Following error monitoring on stepper machines

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20 Jan 2017 18:35 #86269 by 109jb
I have searched the forum and can't seem to find what I am looking for. Probably using the wrong search terms or something because I don't think my idea is in any way revolutionary or has not been done or tried before.

I have a 3 axis CNC milling machine that I converted using open loop steppers for the axis drives and it works very well. However, I am wondering is it possible on a stepper driven machine using the parallel port to set up following error detection that could halt the machine. I am not looking for on they fly correction of a following error essentially closing the loop, but just detection and then a machine halt. I realize that with a standard stepper system that if a following error happens that steps have been lost and that the maximum available torque has been exceeded so you can't really get back the lost steps with a standard stepper system.

So, I am not looking to make a closed loop stepper system, or change to a closed loop servo or stepper system. I envision the stepper still operating in open loop and I only want to detect if a following error/missed steps has happened.

For example, lets say I have a part program that pushes the machine beyond what the steppers can handle. What I would like is to detect the following error with a rotary encoder mounted on the stepper and if detected, just stop all operation to prevent something bad happening.

My main concern is not saving a part, but preventing the following error from resulting in a crash that damages the machine itself.

Thanks in advance.

John Brannen

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20 Jan 2017 19:00 #86272 by tommylight
Steppers are open loop, so there is no feedback from them at all, hence no way to detect missed steps.
It can be done using encoders on the stepper shafts, and that is a cost that can not be justified easily.
With steppers you have to set the machine as follows: find the max feed and acceleration it can do without missing steps and back off at least 20-30% from those values, all this under load.
If using encoders is the choice, it can be done several ways, depending on your setup. If using parallel port, you can wire the encoders directly to paraport and edit the hal file accordingly. If using a dedicated board ( Mesa 7i76 most probably ) you can also add a dedicated encoder board, that can read encoders much much faster.
There are also drives that do steppers with encoders and have also a bit of control I.E. pushing more current when a missed step is detected, but the price comes very near servo systems, so why bother.
Regards.

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20 Jan 2017 19:07 #86273 by 109jb
Thank you for the response.

I realize that I could go with servos or closed loop stepper systems, and mentioned them in my post, but do not want to do this for various reasons, one being cost.

The encoders are not costly, and my steppers are dual shaft with a stub shaft sticking out the back end, so mounting encoders is a not a problem.

What I am looking for is some help on how this would wire up to the parallel port and how the HAL file would need to be modified to accomplish it.

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20 Jan 2017 19:21 #86275 by tommylight
Well in that case you will need :
encoders rated for 5V, and not to much CPR as the paraport can not read fast, but might just be enough for you case
a usb cable that needs one end to be cut, used for supplying 5V to encoders, can use a separate 5V source, but i would not recommend due to possibility of different potentials.
A lot of hal editing to change the feedback to the motion controller.
Buckle up, it's going to get bumpy.

Attach your actual files, if you have them just to get a sense of a starting point, pictures of the machine just in case, more info about drives, bob if any etc etc.

As i mentioned above, i do not use steppers with encoders so i do not have a config for it, but with a bit of research and a lot of reading of this forum, that is quite easy to manage.

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20 Jan 2017 19:31 #86277 by 109jb
Thank you. It may be a few days before I can get all that together as I have to got out of town for work. I will post those things when I can. I do want to try this and if successful put together kind of a guide to what is done for others who may want to go down the same road. I will probably start with just the X axis and go from there.

thanks again.

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20 Jan 2017 19:38 #86279 by tommylight

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20 Jan 2017 20:10 #86283 by 109jb
As a start, the link below is to one encoder option that I am looking at. It is advertised as 5V-24V, so hoping it works well on 5V. It is 600 pulses per rev, which is enough for the following error detection I desire. At 600 P/R, the encoders should be good for about 0.0003" resolution but I only want to check for following errors greater than about 0.002" to 0.004". My mill has 200 step/rev steppers and I run at 1/8 microstepping, and each motor revolution equals an axis movement of 0.200"

Given the above, does the encoder P/R need to equal the microstepping, which would be 1600 /rev? I wouldn't think so, but if it does I need to look further for an encoder.

The mill is a typical benchtop mill and is a Grizzly G0704 model. Below is a picture of the mill I found on my PC from when I was building it. The picture shows the motor attachment which is typical and the extended shaft can be seen.

As for my "files", what will you need? The ini file for my LinuxCNC setup?

Thanks again

www.aliexpress.com/item/1pc-New-Encoder-...ing/32669741048.html

Attachments:

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20 Jan 2017 20:11 #86284 by 109jb
Thanks again. Those links will give me something to chew on while cooling my heels in the hotel room.

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20 Jan 2017 20:17 #86285 by 109jb
Wow. Great assistance. A quick look at those 2 links and I think I may be able to muddle through this.

Thank you very much for the great and fast support.

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20 Jan 2017 20:21 #86286 by tommylight
You are welcomed, always.

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