Optimal Z Sensor Location

Hello all. And Happy Easter or Passover or whatever holiday you might be celebrating this weekend. I am working to get octopi up and running with power control and camera on all 4 of my printers so I can start and monitor prints from my job. For this I need the reliability on my machines to be as good as it possibly can be. One problem I sometimes have is a changing Z homed position. And sometimes even the print head crashing into the build plate. For me this is because I am weak on the G code and when I start a print my machines all warm the bed, then home, then heat the nozzle, then start the print. If I remove any plastic hanging from the nozzle before I start the print I am good every time. However, if there is any filament hanging from the nozzle it contacts the bed before the z sensor triggers. On its own this changes the Z position, But the big problem, the force multiplier that sends the print head crashing into the bed is my placement of the Z probe. Look at my GEEETech i3 printer..

https://oi1050.photobucket.com/album...oWiz/51_16.jpg

Yes the specific configuration I am working with here is aluminum beds and npn inductive proximity switches, a very cheap touchless solution from ebay. But reliable. This problem is not limited to this style sensor. This is a Z probe thing and it applies to bltouch or any other z sensor mounted to the print head. From the above picture it is clearly obvious that I chose to mount the sensor tucked in as close to the nozzle as I could get it. In my mind this was the best possible location. The problem can better be seen from a side profile of the print head..

https://oi1050.photobucket.com/album...oWiz/52_17.jpg

Because of the specific positioning of the nozzle in between the carriage and the z sensor if there should ever be anything hanging off the nozzle it turns the whole print head into a see saw and as the threaded rods drive the z axis down it pulls the z sensor away from the bed. Once this process starts there can be no self correction. No chance of the sensor triggering. Yes I can figure out how to make my printers warm the nozzle before homing the z. But even still I have identified a huge design flaw here.

This is not a new problem. I have identified this a little while back and already made the correction to my home made printer, Printalicious..

https://oi1050.photobucket.com/album...oWiz/141_2.jpg

It is not necessary to mount the z probe on the opposite side of the rails. It just needs to be inboard of the nozzle. Just to make sure that if things do go sideways the whole time things are going sideways if the print head is coming down, so is the sensor. And so long as the sensor is inboard of the nozzle should the nozzle bottom out on the bed the z probe will be driven down at a faster rate than the print head. It will become a see saw in the right direction.

I had a similar delima an solved with a low profile flat proximity sensor. An will in this post explain why I chose this new style proximity sensor.
In the photo below the White mount is a thumb screw adjustable Mounting block, my Proximity sensor a TL-W3MC2 using 5Vdc.

Attachment 12889

As you can see I mounted the proximity sensor behind the X-Axis bearing mount, then using a design that was already created, I re-designed to more suit my needs.
Allowing the low profile shape of the new proximity sensor to be mounted within 15 mm of the nozzle, though I finally decided on a 25mm distance.

By looking at the image you posted I would venture to guess that your Proximity sensor is about 65mm from the nozzle, and shorting this distance would be beneficial
Using my logic I somehow decided against this same design by the manufacture of my 3D Printers, having the sensor the same 65mm distance away from the nozzle.
An am happy with my decision as the auto bed leveling correction has never worked better.

Spring tension thumb screw mount
Attachment 12890

I like it. My Tevo Black Widow printer came with those type of sensors. And I replaced the z- with the BLtouch so I even have a sensor to use..https://oi1050.photobucket.com/album...oWiz/53_12.jpg I will report back when the conversion is complete.