Servo Motor question

[jaslep13]

This may be a silly question but I am new to this. How come nobody is using servo motors on these X3 conversions? I thought a closed loop system was surperior to stepper motor systems.

Jason

[Deviant]

Cost.... usually.

Servo's cost more, and the controllers for them.

Alot of people are of the opinion that with the new stronger steppers. You don't miss steps and don't need to worry as much about the servos.

I have servos for my x2, but they are in a box somewhere, waiting for my garage to be unpacked.

[jl123]

One thing I don't understand about servos is how they keep a position. Do they just do a balancing act or what?

[Deviant]

There is an encoder on the end of the shaft.

It's a two part device. One part consists of a disc with ultra small index marks. Then other is the encoder which reads where those marks are and if they have moved.

If it detects a move that wasn't expected the driving software will readjust the servo back to where it's suspose to be.

[jl123]

The thing I don't get is wouldn't this lead to inaccuracies? Or is the encoder that sensitive to where that movement wouldn't matter?

[Deviant]

I'll let you know when I get mine going.

*grins*

However I believe that you set the amount of backlash with in your system. Then it takes that into account.

[Al_The_Man]

With say a 1000 line quadrature encoder the quadrature edge pulses are often detected to increase the resolution x4 so now you have 4000 counts/turn. say with gearing or ball screw, 1 inch represents 4 turns of the encoder, this will then result in a resolution of 16000 counts/inch = 0.0000625/count.
If the position error is 1 or 2 pulses, then you can see the kind of error or accuracy you would have.
Al.

[MacCNC]

I have servos on my X3 and they are working really well. The motors cost quite a bit more but the drives are about the same (assuming quality parts like Geckos). I really overkilled it on mine, have 400oz on X & Y and 800oz on the Z. I feared it would not be enough torque to lift the Z in rapids without a spring attachment but it has enough to lift the head plus a buddy off the ground hanging on for dear life!

[phantomcow2]

Your steppers will only miss steps when they are not able to supply the necessary torque. Get motors powerful enough, and it will be okay. With ballscrews, the need is greatly reduced still. You will see the kits available all make use of 23 frame motors, and I believe have good results.

[jl123]

Mac why haven't you installed a gas spring on it? That was the easiest part for me.

I wouldn't dare leave the Z unlocked without the gas spring. Do you have to lock it?

[MacCNC]

LOL
I should have, could of used smaller servos!

[MacCNC]

The Z gib is tight enough to keep it from falling (usually...).