steppers vs. servos

[twombo]

What are the relative merits of steppers versus servos? Let's assume that comparable torque rated units and their associated Gecko drives/amps are available for roughly the same price (my example -- 520 in. oz steppers/600 in. oz servos).

Are servos capable of faster and more accurate movement to a great degree ( especially with 3d contouring)?

What are the dependencies in the way of software, computer hardware?

I will be using Visualmill 4.0, Mach 1, and a 1 Ghz Pentium III class cpu.

Of course 3 more cables are required... that is no biggy to me).

Your collective thoughts please?

[HuFlungDung]

I think servos offer better control and more consistent torque. I suppose an oversize stepper would be just as reliable as "the right size servo".

I haven't researched all the diff, but I did build a little device using a stepper drive once, and I wasn't all that happy with the stall characteristics of the stepper I was using. That system had no feedback loop.

A modern servo controller has software settings for integral, differential and proportional gain. I do not know if a stepper drive can simulate this or not (someone who knows can pitch in here ) Fine tuning these settings allows for extreme control of the motor as it accelerates and decelerates, to get rid of jerk, and overshoot, and yet gives good holding torque and speedy response to positioning errors ( as work load is suddenly applied or released)

[CNCadmin]

For accuracy servo is the only way to go. Servos are much faster running that steppers and produce torque from the start and can accelerate much faster than a stepper.

[Zephrant]

Is it possible to purchase standard servo motors with encoders, and a Gecko G340, (and a crude power supply) and have everything needed for a full servo control system? I think the G320/340 take "step" inputs so software like Mach2 would work fine with the above setup?

Anything I am missing here?


Thanks-

Zeph

[HuFlungDung]

Zephrant,

You'll need a multi-axis motion control card in the PC that is capable of driving servo motors. Step output is no good for servos.

Most of the "intelligence" for a servo system is the firmware on the motion control card that provides control of the various tuning features.

[Zephrant]

The G320/340 claim that they take a step/direction input though, so would they not perform just like a stepper system, but with the positional (auto correcting) feedback and speed of a servo system?

If the above is true, I believe that Mach2 fulfills the needs of a motion controller card.

Thanks-

Zeph

[Kookaburra]

Step input to a servo drive is fine if the if the smarts of the hardware performs a quality interface between the step input and the PID control of a servo motor. The beauty of step input servo drives is that you are not locked into purchasing expensive control software and hardware. Our servo drives have step input with a dual mode output where you can control a sevro motor via PID and current limit trim pots as well as the ability to control a servo amplifier via a +/- 1 -10V scalable output and will work with any step and direction software, so far we have not encountered any inconsistancies or inaccuracies. I think Mach2 will be perfect for your application Zeph

[twombo]

Zeph

Dan Mauch at Camtronics has a pretty good looking package of servos and power supply/nc controller hardware that is along the lines of what you are describing. 620 oz. in. servos, cables, powersupply , case, gecko 320/340 drivers, "the whole enchilada". Exceptionally nice looking stuff for around USD 1200/1300, complete and ready to assemble. Should be a pretty potent setup for a very versatile machine. find him at:
www.seanet.com/~dmauch/Products2.htm

Dan is also very knowledgeable and most willing to discuss your specific requirements to tailor your solution.

[Zephrant]

Thanks guys- I've been out of town for a few weeks, just got back and am in the scheming stage again.

Dave- That helps clear up what is going on, thanks!

I've looked at Dan's pages, and will keep him in mind when I get to the ordering stage. I like his box for the Gecko's, but I'm probably too cheap to buy it.


Zeph

[Kookaburra]

Zeph,

Have a look at our website and contact us via e-mail if you wish.

[Zephrant]

Thanks Kook but I don't do flash, so could not navigate your web page.

Zeph

(90% of flash use on the web is for annoying advertising. By not installing flash, I omit a good portion of the adds, and almost all of the obnoxious ones.)