Good afternoon everyone!
My name is Hector and im new in this forum, nice to meet you!
Im currently facing a mechanical problem with a project, and thought maybe someone in the Robotic forum section has faced this before and could give me some advice

I built a mechanism for a project, and designed an engine connecting rod that connects my mechanism to a Nema 23 stepper motor.

Long story short, my engine connecting rod and motor (that are constantly in a 360 degree rotary movement loop) stops / stalls for a micro instance, when it reaches this specific angle:


What seems to happen is that when the connecting rod is in this specific angle, thats the moment when then connecting rod begins to "raise up" all the mechanism weight upwards (around 1,2kg of weight) , and therefore that's when the motor has to do the most amount of force/ torque.


On the contrary, a second problems appears when the connecting rod hits this exact angle:




This angle seems to be the exact point where all the weight of the mechanism that previously was lifted up, begins to fall down together with gravity, making the steppermotor loose steps / jump steps.

Here's a couple videos i recorded in different camera perspectives, to understand better everything i explained

https://www.youtube.com/shorts/3j9Ynp7V-oM
https://youtube.com/shorts/1JO0D7Z94GA


I am currently using a Nema 23 Bipolar 3Nm (425oz.in) 4.2A, the DM542T Digital Stepper driver, a 36V - 10A PowerSupply and an Arduino One to program my stuff. Everything is connected correctly

Before using this motor, I was using a Nema17 with a gearbox that gave me 10Nm torque, and had the exact same problem going on.
I thought that the <1 Backlash degree from my gearbox was causing all of this this, so decided to move to a Nema23 without gearbox but with the necessary torque.
I thought that with the highest microstepping resolution on this Nema23, i would reach 0 motor Backlash and therefore fix the problem, but i was so wrong; Looks like motor Backlash has nothing to do with this problem..?

Would really appreciate if someone who has ever faced something like this could give me some advice on what to do to fix it.
Thanks so much for taking the time to read this, really appreciate

Hector.

 

Welcome to AAC.

The issue with microstepping is that you have less torque than at the major steps. What level of microstepping are you using? Do you get the same problem with less steps?

 

Have you considered using a counterweight or return spring to overcome this problem?

 

idk, It's got to be a mechanical problem. It looks like your motor might be loose at the spline, but I'm sure you already checked that.

 

Are your rod ends bearing’d onto the pivots. The spherical bushings won’t perform as radial bearings. If you plotted the torque over degrees of rotation, what would that look like.
Loosen the lock nuts on the tie rods to see if the action persists

 

thanks so much everyone! really appreciate

 

I thik that the controller is braking the motor during the descending cycle because the gravity. When he start lifting the controller is "realizing" that he need to "work". To confirm my theory you can turn 90 or 180 degree your motor and you will no see difference.
I mean rotate whole motor casing. I think you need more torque. Other solutions may be that you can integrate an torque sensor on your project.
Counterweight @KeithWalker is good solutions, inthis way you engine will do always the same work. No more variation of rotation speed.

 

I thik that the controller is braking the motor during the descending cycle because the gravity. When he start lifting the controller is "realizing" that he need to "work". To confirm my theory you can turn 90 or 180 degree your motor and you will no see difference.
I mean rotate whole motor casing. I think you need more torque. Other solutions may be that you can integrate an torque sensor on your project.
Counterweight @KeithWalker is good solutions, inthis way you engine will do always the same work. No more variation of rotation speed.

Hi, thanks so much for your reply! really appreciate
Yep, i tried turning 90 or 180 degree's the motor and i see no difference: same problem going on in the same exact angles

I will try buying a motor with higher torque then, since adding a counterweight is much more complicated.
Will let you know if a higher torque motor fixes this

thanks so much

 

You can go to car demolition company and buy one electric power steering, even broken. Then replay the motor with your own. Whenever you need to much precision you can chore a electric power steering with steel ball recirculation. This kind is used on some Ford, BMW car.
In the picture the eps of citroen c3
https://www.varaosahaku.fi/image/900/a/85/688617/path/mnt/newftp/800/fibdb/bild/lgnr/97516(1).jpg