metallurgical objective vs biological objective

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rjlittlefield
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Post by rjlittlefield »

mjkzz, I admire your courage in exposing yourself to the level of critique that your effort is promising to attract.

Here is my contribution to the collective effort of increased understanding. This is your data, but plotted to show the variation in measured step size versus the average step size. The up/down pattern in this graph is identical to that shown in yours. The difference is that in this graph, it is evident that the measured step sizes vary quite a lot with respect to their average value (some of the steps are backward!), while in your graph it is impossible to tell whether 1 standard deviation is trivial, significant, or overwhelming.

Image

In terms of the application, I basically agree with the position of Chris S. It's not a good match to use this sort of translation platform for step sizes in the submicron range. I am a huge fan of the StackShot, but that's because it's an off-the-shelf solution that can work in the range of 2 micron step sizes for no more investment of time than providing a credit card and screwing the parts together, and no more investment in money than that of a good macro lens. It is incredibly efficient from that viewpoint.

That said, it's also true that since I have both a StackShot and an automated focus block, I'm generally not going to use the StackShot above 10X because the focus block is clearly better controlled in that range.

I hope this is helpful in some respect. Thank you for participating in our community!

--Rik

mjkzz
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Post by mjkzz »

Chris, nice metaphor, I understand this. As you said, I am trying to find a better implementation :)

Rik, I think I know what is going on here. The coupler I am using is elastic type, this is why we are seeing some backward movements, big jumps after flat or backward displacement. And all the samples seem "elastic" as well :)

It will be fixed soon.

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Post by rjlittlefield »

I would be interested to hear more about what sort of elastic coupler you are using. They vary over a large range of stiffness.

--Rik

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Post by Bushman.K »

Oh, that probably explains everything...

mjkzz
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Post by mjkzz »

Rik and Bushman, I am so embarrassed now :)

The coupler I am using is the flexible type that tolerates large misalignment but can not handle large load. It is used for connecting an optical rotary encoder in my other project. Optical rotary encoder does not need much torque, so this type of coupler is enough without losing track.

It is similar to this:

http://www.ebay.com/itm/10pcs-lot-CNC-M ... 3a8623f5bc


The knob of XEG60 takes some torque to turn as I can feel it and as it should be. So this type of coupler is not good.

I have ordered rigid coupler as the shafts are very well aligned, so it should not be a problem. Anyways, fingers crossed.

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Post by rjlittlefield »

Thanks for the link. I bought one of those for evaluation and found it quite stiff on the torque axis. I will be interested to hear how much difference the rigid coupler makes.

--Rik

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Post by dolmadis »

Hi

Which type of rigid coupler did you go for please?

URL for example would help me too.

Many thanks.


John

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Post by mjkzz »

Rik, yeah, the rigid coupler removed all negative displacements, so it definitely helps. After some more studying of data, it seems under high microstepping, slipage happens a lot -- displacement is around zero. So I reduced microstep to 1/2, I think I will get a larger motor with higher torque and experiment more.

I am using a 10X objective, so to do sub um measurement, I think I need a 40X objective. For example at one 1/8 microstep is only 0.3125um with 1.8 degree stepper, 10X is only 3.125um, less than pixel width.

John, I am using this type of coupler but with 5mm on motor end and 12mm on the XEG60 end.

http://www.ebay.com/itm/5mm-x-10mm-Rigi ... 2a4be23d89

Here is a picture of two full step (4 half steps) and it is pretty good -- each sample represents 5um. I will try larger motor, 40X objectives, maybe another motor driver, but it will take a while.


Image

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Last edited by mjkzz on Tue Sep 22, 2015 10:08 am, edited 1 time in total.

ChrisR
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Post by ChrisR »

What motor are you using ( which is giving you too little torque while microstepping)?

mjkzz
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Post by mjkzz »

Actually, come to think of it, the displacement errors in the first graph are confined between 0.0025 and 0.0030, if this band represents system error, it will be there when we do one full turn (2 half steps), but the center of band will be lowered to 0.0010 and 0.0015 . . . I will try this to confirm and if confirmed, this means, we should not go lower because the center will be lowered close to zero and data collected will be meaningless.

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Post by mjkzz »

Hi Chris, it is NEMA 14, 25mm in length, rated 0.5A 1.8 degree per step. I think I have a 40mm long one somewhere.

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Post by mjkzz »

Here is two half step result (no cheating, images wrapped around 10000 and the second folder only got 90 images). It seems to confirm what I suspect -- data from one half step will be meaningless.

Image

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0.12039636815139002,3.228991088948807E-4

ChrisR
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Post by ChrisR »

A question - which may tell you I don't quite understand what I'm seeing :
If your problem is related to lack of torque, I would expect to see good consistent measurements every 2 or 4 microsteps, and I don't.

If I thought I had a torque problem, I'd load the system with a brake of some sort to make it slightly worse, and look for that to be reflected in worse results.

If I were looking for a problem coming from roughness in the screw thread, I'd go backwards to repeat the same (say) 16 steps over the same part of the screw, several times, to see if a pattern repeated.

You could try other things, such as a torsion spring to hold the carriage "back" against the motor - as opposed to a plain friction brake?

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Post by rjlittlefield »

mjkzz, what I'm seeing in your graphs makes sense to me.

Remember that your new coupler only makes a rigid connection between the physical shafts of the motor and the drive screw. In the system as a whole, there is still a springy coupling -- the magnetic field -- that links the physical rotor of the stepper motor with the virtual rotor that is defined by the drive currents. You can make that spring stiffer by using larger drive currents and/or a different motor, but no matter what you do, it will remain springy to some extent.

My guess is that it's this remaining springiness that is causing the problems you see now.

But that guess could be wrong, so you might want to run some experiments to check it out. One simple test could be to reduce the drive current to say half what it is now, and see if the problem gets worse.

--Rik

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Post by mjkzz »

Chris, I do not know if problem is related to torque ONLY, I just did some experiments and you and Rik suggested, I reduced current to 0.35A and a lot of zero displacements, so torque is an issue.

Rik, yes, with reduced current, a lot more zero displacement due to slipage. I agree, the whole system is "springy" and this springiness will be there and is inherent to such system.

To conclude, I am happy, now I got TWO motorized sliding stages with known performance (down to 2.5um), though the two XEG60s are used ones, they are pretty good and all of these under 600USD and I had a lot of fun.

For what it is worth, here are some interesting results when I use a 0.9 degree step motor making 2.5um movements, it seems much better than using 1.8 degree motor.

Image

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