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Member postings for Charles Shearer 1

Here is a list of all the postings Charles Shearer 1 has made in our forums. Click on a thread name to jump to the thread.

Thread: Fibre Optics / Microscope objectives
23/04/2019 12:22:53

I'm looking to buy or make a flexible cable which I can fit to my flashguns to make a concentrated and 'pointable' output. Not wishing to reinvent the wheel but is there such a device already? if not any ideas on construction - I was thinking of a microphone gooseneck as the outer shell.

I'm also looking for a source of second user microscope objectives - again any ideas?

Best

Charles

Thread: Backlash
21/04/2019 17:05:42

platform.jpgFinished tweaking code and recalibrated stepsizes / backlash after higher precision calculations after switch to Arduino DUE. Now getting < than 0.05 backlash across all microsteps in 10mm travel.

Happy to work with this until I find that absolute encoder !

Couple of pics - lazy - just used compact camera so poor in quality. One of the controller screens and the linear platform.

Thanks to all for suggestions

Best Charlescontroller.jpg

21/04/2019 15:02:28

Hi,

It's being tested as we speak after some final tweaks to the code (until v 1.1). From a couple of runs so far - bearing in mind my DTI's have divisions of 0.01 mm (which I believe is pretty cheap and cheerful) - I can see slight errors now.

The microcontroller is still unboxed (3dPrint project next) so it looks untidy) - but photos will be nevertheless be forthcoming asap.

Best

Charles

Round 2 testing:

1) Full steps i.e. 0.005mm out 2000 steps DTI 10.00 mm Back 2000 steps DTI 0.00

2) Half steps i.e. 0.0025mm out 4000 steps DTI 10.015mm Back 4000 steps DTI 0.015 mm

3) Quarter steps i.e. 0.00125mm out 8000 steps DTI 10.015 mm Back 8000 steps DTI 0.015 mm

4) Eighth steps i.e. 0.000625mm out 16000 steps DTI 10.015mm Back 8000 steps DTI 0.045mm ???? Re-test

5) Sixteenth steps i.e. 0.0003125 out 32000 steps DTI 10.015mm Back 32000 steps DTI 0.05 ?? Re-test

As you say the processing software doesn't care about where the the plane of focus was - just extracts the sharpest detail from each image in the stack - but, the important point is that if you miss a plane of focus you spoil the overall image by having even a tiny element out of focus. As an example, at a modest magnification i.e. 5:1 the depth of field i.e. the depth of the plane of focus at, f2,8 on a full frame sensor, is a mere 0.0030mm deep (reduced by 20% to allow for image overlap). So it is important that there are steps between shutters of no more than that amount .

Update - I know why the 1/8th and 1/16 are out - mathematical precision of the microcontroller - fixing it!

Charles

21/04/2019 11:53:55

Hi John,

Oh no! I feel project creep kicking in here. If I understand you correctly I need to physically add said encoder to the linear rail?

Much as I'd like to declare version 1.0 - this intrigues me, I'll have to look into this.

Can they actually have the resolution to detect the microstep sizes I'm using?

Any further detail appreciated.

Thanks

Charles

21/04/2019 11:11:33

Hi,

Understood - but from initial testing I appear to be very lucky.

That being said - because of the nature of the work I am putting it to the distances travelled are very small. A total run of 10mm is a long run. Vibration at high magnification is a serious problem - so the speed of travel i.e. the speed between steps is set by variable and during the run is currently 200 milliseconds between steps with a 2 second delay before any shutter actuation to ensure any vibrations have subsided. This combined with flash recharge delays makes for a lengthy run time given there'll be 200-300 images each run. The rare microstep loss shouldn't be an issue - at a stepsize of 0.0003125mm I don't even have the hardware to measure it.

During resets, returns to 'home' or 'in points I ramp the speed up.

I've finally finished the code and I'm testing with temporarily attached DTI's - will know soon enough if things are indeed reliable.

Best

Charles

20/04/2019 23:26:51

Michael

Don't suppose you've got a spare one going cheap!!!!!

Chas

20/04/2019 22:59:01

Michael,

Understood.

You are of course correct that there is no absolute necessity to find the exact IN and OUT points - but when at 20x magnification even being slightly out can result in many wasted shutter actuations - and wasted images!

With some runs in the hundreds of images for stacking and a touch of OCD I'm just seeking to make things as accurate and as automated as I can within the mechanical limitations of the linear rails / stepper.

Thanks again

Charles

20/04/2019 22:35:01

Good Evening to all - and thanks for the responses. I had no idea the forum was so active!

To clarify - I am very aware of the commercial offerings for macro rails - the problem with them is the controllers are pathetic in the functionality they offer in that they expect you to do all of the Depth of Field calculations, efStops, magnification factors, CoC etc etc etc and then input the distance to travel.

I've written the code that allows you to input numerous variables via a 3.5" touchscreen tft panel - such as size of sensor, leadscrew pitch, stepper angle, microstepping, fstop used, acceleration, deceleration, flash recharge times. The microcontroller now does all the calculations such as steps between shutter actuations, effective fStop etc etc and controls the stepper movements, fires the flashes, lights etc.

I'm delighted with the accuracy of the movements, particularly when microstepping - no 'jitter' and amazing accuracy and repeatability.

To set up a 'run' I need to find the exact IN spot for the start of a run and the exact OUT point for the end of the run. This usually involves some forward / backwards movements while I study the target images and adjust the points. It is here that I'm coding in precise measurements for backlash so that when I set the aforementioned points the precise and actual microsteps for the IN and IN to OUT is calculated - even 'unused' backlash not taken up is a factor.

A straight run in one direction is achieved after the run set-up and all I need to worry about is the backlash when returning to IN if I want a repeat run.

In any event - Silly Old Duffer confirmed my over-thinking! Shame you adopted that nickname - it'd suit me nicely!

Best to all.

Charles

20/04/2019 15:28:29

I'd really appreciate some help with my ageing brain - what I thought was simple is causing me a lot of confusion!

I'm designing a microcontroller for an extreme macro platform using a linear rail driven by a stepper motor. I have the rail finished and the code for the microcontroller is virtually complete.

However, during early testing I found that the linear rail did not always come back to the exact same spot (I'm working in fractions of a millimetre).

The answer was obvious - backlash on the leadscrew - which I measured with a DTI as 0.065 mm. In 'full' steps ( I can microstep to 1/16th of this) the backlash is measured as 13 steps.

To describe a typical situation.

I start with all backlash taken up and move in the Forward direction.

As an example let us say 500 steps Forward.

I've overshot so I step back 10 steps in Reverse.

Still not right so I move 20 steps Forward.

Finally, I move 5 steps back.

Please ignore the obvious fact that the linear hasn't physically moved if less than 13 steps - the figures are for calculations only.

My question is how many 'real' steps Forward have I taken and do I have to cater for any 'unused' backlash at the end of my travels?

Many thanks in advance

Charles

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