Usage of closed loop servo motors
Posted by Svensson
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Usage of closed loop servo motors March 11, 2017 02:44AM |
Registered: 8 years ago Posts: 9 |
Hello!
While redesigning my large core-xy printer to use ball screws instead I came across some nice 200W ABB servo motors with appropriate drivers that should provide around 2NM of torque. The drivers use standard step/direction inputs and runs off around 50-80 volts.
Are there any direct drawbacks on using real servos other than the fact that I could get about five cheap I3 clones for the cost of three motors+drivers and that they will need PSU of their own? My intention is to have the motors power 1605 ballscrews, mounted directly to the motors. Total moving mass will top out at about 8 kg with the tool changer that I'm currently working on. Final goal is to be able to print parts a bit oversize and then use a fixe axis milling head (reason for the automated tool changer) to mill the prints to final dimensions, but this lies way ahead in the future...
Any thoughts?
While redesigning my large core-xy printer to use ball screws instead I came across some nice 200W ABB servo motors with appropriate drivers that should provide around 2NM of torque. The drivers use standard step/direction inputs and runs off around 50-80 volts.
Are there any direct drawbacks on using real servos other than the fact that I could get about five cheap I3 clones for the cost of three motors+drivers and that they will need PSU of their own? My intention is to have the motors power 1605 ballscrews, mounted directly to the motors. Total moving mass will top out at about 8 kg with the tool changer that I'm currently working on. Final goal is to be able to print parts a bit oversize and then use a fixe axis milling head (reason for the automated tool changer) to mill the prints to final dimensions, but this lies way ahead in the future...
Any thoughts?
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Re: Usage of closed loop servo motors March 11, 2017 02:53AM |
Registered: 10 years ago Posts: 14,672 |
How many step pulses do they take per revolution?
- Too few and you won't get enough resolution, also the machine will be noisy. You may be able to mitigate this by using fine-pitch ball screws.
- Too many and you will need fast 32-bit electronics to generate the step pulses fast enough.
Ordinary stepper motors give you 3200 or 6400 microsteps per revolution when used with x16 microstepping. I suggest you aim for steps/mm to be at least 50 for X and Y and at least 1000 for Z.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
- Too few and you won't get enough resolution, also the machine will be noisy. You may be able to mitigate this by using fine-pitch ball screws.
- Too many and you will need fast 32-bit electronics to generate the step pulses fast enough.
Ordinary stepper motors give you 3200 or 6400 microsteps per revolution when used with x16 microstepping. I suggest you aim for steps/mm to be at least 50 for X and Y and at least 1000 for Z.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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Re: Usage of closed loop servo motors March 11, 2017 08:01AM |
Registered: 8 years ago Posts: 9 |
Thanks for the input!
32 bit electronics are already in place, unfortunately though not a duet.
The servos come equipped with 2500 line quadrature encoders which the drivers run in full quadratic mode (so if I got everything right it should be 10000 pulses in total per revolution). Moreover, for the Z-axis a belt is used to drive my ballscrews giving an additional reduction of about 1:2 (haven't decided yet on tooth-count for the pulleys). On the x-axis I'll have a gearbox driving a shaft to power the two ball screws. This will most likely be a worm gear with about 1:10 reduction, so resolution shouldn't be much of a problem. And given that the motors are capable of up to 2500 rpm I think speeds should be acceptable as well.
32 bit electronics are already in place, unfortunately though not a duet.
The servos come equipped with 2500 line quadrature encoders which the drivers run in full quadratic mode (so if I got everything right it should be 10000 pulses in total per revolution). Moreover, for the Z-axis a belt is used to drive my ballscrews giving an additional reduction of about 1:2 (haven't decided yet on tooth-count for the pulleys). On the x-axis I'll have a gearbox driving a shaft to power the two ball screws. This will most likely be a worm gear with about 1:10 reduction, so resolution shouldn't be much of a problem. And given that the motors are capable of up to 2500 rpm I think speeds should be acceptable as well.
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Re: Usage of closed loop servo motors March 11, 2017 10:11AM |
Registered: 10 years ago Posts: 14,672 |
10000 pulses per revolution along with a 10:1 worm drive on X and Y may give you such a high steps/mm that even 32-bit electronics can't generate the pulses fast enough to give you a reasonable travel speed.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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