Stepper motor vs. Servo motor

[Tony611]

I have read several articles on cnc retrofits, and building cnc machines from scratch. Some authors choose to use stepper motors and others choose to use servo motors.
Could some tell me what the advantage and disadvantages are of one vs. the other.

[epineh]

There are a lot of threads about this, try this one from the nice people at Gecko, scroll down to :

What are the advantages/disadvantages between steppers and servos?

This should cover all you need.

http://www.cnczone.com/forums/showthread.php?t=52090

Cheers.

Russell.

[alexccmeister]

The reason most people here choose stepper over servo is the price.

And for most hobbyists, stepper will do for most cnc build. But if you want accuracy and the ability to prevent lost steps, servo is the way to go. But this will be in the realm of industrial production machine.

[Kipper]

Originally Posted by alexccmeister The reason most people here choose stepper over servo is the price. And for most hobbyists, stepper will do for most cnc build. But if you want accuracy and the ability to prevent lost steps, servo is the way to go. But this will be in the realm of industrial production machine.

Utter rubbish!

[nwrepair1]

Kipper. What do you mean by that remark?

[Kipper]

Originally Posted by nwrepair1 Kipper. What do you mean by that remark?

Have a read of the link epineh posted....Basically "horses for courses" ie if the load is too much for a stepper motor to move at an acceptable rate...go servo.....otherwise steppers are just fine.

Now from my own understanding...A servo with feedback encoders will not prevent missed steps....however if it misses too many it will error and stop.....the end result is the same ie a ruined part, further as I understand it if the stepper is correctly sized a missed step is highly unlikely.

As for resolution how many steps does a servo motor have per revolution V a stepper......It seems similar to the debate of ATI V NVIDIA...fanboy material.

And I thought the reply by Alex was total BS

[LeeWay]

From my point of view, Alex and Kipper have both pointed out reasons one might choose one or the other. The opinion of one obviously differs, but the BS flag should not have been planted.
Another reason and really the reason I don't yet use servo's is the simplicity of steppers. They are just less work to set up. I may one day, delve into servo's, but steppers run my 3 machines just fine.

[dirtmover]

Originally Posted by Kipper A servo with feedback encoders will not prevent missed steps

But this is the whole point of the servo loop! Unless the encoder is damaged the shaft position should always be known by the cotroller down to the resolution, or "step size" of the encoder.

[Bubba]

I run servos on my machine and from what I have read, I don't see much difference in the amount of work to setup! Sure, I had to tune my servos, but I have read that you have to "tune" your steppers because of resonance also. To hook up a servo, I have my power leads (2) and encoder leads (4 as mine is single ended), total of 6 wires. For the stepper, you have the stepper wires (4 minimum) and then usually add the resistor to the driver for current limit. Not a big deal.
I also feel the "resolution" of the servo can be superior as a quadrature step is truly a step whereas with microstepping of stepper is not a guarantee!
In other words, to me the simplicity argument does not hold water and the only real good one is as Marris says about power requirement.

[LeeWay]

The simplicity argument for me can carry 2.5 gallons of water. It was simply easier for me to go with what I already knew. I know nothing yet about encoders. Servo's themselves are rather simple. More so than steppers I think.
It is the encoders that are complicating the issue.

You can't tell me I am not making a valid point, because it was absolutely the reason I chose one over the other for my second, third and eventually fourth machines. It was already in my bag of tricks once my first was completed. The first stepper system was chosen mainly because of price and ease of installation and wiring.

Anything is simple once you know how. Will steppers do the job for the Hobbiest? Most of the time, yes.
Will servo's? I'll take a wild guess at that one and say yes.

[Kipper]

Originally Posted by dirtmover But this is the whole point of the servo loop! Unless the encoder is damaged the shaft position should always be known by the cotroller down to the resolution, or "step size" of the encoder.

It won't prevent missed steps....it will detect that they are missing....then take action....accuracy is gone.....of course if the system is set and specified correctly as has been pointed out power requirement is the only difference.....And welcome to the forum


I should qualify my post of BS by saying that the internet has too much of it...misinformation is misleading, I'd stake my house on Marris knowing what he is talking about....

I'm a Yorkshireman what can anyone expect

Now back to finding my staple gun...............

[lclore]

Hi- Been reading this thread. Steppers and servos are inherently different. I integrate them to custom machinery all the time.

A stepper converts a pulse into a percentage of shaft rotation, typically without encoder feedback to close the position or velocity loop. A stepper has full torque available all the time, even at rest (which is why they are typically hot to the touch, unless the amp has an awo feature). Without encoder feedback, an open loop system assumes the motors have enough torque to do all the work required, therefore assuming it will hold position from now until the next time it is homed. If torque demand exceeds available from the Stepper Motor at the required velocity, the shaft will slip and loose position with no recourse for correction. Micro-stepping is a way of minimizing the "step" transition change in pulsing position to position.

Servo's use the encoder for feedback and position/velocity correction. They can produce, typically, 3x more torque than their continuous rating for short periods of time (handy for rapid acceleration or minor load changes). They run 3-4x faster than a stepper. The "step" transition in shaft rotation is basically eliminated. Encoder resolution (pulses per revolution) and the quality of the amplifier (digital sinusoidal type are best) dictate performance (and price).

For these routers and other light duty equipment where the loads are well defined (ballscrews don't take much torque and do all the real work), steppers are fine so long as they are oversized (50% extra torque is rule of thumb).

In general, steppers are easier and cheaper to work with- however, are a big pain in the rear if higher performance is required. Resonance, loss of position, slower speeds are all the negatives. Servo's cost more, but eliminate those issues in many cases. Many general purpose servos used in automation are designed as drop in replacements to steppers where the machine performance is looking to be improved (the motors have same flange/shaft dimensions, and the servo amplifiers can accept step/dir commands instead of analog commands).

Good Luck