Hi,
OK, that explains quite a bit about what you are after. I understand why you think closed loop servos or steppers would be advantageous
but the truth is that a well-designed and well spec'ed open loop system will be every bit as good.
Prior to my new mill (18-24 months old) I used an own design mini-mill for eight years. It had open loop Vexta steppers matched to low lash planetary gearboxes
and C5 ballscrews. Once I got it set up and tuned it NEVER missed a step except when I did something stupid.
All steppers lose torque the faster they go, that's just plain physics and there is no way around it. Thus, any given stepper may have lots
of torque at low speed but at 1000 rpm its very inclined to miss a step due to the decreased torque. If you limited the stepper speed to
500rpm it would probably never miss a step. So by choosing 1) the right steppers and 2) the right limiting accelerations and speeds a
stepper driven machine will be as good as any closed loop machine...and a damn sight cheaper!!!
The manufacturers of closed loop steppers will tell you that their products 'never miss a step, go faster, more power, more this that and the other thing', nearly
all BS. A closed loop stepper loses torque the faster it goes....just like an open loop stepper, closing the loop changes not one jot the physics of how steppers work.
If a closed loop stepper is running fast and now only has marginally enough torque to power the load, if it misses a step then the stepper drive will insert an extra step
to catch up....but guess what, that step is just as likely to be missed as any other. If it misses too many steps then it will recognize that its lagging behind and will fault out.
The bottom line is that closed loop steppers can get overloaded and start missing steps FOR EXACTLY THE SAME REASONS as an open loop stepper. A closed loop stepper
will valiantly try to catch up and may even succeed, but ultimately its being asked to go too fast against to much load than its capable of. Neither does closing the loop make
a stepper faster or more powerful.
The two advantages that closed loop steppers do have is that they can reliably and accurately do microsteps where open loop steppers fail, and also that they will fault
out if they lag to far behind rather than carry on making an out-of-tolerance part. Quite frankly the advantages offered by closed loop steppers are too slim to justify
the expense. Well-chosen open loop steppers and drives will be every bit as good. As to what constitutes the 'right' stepper is all about inductance. If you want to know
more I will post if you wish.
If you want genuine closed loop performance then you want AC servos. The main advantages of servos is they have a flat torque curve all the way out to 3000rpm, and
also they have three to four times their rated torque as a short term overload. This overload feature means that they seem to perform much better than the specs would suggest.
In short they kick arse and humble any stepper ever made...shame they cost as much as they do.
I use Delta servos, a Taiwanese brand made in China. DMM are a Canadian brand made in China. Both brands are good quality, support, documentation and most importantly
free set-up and tuning software at fair prices. There are cheaper Chinese brands and they are attractive because of price. By-and-large they are good quality and performance but
questionable support and worse documentation and no set-up software. If you've never used servos before don't go there, servos take quite a bit to set up and tune, you
don't want to start with one of these Chinese things with s******t documentation.
I use Delta B2 series servos, they have a 160,000 count per rev encoder, and are considered the entry level for Delta. There are even better servos, the A2 and B3 series
particularly....but even my B2's are absolutely fantastic. The servo pictured in the fourth axis above is:
https://www.fasttobuy.com/flange-80mm-239nm-ac-motor-driver-kits-with-3m-cable-220v-075kw-cnc-servo-motor-for-cnc-router_p28084.htmlIts about a fortnight old!
The truth is that you can make a perfectly good machine with either steppers or servos if you choose well. I would defer that decision until you look at various other
components and the rigidity, or lack thereof, of the machine you want to build, and they will in turn direct your thinking about the axis motors.
What sort of accuracy do you want to achieve......and most importantly what materials do you want to cut?. If you want to cut wood and plastics, maybe a little
bit of aluminum, very VERY slowly then you'll need a machine of rigidity R. If you want to do a good job in aluminum without taking all bloody day then you'll need a machine
of rigidity 5R. If you want to cut steel the you'll need a machine of rigidity 20R at the absolute minimum. Cutting steel is very VERY demanding and most hobby machines
suck at it.
For accuracy you need to choose between rolled C7 ballscrews (cheap) or C5's (or even C3's) at ten or more times the price.
You also need to decide on round rails or square. Round rails are OK for wood but nowhere for metals, even only aluminum, for that you need square rails.
Craig
Guest
