Originally Posted by
TeknicTom
These are good questions. Dual motor axes are very common and are not a problem for servo motors when the integration is done correctly.
Out-Of-Square
It is true that closed loop servos will not lose steps or stall so mispositioning based on these events is prevented. However, if one motor shuts down and the other motor does not also shut down in a timely fashion, it could cause some tension on the gantry (not excessive because one motor is disabled and free to move, but some). The goal here is to have both motors following the same command in real time and to also to be enabled and disabled at the same time. The way to do this is to wire the HLFB output signal ("Servo On" option) from each motor in series with each other. Wiring them in series makes them act like a gate at each motor so if any motor gets a shutdown, it will interrupt the continuity of the circuit the resulting voltage back to your controller will be 0VDC. The controller will see this low state and will disable all the motors.
"HLFB+" from your controller into motor 1, "HLFB-" out of motor 1 and into "HLFB+" for motor 2, "HLFB-" from motor 2 back to your controller
This technique means that both motors will be disabled if one motor gets a shutdown. When you resume machine operation, you will need to re-home (square-up) the machine before cutting.
Tuning Dual Motor Axes
Yes, there is advice on tuning a dual motor axis on Teknic's website but this process was used more in the past for OEM customers with two trained engineers using manual tuning techniques. Now with the ClearPath auto-tuner, there is actually an easier process that can be used. The basic steps are listed below.
1) X and Y axes almost always have the exact same mechanical design (same dia pinion, same rack pitch, etc.) to make the machine easier to build and to make the command generation easier
2) The vertical Z axis is almost always a different mechanical design than the X and Y so generally speaking, it should not be used to create a dual axis motor file
3) The optimal way to create a dual X file using the auto-tuner is to load 1/2 the total X gantry weight onto the single Y axis and then run the auto-tuner. You usually would not use the Z axis because it is biased downwards based on gravity (which dual X axis is not) and because of the mechanical differences.
4) After auto-tuning the Y axis with the additional weight, load the Y file into both X motors.
5) Click the "Reverse Direction" checkbox for one of the X motors and save the two X motor files with different names (e.g. XA_SDSK-3421S_rev1 and XB_SDSK-3421S_rev1)
6) Run the machine. If the X axis does not seem to have optimal tuning, trying moving the "Fine Tuning" slider to the left to see if your results are better. You will need to do this with both X motor files before evaluating the effectiveness of this step. If you make any changes, re-save the new files (overwrite the old files or create new names) and save the files in a safe location for the future.
7) If you are still having problems with your dual motor axis after following this technique, you can contact Teknic directly for technical support.