macro-rail, micro-processor, nano-steps

Adalbert · Post by **Adalbert** » Fri Mar 04, 2016 11:49 am

Hello everybody.

Yesterday I received the LCD-shield and was able to complete my controller.

So, I have added the controller-slat to my macro-slat and have created the Macro-Station :-)

Main features:
1.) Arduino based controller

2.) Open interface programmed easily in C++

3.) Standalone mode and connected to the PC
4.) LCD and 6 keys for the entering of the data
4.1.) Shutter Speed

4.2.) Time for loading of the flashes

4.3.) Aperture for the calculation of the step-size

4.4.) Start point

4.5.) End point

4.6.) Run stack

5.) Connection to the camera remotely by the IR
6.) Size of the steps calculated on the basis of the aperture
7.) Number of the steps calculated on the basis of the distance between the two selectable points ( and step size detected before )
8.) The gear-box 100:1 enables step-size in nano-meters
8.1.) NEMA 17 step angle of 1.8 degree ==> 200 steps for the 360 degrees
8.2.) TB6560 16 micro steps, 1.4 A
8.3.) Gear-box (99 + 1044/2057): 1 ratio
8.4.) Full rotation 318424 (200 * 16 * 99.5075352455031599) micro-steps
8.5.) Mitutoyo screw: scale 25 positions a 0.0254 mm
8.6.) One full rotation ==> movement of the rail = 0,635 mm (25 * 0.0254)
8.7.) Full rotation of the micrometer screw shifts the carriage 635 micro-meters.

At the moment I’m implementing a photo-sensor for the checking if the flashes have fired.
If not then the stacking-session will be stopped.

BR, Adi

ChrisR · Post by **ChrisR** » Sun Mar 06, 2016 3:49 am

Neat piece of work Adi.

How much play/backlash is there in the gearbox?

Adalbert · Post by **Adalbert** » Sun Mar 06, 2016 4:19 am

Hello Chris,
Spontaneously I would say there is no backlash in this solution :-)
This gearbox has a capillary drive, finely graduated and the step-motor holds the position (the sluggish micrometer-screw too).

So, the backlash is not really measurable.
BR, Adi

rjlittlefield · Post by **rjlittlefield** » Sun Mar 06, 2016 11:08 am

Adalbert wrote:This gearbox has a capillary drive

What is a "capillary drive"?

--Rik

Adalbert · Post by **Adalbert** » Sun Mar 06, 2016 1:10 pm

Hi Rik,
This planetary gear-box contains very precise gear-wheels as in a Swiss watch :-)
BR, Adi

rjlittlefield · Post by **rjlittlefield** » Sun Mar 06, 2016 1:45 pm

Adalbert wrote:This planetary gear-box contains very precise gear-wheels as in a Swiss watch

Thanks for the clarification. It will be remarkable if those are actually free of backlash. The unit that I have is spec'd as<=1° . That's for a 14:1 system, where 1° corresponds to about 7 full steps of the 200 steps-per-rotation motor.

So, the backlash is not really measurable.

It would be very interesting to see a plot of carriage movement versus motor steps as the rail is first driven forward and then reversed and driven backward. My guess is that backlash would be easily apparent, and much more than you expect. As a matter of practice this doesn't have much effect except for requiring pre-run when setting limits and starting the stack, but I think it's always good to know what the hardware is really doing.

--Rik

BugEZ · Post by **BugEZ** » Sun Mar 06, 2016 4:31 pm

I usually think of backlash as the space between meshed gear teeth for an individual mesh. A planetary gear drive normally has several gears in mesh, some in series, some in parallel depending on the design. For a planetary gearbox the effective backlash is the sum of the backlash of each mesh with the mesh ratio properly applied. A backlash check of a planetary gear set would be to hold one end fixed (input or output) then to measure the rotation of the opposite end. When the reduction ratio is large the backlash is likely to be larger on the input side than on the output side.

Some backlash is usually desireable in high torque gearboxes to accomidate load deflections or thermal deflections. Backlash also allows interchangeable parts with normal production tolerances. Highly accurate instrumentation gearing that sees light loading can be designed with minimal backlash, and occasionally light interference.

The effective backlash can also be reduced by providing a large bias load (I have seen bungee cords and weights used by others on this forum) so that gear teeth and actuator lead screws are loaded on the same side and do not shift through the backlash.

On my friction belt driven rig I have little backlash, but I have lots of hysteresis. The hysteresis results from the tension needed in the rubber band "belts" to get the pulleys to move. I counter this in my stepper control by deliberately pushing the actuator ~50 steps farther than the starting point then pulling it to the position for the first shot. I then pull it through each successive shot till I reach the hast one. Many stacking rigs do something similar I believe.

You may discover a bit of hysteresis in your rig also. Your build looks very clean and should work well!

Keith

rjlittlefield · Post by **rjlittlefield** » Sun Mar 06, 2016 5:24 pm

BugEZ wrote:I usually think of backlash as the space between meshed gear teeth for an individual mesh. A planetary gear drive normally has several gears in mesh, some in series, some in parallel depending on the design. For a planetary gearbox the effective backlash is the sum of the backlash of each mesh with the mesh ratio properly applied. A backlash check of a planetary gear set would be to hold one end fixed (input or output) then to measure the rotation of the opposite end.

Agreed. And with the unit that I have in hand, when I clamp the motor+gear assembly and turn the gear output shaft, there is approximately 0.1" of free movement at the end of a 6" lever arm. Unless I've blown the calculation, that amounts to something between 0.9 and 1.0 degrees of rotation, so right on spec.

In Adalbert's system, whatever angular backlash there is in the gearing will be multiplied by the screw pitch. At 0.635 mm per turn (635 micro-meters), 1 degree backlash would be only about 1.7 microns at the screw. On the other end, with 100:1 gearing and 200 steps per rotation, the motor would see something like 55 full steps of dead rotation.

--Rik

Adalbert · Post by **Adalbert** » Mon Mar 07, 2016 3:40 am

Hello Rik,
According to the description of my gear-box it has a “backlash at no-load <=1 ° ”
Probably it is a property of the motor because you have the same value.
So, I only can observe some backlash during the changing of the direction of the rotation ( I can hear it :-)
If the rail is driven in one direction I cannot notice any backlash ( probably because of the hard-steering micrometer-screw ).
What does “pre-run when setting limits” mean?
I only set the first point then the second point and finally run the stack from the second point to the first point.
BR, Adi

ChrisR · Post by **ChrisR** » Mon Mar 07, 2016 5:01 am

I only can observe some backlash during the changing of the direction of the rotation ( I can hear it :-)

That's the (undesirable) parameter I was originally thinking of. Perhaps there is a difference in interpretaion of backlash/play/free movement...?

I think "the point" is that microscope focus blocks, don't effectively have any backlash. The statement is valid though, when they're pre-tensioned by gravity or a spring. I have a simple adjuster which has about a 3.3:1 planetary gear reduction. With that, tension on the output is maintained through the gears in a way that a spring would hold them all in tension. Perhaps your gearbox is two of those in series?
If you have enough sets in series, eg Here:
https://www.youtube.com/watch?v=MUH0MGP7AGc
then I doubt the tension would be transmitted back to the motor.

Even if there's no gap between the gears, I'd still expect there still to be a measurable elasticity, depending on the force and number of gears, on change of direction.

Adalbert · Post by **Adalbert** » Mon Mar 07, 2016 7:18 am

Hello Keith,
Many thanks for the info!
In my case is very difficult to measure something because of the big translation.
Maybe you can find some information on the following site http://www.omc-stepperonline.com/
BR, Adi

Adalbert · Post by **Adalbert** » Mon Mar 07, 2016 7:29 am

Hello Chris,

“Even if there's no gap between the gears, I'd still expect there still to be a measurable elasticity, depending on the force and number of gears, on change of direction.”

Yes, for sure but it is too small for me. I don’t have the equipment in order to measure something in NANO-meters

BR, Adi

ChrisR · Post by **ChrisR** » Mon Mar 07, 2016 7:51 am

Sure Adi, I thought someone would say it though!

How small a step do you need?
The smallest movement I can work out I might ever need is something like 0.25 micron (For NA 1.4)

Adalbert · Post by **Adalbert** » Mon Mar 07, 2016 8:05 am

Hello Chris,
The smallest step of my rail is about 2nm and good enough for me :-)
My current gear-box is 100:1

BTW, for the macro-photography I will use the gear-box 5:1.
BR, Adi

ChrisR · Post by **ChrisR** » Mon Mar 07, 2016 8:27 am

Do you have an electron microscope?