New guy HI! and hopefully intelligent question

[eaglej]

I have been reading this forum for some time now, and trying to piece together enough info to feel confident with a conversion.

I have been wanting tools for a long time- I am the engineer who wishes he got to make and touch the things he designs. I think I am going to have a lot of fun with this... Planning to get a small mill- probably X3 or Grizzly, then convert to ballscrews and CNC. Space is a concern for me, so that is about the biggest machine I want to schlep into my basement. Someday, there will be a new house with a dedicated shop, but I would rather get started now than wait for that!

On to the question:

Having a general understanding of how servos use feedback control, and steppers have none (other than hopeful thoughts), I was wondering why DRO output cannot be used in place of an optical encoder?

If you have a DRO with RS232 outputs, can you use the DRO output as "true position" feedback info, as the encoders on a servo motor would? Or is there a time issue with this (ie. feedback lag time etc). It seems that (if it could work) this would give the main benefit of servos without the expense...

Hopefully this isn't a stupid question- I would love to know the answer.

Thanks!
J

[bborb]

If your readout resolution is in half-thousandths, and your rapid travel is 200_inches per minute....... the display must update at a frequency over 6.6Khz.
Probably the highest baud (bits per second) is 115,200
Figure 4 bytes of data (32 bits), the serial connection could only update at 3.6Khz
There may be more "apples to oranges" reasons why this wouldn't work on the surface but I like the concept.
A better pursuit would be to incorporate the existing scales of the DRO system for positional feedback, taking the DRO out of the equation.

[pointcloud]

You can set up a linear scale to work like that... Althought you'd be cutting those db25's off... and hookin' streight to the drive...

BUT if your going with servos and ballscrews, why change? You will not be any more accurite, due to the fact the mills your are looking at will not hold the tolerances that a good ballscrew and a anti-backlash nut will...

Good luck.

[eaglej]

Thanks for the replies guys-

I am planning a ballscrew conversion, and running stepper motors. I guess my main interest is thinking of a way to have a closed loop scheme and run steppers... rather than incurring the expense of servos/ controllers. It also seems like this would be more accurate than servo encoder control- a readout of this type is actual table movement, rather than motor movement (from which table movement can be derived, but isn't the same).

I plan to use a rolled precision ballscrew, and a double nut (preloaded) with some type of homemade bracket for the preload springs.

Bborb- so you are saying that a serial connection could update fast enough if rapids were run at 100ipm?

None of this is super critical right now, I am just trying to think around this issue and see if there isn't a way to incorporate some feedback into the design. I figure I will need a dro for manual work anyhow eventually. I would like to use my computer as a dro, but because there is no closed loop, it seems like this would be impossible. I guess some sort of encoder would be needed to make that arrangement work - unless you could count motor steps, and translate that into linear motion.

Or maybe I have my head up my butt... Either way, this stuff has my nerves tingling with curiosity.

J

[bborb]

er, um,.... I didn't mean to say that, I just threw out the first thing that came to mind on why I wouldn't do it. We'd need to know WHAT data the DRO sends out the serial port and start there (we can't change it's firmware). Is there just one serial port or is there one for each axis. What are the capabilities of the DRO's serial port (baud, data, parity,stip bits).
As I'm writing this, I went to
http://www.newall.com/DROs/c80_rs232.htm
and it gives good technical data about what we're discussing and answered the questions I had. Doesn't look feasable for your application.
Sorry, I was excited there for a second.

[ger21]

You didn't say what software you would be using. The control software may dictate what you will be able to do.