Closed loop software

[cbass]

Hi,

I understand that Mach2 can not receive feedback from a servo drive (it doesn't do closed loop functions).

Can anyone suggest software that will do closed loop and is...

1-Windows based
2-Easy enough to learn
3-Affordable

The emphasis here is on affordability as I can't spend $3000...


Thanks,

Carlo

[ger21]

I don't believe anything Windows and affordable exists. EMC is free for Linux. I've heard hints that Mach3 or 4 may support closed loop in the future, but no telling when (or how).

[unterhaus]

Closed loop and windows is bad unless you buy one of the real time extensions which is large $$$$. I think that the future of closed loop Mach is going to be with the Gecko external box so the loop will be closed outside the PC. I don't think Microsoft is ever going to modify windows so that real-time is really possible, although they do have embedded windows, which has fairly good latency. Right now they are much more interested in user-friendliness and security than making the machine more responsive.

So right now you are looking at EMC and Linux or something like Galil, which is also big $$$

There are people who run EMC on a linux box and have the user interface on Windows. This isn't done a lot so the setup is a little on the difficult side, but it's doable because it was built into the architecture of EMC from the beginning.

If money is an issue, step and direction is much more affordable, and you can still use servos. Then you have a full range of software available to you.

[cbass]

Thanks for your responses,

I had a feeling Windows OS might pose a latency problem. Well that's good then because it makes my decision much easier...Mach 2, here I come

One question remains for me. I've read some postings here that talk about closing the loop within the hardware as opposed to through the software. I imagine there is feedback communication between the servo drives and the motors. Does this help with positioning accuracy? In general, what are the pros or cons of closing the loop using hardware as opposed to software?

Thanks for your patience,

Carlo

[Halfnutz]

Why do you need windows to do pid? Gecko does the pid function at the controller card. The loop is closed but outside of windows. The geckos are stand alone pid controllers that take step and direction input comands I think.

[unterhaus]

There isn't a lot of performance to be gained by using direct servo control over using servos with step/direction drives. Either way, somebody is keeping track of the position of the motors. With steppers, there is always the possibility of missing steps.

[cbass]

Unterhaus,

There isn't a lot of performance to be gained by using direct servo control over using servos with step/direction drives.

I understood the opposite. That you can run servos faster and w/o missing steps (unlike steppers) because of the feedback.

Either way, somebody is keeping track of the position of the motors.

Who or what keeps track of these steps? The servodrive board? Without positional feedback of some sort, servos seem to be just glorified steppers.

I think I'm going to call Gecko and find out... I'll post back soon.

Carlo

BTW, what does "pid" stand for?

[CNCadmin]

Shopbot has a stepper close loop system and it runs about 1,500.00 a axis.

[Al_The_Man]

BTW, what does "pid" stand for?

Proportional integral derivative.
If you want to see the tuning of PID in action see see some of the tuning instruction video's on the Galil site.
Al.

[Halfnutz]

PID is proportional integral derivative and it refers to the

a) the proportion of the feedback signal you are monitoring that is used in the PID formula, ie the amount of the current value

b) the amount of the slope, or integral of the derivative of the signal, or how long of a period of the feedback signal you are considering.

c) the derivative or slope or rate of change of the feedback signal or how fast is it changing.

These are used to calculate how much power to send to the motor. I might be wrong on some of this, feel free to correct me, but it is basically along these lines.

Thats exactly what unterhaus is saying cbass, that the pid loop can be closed at the controller just as effectively as in the computer software, and that servos ARE better than steppers for high speed applications.

[unterhaus]

I probably wasn't too clear.

If you use a gecko drive with servos, the gecko keeps tracks of the steps requested and the steps moved. This is true of any step and direction controlled drive.

A full servo system keeps track of the encoder counts in the computer and knows the position of all the motors.

In a stepper motor system without encoder feedback, nobody really knows how many steps you've taken, which is only a problem if the system can't respond to what was asked of it.

Other solutions exist.

[cbass]

I spoke with Mariss (great guy btw) at Gecko yesterday and he confirmed what you guys have been saying. I guess I just needed to hear it straight from the horses mouth. He explained how the system works and most of it made sense to me.

One thing he emphasized was that if there is any sort of malfunction in the loop (ie. a component on the drive board bites it, encoder on the servo breaks etc) the servos will "take off", which could cause serious damage to the moving axis. He was emphatic about installing limit switches. But I digress.

Thanks for all your replies.

Carlo