Nozzle pitch

The main reason I went to NEMA14 for the  Z axis was the need to pack the nozzles in tighter. NEMA14 body is 35mm versus ’17 which is ~43.

Ideally my nozzles will be the same pitch as the tape feeders, or the same pitch as the strip feeder tape component pitch- IE 4mm or 8mm multiples. This way multiple nozzles can do a simultaneous plunge and pick  without moving the gantry .

I will likely use NEMA8 rather than NEMA11  for the rotation, so weight will drop 60 grams.

Also I have assumed a accel=decel curve for maximums.. but -if decel is to be faster, the stepper is going to have to be bigger, which helps for it to have more authority. This means going to the next NEMA14 up  , the 0.22Nm.
There IS a 0.4Nm one, also.  You are well and truly into ’17 territory there. A ’17  can do bigger work with a shorter body , but the nozzle pitch suffers. 0.4Nm is approx midrange ’17 territory.
On the DOWN plunge is easy for the Z, because the UP is lifting against gravity.  Of course on decel, the opposite applies- lots of work required to slow the payload as the nozzle approaches the Z=0 at mach2.

Advertisements
Posted in Pick and place | Leave a comment

Stepper component rotation accuracy mitigation

More on this subject

(see other post at component rotation…

 

Did a bit more playing. Got some Nema11s today, also (hollow) . These seem to be worse on  in accurate microsteps than the nema8s or 17s I have.

But – the microstep error is always deterministic it appears, which makes mechanical sense.  When I say “always”- on a few tries.

The idea is,  if you are not satisfied with the rotation step that you receive  from the stepper for say 1/16 step, put the component down, turn the nozzle so the stepper is in a different spot. , and then pick the component back up and rotate again.

The microstep errors are in a pattern, so the steps you will need  do not have to be a total guess.

The assumption is, the UP camera will take a look at the rotation you can, and what you get after  the component is put back down, nozzle moved, and picked back up….and rotate again from a different stepper position

So this way, really any step to whatever the stepper drive controller is capable of can be provided. Optionally , I would suggest the step point be ‘current held’ so avoid anything moving, if the controller permits it.

 

 

 

 

Posted in Pick and place | Leave a comment

Nozzle Z probing ideas

Using pressure sensitive resistor.
At about 10% spring compression, resistor drops from 10M to about 50k,  and about 10k >50%.

The nema8+adaptor (85g) sitting up top  compressed the nozzle spring about 85%

nozzle resistor

Posted in Pick and place | Leave a comment

PnP Z axis lift requirements- what size stepper

Z axis must be fast. ideally I want 50mS for 20mm travel.
12.5 mS to full velocity
maybe use a big NEMA14 stepper with rotor inertia  18g/cm2
Total lift weight  – 16g (carriage) +20g( stepper mtging plate) + 120g (NEMA11 hollow stepper) + juki adaptor and nozzle (36g) + air fittings (20g)  .

call it 220g
Driving sprocket size : 24 teeth of GT2

I plug the numbers into my spreadsheet-
Stepper pull out torque , lifting- 0.093Nm
Rotational acceleration = 5235 rad/sec2
Nozzle change – requires a >700g push down… = ~ 7N on the belt or 0.052Nm on the motor shaft.

Choose NEMA14 (35mm square) with 2x lifting = 2 * 0.093 = 0.186Nm…
http://www.omc-stepperonline.com/download/pdf/14HS13-0804S.pdf

Not included in calcs- rotational inertia of pulleys (not much) , and belt weight (not much) , frictional loads of the MGN9 linear guideway (not much).

 

 

 

 

Posted in Pick and place | Leave a comment

PnP frame – Aluminium cut

2020 and 8020 extrusion. wow it is big. big because while travel is 800×600, you need a fair bit each side of the travel.  The single bits of 2020 on top are the top table supports- three tables. Not shown are the bracing plates , guideway mounting plates.

frame.

Image | Posted on by | Leave a comment

component rotation…

On Friday I investigated reported forum issues of not being able to rotate a component with the component on the end of the hollow stepper with sufficient accuracy and resolution.
I set up a job to test this on a basic driver with 1/16 stepping capability.

In all, as in the documentation , there is a periodic sequence between full steps.
Most certainly, in 1/16 mode, I’ll go into 1/32 resolution for the discussion :
you  might get something like 2/32, 2/32 , 1/32, 1/32, 3/32, 2/32, 2/32, 1/32,1/32, 1.32, 3/32, etc .  Sometimes the errors was less than 1/16.

In all cases, the full step is preserved . That’s why microstepping is still accurate over multiple microsteps for a integer number of full steps

1/8 stepping similar- (using 1/16 to demonstrate) 2/16, 2/16, 2/16, 1/16, 1/16, 3/16, 2/16 etc

The different sizes (8,17,23s I had on hand) behaved all differently. Under load with inertia, the 23s where right on 1/16 precisely. On my 6040Z with planet cnc controller- driver, even 1/32 steps were near perfect. So,  different behaviour with different loads.

Which is expected- with no load they are a unloaded and hence underdamped system.

The datasheets say +/-5 % of a full step. OK that us 0.05 * 1.8 = 0.09 degrees. Or 1/20 of a full step. And that is about what I am seeing approx. Maybe a little worse.

Another thing I noticed- behaviour is different going forwards or backwards = a 1/32  step forward in 1/16 mode (under stepped)   might be 3/32 backwards  from the same location- so there is opportunity to iterate to get the right step.
So what is required ?
To get a 20 x 20 LQFP144 down, I want placement better than 1/5 of a pad width (0.35mm)  so 70um..

The outer pins are on a 22mm square – 15.55mm radi circle-  a circumference of 97.7mm.
So, 70um on that is 97.7/0.070 = 1395 steps around the circle or 360/same = 0.257 degrees.

0.257 degrees  is 1/7 step. So I think we are OK. People have reported bad steps in 1/8. So this required more investigation.

In OpenPNP- I wonder if there is enough certainly for the placement estimation ?
In signal processing world, I would take multiple snapshots, and then do multiple estimations and as a starting point, just average the estimations (yes, sure you can get alot more rigorous about the sort of estimation you generate).

I think the direct stepper control should be sufficient. There are a few mitigation strategies for interpolation/ iterative solutions I think. especially if the up vision is involved. You’d need a 2000 step encoder on the stepper shaft to otherwise see what was going on- and that could be printed fairly easily for a largish circle.

The alternative is a reduction drive and a belt , a 1:3 would do it nicely.
If we said the GT2 belt uncertainly was 50 micros, then a simplistic way so look at how big a sprocket we need is  if we need say half of our 0.257degrees with is 0.128 degrees, this is 2801 steps of 0.05mm (belt error)  an a circumference of 140mm (70 teeth at 2mm) .
That would be likely be plenty. Less would likely suffice.
With a 12 tooth drive, say 40 teeth takeup, 1/8 steps would be more than  sufficient.

Is this super resolution required for all components- no- I don’t think do, anything up to about 10×10 should be fine even with +/- 1/8th precision steps.

 

 

 

 

 

 

 

 

 

 

Posted in Pick and place | 1 Comment

Head prototypes

latest head design

head1

The intended placement of the two cameras is visible, also
Belts are not shown, have not figured out how to do belts in Fusion360 yet…

 

 

 

Posted in Pick and place | Leave a comment