Need Help For New Cnc Retrofit

[jdelaney44]

OK, I think it's sinking in to my thick head.....;-) Thanks.

Torque constant. Key word constant.

-jd

[NC Cams]

Al: not only is there a torque constant (Kt = X in-oz/amp) but also there is a RPM-vs-voltage constant of "Yrpm/volt", yes/no?? Naturally the RPM constant is for UNLOADED rotor.

As you apply load, rpm drops, the back emf drops in proportion to rpm drop and current flow increases until you ultimately reach locked rotor torque. At locked rotor, absolute max torque and max amp draw occurs (of course assuming terminal voltage remains constant).

My understanding - oversimplified perhaps but my understanding. Please correct or verify.

[Al_The_Man]

Yes but as I mentioned earlier if the occurence in the last statement happens, you destroy the motor, if max voltage is applied.
I am still putting together a check sheet for empirically coming up with the motors specs when they are not available from the manuf. It will hopefully have some of the above questions made plainer.
Al.

[machworx]

I have a question: does anyone know if there is a system or components available that would allow one to use their existing DRO scales as a feeback loop if driving your axes via stepper motors VS the traditional encoders in servo motors?

[ger21]

It's supposed to be implemeted in a future release of Mach3 with the G100, but I believe it's dependant on a G100 firmware upgrade, and may be quite a ways off. (maybe up to 1 year? but hard to say)

[NC Cams]

Word of caution re: optical scales for F/B loop =

The original Bridgeport Eztraks used a DRO scale on the Z for feeback in lieu of the encoder. They found that they had stability problems when/if vibration were to occur - system hysterisis/slop was supposedly the root cause of the problem. Ultimately they had to add an encoder and ran simultaneous position sensing.

The servo ecoders to deal with position F/B while machine was controlling Z axis - the linear scale was used in manual mode (servo disconnected) for encoding of position for DRO.

This does not mean do imply that linear scales won't work, rather, it only re-emphasises the fact the need for a robust, rigid system that is essentially as free of slop/backlash as you can make it.

Otherwise, you could be plagued with oscillations as the system "hunts" to find/correct itself.

[Al_The_Man]

Originally Posted by machworx I have a question: does anyone know if there is a system or components available that would allow one to use their existing DRO scales as a feeback loop if driving your axes via stepper motors VS the traditional encoders in servo motors?

Unless you have almost zero backlash there can be a problem with just using one feedback loop, If you need a very good explanation of dual loop feedback on systems that have inherent backlash and require accurate positioning by using linear scales, check out the instructional videos on the Galil Motion site (Dual feedback positioning), to see what it entails for tuning the PID loop.
This can only be done successfully however with servo's fitted with an encoder together with the scales.
Al.