> To the point, though, I am having one not-so-minor problem. When I enable > the servos, they are not completely stable, and I'll get following error > almost immediately after enabling them. They don't run away, but the > encoders show them moving ever so slightly, and physically the motors > actually are moving. > > The Anilam system I'm converting is a servo drive with velocity feedback > directly to the servo amps and rotary encoder output that I've wired to the > 7i33TA board. The servo amps take +/-10V for direction and velocity. I can > move the axis using the pncconf tuning page, and have tried to stabilize > them using the DAC compenstation, but there is not a fixed amount of DAC > compensation that seems to work. The Anilam controls still work perfectly > fine and the servos are stable when that control is running. > > I suspect a problem like ground differential, but the cable to the servo amp > has ground, signal and shielding, so I was thinking I should be OK. Despite > the schematic on the wiki above, I am not powering the 7i33TA or 7i37TA > on-board, but instead am using cable power (but I believe that is the only > inaccuracy in the schematic). The 50 pin ribbon cables are ~3 feet. > > Would it be better to power the boards from the 5V supply already in the > chassis? Is this a problem that can be solved with PID tuning? Has anyone > seen anything like this? I've searched the email list and have not been > able to find anyone with the same problem.
If I'm understanding the setup correctly, you have encoders providing position feedback. It's possible there is some offset in the servo amps which is driving the motors slowly even though there is zero at the amp inputs. The PID loops, once tuned, should stop that motion but it likely requires some non-zero value for the Igain (integral) parameter. If you short out the inputs to the servo amps right at the amplifiers themselves, do the motors still move? If so, is there a zero adjustment on the amp? If you've already got a reasonable PID Igain parameter, another possibility is that there is noise in the encoder signal and it is accumulating counts when there is actually no movement. The PID loop thinks it's seeing movement and drives the motor to compensate (trying to maintain zero position on the encoder output). The result is motor movement. One way to determine this is to put a load on the motor. If it's the PID loop driving the motor, it should increase the drive when load is applied, trying to maintain speed. A good way to tell is to monitor the voltage to the motor and see if it increases when the motor is loaded. If it's not the PID integral parameter doing it, the voltage will stay the same and the motor will likely stop under load. I can't say what would be reasonable PID parameters without knowing the scale factors for the PWM output going to the servo amps. Karl ------------------------------------------------------------------------------ Get a FREE DOWNLOAD! and learn more about uberSVN rich system, user administration capabilities and model configuration. Take the hassle out of deploying and managing Subversion and the tools developers use with it. http://p.sf.net/sfu/wandisco-d2d-2 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
