Non linear Steps/mm configuration

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1960FL
Posts: 2
Joined: Tue Jan 05, 2021 10:55 am

Non linear Steps/mm configuration

Post by 1960FL » Tue Jan 05, 2021 1:39 pm

Hello;
I am Rick and a noob to CNC but not equipment, computers, or machining; I am making my foray into CNC and at this point in the learning mode so I purchased an RPi-4, a Protoneer hat, a StepperOnline MW542T driver, and an import Nema23 5mm ball screw dual-rail linear actuator. Goal to test configurations GRBL etc. for future conversion of a surface grinder to 2/3 axis automation.
Here are my steps so far and the eventual issue at hand, please excuse me but I will English measurements even though I have done all the testing in Metric as well since my eventual goal is agnostic. I set up all components, downloaded the latest image from Protoneer site, imaged SD card, and booted Pi; All good. I then wired the motor, driver, power supply; checked dip switch setting, and started to see if something would move? Alas movement but?? Then learned of $100= setting and on we went. Now at this point Driver set to 400 steps (1/2 step) , ball screw is 5mm (I use indicator to check 1rev = 196.7 =/- thousandth) so do the math $100=steps/mm (200/.5)/5 = 80 steps/mm. Now the issue $100=80.00 or even just 80, jog + jog – in MM mode movement is only .025 instead of .0397 ??? check setting WTF?, check ball screw??, check stepper specs??. I dig deeper and know I am doing something wrong download GRBL setting PDF to read and don’t see anything obvious. I convert everything to English and try testing at .100 increments and different step fractions so at my current config I am at 1/8 or 1600 and should have a $100 of about 320.00 but to get the movement of .100 it has to set to 364.350. Not right but will repeat +/- direction .10 - .10 within .0005 multiple times, so you might call it good, so here is the twist, i know it’s not right so I move out +.5 in .1 inc. each step out of .1 I lose about .0007. I have tried to tune around this but then my .1+ <> .1- goes off so obviously even though I can “Fake” it by playing with the $100 something in not linear and just wrong. I have plans for this evening's testing of trying a different stepper motor and if no change then put the whole setup on my Acorn board and see what happens there.
I would like to know obviously what am I missing here in my config/setup (again I am a noob)

Thanks, Rick

Below are the linear actuator specs.
Product description
Size:200mm | Style:SFU1605S
Stepper motor Parameter:
Step angle: 1.8 degree
Motor flange size: 57mm
Motor length: 56mm
Holding torque: 1.2N.m
Current: 3A
Resistance: 0.8 ohm
Inductance: 2.4 mH
Rotor Inertia: 440g.cm
Lead wire: 4pcs
Wiring:
Red: A+
Green: A-
Yellow: B+
Blue: B-
Linear Stage Actuator Parameter:
Effective Travel Length: 200mm
Total length :566mm
Type: linear system module
Ballscrew type: SFU1605 (diameter:16mm, pitch of screw:5mm) Level ballscrew: c7
Weight: 1580g
Package Content:
1pcs Linear Stage Actuator with NEMA23 Stepper Motor
Thanks, Rick

Bertus Kruger
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Posts: 1738
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Location: Wellington , New Zealand
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Re: Non linear Steps/mm configuration

Post by Bertus Kruger » Wed Jan 06, 2021 8:19 pm

1960FL wrote:
Tue Jan 05, 2021 1:39 pm
Then learned of $100= setting and on we went. Now at this point Driver set to 400 steps (1/2 step) , ball screw is 5mm (I use indicator to check 1rev = 196.7 =/- thousandth) so do the math $100=steps/mm (200/.5)/5 = 80 steps/mm. Now the issue $100=80.00 or even just 80, jog + jog – in MM mode movement is only .025 instead of .0397 ??? check setting WTF?, check ball screw??, check stepper specs??. I dig deeper and know I am doing something wrong download GRBL setting PDF to read and don’t see anything obvious. I convert everything to English and try testing at .100 increments and different step fractions so at my current config I am at 1/8 or 1600 and should have a $100 of about 320.00 but to get the movement of .100 it has to set to 364.350. Not right but will repeat +/- direction .10 - .10 within .0005 multiple times, so you might call it good,


so here is the twist, i know it’s not right so I move out +.5 in .1 inc. each step out of .1 I lose about .0007. I have tried to tune around this but then my .1+ <> .1- goes off so obviously even though I can “Fake” it by playing with the $100 something in not linear and just wrong. I have plans for this evening's testing of trying a different stepper motor and if no change then put the whole setup on my Acorn board and see what happens there.
I would like to know obviously what am I missing here in my config/setup (again I am a noob)
My feeling is if you increase the micro stepping you will get more accuracy at the cost of speed.

1/4 or 1/8 micro stepping will increase the number of steps needed per mm/inch.Having $100 in tiny factions will result in inaccuracy because factions will likely be rounded.

1960FL
Posts: 2
Joined: Tue Jan 05, 2021 10:55 am

Re: Non linear Steps/mm configuration

Post by 1960FL » Thu Jan 07, 2021 12:10 am

Bertus; Thank you for the reply and for clearing my first post;

I may not have made my point as clear as I could have I am concerned that I am either consistently missing steps or the board is not communicating with the driver in some manner. 5mm lead screw should move +/- .197 per revolution at 1600 steps that would mean 1mm = 1600/5 or 320 steps to move the one mm (.394th). If I set $100 to 320 it only moves .025 -.027th or .63mm; to get to 1 full mm I must set the $100 to =/- 365 or 45 steps more! something is wrong here and it only gets worse as I increase micro-steps. I have checked this with bCNC and as expected it is same-same.

any thought on where to look? it is currently set to GRBL 1.1
anyone else have any insight?

Rick
Thanks, Rick

Bertus Kruger
Site Admin
Posts: 1738
Joined: Wed Feb 03, 2016 10:26 pm
Location: Wellington , New Zealand
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Re: Non linear Steps/mm configuration

Post by Bertus Kruger » Thu Jan 07, 2021 6:49 pm

I don't have a clear answer for you.

I do suggest:
  • swapping out the stepper driver
    try another axis
    Reduce current in case the stepper driver thermally cuts out.(Loosing steps)

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