Hi, I was about to start a project using a Nema 23 closed loop stepper.
I was wondering if these type of "China" motors with an encoder on the back have any way of knowing their absolute position or only their relative position.
I say this because I need to know the start angle of the drive shaft on "power on" and I was wondering how this is usually calibrated? Do I need to resort to an additional encoder for a reference position or do the encoders on the back normally have a reference point that is readable?
I have another more expensive motor with a much more sophisticated 20,000 point encoder on it which does also output the shaft angle at any time but I was hoping for a more budget solution for more of a hobby project.
I haven't yet purchased a motor but I have attached a PDF of the sort of thing I was looking at.
The PDF file at the link you posted contains no reference to an encoder output or absolute position input or output. Very confusing documentation!
For motors with incremental encoders, you need a separate limit switch to define the "home" position. High resolution absolute shaft position encoders are quite expensive.
krishpants:
I was wondering if these type of "China" motors with an encoder on the back have any way of knowing their absolute position or only their relative position.
Ahh indeed, its quite possible that this cheaper one does not have a Z position then? I don't need to actually know the absolute position I just need to be able to seek an origin point and then rotate to a known angle thereafter. Ordinarily, I can just stop the motor in the right place each time but if power was lost mid-rotation then when everything turned on again the "home" position would be lost.
Perhaps as you suggested I will go with just a single reference mark and an optical sensor of some sort. I can just rotate the motor to that known start position apart of the setup loop.
Robin2:
My reading of the datasheet is that it is simply an integrated package of stepper driver and stepper motor.
...R
No, this is definitely a DC servomotor with a servo controller that pretends to be a stepper controller.
Otherwise there's little point! Note its got PID settings for position and speed and a 20kHz current
sampling rate for the inner current control loop.
In other words it should be pretty gnarly replacement for any stepper based CNC system... Do
people still say gnarly?
Unlike a "real" servomotor it doesn't have absolute positioning nor a electromagnetic brake to hold it
when powered down, but it doesn't need those if its basically for upgrading a stepper based motion
control system.
Just to be clear, my application does not need precision or the fancy stuff that comes from a true servo nor will I need mm precision or holding brakes.
Having worked with this style of the motor before I prefer their ability to auto correct lost steps by increasing power and alarm out if step loss gets too high.
But I think you guys are right, the lack of Z-axis wires likely means this style of motor is fitted with a cheaper encoder which is only designed to give relative feedback of step counts against what the motor was instructed to do.
I think having opened up my more expensive motor, the Z axis encoders have further rings of slots and additional optical sensors to report back Z position to the controller.
Part of the "fancy stuff" you get is far lower power consumption overall, cooler running motor,
no problems of mid-band resonance.
The section in the manual that says:
Avoid dust, oil fog and corrosive gases
is
basically saying "this motor has a commutator".
Unlike a stepper the position accuracy is determined by the encoder, not the rotor lamination stampings,
so microstepping implies more precision, not just smoother running.
The downside is that the brushes will eventually wear out, whereas a stepper keeps going till
the bearings fail. Modern brushes are usually very good though.