[skubajon]

I tried to search for info on this but I wasn't too successful. Sorry If Im posting a thread that was alread discussed. Possibly someone could link me to some info regarding this!

Anyhow.... Im in the process of designing my first CNC build. Ive been working on ideas for about 2 years lol.. Needless to say I am a bit obsesive compulsive when it comes to things lol.

My question is stepper motors with encoders. As a cost issue going servo may be out of the question. But I have read about hybrid steppers and steppers with encoders. Some info I have read was they run a lot smoother, quieter and have better torque etc. Is this true? Can I buy a stepper motor and outfit it with an encoder and supply it to the correct controller? Am I wasting my time with this? Is there any benefits?

[H500]

There are no low cost controllers capable of full speed closed loop control of a stepper.

[fmederos]

skubajon,
Have you looked at Linuxcnc?
http://wiki.linuxcnc.org/cgi-bin/wiki.pl

Regards!
Fernando

[skubajon]

I did just now. You talking about the article where it will stop the process if it hits a certain amount of error?

Anyone have any information on closed loop controls? I couldn't find much anywhere

[Al_The_Man]

I think you may have a problem finding a PID closed loop drive for steppers?
Al.

[109jb]

The problem with closed loop control with steppers is that it is useless for anything more that halting the program if a preset position error is exceeded. The reason for this is because if you have a position error with a stepper it means that you have lost steps. If you have lost steps, you have exceeded the torque capability of the motor. There isn't a way to increase torque to get back to where you should be with steppers other than slow down. Steppers have max torque at 0 rpm and the torque decreases as RPM increases. A servo has pretty much constant torque across its rpm range. Also, the servo controller can provide more power to the servo for short periods if required thereby boosting power (reserve power).

Having said that, a properly set up stepper system can work very well and very reliably. In addition, adding the encoders to monitor for lost steps can provide piece of mind that the machine is performing as desired. I myself am working on my second stepper based machine and both work very well. I also intend to implement the encoders on the steppers as an error monitoring system, not a closed loop system. I personally feel that this is a good compromise. I am in no rush however to implement this because as said, a properly set up stepper system can be very reliable even without any type of monitoring or feedback.

My 2 cents

[skubajon]

After doing a little more research I feel that you are all right lol. Thats why I love this site.

My next thing would be stepper size. I plan on doing some aluminum routing with this machine. It will be a quite heavy machine being contructed out of steel. Any motor size recommendations. I estimate that my gantry will be close to 100 lbs

Also this will be running rack and pinion. Very similar to cncrouterparts setup with a 3:1 ratio

[The Blight]

[ame=http://www.youtube.com/watch?v=ZMZdCcLQc4M]Leadshine Closed loop stepper - YouTube[/ame]

[SwampDonkey]

I mean, how hard can it be?
- Controller sends signal to driver to move 20,000 steps
- Encoder sends feedback that 19,500 steps were achieved
- Controller responds by commanding the stepper to perform 500 additional steps.

And this could all happen at kernal speed so thousands of times a second, constantly correcting missed steps. Cheap steppers instantly become much more robust, reliable, and accurate thanks to a cheap 200 line encoder and a bit of software.

But wait, it could bet better! The magnetic Iguaging DRO packages that many of us have on our X2s' could become real-time linear encoders, ensuring accurate (to the extent the DRO's can perform) placement and even velocity.

Someone give me some money and I'll begin hiring the people to make this happen immediately.

[SignTorch]

I mean, how hard can it be?
- Controller sends signal to driver to move 20,000 steps
- Encoder sends feedback that 19,500 steps were achieved
- Controller responds by commanding the stepper to perform 500 additional steps.

....

It's not that simple when more than one axis is moving, the other axii will have completed the last step by the time the controller realizes one axis missed that step, the other axii would have to stop and wait for the one axis to catch up, or have a PID to speed up the step signals to that axis, while the other axii slow down or continue at normal speed, which means each axis requires independently variable step rate.

That's what servos do, but their speed is not tied to the step rate, if one falls behind (or runs ahead) the controller can simply adjust the voltage to affect the speed and then that motor will catch up all by itself, which all happens on each and every encoder pulse on each and every axis independently, no step rate has to vary, the step rate on all axii remains in sync with the original calculated step rate

[H500]

I mean, how hard can it be?
- Controller sends signal to driver to move 20,000 steps
- Encoder sends feedback that 19,500 steps were achieved
- Controller responds by commanding the stepper to perform 500 additional steps.

That won't work under all conditions. It's similar to what happens when car tires lose traction. Flooring the gas pedal will not result in motion.

A servo can be implemented by treating the stepper like a brushless dc motor and applying vector control techniques. The basic code does not look too complex. However, from what I heard, a pretty powerful (ie: expensive) micro-controller is needed to do the calculations.

I have a stepper board capable of interfacing to an encoder. When I have time, I will see if I can give it closed loop control.

[spank]

Dynomotion Kflop....