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Vibration Test

 
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jin



Joined: 26 Aug 2012
Posts: 88
Location: Singapore

PostPosted: Mon Mar 16, 2020 11:49 pm    Post subject: Vibration Test Reply with quote

Hi All,

What is the accurate and simple way of checking system vibration. My set up consists of:

SLR camera+
extension tube+
objective+
Microscope stand+
stepper motor direct coupled to microscope's focus knob+
StackShot controller driven

Purpose of asking so that i can optimize (minimize) some timings in the StackShot to shorten the whole stacking process.

Cheers!
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Chris S.
Site Admin


Joined: 05 Apr 2009
Posts: 3481
Location: Ohio, USA

PostPosted: Tue Mar 17, 2020 12:26 am    Post subject: Reply with quote

Jin,

I don't know a single accurate and simple test. Perhaps someone else does. I can suggest a number of tests that I would use together.

Are you going to light your images with flash or continuous light? Of course, continuous light is much less tolerant of vibration. One test here would be to shoot the same subject first with short-duration flash in a dark room, and then shoot the same subject with continuous light. If both images are equally sharp, vibration is probably well-controlled enough for continuous light.

Another thing worth looking at is to put on your highest magnification objective, turn on live view (best with an external monitor or tethered to a computer), and zoom the display in as far as you can. Do you see any movement?

It's worth considering vibration as two sets: One is vibrations coming from outside your photo setup, such as floor vibration, sound from furnaces, footsteps, your hand on the table, your heartbeat, etc. The other set is vibrations coming from inside your system, such as vibration from your stepper motor (insignificant, in my experience), vibration from your SLR's mirror coming up (can be eliminated with either mirror up mode or shutter delay mode), and shock from your shutter opening (can be eliminated by using a camera with an effective electronic first curtain shutter such as some Canon bodies, some Nikon bodies, or fully electronic shutter as in some other cameras).

Vibration from outside the system can be mitigated by mounting the system (camera and subject) on something heavy and rigid (I use steel, some use aluminum, some use wood, some use concrete or granite). Then float that heavy base on something like Sorbothane, choosing the proper density, squishiness level, shape, size, and number of Sorbothane feet for your rig. The idea here is to have the Sorbothane feet deformed by squishing about 20 percent under static load. In this state, most outside vibration will have to travel through the feet and get dissipated as heat along the way. Meanwhile, your heavy baseplate should float in serene stillness above.

For mitigating vibration from inside the system, make sure the camera and subject stage are tightly coupled to the base, so they vibrate together. Then use mirror-up, shutter delay, or electronic shutter to minimize internal sources of vibration.

Since I use continuous light and do not have electronic first curtain shutter (let alone fully electronic shutter), I shoot in a dark room with dim continuous light, with a one-second delay between mirror and shutter opening, and long shutter speeds that reduce the effect of shutter shock to an indiscernible level. Timing for this came from testing. I shot stacks a 1/30 sec, 1/20, 1/10, 1/5 etc. through 16 seconds. I found that longer exposures were sharper until I got to eight seconds, at which point no further improvement occurred with increased exposure. So I've used eight-second exposures ever since on my rig.

--Chris S.


Last edited by Chris S. on Tue Mar 17, 2020 3:45 pm; edited 1 time in total
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mawyatt



Joined: 22 Aug 2013
Posts: 2452
Location: Clearwater

PostPosted: Tue Mar 17, 2020 7:56 am    Post subject: Reply with quote

Jin,

To add to what Chris mentioned, some stepper controllers introduce a sudden motor/stage "jerk" with motor starting and stopping. I recall (don't have a Stackshot now) your Stackshot has a parameter that allows acceleration and deceleration control, this was under the Stackshot control from Zerene. This parameter can help smooth out the sudden jerk and is very effective.

If your system is underdamped and has a long time constant this sudden jerk can excite the system resonances and linger for some time, generally called "ringing". If your system is rigid this time constant will be shorter and the "ringing" period is shorter, so it's generally a good idea to try and develop a very rigid setup.

A simple test to see how sensitive your system is, setup with a high magnification and digital zoom as Chris mentioned and tap the setup with your finger. Watch the resultant "ringing" on the external monitor in Live View. Make adjustments/modifications to your system to try an minimize the length of this "ringing". Now tap the area around the setup to see how effective your vibration isolation is, and likewise make adjustments/modifications to minimize the "ringing".

Best,
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ray_parkhurst



Joined: 20 Nov 2010
Posts: 2560
Location: Santa Clara, CA, USA

PostPosted: Tue Mar 17, 2020 8:33 am    Post subject: Reply with quote

My preferred method begins by taking a single shot and viewing it at 100% vs a 100% Live View image. If the single shot is as sharp as the Live View, then shoot the same shot a few times with different timing between shots. If all those are sharp vs 100% Live View, then move the lights closer/farther, forcing a variation in shutter speeds, and take single shots to compare with Live View.

I used this method to dial-in my system with good results up to 50x. At this point my main bugaboo is (believe it or not) airflow in the room. Even a small draft seems to have an oversized impact on movement in the Live View image for some reason, and even as low as 10x. I presume it's due to the continous nature of the air flow. Other vibration sources are generally transitory, and as long as the system has a high resonant frequency and decent damping, these sources simply require sufficient time to settle between shots.

I use a camera with EFSC, and continuous lighting. I suspect folks using non-EFSC and/or flash would do things differently.
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mawyatt



Joined: 22 Aug 2013
Posts: 2452
Location: Clearwater

PostPosted: Tue Mar 17, 2020 9:07 am    Post subject: Reply with quote

Ray,

What you might be experiencing is actually a thermal drift due to the air flow. A quick "hot hair dryer" test can revel just how sensitive these focus rail setups are to slight temperature transients.

Originally, we did this "hot hair dryer" test to check the closed loop piezo stages and then just decided to try the excellent THK KR20 rails. The closed loop kept the piezo stages locked in position (the piezo control voltage was changing dramatically to keep the stage put), but the THK KR20 jumped off the screen in a few seconds!!

Best,
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 20643
Location: Richland, Washington State, USA

PostPosted: Tue Mar 17, 2020 9:53 am    Post subject: Reply with quote

Mike, I've been curious about the system implications of that "hot hair dryer" test.

My guess is that what you heated up in both cases was just the translation mechanism -- the piezo stage and the rail.

It's great, but not surprising, that the closed loop system could monitor and control itself to compensate.

But then what about the rest of the system? In my own setups, when I haul out a tape measure and look carefully, I usually find two or three feet of connecting mechanics between the objective tip and the subject. All of that material is also subject to flexure under mechanical forces or temperature changes. Given all that, I find that I'm a lot less worried about just the translation mechanism than I am the system as a whole.

Is your setup somehow designed to avoid all those other issues? If so, then how? If not, then what happens if you turn that hair dryer on, say, the post that is supporting the camera? Or was that your point, that the whole system is subject to flexure except for the parts under closed loop control?

--Rik
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ray_parkhurst



Joined: 20 Nov 2010
Posts: 2560
Location: Santa Clara, CA, USA

PostPosted: Tue Mar 17, 2020 10:01 am    Post subject: Reply with quote

mawyatt wrote:
Ray,

What you might be experiencing is actually a thermal drift due to the air flow. A quick "hot hair dryer" test can revel just how sensitive these focus rail setups are to slight temperature transients.


I don't think there is any thermal effect going on in my observations. The drafts are just normal air temp, same as the stabilized temp of the rest of the system. Only thing not at temp is the camera itself. Plus the effect is an actual vibration, not a net movement.
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mawyatt



Joined: 22 Aug 2013
Posts: 2452
Location: Clearwater

PostPosted: Tue Mar 17, 2020 12:38 pm    Post subject: Reply with quote

Rik,

My setup is probably typical of most and not specifically designed for thermal management. When I ran the crude test and it was just to see the thermal effects on the Piezo Stage under Closed Loop control, then just decided to compare against the focus rail. I wanted to see how the closed loop system dealt with this condition and how well the feedback sensors and system behaved, quite well indeed!! Never tried other sections of the setup though, just the THK KR20 rail..

Certainly everything associated with the setup is thermally sensitive at these levels, and longer things like the stage mounting bars are very susceptible since they are so long. Of course the heavier bars are better for both rigidity and have more thermal mass, but due to length also more sensitive to temperature change.

This was a severe test with the hot air blowing across the THK rail, suspect that if this was done to other parts of the setup one would expect same result, maybe even more movement.

One interesting possibility of the benefit for the PI stage response as I have it setup (moves the subject) would be where a close localized heat source from illumination sources would only heat up the stage with subject holder, but not the rest of the overall setup. Of course a typical setup where the camera/lens rides on the focus stage and the subject is fixed, the stage is already removed somewhat from the close in illumination sources and you wouldn't expect much thermally induced movement.

Anyway, it seems Ray's problem isn't attributed to thermal transients induced by air currents, something else is at play.

Best,
_________________
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mawyatt



Joined: 22 Aug 2013
Posts: 2452
Location: Clearwater

PostPosted: Tue Mar 17, 2020 1:06 pm    Post subject: Reply with quote

rjlittlefield wrote:
Mike, I've been curious about the system implications of that "hot hair dryer" test.

Or was that your point, that the whole system is subject to flexure except for the parts under closed loop control?

--Rik


Rik,

Forgot to answer this. Yes, the parts under closed loop control don't have the same effects to "outisde" influences as open loop systems. For this to be true under these conditions the feedback sensor in the PI stages must be somewhat temperature insensitive, and the PI stages employ a Strain Gauge Bridge which utilizes 4 strain gauge resistors as a differential voltage divider which has a first order temperature insensitivity because the Bridge output is sensed differentially.

Best,
_________________
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~Mike
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