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stepper motor and mini engine questions
#1
Good evening!

Finished my dolly based on the great work of Airic, thank you very much for doing all this!
I started out with a mini e shield, for the current project I am using a big easy driver and a Nema 17 stepper, rated at 2.55V, 1.7A, 43Ncm. The dolly is based on an Igus rail, the motor is strong enough to just allow for vertical climbs of my Canon 5D3 plus lens.

I noted the following issues and would greatly appreciate any advice.

1. Question
I can adjust the current via the pot on the BED to achieve a smooth movement at lower speeds, but then the stepper looses a lot of torque. When I adjust the current on the BED pot to allow for high torque at low speeds, I am getting quite rough movements. Is this just the performance curve of a typical stepper or can I do something about this? The stepper runs in sixteenth step mode.

2. Question
Generally, it seems like as long as the no. of steps can be processed within the cycle time, I get quite a rough movement of the stepper (when giving it enough current for vertical climbs as described in 1. question). But I noted that as soon as a limit switch was triggered, the mini engine default reverse cycle was very smooth and faster, with high torque. So I increased the no. of steps while maintaining the same cycle time, movement was perfect with high torque, but limit switches were not functional nor could I stop the movement other than via the reset button. Is there any way I can get the smooth movement with high torque while still using the mini engine features like limit switch?

I hope my description is not too confusing and yes, I am pretty new to all of this, thanks for your patience Cool

Cheers from the Swiss Alps
Juerg
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#2
Hello Juerg,

I have one short question - are you talking about the continuous mode? If yes, then the rough movement is caused by the non-parallel processing of the Arduino. This means that everything needs to be done in a big sequence (sequential). This forced me to regularly check how much steps should have been done until the moment of checking versus the real amount of steps done (besides all the other things that need to be done like triggering the camera at the right moment and updating the display). The difference in steps is then executed. This results in a non-perfect stepping pattern which can become quite recognizable with higher speeds and motor torques. The continuous mode was originally designed for very slow set-ups (e.g. several hours in a night shot) where the single steps are widely spaced out.

The limit switch retract-move is so smooth because it is done at once and not integrated into the big software loop (as a simple for-to loop). This makes the stepping much smoother because the steps happen with a well defined frequency.

As a final consequence of this problem I moved to the much faster and more performant Arduino DUE for the miniEngine 2 to be able to get a much smoother stepping for continuous moves (even with 2 motors simultaneously).

If you have the problem in the regular S-M-S mode then I guess the problem is caused buy the very nature of stepper-motors. If the motor current gets high, the steps are executed much more aggressively and thus cause the moves to become less smooth.

Cheers,
Airic
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#3
Thanks Airic for your prompt answer!

Yes, I am talking about continuous mode. I was hoping to be able to use this for both s-m-s timelapse photography and video continuous mode, the latter would have been used much more frequently.

E.g. cycle length 2" and 12'000 steps result in a perfectly smooth continuous movement at full torque, allowing for vertical climbs, but can only be stopped by the reset button or the on/off switch of my system.

Does the miniEngine 2 allow for vibration free, different continuous speeds at maximum torque with my motor?

Thanks for your help!

Cheers
Juerg
Reply
#4
(05-26-2014, 10:40 PM)xlwksc Wrote: Good evening!

Finished my dolly based on the great work of Airic, thank you very much for doing all this!
I started out with a mini e shield, for the current project I am using a big easy driver and a Nema 17 stepper, rated at 2.55V, 1.7A, 43Ncm. The dolly is based on an Igus rail, the motor is strong enough to just allow for vertical climbs of my Canon 5D3 plus lens.

I noted the following issues and would greatly appreciate any advice.

1. Question
I can adjust the current via the pot on the BED to achieve a smooth movement at lower speeds, but then the stepper looses a lot of torque. When I adjust the current on the BED pot to allow for high torque at low speeds, I am getting quite rough movements. Is this just the performance curve of a typical stepper or can I do something about this? The stepper runs in sixteenth step mode.

2. Question
Generally, it seems like as long as the no. of steps can be processed within the cycle time, I get quite a rough movement of the stepper (when giving it enough current for vertical climbs as described in 1. question). But I noted that as soon as a limit switch was triggered, the mini engine default reverse cycle was very smooth and faster, with high torque. So I increased the no. of steps while maintaining the same cycle time, movement was perfect with high torque, but limit switches were not functional nor could I stop the movement other than via the reset button. Is there any way I can get the smooth movement with high torque while still using the mini engine features like limit switch?

I hope my description is not too confusing and yes, I am pretty new to all of this, thanks for your patience Cool

Cheers from the Swiss Alps
Juerg

Hi, All
My motor isn't strong enough to just allow for vertical climbs of my Nikon.I'm used Nema 17 stepper, rated at 12V, 1.7A, 55Ncm.Please post your dolly+control pictures.

Thanks,
Axebuddy
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#5
(05-27-2014, 11:22 AM)xlwksc Wrote: Thanks Airic for your prompt answer!

Yes, I am talking about continuous mode. I was hoping to be able to use this for both s-m-s timelapse photography and video continuous mode, the latter would have been used much more frequently.

E.g. cycle length 2" and 12'000 steps result in a perfectly smooth continuous movement at full torque, allowing for vertical climbs, but can only be stopped by the reset button or the on/off switch of my system.

Does the miniEngine 2 allow for vibration free, different continuous speeds at maximum torque with my motor?

Thanks for your help!

Cheers
Juerg

Hello Juerg,

the reason why the engine is so smooth with your "special setup" is because the theoretical step-count to be done in every loop is bigger then the achievable amount. This causes the step count per software loop to become bigger and bigger. Finally the engine is busy to do steps only so that no time is left for user inputs.

The miniEngine 2 offers much smoother movements as it is able to compute the motor-control-routines about 10 times faster than the original miniEngine. This results in much finer (time-wise) granularity and thus in more regular and better timed stepping. It is still based on stepper motors and will therefore always be based on tiny steps and never be a perfectly smooth move. But one can achieve almost perfect results with a good mechanical transformation of motor-steps to camera-motion. The goal is to change the step-movement-ratio so that the movement per step falls below the the recognizable limit (while lowering the maximum reachable speeds). In the end, stepper motors are the best compromise between cost and move-repeatability (between simple DC motors with the lowest cost and expensive servo motors)

I hope that made everything a bit clearer.

Airic
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#6
Thanks Airic, this made it a lot clearer!

Axebuddy, here are the pics:
   
   

Cheers to all
Juerg
Reply
#7
(05-27-2014, 07:50 PM)xlwksc Wrote: Thanks Airic, this made it a lot clearer!

Axebuddy, here are the pics:



Cheers to all
Juerg

Thanks to all,
Axebuddy
Reply
#8
(05-27-2014, 07:50 PM)xlwksc Wrote: Thanks Airic, this made it a lot clearer!

Axebuddy, here are the pics:



Cheers to all
Juerg

Looks really nice. I am also using NEMA 17 stepper motor but with a 27:1 gearbox attached to it. Also I am using big easy driver just like you are doing. I have some questions if you don't mind. Could you please describe me how did you fit big easy driver on Airic Lenz shield or did you figure out some other way? This is how I am planning to put big easy driver. Put it on the side of shield, and run wires and solder (see pic attached). 

Also, are you using gearbox attached to your stepper motor? Is gearbox necessary, could NEMA 17 without gearbox support the dolly? 

Thank you.


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