Homing / limits - the manual is misleading, possibly wrong

Hey Rod - can you explain that.
Do you mean during homing?
Shouldn't hit the hard limit at all should it?

Or maybe you mean it shouldn't be allowed to coast past the far side of the trip point.

Chris

Chris, I covered it in this thread very recently
forum.linuxcnc.org/49-basic-configuratio...tiple-slaves?start=0

There are 2 use cases at the commencement of homing. axis on left of home switch, axis on right side of home switch.
So if the home switch remains triggered on the right side, and you home from there, LCNC knows which way to move to find the edge triggering point. Leave it untriggered to the right and it will think its to the left of the axis and a massive crash into the limit WILL happen.

EDIT: Very messy if ignore limits is set while homing

Ahh yes now I get it. if you are not using shared limits it's possible to home on the wrong side of the homing switch unless it stays tripped.

I was reading a bit about your reasons for setting up like this (homing speed) and thought it would be nice to have a switch that could slow down homing.
The you could use shared homing and use the other switch to automatically swicth to latching speed....

Anyways I'll write something up about that - thanks.

How about this:

* If using a separate homing switch, it's possible to start homing on the wrong side of the home switch, which combined
with IGNORE_LIMITS option will lead to a hard crash. You can avoid this by making the home switch keep it's state when the trip dog
is on a particular side until it passes the trip point again. Said another way, the home switch state must represent the position of the dog
relative to the switch (ie _before_ or _after_ the switch), and must stay that way even if the dog moves past the switch in the same direction.

cmorley wrote: I was reading a bit about your reasons for setting up like this (homing speed) and thought it would be nice to have a switch that could slow down homing.

I'm not sure that any more switches are required as the velocities cover it all. I will say it is real cool to push a gantry way out of square and slow the velocities right down and watch it home.

My (metric) settings - velocities are mm per second
You can see my home offset is 45mm which give you an idea of how far it "coasts". Its not really coasting. Surely its just decelerating slowly for max accuracy. The offset on the other side is 43mm from memory to get it square. I tried to get it homing at 12-15 m/min but it took over 100mm to slow down. so 6 m/min used here was a bit of a compromise.

I set my machine up this way because I knew no better and did not want to take short cuts. BUt I have found that the axes with separate switch is much easier to setup and maintain.

cmorley wrote:

B<->C is the home_offset (HOME_OFFSET) = -0.7 units Should this be a +ve value as used for the gantry square as I feel using -ve would drive back onto the switch. Just a thought!!

If you used +0.7 that would move the Origin to the right (- side) by 1.4 units ( ie .7 units to the right of 'C') and you would hit the negative hard limit switch before the soft limit stopped the axis, then next time you moved to the minimum limit after homing.
keeping in mind if you change the home_offset number you move the 'ruler' in the pic too.

As for gantry homing, I don't think I understand what you are getting at.
But maybe I should make a drawing for gantry homing for reference too...

Chris

Chris, I agree you have the unit sign (negative) correct in your example.

My feedback:
Nice improvement on the docs!

• Given most of us read from left to right, it makes most intuitive sense to me to have the scale increase from left to right (zero on left, positive on right).
• The use of H to indicate "hard limits". A lot of people use the term "Hard limits" to refer to mechanical stops. That is how I interpret "hard limits". "Limit switch" makes more sense to me. When I look at the image, I first assumed H was short for Home, given the doc is about homing.
• Can you put in a clear sub-heading to divide the shared home/limit example and the separate home / limit example?

"(If using switch based homing) the homing switch(es) utilize the hard limit switches or are inside the hard limit switch area."
Suggest: (If using switch based homing) the homing switch(es) utilize the hard limit switch(es) (shared home / limit switch) or are inside the hard limit switch area when using a separate home switch.