[Fastest1]

Many of us use open loop with no loss of steps or issues. Could there be something else?

[rotec]

Many of us use open loop with no loss of steps or issues. Could there be something else?

Something else? There was no issue with open loop other than if you over loaded the axis or accidentally stalled the stepper motor it would lose steps.

Im not saying you cant build an open loop system using steppers that wont lose steps. All I'm saying is that very few professional CNC set ups use open loop and for good reason. Under normal circumstances your open loop system would work just fine, however the possibility to losing steps is always there.

I assessed my options carefully and with closed loop systems being very affordable today I chose closed loop and Im glad I did and would again for any future CNC project.

I even converted an older open loop stepper my adding my own encoder to the rear shaft and then purchased a Chinese closed loop stepper drive. This is on the 4 axis now and I love it.

[Fastest1]

Understood. My experience with steppers has made me monitor the EStop more than needed, though I can achieve good results if I am patient.

Maybe I dont realize the potential of the closed loop on a stepper, did you say your system corrects itself like a closed loop servo or just fault? Which is still better than a crash.

[rotec]

Understood. My experience with steppers has made me monitor the EStop more than needed, though I can achieve good results if I am patient.

Maybe I dont realize the potential of the closed loop on a stepper, did you say your system corrects itself like a closed loop servo or just fault? Which is still better than a crash.

Yes it will correct itself, in other words If you stall the axis (hard to do) but if you did the servo or stepper with its encoder would stall all at the same time, if the overload cleared it would "catch up" and get back to where it needs to be, this is what a true servo should do.

You can set the drive parameters so that if the stall draws "X" amount of current or the stall duration is too long. It will alarm out and just stop. You can set all these in the drives. I just run the default which is "catch up" mode and current limited.

The Chinese do a "leadhsine" style copy of a closed loop stepper drive. See em on ebay with closed loop stepper cheap. I found the supplier and purchased just the drives very cheap on there own then converted a stepper motor myself using those solid state encoder you can buy (ATM?). I drilled the rear shaft of the armature of the stepper and the just loctite in a 1/4 dowel that stuck out the rear, I then used this to attach the encoder to and then mounted the encoder to the rear plate of the stepper motor. The (ATM sic?) encoder comes with collets to mount the 1/4 shaft and other sizes.

I shocked myself when it worked! Hahaha

[rotec]

Encoder I mounted to the stepper was part number AMT 102 CNC4Pc has them but they are around others too.
I used the line driver with the AMT also CUI-102E-10 CUI Inc. | Sensors, Transducers | DigiKey

[CL_MotoTech]

You have a link for the stepper drivers you are using? I wouldn't mind taking a look.

[Fastest1]

I had seen people adapt the encoders to the back of the steppers before. Sending some form of feedback to the driver. I was under the impression it just ceased motion.

I am still having issues trying to understand if the trajectories would have to change due to a slowing of an axis. Its over my pay grade atm.

[phomann]

Hi,

In almost all cases there is very little advantage in using closed loop stepper drivers over open loop ones.

Consider this situation, you are cutting a part and the X axis stepper gets overloaded.

1. The open loop stepper stalls ruining the job
2. The closed loop stepper lags behind then catches up still ruining the job, but you don't know it's ruined the job.

Unless you have a closed loop motion controller rather than a closed loop drive you are not gaining much and may even end up in a worse place.

With a closed loop motion controller, the encoders go back to the motion controller so that when it sees an axis lag behind, it slows down the feed rate of the other axes so that the cut is still correct. without this you have closed loop drivers with an open loop controller, arguably the worst of both solutions.

When an axis lags behind, then catches up, there is no indication that a problem is occurring and you have no assurance that your part has been cut properly.

Yes, the closed loop will run the steppers cooler as it can vary the current based on the encoder position but that's not a problem that really needs solving. If a stepper starts to lag behind, it's being driven too hard whether it's open or closed loop. A good quality open loop stepper drive will perform as well as your closed loop one when not over-driven. A good quality stepper drive such as a Gecko has proper anti-resonance compensation and morphs to full stepping at higher speeds to increase high speed torque.

If you really want a closed loop stepper system use LinuxCNC. If I recall correctly it can read motor encoders and and adjust the feedrate of the other axes to ensure that synchronisation is not lost when an axis starts to bog down.

BTW, I've never used LinuxCNC

Cheers,

Peter

[phomann]

I had seen people adapt the encoders to the back of the steppers before. Sending some form of feedback to the driver. I was under the impression it just ceased motion.

I am still having issues trying to understand if the trajectories would have to change due to a slowing of an axis. Its over my pay grade atm.

yes, Romaxx have a breakout board that fed the encoders into Mach3. A script (or plugin) monitored the encoders position against the Mach3 commanded position. When a axis lagged too (user definable) far behind, Mach3 would pause the motion, allowing the operator time to decide what to do.


Cheers,

Peter

[merc-blue]

Hey Rotec,
could you empty your PM inbox? just tried messaging you and your inbox is full.

[rotec]

All done! sorry about that.

you can email me direct [email protected]

[rotec]

The servos and steppers I use all have encoders, they are all coordinated, so if one slows or starts to labor the other axis if involved will slow also to maintain coordination. Ive been using CNC for 20 years and all my pro machines are closed loop for good reason.

My first shop made CNC project used steppers open loop, It worked ok but was no where near as good as my closed loop set up. Not even close.

IMO folks want to kid themselves into believing open loop is as good. Its not.

But hey its a FREE world, if you want to build an inferior machine by all means. But dont tell me its better, Ive used both, its not.

[frankleng]

45 degree lock miter router bit
45 degree lock miter router bit for woodworking miter and glue joint-$18.51-Tayoutec CNC

[Fastest1]

45 degree lock miter router bit
45 degree lock miter router bit for woodworking miter and glue joint-$18.51-Tayoutec CNC

Get a life