On 03/01/2017 07:58 AM, dragon wrote: > What are the advantages and disadvantages of using PWM input vs. analog > input servo amplifiers? Simpler, no DAC on the control, no analog input circuitry on the servo amp. No drift when the servo amp is enabled but the LinuxCNC positioning loop is "off". > Unless I am mistaken, it looks like it is easier to drive the PWM input > ones as they can be driven directly off of something like a Mesa 6i24 > without the need for the additional analog interface card. I have worked > with Pico Systems PWM drives before, as well as Mesa 7i29 amplifiers > too. I have no complaints with how those performed. Can I expect the > same thing if using, say an AMC drive that has PWM/Dir input? Or would > it be better to use an analog input AMC amp combined with an analog > interface such as a 7i33 or 7i48? > > However, if you have a DC tach on the motor, it can give very fine velocity feedback to the servo amp, and so the PID loop gain can be turned down to give VERY smooth motion. I got my Bridgeport set up so I could go down to .01 IPM before the stick/slip became dominant. This was getting about 3 encoder counts/second, and the DC tach was calculated to be producing 7 uV (no way I could measure that). I shorted the tach with a screwdriver and the servo amp immediately faulted. So, it REALLY was using the tach constructively.
So, to boil that down, the tach feedback allows the use of lower resolution encoders and still permits smooth movement. With our PWM system, it pretty much requires a higher resolution encoder. However, now that we have the timestamp velocity estimation in both the PWM and PPMC interfaces, it is not quite so critical. Jon ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, SlashDot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
