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Stepper motor stops then speeds/catches up

Hi,

Firstly i would like to say this is my first electronics project so apologies if this doesn’t warrant a post but after quite a few hours of searching I haven’t found an answer.

I have a telescope tracking platform and have just upgraded the motor to a stepper motor for more precision. The motor is 1.68A, 200 step with a 27:1 planetary gear box run buy an Arduino Nano and a Pololu A4988 driver with the current limit set at 1.2A.

I run 1/16 microsteps and the motor runs very slow at 1 step every 11390 micro seconds. The problem I am having is randomly the motor seems to stop for a second and then speeds up to seemingly catch up. for e.g if a star is in dead center of the eyepiece it will suddenly begin to drift out of center for a second then drift back into center stop and track normally. until the processes randomly repeats.

I would like to point out that the error must be extremely small just exaggerated due to the high magnification.

Not sure if there is anything i can do to fix this like increase the current limit on the A4988 to the full 1.68A or is this dangerous for the motor?

I guess another question is, its a pretty standard motor, would a high precision motor designed for low speed and high torque resolve this issue?

Thanks in advance,

Greg

Took advantage of a sleeping baby and a clear night at the same time. Shock haha.

Increased the current limit to 1.5 amps and it seems to have solved the issue thankfully.

Only issue now is the A4988 is only rater for 1 amps without cooling. Running at 1.5A will heat be an issue even when running so slowly? It will be running for acouple hours at a time.

Alternative would be to change to the DRV8825 my only concern is if I am going to be able to desolder the old driver as I’ve never desoldered anything before.

Cheers

Greg

Yes, change the driver (I prefer the MP6500). You need to learn to solder. Sparkfun has a good tutorial, and you can practice on discarded circuit boards from toys, etc. Beware that newer boards use high melting point solder!

Microsteps are not accurate, and for any accuracy at all it is critical that the current limit be properly adjusted, and working properly (which means the driver can’t be allowed to overheat and the power supply also has to be adequate).

Microstepping is generally used for smooth running and to reduce problems with vibration and motor/mount resonance. For an application like yours, most people would use a higher gear reduction and the much more accurate half or full step mode. Then you can use a lower torque, lower current motor as well.

Thanks Jim.

When you say inaccurate does that mean the motor will randomly slow down and speed up like I was observing?

With the premise that full and half steps are much more accurate, does that mean the more microsteps the more inaccurate? E.g is 32 miceosteps more inaccurate than 16 microsteps?

Correct. If the microstepping is not working as intended, as in your case, some steps are skipped entirely, then when the magnetic field increases to the extent that bearing and gear friction, etc. can be overcome, the motor “catches up”. The problem becomes worse with higher microstepping fractions.

Even if everything IS correctly adjusted and properly designed, adjacent microsteps will tend to have different angular increments. For that reason, experienced people do not rely on microstepping for accurate positioning.

Thanks Jim,

While I look into modifying my platform so I can use full or half step geared down I rushed and bought a genuine drv8825 as a quick and easy drop in replacement as I have a bunch of observing planned for this weekend.

Now ofcourse I have just read that the drv8825 has a bad issue with missing steps microstepping at low speed. So I fear i will be making matters worse for myself. Just wondering If this is still an issue with the latest drivers?

If so I may just force fast decay mode straight off the bat.

Alternatively does pololu have any other drivers that can just drop in? I had a look at the driver you mention earlier but that only goes to 1/8 microstep. With current gearing under 1/16 microsteps vibrations/steps are visible in the eyepeice.

Cheers

Greg

Hi, Greg.

There have not been any changes to the DRV8825 chips, so they still have the same issues with low current levels that lead to uneven microstepping. I recommend the TB67S279FTG. It uses a dynamic mixed decay which changes decay mode based on the measured current, so in our experience it offers fairly smooth microstepping.

-Claire