Newbie- linear scale

[Farmers Machine]

Could you use a linear scale like on a mill or lathe DRO system with just a DC motor or a stepper motor with no feed back from the motor? would this make your system as accurate as the scale with out acculmation errors? how would you set up the scale in the set up page? Thinking about building a multi function gantry machine and SICK - STEGMANN makes a magnetic scale and a winding cable scale for my 40 foot application. I am concerned with wear and differences on the rack and pinion gears.
Thank You
The Farmer

[Al_The_Man]

Not sure how you are applying this, this is posted in the Camsoft forum, are you integrating this in Camsoft or?
If requiring servo control, then you need at least one form of feedback, but when a linear scale is used, it is recommended that an encoder on the motor as well as a linear scale is used, especially if backlash is present.
See the Galil site that has a video that explains the PID tuning for this type of control using dual feedback.
I have used a scale alone on some simple systems using a rolling ring type drive, this possesses virtually no backlash, so it was inherently very stable.
Al.

[Karl_T]

If you have no backlash from the motor to the scale you can set it up just like any other encoder. If you have backlash, it will be hard to impossible to tune the servo.

Galil also has dual loop capability by two different methods. There's info about this on the Galil website. You should be able to have one super accurate machine with ths setup.

The scale would have to have quadrature outputs to be compatible.

I've not seen a camsoft .cbk with this option. I'm sure the fine folks at camsoft tech. support could show the way. or buy me a couple scales and I'll figure it out for you <VBG>

Karl

[Karl_T]

Originally Posted by Karl_T Galil also has dual loop capability by two different methods. There's info about this on the Galil website. You should be able to have one super accurate machine with ths setup.

I did just a short bit of research on dual loop. It would be difficult to implement under Camsoft. Linear scales with Camsoft is a great way to go IF you have no backlash so that servo tuning is possible.

I didn't check into steppers and scales but i can see some difficulties there as well.

In short, if you're going with scales, make sure your drive linkage is perfect.

karl

[Farmers Machine]

So, there is no easy way to tell the servo to go until the scale says we are here? the machine was originally equiped with DC servo motors with encoders. The CNC is fried and I want to retro fit with cam soft. the rack and pinion is spring loaded so it will always be tightly meshed but he rack has issues in spots where it was previously used, if the linear scale had a quadature output capability I am assuming that the motor turning would change the display reading just as the shaft encoder would except that you would be in exact true position as accurate as the scale would be.



Thanks
The Farmer

[Al_The_Man]

The scale would be equivalent to a rotary encoder, electronically speaking, and appears the same to the controller.
The wire draw type mechanism uses rotary encoders anyway.
But you mention building a 40ft multi function gantry, is this for plasma/gas or router application? If so do you need high precision for an application like this?
If you have existing rack that is worn, sections can always be replaced.
Al.

[Karl_T]

Originally Posted by Farmers Machine So, there is no easy way to tell the servo to go until the scale says we are here? ... I am assuming that the motor turning would change the display reading just as the shaft encoder would except that you would be in exact true position as accurate as the scale would be.

This is easy in that it is exacty the way a servo works... You are also correct that your linear encoder will give the exact true position.

The problem to deal with going this route is servo instability, a real problem if you got backlash. You get into a chicken and egg thing here: If you eliminate all backlash, the encoder on the motor will be just as accurate as the linear scale.

[Farmers Machine]

I build structural steel parts and the other day a had to weld shear tabs to 8x8x5/8 wall square tubing, each end had a 9 hole grid bolt pattern the distance between the holes was 28 feet 9-3/4" +/- 1/32"
I want to mount a mill head to do drilling with accuracy. My plan is for it to be an oxy fuel,and plasma cutter with a mill head for drilling and a pulsed mig welder machine to make long straight welds.

[Al_The_Man]

It may work with just one feedback via the wire draw encoder, I have seen a similar set up on Burny tables where the encoder was mounted after the motor gearbox directly on the rack.
I would make sure you maintain good mesh and use low backlash planetary gearboxes.
You could use a motor and gearbox on either side to avoid racking, in a master and slave set up, the slave could just have an encoder just on the motor.
After some thought though, the wire draw is not going to give you any edge over an encoder on both motors as that method will not compensate for rack wear etc.
And it would be truly impractical and probably not possible to obtain a scale that length.
Al.