configuring for servomotors
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@Pritt I have no experience with other servos, but I think the problem I'm having with the iHSV motors is partly motor and partly encoder, but not the Duet board.
The iHSV motors worked perfectly with factory settings in my sand table- that's a much higher speed mechanism that does not require the precision and accuracy of a 3D printer- drawing lines in sand with a 12 mm steel ball is pretty sloppy.
Servos are made to run at high speeds. 3D printers are low speed machines. With 20 tooth drive pulleys, moving the mechanism at 200 mm/sec is only 5 revs per sec from the motor. Servos (at least, the ones I have) might work better by gearing them down with belts and pulleys so the motors are spinning fast even as the mechanism moves slowly. That would increase resolution, as well as the already more than sufficient motive force.
I watch the machine when it's homing and the motion doesn't look smooth which leads me to think that here may be some parameter tweaks that can improve the performance at low speeds.
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I think it is a very interesting topic, so I open a thread, see if we can get something clear.
https://forum.duet3d.com/topic/17535/solved-leadshine-closed-loop-external-motor-drivers-with-duet2
Thanks you.
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I just ordered the Nema 23 model.
Somebody in this forum mentioned that 1 to 3 gearing fixes the problem.
I also ordered 1 to 10 planetary gearbox. As far the servo torque is constant up to 3000 rpm I think this will give results. what do you all think? -
@martin7404 Wow! A NEMA-23 servo is definitely overkill for a 3D printer. The only thing I'd be concerned about with a planetary gear box is the possibility of backlash. That could cause some print artifacts.
The typical steppers used in 3D printers are only 5-10 watt devices. The NEMA-17 servos I used in the sand table are 78W motors. I would not want to get my fingers in the way of the moving mechanism. The NEMA-23 motors are probably >100W motors and if you add a gearbox, they will drive the printer mechanism through anything that gets in the way, including your hand.
GeckoDrives has an app note with a protection circuit that will protect your power supply and anything else you connect to the same power supply as the motor. I had an expensive lesson in operating servos at high speed and acceleration and managed to kill a 200W power supply and Duet wifi board. You need oversized power supplies for the iHSV servomotors because they will draw up to 3x rated current.
I have created a PCB layout for the protection circuit that I'm sure is good for the 24V motors I used and probably OK up to about 36V, but not sure how much higher than that. Unfortunately the app note isn't very detailed and I have no idea about the peak current/back EMF the motors can generate when stopped suddenly or driven beyond rated maximum rpm (that spec is supposed to be based on back-emf generated by the spinning motor).
The PCB is easy enough, but some of the component selection is a little "iffy". I will be using 1W wirewound resistors for R1 and R2, but it may not be necessary. 1/2 W resistors may be more than adequate. I also don't really know if the transistor should have a heatsink. I am using a 100V 15A Schottkey diode for D1, and put wide copper areas at the connector and diode so they don't burn up. You may need a bigger diode and more copper for NEMA-23 motors. The crimp lugs for the connector are back ordered until late March, so I'll probably build and test one by soldering wires to the board instead of the connector.
The board layout files are here.
My parts list is here, subject to change when I get a chance to test the board.It is safest not to connect the duet board to the same power supply as the servomotor(s).
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@mrehorstdmd I did build a Special machine for drilling 6 meters long Alu profile, using old USSR universal milling machine and 2 axis X and Z. Initial test was witn Nema17 with 1:10 planetary , Now it is with Nema23 closed loop 3 Nm drives.
I am doing 20 countersinked holes for in 7 minutes now and the stepper is at about 150 rpm, but I want to make it faster . If I do It starts to work unreliably
My thoughts is Nema23 servo 0.6 Nm with 1:5 will give me about 3NM and about 5 NM peak all the way up to 600 rpm at the belt pulley, that is more than enough
here is photo of the milling machine
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@mrehorstdmd
I know, it's been a while since you tried to fit servos to a 3D printer. I gathered from the reading, it's mainly related to low resolution of the encoder. Your solution was to add gears to the output shaft, but why not add a 1:5 gearing beweeen motor and encoder? The encoder would run faster, reaching it's speed limit earlier, but that's not important here. If it's not possible to separate the encoder from the servo, you could use external encoders as well and route them to the servo controller.
Question is: Is it worth the effort, just to have a more silent 3D printer? -
@mrehorstdmd My motors now are at separate 48V powersuply
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@o_lampe The iHSV servos are not easily separated from the encoders. They are "integrated" with motor encoder and driver all in one package. I suspect the encoders are on the same PCB as the drive electronics, but I haven't taken one apart to check. When I do I shoot a bunch of photos and post them.
@martin7404 Interesting machine. It looks like the two clamps at the back are there to align the extrusion for the drilling operation and the one up front grabs the extrusion and slides it along to set up the next drill position. I can see why you would need a NEMA-23 size motor.
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@mrehorstdmd
Yes, pictures would be nice. Maybe you can add a 2-stage reduction or a planetary gear between motor and encoder? -
@o_lampe There may be other advantages to using servos in a printer, beyond quieter operation. Servos will enable much higher acceleration and jerk settings (but increased operating noise, and probably ringing) which might improve extrusion by keeping the extruder running at full print speed for a larger percentage of the print time. That has to have some effect on the pressure in the extruder.
If someone is trying to run their printer hard, and runs into the limits of the steppers, they might have skipped steps, etc. The servomotors absolutely will not skip steps unless a solid object gets in the way of the mechanism.
Closed loop steppers are probably better for printers than servos, and probably wouldn't require pulley or gear reduction to get sufficient resolution (compared to cheap iHSV motors). Other servos may have optical encoders and provide the necessary resolution without belt or gear reduction.
