AC Servo: Analog or Step/Dir?
28 Oct 2015 11:00 #64381
by buckym1a
AC Servo: Analog or Step/Dir? was created by buckym1a
Hello,
Im working on setting up to build a high precision CNC lathe using LCNC, Mesa interface cards, and AC servos. This is not my first CNC build but this is my first time using LCNC, Mesa, and AC servos. The servo drives I have are Mitsubishi MR-J3 drives and will take either a +- 10v analog or step/dir control signal.
So my question now is: which would be the better of the two for CNC machine control?
Ive done some digging around here and elsewhere but have not yet come up with any solid information on the subject, though Its possible Im just being dense or not looking in the right places. Most of what Ive found is either subjective, or not quite relevant to my setup, but I have noticed a general leaning toward analog control, which is what I had originally intended to go with. Usually the reason given for using analog is because its easier. Now Im all for easy but I also recognize that easier doesn't always mean better.
So Im looking for some solid "good, bad, and ugly" for both control schemes.
To help add some context, this is not a production machine that I need to have up and running to meet an order deadline, or a simple weekend hobby machine. This is a "Because I can" long term project, I don't care if it takes a couple years to bring it to completion, in fact I'm quite sure it will take that long or longer. My goal is to end up with a specialized high precision machine, and expand my machine building skill and knowledge base.
Some things Im looking at:
The MR-J3 drives come with three control modes: Position, Speed, and Torque. Of these the position and speed modes are what Im considering.
I like the speed mode option as it is simple to set up, uses analog control, and allows easy encoder feedback to LCNC.
I'm interested in the position mode as it gives me some options including the drives on board Absolute Positioning System. The APS will take some bit of work to adapt to LCNC but I believe it is doable. I don't mind putting the extra work into it if I there are some other solid advantages. This is where my interest in step/dir control comes in as the position mode requires it.
Im working on setting up to build a high precision CNC lathe using LCNC, Mesa interface cards, and AC servos. This is not my first CNC build but this is my first time using LCNC, Mesa, and AC servos. The servo drives I have are Mitsubishi MR-J3 drives and will take either a +- 10v analog or step/dir control signal.
So my question now is: which would be the better of the two for CNC machine control?
Ive done some digging around here and elsewhere but have not yet come up with any solid information on the subject, though Its possible Im just being dense or not looking in the right places. Most of what Ive found is either subjective, or not quite relevant to my setup, but I have noticed a general leaning toward analog control, which is what I had originally intended to go with. Usually the reason given for using analog is because its easier. Now Im all for easy but I also recognize that easier doesn't always mean better.
So Im looking for some solid "good, bad, and ugly" for both control schemes.
To help add some context, this is not a production machine that I need to have up and running to meet an order deadline, or a simple weekend hobby machine. This is a "Because I can" long term project, I don't care if it takes a couple years to bring it to completion, in fact I'm quite sure it will take that long or longer. My goal is to end up with a specialized high precision machine, and expand my machine building skill and knowledge base.
Some things Im looking at:
The MR-J3 drives come with three control modes: Position, Speed, and Torque. Of these the position and speed modes are what Im considering.
I like the speed mode option as it is simple to set up, uses analog control, and allows easy encoder feedback to LCNC.
I'm interested in the position mode as it gives me some options including the drives on board Absolute Positioning System. The APS will take some bit of work to adapt to LCNC but I believe it is doable. I don't mind putting the extra work into it if I there are some other solid advantages. This is where my interest in step/dir control comes in as the position mode requires it.
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28 Oct 2015 18:34 #64388
by BigJohnT
Replied by BigJohnT on topic AC Servo: Analog or Step/Dir?
When ever I have the option of using analog control with feedback I will. Step and direction is usually provided for controls that can't output velocity commands.
JT
JT
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28 Oct 2015 19:36 #64397
by andypugh
Replied by andypugh on topic AC Servo: Analog or Step/Dir?
There is no objective difference between sending commands by pulse-rate or analogue voltage. They are both ways to send a command to the drive.
There may be an advantage in running the system in velocity control mode, as LinuxCNC can then use velocity feedforward to improve the control loop.
If you are using the position loop in the drive in step/dir mode then there is no way to pass the velocity information to the drive (feedforward velocity is something that only LinuxCNC "knows")
However, the position loop in the drive is likely to be a lot faster than the LinuxCNC loop, so it might all balance out.
If the position loop is in LinuxCNC then you can watch it for tuning more easily.
There may be an advantage in running the system in velocity control mode, as LinuxCNC can then use velocity feedforward to improve the control loop.
If you are using the position loop in the drive in step/dir mode then there is no way to pass the velocity information to the drive (feedforward velocity is something that only LinuxCNC "knows")
However, the position loop in the drive is likely to be a lot faster than the LinuxCNC loop, so it might all balance out.
If the position loop is in LinuxCNC then you can watch it for tuning more easily.
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