Automatic Tool Changer R&D

After doing some more R&D on the tool changing, perhaps there's a better way to change a toolhead than simply prying it off via plastic at the tool holder location, especially if the magnets are supposed to be strong to hold something as precise as a hotend.

I came across a maglock and thought about its application here. Creating a fail-safe magnetic lock would be perfect for this situation(disengages when power is cut).

At first, I thought a toroid with an air gap would work since its circular by design, but the magnetic field wouldn't apply very well here.

So back to the basic, tried and true solenoid with a hollow (soft iron) core. As an example it would look like

http://i.stack.imgur.com/DGdo6.png

which should work.

This would be located on the effector side. The effector would have the solenoid slightly recessed, with raised plastic notches every 5-10mm or so to help prevent rotational forces against the tool, specifically something as precise as the hotend. On the tool end side, there would be an evenly distributed pieces of metal to "lock" into the recesses.

the basic magnetic force equation for a solenoid is:

F = (N*I)2 μ0 A / (2 g2)

N being # of turns of wire (will probably be 22-26 gauge wire)
I = Current flowing through
μ0 = 4π×10-7
A = Area
g = gap between core and wire

I am not sure of the precise mathematical equation to deal with a hollow core material compared to that of a typical solid core, it is logically less permeable and therefore weaker.

1.) I am unsure of any effects the magnetic field will have on the hot end as it will be in the direct center of it. I am using a jhead with a brass nozzle and ptfe. How that compares to something more complicated as having a E3D hotend which is (aluminum?) inside of a hollow iron core (target core material at this time due to low cost) is beyond my scope of E and M physics off the top of my head.

2.) To control the attach/detach, it is possible to use a non-pwm fan input which are 12V. Does anyone know the current limits on the fan inputs for Ramps?

3.) Too many variables to determine ideal current load at this time. How much force required to keep hotend attached while moving at high speed and printing in the Z direction? Maximum current load for the solenoid? How many N turns of wire? Wire Gauge? Experimentation Required.

4.) An alternate method is to use the 2nd extruder input (RAMPS 1.4) to power the electromagnet, but I am trying to avoid that as that seems like quite a waste of an extruder slot, but it is available.


Ideally, a script from Repetier Host / macro button from Pronterface would direct the tool to the tool holder mounted in the vertical space of the delta printer ( might as well use as much as we can right since deltas waste so much empty vertical height). Fan input would turn off, electromagnet would disengage and move to another discrete point to pick up a new tool.