Author Topic: [Solved] How Are The "Voltage Levels" Achieved By A Stepper Motor Driver?  (Read 1356 times)

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Offline DarkwingTopic starter

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Hi folks!

I'm currently diving a little bit deeper into stepper motors and controllers and drivers; I read this blog post and saw this video:



and in it there was this graph displayed on a scope:



It kind of "shows the output of the stepper driver DRV8811 to a coil producing the microsteps." The graph is used to display different decay modes, but this doesn't matter.

I'm not exactly sure what I see here. Can you please help:


I understand the "small ripples", this is the current chopping from the driver. But it seems, that this current chopping is always happening on a "base plateau voltage", forming the "big stairs" resulting in this near sine wave shape. Because current follows voltage, you can only "current chop" if you "voltage chop" with some sort of PWM'ed signal. PWM always goes rectangular from 0V to HIGH (or the signal level of your choice). But here, it seems to exist some sort of sine wave generation. A modified sine wave of voltage levels. I looked into the datasheet but found no hint of such a part ...


(http://www.ti.com/lit/ds/symlink/drv8811.pdf)

If it is an oscilloscope it can only measure voltage over time (directly). It can't measure current. So is this graph maybe a calculation from the scope, a resulting graph from other inputs? Hence the unit label "500mA Ω" ...? What was done to achieve this calculated graph in the scope then?


These are my thoughts ...  ???
I actually don't really know how to put my question.  ::)
Can anyone understand and knows what I need to know?   :o
There must be a darn easy explanation ...  ;D


Thanks!
Darkwing   :)
« Last Edit: January 17, 2019, 11:19:46 pm by Darkwing »
 

Offline mvs

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Re: How Are The "Voltage Levels" Achieved By A Stepper Motor Driver?
« Reply #1 on: January 17, 2019, 08:50:45 am »
If it is an oscilloscope it can only measure voltage over time (directly). It can't measure current. So is this graph maybe a calculation from the scope, a resulting graph from other inputs? Hence the unit label "500mA Ω" ...? What was done to achieve this calculated graph in the scope then?
Current can be measured indirectly by using current probe or external shunt. DSOs have usually settings for unit and unit/V ratio of probes to display units and values correctly. DSOs in middle and higher price range may also have data interface to probe and can change this settings automatically.


 

Offline DarkwingTopic starter

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Re: How Are The "Voltage Levels" Achieved By A Stepper Motor Driver?
« Reply #2 on: January 17, 2019, 03:37:53 pm »
This must be it, yes. It is the measurement of a clamp-on current probe, that is available for oscilloscopes. This explains everything.

I noticed (googled) that in the past some guys where asking kind of the same question like above – with no satisfying answer. So I reframed the question to: "how to measure the coil current of a stepper motor", finally resulting in: "how to measure current with an oscilloscope" in general.

This lead me to current probes (expensive), shunt resistors (cheap) and the concept of DIY current probes using a small toroid core with a few turns of wire (clever). Found this forum post here, very helpful.


Well, anyway, thanks!  :)  Solved, I guess.  :-+
« Last Edit: January 17, 2019, 11:20:55 pm by Darkwing »
 


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