to make best use of them. I'll probably do a quick lash, up install on the x,y base I'm using with the stock lead screws simply to familiarise myself with the whole deal before buying new screws.

[m.marino]

Peter it depends on the type of BOB that you use. Some use very little voltage or current at all. The problem a lot of those have is making sure a good signal gets handed on to the Drivers. With you wanting to do steel, I would seriously look at a slightly higher end BOB which in turn will most likely require a higher voltage to run. I have gone up through the BOB line starting with one that I won't mention and then moving on to Roy of DIYCNC PPCPSv2 unit (gave that away to a useful home) and then on to his system 4 board set up (Which I have for sale with extension cards but not sure it is the best for your needs even though it is opto-isolated). I used that with a borrowed USB Smooth Stepper and got huge improvements in control and accuracy (Parallel port on that computer has issues).

Currently (and pretty much the last one I will ever need) using a PMDX-126 board (mains powered) with an Ethernet Smooth Stepper (was in the States and could buy them without the huge postage costs). The difference is massive. Now I am also running AM882 drivers which also make a huge difference. You are looking at running servos and therefore really want to keep as clean and solid signal reaching them as possible. So look around and ask a few folks on what would be best for you. Good luck with the build.

Michael

[JAZZCNC]

The BoB draws such a tiny current you can tap off a spare HDD (or similar) connector inside the PC.

I wouldn't take that route.? The drives are industrial grade and servo's are powerful beast's that unlike steppers tend not to stall when the shit hits the fan.!
For this reason things like limit switch's and E-stops become very important with servo's. They are sensitive to electrical noise and for this reason I wouldn't run any e-stop or limit switch's with 5V, instead using 24V which is more industry standard. So you'll need a 24V PSU so then I'd run the BOB from that, most decent bob's will run off 24V.

[boldford]

I wouldn't take that route.? The drives are industrial grade and servo's are powerful beast's that unlike steppers tend not to stall when the shit hits the fan.!
For this reason things like limit switch's and E-stops become very important with servo's. They are sensitive to electrical noise and for this reason I wouldn't run any e-stop or limit switch's with 5V, instead using 24V which is more industry standard. So you'll need a 24V PSU so then I'd run the BOB from that, most decent bob's will run off 24V.I wasn't suggesting he drive the CNC M/C side of the BoB from the PC. Only the PC side of the optos would come from the HDD 5v. I'm sure your PC wouldn't want you to put 24v or more up its paraport. As I stated the EL817s provide the isolation; on this BoB inbound and outbound, so your e-stop and limits etc. can go up to, say, 30v.

[Peter.]

So that board can take 5v signals from the PC and send out 24V signals to the drives? Is this an automatic thing or is it something that need to be set up via dip-switches or parameters etc?

I have some specs of the hardware now too, just pulling out the info. Seems I've been very lucky and got some nice skip-bound hardware to play with.

I have another of the one I already posted coming to me, which a 750W servo motor acting as a stepper with a step&direct drive, no resolver. A pair of Lexion LMx05 drives with 0.4kw servos & encoders and a pair of Mitsubishi MR-J2S-40A servo drives with Mitsubishi 750W (I think) servo motors & encoders. Plus a bag of other bits - pulleys,belts,rails, canbus leads etc.

I think I will start off by sourcing a breakout board and using the stepper drives for my (vertical) learning-curve. I'm not sure exactly how the servo drives will interface with the PC but I can figure the step & direct out pretty easily. I will, of course, keep a log of my progress right here, though exactly how fast that progress will be I don't know as I have a very high workload in my regular job.

[Peter.]

By the way - what does 'optically isolated' actually mean? Does it mean that the electrical signal from the PC is passed through an emitter-collector pair and out to the drives in order to keep the two circuits isloated, perhaps so that a fault on the machine can't send power back through the port?

[JAZZCNC]

I wasn't suggesting he drive the CNC M/C side of the BoB from the PC. Only the PC side of the optos would come from the HDD 5v. I'm sure your PC wouldn't want you to put 24v or more up its paraport. As I stated the EL817s provide the isolation; on this BoB inbound and outbound, so your e-stop and limits etc. can go up to, say, 30v.

Ok don't want to send this thread off course too much but think best to clear this up so not to confuse.!!

Yes I under stood what you meant but don't think you did me.? I wasn't suggesting sending 24V up the PP's chuff.!!

The 24V supply has multiple uses.!!. . . It's sent thru the limits Switch's and E-stop to help with switch bounce and noise. This is done in conjunction with 24V relays, so it powers them has well.
These control the 5V signals from the BOB, they also act has safety for other things killing power to anything attached etc, enabling a hardware driven fully safe latching E-stop system that can only be restarted when in a safe condition and the master relay engages even then this only engage after pushing a reset button.
If any of the safety switch's are tripped or some things in an unsafe condition then nothing happens.! . . . . Also In this case and the point of my comment meant to also use it to power the BOB and therefore save messing about with the 5V from HDD.

[Peter.]

So from that I gather that whatever voltage I use to power the BOB, that's the signal voltage sent to the drive? Will that BOB linked in post 3 definately run safely off 24v?

[irving2008]

So from that I gather that whatever voltage I use to power the BOB, that's the signal voltage sent to the drive? Will that BOB linked in post 3 definately run safely off 24v?

The problem I have with running this BOB at 24v is this:

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The output circuit has a pull up to 'Isolating + supply'. Without knowing the value of that resistor you can't be 100% sure its not going to fry. In the manual it shows 'Isolating Supply +5v'so I would assume they are sized for 5v and not 24v supply...

[JAZZCNC]

So from that I gather that whatever voltage I use to power the BOB, that's the signal voltage sent to the drive? Will that BOB linked in post 3 definately run safely off 24v?

I wasn't referring to that particular Bob or any Bob really and so obviously it would need to support 24V but most decent Bob's feature a range of input voltages.

I wouldn't use a cheap BOB with quality Servo's and to me this BOB isn't really a quality Item, it doesn't have a Charge pump for one which is nice feature and it looks a bit small and cramped.
Cheap nasty BOB's are a pain in the arse and a large source of problems and they don't always show has plain "works or doesn't" so can cause any thing from weak signals to dropped signals to slow acting Opto's etc all of which if your unlucky to drop for it will drive you crazy. . :hopelessness:

[boldford]

Ok don't want to send this thread off course too much but think best to clear this up so not to confuse.!!

Yes I under stood what you meant but don't think you did me.? I wasn't suggesting sending 24V up the PP's chuff.!!

The 24V supply has multiple uses.!!. . . It's sent thru the limits Switch's and E-stop to help with switch bounce and noise. This is done in conjunction with 24V relays, so it powers them has well.
These control the 5V signals from the BOB, they also act has safety for other things killing power to anything attached etc, enabling a hardware driven fully safe latching E-stop system that can only be restarted when in a safe condition and the master relay engages even then this only engage after pushing a reset button.
If any of the safety switch's are tripped or some things in an unsafe condition then nothing happens.! . . . . Also In this case and the point of my comment meant to also use it to power the BOB and therefore save messing about with the 5V from HDD.

I understood you perfectly well hence the reason why I said "I'm sure your PC wouldn't want you to put 24v or more up its paraport",
Whatever vendors BoB is used, assuming it is capable of complete opto-isolation, and you wish to maintain it, the PC side will almost certainly need to find a logic 5v since most paraports can usually sink far more current than they can source.
I realise I mistakenly suggesting the e-stop be routed back into the PC - Bad move to rely on S/W for that function!. If I did mislead anyone I apologise.

[boldford]

The problem I have with running this BOB at 24v is this:

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The output circuit has a pull up to 'Isolating + supply'. Without knowing the value of that resistor you can't be 100% sure its not going to fry. In the manual it shows 'Isolating Supply +5v'so I would assume they are sized for 5v and not 24v supply...They are 1K.

[irving2008]

They are 1K.


Ok, so 5mA collector current on 5v, 24mA on 24v.. so it wont fry :)

EL817 optos are rated 35v, 50mA so should be ok.

However, as I recall the diode has a 330R series resistor so forward current is about 10mA and those devices have a transfer ratio of about 1.1. So with 10mA in the diode the collector current is going to struggle to be more than 11-12mA which means the output volts wont drop below about 12 - 13v, so though it won't fry its not going to work. No good on 24v :(

[Peter.]

Help me out here a bit folks.

After trawling through 300-odd pages of servo amplifier manual which seems to have every bit of information you could possibly want for the Mitsubishi drive - I can't for the life of me get the faintest idea how it could be connected to accept step & direction commands from Mach 3. What is the usual command process for controlling servos?

[m_c]

Not all servo drives can handle step/dir. Analogue control is far more common in industrial drives, so you need a suitable interface to take the step/dir and convert them.

Search/ask on the Mach forum (www.machsupport.com/forum (http://www.machsupport.com/forum)), as there are a couple people on there who know about servos.

[JAZZCNC]

Help me out here a bit folks.

After trawling through 300-odd pages of servo amplifier manual which seems to have every bit of information you could possibly want for the Mitsubishi drive - I can't for the life of me get the faintest idea how it could be connected to accept step & direction commands from Mach 3. What is the usual command process for controlling servos?

Give us some model numbers etc and I'll have look for you.

[Peter.]

Thanks Jazz

Lexium LXM05CD10M2
and
Mitsubishi MR-J2s-10A