Homing of nozzles on power up?

[Shai Schechter]

I have yet to set up my machine as I'm still in the design stage but I was wondering after seeing this: https://github.com/Smoothieware/Smoothieware/pull/1349

How does openpnp home two nozzles during power up if I use a "lever" mechanism with an optical sensor as shown below? How would the motor know which way to turn so as not to crash one of the nozzles or the lever? In other words, if the left nozzle is already lower, I would want to ensure that the lever doesn't keep pushing it down, but instead turns clockwise.

[bert shivaan]

well there is prolly a much better way, but my first impression is to extent the "flag" all the way on one side. that way during homing routine if the optic is blocked, move one way to the edge, in open move the other. This will allow the motor to move to a known position then start the homing routine.

second option is place the flag so it is at end of travel in one direction.

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[Shai Schechter]

I see, that would make more sense to have it be a quarter closed and quarter open circle. It would define the direction for sure. Does openpnp support this option? To tell it which direction to turn based on if the switch is closed or open? Or would that just be done with general wiring. I guess one can also set the machine to automatically home on power down, but then if you want to do maintenance it doesn't work well I think.

On Saturday, October 6, 2018 at 12:57:38 PM UTC-4, cncmachineguy wrote:

well there is prolly a much better way, but my first impression is to extent the "flag" all the way on one side. that way during homing routine if the optic is blocked, move one way to the edge, in open move the other. This will allow the motor to move to a known position then start the homing routine.

second option is place the flag so it is at end of travel in one direction.

On Sat, Oct 6, 2018 at 12:44 PM, Shai Schechter <shai...@gmail.com> wrote:

I have yet to set up my machine as I'm still in the design stage but I was wondering after seeing this: https://github.com/Smoothieware/Smoothieware/pull/1349

How does openpnp home two nozzles during power up if I use a "lever" mechanism with an optical sensor as shown below? How would the motor know which way to turn so as not to crash one of the nozzles or the lever? In other words, if the left nozzle is already lower, I would want to ensure that the lever doesn't keep pushing it down, but instead turns clockwise.

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[Mike Menci]

Homing, endstops, acceleration....... is set in Smoothie configuration file. You can find details here; http://smoothieware.org/configuring-smoothie
It is advisable to have a limit switch here due to reverse movement which can be pre-set - for fine tuning of homing.
Follow configuration file!

Mike

[Shai Schechter]

The chinese PNP machines all have a cutout in the middle of a half circle, but that's just a reverse of my setup. I would think that they also didn't think of this or am I wrong? What's the typical chinese PNP setup to avoid nozzle collision on initial homing after power up?

[Олег Перевышин]

Ideally, look for 2 extreme positions and calculate the center.

- I have a sensor and 2 notches on the sides

- I want to turn first to the side (remember the value before the sensor goes off)

- Then turn to the other side (before the sensor is activated)

- Find the center between the coordinates, move to this value and reset the value of the axis

With this approach, basing will always be perfect.

But I do not know how to do this with the G code. I think need to make a script.

[Jason von Nieda]

My $0.02:

* On my OpenBuilds design, I removed the Z home switch entirely. It wasn't that useful. The springs return the nozzle to home and with spring loaded nozzles if center is off by a bit it doesn't really matter.

* On my CharmHigh CHMT36VA the springs also do most of the work, but there is an optical interrupter with a slot in a disk. As far as I can tell it's only used to make sure the nozzles did in fact return to center before moving X/Y so that it doesn't move with the nozzle down. It doesn't appear to use it for homing specifically.

Jason

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[Marek T.]

Hi Jason
I've moreless the same as you say about your CHMT. Pneumatically lowered nozzle returned by spring and optical sensors telling the nozzle is not at down position. I'm checking status of this sensor before every move_to command to don't execute xy-move if it's telling the "nozzle not raised".

It makes the trick if the nozzles are spring returned up. But not if raised with stepper isn't it? Then some kind of homing seems be necessary...

[Mark]

Just as a thought:

 

If you’re really constructing something new, consider adding Z-probing instead. The all you need to home Z is a flat surface at a known X/Y position. You’d probe Z with both nozzles and the mid-point is zero/home.

 

Personally I think Z-probing is essential for pragmatic PNP. Otherwise you end up spending a lot of time entering part heights and/or troubleshooting machine table/feeder height imperfections.

 

Some solutions and ideas:

·         The Liteplacer has a proven spring loaded Z axis plus end-switch (I’m using that, see it in action here)

·         Mike Mencinger has shown a very nice hall sensor solution (although he is not using it for that purpose)

·         I assume you could use Trinamic’s StallGuard™  with their drivers (maybe that’s too slow).

·         Personally, if I’d ever construct a head, I’d look into a piezo sensor that measures the stress on some mechanical joint/axis.

 

_Mark

 

 

 

 

[Mike Menci]

Hello,
Thank you Mark - there is one more video of this hight sensing here- https://youtu.be/-1168zeOEH8
This method involves more mechanical parts and 2x sensor (Reed or hall switch) output to Smoothie for 2 nozzles.
I am hoping that Jason will come up soon with vacuum sensing and use vacuum for hight sensing - this is than more simple without any additional mechanical parts needed.
Mike

[Marius Liebenberg]

Mark does this imply that you probe every time you pick up a part? Something like - move to part location, do a g28 or whatever it takes, go to the zero, pick up the part and set offset.

Do we know where the board height is?

[Marius Liebenberg]

I can't see very well but it looks like the Liteplacer probe only lifts the nozzle. The stepper does not move up and down. Can someone concur?

On Sunday, October 7, 2018 at 11:17:01 AM UTC+2, ma...@makr.zone wrote:

[Shai Schechter]

Hi Jason,

Thank you for the feedback. How does the chinese machine know then which way to turn if the nozzle is not homed? My design also has springs (not shown in the screenshot) but if the indicator of home is in the center of a half circle, how does the motor know which way to turn to ensure it does not keep pushing the nozzle down?

[Mike Menci]

Marius
The purpose is just to give a signal to micro controller that nozzle has landed. This signal can be used later for any other application...for exsample hight sensing, bed level Zerro, PCB elevation, feeder part hight etc...
Mike

[Mark]

> Mark does this imply that you probe every time you pick up a part? Something like - move to part location, do a g28 or whatever it takes, go to the zero, pick up the part and set offset.

 

Marius

 

Disclaimer: First let me remind you, that this is still only in testing phase. It works very well with sticky tape on the PCB but I have not placed on solder paste. Maybe I will need to slow down probing with paste.

 

But yes, the probe works for every pick and every place. It just tells the nozzle to go until the Z sensor is triggered. No coordinates involved. OpenPNP never knows the Z coordinate and never needs to know (some relative motion employed, see below).

 

I don’t need to enter (precise) part heights, a rough Z value one or two Millimeters above the part suffices. The rough value is only needed because I want to move Z down faster than the final probing motion allows.

 

I solved all this by grouping some of the pick and place steps (i.e. vacuum probing, dwell times, a small relative retract move to better measure the “picked” vacuum level, etc.) from OpenPNP into larger Gcode fragments.

 

Ask, if I should dig out more details.

 

_Mark

 

 

[Jason von Nieda]

Hi Shai,

I don't think the Chinese machine homes Z. It basically works the same as my OpenBuilds build - it removes power from the stepper and the springs return the nozzles to center. Then it checks if the hole / slot is detected and if it's not it throws an error.

Jason

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[Mike Menci]

I use a stepper (Nema11)for Z and my Z +/- values can be measured and stored in 0.02 mm precision and recorder if and when needed.
Mike

[Shai Schechter]

Hmm, must be a weak stepper motor then? My stepper motors would theoretically hold enough torque even when power is removed. I don't think a spring would be able to pull back when it is off, but perhaps I need to test it in real life. I didn't realize that the spring pulls it back up AND rotates the motor to neutral via spring load.

[Mike Menci]

My steppers are powered and for Z there is no need to have large torque load on Z  movement is balanced for the two A drives Nema8  rotating nozzles 

BR, 

Mike

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[Shai Schechter]

Yes agree. I was going to use Nema 14 motor from Wantai. What type of motor for Z is everyone else using?

[Mike Menci]

Hi - Most use Nema17 for this 3 types of heads- see picture enclosed.

There is as well this page - https://www.briandorey.com/post/diy-pick-and-place-v2-project-complete with two Nema 11...

Mike

[Marius Liebenberg]

Thanks Mark. I will be watching this development closely as I think it is a great idea to save a lot of time and effort.