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What's stopping us from using Servo Motors?

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It seems to me this would solve a lot of issues with steps being skipped. I don't know a lot about it except that servos are a closed loop and steppers are open, so basically a stepper never really knows whether it's in the right position if a step is skipped. A servo counts the number of steps so it is always on target.

Is this something that could be done with the current PCB electronics? Would the firmware work?

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Servos with the same strength as step motors cost an arm and a leg. Also, a server doesn't "count" the number of steps, a server has a feedback of it's position, normally a pot meter. Which also causes it to have a limited move range in most cases. Only commonly available servos that I know of are those used in model airplanes/cars. And those are not suitable at all.

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It seems to me this would solve a lot of issues with steps being skipped.

Just out of curiosity, what makes you think that skipping steps is an issue, or in other words, why does your UM skip steps, because IMO that hasn't been a problem in months.

My UM is not skipping steps, the conditions that did lead to skipped steps are either 1. low grease 2. too high of speed 3. voltage too high or too low 4. axes out of alignment 5. loose wires 6. excess plastic hitting nozzle

I guess my original comment was not really fair. Actually, I was thinking of using multiple motors and that a servo would be easier because they would always stay in sync. Then I saw the price tag - wow, over $400 for an equivalent nema17 servo!

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Most servos do not have 360 degree rotational capabilities, and their positioning accuracy is nowhere near what a stepper motor can provide.
Some have more then 360 degree rotation, and some also have high accuracy. But then you are no longer talking about relatively cheap hobby servos.

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My company develops flight control systems for UAVs and other "things", so I have a vast knowledge of what servos are available and their accuracy. We are starting to move more towards stepper motors with index sensors (like linear actuators) for these applications because they are more accurate and can provide faster response times over the high-end servos that we have used in the past.

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R/C servos and servo motors for industrial control are not the same thing, don't get the two confused :) An industrial servo is usually a DC motor, or even a stepper motor, with an encoder on the back. They are extremely accurate and do not lose steps, as it has the quadrature to be able to tell it how many steps it has moved in which direction to extreme accuracy.

Pretty much all industrial CNC machines will use servo motors, they are not that much more expensive (about 3x-5x) as steppers, but they do need a lot more control hardware. Even a cheap servo will have 1000+ lines in its encoder giving you 0.36degree-1.44degree per line, depending on the encoder implementation. Some encoders can even get more accuracy out of these encoders.

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they are not that much more expensive (about 3x-5x) as steppers, but they do need a lot more control hardware.
The steppers and drivers are currently quite a chunk of the total cost of the machine. For high end industrial hardware the cost is not an issue, for something like the Ultimaker it is.

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As far as I know, there are two technical thingies that people call "servos". You could consider it "homonyms". Two things that share the same word.

First there are the remote controlled airplanes and trucks servos. Those have a potmeter that provides feedback to the controller in the servo to where the hand is. Due to this potmeter, the travel is limited to about 270 degrees. (Some don't go farther than 180 degrees).

On the other hand, there are industrial servos. Those have an encoder that "counts" (as mm_build puts it) the position of the rotor.

On a different level of course these two things are the same: They both have a motor and a position sensing device.

But when you say the positioning is inaccurate and that they do not offer full rotation, then you're thinking about the hobbyservos with the potmeter, while you should be thinking about the ones with an encoder.

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It's up to the specifics of the application, even industrial machinery quite often uses stepper motors. For laser cutting machines up to ~200W for example it's almost ideal cost/benefit wise, there is only the mass of the head and gantry to contend with and steppers are cheap and predictable as long as the loading is predictable. In fact there are many parallels between the Ultimaker's mechanism and the one you would find inside many laser cutting machines - it's just that one is optimised for the delivery of laser light and the other optimised for delivery of plastic and electricity over a cable.

As said there seems to be a huge variation of what servo actually means to people.

'Servo' in general describes a complete control system that operates via feedback. If the stepper motors that we used had a position feedback system, then this would make it a stepper servo and such things do actually exist for various reasons.

The cheapest way of going servo is really the same way that some RepRaps are done with DC motors and encoder strips. I'm not sure about the exact resolution, but something like 0.08mm is probably within reach for those methods.

With the same drive (belt) ratios the RS-555 equivalent DC motor used in most printers wouldn't quite match the holding torque offered by the current NEMA17 - you would have to go to a RS-755 and also put up with occasional 20 ampere spikes in current when your head really does lock up dead or you have the need for speed and want to negotiate a full reversal at full power for whatever reason. Inkjet printers get away with it because they usually decelerate 'off the page' - not really a facility available for our extruders where we don't have the luxury of microsecond flow control.

Even then there's probably going to be issues related to the overall rotation speed of the motors, which determines the intrinsic cooling available to the windings (limiting long term power at low speeds).

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Some people use a stepper motor as a cheap rotary encoder to provide feedback. Simple electronics are added to provide a clean signal that can then be used by e.g. an Arduino to tell the number of steps and the direction, and thus detect missed steps. So you have on the same axis:

- on one side, let say a nema 17, to move the axis

- on the other side a cheap weaker motor, to sense the axis movement

Better if both motors have the same number of full steps, albeit you can't anyways sense missed steps when doing microstepping

For instance:

http://www.piclist.com/techref/io/senso ... ncoder.htm

http://www.webx.dk/oz2cpu/20m/encoder.htm

http://www.4qdtec.com/stpen.html

 

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