Servos instead of steppers?

Has anyone ever built a printer using servos instead of steppers ?

I'm wondering why steppers are the norm over servos. I would think good quality servos with positioning capability would have some advantages over steppers, but I don't really know enough about steppers to understand their advantages over servos.

i think it pretty much boils down to cost, both type of motors have there benefits and drawbacks.

i think you could find more on this discussion in the mechanics part of the forum.

the main reason is, servos of similar cost to steppers are only 180 degrees, and low quality usually.

low quality and precision, even with the ones that are 360 deg and more. pwm controls the rotation. position accuracy is the issue i read about. this does not mean it can not be done. Rc servos can be found new for less that 7$

Are you talking about hobby servos (with pots for feedback, which limit them to less than 360 deg. of rotation) or are you talking about a more conventional industrial servo motor (with encoder feedback and unlimited rotation)? The hobby type are not really suitable given the limited range of rotation. There are two main drawbacks for more conventional industrial servos: 1) the cost of the feedback sensor (the cheapest add-on encoder I know of is about $20), and 2) most DC motors are designed to operate at 3x-5x the speed of a stepper, but with 1/3rd to 1/5th the torque. This means they require some sort of reduction or perhaps a screw drive for printer applications.

That said, servo motors operate more smoothly, run cooler, and use less power than open-loop steppers. They only use just as much current as is required by the load. That is, if an axis is not moving and there is no load, the motor current is zero.

I have done some controllers using encoders on stepper motors - essentially turning the stepper motor into a closed-loop brushless servo motor. This retains the high-torque/low-speed characteristics of a stepper, but the motor runs much cooler and quieter. Driving steppers open-loop at high speed is tricky because if you ever exceed the torque limit, the motor just stalls out. In practice, I have been able to run steppers closed-loop at about twice the speed that I can run the same motor open-loop using a conventional stepper driver.

If anyone's interested, you can find more details at http://lobocnc.com/motorcontrol.html
-Jeff

there is currently a bit of work going into servo motor usage,but currently it' more about cost and suitability,

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Anything can be done but it will be expensive and the extra accuracy is probably not going to come into play with belts and threaded rod and flexing frame buy servo motors with encoders buy standard pulse servo drivers and using CNC machine operating software such as Mach 3 or EMC two with separate temp controllers can be done

I use servo on three of my cnc mills. They run great, reliable, but so much more than a equivalent stepper motor system. I mostly use gecko drivers, they are about $140 each new. I keep a look out on eBay and have bought a few for about $50 each. There is also more wiring to hookup. The 3d printer I am building will be stepper since I don't think using servos are neccasary for this application. You just don't need the higher torque/speed that a well designed servo system is capable of. I will be using industrial stepper drivers made by Parker compumotor for my printer just because I got them for $35 each on eBay. Easy to hook up to the ramps board too.

On a 3D printer, steppers, for the most part, work fine for the X, Y and Z axes. There is little variation in the load, and because the motors are usually stationary, you can usually just use bigger motors if you need to accelerate faster. I'd only bother using servos on a really high-performance printer. (I've worked with servo systems for years and generally dislike steppers, but I used steppers on my own 3D printer.)

The extruder, however, is an excellent candidate for a servo motor. The load can vary considerably (which can play havoc with steppers), and the feedback from the encoder can tell you if your extruder is operating smoothly, or close to jamming for some reason. You can also look a the motor current and servo positioning error to determine if your filament has been stripped or not. When I get the time, I plan on converting my extruder to a servo drive.
-Jeff