Closed Loop Stepper motor chatter at low speed causes?

[WobblyHand]

Second response from Stepper Online China wasn't encouraging either. Claimed it was not possible to change micro step settings via this port. The answer was in not well formed English, so perhaps it is true, or there's a communication problem. Or as customer service they are trained to up sell to a unit that does offer the ability. In this case there was no up sell, just a comment, to paraphrase, "unit can't do that". They never told me pin out or voltage levels. Just feels like a language issue.

They ended with "if you have any questions, please ask". I did, but didn't get all the answers, nevermind the ones I wanted to hear. Don't think this is a company that you go to that caters to hobbyist requests. Probably need to spend the big bucks at a domestic firm to get support. At least they will tell you point blank that what you want can't be done that way, but here's a different way to do it.

Let's say this approach is looking less likely. I could deadbug something and try it just for the heck of it. Or at least connect the port pins to the oscilloscope for a quick look. Don't think 4 TPI is worth the hassle of engineering a good solution at the moment on a G0602/G0752Z. May change my mind later, should the need arise.

 

[ttabbal]

That's too bad. It would be nice if these places would be willing to give out info, but many places seem to think it's a competitive advantage or something. I don't see how, it's not like there's some mystery in driving steppers.

On the other hand, I don't think I've ever wanted a 4 TPI thread. So I guess I wouldn't feel like I was losing anything. Of course, I can't speak for you or anyone else. And at least you know you CAN get it if you need it, it's just a pain. One possibility might be to use the gearbox on the machine if you have one. My PM1127 has a 3 speed selector. If you have an option for x2 or /2 you might be able to switch it and set the ELS for the multiple of what you want. The downside is that I would forget I did that and spend hours wondering why the next thread is so wrong...

 

[WobblyHand]

The downside is that I would forget I did that and spend hours wondering why the next thread is so wrong...

This ^^^

I would forget the settings as well. If I had sensor feedback to my controller with the gear box settings, it would be ok. But I forget my name in the morning, so an open loop design is not good for me. Basically, I am limited to 6 TPI and higher. That's not terrible.

Another way of solving the problem is to increase the encoder resolution a little. 1500P/R will give me 4 TPI, and 6mm. I have 1024P/R. 1024 will do 5 TPI and higher. (5mm pitch and lower)

The eBay purveyors of these encoders usually don't know what they are selling. Or they know they are selling 2nds, but aren't telling you. I am looking at a 1500P/R encoder, but the picture is unclear, and I can't tell what I am purchasing. The listing is for 1500P/R, but the picture, and the product could be 50P/R, the picture is that out of focus.

The "1500P/R" encoder costs $44. May be inauthentic, a kind way of saying counterfeit. Is 4 TPI and 6mm worth that? Not at the moment. I was looking for a cheap way to get a little more range in threading. Haven't found that cheap way yet, so I will stay put for now.

 

[jwmelvin]

If I had sensor feedback to my controller with the gear box settings, it would be ok.

Seems like this approach would be about as straightforward as trying to dynamically change the microstepping. Also, your system depends on the gearbox being in a certain setting, so confirmation of that setting would be nice. Once you sense one gear, sensing another is pretty trivial.

 

[WobblyHand]

Seems like this approach would be about as straightforward as trying to dynamically change the microstepping. Also, your system depends on the gearbox being in a certain setting, so confirmation of that setting would be nice. Once you sense one gear, sensing another is pretty trivial.

Exactly. It also assumes that the gear box actually has the correct ratio! I haven't even checked if the gear box has favorable ratios for this.

Doesn't seem like it is worth the effort for a hobbyist on a 10 x 22 lathe. Lots of effort for a minimal benefit. Was trying to see if adding capability was practical for me to do. Don't see that it makes sense right now. My ELS is supposed to be a simple add on that gives some improvements over the traditional geared way. A base G0602 won't do less than 8 TPI, at least with the provided gearing and QCGB. I can actually do 5 TPI and up electronically, without touching a gear box or touching gears, so that is not too bad.

Due to screen size limitations, (320x240 pixel display) I can only fit 25 pitches on a screen at a time. Could go denser, but then it becomes very difficult for a finger press to select the correct pitch. The fatter your finger, the harder it would be to select a pitch. Would not want to rely on the use of a stylus. I haven't implemented a secondary screen for pitches, just simply because its work. The 25 values I chose cover everything that I have ever run into. However, there's no reason that the table couldn't be edited to replace a value.

I suppose if I worked at it, I could implement a user input thread. The problem with that is there are a lot of checks you need to do to avoid non-integer values. For the moment, I have implemented Bresenham using integer math. It is fast, just the thing you need to do in an interrupt service routine. Going to float would slow things down a bit. At 2000 RPM with my current encoder, there is 7.3us per interrupt. (136kHz) I rarely have even operated the lathe that fast. Usually 1/2 that speed and down near 100-300 for threading.

The custom thread algorithm would run when the spindle is off, or we are no longer tracking the spindle. (I can disable the encoder) The algorithm would have to do all those non-integer value tests. A lot of the fudges and workarounds are really easy to see as a human, but tough to implement with a machine, because they have no common sense, they only do what we tell them to do.

 

[jwmelvin]

I agree with your approach. Sure, it might be nice to develop a way to enter custom threads, but it's not a first-order requirement. Also - for sure the super coarse threads are a fringe use case. And one might arrive there through a gearing change even without changes to the software. I can imagine that super coarse threads might be fun for using live tooling to create a decorative helix of sorts, maybe in wood etc.

 

[WobblyHand]

I agree with your approach. Sure, it might be nice to develop a way to enter custom threads, but it's not a first-order requirement. Also - for sure the super coarse threads are a fringe use case. And one might arrive there through a gearing change even without changes to the software. I can imagine that super coarse threads might be fun for using live tooling to create a decorative helix of sorts, maybe in wood etc.

I did come up with a way to get coarser threads. I need to try it tonight. For some reason, I stuck with thinking 8 microsteps was the right number.

There is no reason why I cannot use 4 instead all the time I will see if it is noisier. Using 4 instead of 8 micro steps should also provide more holding torque, nearly double that of 8. If it works, then I can get to 10mm threads or down to 2.5 TPI. Will I ever try 2.5 TPI? Only for giggles.

My PEC pitch gauge is from 4-84, TPI. I will try 4 TPI, then 20, then 40 at 4 micro-step setting. Have a program that calculates the tables for me. Think I have a P4 blade. 4 TPI is on 2.5" diameter screws. That's quite a big screw. Not sure I want to waste stock on that size...