Thread: servo control vs stepper control
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.
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.
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.
The Following User Says Thank You to JAZZCNC For This Useful Post:
breakout board | eBay 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.
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?
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.
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.
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.
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