Servo, ac or dc versus stepper

[zoeper]

Hi All,

Getting geared to perform a bridgeport to CNC operation but all the options is blurring my vision...
Originally put my heart on dc servo's, then advised to go stepper and now i saw all the good looking ac servo's available as surplus. My project is rather cost sensitive but if it's gonna be worth it i'd rather save/hunt awhile longer and busy myself with sorting out other areas of the conversion first while saving for the propper stuff.
Please also coment on availability and cost of drivers for all the above.
any guidance/opinions would be much appreciated
Pieter

[NC Cams]

The Bridgeport Eztrak was a DC servo based system. It worked fine and mine, which is over 10 years old is still doing exemplary service. As BPT was QUITE cost concious, it says a lot that they went with DC servos when steppers could have probably sufficed. THen again, it was a high end machine at the time....

The charm about DC is that it is relatively simple and quite robust. Two wire power feed and the amps switch polarity. Tach/encoder feed back and thats it.

I prefer it (DC) cuz I UNDERSTAND IT. I also found that you can buy bolt together kits for mills from AjaxCNC or the like and they come from the factory with DC servos that will interface with the stuff I have.

I have stepper drive for my cam grinder because that's what the guy came up with when he designed it - it is lame but it works but it isn't a servo. ON the other hand, my BPT mill and my 3 Fanuc lathes all have servos. You can probably tell my preference.

However to be fair, check out the following:

Servo vs Stepper:
http://www.cnczone.com/forums/showthread.php?t=17419

Simply stepper info:
http://www.parkermotion.com/catalog/catalogA/A12.pdf

Stepper reverse engineering:
http://www.doc.ic.ac.uk/~ih/doc/stepper/others/

Stepper sizing:
http://www.cnczone.com/forums/showthread.php?t=17707

Small stepper P/S
http://www.campbelldesigns.com/files...ply-part-1.pdf


The stepper vs servo question will rage on and on. Unless you provide some sort of postion feedback, steppers DON'T/CAN'T compensate for "lost steps" due to binding, electron stupidity or whatever. THere is a link that deals with lost step prevention equipment drive/interface but I don't have it

Steppers are usually a bit less costly - high end machines pretty much all use servos.

Hint: go with servos. When you get them dialed in right, you won't be disappointed.

[unterhaus]

Brushless servo is the way of the future. I don't think that people are going to be building very many high quality brushed servos. Like you say, brushless servos are already plentiful on the surplus market. I don't like the runaway problem with DC servos. From that point of view, steppers and brushless are the way to go.

After an expensive education on ebay, I have a set of brushless motors and drives that didn't cost any more than a set of steppers and drives. But it's hard work.

[zoeper]

Thank you for the reply's. The "lost step factor" is the one thing that keeps me away from steppers and i feel stronger about it now.
How does the drive for a ac servo differ from that of a dc servo??
I am weary of buying servo drives on e-bay and had my eyes on Gecko for some new ones. Are there similar priced versions available for ac servo's?
Pieter

[unterhaus]

yes, you can get Rutex drives for Brushless, although they are more expensive.

Brushless motors work more like a very coarse stepper. They only have a few steps per rotation. Thus, the voltages in the three windings are varied in such a way as to produce the motion that is needed. There needs to be a change in voltage in order for the motor to move. This voltage change is known as commutation.

A brushed DC motor is voltage controlled. A particular voltage will result in a given speed. They are mechanically commutated.

[zoeper]

Thank you for that explanation, does AC servos work in a similar fashion?
Once you've got them hooked up and running, does any one outperform the others?
Thanx
P

[NC Cams]

We did some testing with both AC and DC motors powered grinders when I worked in the bearing industry - reason: find root cause of bearing noise.

DC motors with high commutator bar count do not have as much "cogging" as an AC motor has. Thus, they have less issues with "cage rattle" due to sine wave speed changes that occur in AC motors within each revolution.

A common trick to get rid of chatter in grinding spindles with AC motors is to run the belts loose - they howl at start up but it helps minimize the problem.

Reason: typical AC current has a 60hz "ripple". This causes motor to minutely accel and decel during each revolution as the current rises and falls in the motor windings during commutation. Same problem doesn't befall DC motors.

For ultra precision cam grinding, DC motors offer superb finishes BUT they are real pricey in high HP applications.

VFD's provide comparable speed control in 3 phase AC motors comparable to that of a SCR driven DC motor at a fraction of the cost. Our testing found, however, that VFD's shake the daylights out of bearing cages due to even more ripple from the VF effect.

When you're looking for ultra smooth and ultra quiet operation, AC motors can drive you nuts. Flywheels help dampen the vibes but that slows down response in abrupt reversal and/or fast speed change applications.

Granted AC's are being used successfully in many applications and I don't know if/or how they get around the AC sine wave "cogging" issue. HOwever, "old timers" know that high bar count overlap wound DC motors is a proven way to go when smooth commutation is critical....

When it comes to brushless vs brushed motors in CNC servo applications, the above become a moot point as it probably isn't that critical unless you're going for mirror like finishes and even the slightest motor vibration matters....

[unterhaus]

That was a very interesting post about AC induction motors. I have heard that people are starting to use such motors as servo motors, but I don't really know any vendors that do so. That's because they haven't hit ebay yet. Some of the AB vfd literature says you can them in servo applications.

What is usually called an AC servo motor is a brushless permanent magnet motor that is very much like a DC brushless servo motor. They are commutated the same way electrically. That statement will probably drive a few people nuts if they see it. I can't get too wrapped up in the terminology, it varies from vendor to vendor depending on what their marketing department wants to sell you. There is a wide variety of feedback and winding configurations for brushless motors, and a lot of study and care must be made matching them to drives.