Micro stepping Myths and Realities 2015-07-25

Paul - Like Wow! I think you should change you name to Doc. microstep! Great education in those pages.

Thanks
Larry

 

It is said that "there's nothing truly new under the heavens" because mostly folks just rehash known science in a effort to "publish or perish". While I don't agree 100% with that statement, I do know that true discovery IS rare and most science is researchers restating what other researchers have said about other researchers work - - with everything carefully documented, of course. In the 9 years I spent in a university research center I saw this over and over, and got so burnt out over it that left to go into business.

I've seen far too many people awarded Phds for far less scientific research than you offered my friend. Your choice is, of course, your to make So I can only offer my praise to you for relaying data that I needed to solve issues I was struggling with understanding. A simple thanks seems far to little, giving how I've been banging my head against the walls of my shop, but since that is the only choice you will allow me to make - Thank you, Sir!

 

Hi, this is my first post on this site. I'll be ordering parts for a C-Beam Machine plate maker soon. Great site here!!!

Sorry for such a long post. You can skip to Conclusions if you don't want the fine details.

I was looking at this thread and reading over the MICROMO white paper because something didn't seem quite right. The white paper is NOT stating that increasing microstepping will decrease torque. It says the INCREMENTAL torque per microstep will drop off. There's a huge difference.

Torque is approximately proportional to the difference in angle the at rest position (the place the shaft would go to if there were zero load) and the actual position - because of the counter torque from the load. Think of it this way...

If you applied a fixed current to the coils (not change the step position) and apply zero load then it produces zero torque. It's not trying to move the shaft since it's where it belongs.

At the other extreme you could put a handle on the shaft and manually force it to slip if you applied enough torque. This is the holding torque limit and is dependent on the motor itself and the current you applied to it.

If you apply a force that's less than this limit then you will move the shaft very slightly from it's resting position. The amount of the displacement angle is roughly proportional to the applied external torque.

Instead of trying to manually move the shaft let's let the shaft freely move to it's resting position then lock it down so it can't move. We'll put a force meter on the end of the handle to measure the torque. The shaft can't move but we can measure the torque.

Now we make 1 microstep. This will cause the torque meter to move from zero. If we continue to microstep the torque will increase. Naturally we can't microstep it too much or it will try to snap to the next/previous indent since we're not letting the shaft move.

This is the incremental torque the white paper is referring to. It's not the same as the holding torque that we all refer to. The holding torque is the amount of torque you can applied before the motor slips sync. Naturally the incremental torque drops as you increase the microstepping. However the holding torque remains the same.

The motor itself is not perfect. If you were to apply 2 perfect sine wave current sources (90 degrees out of phase with each other) to each a coil, the shaft angle would not follow exactly the instantaneous phase of the applied current. That's what the "Torque Vs. shaft position" plot is trying to show.

The main point of the white paper is that the accuracy does not track with the increase in microstep resolution. For example let's say you have an 8 mm pitch and you are using quarter steps you have a total of 200*4=800 steps. The linear resolution is 8/800=0.01 mm. If you decide to go to 32 microsteps you would have 200*32=6400 steps or 8/6400=0.00125 mm resolution. But the reality is that since the motor doesn't follow the perfect sinewave, and because of detent torque you will not see the system move 0.00125 for each step. For some steps it won't move any, then after sending enough steps it will catch up. If you apply the full 6400 steps the system will move 8 mm. However if you were to accurately measure the position after each of the steps and plot the result on a graph it would not be an ideal straight line. There would be points where there's little or no movement and points where there's a jump to "catch up".

However microstepping with 32 vs 4 cannot decrease the overall accuracy. The system's accuracy just won't get better by that factor of 8 you were hoping for.

I have done zero load tests with disc containing a micro image pattern on the end of the shaft, and a microscope positioned to observe the real step change. I used a TI DRV8711EVM as a test bed configuring it as high as 256 microsteps.

The conclusion is that increasing microstepping after a certain point (dependent on the motor) doesn't gain you any accuracy or usable resolution. But is doesn't make it any worse either. This is consistent with what the paper says in its summary:

"In summary, although Microstepping gives the designer more resolution, improved accuracy is not realized."

The torque remains the same.

Conclusions

So you can use high microstepping with no fear of loss of torque. Just don't expect the useable resolution to improve at the same rate as the increase in steps.

If you read the rest of the white paper's summary you will realize that increased microstepping is generally a good thing:

"Reduction in mechanical and electromagnetically induced noise is, however, a real benefit. The mechanical transmission of torque will also be much gentler as will a reduction in resonance problems. This gives better confidence in maintaining synchronization of the open loop system and less wear and tear on the mechanical
transmission system."

Do not misinterpret "torque will also be much gentler" as there will be less torque. What this is referring to is the applied torque will have finer steps, hence less abrupt changes. It will be smoother. The total holding torque does not change.

Cheers,
Bob