Multi homing switches

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26 Mar 2019 06:34 #129646 by JTknives
Is there a way to have have multiple limit switches on each axis. I’m thinking a soft limit switch of the magnetic reed kind then after then a highly precise Metrol switch. I’m wanting it to home at high speed and once the reed switch is tripped it would slow to a crawl till it contacts the last switch. My gole is to have a repeatable home position that’s accurate down to the nats ass.
Thanks guys

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26 Mar 2019 08:36 #129654 by Leon82
Replied by Leon82 on topic Multi homing switches

Is there a way to have have multiple limit switches on each axis. I’m thinking a soft limit switch of the magnetic reed kind then after then a highly precise Metrol switch. I’m wanting it to home at high speed and once the reed switch is tripped it would slow to a crawl till it contacts the last switch. My gole is to have a repeatable home position that’s accurate down to the nats ass.
Thanks guys


The home sequence does a moderate speed touch and then a slow final touch.

I used the 3 dollar lever switches on some pla printed brackets. With a .0005inch indicator I zeroed on the ground vise jaw. After shutting the machine down and back on it always repeated with in less than .0005 and I have cheap rolled ball screws.

Mine homes at about 30ipm and then does a very slow final touch. 1.5ipm I think

They use home search and home latch velocity and you can adjust them in the ini to suit your needs

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26 Mar 2019 14:56 #129694 by islander261
Replied by islander261 on topic Multi homing switches
Don't worry about homing time, on small machines it doesn't take long even when done properly. I use a combined limit/homing switch on each axis and have never had a crash from the drive running off the other end.

1. Seek at a moderate speed which is dependent on your hardware.
2. Back off at a very low speed until the switch releases, this won't take long as the move is short. You will good repeat ability doing this.
3. My table with rack and pinion drive running on cam rollers will do better than .001" homing repeat ability. I used a 1"/.001" dial indicator for testing.

The real thing you should be concerned with is how accurately you can setup your work piece (G54). For plasma cutting a flame cut or distributor cut piece of stock this will be your limiting factor. I just align the stock on my table using a laser pointer. The sheet stock I get from the distributor can be quite parallelogram shaped and the dimensions are usually -0"/+.5". I tried using jigs attached to the table way back when to get easy repeat able stock placement and just gave up as the sheets varied too much. If you can't live with this kind of accuracy out of plasma cutting then you need to have your material waterjet cut and machined because no air plasma will be better than about +/-.032" and worse over time.

John

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27 Mar 2019 11:30 #129760 by andypugh
Replied by andypugh on topic Multi homing switches

Is there a way to have have multiple limit switches on each axis. I’m thinking a soft limit switch of the magnetic reed kind then after then a highly precise Metrol switch.


I don't think that there is a built-in way to do this, but I think it could be configured in HAL.

One version of the homing sequence does a rapid to the switch, backs off, and then does a slow move back towards the switch.

So, a HAL component that would set an output the first time the first switch set, reset it when the machine backs off, then leave it reset until the precise switch triggers would work.

In the dev version of linuxCNC there is a pin called joint.N.home state , and this provides an easy way to choose between the two switches.

Use the home-state value to select between the inputs of a bit-type mux-generic component. Connect the low-precision switch to the inputs that correspond to all homing states other than the final latch and rapid to home.

If you are running with 2.7 I don't think that pin exists (it would be called axis.N.home-state) but it might be worth looking.
I think in 2.7 you might need a custom HAL component. flipflop is close to what is needed and might make a good starting point.

Alternatively: You can arrange that the accurate switch it operated by a long actuator that it can over-run (roller lever like this:

_______________/ o

> direction of movement

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