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View Full Version : Another which should I get Servo or Stepper Question.



mysterious
03-27-2004, 10:24 AM
I will be doing a CNC conversion on a Shoptask 3 in 1 machine. I know servos are supposed to be faster and more accurate than steppers, but I am doing this as a hobby and don't need super high speed. Also are servo systems harder to use as far as programming or are they about the same? I haven't heard this discussed too often, so is it not even an issue. I can get servo versus stepper for about $600 more, but is it really worth it? This is a mill lathe conversion and I think steppers would be fast enough, I don't think the machine can keep up with servos top speed anyways. Tell me what you guys think.

ger21
03-27-2004, 10:42 AM
Steppers can be every bit or even more accurate than servos. Spend the $600 on other areas and go with the steppers. As long as the steppers can give you enough power.

Gerry

HuFlungDung
03-27-2004, 10:53 AM
I would recommend that you use a closed loop system, whichever way you go. That is my main objection to the less expensive stepper systems, is that error can creep in and you will be totally unaware of it until it is too late. This means some kind of encoder feedback, and software that will make use of the information to keep the current commanded position correct.

ger21
03-27-2004, 11:06 AM
While Hu raises a valid point, it all depends on your budget and what you're planning on doing. Are you planning on using Geckos to drive the steppers? What size are the steppers? And what software? You can use Geckos, steppers and Mach2 all for typically well under $1000. I'm assuming that since this is a hobby it would be more than adequate. Unless I'm mistaken, the software alone for a system like Hu is talking about can cost more than that. And the hardware even more. Care to elaborate on this Hu?

Gerry

HuFlungDung
03-27-2004, 11:42 AM
I can't say that I know much about the detailed workings of the low budget systems. Its up to the buyer to try to find the most performance for the least money. But, part of the equation is knowing what you should be looking for, and knowing the ultimate limitations of the machine that you end up with. Machining without feedback to the controller is like driving blindfolded, IMO ;)

I really don't believe that there is much of a difference between the hobbyist and the professional in wanting the machining job to be carried out successfully. A wide tolerance range is not the same as random errors due to mispositioning.

Swede
03-27-2004, 11:48 AM
I tend to agree with Hu in that closing the loop via encoding ensures the accuracy of the program. Importantly, it also provides peace of mind. Consider, you are on machining step 6 of a complex, expensive part. Steps 1 through 5 went great, but during step six, you lose position, the part is scrap.

It is much the same in manual machining. I have had parts that were literally chucked for a week in a dividing head doing numberless operations. The closer I got to finishing the part, the more focused I became in not screwing it up. Scrapping some aluminum round you chucked 10 minutes ago doesn't hurt, but scrapping a radial engine crankcase, or ANY complex part with 3 weeks of work behind it, well let's just say "hurts" is an understatement.

It's all budget, of course, do what you can with what you've got. Best of luck!

Stevie
03-27-2004, 08:15 PM
couldn't he use Rutex drivers with servos and then a low cost controler like Mach2 do just as well for him

mysterious
03-27-2004, 09:58 PM
I am planning on going with geckos, 500 oz. steppers, and Mach 2. I thought only servos could give feedback and with steppers you just crossed your fingers and hoped that there are no mistakes. I don't hink I will be machining anything too complex, mostly some goped aluminum parts. Back to the feedback thing, so you can have steppers and get feedback on what they are doing? Very interesting. Tell me more on how you set this up. Just when I thought that I was beginning to understand something, I read something else. Also with servos I know you have to be careful with how you wire stuff because you can get cross feed and your coordinates would get lost, how do you guys avoid this?

mysterious
03-27-2004, 10:01 PM
Oh also, from what I here geckos with good steppers being run by Mach2 rarely mis any steps. How true is this and if it is would it be more dependable than servos and having to worry about cross feed messing everything up.

ger21
03-27-2004, 10:13 PM
With the exception of electrical interference, generally the only reason a stepper will lose steps is if it's not powerful enough for the application. Or, since steppers lose torque as speed increases, because you're going to fast.

Stevie
03-27-2004, 10:19 PM
Hey Gerry; your pretty darn close to me; just a hop-skip and a jump over the big river

ger21
03-27-2004, 10:21 PM
Actually, I'll be there tomorrow picking my wife up from the airport (she's in Halifax visiting a friend). You wouldn't know off hand which exit the airport is off, would you? I've only been there once before. Thanks.

mysterious
03-27-2004, 10:28 PM
Yes, I don't think steppers are as sensitive to interference, but I thought that servos were a little more sensitive.

Hey Stevie are you trying to hijack my thread? :D

Stevie
03-27-2004, 10:33 PM
No problem; just stay on the HYW 402 untill you see the Airport Road turn off or Modeland Rd and get onto #7 (it's called London Rd also) the airport is clearly signed

Or you could get off at Idian Road drive up Exmouth; onto Murphy Rd then turn into Fraser Ave in Twin Lakes and I'll make you a coffee; she can find her own way home :D (Joke)

Stevie
03-27-2004, 10:34 PM
Try steppers with encoders

ger21
03-27-2004, 10:45 PM
Yes, I don't think steppers are as sensitive to interference, but I thought that servos were a little more sensitive.

It's not the steppers or servos that can suffer interference. More like the wiring and/or drives. Interference with the step and/or dir lines can cause missed and/or gained steps with either servos, or steppers. If you use Geckos for either steppers or servos, I don't think makes any difference in regards to interference. With proper grounding and shielding, and the right size steppers, you shouldn't have any problems.


Back to the feedback thing, so you can have steppers and get feedback on what they are doing? Very interesting. Tell me more on how you set this up

You can't do this with Mach2 or Geckos. You need special (expensive) drivers and software to go with them (more expensive).

Gerry

Stevie
03-27-2004, 10:59 PM
Gerry; on second thoughts just turn off on Modeland Rd; then follow the signs; I'm not sure about a direct turn into the airport; i think it's actually past and then a dubble back

mysterious
03-27-2004, 11:00 PM
Sounds like I am going with steppers. Thanx again guys.

Stevie
03-28-2004, 12:24 PM
http://www.aaaim.com/cgi-local/shop991/shop.pl/SID=166811070718/page=cart_contents.htm/buy=1/item=1814

Stepper with encoder; not a bad price; might be just the ticket

ynneb
03-28-2004, 06:48 PM
Im eight weeks into been an expert on the subject. Before that I had never heard of CNC.

The reason I chose to use servos was the uniform torque across the speed range, unlike steppers that lose their torque at higher speeds and therefore may loses steps. Servos been a DC motor have a smooth motion. Steppers move by a series of jumps. Although gearing down and micro stepping can reduce this to a very small amount. I read a post in this forum where this guy after building his machine regretted using steppers and wished he had used servos. That was enough to convince me. I am sure current stepper users will jump to thier defence though.
I guess if most of your cutting is going to be straight lines then steppers will be fine. I gather a servo will do nice rounds.

As I said before I am no expert and am just going off the readings in this forum. But the general consensus seems to be servos are better. With the occasional stepper user who will dissagree.

ger21
03-28-2004, 10:08 PM
I guess if most of your cutting is going to be straight lines then steppers will be fine. I gather a servo will do nice rounds.

This is not a fair statement. Both steppers and servos will give you the exact same results up to a certain point. That point being when you need more power, or higher speeds than steppers are capable of.


Servos been a DC motor have a smooth motion. Steppers move by a series of jumps.

If you command either a stepper or servo to move 500 steps, they will both accelerate and move 500 steps. Tell them both to move 1 step, they'll move 1 step. My steppers move very smoothly, and seem to spin just like any other type of motor I've ever seen. Even at very slow speeds, there is no stop and start jerking, just a slowly rotating motor.

Sometimes, steppers can suffer from resonance problems. With microstepping, this can be mostly eliminated. And Gecko drives, I believe, have some additional features to reduce resonance problems.

The bottom line is, a properly set up and designed stepper system should perform flawlessly. And, a properly set up and designed servo system should perform flawlessly. And generally, stepper systems are cheaper. So, if a stepper system can provide the performance your system requires, why bother with the extra expense and complexity. jmo

vacpress
03-29-2004, 12:19 AM
Another note on this debate - the big expensive CNC we use at school\work is driven by stepers without encoders. It has run dozens or programs, and hasnt had any reports of lost step related failure. These programs are usually very complex and large 3d shapes. I have seen almsot nothing like them on the forum. The device is used constantly for finishing 3d surfaces. It does this just fine. I think the bigest problem we have is ocassionaly breaking a bit by running to fast in a material we dont know. If you setup the stepers right they seem to work well.. Some notes on right:

1-a good power supply is key - no switching computer supplies for obvious reasons.

2-the faster you go the less torque, the more likely the thing will stall.. a stall is the worst because it can trash a part if nothing stops the program. we have never stalled our machine, running unto 180ipm in foam.. 85-120ipm in REnboard.

3-good wiring, grounding, etc.

radio-op
04-03-2004, 07:47 PM
I agree that a closed loop system is better, and I haven't understood why there aren't more stepper systems with encoders. I have been working on small robot projects for a wile now and it doesn't seem that costly or hard to close the loop for a CNC table. Is the problem the added cost or the control software? Or is it a case of " if it anit broke..." ?
I'm still working on my table so I haven't played with TurboCNC yet, but I thought that it could handle encoder input. I'm not really planing to add encoders right away, just want to get things spinning first. But would it be worth the extra work of adding a feedback system?
Bill

ger21
04-03-2004, 09:12 PM
None of the inexpensive "hobbiest" software can handle feedback from an encoder. I'm sure that is the limiting factor here. MaxNC sells closed loop stepper systems as well as open loop. The cost difference is about $1000. Everything else is the same.

radio-op
04-03-2004, 09:22 PM
Yea that would explain it! Thanks
Bill

imserv
04-04-2004, 02:51 PM
Originally posted by ger21
None of the inexpensive "hobbiest" software can handle feedback from an encoder. I'm sure that is the limiting factor here. MaxNC sells closed loop stepper systems as well as open loop. The cost difference is about $1000. Everything else is the same.

In the case of the DeskCNC servo drives, the PID servo loop is closed in the software contained on the driver board, rather than within the PC. The drivers work with brushed DC servo motors with quadrature encoders, not steppers. The system provides precise positioning from step and direction signals. They are very competitively priced with equivalent stepper systems. Under $150 per axis for the Kit-2 which includes drivers, servo motors w/ encoders, mounting plates and a paid license for TurboCNC (parallel port).

http://www.cadcamcadcam.com

Fred Smith - IMService

vacpress
04-04-2004, 03:25 PM
hey Imserv- do you offer just the servo amps?

imserv
04-04-2004, 03:50 PM
Yes, just the drivers, just the motors, just one axis, w/PS kit, extra and combined 6 ways to Sunday, due to popular demand.

Fred Smith - IMService

vacpress
04-05-2004, 04:55 AM
imserv -

tell me, if you would be such a good fellow, would your drivers make this guy:
http://www.automec-direct.netfirms.com/html/motor_details.html

move well if it was given an encoder?

imserv
04-05-2004, 10:12 AM
Short of buying one and trying it out, I will qualify that it probably will work very well with the DeskCNC servo driver. It's too bad that encoders are so expensive. $10 for a motor and $35 for an encoder.

The web site does not list the duty cycle. If they are only showing peak load, it may not be the right motor for a CNC milling or routing machine.

They may also be too powerful for some desktop machines, they weigh 3 lbs. I am evaluating some new motors to replace the surplus Pittmans that we use with our kits. They only weigh 2 lbs and look enormous on a Sherline mill.

Fred Smith - IMService

http://www.cadcamcadcam.com

vacpress
04-05-2004, 10:40 AM
ah sweet. well. i ordered one, and have some encoders from some oldschool okidata dotmatrix printers which had big servos and encoders... kinda wish i knew how to hack the driver on that thing... anyhow, perhaps i will order a ims drive... thanks for the information.

robert

HomeCNC
04-05-2004, 01:02 PM
mysterious:

Have you seen Bloy2004's post on his shoptask conversion? It has lots of good information in there. He went with Servos on his system. I also like Servos over steppers.